A study on the multifunction of ferrous chloride in the formation of poly(vinylidene fluoride) ultrafiltration membranes

Ferrous chloride was employed as additive in preparing flat sheet poly(vinylidene fluoride) (PVDF) membranes by phase inversion method, N,N-dimethylacetamide (DMAc) and water were used as solvent and coagulant, respectively, the role of ferrous chloride in membrane formation process and its effects on membrane property were investigated. Ultrafiltration tests showed that the introduction of ferrous chloride in casting solution could effectively increase membrane permeability while acceptable separation performance was maintained. The prepared membranes exhibited satisfactory compaction resistant property. Instrumental analysis found certain chemical traces left by ferrous chloride on the membranes: from scanning electron microscopy (SEM) observation, a large number of particulates with diameter of about 0.3 μm were found on membrane cross-section which might be quite related to the compaction resistant behavior; energy dispersive X-ray (EDX) analysis indicated that those particulates were composed of ferric oxide and PVDF chains; X-ray photoelectron spectroscopy (XPS) analysis revealed that the iron element entrapped on membrane surface stayed in three different chemical states: ferric hydroxide (FeOOH), ferric oxide (Fe2O3) and another unidentified one, while FeOOH was the main component of them. The presence of FeOOH could not only enhance membrane hydrophilicity but also combine separation and catalyzation as the membrane's two distinct functions.