Effects of organic ligands on enhancing the solubility of silica in aqueous solution: Implications on silica fouling in membrane filtration

Membrane technology is widely used in the purification of drinking water, desalination, and in the treatment of wastewater to meet the needs of a growing population. It is used in various industries such as biotechnology, pharmaceutical, chemical, and electronics. Reverse osmosis systems were employed in desalination process. Silica fouling is reported to be one of the major unresolved problems in reverse osmosis systems. Silica fouling reduced the efficiency of the system and increased the energy loss and costs of operation. In this study, special emphasis was given to silica fouling and experiments were performed to study the effects of ligands on enhancing the solubility of silica. Few studies in the past have been made concerning the precipitation mechanism of silica. Previous investigations reported that the silanol group (= Si-OH) of silica formed hydrogen bonds with the electron donor atoms of polar organic ligands. Silica solubility in the presence of ligands, including ethyelenediaminetetraacetic acid (EDTA), citric acid, alpha- alanine, beta-alanine and diaminopropanoic acid, non-ionic surfactants (Triton X-100 and Surfonic N-95), anionic surfactant (sodium dodecyl sulfate), lactose, and trimethyl phosphate, is studied at pH 2-12 in batch experiments. - Results from the batch experiments showed an approximate 70% decrease in the initial concentration (10 mM) of soluble silica after 10 hours for the control (silica). The presence of non-ionic surfactants enhanced the dissolved silica concentration to about 24- 27% compared to the control at the end of the run. This occurs because of the adsorption of surfactants on the surface of silica by the formation of hydrogen bond between the surface silanol groups and the hydrophilic polyethoxylate groups of the non-ionic surfactants. Membrane filtration experiments using thin film NF DL nanofilter were conducted to illustrate the results of the batch experiments. In the presence of Triton X-100 in the feed silicate solution. the flux decline was reduced from 47% (control) to 12%. This shows the effect of non-ionic surfactants on enhancing the solubility of silica by membrane filtration experiments. Future studies should show the evidence of hydrogen bonding and the adsorption behavior of non-ionic surfactants on the silica surface.