Organosulfur compound-driven mobilization of colloidal metals and organic matter from acid mine drainage-contaminated paddy soils.

Although organosulfur compounds dominate the speciation of sulfur in acid mine drainage (AMD)-contaminated paddy soils, their impact on the migration and transformation of heavy metals remains unclear. We conducted soil incubation experiments to investigate organosulfur compound effects on the speciation of mobilized dissolved and colloidal cadmium (Cd), copper (Cu) and organic matter. The addition of cysteine (Cys) as a model thiol or cysteic acid (Cya) as a model sulfonate significantly increased the mobilization of Cd and Cu from the iron (Fe)-manganese (Mn) oxides fraction of soils compared with the control. Mobilization of Cd and Cu increased at higher concentrations of Cys and Cya, but rising concentrations of Cys caused an initial decrease followed by an increase in the mobilization of colloidal Cu. Fourier transform-ion cyclotron resonance mass spectrometry analysis of mobilized colloids revealed a higher release of CHONS compounds in Cys than Cya soil incubations, indicating a possible thiol-driven mobilization of sulfur-rich organic compounds and associated metals. Disaggregation of Fe oxides and sulfides occurred in low and high Cys treatments, respectively, while released soil organic carbon stabilized dissolved Fe at intermediate Cys concentrations. In Cya incubations, composite Fe, Mn and organic compound colloids were dominant carriers of mobilized colloidal Cd and Cu. X-ray absorption fluorescence spectroscopy showed concentration-dependent coordination of colloidal Cu in incubations with Cys and Cya. Our results indicate that the release of heavy metals from soils was primarily driven by thiol coordination and perhaps the dissolution and disaggregation of Fe and Mn oxides and Fe sulfides with Cys, and by metal-organic colloid formation and acidification with Cya.