Difference between revisions of "Munitions Constituents - Abiotic Reduction"

Latest revision as of 01:27, 28 April 2022

Munition compounds (MCs) often contain one or more nitro (-NO₂) functional groups which makes them susceptible to abiotic reduction, i.e., transformation by accepting electrons from a chemical electron donor. In soil and groundwater, the most prevalent electron donors are natural organic carbon and iron minerals. Understanding the kinetics and mechanisms of abiotic reduction of MCs by carbon and iron constituents in soil is not only essential for evaluating the environmental fate of MCs but also key to developing cost-efficient remediation strategies. This article summarizes the recent advances in our understanding of MC reduction by carbon and iron based reductants.

Introduction

Figure 1. Common munitions compounds. TNT and RDX are legacy explosives. DNAN, NTO, and NQ are insensitive MCs (IMCs) widely used as replacement for legacy explosives.

Legacy and insensitive MCs (Figure 1) are susceptible to reductive transformation in soil and groundwater. Many redox-active constituents in the subsurface, especially those containing organic carbon, Fe(II), and sulfur can mediate MC reduction. Specific examples include Fe(II)-organic complexes^[2]^[3]^[4]^[5]^[6], iron oxides in the presence of aqueous Fe(II)^[7]^[8]^[9]^[10]^[11]^[12]^[13]^[14]^[15]^[16]^[17], magnetite^[12]^[14]^[18]^[19]^[20], Fe(II)-bearing clays^[21]^[22]^[23]^[24]^[25]^[26]^[27], hydroquinones (as surrogates of natural organic matter)^[4]^[28]^[29]^[30]^[31]^[32]^[33], dissolved organic matter^[34]^[35]^[36], black carbon^[37]^[38]^[39]^[40]^[41]^[42], and sulfides^[43]^[44]. These geo-reductants may control the fate and half-lives of MCs in the environment and can be used to promote MC degradation in soil and groundwater through enhanced natural attenuation^[45].

Figure 2. General mechanism for the reduction of NACs/MCs.

Figure 3. Schematic of natural attenuation of MCs-impacted soils through chemical reduction.

Although the chemical structures of MCs can vary significantly (Figure 1), most of them contain at least one nitro functional group (-NO₂), which is susceptible to reductive transformation^[46]. Of the MCs shown in Figure 1, 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), and 3-nitro-1,2,4-triazol-5-one (NTO)^[47] are nitroaromatic compounds (NACs) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ) are nitramines. The structural differences may result in different reactivities and reaction pathways. Reduction of NACs results in the formation of aromatic amines (i.e., anilines) with nitroso and hydroxylamine compounds as intermediates (Figure 2)^[1].

Although the final reduction products are different for non-aromatic MCs, the reduction process often starts with the transformation of the -NO₂ moiety, either through de-nitration (e.g., RDX^[48]^[49]) or reduction to nitroso^[33]^[50] followed by ring cleavage^[6]^[49]^[50]^[51].

Figure 3 illustrates a typical MC reduction reaction. A redox-active soil constituent, such as organic matter or iron mineral, donates electrons to an MC and transforms the nitro group into an amino group (R-NH₂). The rate at which an MC is reduced can vary by many orders of magnitude depending on the soil constituent, the MC, the reduction potential (E_H) and other media conditions^[52].

The most prevalent reductants in soils are iron minerals and organic carbon such as that found in natural organic matter. It has been suggested that Fe(II)_aq and dissolved organic matter concentrations could serve as indicators of NAC reducibility in anaerobic sediments^[53]. The following sections summarize these two classes of reductants separately and present advances in our understanding of the kinetics of NAC/MC reduction by these geo-reductants.

Carbonaceous Reductants

Figure 4. Chemical structure of commonly used hydroquinones in NACs/MCs kinetic experiments.

The two most predominant forms of organic carbon in natural systems are natural organic matter (NOM) and black carbon (BC)^[54]. Black carbon includes charcoal, soot, graphite, and coal. Until the early 2000s black carbon was considered to be a class of (bio)chemically inert geosorbents^[55]. However, it has been shown that BC can contain abundant quinone functional groups and thus can store and exchange electrons^[56] with chemical^[57] and biological^[58] agents in the surroundings. Specifically, BC such as biochar has been shown to reductively transform MCs including NTO, DNAN, and RDX^[42].

NOM encompasses all the organic compounds present in terrestrial and aquatic environments and can be classified into two groups, non-humic and humic substances. Humic substances (HS) contain a wide array of functional groups including carboxyl, enol, ether, ketone, ester, amide, (hydro)quinone, and phenol^[59]. Quinone and hydroquinone groups are believed to be the predominant redox moieties responsible for the capacity of HS and BC to store and reversibly accept and donate electrons (i.e., through reduction and oxidation of HS/BC, respectively)^[28]^[34]^[56]^[60]^[61]^[62]^[63]^[64]^[65]^[66]^[67]^[68]^[69]^[70]^[71].

Hydroquinones have been widely used as surrogates to understand the reductive transformation of NACs and MCs by NOM. Figure 4 shows the chemical structures of the singly deprotonated forms of four hydroquinone species previously used to study NAC/MC reduction. The second-order rate constants (k_R) for the reduction of NACs/MCs by these hydroquinone species are listed in Table 1, along with the aqueous-phase one electron reduction potentials of the NACs/MCs (E_H^1’) where available. E_H^1’ is an experimentally measurable thermodynamic property that reflects the propensity of a given NAC/MC to accept an electron in water (E_H¹(R-NO₂)):

Equation 1: R-NO₂ + e^- ⇔ R-NO₂^•-

Knowing the identity of and reaction order in the reductant is required to derive the second-order rate constants listed in Table 1. This same reason limits the utility of reduction rate constants measured with complex carbonaceous reductants such as NOM^[34], BC^[37]^[38]^[39]^[72], and HS^[35]^[36], whose chemical structures and redox moieties responsible for the reduction, as well as their abundance, are not clearly defined or known. In other words, the observed rate constants in those studies are specific to the experimental conditions (e.g., pH and NOM source and concentration), and may not be easily comparable to other studies.

Table 1. Aqueous phase one electron reduction potentials and logarithm of second-order rate constants for the reduction of NACs and MCs by the singly deprotonated form of the hydroquinones lawsone, juglone, AHQDS and AHQS, with the second-order rate constants for the deprotonated NAC/MC species (i.e., nitrophenolates and NTO^–) in parentheses.
Compound	E_H^1' (V)	Hydroquinone [log k_R (M^-1s^-1)]
(NAC/MC)	E_H^1' (V)	LAW^-	JUG^-	AHQDS^-	AHQS^-
Nitrobenzene (NB)	-0.485^[28]	0.380^[28]	-1.102^[28]	2.050^[31]	3.060^[31]
2-nitrotoluene (2-NT)	-0.590^[28]	-1.432^[28]	-2.523^[28]	0.775^[4]
3-nitrotoluene (3-NT)	-0.475^[28]	0.462^[28]	-0.921^[28]
4-nitrotoluene (4-NT)	-0.500^[28]	0.041^[28]	-1.292^[28]	1.822^[4]	2.610^[31]
2-chloronitrobenzene (2-ClNB)	-0.485^[28]	0.342^[28]	-0.824^[28]	2.412^[4]
3-chloronitrobenzene (3-ClNB)	-0.405^[28]	1.491^[28]	0.114^[28]
4-chloronitrobenzene (4-ClNB)	-0.450^[28]	1.041^[28]	-0.301^[28]	2.988^[4]
2-acetylnitrobenzene (2-AcNB)	-0.470^[28]	0.519^[28]	-0.456^[28]
3-acetylnitrobenzene (3-AcNB)	-0.405^[28]	1.663^[28]	0.398^[28]
4-acetylnitrobenzene (4-AcNB)	-0.360^[28]	2.519^[28]	1.477^[28]
2-nitrophenol (2-NP)		0.568 (0.079)^[28]
4-nitrophenol (4-NP)		-0.699 (-1.301)^[28]
4-methyl-2-nitrophenol (4-Me-2-NP)		0.748 (0.146)^[28]
4-chloro-2-nitrophenol (4-Cl-2-NP)		1.602 (1.114)^[28]
5-fluoro-2-nitrophenol (5-Cl-2-NP)		0.447 (-0.155)^[28]
2,4,6-trinitrotoluene (TNT)	-0.280^[1]		2.869^[30]	5.204^[4]
2-amino-4,6-dinitrotoluene (2-A-4,6-DNT)	-0.400^[1]		0.987^[30]
4-amino-2,6-dinitrotoluene (4-A-2,6-DNT)	-0.440^[1]		0.079^[30]
2,4-diamino-6-nitrotoluene (2,4-DA-6-NT)	-0.505^[1]		-1.678^[30]
2,6-diamino-4-nitrotoluene (2,6-DA-4-NT)	-0.495^[1]		-1.252^[30]
1,3-dinitrobenzene (1,3-DNB)	-0.345^[30]		1.785^[30]
1,4-dinitrobenzene (1,4-DNB)	-0.257^[30]		3.839^[30]
2-nitroaniline (2-NANE)	< -0.560^[30]		-2.638^[30]
3-nitroaniline (3-NANE)	-0.500^[30]		-1.367^[30]
1,2-dinitrobenzene (1,2-DNB)	-0.290^[30]			5.407^[4]
4-nitroanisole (4-NAN)		-0.661^[31]		1.220^[31]
2-amino-4-nitroanisole (2-A-4-NAN)		-0.924^[31]		1.150^[31]	2.190^[31]
4-amino-2-nitroanisole (4-A-2-NAN)				1.610^[31]	2.360^[31]
2-chloro-4-nitroaniline (2-Cl-5-NANE)		-0.863^[31]		1.250^[31]	2.210^[31]
N-methyl-4-nitroaniline (MNA)		-1.740^[31]		-0.260^[31]	0.692^[31]
3-nitro-1,2,4-triazol-5-one (NTO)				5.701 (1.914)^[36]
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)				-0.349^[48]

Figure 5. Relative reduction rate constants of the NACs/MCs listed in Table 1 for AHQDS^–. Rate constants are compared with respect to RDX. Abbreviations of NACs/MCs as listed in Table 1.

Most of the current knowledge about MC degradation is derived from studies using NACs. The reduction kinetics of only four MCs, namely TNT, N-methyl-4-nitroaniline (MNA), NTO, and RDX, have been investigated with hydroquinones. Of these four MCs, only the reduction rates of MNA and TNT have been modeled^[30]^[31]^[73]^[74].

Using the rate constants obtained with AHQDS^–, a relative reactivity trend can be obtained (Figure 5). RDX is the slowest reacting MC in Table 1, hence it was selected to calculate the relative rates of reaction (i.e., log k_NAC/MC – log k_RDX). If only the MCs in Figure 5 are considered, the reactivity spans 6 orders of magnitude following the trend: RDX ≈ MNA < NTO^– < DNAN < TNT < NTO. The rate constant for DNAN reduction by AHQDS^– is not yet published and hence not included in Table 1. Note that speciation of NACs/MCs can significantly affect their reduction rates. Upon deprotonation, the NAC/MC becomes negatively charged and less reactive as an oxidant (i.e., less prone to accept an electron). As a result, the second-order rate constant can decrease by 0.5-0.6 log unit in the case of nitrophenols and approximately 4 log units in the case of NTO (numbers in parentheses in Table 1)^[28]^[36].

Ferruginous Reductants

Table 2. Logarithm of second-order rate constants for reduction of NACs and MCs by dissolved Fe(II) complexes with the stoichiometry of ligand and iron in square brackets
Compound	E_H^1' (V)	Cysteine^[3] [FeL₂]^2-	Thioglycolic acid^[3] [FeL₂]^2-	DFOB^[5] [FeHL]⁰	AcHA^[5] [FeL₃]^-	Tiron ^a [FeL₂]^6-	Fe-Porphyrin ^b
Compound	E_H^1' (V)	Fe(II)-Ligand [log k_R (M^-1s^-1)]
Nitrobenzene	-0.485^[28]	-0.347	0.874	2.235	-0.136	1.424^[75] 4.000^[74]	-0.018^[28] 0.026^[74]
2-nitrotoluene	-0.590^[28]						-0.602^[28]
3-nitrotoluene	-0.475^[28]	-0.434	0.767	2.106	-0.229	1.999^[75] 3.800^[74]	0.041^[28]
4-nitrotoluene	-0.500^[28]	-0.652	0.528	2.013	-0.402	1.446^[75] 3.500^[74]	-0.174^[28]
2-chloronitrobenzene	-0.485^[28]						0.944^[28]
3-chloronitrobenzene	-0.405^[28]	0.360	1.810	2.888	0.691	2.882^[75] 4.900^[74]	0.724^[28]
4-chloronitrobenzene	-0.450^[28]	0.230	1.415	2.512	0.375	3.937^[75] 4.581^[2]	0.431^[28] 0.289^[74]
2-acetylnitrobenzene	-0.470^[28]						1.377^[28]
3-acetylnitrobenzene	-0.405^[28]						0.799^[28]
4-acetylnitrobenzene	-0.360^[28]	0.965	2.771		1.872	5.028^[75] 6.300^[74]	1.286^[28]
RDX	-0.550^[76]					2.212^[75] 2.864^[6]
HMX	-0.660^[76]					-2.762^[75]
TNT	-0.280^[1]					7.427^[75]	2.050^[74]
1,3-dinitrobenzene	-0.345^[30]						1.220^[74]
2,4-dinitrotoluene	-0.380^[1]					5.319^[75]	1.156^[74]
Nitroguanidine (NQ)	-0.700^[76]					-0.185^[75]
2,4-dinitroanisole (DNAN)	-0.400^[76]						1.243^[74]
Notes: ^a 4,5-dihydroxybenzene-1,3-disulfonate (Tiron). ^b meso-tetra(N-methyl-pyridyl)iron porphin in cysteine.

