Influence of temperature on anionic surface active agent removal from a water solution by ultrafiltration

Ultrafiltration separation of an anionic surface active agent belonging to the alkylsulfonate family was studied. Asymmetric membranes made of polyethersulfone, polysulfone, regenerated cellulose, cellulose acetate and polyamide with a molecular weight cut-off equal to 5, 10 and 30 kDa were used. The temperature of tested solutions ranged from 25EC to 55EC. The CMC value of sodium dodecyl sulfate (SDS) at different temperatures was determined by conductivity measurements. The influence of polymer material, membrane cut-off and surface active agent concentration on transport and separation properties of UF membranes under different temperature conditions was investigated. It was found that the SDS separation efficiency was influenced by membrane material. The membranes with the best separation characteristics were made of polysulfone and polyethersulfone. Deterioration in separation and transport properties of UF membranes with an increase of SDS concentration was observed. The temperature increase of SDS solutions resulted in a small reduction of the SDS retention coefficient. The permeate flux increased with temperature rise due to the solution viscosity decrease and thermal expansion of the membrane material.

Fig. 7. Separation properties of Intersep Nadir membranes (cut-off 5, 10, 30 kDa) vs. SDS solution temperatures (300 g SDS/m3, ΔP = 0.20 MPa). increasing surfactant concentration in aqueous solutions. According to Mizoguchi et al. [17], for surfactant concentration below the CMC, some of the pre-micelles can convert to micelles. This is due to the concentration polarization effect on the membrane surface. Taking into account the micelle radius of about 2.5–2.7 nm [18,19] and the pore characteristics of tested membranes (mean pore radius in the range from 0.8 nm for PS5 membrane to 11 nm for PA30 membrane), it seems that the molecular sieving effect also plays an important role in surfactant separation. The decrease in retention could be connected with the thermal expansion of the membrane material and also with the increase in CMC value 1. Transport and separation properties of UF membranes are influenced by SDS concentration, solution temperature, membrane cut-off value and hydrophilic/hydrophobic properties of the polymer. 2. Deterioration in membrane permeability for SDS solutions (in relation to distilled water) can be attributed to the concentration polarization at the membrane surface. Reduction in UF membrane permeability was more pronounced for less hydrophilic membranes (polyamide, polysulfone and polyethersulfone) of higher cut-off values. 3. The permeate volume flux increased with temperature growth due to the solution viscosity decrease and thermal expansion of the membrane material. 4. The increase in the CMC value of SDS with a rise in temperature is an indication of the demicellization process caused by palisade layer disruption of the micelle. 5. The membranes that showed the best separation characteristics were made of polysulfone and polyethersulfone.