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 (Δ G chem = Δ G chem,1 + Δ G 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, Δ G chem,1 ) and the formation of new phases consisting of M and X (Δ G chem,2 ). The concept of Δ G 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 , MnO 2 and Ni(OH) 2 and the mechanism of concentration of Co 2+ , Tl + , Pb 2+ , Pt 2+ , Au + , and Pd 2+ in marine ferromanganese crusts.