Desalination is the most common technology for producing potable water from brackish water and seawater. However, mineral scaling – precipitation of sparingly soluble salt on the membrane surface – limits the recovery ratio during desalination. This is extremely important in inland desalination, where the concentrate cannot be discharged back to the sea. Specifically, as there is no antiscalant against Si scaling, Si scaling presents an additional challenge in brackish water containing high Si concentrations. We present a modification process to achieve polyzwitterion pseudo bottle brushes on reverse osmosis (RO) membranes to mitigate mineral scaling propensity.
First, the membrane surface was finalized with poly2-hydroxyethyl methacrylate (polyHEMA) as an active site for grafting the brush. Then, atom transfer radical polymerization (ATRP) initiators were incorporated into the poly(HEMA). Finally, the low-fouling polyzwitterion poly(sulfobetaine methacrylate) was grafted by ATRP as polymer brushes at three different thicknesses. The successful grafting and the differences in brush yield at different reaction times were verified by surface characterization methods. Atomic force microscopy and contact angle measurements revealed that the surface roughness, surface stiffness, and hydrophilicity greatly decreased with prolonged reaction time. A higher grafting yield resulted in lower permeability but a higher salt selectivity. Desalination experiments with synthetic gypsum and silica scaling solutions mimicking 80% recovery of brackish water demonstrated that the scaling of the modified membranes was significantly reduced compared to the pristine RO membrane and that the anti-scaling tendency correlated with brush thickness. The highest-yield modified membrane was further studied for filtration of semisynthetic brackish water, mimicking 90% recovery. While severe scaling appeared on the pristine RO membrane, the modified membrane showed an excellent anti-scaling performance.
Overall, this work provides a convenient approach for preparing bottle brush-like grafts on RO membrane surfaces and expands on the implications of brush-decorated TFC surfaces for mineral scaling.