The role of dissolved aluminum in silica chemistry for membrane processes

The use of aluminum sulfate (alum) coagulation prior to reverse osmosis (RO) treatment has been shown to be problematic. Membrane fouling was theorized to occur through soluble aluminum (Al3+) reacting with ambient silica (H4SiO2) to form kaolinite (Al2Si2O5(OH)(4)) within the RO unit. Chelating agents (citrate at 34 mg/L and ethylenediaminetetraacetic acid [EDTA] at 16 mg/L) were tested for their efficacy in controlling aluminum silicate fouling. The results of bench-scale testing demonstrated that both citrate and EDTA did control aluminum silicate formation, citrate more so than EDTA. Additional aluminum-based fouling was encountered when a commercial, phosphonate-based antiscalant - used to control barium sulfate scaling - reacted with the excess aluminum despite the presence of either citrate or EDTA.