Cadmium(Ii) Removal By Mackinawite Under Anoxic Conditions

Ferrous monosulfide mackinawite (FeS) forming under iron-rich sulfate-reducing conditions is an effective scavenger of heavy metals and metalloids. In this work, we studied the mechanisms of cadmium(II) [Cd(II)] (a toxic chalcophile metal generally considered redox-inactive) reactions with FeS under an anoxic condition over ranges of geochemical variables. The batch uptake experiments show that dissolved Cd(II) is almost completely removed from the FeS suspensions without a distinct pH or ionic strength dependency under all the experimental conditions studied. The batch uptake results do not support (Cd,Fe)S solid-solution formation. Instead, identification of the solid-phase reaction products by a suite of surface chemical probes suggests that the major Cd reaction product is CdS, with minor amounts of metallic Cd (Cd-0) and surface-complexed Cd(II). Density functional theory calculations support the favorability of pure CdS formation compared to Fe-doped CdS or Cd-doped FeS. This study shows that sulfide precipitation largely controls the mobility of Cd, similar to previously reported Hg and As, implicating the presence of dissolved or solid-phase sulfides to be a key parameter in sulfate-reducing natural and engineered environments.