The aggregation/sedimentation potentials of TiO2 nanoparticles were studied in mono- and binary systems of Suwannee River fulvic acids (SRFA) and Fe(III) at different pH values. SRFA adsorption significantly enhanced the stability of TiO2 nanoparticles at pH 4, 6 and 8, mainly due to the dramatic increase in negative surface charges. The presence of Fe(III) stabilized aggregation/sedimentation of TiO2 nanoparticles at pH 4 due to the increasing positive charges after Fe(III) sorption, but destabilized aggregation/sedimentation at pH 6 and 8 attributed to the bridging effect of Fe(III)-hydroxy. The formation of COO-Fe(III) complex was found in binary system of SRFA and Fe(III). Thus, the positive charges that Fe(III) imparted to nanoparticles were neutralized by SRFA. Compared with those in mono-system of Fe(III). SRFA enhanced aggregation/sedimentation at pH 4, while stabilized TiO2 nanoparticles at higher pH in binary system of SRFA and Fe(III). The sedimentation rates of TiO2 nanoparticles showed relatively lower coefficient with zeta potentials (-0.883, P < 0.01) than with hydrodynamic particle sizes (0.964, P < 0.01), due to the steric hindrance of SRFA and the bridging effect of Fe(III)-hydroxy, which was confirmed by DLVO calculation. These results have important implications for prediction of the stability and fate of nanoparticles in natural water. (C) 2011 Elsevier B.V. All rights reserved.