Table 3. Rate constants for the reduction of MCs by iron minerals
MC	Iron Mineral	Iron mineral loading (g/L)	Surface area (m²/g)	Fe(II)_aq initial (mM) ^b	Fe(II)_aq after 24 h (mM) ^c	Fe(II)_aq sorbed (mM) ^d	pH	Buffer	Buffer (mM)	MC initial (μM) ^e	log k_obs (h^-1) ^f	log k_SA (Lh^-1m^-2) ^g
TNT^[30]	Goethite	0.64	17.5	1.5			7.0	MOPS	25	50	1.200	0.170
RDX^[77]	Magnetite	1.00	44	0.1	0	0.10	7.0	HEPES	50	50	-3.500	-5.200
RDX^[77]	Magnetite	1.00	44	0.2	0.02	0.18	7.0	HEPES	50	50	-2.900	-4.500
RDX^[77]	Magnetite	1.00	44	0.5	0.23	0.27	7.0	HEPES	50	50	-1.900	-3.600
RDX^[77]	Magnetite	1.00	44	1.5	0.94	0.56	7.0	HEPES	50	50	-1.400	-3.100
RDX^[77]	Magnetite	1.00	44	3.0	1.74	1.26	7.0	HEPES	50	50	-1.200	-2.900
RDX^[77]	Magnetite	1.00	44	5.0	3.38	1.62	7.0	HEPES	50	50	-1.100	-2.800
RDX^[77]	Magnetite	1.00	44	10.0	7.77	2.23	7.0	HEPES	50	50	-1.000	-2.600
RDX^[77]	Magnetite	1.00	44	1.6	1.42	0.16	6.0	MES	50	50	-2.700	-4.300
RDX^[77]	Magnetite	1.00	44	1.6	1.34	0.24	6.5	MOPS	50	50	-1.800	-3.400
RDX^[77]	Magnetite	1.00	44	1.6	1.21	0.37	7.0	MOPS	50	50	-1.200	-2.900
RDX^[77]	Magnetite	1.00	44	1.6	1.01	0.57	7.0	HEPES	50	50	-1.200	-2.800
RDX^[77]	Magnetite	1.00	44	1.6	0.76	0.82	7.5	HEPES	50	50	-0.490	-2.100
RDX^[77]	Magnetite	1.00	44	1.6	0.56	1.01	8.0	HEPES	50	50	-0.590	-2.200
NG^[78]	Magnetite	4.00	0.56	4.0			7.4	HEPES	90	226
NG^[79]	Pyrite	20.00	0.53				7.4	HEPES	100	307	-2.213	-3.238
TNT^[79]	Pyrite	20.00	0.53				7.4	HEPES	100	242	-2.812	-3.837
RDX^[79]	Pyrite	20.00	0.53				7.4	HEPES	100	201	-3.058	-4.083
RDX^[51]	Carbonate Green Rust	5.00	36				7.0			100
RDX^[51]	Sulfate Green Rust	5.00	20				7.0			100
DNAN^[80]	Sulfate Green Rust	10.00					8.4			500
NTO^[80]	Sulfate Green Rust	10.00					8.4			500
DNAN^[81]	Magnetite	2.00	17.8	1.0			7.0	NaHCO₃	10	200	-0.100	-1.700
DNAN^[81]	Mackinawite	1.50					7.0	NaHCO₃	10	200	0.061
DNAN^[81]	Goethite	1.00	103.8	1.0			7.0	NaHCO₃	10	200	0.410	-1.600
RDX^[82]	Magnetite	0.62		1.0			7.0	NaHCO₃	10	17.5	-1.100
RDX^[82]	Magnetite	0.62					7.0	MOPS	50	17.5	-0.270
RDX^[82]	Magnetite	0.62		1.0			7.0	MOPS	10	17.6	-0.480
NTO^[83]	Hematite	1.00	5.7	1.0	0.92	0.08	5.5	MES	50	30	-0.550	-1.308
NTO^[83]	Hematite	1.00	5.7	1.0	0.85	0.15	6.0	MES	50	30	0.619	-0.140
NTO^[83]	Hematite	1.00	5.7	1.0	0.9	0.10	6.5	MES	50	30	1.348	0.590
NTO^[83]	Hematite	1.00	5.7	1.0	0.77	0.23	7.0	MOPS	50	30	2.167	1.408
NTO^[83]	Hematite ^a	1.00	5.7		1.01		5.5	MES	50	30	-1.444	-2.200
NTO^[83]	Hematite ^a	1.00	5.7		0.97		6.0	MES	50	30	-0.658	-1.413
NTO^[83]	Hematite ^a	1.00	5.7		0.87		6.5	MES	50	30	0.068	-0.688
NTO^[83]	Hematite ^a	1.00	5.7		0.79		7.0	MOPS	50	30	1.210	0.456
RDX^[50]	Mackinawite	0.45					6.5	NaHCO₃	10	250	-0.092
RDX^[50]	Mackinawite	0.45					7.0	NaHCO₃	10	250	0.009
RDX^[50]	Mackinawite	0.45					7.5	NaHCO₃	10	250	0.158
RDX^[50]	Green Rust	5					6.5	NaHCO₃	10	250	-1.301
RDX^[50]	Green Rust	5					7.0	NaHCO₃	10	250	-1.097
RDX^[50]	Green Rust	5					7.5	NaHCO₃	10	250	-0.745
RDX^[50]	Goethite	0.5		1	1		6.5	NaHCO₃	10	250	-0.921
RDX^[50]	Goethite	0.5		1	1		7.0	NaHCO₃	10	250	-0.347
RDX^[50]	Goethite	0.5		1	1		7.5	NaHCO₃	10	250	0.009
RDX^[50]	Hematite	0.5		1	1		6.5	NaHCO₃	10	250	-0.824
RDX^[50]	Hematite	0.5		1	1		7.0	NaHCO₃	10	250	-0.456
RDX^[50]	Hematite	0.5		1	1		7.5	NaHCO₃	10	250	-0.237
RDX^[50]	Magnetite	2		1	1		6.5	NaHCO₃	10	250	-1.523
RDX^[50]	Magnetite	2		1	1		7.0	NaHCO₃	10	250	-0.824
RDX^[50]	Magnetite	2		1	1		7.5	NaHCO₃	10	250	-0.229
DNAN^[84]	Mackinawite	4.28	0.25				6.5	NaHCO₃	8.5 + 20% CO₂(g)	400	0.836	0.806
DNAN^[84]	Mackinawite	4.28	0.25				7.6	NaHCO₃	95.2 + 20% CO₂(g)	400	0.762	0.732
DNAN^[84]	Commercial FeS	5.00	0.214				6.5	NaHCO₃	8.5 + 20% CO₂(g)	400	0.477	0.447
DNAN^[84]	Commercial FeS	5.00	0.214				7.6	NaHCO₃	95.2 + 20% CO₂(g)	400	0.745	0.716
NTO^[84]	Mackinawite	4.28	0.25				6.5	NaHCO₃	8.5 + 20% CO₂(g)	1000	0.663	0.633
NTO^[84]	Mackinawite	4.28	0.25				7.6	NaHCO₃	95.2 + 20% CO₂(g)	1000	0.521	0.491
NTO^[84]	Commercial FeS	5.00	0.214				6.5	NaHCO₃	8.5 + 20% CO₂(g)	1000	0.492	0.462
NTO^[84]	Commercial FeS	5.00	0.214				7.6	NaHCO₃	95.2 + 20% CO₂(g)	1000	0.427	0.398
Notes: ^a Dithionite-reduced hematite; experiments conducted in the presence of 1 mM sulfite. ^b Initial aqueous Fe(II); not added for Fe(II) bearing minerals. ^c Aqueous Fe(II) after 24h of equilibration. ^d Difference between b and c. ^e Initial nominal MC concentration. ^f Pseudo-first order rate constant. ^g Surface area normalized rate constant calculated as k_Obs / (surface area concentration) or k_Obs / (surface area × mineral loading).

