Superhydrophobic PVDF/micro fibrillated cellulose membrane for membrane distillation crystallization of struvite

Abstract

Phosphate recovery is essential due to the depletion of phosphate resources and the phosphate loss by surface run-off. Membrane distillation crystallization (MDC) system with macroporous membrane provides a large interfacial surface for water removal at low temperature and phosphate concentration for crystal growth in the phosphate recovery strategy. However, the water vapour removal rate is limited by the membrane porosity and pore size. Membrane wetting and fouling on the hydrophobic membrane surface further reduce phosphate recovery in the long operation. In this work, polyvinylidene fluoride (PVDF) membrane was enhanced by adding micro fibrillated cellulose (MFC) filler to enhance phosphate recovery through struvite precipitation in MDC. The membrane was further modified by non-fluorinated silane (hexadecyltrimethoxysilane or octadecyltrichlorosilane) after phase inversion to attain superhydrophobicity. MFC promoted pore growth and porosity creation in PVDF/MFC membrane as the solvent-non-solvent exchange rate was boosted during phase inversion. In silane modification, abundant moieties of MFC further promoted vertical polymerization for creating superhydrophobic surface. The PVDF/MFC membrane modified with octadecyltrichlorosilane obtained the highest water contact angle of 154.4°, liquid entry pressure of 1.51 bar, relatively large pore size of 0.40 μm and satisfactory porosity of 45.1%, resulting in the stable permeate flux in MDC. The needle-like struvite crystals formed after pH adjustment and the modified membrane surface was not fouled even after 24 h of operation.