Ligand-induced dissolution and release of ferrihydrite colloids

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.