Table 4. Rate constants for the reduction of NACs by iron oxides in the presence of aqueous Fe(II)
NAC ^a	Iron Oxide	Iron oxide loading (g/L)	Surface area (m²/g)	Fe(II)_aq initial (mM) ^b	Fe(II)_aq after 24 h (mM) ^c	Fe(II)_aq sorbed (mM) ^d	pH	Buffer	Buffer (mM)	NAC initial (μM) ^e	log k_obs (h^-1) ^f	log k_SA (Lh^-1m^-2) ^g
NB^[12]	Magnetite	0.200	56.00	1.5000			7.00	Phosphate	10	50	1.05E+00	7.75E-04
4-ClNB^[12]	Magnetite	0.200	56.00	1.5000			7.00	Phosphate	10	50	1.14E+00	8.69E-02
4-ClNB^[30]	Goethite	0.640	17.50	1.5000			7.00	MOPS	25	50	-1.01E-01	-1.15E+00
4-ClNB^[14]	Goethite	1.500	16.20	1.2400	0.9600	0.2800	7.20	MOPS	1.2	0.5 - 3	1.68E+00	2.80E-01
4-ClNB^[14]	Hematite	1.800	13.70	1.0400	1.0100	0.0300	7.20	MOPS	1.2	0.5 - 3	-2.32E+00	-3.72E+00
4-ClNB^[14]	Lepidocrocite	1.400	17.60	1.1400	1.0000	0.1400	7.20	MOPS	1.2	0.5 - 3	1.51E+00	1.20E-01
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3500	0.0300	7.97	HEPES	25	15	-7.47E-01	-8.61E-01
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3700	0.0079	7.67	HEPES	25	15	-1.51E+00	-1.62E+00
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3600	0.0200	7.50	MOPS	25	15	-2.15E+00	-2.26E+00
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3600	0.0120	7.28	MOPS	25	15	-3.08E+00	-3.19E+00
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3700	0.0004	7.00	MOPS	25	15	-3.22E+00	-3.34E+00
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3700	0.0024	6.80	MOPSO	25	15	-3.72E+00	-3.83E+00
4-CNNB^[7]	Ferrihydrite	0.004	292.00	0.3750	0.3700	0.0031	6.60	MES	25	15	-3.83E+00	-3.94E+00
4-CNNB^[7]	Ferrihydrite	0.020	292.00	0.3750	0.3700	0.0031	6.60	MES	25	15	-3.83E+00	-4.60E+00
4-CNNB^[7]	Ferrihydrite	0.110	292.00	0.3750	0.3700	0.0032	6.60	MES	25	15	-1.57E+00	-3.08E+00
4-CNNB^[7]	Ferrihydrite	0.220	292.00	0.3750	0.3700	0.0040	6.60	MES	25	15	-1.12E+00	-2.93E+00
4-CNNB^[7]	Ferrihydrite	0.551	292.00	0.3750	0.3700	0.0092	6.60	MES	25	15	-6.18E-01	-2.82E+00
4-CNNB^[7]	Ferrihydrite	1.099	292.00	0.3750	0.3500	0.0240	6.60	MES	25	15	-3.66E-01	-2.87E+00
4-CNNB^[7]	Ferrihydrite	1.651	292.00	0.3750	0.3400	0.0340	6.60	MES	25	15	-8.35E-02	-2.77E+00
4-CNNB^[7]	Ferrihydrite	2.199	292.00	0.3750	0.3300	0.0430	6.60	MES	25	15	-3.11E-02	-2.84E+00
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3320	0.0430	7.97	HEPES	25	15	1.63E+00	1.52E+00
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3480	0.0270	7.67	HEPES	25	15	1.26E+00	1.15E+00
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3470	0.0280	7.50	MOPS	25	15	7.23E-01	6.10E-01
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3680	0.0066	7.28	MOPS	25	15	4.53E-02	-6.86E-02
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3710	0.0043	7.00	MOPS	25	15	-3.12E-01	-4.26E-01
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3710	0.0042	6.80	MOPSO	25	15	-7.75E-01	-8.89E-01
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3680	0.0069	6.60	MES	25	15	-1.39E+00	-1.50E+00
4-CNNB^[7]	Hematite	0.038	34.00	0.3750	0.3750	0.0003	6.10	MES	25	15	-2.77E+00	-2.88E+00
4-CNNB^[7]	Hematite	0.016	34.00	0.3750	0.3730	0.0024	6.60	MES	25	15	-3.20E+00	-2.95E+00
4-CNNB^[7]	Hematite	0.024	34.00	0.3750	0.3690	0.0064	6.60	MES	25	15	-2.74E+00	-2.66E+00
4-CNNB^[7]	Hematite	0.033	34.00	0.3750	0.3680	0.0069	6.60	MES	25	15	-1.39E+00	-1.43E+00
4-CNNB^[7]	Hematite	0.177	34.00	0.3750	0.3640	0.0110	6.60	MES	25	15	3.58E-01	-4.22E-01
4-CNNB^[7]	Hematite	0.353	34.00	0.3750	0.3630	0.0120	6.60	MES	25	15	9.97E-01	-8.27E-02
4-CNNB^[7]	Hematite	0.885	34.00	0.3750	0.3480	0.0270	6.60	MES	25	15	1.34E+00	-1.34E-01
4-CNNB^[7]	Hematite	1.771	34.00	0.3750	0.3380	0.0370	6.60	MES	25	15	1.78E+00	3.03E-04
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3460	0.0290	7.97	HEPES	25	15	1.31E+00	1.20E+00
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3610	0.0140	7.67	HEPES	25	15	5.82E-01	4.68E-01
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3480	0.0270	7.50	MOPS	25	15	4.92E-02	-6.47E-02
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3640	0.0110	7.28	MOPS	25	15	-3.77E-01	-4.90E-01
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3640	0.0110	7.00	MOPS	25	15	-1.25E+00	-1.36E+00
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3620	0.0130	6.80	MOPSO	25	15	-1.74E+00	-1.86E+00
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3740	0.0015	6.60	MES	25	15	-2.58E+00	-2.69E+00
4-CNNB^[7]	Lepidocrocite	0.027	49.00	0.3750	0.3700	0.0046	6.10	MES	25	15	-3.80E+00	-3.92E+00
4-CNNB^[7]	Lepidocrocite	0.020	49.00	0.3750	0.3740	0.0014	6.60	MES	25	15	-2.58E+00	-2.57E+00
4-CNNB^[7]	Lepidocrocite	11.980	49.00	0.3750	0.3620	0.0130	6.60	MES	25	15	-5.78E-01	-3.35E+00
4-CNNB^[7]	Lepidocrocite	0.239	49.00	0.3750	0.3530	0.0220	6.60	MES	25	15	-2.78E-02	-1.10E+00
4-CNNB^[7]	Lepidocrocite	0.600	49.00	0.3750	0.3190	0.0560	6.60	MES	25	15	3.75E-01	-1.09E+00
4-CNNB^[7]	Lepidocrocite	1.198	49.00	0.3750	0.2700	0.1050	6.60	MES	25	15	5.05E-01	-1.26E+00
4-CNNB^[7]	Lepidocrocite	1.798	49.00	0.3750	0.2230	0.1520	6.60	MES	25	15	5.56E-01	-1.39E+00
4-CNNB^[7]	Lepidocrocite	2.388	49.00	0.3750	0.1820	0.1930	6.60	MES	25	15	5.28E-01	-1.54E+00
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3440	0.0310	7.97	HEPES	25	15	9.21E-01	8.07E-01
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3660	0.0094	7.67	HEPES	25	15	3.05E-01	1.91E-01
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3570	0.0180	7.50	MOPS	25	15	-9.96E-02	-2.14E-01
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3640	0.0110	7.28	MOPS	25	15	-8.18E-01	-9.32E-01
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3670	0.0084	7.00	MOPS	25	15	-1.61E+00	-1.73E+00
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3750	0.0004	6.80	MOPSO	25	15	-1.82E+00	-1.93E+00
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3730	0.0018	6.60	MES	25	15	-2.26E+00	-2.37E+00
4-CNNB^[7]	Goethite	0.025	51.00	0.3750	0.3670	0.0076	6.10	MES	25	15	-3.56E+00	-3.67E+00
4-CNNB^[7]	Goethite	0.020	51.00	0.3750	0.3680	0.0069	6.60	MES	25	15	-2.26E+00	-2.27E+00
4-CNNB^[7]	Goethite	0.110	51.00	0.3750	0.3660	0.0090	6.60	MES	25	15	-3.19E-01	-1.07E+00
4-CNNB^[7]	Goethite	0.220	51.00	0.3750	0.3540	0.0210	6.60	MES	25	15	5.00E-01	-5.50E-01
4-CNNB^[7]	Goethite	0.551	51.00	0.3750	0.3220	0.0530	6.60	MES	25	15	1.03E+00	-4.15E-01
4-CNNB^[7]	Goethite	1.100	51.00	0.3750	0.2740	0.1010	6.60	MES	25	15	1.46E+00	-2.88E-01
4-CNNB^[7]	Goethite	1.651	51.00	0.3750	0.2330	0.1420	6.60	MES	25	15	1.66E+00	-2.70E-01
4-CNNB^[7]	Goethite	2.196	51.00	0.3750	0.1910	0.1840	6.60	MES	25	15	1.83E+00	-2.19E-01
4-CNNB^[7]	Goethite	0.142	51.00	0.3750			6.60	MES	25	15	1.99E-01	-6.61E-01
4-AcNB^[7]	Goethite	0.142	51.00	0.3750			6.60	MES	25	15	-6.85E-02	-9.28E-01
4-ClNB^[7]	Goethite	0.142	51.00	0.3750			6.60	MES	25	15	-5.47E-01	-1.41E+00
4-BrNB^[7]	Goethite	0.142	51.00	0.3750			6.60	MES	25	15	-5.73E-01	-1.43E+00
NB^[7]	Goethite	0.142	51.00	0.3750			6.60	MES	25	15	-7.93E-01	-1.65E+00
4-MeNB^[7]	Goethite	0.142	51.00	0.3750			6.60	MES	25	15	-9.79E-01	-1.84E+00
4-ClNB^[11]	Goethite	0.040	186.75	1.0000	0.8050	0.1950	7.00				5.53E-01	-3.20E-01
4-ClNB^[11]	Goethite	7.516	16.10	1.0000	0.9260	0.0740	7.00				2.08E+00	0.00E+00
4-ClNB^[11]	Ferrihydrite	0.111	252.60	1.0000	0.6650	0.3350	7.00				-1.12E-01	-1.56E+00
4-ClNB^[11]	Lepidocrocite	2.384	60.40	1.0000	0.9250	0.0750	7.00				1.30E+00	-8.60E-01
4-ClNB^[10]	Goethite	10.000	14.90	1.0000			7.20	HEPES	10	10 - 50	2.26E+00	8.00E-02
4-ClNB^[10]	Goethite	3.000	14.90	1.0000			7.20	HEPES	10	10 - 50	2.38E+00	7.30E-01
4-ClNB^[10]	Lepidocrocite	2.700	16.20	1.0000			7.20	HEPES	10	10 - 50	9.20E-01	-7.20E-01
4-ClNB^[10]	Lepidocrocite	10.000	16.20	1.0000			7.20	HEPES	10	10 - 50	1.03E+00	-1.18E+00
4-ClNB^[13]	Goethite	0.325	140.00	1.0000			7.00	Bicarbonate	10	100	-1.32E-01	-1.79E+00
4-ClNB^[13]	Goethite	0.325	140.00	1.0000			6.50	Bicarbonate	10	100	-4.42E-01	-2.10E+00
NB^[16]	Goethite	0.500	30.70	0.1000	0.1120	0.0090	6.00	MES	25	12	-1.42E+00	-2.61E+00
NB^[16]	Goethite	0.500	30.70	0.5000	0.5150	0.0240	6.00	MES	25	15	-7.45E-01	-1.93E+00
NB^[16]	Goethite	0.500	30.70	1.0000	1.0280	0.0140	6.00	MES	25	19	-7.45E-01	-1.93E+00
NB^[16]	Goethite	1.000	30.70	0.1000	0.0960	0.0260	6.00	MES	25	13	-1.12E+00	-2.61E+00
NB^[16]	Goethite	1.000	30.70	0.5000	0.4890	0.0230	6.00	MES	25	14	-5.53E-01	-2.04E+00
NB^[16]	Goethite	1.000	30.70	1.0000	0.9870	0.0380	6.00	MES	25	19	-2.52E-01	-1.74E+00
NB^[16]	Goethite	2.000	30.70	0.1000	0.0800	0.0490	6.00	MES	25	11	-8.86E-01	-2.67E+00
NB^[16]	Goethite	2.000	30.70	0.6000	0.4890	0.0640	6.00	MES	25	14	-1.08E-01	-1.90E+00
NB^[16]	Goethite	2.000	30.70	1.1000	0.9870	0.0670	6.00	MES	25	14	2.30E-01	-1.56E+00
NB^[16]	Goethite	4.000	30.70	0.1000	0.0600	0.0650	6.00	MES	25	11	-8.89E-01	-2.98E+00
NB^[16]	Goethite	4.000	30.70	0.6000	0.3960	0.1550	6.00	MES	25	17	1.43E-01	-1.95E+00
NB^[16]	Goethite	4.000	30.70	1.0000	0.8360	0.1450	6.00	MES	25	16	4.80E-01	-1.61E+00
NB^[16]	Goethite	4.000	30.70	5.6000	5.2110	0.3790	6.00	MES	25	15	1.17E+00	-9.19E-01
NB^[16]	Goethite	1.000	30.70	0.1000	0.0870	0.0300	6.50	MES	25	5.5	-1.74E-01	-1.66E+00
NB^[16]	Goethite	1.000	30.70	0.5000	0.4920	0.0300	6.50	MES	25	15	3.64E-01	-1.12E+00
NB^[16]	Goethite	1.000	30.70	1.0000	0.9390	0.0650	6.50	MES	25	18	6.70E-01	-8.17E-01
NB^[16]	Goethite	2.000	30.70	0.1000	0.0490	0.0730	6.50	MES	25	5.2	3.01E-01	-1.49E+00
NB^[16]	Goethite	2.000	30.70	0.5000	0.4640	0.0710	6.50	MES	25	14	8.85E-01	-9.03E-01
NB^[16]	Goethite	2.000	30.70	1.0000	0.9130	0.1280	6.50	MES	25	16	1.12E+00	-6.64E-01
NB^[16]	Goethite	1.000	30.70	0.1000	0.0630	0.0480	7.00	MOPS	25	5.3	6.12E-01	-8.75E-01
NB^[16]	Goethite	1.000	30.70	0.5000	0.4690	0.0520	7.00	MOPS	25	9	1.51E+00	2.07E-02
NB^[16]	Goethite	1.000	30.70	1.0000	0.9360	0.1090	7.00	MOPS	25	18	1.33E+00	-1.53E-01
NB^[16]	Goethite	2.000	30.70	0.1000	0.0290	0.0880	7.00	MOPS	25	12	6.85E-01	-1.10E+00
NB^[16]	Goethite	2.000	30.70	0.5000	0.3950	0.1450	7.00	MOPS	25	15	1.59E+00	-1.95E-01
Notes: ^a The NACs are Nitrobenzene (NB), 4-chloronitrobenzene(4-ClNB), 4-cyanonitrobenzene (4-CNNB), 4-acetylnitrobenzene (4-AcNB), 4-bromonitrobenzene (4-BrNB), 4-nitrotoluene (4-MeNB). ^b Initial aqueous Fe(II). ^c Aqueous Fe(II) after 24h of equilibration. ^d Difference between b and c. ^e Initial nominal NAC concentration. ^f Pseudo-first order rate constant. ^g Surface area normalized rate constant calculated as k_Obs / (surface area × mineral loading).

Iron(II) can be complexed by a myriad of organic ligands and may thereby become more reactive towards MCs and other pollutants. The reactivity of an Fe(II)-organic complex depends on the relative preference of the organic ligand for Fe(III) versus Fe(II)^[5]. Since the majority of naturally occurring ligands complex Fe(III) more strongly than Fe(II), the reduction potential of the resulting Fe(III) complex is lower than that of aqueous Fe(III); therefore, complexation by organic ligands often renders Fe(II) a stronger reductant thermodynamically^[85]. The reactivity of dissolved Fe(II)-organic complexes towards NACs/MCs has been investigated. The intrinsic, second-order rate constants and one electron reduction potentials are listed in Table 2.

In addition to forming organic complexes, iron is ubiquitous in minerals. Iron-bearing minerals play an important role in controlling the environmental fate of contaminants through adsorption^[86]^[87] and reduction^[88] processes. Studies have shown that aqueous Fe(II) itself cannot reduce NACs/MCs at circumneutral pH^[12]^[77] but in the presence of an iron oxide (e.g., goethite, hematite, lepidocrocite, ferrihydrite, or magnetite), NACs^[7]^[12]^[13]^[14]^[21] and MCs such as TNT^[30], RDX^[77], DNAN^[81], and NG^[78] can be rapidly reduced. Unlike ferric oxides, Fe(II)-bearing minerals including clays^[21]^[22]^[23]^[24]^[25]^[26]^[27], green rust^[51]^[80], mackinawite^[14]^[81]^[84] and pyrite^[14]^[79] do not need aqueous Fe(II) to be reactive toward NACs/MCs. However, upon oxidation, sulfate green rust was converted into lepidocrocite^[80], and mackinawite into goethite^[84], suggesting that aqueous Fe(II) coupled to Fe(III) oxides might be at least partially responsible for continued degradation of NACs/MCs in the subsurface once the parent reductant (e.g., green rust or iron sulfide) oxidizes.

The reaction conditions and rate constants for a list of studies on MC reduction by iron oxide-aqueous Fe(II) redox couples and by other Fe(II)-containing minerals are shown in Table 3^[30]^[51]^[77]^[81]^[79]^[82]^[83]. Unlike hydroquinones and Fe(II) complexes, where second-order rate constants can be readily calculated, the reduction rate constants of NACs/MCs in mineral suspensions are often specific to the experimental conditions used and are usually reported as BET surface area-normalized reduction rate constants (k_SA). In the case of iron oxide-Fe(II) redox couples, reduction rate constants have been shown to increase with pH (specifically, with [OH^–]²) and aqueous Fe(II) concentration, both of which correspond to a decrease in the system's reduction potential^[7]^[9]^[83].

For minerals that contain structural iron(II) and can reduce pollutants in the absence of aqueous Fe(II), the observed rates of reduction increased with increasing structural Fe(II) content, as seen with iron-bearing clays^[23]^[24] and green rust^[51]. This dependency on Fe(II) content allows for the derivation of second-order rate constants, as shown on Table 3 for the reduction of RDX by green rust^[51], and the development of reduction potential (E_H)-based models^[23]^[89]^[90]^[91], where E_H represents the reduction potential of the iron-bearing clays. Iron-bearing expandable clay minerals represent a special case, which in addition to reduction can remove NACs/MCs through adsorption. This is particularly important for planar NACs/MCs that contain multiple electron-withdrawing nitro groups and can form strong electron donor-acceptor (EDA) complexes with the clay surface^[21]^[25]^[26].

Although the second-order rate constants derived for Fe(II)-bearing minerals may allow comparison among different studies, they may not reflect changes in reactivity due to variations in surface area, pH, and the presence of ions. Anions such as bicarbonate^[51]^[82]^[92] and phosphate^[51]^[93] are known to decrease the reactivity of iron oxides-Fe(II) redox couples and green rust. Sulfite has also been shown to decrease the reactivity of hematite-Fe(II) towards the deprotonated form of NTO (Table 3)^[83]. Exchanging cations in iron-bearing clays can change the reactivity of these minerals by up to 7-fold^[21]. Thus, more comprehensive models are needed to account for the complexities in the subsurface environment.

The reduction of NACs has been widely studied in the presence of different iron minerals, pH, and Fe(II)_(aq) concentrations (Table 4)^[7]^[12]^[13]^[14]^[21]. Only selected NACs are included in Table 4. For more information on other NACs and ferruginous reductants, please refer to the cited references.

