Composite distillation membranes with highly water-permeable and anti-crystallization polyzwitterionic layers for deep concentration of hypersaline brine

Abstract

Inorganic crystallization exhibits lethal effects on membrane efficiencies during membrane distillation of hypersaline wastewater toward zero liquid discharge (ZLD). Constructing a dense surface layer was a promising strategy to resist crystal-induced membrane deterioration, while conventional methods cannot improve membrane anti-scaling/wetting abilities without compromising membrane water permeance. Herein, we developed a composite membrane (CM) with a dense and highly water-permeable surface layer using PVA and polyzwitterionic polymers (PZ) for deep concentration of hypersaline water toward ZLD. The effect of ionic-PZ and nonionic-PVA polymer chains in the surface layer was systematically investigated. Compared to the CM with an imporous PVA layer, the integration of PZ chains notably enhanced the water permeance (∼61 %) while maintaining the anti-scaling/wetting ability of the membrane (distillate conductivity: <10 μS cm⁻¹). The PZ chains enlarged the space between hydrogel networks (from 0.53 to 1.15 nm) and induced more salt ions permeating into the surface layer (up to 3.5 M while below saturation), increasing surface osmotic pressure and avoiding inorganic crystallization, simultaneously. This promoted water diffusion from hypersaline feed to the CM surface. The water in the PVA@PZ surface layer also exhibited a notably higher evaporation rate (∼138.9 g m⁻² h⁻¹) and lower evaporation enthalpy (896.5 J g⁻¹) than that in the PVA surface layer (∼64.6 g m⁻² h⁻¹; 1932.8 J g⁻¹) due to the promoted water transformation to intermediate states, demonstrating PZ integration enhanced water evaporation at the surface layer/air interface. These lowered the detrimental effect of feed salinity on the water permeance of the CM and enhanced membrane water flux during the deep concentration of hypersaline water. The operation performances of membranes under actual complex water quality conditions were also evaluated using actual coal-chemical hypersaline wastewater, further demonstrating the application potential of the PZ-based CM.