Treatment of simulated saline brine water by membrane distillation process enhanced through alternating current electric field

Treatment of saline brine has been a tricky challenge in recent years, and membrane distillation (MD) is considered as a promising alternative technology. But it was found that the normalized flux significantly decreased to 0.6 within 15 h when treating simulated salinity brine, which was caused by the deposition of the humic acids (HA), CaSO4, and HA-Ca complex, suggesting the severe membrane fouling. In this study, a composite membrane was fabricated by coating carbon nanotubes (CNT) on the commercial PTFE membrane, and an alternating current (AC) electric field was subsequently applied that aimed to improve MD performance. Specially, the application of 50 Hz square wave dramatically retarded the inorganic fouling and the intensity of membrane organic matter was reduced by 60.6% due to its hydrophilic surface and instantaneous polarity conversion, thus its normalized flux even maintained around 0.9 after 15 h operation. Coupled with the modified XDLVO analysis, the coating CNT layer exhibited good resistance of HA due to their electrostatic repulsions, both of which carried a strong negative charge and high density hydrophilic functional groups. It was also found that except the movement distance of charged substances far greater than the thickness of the fixed electrostatic double layer, AC electric field with certain frequency significantly reduced the coexistence time for substances with opposite charge by keeping them in motion that could further alleviate the fouling. This study suggested that a hybrid system of AC electric field and MD would provide a potential alternative for treating the salinity brine water.