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(Carbonaceous Reductants)
(Carbonaceous Reductants)
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! AHQS<sup>-</sup>
 
! AHQS<sup>-</sup>
 
|-
 
|-
| Nitrobenzene (NB) || -0.485 || 0.380 || -1.102 || 2.050 || 3.060
+
| Nitrobenzene (NB) || -0.485<ref name="Schwarzenbach1990"/> || 0.380<ref name="Schwarzenbach1990"/> || -1.102<ref name="Schwarzenbach1990"/> || 2.050 || 3.060
 
|-
 
|-
| 2-nitrotoluene (2-NT) || -0.590 || -1.432 || -2.523 || 0.775 ||  
+
| 2-nitrotoluene (2-NT) || -0.590<ref name="Schwarzenbach1990"/> || -1.432<ref name="Schwarzenbach1990"/> || -2.523<ref name="Schwarzenbach1990"/> || 0.775 ||  
 
|-  
 
|-  
| 3-nitrotoluene (3-NT) || -0.475 || 0.462 || -0.921 ||  ||
+
| 3-nitrotoluene (3-NT) || -0.475<ref name="Schwarzenbach1990"/> || 0.462<ref name="Schwarzenbach1990"/> || -0.921<ref name="Schwarzenbach1990"/> ||  ||
 
|-
 
|-
| 4-nitrotoluene (4-NT) || -0.500 || 0.041 || -1.292 || 1.822 || 2.610
+
| 4-nitrotoluene (4-NT) || -0.500<ref name="Schwarzenbach1990"/> || 0.041<ref name="Schwarzenbach1990"/> || -1.292<ref name="Schwarzenbach1990"/> || 1.822 || 2.610
 
|-
 
|-
| 2-chloronitrobenzene (2-ClNB) || -0.485 || 0.342 || -0.824 ||2.412 ||
+
| 2-chloronitrobenzene (2-ClNB) || -0.485<ref name="Schwarzenbach1990"/> || 0.342<ref name="Schwarzenbach1990"/> || -0.824<ref name="Schwarzenbach1990"/> ||2.412 ||
 
|-
 
|-
| 3-chloronitrobenzene (3-ClNB) || -0.405 || 1.491 || 0.114 || ||
+
| 3-chloronitrobenzene (3-ClNB) || -0.405<ref name="Schwarzenbach1990"/> || 1.491<ref name="Schwarzenbach1990"/> || 0.114<ref name="Schwarzenbach1990"/> || ||
 
|-
 
|-
| 4-chloronitrobenzene (4-ClNB) || -0.450 || 1.041 || -0.301 || 2.988 ||
+
| 4-chloronitrobenzene (4-ClNB) || -0.450<ref name="Schwarzenbach1990"/> || 1.041<ref name="Schwarzenbach1990"/> || -0.301<ref name="Schwarzenbach1990"/> || 2.988 ||
 
|-
 
|-
| 2-acetylnitrobenzene (2-AcNB) || -0.470 || 0.519 || -0.456 || ||  
+
| 2-acetylnitrobenzene (2-AcNB) || -0.470<ref name="Schwarzenbach1990"/> || 0.519<ref name="Schwarzenbach1990"/> || -0.456<ref name="Schwarzenbach1990"/> || ||  
 
|-
 
|-
| 3-acetylnitrobenzene (3-AcNB) || -0.405 || 1.663 || 0.398 || ||
+
| 3-acetylnitrobenzene (3-AcNB) || -0.405<ref name="Schwarzenbach1990"/> || 1.663<ref name="Schwarzenbach1990"/> || 0.398<ref name="Schwarzenbach1990"/> || ||
 
|-
 
|-
| 4-acetylnitrobenzene (4-AcNB) || -0.360 || 2.519 || 1.477 || ||
+
| 4-acetylnitrobenzene (4-AcNB) || -0.360<ref name="Schwarzenbach1990"/> || 2.519<ref name="Schwarzenbach1990"/> || 1.477<ref name="Schwarzenbach1990"/> || ||
 
|-
 
|-
| 2-nitrophenol (2-NP) || || 0.568 (0.079) || || ||
+
| 2-nitrophenol (2-NP) || || 0.568 (0.079)<ref name="Schwarzenbach1990"/> || || ||
 
|-
 
|-
| 4-nitrophenol (4-NP) || || -0.699 (-1.301) || || ||
+
| 4-nitrophenol (4-NP) || || -0.699 (-1.301)<ref name="Schwarzenbach1990"/> || || ||
 
|-
 
|-
| 4-methyl-2-nitrophenol (4-Me-2-NP) || || 0.748 (0.176) || || ||  
+
| 4-methyl-2-nitrophenol (4-Me-2-NP) || || 0.748 (0.176)<ref name="Schwarzenbach1990"/> || || ||  
 
