Retention and induced aggregation of Co(II) on a humic substance: sorption isotherms, infrared absorption, and molecular modeling

The solubility of humic substances (HS) increases with pH. However, when in the presence of appropriate concentrations of transition metals in solution, soluble HS flocculate as a consequence of the aggregation processes promoted by the metallic ions. This is of great environmental interest as HS can retain large amounts of metallic ions at low pH. They can also form soluble complexes with these metallic ions as pH increases, even at pH values greater than the precipitation pH of the cation. The aim of this work is to model isotherms and combine macroscopic data with Fourier Transform infrared absorption (FTIR), and molecular modeling techniques, in order to study the interaction of Co(II) with HS. The variation of the shape of the isotherms from L- to S-type, with pH, indicates a change in the retention processes of Co(II) on HS. Since the solubility of HS, and the shift of the precipitation edge, both increase with pH, the S-shaped isotherms are probably due to a mixed mechanism. This mechanism involves the complexation of metallic ions by soluble HS, the aggregation of these soluble HS macroions (due to the decrease in their charge arising from the complexation of the Co(II) ions), and finally, the surface precipitation of cobalt hydroxide. The experimental data (ns vs Ce) agree well with a model that considers the global sorption process to be the sum of single sorption processes. Speciation diagrams and FTIR results show that, from pH 2 to 6, [Co(H2O)6]2+ is the preferred adsorbed species for the retention of Co(II), while [Co(H2O)5(OH)]+ adsorption increases with n0 and pH. At pH 8, a surface precipitation process of an amorphous Co hydroxide begins. The pH-variable sorption isotherms present a hyperbolic shape reflecting adsorption, aggregation, and flocculation processes. Molecular modeling simulations also support the proposed interaction mechanism between Co(II) and HS.