Environ. Sci. Technol. 3993, 27, 2783-2788
Enhanced Solubilization of Aromatic Solutes in Aqueous Solutions of IIFVinylpyrrolidone/Styrene William R. Hauibrook,' Jeffrey L. Feerer, T. Alan Hatton, and Jefferson W. Tester
Department of Chemical Engineering and Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139 The partitioning behavior of several aromatic hydrocarbons between N-vinylpyrrolidonelstyrene(NVPS) and water is reported. Values of log K,,, concentration-based polymer-water partition coefficient, in this study are 3.39 f 0.04, 3.38 f 0.01, and 4.69 f 0.02 for toluene, naphthalene, and phenanthrene, respectively,in aqueous NVPS solution. A relationship between log Kpwand log KO,, octanol-water partition coefficient, is proposed to extend predictive analysis to other solutes. The high molecular weight (-3.4 million g/mol) and high organic solubilization of NVPS make it attractive for use in organic-contaminated water remediation. Potential applications are briefly discussed.
Introduction The contamination of surface water and groundwater by dilute organic compounds is a worldwide environmental problem. Organic compounds from gasoline,oils, solvents, and other heavily-used industrial products reach streams, lakes, rivers, and groundwater via runoff from improper waste disposal. Many of the most harmful of these compounds have low solubility in water (ppm and below), yet remain toxic even at these low concentrations. For example, the organic pesticides endrine, lindane, and toxaphene have water solubilities of 0.2, 7, and 3 mg/L, respectively; but they are hazardous at much lower concentrations-0,0002, 0.004, and 0.005 mg/L, respectively (I). The removal of this dissolved organic is very costly since a small amount of organic can contaminate a large volume of water. A widely-used method for removing organic from water is incineration. Other methods such as supercritical water oxidation are currently being used as alternatives to incineration (2). Incineration of wastebearing liquid is expensive-a representative figure is $1 per gallon (3)-therefore, concentration of the waste is needed prior to incineration. Remediation of organic-contaminated soils also poses unique problems. Contaminant concentrations on soil can be orders of magnitude higher than their concentrations in the surrounding liquid water phase due to the preferential partitioning of contaminant. This is primarily due to the attraction of hydrophobic organic contaminant to the organic components of the soil. The use of surfactants such as sodium dodecyl sulfate to facilitate transfer of soil-adsorbed contaminants into a mobile aqueous phase is under investigation (4, 5 ) . A major goal in the remediation of organic-contaminated soils and dilute aqueous phases is the concentration of the contaminant into an easily-mobilized phase. Current methods of liquid waste concentration include carbon adsorption and solvent extraction. In carbon adsorption, organic contaminant is filtered from water passing through an activated carbon bed. Bulk handling and pumping concerns have made this technique unattractive for some 0013-936X/93/0927-2783$04.00/0
0 1993 American Chemical Society
applications (6). The solvent extraction technique attempts to remove the organic contaminant by contacting the contaminated water with an organic solvent, allowing the organic contaminant to preferentially partition into the organic phase. The use of traditional solvents such as benzene and toluene in this technique is not favorable, since counter-contamination of the water with the solvent may be a problem (7).More desirable routes of aqueous waste separation and concentration which employ nontoxic, easily separable compounds are currently under investigation. One such process involves ultrafiltration with high molecular weight copolymer surfactants (8). Recently, the use of surfactants whose molecules form micelle structures has been investigated for the enhanced solubilization of hydrophobic organic compounds in water (4,5,8). In light of new membrane separation techniques (8), the use of easily-separable, high molecular weight surfactants provides an alternative to carbon adsorption and solvent extraction. The molecular weight of the surfactants used in the above studies range from a few hundred to 13000 g/mol. This paper investigates a particular copolymer surfactant, N-vinylpyrrolidonelstyrene (NVPS), of approximate molecular weight 3.4 million g/mol. The primary objective of this work was to quantify the capacity of NVPS to solubilize several model hydrophobic aromatic compounds in aqueous solution. Thermodynamic analysis was used to develop a methodology for generalizing the results so that the solubilization capacities of NVPS and other copolymerscan be estimated for various hydrophobic compounds of interest. NVPS is nontoxic and is generally no more expensive than sodium dodecyl sulfate, a commonly-usedsurfactant. Since NVPS has very high molecular weight, it can be separated from wash solution by simple mechanical filtration. Regeneration studies of organic-saturated NVPS have not yet been conducted.
Experimental Methods
Materials. The copolymer used in this study, Nvinylpyrrolidone/styrene, was obtained as a 40 wt % copolymer solution in water. The supplier, Scientific Polymer Products, Inc., reported that the copolymer is composed of styrene and N-vinylpyrrolidone monomers in a weight ratio of 60:40. The hydrophilic character of the copolymer allows it to form a stable suspension in water, and its hydrophobic nature lends it organic solubilization capacity. The average molecular weight of NVPS was reported as approximately 3.4 million g/mol. NVPS is a random copolymer as shown in Figure 1. The hydrophobic model organic compounds used in this study were toluene (99.8% pure), naphthalene (99+% pure), and phenanthrene (98% pure). These were chosen as representative one-, two-, and three-ring aromatic compounds. The toluene was obtained from Aldrich Chemical Co., Inc., and the naphthalene and phenanthrene were Envlron. Sci. Technol., Vol. 27, No. 13, 1993
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N-viny Ipyrrolidonehtyrene:
x
y
y
where x =
x
Table I. Saturated Concentrations of Solutes in Aqueous N-VinylpyrrolidonelStyreneSolutions
x
x
y
x
@ 0
Flgure 1. Structure of copolymer, Mvinylpyrrolidone/styrene (NVPS). A random sequential arrangement of ring structures x and y on the copolymer chain with x to y weight ratio of 60:40 is depicted.
obtained from Sigma Chemical Co. These chemicals were used without further purification. Partitioning Experiments. Since organics tend to adsorb onto solid surfaces such as the containers used in concentration measurements, careful handling techniques must be employed. Glass flasks (25 mL) were filled with aqueous NVPS copolymer solutions of known concentration and about 8 g of either solid naphthalene or phenanthrene. Other 25-mL glass flasks were filled with aqueous NVPS solution and about 10 mL of liquid toluene. Very low headspace was allowed in the flasks (
