Adsorption at the mineral-water interface is ail important process governing metal ion concentration and mobility in aqueous systems. Ferric iron oxyhydroxide nanoparticles possess a large capacity for the adsorption of heavy metals but quantification of metal uptake and sequestration is challenging due to the tendency of natural nanoparticles to aggregate in natural waters. We studied the effects of aggregation via pH, ionic strength, drying, and freezing on the uptake and release of copper from ferrihydrite nanoparticles employing small-angle X-ray scattering (SAXS) studies of aggregate morphology, macroscopic Cu(II) sorption and desorption batch experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopic studies of copper sorption geometries. Results show that the mechanism of aggregation has a large effect upon aggregate morphology and consequently on the net sorption/retention of ions from solution. While aggregation reduces the total amount of copper that can be adsorbed, it also may introduce physical constraints to desorption and/or increased proportions of higher strength binding sites that lead to greater retention, and hence more effective sequestration, of metal ion contaminants. (C) 2009 Elsevier Inc. All rights reserved.