Adsorption on silica in Pb[sbnd] and Ca[sbnd]SO4[sbnd]CO3 systems

Abstract

Formation of Pb and Ca precipitates on a silica surface in the presence of sulphate and carbonate under neutral to alkaline condition is studied using zeta potential measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy. The effect of carbonate on displacing sulphate is discussed.In the Pb/sulphate system, precipitates of predominantly Pboxide/hydroxide formed on the silica surface. In the Pb/carbonate system, the precipitates were predominantly Pbcarbonate/hydroxy carbonate. Precipitates in both the Pb/SO4 and Pb/CO3 systems were dispersed by addition of sufficient carbonate, the silica surface eventually becoming free of precipitates. This dispersion is attributed to a high negative surface charge density on both lead carbonate and silica.In the Ca systems, calcium sulphate precipitates did not form on the silica particles whereas calcium carbonate precipitates did. The precipitates were not removed by increasing carbonate concentration. The difference from the Pb system is attributed to a low negative surface charge density on calcium carbonate compared with lead carbonate, leading to less dispersing conditions, and the higher solubility of calcium sulphate compared with lead sulphate.