Gypsum scaling mechanisms on hydrophobic membranes and its mitigation strategies in membrane distillation

Abstract

Mineral scaling is one of the significant obstacles in hypersaline wastewater treatment using membrane distillation (MD) technology. A deeper understanding of the scaling mechanism can help to solve mineral scaling problems. This work investigated three porous membranes with different surface pore sizes and hydrophobic properties, i.e., PTFE-0.22, PTFE-0.10, and PVDF-0.22, used in the direct-contact membrane distillation (DCMD) process. We selected gypsum as a model contaminant in the scaling experiment and focused on its homogeneous and heterogeneous nucleation effects on membrane scaling. The morphology analysis of the scaled membrane revealed that homogeneous nucleation might be the main reason for scaling. In addition, gypsum crystallization in the membrane pores was the main reason for the significant flux decline and low physical cleaning efficiency (PTFE-0.22). When the surface pore size of the membrane (PTFE-0.10 and PVDF-0.22) was reduced, gypsum crystalization in the pores could be alleviated. Such phenomenon was mainly initiated by homogeneous nucleation. Nonetheless, the surface scaling was still severe for PVDF-0.22 because its hydrophobicity was too low to inhibit the heterogeneous nucleation on the membrane. However, PTFE-0.10 with a small surface pore size and high hydrophobicity could simultaneously alleviate the effect of two nucleations. We could easily remove the gypsum crystals deposited on the membrane surface by physical cleaning and achieve more than 90% flux recovery efficiency. Therefore, reducing the surface pore size and increasing the surface hydrophobicity should be two main strategies in preparing the anti-scaling hydrophobic membrane used in MD.