Silica fouling in reverse osmosis systems : the role of chemical interaction with the membrane surface
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Silica fouling of reverse osmosis (RO) membranes occurs by the deposition of silica and the subsequent formation of a silica film on the membrane surface. Typically, RO membranes are polyamide membranes made of 1,3-benzenediamine (m-phenylenediamine) and trimesoyl chloride (1,3,5-benzentricarbonyl chloride), having carboxyl (-COOH) and amide (-CONH-) functionalized groups. However, understanding of silica deposition on the carboxyl/amide functionalized surface of the membrane is still unclear. The goal of this study was to advance the understanding of how the chemical bonding groups of polyamide membranes are affected by dissolved silicates and how the structural change of functionalized groups by silicates consequently influences fouling. To achieve this goal, silica fouling tests were carried out by using a crossflow reverse osmosis (RO) system, and the membrane coupons were analyzed by X-ray photoelectron spectrometer (XPS) and Attenuated total reflection – Fourier transform infrared spectroscopy (ATR – FTIR). The XLE membrane mainly used in this study is the fully aromatic polyamide membrane, composing of the aromatic/aliphatic (C-C/C-H), amide (N-C=O), and carboxyl (O-C=O) group. The silica uptake was accompanied with an increase of oxygen, indicating the formation of SiO₂ structure on the membrane surface. Due to the concentration polarization effects, highly concentrated silicates had great propensity to form a silica gel layer. As a result, a soft gel layer with high water content was formed. Then, the accumulated silica gel aged and became a more condensed (SiO₂)[subscript n] structure by expelling water molecules. Therefore, the ratio of oxygen to silica (O/Si) of condensed (SiO₂)[subscript n] network structure would be close to 2.0. This ratio of membrane exposed in a fouling test to oversaturated silica feed water was 1.99, while a similar membrane exposed to undersaturated silica feed water was 11.38. After silica deposition, the aromatic/aliphatic group decreased significantly and the silicon carbide (Si-C) and hydroxyl (C-OH) groups newly appeared. These bonding changes showed that the aromatic /aliphatic group partially transformed to the hydroxyl group and the silica bonded to the carbon source in the carboxyl group by substituting for the oxygen. The carboxyl group decreased as the silica concentration in the feed water increased. Interestingly, the decrease of carboxyl group involved the flux decline. The carboxyl group came from the hydrolysis of trimesoyl chloride, giving hydrophilicity and consequently increasing water permeability. The carboxyl group was not recovered by the membrane cleaning. This basic (pH > 10.0) cleaning procedure decreased the ratio of O/Si from 11.38 to 1.41, indicating the oxygen lost in the silica deposition. The results of this study could contribute to the fundamental understanding to improve antifouling property through appropriate surface modifications that can alleviate the deposition of silica foulants or prevent the deformation of chemical bonding groups of polyamide membranes.