The effect of natural organic matter on the formation and solubility of M(OH)4 solid phases

In this study the formation of M(OH)4 solid phases (e.g. Th(IV), Ce(IV) and Zr(IV)) has been investigated as a function of the natural organic matter (NOM) concentration in 0.1 M NaClO4, in the pH range between 2 and 5, and under normal atmospheric conditions. M(OH)4 has been prepared by alkaline precipitation and characterized by TGA, ATR-FTIR, XRD, XPS and solubility measurements. According to the experimental data M(OH)4 is stable and is the solubility limiting solid phase even in the presence of increased humic acid concentration in solution. In contrast to MO2(OH)2 and M(OH)3 solid phases, increasing humic acid concentration does not affect the crystallite size and the solubility product of M(OH)4. The M(OH)4 solubility is basically pH depended and governed by the presence of colloidal species. Regarding the effect of NOM on the solid phase formation of redox-sensitive M(IV) ions (e.g. Ce(IV)), it could be shown that the amount of the reduced metal-ion (e.g. Ce(III)) in the solid phase depends linearly from the NOM concentration in the test solutions. (author)