Carbon black-modified polyvinylidene fluoride vapor-induced phase separation membranes for effective direct contact membrane distillation

We report the formation of polyvinylidene fluoride (PVDF)-based membranes by the vapor-induced phase separation (VIPS) process, from casting solutions containing controlled amounts of carbon black (CB), a non-toxic and inexpensive material, and utilizing triethyl phosphate as the solvent. A relative humidity of 75 % and an exposure time of 20 min led to the best membrane in terms of hydrophobicity (water contact angle between 143 degrees and 147 degrees). The target mean pore size (<0.5 mu m) and liquid entry pressure (>200 kPa) could be reached from a 2 wt%-CB content. The positive influence of CB on mechanical strength and thermal retention was also demonstrated. During 3h-direct contact membrane distillation (DCMD), setting the feed and permeate temperatures to 60 degrees C and 25 degrees C, respectively, the flux was steady to 24 LMH with synthetic sea water (NaCl 35 g/L) and real sea water, and 22 LMH using a concentrated salt solution. The rejection was in the range 99.96-99.99 %. During similar tests run while exposing the membrane to light under light power (40W), the flux of the CB-loaded membrane reached up to 30 LMH due to the heat retention properties of the particles. It also demonstrated better wetting and scaling resistances than the virgin membrane. The CB-modified VIPS membranes exhibit competitive flux and rejection rates in various desalination scenarios under relatively low-energy conditions (i.e., moderate feed temperatures), and their performance can be further enhanced under low-power light exposure, making them a promising candidate for DCMD processes.