LIGAND-INDUCED DISSOLUTION AND RELEASE OF FERRIHYDRITE COLLOIDS

Abstract

This laboratory study attempted to delineate the processes of iron oxide particle release from a sandy aquifer as influenced by electrostatic repulsion and chemical dissolution. The release of ferrihydrite particles by 5 mM citrate was studied in flow-through columns that contained ferrihydrite-coated quartz. Results indicated two major mechanisms for the release of ferrihydrite colloids by citrate: (1) the repulsive interfacial forces were the primary cause for the peak output of colloids at the beginning of the breakthrough, and (2) the release of colloids at longer run-times was induced mainly by bondbreaking at the Fe oxide-quartz interface that resulted from the dissolution of ferrihydrite. The rate of chemical dissolution was investigated in batch experiments with 0.1 to 5 mM organic ligands (ascorbate and citrate) and 0.4 gL-1 ferrihydrite in a pH 4, 10 mM NaCl solution at ~21°C. The results of the adsorption and dissolution study showed that citrate dissolved ferrihydrite with initial rates positively related to the adsorption density, and an initial rate up to 1.86 μmol m-2h-1 was derived at ~4.5 mM citrate. Ascorbate dissolved ferrihydrite at an initial rate ~4 times faster than citrate. At pH 4, a near complete dissolution occurred at ~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 dissolution rates in the batch experiment may not be the best measure of dissolution occurring in a flow-through system, where the steady dissolution rate was substantially lower than the batch prediction. The study suggests that, in an Fe-chemistry-dominated aquifer, a chemical perturbation (e.g., a plume of organic ligands) is likely to induce colloid release initially via electrostatic repulsion. Over time, dissolution will take a controlling role, changing the ratio of dissolved to colloidal Fe.