Speciation of toxic metals in metal finishing filter cake by X-ray absorption spectroscopy.

The speciation of Cr, Zn, Cu and Pb in two metal finishing filter cakes (TX and ST) was investigated by X-ray absorption spectroscopy (XAS) complemented by X-ray fluorescence (XRF) and X-ray diffraction (XRD). XRF showed that concentrations of Cr, Zn, Cu and Pb were 1.4%, 0.19%, 0.20% and 0.01%, respectively, in TX, and 12.6%, 3.3%, 1.3% and 0.21% in ST. No crystalline phases were detected in TX by XRD whereas ST was dominated by calcite. Cr and Fe K edge XAS showed Cr to be trivalent and octahedrally coordinated, co-precipitated with Fe as CrFe-(oxy)hydroxides in both filter cakes. Zn, P and Ca K edge XAS showed that 2ZnCO∙3Zn(OH) and Zn(PO) were the dominant zinc-containing phases, with combined tetrahedral and octahedral coordination; Zn phases were slightly more crystalline in TX than ST. Pb L edge X-ray absorption near edge spectroscopy (XANES) found that Pb was likely adsorbed on amorphous SiO. Cu, Si and S K edge XAS showed that all Cu was divalent, and the dominant copper phases were found to be CuCl(OH), Cu(OH) and CuSO·5HO for ST, whereas Cu appeared to adsorb to amorphous SiO for TX, which contained much less Pb. Cr is thus immobilized in the filter cakes in a phase with low solubility at environmentally feasible pH values, whereas Zn, Cu and Pb could be released when the pH decreases below 8 or above 11. These findings are significant for the development of waste management regulations and/or metal recovery methods (e.g., hydro/pyrometallurgy).