References

^ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 ^1.7 ^1.8 Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M., 2016. Environmental Organic Chemistry, 3rd Edition. John Wiley and Sons, Ltd, 1024 pages. ISBN: 978-1-118-76723-8
^ ^2.0 ^2.1 Naka, D., Kim, D., and Strathmann, T.J., 2006. Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II)−Catechol Complexes. Environmental Science and Technology 40(9), pp. 3006–3012. DOI: 10.1021/es060044t
^ ^3.0 ^3.1 ^3.2 Naka, D., Kim, D., Carbonaro, R.F., and Strathmann, T.J., 2008. Abiotic reduction of nitroaromatic contaminants by iron(II) complexes with organothiol ligands. Environmental Toxicology and Chemistry, 27(6), pp. 1257–1266. DOI: 10.1897/07-505.1
^ ^4.0 ^4.1 ^4.2 ^4.3 ^4.4 ^4.5 ^4.6 ^4.7 Hartenbach, A.E., Hofstetter, T.B., Aeschbacher, M., Sander, M., Kim, D., Strathmann, T.J., Arnold, W.A., Cramer, C.J., and Schwarzenbach, R.P., 2008. Variability of Nitrogen Isotope Fractionation during the Reduction of Nitroaromatic Compounds with Dissolved Reductants. Environmental Science and Technology 42(22), pp. 8352–8359. DOI: 10.1021/es801063u
^ ^5.0 ^5.1 ^5.2 ^5.3 Kim, D., Duckworth, O.W., and Strathmann, T.J., 2009. Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants. Geochimica et Cosmochimica Acta, 73(5), pp. 1297–1311. DOI: 10.1016/j.gca.2008.11.039
^ ^6.0 ^6.1 ^6.2 Kim, D., and Strathmann, T.J., 2007. Role of Organically Complexed Iron(II) Species in the Reductive Transformation of RDX in Anoxic Environments. Environmental Science and Technology, 41(4), pp. 1257–1264. DOI: 10.1021/es062365a
^ ^7.00 ^7.01 ^7.02 ^7.03 ^7.04 ^7.05 ^7.06 ^7.07 ^7.08 ^7.09 ^7.10 ^7.11 ^7.12 ^7.13 ^7.14 ^7.15 ^7.16 ^7.17 ^7.18 ^7.19 ^7.20 ^7.21 ^7.22 ^7.23 ^7.24 ^7.25 ^7.26 ^7.27 ^7.28 ^7.29 ^7.30 ^7.31 ^7.32 ^7.33 ^7.34 ^7.35 ^7.36 ^7.37 ^7.38 ^7.39 ^7.40 ^7.41 ^7.42 ^7.43 ^7.44 ^7.45 ^7.46 ^7.47 ^7.48 ^7.49 ^7.50 ^7.51 ^7.52 ^7.53 ^7.54 ^7.55 ^7.56 ^7.57 ^7.58 ^7.59 ^7.60 ^7.61 ^7.62 ^7.63 ^7.64 ^7.65 ^7.66 ^7.67 ^7.68 Colón, D., Weber, E.J., and Anderson, J.L., 2006. QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 40(16), pp. 4976–4982. DOI: 10.1021/es052425x
^ Luan, F., Xie, L., Li, J., and Zhou, Q., 2013. Abiotic reduction of nitroaromatic compounds by Fe(II) associated with iron oxides and humic acid. Chemosphere, 91(7), pp. 1035–1041. DOI: 10.1016/j.chemosphere.2013.01.070
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^ Gorski, C.A., Klüpfel, L., Voegelin, A., Sander, M., and Hofstetter, T.B., 2012. Redox Properties of Structural Fe in Clay Minerals. 2. Electrochemical and Spectroscopic Characterization of Electron Transfer Irreversibility in Ferruginous Smectite, SWa-1. Environmental Science and Technology, 46(17), pp. 9369–9377. DOI: 10.1021/es302014u
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@@ Line 1: / Line 1: @@
-Munition compounds (MCs) often contain one or more nitro (-NO<sub>2</sub>) functional groups which makes them susceptible to abiotic reduction, i.e., transformation by accepting electrons from a chemical electron donor. In soil and groundwater, the most prevalent electron donors are natural organic carbon and iron minerals. Understanding the kinetics and mechanisms of abiotic reduction of MCs by carbon and iron constituents in soil is not only essential for evaluating the environmental fate of MCs but also key to developing cost-efficient remediation strategies. This article summarizes the recent advances in our understanding of MC reduction by carbon and iron based reductants.
+[[Munitions Constituents|Munition compounds]] (MCs) often contain one or more nitro (-NO<sub>2</sub>) functional groups which makes them susceptible to abiotic reduction, i.e., transformation by accepting electrons from a chemical [[wikipedia:Electron_donor|electron donor]]. In soil and groundwater, the most prevalent electron donors are natural organic carbon and iron minerals. Understanding the kinetics and mechanisms of abiotic reduction of MCs by carbon and iron constituents in soil is not only essential for evaluating the environmental fate of MCs but also key to developing cost-efficient remediation strategies. This article summarizes the recent advances in our understanding of MC reduction by carbon and iron based reductants.
 <div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
@@ Line 6: / Line 6: @@
 *[[Munitions Constituents]]
 *[[Munitions Constituents - Alkaline Degradation]]
-*[[Munitions Constituents – Photolysis]]
+*[[Munitions Constituents - Composting]]
+*[[Munitions Constituents - Deposition]]
+*[[Munitions Constituents - Dissolution]]
+*[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
+*[[Munitions Constituents - Sorption]]
-'''Contributor(s):'''
-*Dr. Jimmy Murillo-Gelvez
+'''Contributor(s):''' [[Dr. Jimmy Murillo-Gelvez]], [[Paula Andrea Cárdenas-Hernández]], Dr. Dominic M. Di Toro, [[Dr. Richard F. Carbonaro]] and [[Dr. Pei Chiu]]
-*Paula Andrea Cárdenas-Hernández
-*Dr. Dominic M. Di Toro
-*Dr. Richard F. Carbonaro
-*Dr. Pei Chiu
 '''Key Resource(s):'''
@@ Line 22: / Line 22: @@
 ==Introduction==
 [[File:AbioMCredFig1.PNG | thumb |left|300px|Figure 1. Common munitions compounds. TNT and RDX are legacy explosives. DNAN, NTO, and NQ are insensitive MCs (IMCs) widely used as replacement for legacy explosives.]]
-Legacy and insensitive MCs (Figure 1) are susceptible to reductive transformation in soil and groundwater. Many redox-active constituents in the subsurface, especially those containing organic carbon, Fe(II), and sulfur can mediate MC reduction. Specific examples include Fe(II)-organic complexes<ref name="Naka2006">Naka, D., Kim, D., and Strathmann, T.J., 2006. Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II)−Catechol Complexes. Environmental Science and Technology 40(9), pp. 3006–3012.  [https://doi.org/10.1021/es060044t DOI: 10.1021/es060044t]</ref><ref name="Naka2008">Naka, D., Kim, D., Carbonaro, R.F., and Strathmann, T.J., 2008. Abiotic reduction of nitroaromatic contaminants by iron(II) complexes with organothiol ligands. Environmental Toxicology and Chemistry, 27(6), pp. 1257–1266.  [https://doi.org/10.1897/07-505.1 DOI: 10.1897/07-505.1]</ref><ref name="Hartenbach2008">Hartenbach, A.E., Hofstetter, T.B., Aeschbacher, M., Sander, M., Kim, D., Strathmann, T.J., Arnold, W.A., Cramer, C.J., and Schwarzenbach, R.P., 2008. Variability of Nitrogen Isotope Fractionation during the Reduction of Nitroaromatic Compounds with Dissolved Reductants. Environmental Science and Technology 42(22), pp. 8352–8359.  [https://doi.org/10.1021/es801063u DOI: 10.1021/es801063u]</ref><ref name="Kim2009">Kim, D., Duckworth, O.W., and Strathmann, T.J., 2009. Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants. Geochimica et Cosmochimica Acta, 73(5), pp. 1297–1311.  [https://doi.org/10.1016/j.gca.2008.11.039 DOI: 10.1016/j.gca.2008.11.039]</ref><ref name="Kim2007">Kim, D., and Strathmann, T.J., 2007. Role of Organically Complexed Iron(II) Species in the Reductive Transformation of RDX in Anoxic Environments. Environmental Science and Technology, 41(4), pp. 1257–1264.  [https://doi.org/10.1021/es062365a DOI: 10.1021/es062365a]</ref>, iron oxides in the presence of aqueous Fe(II)<ref name="Colón2006">Colón, D., Weber, E.J., and Anderson, J.L., 2006. QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 40(16), pp. 4976–4982.  [https://doi.org/10.1021/es052425x DOI: 10.1021/es052425x]</ref><ref name="Luan2013">Luan, F., Xie, L., Li, J., and Zhou, Q., 2013. Abiotic reduction of nitroaromatic compounds by Fe(II) associated with iron oxides and humic acid. Chemosphere, 91(7), pp. 1035–1041.  [https://doi.org/10.1016/j.chemosphere.2013.01.070 DOI: 10.1016/j.chemosphere.2013.01.070]</ref><ref name="Gorski2016">Gorski, C.A., Edwards, R., Sander, M., Hofstetter, T.B., and Stewart, S.M., 2016. Thermodynamic Characterization of Iron Oxide–Aqueous Fe<sup>2+</sup> Redox Couples. Environmental Science and Technology, 50(16), pp. 8538–8547.  [https://doi.org/10.1021/acs.est.6b02661 DOI: 10.1021/acs.est.6b02661]</ref><ref name="Fan2016">Fan, D., Bradley, M.J., Hinkle, A.W., Johnson, R.L., and Tratnyek, P.G., 2016. Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals. Environmental Science and Technology, 50(4), pp. 1868–1876.  [https://doi.org/10.1021/acs.est.5b05800 DOI: 10.1021/acs.est.5b05800]</ref><ref name="Jones2016">Jones, A.M., Kinsela, A.S., Collins, R.N., and Waite, T.D., 2016. The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate. Journal of Hazardous Materials, 320, pp. 143–149.  [https://doi.org/10.1016/j.jhazmat.2016.08.031 DOI: 10.1016/j.jhazmat.2016.08.031]</ref><ref name="Klausen1995">Klausen, J., Troeber, S.P., Haderlein, S.B., and Schwarzenbach, R.P., 1995. Reduction of Substituted Nitrobenzenes by Fe(II) in Aqueous Mineral Suspensions. Environmental Science and Technology, 29(9), pp. 2396–2404.  [https://doi.org/10.1021/es00009a036 DOI: 10.1021/es00009a036]</ref><ref name="Strehlau2016">Strehlau, J.H., Stemig, M.S., Penn, R.L., and Arnold, W.A., 2016. Facet-Dependent Oxidative Goethite Growth As a Function of Aqueous Solution Conditions. Environmental Science and Technology, 50(19), pp. 10406–10412.  [https://doi.org/10.1021/acs.est.6b02436 DOI: 10.1021/acs.est.6b02436]</ref><ref name="Elsner2004">Elsner, M., Schwarzenbach, R.P., and Haderlein, S.B., 2004. Reactivity of Fe(II)-Bearing Minerals toward Reductive Transformation of Organic Contaminants. 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+[[Munitions Constituents|Legacy and insensitive MCs]] (Figure 1) are susceptible to reductive transformation in soil and groundwater. Many redox-active constituents in the subsurface, especially those containing organic carbon, Fe(II), and sulfur can mediate MC reduction. Specific examples include Fe(II)-organic complexes<ref name="Naka2006">Naka, D., Kim, D., and Strathmann, T.J., 2006. Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II)−Catechol Complexes. Environmental Science and Technology 40(9), pp. 3006–3012.  [https://doi.org/10.1021/es060044t DOI: 10.1021/es060044t]</ref><ref name="Naka2008">Naka, D., Kim, D., Carbonaro, R.F., and Strathmann, T.J., 2008. Abiotic reduction of nitroaromatic contaminants by iron(II) complexes with organothiol ligands. Environmental Toxicology and Chemistry, 27(6), pp. 1257–1266.  [https://doi.org/10.1897/07-505.1 DOI: 10.1897/07-505.1]</ref><ref name="Hartenbach2008">Hartenbach, A.E., Hofstetter, T.B., Aeschbacher, M., Sander, M., Kim, D., Strathmann, T.J., Arnold, W.A., Cramer, C.J., and Schwarzenbach, R.P., 2008. Variability of Nitrogen Isotope Fractionation during the Reduction of Nitroaromatic Compounds with Dissolved Reductants. Environmental Science and Technology 42(22), pp. 8352–8359.  [https://doi.org/10.1021/es801063u DOI: 10.1021/es801063u]</ref><ref name="Kim2009">Kim, D., Duckworth, O.W., and Strathmann, T.J., 2009. Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants. Geochimica et Cosmochimica Acta, 73(5), pp. 1297–1311.  [https://doi.org/10.1016/j.gca.2008.11.039 DOI: 10.1016/j.gca.2008.11.039]</ref><ref name="Kim2007">Kim, D., and Strathmann, T.J., 2007. Role of Organically Complexed Iron(II) Species in the Reductive Transformation of RDX in Anoxic Environments. Environmental Science and Technology, 41(4), pp. 1257–1264.  [https://doi.org/10.1021/es062365a DOI: 10.1021/es062365a]</ref>, [[wikipedia:Iron_oxide|iron oxides]] in the presence of aqueous Fe(II)<ref name="Colón2006">Colón, D., Weber, E.J., and Anderson, J.L., 2006. QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 40(16), pp. 4976–4982.  [https://doi.org/10.1021/es052425x DOI: 10.1021/es052425x]</ref><ref name="Luan2013">Luan, F., Xie, L., Li, J., and Zhou, Q., 2013. Abiotic reduction of nitroaromatic compounds by Fe(II) associated with iron oxides and humic acid. Chemosphere, 91(7), pp. 1035–1041.  [https://doi.org/10.1016/j.chemosphere.2013.01.070 DOI: 10.1016/j.chemosphere.2013.01.070]</ref><ref name="Gorski2016">Gorski, C.A., Edwards, R., Sander, M., Hofstetter, T.B., and Stewart, S.M., 2016. Thermodynamic Characterization of Iron Oxide–Aqueous Fe<sup>2+</sup> Redox Couples. Environmental Science and Technology, 50(16), pp. 8538–8547.  [https://doi.org/10.1021/acs.est.6b02661 DOI: 10.1021/acs.est.6b02661]</ref><ref name="Fan2016">Fan, D., Bradley, M.J., Hinkle, A.W., Johnson, R.L., and Tratnyek, P.G., 2016. Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals. Environmental Science and Technology, 50(4), pp. 1868–1876.  [https://doi.org/10.1021/acs.est.5b05800 DOI: 10.1021/acs.est.5b05800]</ref><ref name="Jones2016">Jones, A.M., Kinsela, A.S., Collins, R.N., and Waite, T.D., 2016. The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate. Journal of Hazardous Materials, 320, pp. 143–149.  [https://doi.org/10.1016/j.jhazmat.2016.08.031 DOI: 10.1016/j.jhazmat.2016.08.031]</ref><ref name="Klausen1995">Klausen, J., Troeber, S.P., Haderlein, S.B., and Schwarzenbach, R.P., 1995. Reduction of Substituted Nitrobenzenes by Fe(II) in Aqueous Mineral Suspensions. Environmental Science and Technology, 29(9), pp. 2396–2404.  [https://doi.org/10.1021/es00009a036 DOI: 10.1021/es00009a036]</ref><ref name="Strehlau2016">Strehlau, J.H., Stemig, M.S., Penn, R.L., and Arnold, W.A., 2016. Facet-Dependent Oxidative Goethite Growth As a Function of Aqueous Solution Conditions. Environmental Science and Technology, 50(19), pp. 10406–10412.  [https://doi.org/10.1021/acs.est.6b02436 DOI: 10.1021/acs.est.6b02436]</ref><ref name="Elsner2004">Elsner, M., Schwarzenbach, R.P., and Haderlein, S.B., 2004. Reactivity of Fe(II)-Bearing Minerals toward Reductive Transformation of Organic Contaminants. Environmental Science and Technology, 38(3), pp. 799–807.  [https://doi.org/10.1021/es0345569 DOI: 10.1021/es0345569]</ref><ref name="Colón2008">Colón, D., Weber, E.J., and Anderson, J.L., 2008. Effect of Natural Organic Matter on the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 42(17), pp. 6538–6543.  [https://doi.org/10.1021/es8004249 DOI: 10.1021/es8004249]</ref><ref name="Stewart2018">Stewart, S.M., Hofstetter, T.B., Joshi, P. and Gorski, C.A., 2018. Linking Thermodynamics to Pollutant Reduction Kinetics by Fe<sup>2+</sup> Bound to Iron Oxides. Environmental Science and Technology, 52(10), pp. 5600–5609.  [https://doi.org/10.1021/acs.est.8b00481 DOI: 10.1021/acs.est.8b00481]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.8b00481 Article pdf]</ref><ref name="Klupinski2004">Klupinski, T.P., Chin, Y.P., and Traina, S.J., 2004. Abiotic Degradation of Pentachloronitrobenzene by Fe(II):  Reactions on Goethite and Iron Oxide Nanoparticles. Environmental Science and Technology, 38(16), pp. 4353–4360.  [https://doi.org/10.1021/es035434j DOI: 10.1021/es035434j]</ref>, [[wikipedia:Magnetite|magnetite]]<ref name="Klausen1995" /><ref name="Elsner2004" /><ref name="Heijman1993">Heijman, C.G., Holliger, C., Glaus, M.A., Schwarzenbach, R.P., and Zeyer, J., 1993. Abiotic Reduction of 4-Chloronitrobenzene to 4-Chloroaniline in a Dissimilatory Iron-Reducing Enrichment Culture. Applied and Environmental Microbiology, 59(12), pp. 4350–4353. [https://doi.org/10.1128/aem.59.12.4350-4353.1993 DOI: 10.1128/aem.59.12.4350-4353.1993]&nbsp;&nbsp;[https://journals.asm.org/doi/reader/10.1128/aem.59.12.4350-4353.1993 Article pdf]</ref><ref name="Gorski2009">Gorski, C.A., and Scherer, M.M., 2009. Influence of Magnetite Stoichiometry on Fe<sup>II</sup> Uptake and Nitrobenzene Reduction. Environmental Science and Technology, 43(10), pp. 3675–3680.  [https://doi.org/10.1021/es803613a DOI: 10.1021/es803613a]</ref><ref name="Gorski2010">Gorski, C.A., Nurmi, J.T., Tratnyek, P.G., Hofstetter, T.B. and Scherer, M.M., 2010. Redox Behavior of Magnetite: Implications for Contaminant Reduction. Environmental Science and Technology, 44(1), pp. 55–60.  [https://doi.org/10.1021/es9016848 DOI: 10.1021/es9016848]</ref>, Fe(II)-bearing clays<ref name="Hofstetter2006">Hofstetter, T.B., Neumann, A., and Schwarzenbach, R.P., 2006. Reduction of Nitroaromatic Compounds by Fe(II) Species Associated with Iron-Rich Smectites. Environmental Science and Technology, 40(1), pp. 235–242.  [https://doi.org/10.1021/es0515147 DOI: 10.1021/es0515147]</ref><ref name="Schultz2000">Schultz, C. A., and Grundl, T.J., 2000. pH Dependence on Reduction Rate of 4-Cl-Nitrobenzene by Fe(II)/Montmorillonite Systems. Environmental Science and Technology 34(17), pp. 3641–3648.  [https://doi.org/10.1021/es990931e DOI: 10.1021/es990931e]</ref><ref name="Luan2015a">Luan, F., Gorski, C.A., and Burgos, W.D., 2015. Linear Free Energy Relationships for the Biotic and Abiotic Reduction of Nitroaromatic Compounds. Environmental Science and Technology, 49(6), pp. 3557–3565.  [https://doi.org/10.1021/es5060918 DOI: 10.1021/es5060918]</ref><ref name="Luan2015b">Luan, F., Liu, Y., Griffin, A.M., Gorski, C.A. and Burgos, W.D., 2015. Iron(III)-Bearing Clay Minerals Enhance Bioreduction of Nitrobenzene by ''Shewanella putrefaciens'' CN32. Environmental Science and Technology, 49(3), pp. 1418–1426.  [https://doi.org/10.1021/es504149y DOI: 10.1021/es504149y]</ref><ref name="Hofstetter2003">Hofstetter, T.B., Schwarzenbach, R.P. and Haderlein, S.B., 2003. Reactivity of Fe(II) Species Associated with Clay Minerals. Environmental Science and Technology, 37(3), pp. 519–528.  [https://doi.org/10.1021/es025955r DOI: 10.1021/es025955r]</ref><ref name="Neumann2008">Neumann, A., Hofstetter, T.B., Lüssi, M., Cirpka, O.A., Petit, S., and Schwarzenbach, R.P., 2008. Assessing the Redox Reactivity of Structural Iron in Smectites Using Nitroaromatic Compounds As Kinetic Probes. Environmental Science and Technology, 42(22), pp. 8381–8387.  [https://doi.org/10.1021/es801840x DOI: 10.1021/es801840x]</ref><ref name="Hofstetter2008">Hofstetter, T.B., Neumann, A., Arnold, W.A., Hartenbach, A.E., Bolotin, J., Cramer, C.J., and Schwarzenbach, R.P., 2008. Substituent Effects on Nitrogen Isotope Fractionation During Abiotic Reduction of Nitroaromatic Compounds. Environmental Science and Technology, 42(6), pp. 1997–2003.  [https://doi.org/10.1021/es702471k DOI: 10.1021/es702471k]</ref>, [[wikipedia:Hydroquinone|hydroquinones]] (as surrogates of natural organic matter)<ref name="Hartenbach2008" /><ref name="Schwarzenbach1990">Schwarzenbach, R.P., Stierli, R., Lanz, K., and Zeyer, J., 1990. Quinone and Iron Porphyrin Mediated Reduction of Nitroaromatic Compounds in Homogeneous Aqueous Solution. Environmental Science and Technology, 24(10), pp. 1566–1574.  [https://doi.org/10.1021/es00080a017 DOI: 10.1021/es00080a017]</ref><ref name="Tratnyek1989">Tratnyek, P.G., and Macalady, D.L., 1989. Abiotic Reduction of Nitro Aromatic Pesticides in Anaerobic Laboratory Systems. Journal of Agricultural and Food Chemistry, 37(1), pp. 248–254.  [https://doi.org/10.1021/jf00085a058 DOI: 10.1021/jf00085a058]</ref><ref name="Hofstetter1999">Hofstetter, T.B., Heijman, C.G., Haderlein, S.B., Holliger, C. and Schwarzenbach, R.P., 1999. Complete Reduction of TNT and Other (Poly)nitroaromatic Compounds under Iron-Reducing Subsurface Conditions. Environmental Science and Technology, 33(9), pp. 1479–1487.  [https://doi.org/10.1021/es9809760 DOI: 10.1021/es9809760]</ref><ref name="Murillo-Gelvez2019">Murillo-Gelvez, J., Hickey, K.P., Di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2019. Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants. Environmental Science and Technology, 53(10), pp. 5816–5827.  [https://doi.org/10.1021/acs.est.9b00910 DOI: 10.1021/acs.est.9b00910]</ref><ref name="Niedźwiecka2017">Niedźwiecka, J.B., Drew, S.R., Schlautman, M.A., Millerick, K.A., Grubbs, E., Tharayil, N. and Finneran, K.T., 2017. Iron and Electron Shuttle Mediated (Bio)degradation of 2,4-Dinitroanisole (DNAN). Environmental Science and Technology, 51(18), pp. 10729–10735.  [https://doi.org/10.1021/acs.est.7b02433 DOI: 10.1021/acs.est.7b02433]</ref><ref name="Kwon2006">Kwon, M.J., and Finneran, K.T., 2006. Microbially Mediated Biodegradation of Hexahydro-1,3,5-Trinitro-1,3,5- Triazine by Extracellular Electron Shuttling Compounds. Applied and Environmental Microbiology, 72(9), pp. 5933–5941.  [https://doi.org/10.1128/AEM.00660-06 DOI: 10.1128/AEM.00660-06]&nbsp;&nbsp;[https://journals.asm.org/doi/reader/10.1128/AEM.00660-06 Article pdf]</ref>, dissolved organic matter<ref name="Dunnivant1992">Dunnivant, F.M., Schwarzenbach, R.P., and Macalady, D.L., 1992. Reduction of Substituted Nitrobenzenes in Aqueous Solutions Containing Natural Organic Matter. Environmental Science and Technology, 26(11), pp. 2133–2141.  [https://doi.org/10.1021/es00035a010 DOI: 10.1021/es00035a010]</ref><ref name="Luan2010">Luan, F., Burgos, W.D., Xie, L., and Zhou, Q., 2010. Bioreduction of Nitrobenzene, Natural Organic Matter, and Hematite by Shewanella putrefaciens CN32. Environmental Science and Technology, 44(1), pp. 184–190.  [https://doi.org/10.1021/es901585z DOI: 10.1021/es901585z]</ref><ref name="Murillo-Gelvez2021">Murillo-Gelvez, J., di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2021. Reductive Transformation of 3-Nitro-1,2,4-triazol-5-one (NTO) by Leonardite Humic Acid and Anthraquinone-2,6-disulfonate (AQDS). Environmental Science and Technology, 55(19), pp. 12973–12983.  [https://doi.org/10.1021/acs.est.1c03333 DOI: 10.1021/acs.est.1c03333]</ref>, [[wikipedia:Black_carbon|black carbon]]<ref name="Oh2013">Oh, S.-Y., Son, J.G., and Chiu, P.C., 2013. Biochar-Mediated Reductive Transformation of Nitro Herbicides and Explosives. Environmental Toxicology and Chemistry, 32(3), pp. 501–508.  [https://doi.org/10.1002/etc.2087 DOI: 10.1002/etc.2087]&nbsp;&nbsp; [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/etc.2087 Article pdf]</ref><ref name="Oh2009">Oh, S.-Y., and Chiu, P.C., 2009. Graphite- and Soot-Mediated Reduction of 2,4-Dinitrotoluene and Hexahydro-1,3,5-trinitro-1,3,5-triazine. Environmental Science & Technology, 43(18), pp. 6983–6988.  [https://doi.org/10.1021/es901433m DOI: 10.1021/es901433m]</ref><ref name="Xu2015">Xu, W., Pignatello, J.J., and Mitch, W.A., 2015. Reduction of Nitroaromatics Sorbed to Black Carbon by Direct Reaction with Sorbed Sulfides. Environmental Science and Technology, 49(6), pp. 3419–3426.  [https://doi.org/10.1021/es5045198 DOI: 10.1021/es5045198]</ref><ref name="Oh2002">Oh, S.-Y., Cha, D.K., and Chiu, P.C., 2002. Graphite-Mediated Reduction of 2,4-Dinitrotoluene with Elemental Iron. Environmental Science and Technology, 36(10), pp. 2178–2184.  [https://doi.org/10.1021/es011474g DOI: 10.1021/es011474g]</ref><ref name="Amezquita-Garcia2013">Amezquita-Garcia, H.J., Razo-Flores, E., Cervantes, F.J., and Rangel-Mendez, J.R., 2013.  Activated carbon fibers as redox mediators for the increased reduction of nitroaromatics. Carbon, 55, pp. 276–284. [https://doi.org/10.1016/j.carbon.2012.12.062 DOI: 10.1016/j.carbon.2012.12.062]</ref><ref name="Xin2022">Xin, D., Girón, J., Fuller, M.E., and Chiu, P.C., 2022. Abiotic Reduction of 3-Nitro-1,2,4-triazol-5-one (NTO) and Other Munitions Constituents by Wood-Derived Biochar through Its Rechargeable Electron Storage Capacity. Environmental Science: Processes and Impacts, 24(2), pp. 316-329.  [https://doi.org/10.1039/D1EM00447F DOI: 10.1039/D1EM00447F]</ref>, and sulfides<ref name="Hojo1960">Hojo, M., Takagi, Y. and Ogata, Y., 1960. Kinetics of the Reduction of Nitrobenzenes by Sodium Disulfide. Journal of the American Chemical Society, 82(10), pp. 2459–2462.  [https://doi.org/10.1021/ja01495a017 DOI: 10.1021/ja01495a017]</ref><ref name="Zeng2012">Zeng, T., Chin, Y.P., and Arnold, W.A., 2012. Potential for Abiotic Reduction of Pesticides in Prairie Pothole Porewaters. Environmental Science and Technology, 46(6), pp. 3177–3187.  [https://doi.org/10.1021/es203584d DOI: 10.1021/es203584d]</ref>. These geo-reductants may control the fate and half-lives of MCs in the environment and can be used to promote MC degradation in soil and groundwater through enhanced [[Monitored Natural Attenuation (MNA)|natural attenuation]]<ref name="USEPA2012">US EPA, 2012. A Citizen’s Guide to Monitored Natural Attenuation. EPA document 542-F-12-014. [https://www.epa.gov/sites/default/files/2015-04/documents/a_citizens_guide_to_monitored_natural_attenuation.pdf Article pdf]</ref>.
 [[File:AbioMCredFig2.png | thumb |450px|Figure 2. General mechanism for the reduction of NACs/MCs.]]
 [[File:AbioMCredFig3.png | thumb |450px|Figure 3. Schematic of natural attenuation of MCs-impacted soils through chemical reduction.]]
-Although the chemical structures of MCs can vary significantly (Figure 1), most of them contain at least one nitro functional group (-NO<sub>2</sub>), which is susceptible to reductive transformation<ref name="Spain2000">Spain, J.C., Hughes, J.B., and Knackmuss, H.J., 2000. Biodegradation of Nitroaromatic Compounds and Explosives. CRC Press, 456 pages. ISBN: 9780367398491</ref>. Of the MCs shown in Figure 1, 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), and 3-nitro-1,2,4-triazol-5-one (NTO)<ref name="Harris1996">Harris, N.J., and Lammertsma, K., 1996. Tautomerism, Ionization, and Bond Dissociations of 5-Nitro-2,4-dihydro-3H-1,2,4-triazolone. Journal of the American Chemical Society, 118(34), pp. 8048–8055.  [https://doi.org/10.1021/ja960834a DOI: 10.1021/ja960834a]</ref> are nitroaromatic compounds (NACs) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ) are nitramines. The structural differences may result in different reactivities and reaction pathways. Reduction of NACs results in the formation of aromatic amines (i.e., anilines) with nitroso and hydroxylamine compounds as intermediates (Figure 2)<ref name="Schwarzenbach2016" />.
