Vacuum film etching effect of carbon alumina mixed matrix membranes

This work investigates the preparation and performance of carbon alumina mixed matrix membranes (CA-MMM) by a novel vacuum-assist method by impregnating phenolic resin into a porous alumina substrate followed by pyrolysis. Increasing the precursor concentration from 1 to 20 wt% led to greater carbon pore filling and a reduction in water flux. Systematic recording of vacuum pressure showed a sharp decrease in pressure for vacuum time (t(v))< 30 s, followed by an over spike and then a steady state pressure for t(v) = 120 s. A more interesting finding was the key role played by the vacuum time as water fluxes increased by almost three-fold when tv increased from 90 to 120 s. For instance, water fluxes reached 25 L m(-2) h(-1) at 3.5% NaCl feed solution (75 degrees C) and NaCl rejection of 90%, though as high as 99.7% at 25 degrees C. A remarkable finding in this work is that the depth of impregnation increased to over 10 mu m for t(v) >= 120 s, producing much thicker impregnated films, though delivering much higher water fluxes in desalting water. To explain this unusual permeation behaviour, we discovered that during vacuum impregnation, the solvent methanol was permeating through the membrane and entraining the resin oligomers. This resulted in the (re) dissolution and (re) deposition of the resin oligomers into a greater depth of the porous alumina substrate, whilst at the same time reducing the effective thickness of the CA-MMM membrane. The vacuum-assisted method seemed to have etched the resin film during impregnation. This resulted in the carbonised structures embedded into the porous alumina substrate with lower resistance to mass transfer, hence higher water fluxes.