Carboxyl antiscalants for flux enhancement and gypsum scaling control in a single-pass conductive heating vacuum membrane distillation system

Membrane distillation (MD) shows great potential in hypersaline water treatment, but it is invariably constrained by temperature polarization. Conductive heating vacuum membrane distillation (CHVMD), a single-pass system without recirculating feed, effectively alleviates temperature polarization by introducing a thermal conducting layer that enhances the transfer of external heat to the membrane-water interface. However, this process exacerbates concentration polarization, raising the scaling potential. Gypsum scaling, a common issue in MD, has been effectively alleviated by using carboxyl antiscalants. However, the mechanisms by which different carboxyl antiscalants influence gypsum scaling in MD remain poorly understood. In this study, the performance and mechanism of three carboxyl antiscalants—polyacrylic acid (PAA), polymethacrylic acid (PMAA) and hydrolyzed polymaleic anhydride (HPMA)—in CHVMD using gypsum solution as the feed were investigated. The results show that these carboxyl antiscalants enhance the membrane durability in the single-pass CHVMD system, while improving both initial and stabilized flux in the order: PAA > HPMA > PMAA. PAA exhibits a more pronounced effect on reducing the evaporation enthalpy of water through the formation of intermediate water, and lowering the resistance to heat and mass transfer with the formation of vertical, large lamellar gypsum crystals, achieving an initial flux of 12.79 kg/m 2  h and a stabilized flux of 10.20 kg/m 2  h. For PMAA, increasing feed salinity significantly reduces the evaporation enthalpy, and lowers the transfer resistance by the formation of larger needle-like gypsum crystals, thereby obtaining the highest initial flux of 15.45 kg/m 2  h and stabilized flux of 11.35 kg/m 2  h.