Role of co-existing ions in the removal of dissolved silica by ceramic nanofiltration membrane

Even low concentrations of nano-sized silica particles should be effectively removed from chemical mechanical polishing (CMP) wastewater as they can corrode the pipes in semiconductor manufacturing processes. In this study, we systematically investigated the factors affecting the silica removal efficiency using ceramic nanofiltration (NF) membranes under different conditions to remove nano-sized dissolved silica, which is difficult with conventional microfiltration (MF) or ultrafiltration (UF) membranes. A tubular ceramic NF membrane with an average pore size of approximately 946 Da (≈1.3 nm) was strongly negatively charged at pH values above 4.0, indicating the existence of repulsive forces between the membrane and monoanionic silica (H₃SiO₄⁻) under basic pH conditions. As the pH level increased from 4 to 12, retention increased from 10.6 % to 62.0 %, and it also increased with increasing concentration of dissolved silica and applied pressure. We also investigated the effect of combined salt solutions (KCl, K₂SO₄, CaCl₂, or CaSO₄) on the removal efficiency for dissolved silica to better understand the mechanism for underlying the operating conditions. When dissolved silica was the only contaminant in the solution, 61.6 % was removed however, the presence of cations such as Ca²⁺ with higher valence reduced the removal efficiency by up to 20.5 %. The presence of calcium compounds that bonded to higher-valence anions had a greater impact on the removal efficiency for dissolved silica. Our findings can serve as guidance for the selection of suitable coagulants and the improvement of the membrane performance through fine control strategies in semiconductor wastewater treatment processes.