Micellar-enhanced ultrafiltration of chromate and nitrate: binding competition between chromate and nitrate

DESALINATION ELSEVIER

Desalination 167 (2004) 111-118

www.elsevier.com/locate/desal

Micellar-enhanced ultrafiltration of chromate and nitrate: binding competition between chromate and nitrate Kitae Baek, Ji-Won Yang* National Research Laboratoryfor Environmental Remediation, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseon-gu, Daejeon 305-701, Republic of Korea Tel. +82 (42) 8693924; Fax +82 (42) 8693910; e-mail: [email protected] Received 26 January 2004; accepted 9 February 2004

Abstract

Simultaneous removal of chromate and nitrate, major anionic pollutants in subsurface water system, was investigated using micellar-enhanced ultrafiltration (MEUF) with cetylpyridinium chloride as a cationic surfactant. Rejection of nitrate and chromate was expressed as a function of molar ratio of surfactant to pollutants. With the molar ratio of 1:1:10 (chromate: nitrate: surfactant), rejection of nitrate and chromate reached 80% and 98%, respectively. Rejection of surfactant was observed to be >95% under the same conditions. The rejection of nitrate was inhibited by co-existence of chromate. It was due to the differences in binding power between ionic micelles and counter ions, which is proportional to the valence of counter ions; the valence of chromate was higher than that of nitrate. Keywords: Competitive binding; Chromate: Nitrate; Groundwater; Micellar-enhanced ultrafiltration

1. Introduction

The increasing contamination of groundwater by toxic inorganic compounds is a serious environmental problem. These inorganic pollut*Corresponding author

ants are o f considerable concern because they are non-biodegradable and highly toxic, and have probable carcinogenic or other negative effects. Chromate and nitrate are frequently-observed pollutants in groundwater. Chromate is one o f the most common groundwater contaminants especially at indus-

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi; 10.1016/j.desal.2004.06.118

K. Back, J-VK. Yang / Desalination 167 (2004) 111-118

112

trial sites and military facilities due to its widespread use as a metal corrosion inhibitor [1]. In the environment, the stable oxidation states of chromium are Cr(m) and Cr(VI). While Cr(II-I) is an essential element for living beings, playing an important role in carbohydrate metabolism, Cr(VI) becomes carcinogenic for long exposures. Hence, Cr(VI) incurs a significant risk to human health when released into the environment [2]. Nitrate, which caused blue baby syndrome to a baby less than 6 months, has also been a major pollutant in groundwater due to development of agricultural fertilizers. Nitrate contamination of groundwater is increasingly breaching safety levels in developed and developing coun-

tries. The maximum level of nitrate in water for human consumption is about 10 mg/1 [3]. Chromate and nitrate removal from water using ion exchange requires frequent regeneration of the resins. Membrane technology such as reverse osmosis and electrodialysis is generally achieved for ion removal, however, reverse osmosis needs high operating pressure, and electrodialysis is a high energy consumer [4]. Micellar enhanced ultrafiltration (MEUF) has been shown to be an effective method for removing inorganic pollutants, including chromate and nitrate, from the aqueous phase [415]. MEUF involves the addition of a surfactant above the critical micellar concentration (CMC) in order to entrap ionic solutes in an aqueous

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K. Baek, J-W. Yang/ Desalination 167 (2004) 111-118

stream. The increased hydrodynamic size of the solutes enables their rejection by ultrafiltration membranes. Fig. 1A shows the conceptual diagram of MEUF for removal of nitrate and chromate with a cationic surfactant. Most researchers on removal of chromate or nitrate have focused on the single pollutant system [4--8]. In the real field, however, chromate commonly exists with nitrate or other pollutants. In multi-pollutant systems, binding phenomena on the surfactant micelle is changed due to competition between one pollutant and the other [9,11-13]. Changes in binding phenomena influenced the rejection of pollutant because rejection of each pollutant depends on the binding of pollutants on the micelles. In the present study, an attempt is made to remove chromate and nitrate simultaneously from aqueous solutions by MEUF using cetylpyridinium chloride (CPC). 2. Materials and methods

CPC, sodium nitrate, and sodium chromate were purchased from Simga Chemicals (USA). Deionized water was used in preparing all solutions. Ultrafiltration experiments were executed with dead-end stirred cell (Millipore, 8400, USA). The schematic diagram of experimental apparatus was shown in Fig. lb. A regenerated cellulose membrane with molecular weight cutoff (MWCO) of 3000 and 10,000 were used for ultrafiltration under 2 bar of transmembrane pressure. Feed solutions were mixed adequately for at least 12 h. The inlet reservoir was initially filled with a 100 ml of feed solution, and the permeate was wasted during the first 3 min to reach the steady state. To evaluate the filtration efficiency in removal of chromate and nitrate from the feed solution, the following equation was used:

