The acid dissolution characteristics of cadmium fixed by a novel Ca-Fe-Si composite material

Ca-Fe-Si material (CIS), a novel composite material rich in calcium, iron, manganese and silicon showed marvelous immobilization properties for heavy metal(loid)s in soils. To elucidate the acid stability of Cd fixed by CIS (CIS-Cd) and the underlying immobilization mechanisms, the acid dissolution characteristics of CIS-Cd were investigated by using acid titration method and X-ray diffraction (XRD) technique. The results showed that CIS-Cd had distinctive acid buffering capacity in different pH ranges. Based on the titration curve between dissolution rate of CIS-Cd and pH, CIS-Cd can be divided into non acid-stable Cd (9.4%), moderately acid-stable Cd (22.5%) and acid-stable Cd (68.1%). XRD analysis of CIS-Cd at different pH intervals and the correlation curves of dissolution rates of Cd and concomitant elements indicated that non acid-stable Cd was mainly bound by carbonate, silicate and sulfate (CdCO₃, Cd₂SiO₄ and CdSO₄) or co-precipitated with the corresponding calcium salts. Moderately acid-stable Cd was mainly bound by magnesium-aluminum-silicon containing minerals or electrically bound by manganese iron minerals. Acid-stable Cd remaining undissolved at pH < 2.42 included CdFe₂O₄ and ferromanganese minerals strongly bound Cd. It was by multilateral fixation mechanisms that Ca-Fe-Si material possessed marvelous immobilization capability for Cd and strong resilience to environmental acidification as well. The findings implicated that proper combination of calcium-iron-silicon containing minerals could develop novel promising amendments with high efficiency in heavy metal(loid)s immobilization and strong resilience to environmental change.