Boron removal and desalination from seawater by PVDF flat-sheet membrane through direct contact membrane distillation

Boron removal and desalination from seawater by direct contact membrane distillation (DCMD) was studied with self-prepared polyvinylidene fluoride (PVDF) flat-sheet membrane. The membrane had an ultra-thin top skin and a porous network sponge-like layer and its effective pores showed a narrow distribution with a mean pore size of 0.22 μm. The membrane exhibited good hydrophobicity and its contact angle was about 82.6 ± 0.7°. During DCMD test of 35 g/L sodium chloride solution, the maximum permeate flux can achieve 47.6 kg/m2 h. The natural seawater containing 4.65 mg/L boron was treated by DCMD for simultaneous boron removal and desalination. Although the permeate flux decreased with concentration factor (CF) value increase, the permeate boron kept below 20 μg/L and salt rejection was over 99.9%. When the CF value exceeded 4.0, there would be scale deposits formed, and the permeate quality and flux declined rapidly. The antiscalant polyacrylic acid could delay the deposit formation and the advisable dosage was 10 mg/L. The membrane showed satisfying performance stability in a 300 h continuous DCMD experiment even with the CF value as high as 7.0, indicating that the self-prepared membrane and DCMD process could be efficiently used for boron removal and desalination from seawater.