ADSORPTION ON SILICA IN PB-SO4-CO3 AND CA-SO4-CO3 SYSTEMS

Formation of Pb and Ca precipitates on a silica surface in the presenc e of sulphate and carbonate under neutral to alkaline condition is stu died using zeta potential measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy. The effect of carbonate on displ acing sulphate is discussed. In the Pb/sulphate system, precipitates o f predominantly Pb-oxide/hydroxide formed on the silica surface. In th e Pb/carbonate system, the precipitates were predominantly Pb-carbonat e/hydroxy carbonate. Precipitates in both the Pb/SO4 and Pb/CO3 system s were dispersed by addition of sufficient carbonate, the silica surfa ce eventually becoming free of precipitates. This dispersion is attrib uted to a high negative surface charge density on both lead carbonate and silica. In the Ca systems, calcium sulphate precipitates did not f orm on the silica particles whereas calcium carbonate precipitates did . The precipitates were not removed by increasing carbonate concentrat ion. The difference from the Pb system is attributed to a low negative surface charge density on calcium carbonate compared with lead carbon ate, leading to less dispersing conditions, and the higher solubility of calcium sulphate compared with lead sulphate. (C) 1998 Elsevier Sci ence B.V.