Mechanistic Study of Polymer Coatings to Control Mineral Scaling During Desalination



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Surface modifications are widely adopted to control interfacial scale formation. However, the underlying mechanisms for sulfate mineral scaling (i.e. gypsum) on surfaces containing biomolecules or zwitterionic materials remain unclear. To provide mechanistic insights on scaling-resistant materials containing biomolecules, such as bovine serum albumin (BSA), studies on gypsum scaling tests on bare and bovine serum albumin (BSA) conditioned membranes were conducted. In this first investigation, mass, crystal growth orientation, and crystallinity of mineral precipitates on membranes, as well as their effects on membrane permeability have been investigated. Results indicated that different permeability decay behaviors of bare and BSA-conditioned membranes were attributed to differences in crystal growth orientations rather than amounts of gypsum precipitates. BSA-conditioned layers with high carboxylic density and specific molecular structure could stabilize bassanite, and disrupt the oriented growth to inhibit the formation of needle-like gypsum crystals as observed on bare membranes, thus slowing down the membrane flux decline rate. In addition to biomolecules, we also investigated the effects of synthetic molecules. In the second part of this study, the scaling-resistant performance of different types of zwitterionic amphiphilic copolymers (ZACs) including poly(trifluoroethyl methacrylate-random-sulfobetaine methacrylate) (PTFEMA-r-SBMA, shorten as PT:SBMA), poly(trifluoroethyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PTFEMA-r-MPC, shorten as PT:MPC), poly(methyl methacrylate-random-sulfobetaine methacrylate) (PMMA-r-SBMA, shorten as PM:SBMA), and poly(methyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PMMA-r-MPC, shorten as PM:MPC). PT: SBMA, and PT: MPC were investigated since they had been previously identified to better inhibit the crystal deposition or adhesion of gypsum and biomolecules than other ZAC coatings. Overall our results showed that ZAC coatings containing MPC showed better surface crystallization resistance than those containing SBMA. Furthermore, in-situ X-ray scattering and quartz crystal microbalance with dissipation (QCMD) techniques were also employed to observe the short-term and long-term kinetic process of heterogeneous CaSO4 formation, respectively, on PT: SBMA, and PT: MPC. Results showed that PT: SBMA promoted crystal growth, which was demonstrated by a faster reaction kinetics observed on PT: SBMA than on PT: MPC. On the other hand, PT: MPC enhanced heterogeneous CaSO4 nucleation. It was determined that electrostatic, and ionic interactions contribute to the kinetic differences at different stages of gypsum scaling. Amorphous bassanite was found to cause the occurrence of the early Ostwald ripening process, meanwhile, they also further affected the differences in crystal growth rate on both coatings. Overall, these results allowed us to improve the understanding of sulfate mineral scaling in the presence of different polymer materials.



Surface modifications, Mineral scaling, Polymer coatings


Portions of this document appear in: Wang, M.; Cao, B.; Hu, Y.; Rodrigues, D. F. Mineral Scaling on Reverse Osmosis Membranes: Role of Mass, Orientation, and Crystallinity on Permeability. Environ. Sci. Technol. 2021, 55 (23), 16110–16119; and in: Wang, M.; Nguyen, H.; Lounder, S. J.; Asatekin, A.; Rodrigues, D. F. Calcium Sulfate Formation on Different Zwitterionic Amphiphilic Copolymer Substrates for Salt Water Treatment. ACS Appl. Polym. Mater. 2022, 4 (10), 7090–7101.