Separation of Volatile Organic Compounds from Aqueous Solutions by Pervaporation Using S-B -S Block Copolymer Membranes BINAY K. DUTTA † AND SUBHAS K. SIKDAR* Sustainable Technology Division, National Risk Management Research Laboratory, U.S. Environm ental Protection Agency, Cincinnati, Ohio 45268 Composite membranes of a block copolymer of styrene and butadiene (S-B -S) were cast on highly porous, hydrophobic thin films of PTFE and used for the separation and recovery of volatile organic compounds (VOCs) from aqueous solutions by pervaporation. Trichloroethane, trichloroethylene, and toluene were the VOCs selected for testing the efficacy of these membranes. An analysis of the pervaporation data showed that the liquid film boundary layer offered the main mass transfer resistance to permeation. The separation factor for the VOCs was as high as 5000 at near-ambient temperatures but decreased substantially at higher temperatures. The water flux was practically independent of the solute concentration. But it increased more rapidly with an increase in temperature as compared to the organic flux, thereby reducing the separation factor. Also, the separation of a multicomponent mixture from the aqueous feed could be predicted well from single-component data. Introduction The need to remove, separate, and recover volatile organic compounds (VOCs) from contaminated media, such as air or water, has steadily grown in importance because of the potential of these compounds to adversely affect human health and the environment. Chlorinated hydrocarbons, as a group, constitute the greatest concern in this regard. Air stripping (1) or adsorption coupled with such destruction methodsascombustion and oxidation technologiesare good optionswhen theyaretheonlyeconomictreatmentmethods available. In these methods, the bulk of the contaminated stream snot just the contaminants sis subjected to the treatmentmethod.Forinstance,in combustingcontaminants in air from an air stripping operation, the entire air is heated to the combustion temperature. In situations where the recoveryofthe VOCsiseconomicallydesirable ora lowenergy approach is needed, the pervaporation process may be a viable alternative with significant advantages (2). This membraneprocessisoperated atnear-ambienttemperatures and pressures and offers very large VOC separation factors with respect to water for low VOC concentrations in a liquid stream. Two principal factors enable a high degree of separation ofnonpolar and weaklypolar light organics byhydrophobic membranesfrom VOC-bearingaqueoussolutions: partition coefficients ofthe VOCs in the membrane and their diffusion coefficients. Successful separation occurs when either the partition coefficients or the diffusion coefficients ofthe VOCs, or both, are significantly superior to those for water. Typical examples of polymers used in membranes for this purpose are poly(dimethylsiloxane) (PDMS), nitrile -butadiene rub- ber, polybutadiene, polyurethane, and ethylene -propylene terpolymer (EPDM) (3). PDMS has long been known as a suitable elastomeric material that can be cast in the form of thin films or tubes and used for separating organics from water bypervaporation,particularlyfor the selective removal of alcohol from an aqueous solution. For instance, Nguyen and Cobe (4) used silicone tubes to explore the removal of halogenated hydrocarbons (e.g., chloroform, bromoform), ethanol, and acetone and found that the halogenated hydrocarbonshad higherpermeabilitiesand selectivitiesthan the latter compounds. Psaume et al. (5) also studied the removaloftrichloroethylene (TCE)from water usingsilicone tubes.These authors observed a significant increase in solute flux through the membrane as the feed Reynolds number increased and concluded that the liquid-phase diffusion resistance largelydetermined the solute flux.Lipskiand Cote (6)further studied the above system usingtwo configurations of feed flowsthe axial flow mode (lumen-side flow) and the transverse flow (shell-side flow) mode. They developed a resistance-in-seriesmodelforboth flowconfigurations.They also reported that the liquid-phase resistance controlled the organic flux. By comparing the economics of the pervapo- ration process with the conventional technologies of air stripping and carbon adsorption, these authors established that pervaporation is an economically competitive technol- ogy.Watson and Pyne (7)analyzed the effectsofdownstream pressure on solute selectivityand claimed that sorption rather than diffusivity had more influence on the separation. For removing organics from water, Blume et al. (8) tested the efficacy of two spiral-wound membrane modules: a two- layer composite ofPDMScoated on a microporous polyester fabric support and a three-layer composite consistingoftwo coatings, one of PDMS and the other of a polyolefin. Compared to a separation factor of 10 for ethanol removal [(see, for example, Takegami et al. (9)], 50-400-fold enrich- ment could be achieved for nonpolar organics such as chloroform or trichloroethane. Raghunath and Hwang (10) used a tubular membrane module fabricated from silicone rubber tubing and measured the permeation of benzene, chlorobenzene, and toluene in the laminar regime of feed flow. They calculated the liquid-phase resistance by solving the Greatz problem and found it to contribute significantly toward the totalmass transfer resistance.Similar conclusions were also arrived at bythe same authors (11)from the results of their work with flat silicone rubber sheets of different thickness. Viswanathan et al. (12) studied the separation of 1,1,1-trichloroethane and its mixture with trichloroethylene in water using a membrane module made ofsilicone tubing that was operated in the sweeping gas pervaporation mode. They observed that TCE permeated faster than TCA. They also observed some fluxcoupling.For instance,an increased concentration of one of the components had a negative influence on the flux of the other. Ji et al. (13) studied pervaporation of a mixture of TCA, toluene, and methylene chloride using membranes made of PDMS, polyether block polyimide (PEBA), polyurethane (PUR), and a silicone polycarbonate copolymer (SPC). They found no coupling of the fluxes.Aresistance-in-series modelwas used to interpret thedata.Theyproposedasimplebutusefulmodeltoaccount *Correspondingauthore-mail: Sikdar.subhas@epamail.epa.gov. † Present address: Department ofChemicalEngineering,Calcutta University, Calcutta 700 009, India; e-mail: bkdutta@cucc.ernet.in. Environ. Sci. Technol. 1999, 33, 1709-1716 10.1021/es980689w CCC: $18.00  1999 American Chemical Society VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1709 Published on Web 04/09/1999