113

where R is the rejection efficiency, Cp the concentration of chromate or nitrate in the permeate, and Ci the initial feed concentration. For the comparison of fluxes measured in different conditions, relative fluxes to the fluxes of deionized water were used. The analysis of nitrate, CPC, and chromate in permeate solution was carried out through UV/VIS spectrophotometer (HP 8452 A, USA) at 232 nm, 258 nm, and 372 nm, respectively. 3. Results and discussion 3.1. Nitrate/CPC system

Fig. 2 shows the performance of MEUF in nitrate/CPC system as a function of molar ratio of CPC to nitrate. The rejection of nitrate was increased gradually from 40% to 62%, to 81% and to 89% as the molar ratio of CPC to nitrate increased from 1 to 2, to 4, and to 6, respectively. When the molar ratio increased from 6 to 10, the increase in the rejection of nitrate was negligible (<4%) and the rejection was saturated to about 93%. As the pore size of the membrane increased from MWCO of 3,000 to MWCO of 10,000, the rejection decreased slightly, but the flux increased three times. With the molar ratio of CPC to nitrate of >6, more than 90% of the nitrates were removed from the aqueous stream. 3.2. Chromate/CPC system

Fig. 3 shows the rejection of chromate in the chromate/CPC system as a function of molar ratio of CPC to chromate. Chromate rejection increased with the molar ratio of CPC to chromate. In the filtration system with the membrane of MWCO of 3,000, the rejection of chromate increased from 59% to 82%, to 93% and to >99% as the molar ratio of CPC to chromate increased from 1 to 2, to 3 and to 5 respectively. The rejection was saturated to 99% with the molar ratio of>5. As the pore size of membrane increased from MWCO of 3,000 to

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Fig. 2. Removal of nitrate in the nitrate/CPC system. The initial concentration of nitrate was 1 mM and the regenerated cellulose membranes with molecular weight cut-off of 3000, upper, and molecular weight cut-off of 10000, lower, were used for filtration at 2 bar of transmembrane pressure. MWCO o f 10,000, the rejection decreased from 58% to 46% at the molar ratio of 1, from 82% to 68% at that o f 2, and from 93% to 85% at that o f 3. At the molar ratio o f <5, the rejection was saturated to 97% in the system with MWCO o f 10,000, while >99% in the system with MWCO o f 3,000. 3.3. Nitrate/chromate/CPC system Fig. 4 shows the rejection o f nitrate in the nitrate/chromate/CPC system. The significant

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Fig. 4. Removal o f nitrate in the nitrate/chromate/CPC system. The initial concentration of nitrate and chromate were 1 mM and the regenerated cellulose membranes with molecular weight cut-off 3000, upper, and molecular weight cut-off o f 10000, lower, were used for filtration at 2 bar o f transmembrane pressure.

to 1:1:5. With the molar ratio of 1:1:10, the rejection of 80% was obtained. Significant differences between MWCO of 3,000 and MWCO of 10,000 were not observed. The presence of chromate decreased the nitrate rejection significantly compared to nitrate/CPC system (Fig. 2).

115

The rejection of chromate in the nitrate/ chromate/CPC system is shown in Fig. 5. The rejection of chromate increased in proportion to the molar ratio of CPC added in the system as shown in chromate/CPC system. The rejection increased from 50% to 71%, to 90%, and to 98% as the molar ratio increased from 1:1:1 to 1:1:2, to 1:1:3, and to 1:1:5 (nitrate: chromate: CPC), respectively. It means that the presence of nitrate in the system does not affect significantly the rejection of chromate, though the rejection slightly decreased by 10% at the low molar ratio of CPC compared to those in the chromate/CPC system (Fig. 3). This phenomenon can be explained by the differences in the binding power between anions and cationic surfactant micelles. Recently, Tangvijitsri et al. [10] reported that differences in the rejection of nitrate, sulfate, and chromate resulted from the differences in valences of ions in the polyelectrolyte-enhanced ultrafiltration. Rejections of sulfate and chromate with valences of 2 were similar, however greater than that of nitrate with valence of 1. In the nitrate/chromate/CPC system, chromate has 2 as valence values, but 1 for nitrate. Based on the valences of each ion, the binding power of chromate is greater than that of nitrate to bind on the CPC micelles. In the present system, chromate bound on the CPC micelles preferentially because of greater binding power. At the molar ratio of 1:1:10, nitrate rejection was slightly lower than the rejection at the nitrate/CPC system with the molar ratio of 1:1:6. At the molar ratio of <1:1:5, nitrate rejection was inhibited because nitrate bound to unbounded CPC with chromate. At the molar ratio of >1:1:5, chromate rejection was saturated and nitrate was not inhibited by chromate. Fig. 6 shows the CPC rejection as a function of molar ratios. The CPC rejection increased slightly as the molar ratio increased because rejected micelle formed secondary membrane on the membrane surface. The rejection was high enough (93%) even at the lowest concentration of CPC because the concentration of CPC was greater than the CMC of CPC (0.9 raM). As the

