Influence of aluminate and silicate on selenate immobilization using alkaline-earth metal oxides and ferrous salt

Effects of aluminate and silicate species on the SeO₄^2- immobilization using alkali-earth metal oxides and ferrous species have not been clearly elucidated. In the present study, Al and Si species were separately added into MgO/Fe(II) and CaO/Fe(II) reactions containing SeO₄^2-, studied by toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray absorption fine structure (XAFS), and PHREEQC simulation. Approximately 42 % of SeO₄^2- was reduced to SeO₃^2- for MgO/Fe(II) reaction in the presence of Al species, being consistent with the case without Al species. The Al species only showed slight inhibition of Se leaching for the MgO/Fe(II) reaction. Most of Se oxyanions were adsorbed onto Mg(OH)₂ through outer-sphere complexation. For CaO/Fe(II) reaction, all of SeO₄^2- was reduced to SeO₃^2- with or without Al species. However, the Se leaching amount (3 %) of sample added with Al species (CE3) is much lower than that (12 %) of sample without Al species (CE2). This is mainly because SeO₃^2- can be sorbed onto the iron-based minerals through binuclear bidentate corner-sharing (²C) complexation instead of monodentate mononuclear corner-sharing (¹V) complexation of the case without Al species. On the other hand, SeO₄^2- was not reduced to SeO₃^2- in the presence of silicate, and almost all of Se was leached out for silicate-contained samples except CaO/Fe(II) reaction with the addition of Al species. This is due to the polymerization of Al and Si species under a high-alkalinity environment, thereby stabilizing SeO₄^2- in the amorphous silicon-aluminum structure and contributing to the decrease of Se leaching.