Effect of silicic acid on the aggregation properties of goethite

Aggregation properties of iron oxide particles have received much attention because of their environmental importance in soil and aquatic environments. The effects of silicic acid on the pH-dependent surface charges that govern the aggregation of goethite have received little attention. In this study we determined the effect of silicic acid as a function of pH and ionic strength (IS) on the aggregation of a synthetic goethite by a combination of particle-size analysis using dynamic light scattering and test tube experiments to quantify particles in suspension. We investigated silicic acid adsorption on goethite quantitatively by batch adsorption isotherms, and the changes in surface charge on the adsorption of silicic acid were identified by the zeta potential (ζ). Our results revealed that silicic acid can lead to a significant decrease in ζ. A change in the pH_iep (iep: isoelectric point) from 8.5 to lower pH values, typically pH 4–5.5, in the presence of silicic acid suggests enhanced aggregation of goethite in the acidic pH range. At alkaline pH, both goethite and silicic acid were negatively charged because of strong deprotonation, and silicic acid adsorption promoted goethite dispersion. Ionic strength affected the aggregation of goethite by shifting ζ towards the iep and it can also obscure the role of silicic acid. Highlights: Effect of silicic acid on surface charge and aggregation of goethite was investigated. Adsorption of silicic acid reduces ζ and allows goethite to aggregate at lower pH. Ionic strength can also affect aggregation of goethite by shifting ζ towards the isoelectric point. Silicic acid in soil might maintain goethite particles in an aggregated state.