Abiotic reduction of nitroaromatic contaminants by iron(II) complexes with organothiol ligands

Abstract

Complexation of Fe(II) by dissolved and surface-bound ligands can significantly modify the metal's redox reactivity, and recent work reveals that Fe(II) complexes with selected classes of organic ligands are potent reductants that may contribute to the natural attenuation of subsurface contaminants. In the present study, we investigated the reactivity of Fe(II)-organothiol ligand complexes with nitroaromatic contaminants (NACs; ArNO(2)). Experimental results show that NACs are unreactive in Fe(2+)-only and ligand-only solutions but are reduced to the corresponding aniline compounds (ArNH(2)) in solutions containing both Fe(II) and a number of organothiol ligands. Observed reaction rates are highly dependent on the structure of the Fe(II)-complexing ligand, solution composition, Fe(II) speciation, and NAC structure. For two model ligands, cysteine and thioglycolic acid, observed pseudo-first order rate constants for 4-chloronitrobenzene reduction (k(obs); 1/s) are linearly correlated with the concentration of the respective 1:2 Fe(II)- organothiol complexes (FeL(2)(2-)), and k(obs) measurements are accurately predicted by k(obs) = k(FeL(2-)(2))[FeL(2-)(2)], where k(FeL(2-)(2)) = 1.70 (+/-0.59) 1/M/s and 26.0 (+/-4.8) 1/M/s for cysteine and thioglycolic acid, respectively. The high reactivity of these Fe(II) complexes is attributed to a lowering of the standard one-electron reduction potential of the Fe(III)/Fe(II) redox couple on complexation by organothiol ligands. The relative reactivity of a series of substituted NACs with individual Fe(II) complexes can be described by linear free-energy relationships with the apparent one-electron reduction potentials of the NACs. Tests also show that organothiol ligands can further promote NAC reduction indirectly by re-reducing the Fe(III) that forms when Fe(II) complexes are oxidized by reactions with the NACs.

Reductive Transformation of 2,4-Dinitrotoluene: Roles of Iron and Natural Organic Matter

“…Similar rate enhancement by added organic ligands in Fe(II) oxidation has been observed previously for a variety of organic and inorganic oxidants such as Cr(IV) (Buerge and Hug 1998), oxime carbamate pesticides (Strathmann and Stone 2002), mononitrobenzenes (Naka et al 2006(Naka et al , 2008, and benzoquinones (Uchimiya and Stone 2009). However, no appreciable reaction between mononitrobenzenes (e.g., 4-chloronitrobenzene) and Fe(II) was observed in the presence of carboxylate and aminocaboxylate ligands (e.g., citrate and EDTA); stronger ligands containing catechol (Naka et al 2006) and thiol (Naka et al 2008) functionalities were required for the reactions to proceed to an appreciable extent.…”

“…However, no appreciable reaction between mononitrobenzenes (e.g., 4-chloronitrobenzene) and Fe(II) was observed in the presence of carboxylate and aminocaboxylate ligands (e.g., citrate and EDTA); stronger ligands containing catechol (Naka et al 2006) and thiol (Naka et al 2008) functionalities were required for the reactions to proceed to an appreciable extent. More positive one-electron reduction potential of 2,4-DNT [E m = -0.38 ± 0.02 V, relative to mononitrobenzenes e.g., -0.45 V for 4-chloronitrobenzene and -0.50 V for 4-nitrotoluene )] and longer monitoring periods in this study most likely enabled empirical observation of 2,4-DNT reduction under less favorable conditions, i.e., slow but appreciable reduction in the presence of carboxylate and aminocaboxylate ligands (Fig.…”

“…Previous studies demonstrated that small changes in Fe(II) speciation cause dramatic shifts in the rate of reaction with Cr(IV) (Buerge and Hug 1998), oxime carbamate pesticides (Strathmann and Stone 2002), mononitrobenzenes (Naka et al 2006(Naka et al , 2008 and benzoquinones (Uchimiya and Stone 2009). Fe(II) has been shown to reduce mononitrobenzenes only in the presence of mineral surfaces (Klausen et al 1995;Hofstetter et al 1999) or catechol (Naka et al 2006) and thiol (Naka et al 2008) (but not carboxylate and aminocaboxylate) ligands that form Fe(II) species having sufficiently low reduction potentials (E h (Fe(III/II)) in volts).…”

“…Fe(II) has been shown to reduce mononitrobenzenes only in the presence of mineral surfaces (Klausen et al 1995;Hofstetter et al 1999) or catechol (Naka et al 2006) and thiol (Naka et al 2008) (but not carboxylate and aminocaboxylate) ligands that form Fe(II) species having sufficiently low reduction potentials (E h (Fe(III/II)) in volts).…”

Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants

“…Although most work to date has focused on contaminant reactions with mineral-associated Fe II species (Myneni et al, 1997;Liger et al, 1999;Butler and Hayes, 1999;Hofstetter et al, 2003;Elsner et al, 2004;Gregory et al, 2004;Klupinski et al, 2004), recent studies show that soluble Fe II complexes with chelating organic ligands can also be potent reductants of many contaminants (Buerge and Hug, 1998;Strathmann and Stone, 2002a;Naka et al, 2006). In particular, Fe II complexes with catecholate and organothiol ligands have been shown to rapidly reduce a variety of nitroaromatic, N-heterocyclic nitramine and haloalkane contaminants (Naka et al, 2006;Bussan and Strathmann, 2007;Kim and Strathmann, 2007;Naka et al, 2008). As mentioned above, catecholates are an important class of siderophores.…”