Studies on treatment of sodium dodecyl benzene sulfonate solution by high shear ultrafiltration system

The dynamic ultrafiltration of anionic surfactant solution, sodium dodecyl benzene sulfonate (SDBS), using a rotating disk system with hydrophilic polyethersulfone (PES) membranes (MWCO 10 kDa) was studied in this paper. The effects of mean transmembrane pressure (TMP) and disk rotating speed on the permeate flux and surfactant retention have been reported. When the rotating speed of the disk with vanes was above 1000 rpm, it was found that the permeate flux kept rising without reaching a plateau up to a TMP of 2100 kPa. The surfactant retention of the 10-kDa membranes was observed to be 81.5% for a micellar SDBS solution of 1 Critical Micelle Concentration (CMC) at the highest TMP of 2100 kPa and a rotating speed of 1000 rpm, and it reached 92.5% at a rotating speed of 2750 rpm. The dynamic filtration system was proved to be very efficient for decreasing concentration polarization, due to high shear rates on the membrane generated by disk rotation. Then, the influence of the feed concentration on the permeate flux and surfactant retention was investigated at different concentrations up to 5 CMC. The unexpected result obtained was that the flux increased with increasing feed concentration, which could be explained by the reduction of concentration polarization by enhanced diffusion of surfactant micelles due to high shear rates. The adsorption of surfactant anions on the membrane surface probably increased the hydrophilicity of PES membrane.

Our results have shown that high shear rates on the membrane surface induced by disk rotation above 1000 rpm allowed the filtration to remain in the pressure limited regime until the highest TMP of 2100 kPa. Meanwhile the membrane retention was rather high and stable, especially for the disk rotating speed of 2750 rpm, since the effect of the concentration polarization was greatly reduced due to the high shear rates on the membrane, even at the highest pressure of 2100 kPa. The adsorption of anionic surfactant on the membrane surface increases membrane hydrophilicity, and consequently the permeate flux. It seems that a higher SDBS concentration gives a bigger increase in the hydrophilicity of PES membrane. Moreover, when the surfactant concentration increased up to 1 CMC and higher, micelle motions caused by high shear rates, electrostatic interaction between the PES membrane surface and the surfactant micelles, and also among micelles themselves, could reduce the concentration polarization on the membrane. This could explain the larger flux at higher feed concentrations.