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

Effects of aluminate and silicate species on the SeO 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 SeO, 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 SeO was reduced to SeO 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) through outer-sphere complexation. For CaO/Fe(II) reaction, all of SeO was reduced to SeO 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 SeO can be sorbed onto the iron-based minerals through binuclear bidentate corner-sharing (C) complexation instead of monodentate mononuclear corner-sharing (V) complexation of the case without Al species. On the other hand, SeO was not reduced to SeO 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 SeO in the amorphous silicon-aluminum structure and contributing to the decrease of Se leaching.