Calcium Sulfate Formation on Different Zwitterionic Amphiphilic Copolymer Substrates for Salt Water Treatment

Zwitterionic (ZI) polymers are a promising class of fouling-resistant and antiscaling coating materials. However, their mineral scaling prevention mechanism is still not well understood, which hampers the applications of ZI polymers. To fill this knowledge gap, we investigated the scaling mechanisms of different types of ZI amphiphilic copolymers (ZACs) synthesized from diverse combinations of zwitterion monomers and hydrophobic monomers using free-radical polymerization (FRP). The selected ZACs included poly(trifluoroethyl methacrylate-random-sulfobetaine methacrylate) (PTFEMA-r-SBMA, shortened as PT:SBMA), poly(trifluoroethyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PTFEMA-r-MPC, shortened as PT:MPC), poly(methyl methacrylate-random-sulfobetaine methacrylate) (PMMA-r-SBMA, shortened as PM:SBMA), and poly(methyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PMMA-r-MPC, shortened as PM:MPC). Coatings of these polymers were tested for calcium sulfate (CaSO4) scaling in simulated brackish water solutions with different saturation indices, defined as the logarithm of the actual dissolved ion product (Q, i.e., calcium and sulfate ions) normalized by the solubility constant of gypsum (Ksp). These experimental results indicated that at high saturation indices (SI), the scaling deposition on ZAC coatings mainly depended on the surface energy. Therefore, PT:SBMA coatings, which had the lowest surface energy, exhibited better-scaling resistance. At lower SI, the hydrophilicity of the ZAC coatings was the dominant factor. Since ZAC coatings containing MPC have higher surface hydrophilicity, they might have a relatively slower scale-forming ion adsorption rate compared to SBMA containing ZACs, thus not markedly enriching the scale-forming ion near MPC coating surfaces and hence inhibiting heterogeneous CaSO4 precipitates. The results demonstrated that the selection of ZAC coatings should consider the feed water chemistry and focus on the optimal prevention of scaling in addition to organic fouling.