Kinetic controls on Cu and Pb sorption by ferrihydrite

Metal partitioning in ferrihydrite suspensions may reach equilibrium only a fter a long reaction time. To determine key factors controlling the kinetic s, we measured Cu and Pb uptake as a function of ferrihydrite morphology, r eaction temperature, metal competition, and fulvic acid concentration over a period of 2 months. X-ray microscopy, which was used to probe ferrihydrit e morphology in suspension, showed that drying irreversibly converted the g ellike structure of fresh precipitate into dense aggregates. These dense ag gregates sorbed Cu and Pb much slower than the gel. Temperature had a more pronounced effect on the kinetics of metal uptake by ferrihydrite gel than by dense ferrihydrite. Independently of treatment and time, Cu and Ph were bound to the ferrihydrite surface by formation of edge-sharing inner-sphere sorption complexes as confirmed by X-ray absorption fine-structure (XAFS) spectroscopy. This invariable binding mechanism, together with the observed effects of morphology and temperature, are in line with surface diffusion limiting the slow sorption process. The quantification of diffusion-limited surface sites in soils and sediments and the subsequent estimation of the effect of reaction time and temperature will be a challenge for properly pr edicting the fate of metals in the environment.