The silicic acid charge varies with protonation and deprotonation at different pH levels. As an adsorbing species, silicic acid could modify surface charge and affect the colloidal stability of HFO. Electrophoretic experiments revealed that silicic acid lowered particle ζ, decreased the isoelectric point (iep), and allowed HFO to aggregate at a lower pH. Reversal of charge was observed at pH , , , and for silicic acid concentrations of 0, , and mM, respectively. | Vietnam Journal of Earth Sciences, 38(4), 345-355, DOI: Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences (VAST) Effect of silicic acid on aggregation of hydrous ferric oxide Nguyen Ngoc Minh*,1,2, Flynn Picardal1 1 School of Public and Environmental Affairs, Indiana University, MSBII, Walnut Grove Ave, Bloomington, USA 2 Faculty of Environmental Science, VNU University of Science, Vietnam National University Received 5 July 2016. Accepted 13 October 2016 ABSTRACT Colloidal properties of hydrous ferric oxide (HFO) have received much attention due to their environmental relevance. In this study, aggregation of HFO was determined by time-resolved dynamic light scattering and test tube experiments, evaluating surface charge via zeta potential (ζ) measurements. The silicic acid charge varies with protonation and deprotonation at different pH levels. As an adsorbing species, silicic acid could modify surface charge and affect the colloidal stability of HFO. Electrophoretic experiments revealed that silicic acid lowered particle ζ, decreased the isoelectric point (iep), and allowed HFO to aggregate at a lower pH. Reversal of charge was observed at pH , , , and for silicic acid concentrations of 0, , and mM, respectively. By demonstrating that silicic acid shifts the iep of HFO to lower pH values, results indicate that silicic acid can change the aggregation properties of HFO. Both light scattering and test tube experiments revealed a “peak aggregation” at pH in the presence of silicic acid. As this pH range is typical for many aqueous systems and soils, we conclude that silicic acid likely plays an important role in HFO transport in water and accumulation of particulate HFO in soil horizons. Keyworks: Silicic acid, hydrous ferric oxide, surface charge, aggregation. ©2016 Vietnam Academy of Science and Technology 1. Introduction1 Hydrous ferric