|-
 
|-
| 4-chloro-2-nitrophenol (4-Cl-2-NP) || || 1.602 (1.114) || || ||
+
| 4-chloro-2-nitrophenol (4-Cl-2-NP) || || 1.602 (1.114)<ref name="Schwarzenbach1990"/> || || ||
 
|-
 
|-
| 5-fluoro-2-nitrophenol (5-Cl-2-NP) || || 0.447 (-0.155) || || ||
+
| 5-fluoro-2-nitrophenol (5-Cl-2-NP) || || 0.447 (-0.155)<ref name="Schwarzenbach1990"/> || || ||
 
|-
 
|-
 
| 2,4,6-trinitrotoluene (TNT) || -0.280 || || 2.869 || 5.204 ||  
 
| 2,4,6-trinitrotoluene (TNT) || -0.280 || || 2.869 || 5.204 ||  

Revision as of 22:23, 4 April 2022

Abiotic Reduction of Munitions Constituents

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.

Related Article(s):

Contributor(s):

  • Dr. Jimmy Murillo-Gelvez
  • Paula Andrea Cárdenas-Hernández
  • Dr. Pei Chiu

Key Resource(s):

  • Schwarzenbach, Gschwend, and Imboden, 2016. Environmental Organic Chemistry, 3rd ed.[1]

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 (EH) 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 (kR) 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 (EH1’) where available. EH1’ is an experimentally measurable thermodynamic property that reflects the propensity of a given NAC/MC to accept an electron in water (EH1(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 EH1' (V) Hydroquinone (log kR (M-1s-1))
(NAC/MC) LAW- JUG- AHQDS- AHQS-
Nitrobenzene (NB) -0.485[28] 0.380[28] -1.102[28] 2.050 3.060
2-nitrotoluene (2-NT) -0.590[28] -1.432[28] -2.523[28] 0.775
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 2.610
2-chloronitrobenzene (2-ClNB) -0.485[28] 0.342[28] -0.824[28] 2.412
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
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 (0.176)[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 2.869 5.204
2-amino-4,6-dinitrotoluene (2-A-4,6-DNT) -0.400 0.987
4-amino-2,6-dinitrotoluene (4-A-2,6-DNT) -0.440 0.079
2,4-diamino-6-nitrotoluene (2,4-DA-6-NT) -0.505 -1.678
2,6-diamino-4-nitrotoluene (2,6-DA-4-NT) -0.495 -1.252
1,3-dinitrobenzene (1,3-DNB) -0.345 1.785
1,4-dinitrobenzene (1,4-DNB) -0.257 3.839
2-nitroaniline (2-NANE) < -0.560 -2.638
3-nitroaniline (3-NANE) -0.500 -1.367
1,2-dinitrobenzene (1,2-DNB) -0.290 5.407
4-nitroanisole (4-NAN) -0.661 1.220
2-amino-4-nitroanisole (2-A-4-NAN) -0.924 1.150 2.190
4-amino-2-nitroanisole (4-A-2-NAN) 1.610 2.360
2-chloro-4-nitroaniline (2-Cl-5-NANE) -0.863 1.250 2.210
N-methyl-4-nitroaniline (MNA) -1.740 -0.260 0.692
3-nitro-1,2,4-triazol-5-one (NTO) 5.701 (1.914)
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) -0.349

Advantages and Limitations of the Technology for PFAS Treatment

Advantages:

  • High removal rates of long-chain PFAS (C5-C8) due to the production of versatile reactive species
  • Requires no chemical additions and produces no residual waste
  • Total organic carbon (TOC) concentration and other non-surfactant co-contaminants do not influence the process efficiency
  • The process is mobile and scalable
  • Versatile: can be used in batch and continuous systems

Limitations:

  • Limited removal of short-chain PFAS due to their inability to concentrate at plasma-liquid interfaces. Addition of surfactants such as CTAB improves their removal and degradation rates.
  • Excessive foaming caused by bubbling argon gas through a solution containing high (>10 mg/L) concentrations of long-chain (surfactant) PFAS may interfere with the formation of plasma.

Summary

PFAS are susceptible to plasma treatment because the hydrophobic PFAS accumulates at the gas-liquid interface, exposing more of the PFAS to the plasma. Plasma-based treatment of PFAS contaminated water successfully degrades PFOA and PFOS to below the EPA health advisory level of 70 ppt and accomplishes the near complete destruction of other PFAS within a short treatment time. PFAS concentration reductions of ≥90% and post-treatment concentrations below laboratory detection levels are common for long chain PFAS and precursors. The lack of sensitivity of plasma to co-contaminants, coupled with high PFAS removal and defluorination efficiencies, makes plasma-based water treatment a promising technology for the remediation of PFAS-contaminated water. The plasma treatment process is currently developed for ex situ application and can also be integrated into a treatment train[73].

References

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