Effect of grafting on microstructure, composition and surface and transport properties of ceramic membranes for osmotic evaporation

The effect of grafting with fluoroalkylsilanes on microstructure, composition and surface and transport properties of alumina membranes having different pore sizes (0.2, 0.5 and 0.8 μm) was studied. Surface and transport properties were evaluated in terms of contact angle, adhesion work and hydrodynamic membrane pore response. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) were employed to evidence the induced microstructure and composition changes after grafting. Contact angle higher than 90°, in addition to low work adhesion and surface tension values confirmed the hydrophobic character of all three membranes after chemical treatment. SEM analysis showed facetted alumina grains in unmodified membranes while grains with rounded edges appeared after grafting. This effect induced compaction on internal and intermediate layers mainly on the 0.2 μm membrane. The insertion of hydrophobic chains on the outer surface and the porous structure of membranes after grafting, were evidenced by detection of fluor in the active and internal structure of treated membranes as well as by increments on the Si/Al ratios followed by EDXS. In addition, water permeability drastically decreased and intrusion pressure rose, mainly on the membrane of lower pore size tested. Hydrodynamic membrane response indicated pore size reductions of 53%, 65% and up to 82% in 0.2, 0.5 and 0.8 μm grafted membranes, respectively. However, silane chains inserted inside of 0.5 and 0.8 μm membrane pores were apparently more susceptible to flow induced deformation, limiting the potential application of these membranes on the osmotic evaporation process. Results from grafted 0.2 μm alumina membrane during OE of clarified cane juice and model sucrose solution indicate that feed and brine layer resistances contribute in less than 10% while membrane resistance contribution to the total process resistance was around 90%. Results from this work indicate that applicability of grafted alumina membranes in the OE process might further improve by using 0.2 μm < d_P < 0.5 μm average pore size membranes and/or by decreasing membrane thickness.