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

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; 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 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 (FeL₂^2-), and k_obs measurements are accurately predicted by k_obs = k_FeL2 ^2-[FeL₂^2-], where k_FeL2^2- = 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^II that forms when Fe^II complexes are oxidized by reactions with the NACs.