BACKGROUND: Phosphate is one of the main contaminants responsible for the eutrophication of surface waters, and adsorption is a potential treatment method for this pollutant. A magnetic adsorbent manufactured from magnetite (Fe(3)O(4)) can be recovered easily from treated water by magnetic force, without requiring further downstream treatment. In this research, the surface of magnetite modified with aluminum and silica (Al/SiO(2)/Fe(3)O(4)) was used to adsorb phosphate in an aqueous solution in a batch system. RESULTS: The optimum solution pH for phosphate adsorption by Al/SiO(2)/Fe(3)O(4) was found to be 4.5. The phosphate adsorption behavior of Al/SiO(2)/Fe(3)O(4) was in good agreement with both the Langmuir and Freundlich adsorption isotherm, and the maximum adsorption capacity (q(m)) and Gibbs free energy of phosphate was 25.64 mg g(-1) and-21.47 kJ mol(-1), respectively. A pseudo-second-order model could best describe the adsorption kinetics, and the derived activation energy was 3.52 kJ mol(-1). The optimum condition to desorb phosphate from Al/SiO(2)/Fe(3)O(4) is provided by a solution with 0.05mol L(-1) NaOH. CONCLUSIONS: Magnetic adsorbent is a potential material for a water treatment method. The results of this study will be helpful in the development of aluminum modified silica magnetic adsorbents that can be used to remove phosphate in aqueous solution. (C) 2011 Society of Chemical Industry