Investigations on adsorption of inorganic ions in aqueous solution to some metal oxides, hydroxides and a carbonate by the X-ray spectroscopic method

Why does the adsorption and concentration of inorganic chemical species proceed at aqueous-solid interfaces? In this review paper, we discuss the use of X-ray chemical state analysis to elucidate the intrinsic adsorption mechanism. Based on the chemical states of the species adsorbed to solids as determined by X-ray chemical state analysis, possible adsorption mechanisms are discussed. The driving forces of adsorption are represented by the Gibbs free energy change (ΔG_chem = ΔG_chem,1 + ΔG_chem,2) resulting from the formation of covalent bonds between metal ions (M) in metal oxides or hydroxides and adsorbed species (X) (M-O-X bond, ΔG_chem,1) and the formation of new phases consisting of M and X (ΔG_chem,2). The concept of ΔG_chem,2 is proposed based on the experimental results from chemical state analyses. As examples, the following investigations are discussed in this review paper: the formation of mullite precursors by the adsorption of monosilicic acid to Al(OH)₃, the spontaneous reduction of Au(III) to Au(0) by adsorption of Au(III) to Al(OH)₃, MnO₂ and Ni(OH)₂ and the mechanism of concentration of Co²⁺, Tl⁺, Pb²⁺, Pt²⁺, Au⁺, and Pd²⁺ in marine ferromanganese crusts. (author)