Ind. Eng. Chem. Res. 2009, 48, 6797–6804
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Determination of Tribromo-neopentyl Alcohol Micelle Distribution Coefficients in Single and Mixture Solutions for Evaluation of Micellar Enhanced Ultrafiltration Feasibility in Treating Contaminated Groundwater M. Krivorot,† Y. Oren,† Y. Talmon,‡ and J. Gilron*,† Zuckerberg Institute for Water Research and Unit of EnVironmental Engineering, Ben Gurion UniVersity of the NegeV, POB 653, Beer SheVa, 84105, Israel, and Faculty of Chemical Engineering, TechnionsIsrael Institute of Technology, Haifa, 32000, Israel
To evaluate the potential of micellar enhanced ultrafiltration (MEUF) for treating groundwater contaminated with organics in northern Negev, Israel, the partition coefficient, KTBNPA, of tribromo-neopentyl alcohol (TBNPA) between aqueous and micellar phases was determined for nonionic surfactants. Two methods were used for the determination: the MEUF method using centrifugal UF equipped with 10 kDa MWCO regenerated cellulose and the solid phase matrix extraction (SPME) method using gas chromatography (GC) analysis. The distribution coefficient of TBNPA for nonionic surfactant Brij 58 (C16EO20) was ∼630 M-1 and fairly constant over a wide range of loadings (mole fraction of TBNPA in micelle from 0.07-0.45) when determined by centrifugal MEUF; but is was only ∼340-400 M-1 when determined by SPME. This difference is attributed to concentration polarization of the micelles in the UF measurements. The partition coefficient was reduced about 10% in the presence of 1% NaCl. The presence of toluene in samples containing TBNPA increases the solubility of TBNPA in the micellar phases of Brij 58 and Igepal CO-720, as determined by SPME. This phenomenon is explained by assuming that toluene (a nonpolar molecule) dissolves in the hydrocarbon core of the micelles resulting in expansion of the micelle and increase of the distance between polar groups in the palisade layer. As a result, the micelle can incorporate more of the polar TBNPA molecules in that layer. Cryogenic-temperature transmission electron microscopy observations of micelles loaded with TBNPA showed that TBNPA loading did not significantly change micelle size, consistent with TBNPA concentrating in the palisade layer. 1. Introduction Groundwater contamination and soil pollution by industrial streams have become recognized as important environmental problems over the last forty years. Such an issue is being confronted in the northern Negev region in Israel near an industrial area. A survey of the brackish groundwater conducted there over a 9-year period showed significant contamination.1 The contaminant levels found in the 2001/2002 survey are displayed in Table 1 for groundwater samples from four monitoring wells in the vicinity of the industrial complex. The contaminants can be divided roughly between the family of halogenated aliphatics and aromatic organics. Members of both classes of compounds can cause significant health hazards.2 The Israeli government’s decision to build a new army training base in the vicinity3 and a recent epidemiological study on Bedouin residents in the area4 add urgency to dealing with the issue. The techniques proposed for treating such contaminated groundwaters include: in situ bioremediation, dual-phase extraction, and various pump-and-treat schemes.5 Micellar enhanced ultrafiltration (MEUF) has been intensively studied as one of the proposed processes for removal of organic contaminants in groundwater. Small molecules not normally rejected by ultrafiltration membranes are taken up in surfactant micelles large enough to be retained by the same membrane. This method was originally developed in the mid-1980s by the group of Scamehorn and Christian6,7 for removing organics and heavy metals and has been extensively studied since then. Recently it has * To whom correspondence should be addressed. Tel.: +972-86563534. Fax: +972-8-6563503. E-mail:
[email protected]. † Ben Gurion University of the Negev. ‡ TechnionsIsrael Institute of Technology.
been used for carrying out remediation of soils contaminated with chlorinated aliphatics.8 As has been shown by previous groups,6 the distribution of an organic component between the aqueous and micellar phases can be described by using a mass action expression: K)
Om , [M-1] SmOw
(1)
where O refers to organic solubilizate concentration and S to surfactant molar concentrations relative to the aqueous solution volume. The subscripts m and w refer to the micellar and Table 1. Composition of Water Sampled from Four Wells in the Negev Region1 well
component TDS, mg/L chlorobenzene, µg/L bromobenzene, µg/L methylene chloride, mg/L 1,2 dichloroethane, mg/L dibromomethane, mg/L tribromoneopentyl alcohol, mg/L benzene, mg/L toluene, mg/L ethylbenzene, mg/L xylene, mg/L
A 5 3.9 0
B 7988 0 0
C
D
16742 21400 21 149 1.4 9.2 4 608
64
47
230
EPA maximum level in drinking water (www.epa.gov/safewater) 0.1 0.005 0.005
292 73
557
130
4.1 1.3 3.4
1.29 3.92 0.61
11 30 6
33 3375
3.4
2.36
11
15
10.1021/ie8011352 CCC: $40.75 2009 American Chemical Society Published on Web 05/28/2009
0.005 1 0.7 10
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Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009
aqueous phases, respectively. While ideally K should be constant, there are cases where it increases or decreases with loading of the solubilizate, expressed as a mole fraction of solubilizate in the micelle.9 Since only aqueous phase organic solute passes freely through the UF membrane, measuring the concentration of organic solute in the UF permeate provides the value of Ow.6 To find the concentration of Om, a mass balance can be simply applied, Om ) Ototal - Ow
(2)
Substituting (2) into eq 1 gives K)
Ototal - Ow SmOw
(3)
in which all of the quantities on the right-hand side are measurable. Sm is usually set equal to the total surfactant concentration, Stot, when surfactant solution is high, and the critical micelle concentration is low. Otherwise, Sm is set equal to Stot - Sw.6,9 Once K is determined for a variety of conditions, Ow can be predicted in an MEUF process based on the composition of the feed solution: Ow )
Ototal 1 + KSm
(4)
The methods used to determine K have included semiequilibrium dialysis (SED),7 partial vapor pressure determination for volatile organic solubilizates,10-12 ultrafiltration,13 centrifugal ultrafiltration,14 and solid phase matrix extraction (SPME).15 With volatile materials, the ultrafiltration based methods are quite problematic due to potential loss to the atmosphere or absorption on solid surfaces. The inconsistent results sometimes reported for distribution coefficients of volatile organics determined by MEUF16 clearly reflect this. Tribromo-neopentyl alcohol (TBNPA), CAS 1522-92-5, MW 324.8, log Kow ) 2.6, water solubility ∼ 2 g/L,17 is used as an intermediate in making fire-retardant materials and is one of the materials found in large concentrations (up to 500 mg/L) in some of the wells sampled from the above-mentioned aquifer.1 It is semivolatile and potentially toxic and will have a long persistence in the aquifer.18 It is not well-retained by nanofiltration membranes as found by preliminary testing in our laboratory. It was therefore decided to study the potential for TBNPA removal by MEUF. It was studied by itself in solution and also in the presence of toluene, another contaminant found in the aquifer at concentrations ∼30 mg/L. In the present work, we determined the partition coefficient of TBNPA between an aqueous phase and micelles by two methods: centrifugal MEUF and SPME in the presence and absence of 1% NaCl and toluene. 2. Methods 2.1. Materials. TBNPA was kindly supplied by Dead Sea Bromine Group, Israel, and was used without further purification. Reagent grade NaCl and toluene were supplied by Frutarom Ltd. (Akko). Water used was from an Elga water purifier with an outlet total organic carbon (TOC) below detection limits and conductivity of
