Understanding the impacts of sodium silicate on water quality and iron oxide particles

Maintaining drinking water quality during distribution requires a range of tools to ensure chemical stability at the tap. Coloured water is a primary concern, and sodium silicates have been reported to reduce colour from iron. However, the impacts of water quality and the interactions between sodium silicate and iron oxides are not well characterized in this context. We conducted a bench-scale factorial experiment (2(4)) to evaluate the effect of sodium silicate on colour and turbidity with pH, humic acid, free chlorine, and sodium silicate as independent variables. Sodium silicate reduced colour and turbidity due to particulate iron by 113.3 Pt-Co and 4.0 NTU, respectively. Sodium silicate reduced the size of iron particles, as determined by dynamic light scattering and laser diffraction, and increased the apparent density, as assessed by electron microscopy. XRD and XPS data were consistent with increased amorphous and oxyhydroxide content in iron precipitates exposed to sodium silicate. Silicate also inhibited the adsorption of humic acid onto model iron oxides due to electrostatic repulsion: the zeta (zeta-) potentials of iron oxides in silicate-treated systems were substantially more negative.