Experimental and theoretical investigation of thin ZIF-8/chitosan coated layer on air gap membrane distillation performance of PVDF membrane

Despite the conventional desalination methods, membrane distillation (MD) with moderate temperature need, low operating pressure, and high salt rejection is a noteworthy choice for seawater desalination. In this study, a novel thin film composite (TFC) membrane with ultrathin zeolitic imidazolate framework (ZIF-8)/chitosan layer coated on the polyvinylidene fluoride (PVDF) membrane surface was synthesized to improve the membrane distillation (MD) performance for water desalination. Water contact angle analysis did not show significant change in hydrophilicity of TFC and unmodified PVDF membranes while the liquid entry pressure of water (LEPw) for TFC membrane increased significantly due to formation of ZIF-8/chitosan layer on the PVDF membrane surface. As a result of air gap membrane distillation (AGMD) tests, the permeate water flux increased about 350% after formation of ultrathin ZIF-8/chitosan layer on PVDF membrane surface compared to the unmodified PVDF membrane under filtration of saline water (3.5 wt% NaCl). Surprisingly, ZIF-8/chitosan modified membrane showed higher NaCl rejection of > 99.5. The antifouling properties of the membranes were also investigated. The TFC membrane presented better antifouling property when real seawater was used as feed solution by flux reduction of 16% and flux recovery of 90% compared with flux reduction of 41% and flux recovery of 67% for the neat PVDF membrane. Moreover, the stability of TFC was investigated by molecular dynamic simulation. Interaction energy computation and diffusion coefficient calculation confirmed good compatibility of ZIF-8 network with chitosan chain and good performance of the TFC membrane in case of water vapor flux.