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

Abstract

AbstractComplexation of FeII by dissolved and surface‐bound ligands can significantly modify the metal's redox reactivity, and recent work reveals that FeII 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 FeII–organothiol ligand complexes with nitroaromatic contaminants (NACs; ArNO2). Experimental results show that NACs are unreactive in Fe2+‐only and ligand‐only solutions but are reduced to the corresponding aniline compounds (ArNH2) in solutions containing both FeII and a number of organothiol ligands. Observed reaction rates are highly dependent on the structure of the FeII‐complexing ligand, solution composition, FeII speciation, and NAC structure. For two model ligands, cysteine and thioglycolic acid, observed pseudo–first order rate constants for 4‐chloronitrobenzene reduction (kobs; 1/s) are linearly correlated with the concentration of the respective 1:2 FeII–organothiol complexes (FeL22−), and kobs measurements are accurately predicted by kobs = kFeL22‐[FeL22−], where kFeL22‐ = 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 FeII complexes is attributed to a lowering of the standard one‐electron reduction potential of the FeIII/FeII redox couple on complexation by organothiol ligands. The relative reactivity of a series of substituted NACs with individual FeII 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 FeIII that forms when FeII complexes are oxidized by reactions with the NACs.