음이온 미쎌을 이용한 o-cresol의 용해와 한외여과

나용식, 최선아, 박상준

경원대학교 화학공학과

Solubilization and Ultrafiltration of o-cresol

by Micelles of Anionic Surfactant

Yong Sik Nah, Sun-Ah Choi, Sang Joon Park

Department of Chemical Engineering, Kyungwon University

Introduction

Micellar-enhanced ultrafiltration(MEUF) is a separation process using surfactants and membranes which can remove dissolved organic solutes or multivalent ions from water. In MEUF, the surfactant can be selected depending on the target material to be removed. For the separation of organics in aqueous solutions, either cationic or anionic surfactant may be used as long as the interior of the micelle contains the hydrocarbon chain and form a hydrophobic environment. In most of MEUF studies, however, cationic surfactants were employed for organics removal due to the disadvantages of using anionic surfactants; small micelle size, high CMC, and relatively low solubilization capacity. On the other hand, for the removal of multivalent heavy metals, anionic surfactants must be employed due to the mechanism of separation. In other words, anionic micelles have a high negative electrical potential on the surface where the metal cations adsorb or bind due to electrostatic attraction.

In a number of industrial operations (including coal refining, textiles dyes, and synfuel processing), wastewater contains unacceptable concentrations of both dissolved organics and heavy metals. In order to remove dissolved organics and metal ions (cations) simultaneously, it is essential that we employ an anionic surfactant in MEUF. Unfortunately, however, systematic studies on MEUF using anionic surfactant for organics removal have not been reported yet. In the presenr report, the equilibrium solubilization of organic solute (o-cresol) in micelles formed by a typical anionic surfactant (SDS) was investigated using the semiequilibrium dialysis (SED) method. To evaluate the performance of MEUF when SDS is employed, stirred-cell ultrafiltration experiments were conducted and the results were reported.

Experimentals

SED experiments were performed using regenerated cellulose membranes(6000 dalton molecular weight cutoff) and ordinary 20-ml equilibrium dialysis cells(Fisher Scientific). Initially, an aqueous solution containing known concentrations of o-cresol and SDS is placed on one side of the membrane(the retentate side) and pure water was added to the permeate side. Sodium dodecyl sulfate(SDS) and o-cresol were obtained from Sigma Chemical Co. The cells were kept in a desiccator, submerged in a thermostated bath at 30 ℃. After 24 h, the concentrations of o-cresol and SDS in the both permeate and retentate sides were  determined. The concentration of o-cresol was measured by UV spectroscopy and the concentration of SDS was determined by employing an ethyl violet spectrophotometric determination method. The ultrafiltration experiments were performed in a 400 ml batch, stirred cell. The ultrafiltration cell was wrapped in tubing containing circulating water to maintain temperature at 30 ℃. Pressure was maintained by nitrogen gas at 414 kPa(60 psig). The membranes were acetate cellulose, 76 mm diameter, membranes (Molecular/Por Type C) from Spectrum. The molecular weight cut-off of the membranes used in the ultrafiltration runs were 10,000 and the flux of pure water through the membranes was 68.1 L/h-m2 at 414 kPa(60 psig). Initially, the cell was filled with 300 mL of feed solution containing known concentrations of o-cresol and SDS. Agitation speed was held at approximately 840 rpm. The permeate flux was measured and samples were taken periodically throughout the run. A run was terminated after 200 mL of permeate had been collected. At any time, the permeate concentration was determined experimentally as described above, and the retentate concentration corresponding to the permeate concentration was calculated by a material balance.

Result and Discussion

The analysis of the SED results is based on the pseudo phase equilibrium model[Bhat et al., 1987, Lee et al., 1990]. In this work, the solubilization equilibrium constant of o-cresol in an aqueous SDS micellar solution (K) is defined as K= Xp/CC

where Xp is the mole fraction of organic solute(o-cresol) in the micellar phase, and  CC is the molar concentration of monomeric organic solute. In this work, the following equation, K = KO (1 - BXp)2, was used to express the relationship between the solubilization equilibrium constant and the mole fraction of organic solute in the micellar phase, where KO is the value of the solubilization constant in the limit as Xp approaches zero and B is an empirical constant..

Fig.1 shows the relationship between the square root of the solubilization equilibrium constant (K1/2) as a function of Xp. In the previous studies [Lee et al, 1990; Morgan et al., 1994], it has been shown that the values of Ko and B can be related to constants in the Langmuir adsorption equation in the form of Xp = KO CC / (1 + 2B KO CC ). Then, 2B may be considered as the number of molecules of the surfactant that constitute a "site" for binding the organic solute. Fig 2 shows the solubilization behavior of o-cresol is very similar to the Langmuir adsorption. In the plot, Xp/(1-Xp) represents the fraction of micelle surface that is occupied by o-cresol. The value of 2B can be obtained from the plot in Fig. 1 and the value is 1.68. Therefore, it can be inferred that approximately two molecules of SDS provide a location for the attachment of a single o-cresol molecule. Fig. 3 shows the percentage rejection of o-cresol in MEUF as a function of total SDS concentration in the retentate and the percentage rejection is defined as Rejection = ( 1 - [solute]per / [solute]ret ) x 100. The rejections of o-cresol by MEUF employing SDS micelles increase gradually with increasing concentration of SDS in the retentate from approximately 65% to about 85%; this is reasonable since at an almost fixed mole ratio of o-cresol and SDS in the retentate, the concentration of SDS in the feed increases in proportion to the unsolubilized solute, i.e., the permeate concentration of the solute. Relative fluxes are plotted in Fig. 4, as a function of SDS concentration in the retentate. As the retentate SDS concentration increases, the relative flux decreases nearly linearly with the log[SDS] which means the concentration polarization occurs. In addition, when the micelles are highly loaded with o-cresol, the relative flux is greatly reduced. The reduction in flux is probably due to the difference in the solution viscosity since the flux is inversely proportional to the solution viscosity. Furthermore, the more concentrated solution provides the higher osmotic pressure on the permeate which can reduce a flux in ultrafiltrations. In addition, the higher loading of solute on micelles can lead to the increase in the size of micelles. Accordingly, the pore blocking effect may be pronounced by the micelles when the loading is very high. Therefore, the reduction in flux could be ascribed to the increases in solution viscosity, osmotic pressure and pore-blocking resistance.

Conclusion

The relationship between the solubilization equilibrium constant and the mole fraction of o-cresol in the SDS micellar phase can be successfully correlated by the expression K = Ko ( 1 - B Xp)2. The analysis of the results from the solubilization equilibrium study supported that approximately two surfactant molecules may provide a location for the solubilized solute within micelle. In the removal of o-cresol by MEUF employing SDS micelles, the maximum rejection of 85% can be attained under the conditions used. In addition, as the o-cresol loading of retentate increases, the permeate flux reduces remarkably.

Reference

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