116

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Fig. 5. Removal of chromate in the nitrate/chromate/ CPC system. The initial concentration of nitrate and chromate were 1 mM and the regenerated cellulose membranes with molecular weight cut-off 3000, upper, and molecular weight cut-off of 10000, lower, were used for filtration at 2 bar of transmembrane pressure.

Fig. 6. Rejection of CPC in the nitrate/chromate/CPC system. The initial concentration of nitrate and chromate were 1 mM and the regenerated cellulose membranes with molecular weight cut-off 3000, upper, and molecular weight cut-off of 10000, lower, were used for illtration at 2 bar of transmembrane pressure.

filtration progressed, the rejection decreased slightly because the retentate in the ultrafiltration cell was concentrated. The rejection o f membrane with MWCO 3,000 was slightly higher than that with MWCO 10,000.

Concentration o f CPC in the permeate as well as the rejection o f CPC should be considered in groundwater treatment because the surfaetant used in MEUF can be a secondary pollutant in the permeate (i.e., treated water).

K. Baek, J-W. Yang / Desalination 167 (2004) 111-118

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Fig. 8. Relative flux of the MEUF process in the nitrate/chromate/CPC system. The initial concentration of nitrate and chromate were 1 mM and the regenerated cellulose membranes with molecular weight cut-off 3000, upper, and molecular weight cut-off of 10000, lower, were used for filtration at 2 bar of transmembrane pressure. The flux of deionized water in MWCO of 3,000 was 22 1/m2/h and 66 1/m2/h for MWCO of

10,000. Theoretically, the concentration o f surfactant in the p e r m e a t e can b e increased to the C M C o f surfactant in M E U F process. Fig. 7 shows the concentration o f CPC in the permeates. T h e concentration increased gradually as the operation

progressed and the molar ratio increased. The concentration decreased in the m e m b r a n e system with lower pore size. In this study, the concentration o f CPC in the permeate was <0.4 m M

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K. Back, J-W. Yang / Desalination 167 (2004) 111-118

for MWCO o f 3,000, while <0.6 mM for MWCO of 10,000. Fig. 8 shows relative flux as a function o f retentate CPC concentration. As shown in the figure the relative flux decreased as the molar ratio increased because o f the increased formation o f secondary membrane increase. As the filtration progressed, the flux also decreased because the retentate was concentrated, which resulted in the similar effect to increase in CPC molar ratio. As the membrane pore size increased, decrease in the flux became more serious. It seems that the pore size o f secondary membrane is between MWCO o f 3,000 and MWCO o f 10,000.

4. Conclusion The MEUF removed chromate and nitrate simultaneously to 99% and 80%, respectively, at the molar ratio o f 1:1:10 (nitrate: chromate: CPC). In the nitrate/chromate/CPC system, the rejection o f nitrate decreased significantly compared to those in the nitrate/CPC system, while the chromate rejection was not affected. This phenomenon shows that chromate anions bind on the CPC micelles preferentially because the binding power o f chromate on the CPC micelles is greater than that o f nitrate. There is no significant difference in rejection o f nitrate and chromate between membrane with MWCO o f 10,000 and membrane with MWCO o f 3,000 because the size o f the CPC mieelle is big enough not to pass through the membrane pore. As a result, the rejection o f CPC was high and the concentration o f CPC in the permeate was low enough. The MEUF process could be an alternative to treating groundwater contaminated with nitrate and chromate.

Acknowledgement This work was supported by a grant (M10203-00-0001) from the Korea Ministry of Science and Technology through the National Research Laboratory program.

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