Dynamic micellar enhanced ultrafiltration: use of anionic (SDS)–nonionic(NPE) system to remove Cr3+ at low surfactant concentration

Ultrafiltration experiments were carried out with tangential cell system. The permeate flux and rejection characteristics of SDS and nonylphenol ethoxylated C9H19C6H4(CH2CH2O)5CH2CH2OH (NPE) micellar solutions for a ceramic tubular membrane (zirconium oxide–Titanium oxide) were investigated. For both surfactants, micelle rejection increased with pressure and reached an asymptotic value at ΔP=250 kPa beyond which micellar rejection remained unchanged (∼+99%). This was explained in terms of concentration polarization occurring in the vicinity of the membrane–solution interface. Time dependence of the permeate surfactant concentration was also investigated. During the first 2 h of ultrafiltration, the surfactant concentration changed with time. Then a steady-state condition was reached and the permeate concentration remained invariant. This trend was attributed to a time-dependent gradual change of the state of packing of the micelles over the membrane surface. Cr3+ rejection by SDS–NPE mixed micelles yielded high Cr3+ rejection (+99.5%) at NPE molar fraction up to 0.2. Within this optimum composition and ΔP=250 kPa, unexpectedly high rejection (33.50%) was measured below the critical micellar concentration (cmc). This was considered as due to a high surfactant concentration in the vicinity of the membrane surface where micelles are present, although the retentate surfactant concentration is below the cmc. Above the cmc, high Cr3+ rejections (+99%) were obtained. Interestingly, this rejection was found to be independent of the feed surfactant concentration within the range investigated in this study (3×cmc–30×cmc), suggesting that Cr3+ rejection is not a function of the surfactant concentration in the bulk retentate solution but a function of the surfactant concentration nearby the membrane surface. The presence of NaCl in the feed surfactant solution showed a drastic adverse effect on Cr3+ rejection by SDS–NPE micelles.