Under sulfate reducing conditions, the iron sulfide mackinawite (FeS1-x) forms, and has been shown to be an effective scavenger of heavy metal ion contaminants. In this study, the reaction of cadmium (Cd) with mackinawite was investigate under anoxic conditions over a range of pH values, initial Cd and background electrolyte concentrations. The batch sorption measurements showed Cd removal by mackinawite suspensions of greater than 99% under all the experimental conditions tested, including pH between 4 and 10, initial Cd concentration between 10-6 and 10-4M, and NaCl background electrolyte concentration between 0.01 and 0.1M. Thermodynamic calculations showed Cd aqueous speciation changes as a function of pH, background electrolyte concentration, and the addition of mackinawite, with CdCl+ the predominant aqueous Cd species in the absence of mackinawite and Cd(HS)2(aq) species dominant when mackinawite is present. The impacts of the geochemical conditions on the Cd removal mechanisms, including adsorption by complexation with the mackinawite surface, formation of a crystalline or amorphous Cd sulfide solid phase along with mackinawite dissolution, and formation of (Fe,Cd)S solid solution, are being confirmed by using a combination of aqueous and surface complexation modeling, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). Consistent with previous studies of Hg, XPS and XAS indicate that the formation of metal sulfide, specifically greenockite (a hexagonal CdS phase) in this case, is the primary Cd removal mechanism at higher initial Cd concentrations while adsorption is implicated at lower Cd concentrations.

Cadmium removal mechanism by mackinawite under anoxic conditions

255th National Meeting (2018)

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