+Although the chemical structures of MCs can vary significantly (Figure 1), most of them contain at least one nitro functional group (-NO<sub>2</sub>), which is susceptible to reductive transformation<ref name="Spain2000">Spain, J.C., Hughes, J.B., and Knackmuss, H.J., 2000. Biodegradation of Nitroaromatic Compounds and Explosives. CRC Press, 456 pages. ISBN: 9780367398491</ref>. Of the MCs shown in Figure 1, [[wikipedia:TNT|2,4,6-trinitrotoluene]] (TNT), [[wikipedia:2,4-Dinitroanisole|2,4-dinitroanisole]] (DNAN), and 3-nitro-1,2,4-triazol-5-one (NTO)<ref name="Harris1996">Harris, N.J., and Lammertsma, K., 1996. Tautomerism, Ionization, and Bond Dissociations of 5-Nitro-2,4-dihydro-3H-1,2,4-triazolone. Journal of the American Chemical Society, 118(34), pp. 8048–8055.  [https://doi.org/10.1021/ja960834a DOI: 10.1021/ja960834a]</ref> are nitroaromatic compounds (NACs) and [[wikipedia:RDX|hexahydro-1,3,5-trinitro-1,3,5-triazine]] (RDX) and [[wikipedia:Nitroguanidine|nitroguanidine]] (NQ) are nitramines. The structural differences may result in different reactivities and reaction pathways. Reduction of NACs results in the formation of [[wikipedia:Aromatic_amine|aromatic amines]] (i.e., anilines) with [[wikipedia:Nitroso|nitroso]] and [[wikipedia:Hydroxylamine|hydroxylamine]] compounds as intermediates (Figure 2)<ref name="Schwarzenbach2016" />.
 Although the final reduction products are different for non-aromatic MCs, the reduction process often starts with the transformation of the -NO<sub>2</sub> moiety, either through de-nitration (e.g., RDX<ref name="Kwon2008">Kwon, M.J., and Finneran, K.T., 2008. Biotransformation products and mineralization potential for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in abiotic versus biological degradation pathways with anthraquinone-2,6-disulfonate (AQDS) and ''Geobacter metallireducens''. Biodegradation, 19(5), pp. 705–715.  [https://doi.org/10.1007/s10532-008-9175-5 DOI: 10.1007/s10532-008-9175-5]</ref><ref name="Halasz2011">Halasz, A., and Hawari, J., 2011. Degradation Routes of RDX in Various Redox Systems. Aquatic Redox Chemistry, American Chemical Society, 1071(20), pp. 441-462.  [https://doi.org/10.1021/bk-2011-1071.ch020 DOI: 10.1021/bk-2011-1071.ch020]</ref>) or reduction to nitroso<ref name="Kwon2006" /><ref name="Tong2021">Tong, Y., Berens, M.J., Ulrich, B.A., Bolotin, J., Strehlau, J.H., Hofstetter, T.B., and Arnold, W.A., 2021. Exploring the Utility of Compound-Specific Isotope Analysis for Assessing Ferrous Iron-Mediated Reduction of RDX in the Subsurface. Environmental Science and Technology, 55(10), pp. 6752–6763.  [https://doi.org/10.1021/acs.est.0c08420 DOI: 10.1021/acs.est.0c08420]</ref> followed by ring cleavage<ref name="Kim2007" /><ref name="Halasz2011" /><ref name="Tong2021" /><ref name="Larese-Casanova2008">Larese-Casanova, P., and Scherer, M.M., 2008. Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Green Rusts. Environmental Science and Technology, 42(11), pp. 3975–3981.  [https://doi.org/10.1021/es702390b DOI: 10.1021/es702390b]</ref>.
-Figure 3 illustrates a typical MC reduction reaction. A redox-active soil constituent, such as organic matter or iron mineral, donates electrons to an MC and transforms the nitro group into an amino group (R-NH<sub>2</sub>). The rate at which an MC is reduced can vary by many orders of magnitude depending on the soil constituent, the MC, the reduction potential (''E<sub>H</sub>'') and other media conditions<ref name="Borch2010">Borch, T., Kretzschmar, R., Kappler, A., Cappellen, P.V., Ginder-Vogel, M., Voegelin, A., and Campbell, K., 2010. Biogeochemical Redox Processes and their Impact on Contaminant Dynamics. Environmental Science and Technology, 44(1), pp. 15–23.  [https://doi.org/10.1021/es9026248 DOI: 10.1021/es9026248]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es9026248 Open access article.]</ref>.
+Figure 3 illustrates a typical MC reduction reaction. A redox-active soil constituent, such as organic matter or iron mineral, donates electrons to an MC and transforms the nitro group into an [[wikipedia:Amine|amino group]] (R-NH<sub>2</sub>). The rate at which an MC is reduced can vary by many orders of magnitude depending on the soil constituent, the MC, the [[wikipedia:Reduction_potential|reduction potential]] (''E<sub>H</sub>'') and other media conditions<ref name="Borch2010">Borch, T., Kretzschmar, R., Kappler, A., Cappellen, P.V., Ginder-Vogel, M., Voegelin, A., and Campbell, K., 2010. Biogeochemical Redox Processes and their Impact on Contaminant Dynamics. Environmental Science and Technology, 44(1), pp. 15–23.  [https://doi.org/10.1021/es9026248 DOI: 10.1021/es9026248]&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es9026248 Article pdf]</ref>.
 The most prevalent reductants in soils are iron minerals and organic carbon such as that found in natural organic matter. It has been suggested that Fe(II)<sub>aq</sub> and dissolved organic matter concentrations could serve as indicators of NAC reducibility in anaerobic sediments<ref name="Zhang2009">Zhang, H., and Weber, E.J., 2009. Elucidating the Role of Electron Shuttles in Reductive Transformations in Anaerobic Sediments. Environmental Science and Technology, 43(4), pp. 1042–1048.  [https://doi.org/10.1021/es8017072 DOI: 10.1021/es8017072]</ref>. The following sections summarize these two classes of reductants separately and present advances in our understanding of the kinetics of NAC/MC reduction by these geo-reductants.
@@ Line 36: / Line 36: @@
 ==Carbonaceous Reductants==
 [[File:AbioMCredFig4.png | thumb |600px|Figure 4. Chemical structure of commonly used hydroquinones in NACs/MCs kinetic experiments.]]
-The two most predominant forms of organic carbon in natural systems are natural organic matter (NOM) and black carbon (BC)<ref name="Schumacher2002">Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments. U.S. EPA, Ecological Risk Assessment Support Center. [http://bcodata.whoi.edu/LaurentianGreatLakes_Chemistry/bs116.pdf Free download.]</ref>. Black carbon includes charcoal, soot, graphite, and coal. Until the early 2000s black carbon was considered to be a class of (bio)chemically inert geosorbents<ref name="Schmidt2000">Schmidt, M.W.I., and Noack, A.G., 2000. Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges. Global Biogeochemical Cycles, 14(3), pp. 777–793.  [https://doi.org/10.1029/1999GB001208 DOI: 10.1029/1999GB001208]&nbsp;&nbsp; [https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999GB001208 Open access article.]</ref>. However, it has been shown that BC can contain abundant quinone functional groups and thus can store and exchange electrons<ref name="Klüpfel2014">Klüpfel, L., Keiluweit, M., Kleber, M., and Sander, M., 2014. Redox Properties of Plant Biomass-Derived Black Carbon (Biochar). Environmental Science and Technology, 48(10), pp. 5601–5611.  [https://doi.org/10.1021/es500906d DOI: 10.1021/es500906d]</ref> with chemical<ref name="Xin2019">Xin, D., Xian, M., and Chiu, P.C., 2019. New methods for assessing electron storage capacity and redox reversibility of biochar. Chemosphere, 215, 827–834.  [https://doi.org/10.1016/j.chemosphere.2018.10.080 DOI: 10.1016/j.chemosphere.2018.10.080]</ref> and biological<ref name="Saquing2016">Saquing, J.M., Yu, Y.-H., and Chiu, P.C., 2016. Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor. Environmental Science and Technology Letters, 3(2), pp. 62–66.  [https://doi.org/10.1021/acs.estlett.5b00354 DOI: 10.1021/acs.estlett.5b00354]</ref> agents in the surroundings. Specifically, BC such as biochar has been shown to reductively transform MCs including NTO, DNAN, and RDX<ref name="Xin2022" />.
+The two most predominant forms of organic carbon in natural systems are natural organic matter (NOM) and black carbon (BC)<ref name="Schumacher2002">Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments. U.S. EPA, Ecological Risk Assessment Support Center. [http://bcodata.whoi.edu/LaurentianGreatLakes_Chemistry/bs116.pdf Article pdf.]</ref>. Black carbon includes [[wikipedia:Charcoal|charcoal]], [[wikipedia:Soot|soot]], [[wikipedia:Graphite|graphite]], and [[wikipedia:Coal|coal]]. Until the early 2000s black carbon was considered to be a class of (bio)chemically inert geosorbents<ref name="Schmidt2000">Schmidt, M.W.I., and Noack, A.G., 2000. Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges. Global Biogeochemical Cycles, 14(3), pp. 777–793.  [https://doi.org/10.1029/1999GB001208 DOI: 10.1029/1999GB001208]&nbsp; [https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999GB001208 Article pdf]</ref>. However, it has been shown that BC can contain abundant [[wikipedia:Quinone|quinone]] functional groups and thus can store and exchange electrons<ref name="Klüpfel2014">Klüpfel, L., Keiluweit, M., Kleber, M., and Sander, M., 2014. Redox Properties of Plant Biomass-Derived Black Carbon (Biochar). Environmental Science and Technology, 48(10), pp. 5601–5611.  [https://doi.org/10.1021/es500906d DOI: 10.1021/es500906d]</ref> with chemical<ref name="Xin2019">Xin, D., Xian, M., and Chiu, P.C., 2019. New methods for assessing electron storage capacity and redox reversibility of biochar. Chemosphere, 215, 827–834.  [https://doi.org/10.1016/j.chemosphere.2018.10.080 DOI: 10.1016/j.chemosphere.2018.10.080]</ref> and biological<ref name="Saquing2016">Saquing, J.M., Yu, Y.-H., and Chiu, P.C., 2016. Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor. Environmental Science and Technology Letters, 3(2), pp. 62–66.  [https://doi.org/10.1021/acs.estlett.5b00354 DOI: 10.1021/acs.estlett.5b00354]</ref> agents in the surroundings. Specifically, BC such as biochar has been shown to reductively transform MCs including NTO, DNAN, and RDX<ref name="Xin2022" />.
-NOM encompasses all the organic compounds present in terrestrial and aquatic environments and can be classified into two groups, non-humic and humic substances. Humic substances (HS) contain a wide array of functional groups including carboxyl, enol, ether, ketone, ester, amide, (hydro)quinone, and phenol<ref name="Sparks2003">Sparks, D.L., 2003. Environmental Soil Chemistry, 2nd Edition. Elsevier Science and Technology Books.  [https://doi.org/10.1016/B978-0-12-656446-4.X5000-2 DOI: 10.1016/B978-0-12-656446-4.X5000-2]</ref>. Quinone and hydroquinone groups are believed to be the predominant redox moieties responsible for the capacity of HS and BC to store and reversibly accept and donate electrons (i.e., through reduction and oxidation of HS/BC, respectively)<ref name="Schwarzenbach1990" /><ref name="Dunnivant1992" /><ref name="Klüpfel2014" /><ref name="Scott1998">Scott, D.T., McKnight, D.M., Blunt-Harris, E.L., Kolesar, S.E., and Lovley, D.R., 1998. Quinone Moieties Act as Electron Acceptors in the Reduction of Humic Substances by Humics-Reducing Microorganisms. Environmental Science and Technology, 32(19), pp. 2984–2989.  [https://doi.org/10.1021/es980272q DOI: 10.1021/es980272q]</ref><ref name="Cory2005">Cory, R.M., and McKnight, D.M., 2005. Fluorescence Spectroscopy Reveals Ubiquitous Presence of Oxidized and Reduced Quinones in Dissolved Organic Matter. Environmental Science & Technology, 39(21), pp 8142–8149.  [https://doi.org/10.1021/es0506962 DOI: 10.1021/es0506962]</ref><ref name="Fimmen2007">Fimmen, R.L., Cory, R.M., Chin, Y.P., Trouts, T.D., and McKnight, D.M., 2007. Probing the oxidation–reduction properties of terrestrially and microbially derived dissolved organic matter. Geochimica et Cosmochimica Acta, 71(12), pp. 3003–3015.  [https://doi.org/10.1016/j.gca.2007.04.009 DOI: 10.1016/j.gca.2007.04.009]</ref><ref name="Struyk2001">Struyk, Z., and Sposito, G., 2001. Redox properties of standard humic acids. Geoderma, 102(3-4), pp. 329–346.  [https://doi.org/10.1016/S0016-7061(01)00040-4 DOI: 10.1016/S0016-7061(01)00040-4]</ref><ref name="Ratasuk2007">Ratasuk, N., and Nanny, M.A., 2007. Characterization and Quantification of Reversible Redox Sites in Humic Substances. Environmental Science and Technology, 41(22), pp. 7844–7850.  [https://doi.org/10.1021/es071389u DOI: 10.1021/es071389u]</ref><ref name="Aeschbacher2010">Aeschbacher, M., Sander, M., and Schwarzenbach, R.P., 2010. Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances. Environmental Science and Technology, 44(1), pp. 87–93.  [https://doi.org/10.1021/es902627p DOI: 10.1021/es902627p]</ref><ref name="Aeschbacher2011">Aeschbacher, M., Vergari, D., Schwarzenbach, R.P., and Sander, M., 2011. Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids. Environmental Science and Technology, 45(19), pp. 8385–8394.  [https://doi.org/10.1021/es201981g DOI: 10.1021/es201981g]</ref><ref name="Bauer2009">Bauer, I., and Kappler, A., 2009. Rates and Extent of Reduction of Fe(III) Compounds and O<sub>2</sub> by Humic Substances. Environmental Science and Technology, 43(13), pp. 4902–4908.  [https://doi.org/10.1021/es900179s DOI: 10.1021/es900179s]</ref><ref name="Maurer2010">Maurer, F., Christl, I. and Kretzschmar, R., 2010. Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding. Environmental Science and Technology, 44(15), pp. 5787–5792.  [https://doi.org/10.1021/es100594t DOI: 10.1021/es100594t]</ref><ref name="Walpen2016">Walpen, N., Schroth, M.H., and Sander, M., 2016. Quantification of Phenolic Antioxidant Moieties in Dissolved Organic Matter by Flow-Injection Analysis with Electrochemical Detection. Environmental Science and Technology, 50(12), pp. 6423–6432.  [https://doi.org/10.1021/acs.est.6b01120 DOI: 10.1021/acs.est.6b01120]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b01120 Open access article.]</ref><ref name="Aeschbacher2012">Aeschbacher, M., Graf, C., Schwarzenbach, R.P., and Sander, M., 2012.  Antioxidant Properties of Humic Substances. Environmental Science and Technology, 46(9), pp. 4916–4925.  [https://doi.org/10.1021/es300039h DOI: 10.1021/es300039h]</ref><ref name="Nurmi2002">Nurmi, J.T., and Tratnyek, P.G., 2002. Electrochemical Properties of Natural Organic Matter (NOM), Fractions of NOM, and Model Biogeochemical Electron Shuttles. Environmental Science and Technology, 36(4), pp. 617–624.  [https://doi.org/10.1021/es0110731 DOI: 10.1021/es0110731]</ref>.
