Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach

At the solid/solution interface of metal (hydr)oxides various types of O(H) and OH(H) groups are present, which differ in the number of coordinating metal ions. The σ0-pH curves of metal (hydr)oxides are strongly determined by the composition and the relative extent of the various crystal planes of (hydr)oxides. The charging behavior is discussed for gibbsite (Al(OH)3), goethite (FeOOH), hematite (Fe2O3), rutile (TiO2), and silica (SiO2). New experimental σ0-pH data for goethite and gibbsite are presented. Several important (hydr)oxides exhibit crystal faces which do not develop surface charge over a relatively wide pH range. An uncharged crystal face may be due to the presence of surface groups which are not reactive (inert) in the pH range under consideration, like the 001 face of gibbsite and the 0001 face of hematite, or caused by the presence of two types of interacting charged surface groups of which the charge of one type is fully compensated by the other like at the 100 face of goethite. The charging behavior of silica and the 001 face of gibbsite is determined by one type of reactive surface group with a large ΔpK for the consecutive protonation steps. The crystal structure imposes the presence of uncharged surface groups and this results in a quite different shape of σ0-pH curves for gibbsite and silica in comparison with the commonly observed σ0-pH curves of metal (hydr)oxides. The MUltiSIte Complexation (MUSIC) model as developed by T. Hiemstra, W. H. Van Riemsdijk, and G. H. Bolt, (J. Colloid Interface Sci.132 (1989)) leads to a rather good prediction of σ0-pH curves for various metal (hydr)oxides using predicted affinity constants for the various types of surface groups and Stern layer capacitance values and pair formation constants estimated from the literature.