HYDROTALCITE-LIKE MINERALS (M_2Al(OH)_6(CO_3)_(0.5) .XH_2O, WHERE M = Mg, Zn, Co, Ni) IN THE ENVIRONMENT: SYNTHESIS, CHARACTERIZATION AND THERMODYNAMIC STABILITY

摘要

Hydrotalcite-like layered double hydroxides (LDH), of the formulation M_2Al(OH)_6(CO_3)_(0.5).H_2O, where M = Mg, Zn, Co, Ni, have been prepared, the products characterized and their solubility products measured at ionic strengths of 0.0065 and 0.0128 M and at 25℃. Steady-state solubility was reached after 100 days. The solubility products have been formulated according to the following reaction: M_2Al(OH)_60.5CO_3.H_2O +6H~+ → 2M~(2+) + Al~(3+) + 0.5CO_3~(2-) + H_2O where K_(so) = [M~(2+)]~2[Al~(3+)][CO_3~(2-)]~(0.5)/[H]~6 Average values of K_(so) for I = 0, estimated using the Davies equation, are 25.43, 20.80, 22.88 and 20.03 for Mg, Zn, Co and Ni, respectively. Model calculations reveal that the thermodynamic stability of the LDHs is greater than that of the corresponding divalent hydroxides for Zn, Co and Ni below a pH of ～10, 9 and 8, respectively, and at least up to pH 12 for Mg.