Mechanism insight into gypsum scaling of differently wettable membrane surfaces with antiscalants in membrane distillation

Scaling in membrane distillation (MD) is inevitable in concentrating hypersaline wastewaters. Although adding antiscalants in hypersaline brines can effectively mitigate membrane scaling, the effectiveness and underlying mechanisms in preventing scaling on differently wettable membrane surfaces with different functional-group antiscalants have rarely been reported. Herein, the hydrophobic, superhydrophobic and Janus membranes with ultrathin scale inhibition layer were elaborately designed to elucidate the efficiencies of functional groups of antiscalants in preventing gypsum scaling on differently wettable membrane surfaces. The results show that the superhydrophobic and Janus membranes are the most effective in retardation of membrane scaling in the presence of a single antiscalant with carboxyl or amino functional groups, respectively. Besides, the performance of the superhydrophobic membrane can be further improved while the Janus membrane was continuously declined as amino and carboxyl-functional-group antiscalants were simultaneously added to the feed gypsum solution. A set of characterization analyses revealed that the significant improvement in membrane gypsum scaling resistance is ascribed to the obstruction of scale precursors to nucleation. Meanwhile, the physicochemical characteristics of antiscalants also contribute to reducing the salt deposited on the MD membrane surface in the supersaturated brine. This work provides new insight into the structure design of membrane and antiscalants tailored to MD for concentrating hypersaline brines with high water recovery and robust salts rejection efficiency.