Sorption of H4SiO4 (including experiments as a function of time, K-d measurement with different m/v ratios and sorption edges) onto different iron (hydro)oxides as goethite (ci-FeOOH), hematite (alpha-Fe2O3), and magnetite (Fe3O4) has been studied with concentration of silicates under solubility limit. A surface complexation model has been used to account for sorption edge of silicates onto these iron oxide surfaces. It reveals that two types of surface complex namely equivalent to FeH3SiO4 and equivalent to FeH2SiO4-, are needed to describe properly the experimental observations. (c) 2007 Elsevier Inc. All rights reserved.