수용성고분자/계면활성제 콤플렉스를 이용한 수용액내

묽은 농도의 유기물 분리

이상웅, 송태승, 박상준*

경원대학교 화학공학과

Ultrafiltration of dilute organics from aqueous solutions

by Polymer/Surfactant Complexes**

Sang Woong Lee, Tae-Seung Song, Sang Joon Park

Department of Chemical Engineering, Kyungwon University, Seongnam City, Korea

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. On the other hand, for the removal of multivalent heavy metals, anionic surfactants must be employed due to the mechanism of separation.

In a number of industrial operations, 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. In operating MEUF, however, the monomeric or free surfactant molecules which are approximately equivalent to the critical micelle concentration(CMC) can penetrate through the membranes. The leakage of surfactant is not favorable since we waste the surfactant to the permeate in MEUF. In addition, a disadvantege of MEUF is the relatively large concentration of surfactant that must be used for an effective separation.

Recently, a method has been proposed by several workers[Uchiyama et al., 1994] to solve the leakage problems. In their work, so-called colloid-enhanced ultrafiltration(CEUF) was employed using polymer/surfactant complexes. In the studies, water-soluble (anionic and cationic)polymers were added to the surfactant system and the polymer/surfactant complexes were formed. Since the polymer can play a role as a "scavenger" for surfactant species that enter the permeate, the leakage of the surfactant could be reduced. In the present work, the performance of CEUF for the dilute organics removal was tested with the complex of a non-ionic polymer and an anionic surfactant. The study was performed as a first phase of research for the simultaneous removal of organics and metal cations. In the study, the polymer chosen was hydroxypropylcellulose(HPC) since cellulose and their derivatives are biodegradable and less toxic. As a target solute, o-cresol was used and sodium dodecyl sulfate(SDS) was selected as a complex forming surfactant with the polymer.

EXPERIMENTAL

1. Materials

Hydroxypropylcellulose(HPC) with a weight-average molecular weight of 400,000 and 100,000 were obtained from TCI, Japan and Aldrich, repectively. The stock solutions of HPC was dialyzed in tube membranes(Spectra/Por, Spectrum, MWCO: 12,000) against Milli Q water(Millipore) for 5 days to remove salts and other low molecular weight materials before the concentration was determined by drying the samples to constant weight at 105 ℃. Sodium dodecyl sulfate(SDS) and o-cresol were obtained from Sigma Chemical Co. They were more than 99% pure and were used as received.

2. Ultrafiltration Experiments

The ultrafiltration experiments were performed in a 400 ml batch, stirred cell(Fisher Scientific). The cell was wrapped in tubing containing circulating water to maintain temperature at 30 ℃. Pressure was maintained by nitrogen gas at 200 kPa(29 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 ultrfiltration runs were 20,000 and the flux of pure water through the membranes was 40 L/h-m2. Initially, the cell was filled with 300 mL of feed solution containing known concentrations of o-cresol, HPC, and SDS. Agitation speed was held at approximately 840 rpm. The 40 ml of permeate was collected and the average permeate flux was measured. At any time, the permeate and the retentate concentrations were determined experimentally. The concentration of SDS was determined by employing an ethyl violet spectrophotometric determination method. The concentration of o-cresol was measured by GC after the solute was completely extracted by toluene.

RESULTS AND DISCUSSION

In Fig. 1, the ultrafiltration results for the removal of o-cresol are given as a function of the total retentate concentration of SDS. In the figure, the percentage rejection is defined as R=(1 - [solute]per/[solute]ret) x 100. The results show that there are three SDS concentration regions in the rejection for the polymer/SDS system. At low [SDS], the rejection linearly increases an the rejection was suddenly drops down  with more increase in [SDS]. From the point, the rejection increases again as [SDS] increases for the system utilizing HPC with the molecular weight 400,000. This result is probably due to the network formation of polymer HPC and SDS molecules. In the previous study [Nilsson, 1995] on a similar system, a model was proposed for the interactions between SDS and hydroxypropylmethylcellulose (HPMC). According to the model, at the critical aggregate concentration, SDS starts to adsorb to the polymer chain in a cooperative manner in the form of small micelles(clusters). Then, the adsorbed clusters increase in size with surfactant concentration up to a certain limiting value, and a cluster is shared by two or more polymer molecules creating a three-dimensional network when the polymer concentrations are high enough. In addition, at high concentrations of SDS, [SDS] > CMC, normal micelles also form into which HPMC may distribute. Therefore, it can be inferred that the low rejection values in the second region of the present system could be ascribed to the distribute of HPC and the formation of normal micelles since the o-cresol could be solubilized not only in clusters on the polymer but in normal micelles(as noted above, the complex gives less free SDS molecules in the solutions). The viscosity measurements shown in Fig. 2 also support the present hypothesis(model). In other words, the sudden viscosity increase at lower [SDS] can be ascribed to the formation of three-dimensional network. In addition, the shift of rejection curve for the HPC with the molecular weight of 100,000 may be due to the difference in number of propyl groups on the polymer surface.

In Fig. 3, the concentration of SDS in permeate as a function of the concentration of SDS in retentate is given for the ultrafiltration runs. The value for the polymer/SDS system was smaller than the value for the MEUF system, thus less amount of SDS permeates in the polymer/HPC system. However, at any concentrations of SDS, the role of polymer as a "scavenger" was not so effective compared with the results for the ionic polymer/ionic surfactant system. In the case of ionic-polymer/ionic-surfactant system, the interaction between the polymer and the surfactant was strong thus the leakage of the surfactant molecules to the permeate could be greatly reduced. Nevertheless, it is very attractive to employ the HPC/SDS complex in CEUF for the removal of o-cresol since the system needs only 5-20 mM of SDS. Thus the more economical process could be utilized. In addition, we lose the less amount of SDS to the premeate in operating CEUF. When the SDS micellar system was utilized for the o-cresol removal, the 50-200 mM of SDS was required for obtaining the same rejection values.

The relative fluxes for the ultrafiltration runs are shown in Fig. 4. In the figure, the relative flux is defined by [the flux of the system]/[the flux of pure water]. As expected, the relative flux was a strong function of [SDS] primarily due to the difference in solution viscosity. However, when the polymer of low molecular weight was employed, the relative flux was remarkably increased. The result shows that the selection of the molecular weight of polymer is very important in optimizing the operation of CEUF employing HPC/SDS complexes.

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Fig. 1. Rejection of o-cresol(0.1 mM)      Fig. 2. Viscosity at 30 C for the

as a fuction of [SDS] in retentate.         HPC/SDS system as a function of

[SDS]

Fig. 3. Permeate SDS concentrations        Fig. 4. Relative flux of permeate

in UF runs as a function [SDS] at         in UF runs as a function of [SDS]

at 30 C.                                at 30 C.