+NOM encompasses all the organic compounds present in terrestrial and aquatic environments and can be classified into two groups, non-humic and [[wikipedia:Humic_substance|humic substances]]. Humic substances (HS) contain a wide array of functional groups including carboxyl, [[wikipedia:Enol|enol]], [[wikipedia:Ether|ether]], [[wikipedia:Ketone|ketone]], [[wikipedia:Ester|ester]], [[wikipedia:Amide_(functional_group)|amide]], (hydro)quinone, and [[wikipedia:Phenol|phenol]]<ref name="Sparks2003">Sparks, D.L., 2003. Environmental Soil Chemistry, 2nd Edition. Elsevier Science and Technology Books.  [https://doi.org/10.1016/B978-0-12-656446-4.X5000-2 DOI: 10.1016/B978-0-12-656446-4.X5000-2]</ref>. Quinone and hydroquinone groups are believed to be the predominant redox moieties responsible for the capacity of HS and BC to store and reversibly accept and donate electrons (i.e., through reduction and oxidation of HS/BC, respectively)<ref name="Schwarzenbach1990" /><ref name="Dunnivant1992" /><ref name="Klüpfel2014" /><ref name="Scott1998">Scott, D.T., McKnight, D.M., Blunt-Harris, E.L., Kolesar, S.E., and Lovley, D.R., 1998. Quinone Moieties Act as Electron Acceptors in the Reduction of Humic Substances by Humics-Reducing Microorganisms. Environmental Science and Technology, 32(19), pp. 2984–2989.  [https://doi.org/10.1021/es980272q DOI: 10.1021/es980272q]</ref><ref name="Cory2005">Cory, R.M., and McKnight, D.M., 2005. Fluorescence Spectroscopy Reveals Ubiquitous Presence of Oxidized and Reduced Quinones in Dissolved Organic Matter. Environmental Science & Technology, 39(21), pp 8142–8149.  [https://doi.org/10.1021/es0506962 DOI: 10.1021/es0506962]</ref><ref name="Fimmen2007">Fimmen, R.L., Cory, R.M., Chin, Y.P., Trouts, T.D., and McKnight, D.M., 2007. Probing the oxidation–reduction properties of terrestrially and microbially derived dissolved organic matter. Geochimica et Cosmochimica Acta, 71(12), pp. 3003–3015.  [https://doi.org/10.1016/j.gca.2007.04.009 DOI: 10.1016/j.gca.2007.04.009]</ref><ref name="Struyk2001">Struyk, Z., and Sposito, G., 2001. Redox properties of standard humic acids. Geoderma, 102(3-4), pp. 329–346.  [https://doi.org/10.1016/S0016-7061(01)00040-4 DOI: 10.1016/S0016-7061(01)00040-4]</ref><ref name="Ratasuk2007">Ratasuk, N., and Nanny, M.A., 2007. Characterization and Quantification of Reversible Redox Sites in Humic Substances. Environmental Science and Technology, 41(22), pp. 7844–7850.  [https://doi.org/10.1021/es071389u DOI: 10.1021/es071389u]</ref><ref name="Aeschbacher2010">Aeschbacher, M., Sander, M., and Schwarzenbach, R.P., 2010. Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances. Environmental Science and Technology, 44(1), pp. 87–93.  [https://doi.org/10.1021/es902627p DOI: 10.1021/es902627p]</ref><ref name="Aeschbacher2011">Aeschbacher, M., Vergari, D., Schwarzenbach, R.P., and Sander, M., 2011. Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids. Environmental Science and Technology, 45(19), pp. 8385–8394.  [https://doi.org/10.1021/es201981g DOI: 10.1021/es201981g]</ref><ref name="Bauer2009">Bauer, I., and Kappler, A., 2009. Rates and Extent of Reduction of Fe(III) Compounds and O<sub>2</sub> by Humic Substances. Environmental Science and Technology, 43(13), pp. 4902–4908.  [https://doi.org/10.1021/es900179s DOI: 10.1021/es900179s]</ref><ref name="Maurer2010">Maurer, F., Christl, I. and Kretzschmar, R., 2010. Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding. Environmental Science and Technology, 44(15), pp. 5787–5792.  [https://doi.org/10.1021/es100594t DOI: 10.1021/es100594t]</ref><ref name="Walpen2016">Walpen, N., Schroth, M.H., and Sander, M., 2016. Quantification of Phenolic Antioxidant Moieties in Dissolved Organic Matter by Flow-Injection Analysis with Electrochemical Detection. Environmental Science and Technology, 50(12), pp. 6423–6432.  [https://doi.org/10.1021/acs.est.6b01120 DOI: 10.1021/acs.est.6b01120]&nbsp;[https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b01120 Article pdf.]</ref><ref name="Aeschbacher2012">Aeschbacher, M., Graf, C., Schwarzenbach, R.P., and Sander, M., 2012.  Antioxidant Properties of Humic Substances. Environmental Science and Technology, 46(9), pp. 4916–4925.  [https://doi.org/10.1021/es300039h DOI: 10.1021/es300039h]</ref><ref name="Nurmi2002">Nurmi, J.T., and Tratnyek, P.G., 2002. Electrochemical Properties of Natural Organic Matter (NOM), Fractions of NOM, and Model Biogeochemical Electron Shuttles. Environmental Science and Technology, 36(4), pp. 617–624.  [https://doi.org/10.1021/es0110731 DOI: 10.1021/es0110731]</ref>.
 Hydroquinones have been widely used as surrogates to understand the reductive transformation of NACs and MCs by NOM. Figure 4 shows the chemical structures of the singly deprotonated forms of four hydroquinone species previously used to study NAC/MC reduction. The second-order rate constants (''k<sub>R</sub>'') for the reduction of NACs/MCs by these hydroquinone species are listed in Table 1, along with the aqueous-phase one electron reduction potentials of the NACs/MCs (''E<sub>H</sub><sup>1’</sup>'') where available. ''E<sub>H</sub><sup>1’</sup>'' is an experimentally measurable thermodynamic property that reflects the propensity of a given NAC/MC to accept an electron in water (''E<sub>H</sub><sup>1</sup>''(R-NO<sub>2</sub>)):
@@ Line 44: / Line 44: @@
 :::::<big>'''Equation 1:'''&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;''R-NO<sub>2</sub> + e<sup>-</sup> ⇔ R-NO<sub>2</sub><sup>•-</sup>''</big>
-Knowing the identity of and reaction order in the reductant is required to derive the second-order rate constants listed in Table 1. This same reason limits the utility of reduction rate constants measured with complex carbonaceous reductants such as NOM<ref name="Dunnivant1992" />, BC<ref name="Oh2013" /><ref name="Oh2009" /><ref name="Xu2015" /><ref name="Xin2021">Xin, D., 2021. Understanding the Electron Storage Capacity of Pyrogenic Black Carbon: Origin, Redox Reversibility, Spatial Distribution, and Environmental Applications. Doctoral Thesis, University of Delaware.  [https://udspace.udel.edu/bitstream/handle/19716/30105/Xin_udel_0060D_14728.pdf?sequence=1 Free download.]</ref>, and HS<ref name="Luan2010" /><ref name="Murillo-Gelvez2021" />, whose chemical structures and redox moieties responsible for the reduction, as well as their abundance, are not clearly defined or known. In other words, the observed rate constants in those studies are specific to the experimental conditions (e.g., pH and NOM source and concentration), and may not be easily comparable to other studies.
+Knowing the identity of and reaction order in the reductant is required to derive the second-order rate constants listed in Table 1. This same reason limits the utility of reduction rate constants measured with complex carbonaceous reductants such as NOM<ref name="Dunnivant1992" />, BC<ref name="Oh2013" /><ref name="Oh2009" /><ref name="Xu2015" /><ref name="Xin2021">Xin, D., 2021. Understanding the Electron Storage Capacity of Pyrogenic Black Carbon: Origin, Redox Reversibility, Spatial Distribution, and Environmental Applications. Doctoral Thesis, University of Delaware. [https://udspace.udel.edu/bitstream/handle/19716/30105/Xin_udel_0060D_14728.pdf?sequence=1 Thesis pdf.]</ref>, and HS<ref name="Luan2010" /><ref name="Murillo-Gelvez2021" />, whose chemical structures and redox moieties responsible for the reduction, as well as their abundance, are not clearly defined or known. In other words, the observed rate constants in those studies are specific to the experimental conditions (e.g., pH and NOM source and concentration), and may not be easily comparable to other studies.
 {| class="wikitable mw-collapsible" style="float:left; margin-right:40px; text-align:center;"
@@ Line 125: / Line 125: @@
 [[File:AbioMCredFig5.png | thumb |500px|Figure 5. Relative reduction rate constants of the NACs/MCs listed in Table 1 for AHQDS<sup>–</sup>. Rate constants are compared with respect to RDX. Abbreviations of NACs/MCs as listed in Table 1.]]
-Most of the current knowledge about MC degradation is derived from studies using NACs. The reduction kinetics of only four MCs, namely TNT, N-methyl-4-nitroaniline (MNA), NTO, and RDX, have been investigated with hydroquinones. Of these four MCs, only the reduction rates of MNA and TNT have been modeled<ref name="Hofstetter1999" /><ref name="Murillo-Gelvez2019" /><ref name="Riefler2000">Riefler, R.G., and Smets, B.F., 2000. Enzymatic Reduction of 2,4,6-Trinitrotoluene and Related Nitroarenes: Kinetics Linked to One-Electron Redox Potentials. Environmental Science and Technology, 34(18), pp. 3900–3906.  [https://doi.org/10.1021/es991422f DOI: 10.1021/es991422f]</ref><ref name="Salter-Blanc2015">Salter-Blanc, A.J., Bylaska, E.J., Johnston, H.J., and Tratnyek, P.G., 2015. Predicting Reduction Rates of Energetic Nitroaromatic Compounds Using Calculated One-Electron Reduction Potentials. Environmental Science and Technology, 49(6), pp. 3778–3786.  [https://doi.org/10.1021/es505092s DOI: 10.1021/es505092s]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es505092s Open access article.]</ref>.
+Most of the current knowledge about MC degradation is derived from studies using NACs. The reduction kinetics of only four MCs, namely TNT, N-methyl-4-nitroaniline (MNA), NTO, and RDX, have been investigated with hydroquinones. Of these four MCs, only the reduction rates of MNA and TNT have been modeled<ref name="Hofstetter1999" /><ref name="Murillo-Gelvez2019" /><ref name="Riefler2000">Riefler, R.G., and Smets, B.F., 2000. Enzymatic Reduction of 2,4,6-Trinitrotoluene and Related Nitroarenes: Kinetics Linked to One-Electron Redox Potentials. Environmental Science and Technology, 34(18), pp. 3900–3906.  [https://doi.org/10.1021/es991422f DOI: 10.1021/es991422f]</ref><ref name="Salter-Blanc2015">Salter-Blanc, A.J., Bylaska, E.J., Johnston, H.J., and Tratnyek, P.G., 2015. Predicting Reduction Rates of Energetic Nitroaromatic Compounds Using Calculated One-Electron Reduction Potentials. Environmental Science and Technology, 49(6), pp. 3778–3786.  [https://doi.org/10.1021/es505092s DOI: 10.1021/es505092s]&nbsp;&nbsp;[https://pubs.acs.org/doi/pdf/10.1021/es505092s Article pdf.]</ref>.
-Using the rate constants obtained with AHQDS<sup>–</sup>, a relative reactivity trend can be obtained (Figure 5). RDX is the slowest reacting MC in Table 1, hence it was selected to calculate the relative rates of reaction (i.e., log ''k<sub>NAC/MC</sub>'' – log ''k<sub>RDX</sub>''). If only the MCs in Figure 5 are considered, the reactivity spans 6 orders of magnitude following the trend: RDX ≈ MNA < NTO<sup>–</sup> < DNAN < TNT < NTO. The rate constant for DNAN reduction by AHQDS<sup>–</sup> is not yet published and hence not included in Table 1. Note that speciation of NACs/MCs can significantly affect their reduction rates. Upon deprotonation, the NAC/MC becomes negatively charged and less reactive as an oxidant (i.e., less prone to accept an electron). As a result, the second-order rate constant can decrease by 0.5-0.6 log unit in the case of nitrophenols and approximately 4 log units in the case of NTO (numbers in parentheses in Table 1)<ref name="Schwarzenbach1990" /><ref name="Murillo-Gelvez2021" />.
+Using the rate constants obtained with AHQDS<sup>–</sup>, a relative reactivity trend can be obtained (Figure 5). RDX is the slowest reacting MC in Table 1, hence it was selected to calculate the relative rates of reaction (i.e., log ''k<sub>NAC/MC</sub>'' – log ''k<sub>RDX</sub>''). If only the MCs in Figure 5 are considered, the reactivity spans 6 orders of magnitude following the trend: RDX ≈ MNA < NTO<sup>–</sup> < DNAN < TNT < NTO. The rate constant for DNAN reduction by AHQDS<sup>–</sup> is not yet published and hence not included in Table 1. Note that speciation of NACs/MCs can significantly affect their reduction rates. Upon deprotonation, the NAC/MC becomes negatively charged and less reactive as an oxidant (i.e., less prone to accept an electron). As a result, the second-order rate constant can decrease by 0.5-0.6 log unit in the case of [[wikipedia:Nitrophenol|nitrophenols]] and approximately 4 log units in the case of NTO (numbers in parentheses in Table 1)<ref name="Schwarzenbach1990" /><ref name="Murillo-Gelvez2021" />.
 ==Ferruginous Reductants==
@@ Line 547: / Line 547: @@
 |}
