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Scheinost, A.C., S. Abend, K. I. Pandya, and D. L. Sparks. 2001. Kinetic controls on Cu and Pb sorption by ferrihydrite. Environ. Sci. Tech.35: 1090 -1096.


Kinetic Controls on Cu and Pb Sorption by Ferrihydrite

Andreas C. Scheinost,* Sven Abend, Kaumudi I. Pandya, and Donald L. Sparks

Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717, Department of Physics and Astronomy, SUNY Stony Brook, New York 11794, and National Synchrotron Light Source, BNL, Upton, New York 11973


Metal partitioning in ferrihydrite suspensions may reach equilibrium only after a long reaction time. To determine key factors controlling the kinetics, we measured Cu and Pb uptake as a function of ferrihydrite morphology, reaction temperature, metal competition, and fulvic acid concentration over a period of 2 months. X-ray microscopy, which was used to probe ferrihydrite morphology in suspension, showed that drying irreversibly converted the gellike structure of fresh precipitate into dense aggregates. These dense aggregates 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 Pb 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. Thequantification 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 predicting the fate of metals in the environment.

Key Words: Arsenic; γ-Al2O3; adsorption mechanisms; XAS; electrophoretic mobility; oxide-water interface.

Copyright 2001 American Chemical Society

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