Engineering high-flux poly (vinylidene fluoride) membranes with symmetric structure for membrane distillation via delayed phase inversion

Membrane distillation (MD), as a new separation technology, has shown great potential in seawater desalination, food concentration, and industrial wastewater treatment. However, traditional polymer membranes prepared by non-solvent phase inversion (NIPS) usually obtain relatively denser membrane layer, making it difficult to obtain efficient and stable separation efficiency. Moreover, the complex procedures involved in optimizing the performance of Poly (vinylidene fluoride) (PVDF) membranes, such as additives and their extra costs, are two obstacles to large-scale application. This work proposed a simple reversion method to obtain PVDF membrane with a highly porous surface by delaying the phase inversion velocity. Furthermore, the effects of different polymer concentrations on the membrane structures and performances were explored. It was demonstrated that the optimized membranes showed the rejection of 99.9 %, 99 %, and ∼100 % in the MD for saline water, ginseng solution, and dye/salt solutions, respectively. The average water flux was up to 34.8 kg m⁻² h⁻¹ in the 35 g/L NaCl solution system and 27.9 kg m⁻² h⁻¹ in the 100 g/L NaCl solution system, which demonstrated a significant high-permeance advantage. The membrane flux when treating dye/salt solution and ginseng solution were 22.7–25.4 kg m⁻² h⁻¹ and 5.6 to 9.9 kg m⁻² h⁻¹, respectively. This work provided a simple and cost-effective method to fabricate PVDF membranes for separation of saline water, food extracts and dye waste water in the chemical industry.