FeS-mediated mobilization and immobilization of Cr(III) in oxic aquatic systems

Reduction of soluble Cr(VI) into insoluble Cr(III) by iron sulfide (FeS) minerals under anoxic conditions has been widely observed in natural and engineered systems. Yet, information has been lacking on the FeS-mediated oxidation and remobilization potential of Cr(III) under varying environmental conditions. The objective of this study was to investigate FeS-mediated redox transformation of Cr(III) to Cr(VI) and the associated mobilization and immobilization when Cr(III)-FeS systems are exposed to atmospheric conditions. The results showed that FeS nanoparticles facilitated rapid and strong Fenton-like reactions during the early-stage oxygenation of FeS, resulting in rapid production of hydroxyl radicals (center dot OH). Consequently, Cr(III) was rapidly oxidized into Cr (VI). Yet, as the reactions proceeded, the oxidative potential was counteracted by competitive scavenging of center dot OH by Fe(II) and S(-II) from FeS and the reduction reactions by these electron donors. At equilibrium, all Cr(VI) was reduced back to Cr(III) at an FeS-Cr(III) molar ratio of 10:1, while a small fraction of Cr(VI) persisted in solid products of Cr(OH)(3)(s) at an FeS-Cr(III) molar ratio of 1:1. Acidic conditions favored the generation of Cr(VI) and the equilibrium concentration of Cr(VI) in oxic FeS NPs systems at pH 5.0 was 1.7 times higher than at pH 9.0. Overall, the FeS-induced Fenton-like reactions and the oxidation of Cr(III) were favored in the early stage, but quenched in the later stage and outcompeted by the reduction of Cr(VI) if sufficient FeS was available. The findings provide new insights into the hydrochemical processes that can affect the speciation, toxicity, and mobility of Cr in aquatic systems containing FeS and Cr.

Automated summary

This study found that FeS nanoparticles facilitated rapid Fenton-like reactions during the early stage of FeS oxygenation, leading to the rapid oxidation of Cr(III) to Cr(VI). However, as the reactions progressed, the oxidative potential was counteracted by competitive scavenging and reduction reactions. In equilibrium, all Cr(VI) was reduced back to Cr(III) at an FeS-Cr(III) molar ratio of 10:1, while a small fraction of Cr(VI) remained in solid products at a ratio of 1:1. Acidic conditions favored the generation of Cr(VI) and the equilibrium concentration was higher at pH 5.0 compared to pH 9.0.