-Iron(II)&nbsp;can&nbsp;be&nbsp;complexed by a myriad of organic ligands and may thereby become more reactive towards MCs and other pollutants. The reactivity of an Fe(II)-organic complex depends on the relative preference of the organic ligand for Fe(III) versus Fe(II)<ref name="Kim2009" />. Since the majority of naturally occurring ligands complex Fe(III) more strongly than Fe(II), the reduction potential of the resulting Fe(III) complex is lower than that of aqueous Fe(III); therefore, complexation by organic ligands often renders Fe(II) a stronger reductant thermodynamically<ref name="Strathmann2011">Strathmann, T.J., 2011. Redox Reactivity of Organically Complexed Iron(II) Species with Aquatic Contaminants. Aquatic Redox Chemistry, American Chemical Society,1071(14), pp. 283-313.  [https://doi.org/10.1021/bk-2011-1071.ch014 DOI: 10.1021/bk-2011-1071.ch014]</ref>. The reactivity of dissolved Fe(II)-organic complexes towards NACs/MCs has been investigated. The intrinsic, second-order rate constants and one electron reduction potentials are listed in Table 2.
+Iron(II)&nbsp;can&nbsp;be&nbsp;complexed by a myriad of organic ligands and may thereby become more reactive towards MCs and other pollutants. The reactivity of an Fe(II)-organic complex depends on the relative preference of the organic [[wikipedia:Ligand|ligand]] for Fe(III) versus Fe(II)<ref name="Kim2009" />. Since the majority of naturally occurring ligands complex Fe(III) more strongly than Fe(II), the reduction potential of the resulting Fe(III) complex is lower than that of aqueous Fe(III); therefore, complexation by organic ligands often renders Fe(II) a stronger reductant thermodynamically<ref name="Strathmann2011">Strathmann, T.J., 2011. Redox Reactivity of Organically Complexed Iron(II) Species with Aquatic Contaminants. Aquatic Redox Chemistry, American Chemical Society,1071(14), pp. 283-313.  [https://doi.org/10.1021/bk-2011-1071.ch014 DOI: 10.1021/bk-2011-1071.ch014]</ref>. The reactivity of dissolved Fe(II)-organic complexes towards NACs/MCs has been investigated. The intrinsic, second-order rate constants and one electron reduction potentials are listed in Table 2.
-In addition to forming organic complexes, iron is ubiquitous in minerals. Iron-bearing minerals play an important role in controlling the environmental fate of contaminants through adsorption<ref name="Linker2015">Linker, B.R., Khatiwada, R., Perdrial, N., Abrell, L., Sierra-Alvarez, R., Field, J.A., and Chorover, J., 2015. Adsorption of novel insensitive munitions compounds at clay mineral and metal oxide surfaces. Environmental Chemistry, 12(1), pp. 74–84.  [https://doi.org/10.1071/EN14065 DOI: 10.1071/EN14065]</ref><ref name="Jenness2020">Jenness, G.R., Giles, S.A., and Shukla, M.K., 2020. Thermodynamic Adsorption States of TNT and DNAN on Corundum and Hematite. The Journal of Physical Chemistry C, 124(25), pp. 13837–13844.  [https://doi.org/10.1021/acs.jpcc.0c04512 DOI: 10.1021/acs.jpcc.0c04512]</ref> and reduction<ref name="Gorski2011">Gorski, C.A., and Scherer, M.M., 2011. Fe<sup>2+</sup> Sorption at the Fe Oxide-Water Interface: A Revised Conceptual Framework. Aquatic Redox Chemistry, American Chemical Society, 1071(15), pp. 315–343.  [https://doi.org/10.1021/bk-2011-1071.ch015 DOI: 10.1021/bk-2011-1071.ch015]</ref> processes. Studies have shown that aqueous Fe(II) itself cannot reduce NACs/MCs at circumneutral pH<ref name="Klausen1995" /><ref name="Gregory2004">Gregory, K.B., Larese-Casanova, P., Parkin, G.F., and Scherer, M.M., 2004. Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine by Fe<sup>II</sup> Bound to Magnetite. Environmental Science and Technology, 38(5), pp. 1408–1414.  [https://doi.org/10.1021/es034588w DOI: 10.1021/es034588w]</ref> but in the presence of an iron oxide (e.g., goethite, hematite, lepidocrocite, ferrihydrite, or magnetite), NACs<ref name="Colón2006" /><ref name="Klausen1995" /><ref name="Strehlau2016" /><ref name="Elsner2004" /><ref name="Hofstetter2006" /> and MCs such as TNT<ref name="Hofstetter1999" />, RDX<ref name="Gregory2004" />, DNAN<ref name="Berens2019">Berens, M.J., Ulrich, B.A., Strehlau, J.H., Hofstetter, T.B., and Arnold, W.A., 2019. Mineral identity, natural organic matter, and repeated contaminant exposures do not affect the carbon and nitrogen isotope fractionation of 2,4-dinitroanisole during abiotic reduction. Environmental Science: Processes and Impacts, 21(1), pp. 51-62.  [https://doi.org/10.1039/C8EM00381E DOI: 10.1039/C8EM00381E]</ref>, and NG<ref name="Oh2004">Oh, S.-Y., Cha, D.K., Kim, B.J., and Chiu, P.C., 2004. Reduction of Nitroglycerin with Elemental Iron:  Pathway, Kinetics, and Mechanisms. Environmental Science and Technology, 38(13), pp. 3723–3730.  [https://doi.org/10.1021/es0354667 DOI: 10.1021/es0354667]</ref> can be rapidly reduced. Unlike ferric oxides, Fe(II)-bearing minerals including clays<ref name="Hofstetter2006" /><ref name="Schultz2000" /><ref name="Luan2015a" /><ref name="Luan2015b" /><ref name="Hofstetter2003" /><ref name="Neumann2008" /><ref name="Hofstetter2008" />, green rust<ref name="Larese-Casanova2008" /><ref name="Khatiwada2018">Khatiwada, R., Root, R.A., Abrell, L., Sierra-Alvarez, R., Field, J.A., and Chorover, J., 2018. Abiotic reduction of insensitive munition compounds by sulfate green rust. Environmental Chemistry, 15(5), pp. 259–266.  [https://doi.org/10.1071/EN17221 DOI: 10.1071/EN17221]</ref>, mackinawite<ref name="Elsner2004" /><ref name="Berens2019" /><ref name="Menezes2021">Menezes, O., Yu, Y., Root, R.A., Gavazza, S., Chorover, J., Sierra-Alvarez, R., and Field, J.A., 2021. Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO). Chemosphere, 285, p. 131409.  [https://doi.org/10.1016/j.chemosphere.2021.131409 DOI: 10.1016/j.chemosphere.2021.131409]</ref> and pyrite<ref name="Elsner2004" /><ref name="Oh2008">Oh, S.-Y., Chiu, P.C., and Cha, D.K., 2008. Reductive transformation of 2,4,6-trinitrotoluene,  hexahydro-1,3,5-trinitro-1,3,5-triazine, and nitroglycerin by pyrite and magnetite. Journal of hazardous materials, 158(2-3), pp. 652–655.  [https://doi.org/10.1016/j.jhazmat.2008.01.078 DOI: 10.1016/j.jhazmat.2008.01.078]</ref> do not need aqueous Fe(II) to be reactive toward NACs/MCs. However, upon oxidation, sulfate green rust was converted into lepidocrocite<ref name="Khatiwada2018" />, and mackinawite into goethite<ref name="Menezes2021" />, suggesting that aqueous Fe(II) coupled to Fe(III) oxides might be at least partially responsible for continued degradation of NACs/MCs in the subsurface once the parent reductant (e.g., green rust or iron sulfide) oxidizes.
+In addition to forming organic complexes, iron is ubiquitous in minerals. Iron-bearing minerals play an important role in controlling the environmental fate of contaminants through adsorption<ref name="Linker2015">Linker, B.R., Khatiwada, R., Perdrial, N., Abrell, L., Sierra-Alvarez, R., Field, J.A., and Chorover, J., 2015. Adsorption of novel insensitive munitions compounds at clay mineral and metal oxide surfaces. Environmental Chemistry, 12(1), pp. 74–84.  [https://doi.org/10.1071/EN14065 DOI: 10.1071/EN14065]</ref><ref name="Jenness2020">Jenness, G.R., Giles, S.A., and Shukla, M.K., 2020. Thermodynamic Adsorption States of TNT and DNAN on Corundum and Hematite. The Journal of Physical Chemistry C, 124(25), pp. 13837–13844.  [https://doi.org/10.1021/acs.jpcc.0c04512 DOI: 10.1021/acs.jpcc.0c04512]</ref> and reduction<ref name="Gorski2011">Gorski, C.A., and Scherer, M.M., 2011. Fe<sup>2+</sup> Sorption at the Fe Oxide-Water Interface: A Revised Conceptual Framework. Aquatic Redox Chemistry, American Chemical Society, 1071(15), pp. 315–343.  [https://doi.org/10.1021/bk-2011-1071.ch015 DOI: 10.1021/bk-2011-1071.ch015]</ref> processes. Studies have shown that aqueous Fe(II) itself cannot reduce NACs/MCs at circumneutral pH<ref name="Klausen1995" /><ref name="Gregory2004">Gregory, K.B., Larese-Casanova, P., Parkin, G.F., and Scherer, M.M., 2004. Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine by Fe<sup>II</sup> Bound to Magnetite. Environmental Science and Technology, 38(5), pp. 1408–1414.  [https://doi.org/10.1021/es034588w DOI: 10.1021/es034588w]</ref> but in the presence of an iron oxide (e.g., [[wikipedia:Goethite|goethite]], [[wikipedia:Hematite|hematite]], [[wikipedia:Lepidocrocite|lepidocrocite]], [[wikipedia:Ferrihydrite|ferrihydrite]], or [[wikipedia:Magnetite|magnetite]]), NACs<ref name="Colón2006" /><ref name="Klausen1995" /><ref name="Strehlau2016" /><ref name="Elsner2004" /><ref name="Hofstetter2006" /> and MCs such as TNT<ref name="Hofstetter1999" />, RDX<ref name="Gregory2004" />, DNAN<ref name="Berens2019">Berens, M.J., Ulrich, B.A., Strehlau, J.H., Hofstetter, T.B., and Arnold, W.A., 2019. Mineral identity, natural organic matter, and repeated contaminant exposures do not affect the carbon and nitrogen isotope fractionation of 2,4-dinitroanisole during abiotic reduction. Environmental Science: Processes and Impacts, 21(1), pp. 51-62.  [https://doi.org/10.1039/C8EM00381E DOI: 10.1039/C8EM00381E]</ref>, and NG<ref name="Oh2004">Oh, S.-Y., Cha, D.K., Kim, B.J., and Chiu, P.C., 2004. Reduction of Nitroglycerin with Elemental Iron:  Pathway, Kinetics, and Mechanisms. Environmental Science and Technology, 38(13), pp. 3723–3730.  [https://doi.org/10.1021/es0354667 DOI: 10.1021/es0354667]</ref> can be rapidly reduced. Unlike ferric oxides, Fe(II)-bearing minerals including [[wikipedia:Clay|clays]]<ref name="Hofstetter2006" /><ref name="Schultz2000" /><ref name="Luan2015a" /><ref name="Luan2015b" /><ref name="Hofstetter2003" /><ref name="Neumann2008" /><ref name="Hofstetter2008" />, [[wikipedia:Green_rust|green rust]]<ref name="Larese-Casanova2008" /><ref name="Khatiwada2018">Khatiwada, R., Root, R.A., Abrell, L., Sierra-Alvarez, R., Field, J.A., and Chorover, J., 2018. Abiotic reduction of insensitive munition compounds by sulfate green rust. Environmental Chemistry, 15(5), pp. 259–266.  [https://doi.org/10.1071/EN17221 DOI: 10.1071/EN17221]</ref>, [[wikipedia:Mackinawite|mackinawite]]<ref name="Elsner2004" /><ref name="Berens2019" /><ref name="Menezes2021">Menezes, O., Yu, Y., Root, R.A., Gavazza, S., Chorover, J., Sierra-Alvarez, R., and Field, J.A., 2021. Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO). Chemosphere, 285, p. 131409.  [https://doi.org/10.1016/j.chemosphere.2021.131409 DOI: 10.1016/j.chemosphere.2021.131409]</ref> and [[wikipedia:Pyrite|pyrite]]<ref name="Elsner2004" /><ref name="Oh2008">Oh, S.-Y., Chiu, P.C., and Cha, D.K., 2008. Reductive transformation of 2,4,6-trinitrotoluene,  hexahydro-1,3,5-trinitro-1,3,5-triazine, and nitroglycerin by pyrite and magnetite. Journal of hazardous materials, 158(2-3), pp. 652–655.  [https://doi.org/10.1016/j.jhazmat.2008.01.078 DOI: 10.1016/j.jhazmat.2008.01.078]</ref> do not need aqueous Fe(II) to be reactive toward NACs/MCs. However, upon oxidation, sulfate green rust was converted into lepidocrocite<ref name="Khatiwada2018" />, and mackinawite into goethite<ref name="Menezes2021" />, suggesting that aqueous Fe(II) coupled to Fe(III) oxides might be at least partially responsible for continued degradation of NACs/MCs in the subsurface once the parent reductant (e.g., green rust or iron sulfide) oxidizes.
 The reaction conditions and rate constants for a list of studies on MC reduction by iron oxide-aqueous Fe(II)  redox couples and by other Fe(II)-containing minerals are shown in Table 3<ref name="Hofstetter1999" /><ref name="Larese-Casanova2008" /><ref name="Gregory2004" /><ref name="Berens2019" /><ref name="Oh2008" /><ref name="Strehlau2018">Strehlau, J.H., Berens, M.J., and Arnold, W.A., 2018. Mineralogy and buffer identity effects on RDX kinetics and intermediates during reaction with natural and synthetic magnetite. Chemosphere, 213, pp. 602–609.  [https://doi.org/10.1016/j.chemosphere.2018.09.139 DOI: 10.1016/j.chemosphere.2018.09.139]</ref><ref name="Cardenas-Hernandez2020">Cárdenas-Hernandez, P.A., Anderson, K.A., Murillo-Gelvez, J., di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2020. Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite–Aqueous Fe(II) Redox Couple. Environmental Science and Technology, 54(19), pp. 12191–12201.  [https://doi.org/10.1021/acs.est.0c03872 DOI: 10.1021/acs.est.0c03872]</ref>. Unlike hydroquinones and Fe(II) complexes, where second-order rate constants can be readily calculated, the reduction rate constants of NACs/MCs in mineral suspensions are often specific to the experimental conditions used and are usually reported as BET surface area-normalized reduction rate constants (''k<sub>SA</sub>''). In the case of iron oxide-Fe(II) redox couples, reduction rate constants have been shown to increase with pH (specifically, with [OH<sup>– </sup>]<sup>2</sup>) and aqueous Fe(II) concentration, both of which correspond to a decrease in the system's reduction potential<ref name="Colón2006" /><ref name="Gorski2016" /><ref name="Cardenas-Hernandez2020" />.

Difference between revisions of "Munitions Constituents - Abiotic Reduction"

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Ferruginous Reductants

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