Ligand-induced dissolution and release of ferrihydrite colloids

This laboratory study attempted to delineate the processes of iron oxide pa rticle release from a sandy aquifer as influenced by electrostatic repulsio n and chemical dissolution. The release of ferrihydrite particles by 5 mM c itrate was studied in flow-through columns that contained ferrihydrite-coat ed quartz. Results indicated two major mechanisms for the release of ferrih ydrite colloids by citrate: (1) the repulsive interfacial forces were the p rimary cause for the peak output of colloids at the beginning of the breakt hrough, and (2) the release of colloids at longer run-times was induced mai nly by bondbreaking at the Fe oxide-quartz interface that resulted from the dissolution of ferrihydrite. The rate of chemical dissolution was investig ated in batch experiments with 0.1 to 5 mM organic ligands (ascorbate and c itrate) and 0.4 gL(-1) ferrihydrite in a pH 1, 10 mM NaCl solution at simil ar to 21 degrees C. The results of the adsorption and dissolution study sho wed that citrate dissolved ferrihydrite with initial rates positively relat ed to the adsorption density, and an initial rate up to 1.86 mu mol m(-2)h( -1) was derived at similar to 4.5 mM citrate. Ascorbate dissolved ferrihydr ite at an initial rate similar to 4 times faster than citrate. At pH 4, a n ear complete dissolution occurred at similar to 5 h, and the measured Fe(II ) to ligand ratio was about 2 at the maximum dissolution, suggesting a two- electron transfer process from ascorbate to Fe(III). However, the initial d issolution rates in the batch experiment may not be the best measure of dis solution occurring in a flow-through system, where the steady dissolution r ate was substantially lower than the batch prediction. The study suggests t hat, in an Fe-chemistry-dominated aquifer, a chemical perturbation (e.g., a plume of organic ligands) is likely to induce colloid release initially vi a electrostatic repulsion. Over time, dissolution will rake a controlling r ole, changing the ratio of dissolved to colloidal Fe. Copyright (C) 2000 El sevier Science Ltd.