Vacuum Photocatalytic Oxidation of Trichloroethylene RAO ANNAPRAGADA, ROBERT LEET, RAJNISH CHANGRANI, AND GREGORY B. RAUPP* Departm ent of Chem ical, Bio & Materials Engineering, Arizona State University, Tem pe, Arizona 85287-6006 The combination of physical removal methods such as soil vapor vacuum extraction or vacuum air stripping with gas- solid heterogeneous photocatalytic oxidation of the off- gases produced may be an effective remediation technology for a variety of soil and water contamination problems, particularly those involving chlorinated ethylenes. To test the hypothesis that reduced pressure operation of the photocatalytic unit could enhance reactor performance, a bench-scale annular photocatalytic reactor operating in the vacuum range was designed, built, and evaluated. The reactor inner wall was coated with sol -gel-derived titania to provide a uniform, adherent, photocatalytically active thin film. Photocatalytic oxidation of trichloroethylene (TCE) in humid airstreams was employed as a model chemistry. Reduction of the operating pressure at fixed feed conditions and molar feed rate significantly enhanced PCO performance as measured by the observed TCEconversion. Higher conversions were obtained in spite of a reduction in the residence time accompanying the lower pressure operation. The greatest enhancements in the TCEdestruction efficiency occurred for low TCE feed concentrations and high water vapor levels. The performance enhancement appears to be linked to reduction in the absolute water vapor concentration and competition between TCEand water vapor for adsorption sites on the catalyst. Introduction Gas-solid heterogeneousphotocatalyticoxidation (PCO)has been shown to be effective in its abilityto oxidize dilute volatile chlorinated organics (1-3) as well as other volatile organic compounds (VOCs) (4-8)in humid airstreams. Acandidate application for this technology is incorporation of a PCO reactor downstream of an air stripper at a groundwater remediation pump-and-treat site to completely oxidize the organics in the air stripper off-gases (9). Unfortunately, several factors make the air stripper application less than ideal. First,airstrippersgenerallyemploylarge air:waterflow ratios, leading to a relatively high air flow to treat for the volume of water remediated. Second, a high level of water vapor in the air significantly inhibits the photocatalytic oxidation rate for several classes of VOCs (1, 2, 4, 6, 8), including the chlorinated ethylenes. This rate inhibition forces the design of larger, and hence more costly, PCO reactors to achieve the desired VOCdestruction and removal efficiency (DRE). In air stripping of VOC-contaminated water streams, reduction of the stripper operating pressure below atmo- spheric can significantly reduce the air:water ratio while achieving the same VOC removal achieved in atmospheric pressure operation (10). This new technology, known as vacuum air stripping, offers a significant advantage for PCO, or any air abatement technique for that matter, since a lower volume ofair needs to be treated. We are therefore interested in exploring the technical viability of vacuum photocatalytic oxidation for direct integration with vacuum air stripping. Vacuum PCO could also be employed in association with soil vapor vacuum extraction in which the contaminants in the soil are volatilized by means of convective forces generated by an applied vacuum (11). One can imagine a number of other applications in which vacuum capability would be advantageous. For example, consider a VOC control system incorporatingphysicaladsorption bedsand downstream PCO. The adsorption beds (e.g., activated carbon) capture VOCs duringcyclicalorintermittent operation ofa chemicalprocess emitting VOCs. During thermal regeneration of the beds, a vacuum is applied and the off-gases are treated in the PCO unit. Through cyclical operation/regeneration of several adsorption beds “load-leveling” is achieved, allowing design of a PCO unit for moderate to low VOC levels rather than the occasional high VOC levels emitted by the process. A bench-scale reactor was designed, built, and tested in our laboratoryto investigate the effectivenessofvacuum PCO. TCEwas selected as the test contaminant because it is a water and soilpollutant ofgreat environmentalconcern and because it has been studied extensively in gas-phase photocatalytic reactors previously (1-3, 9, 12-14). In addition, it is known that TCE converts fairly rapidly, thus providing a rigorous test of operation at reduced pressures. Experimental Section Conversion Apparatus and Data Collection Procedures. Dilute TCE in water is oxidized according to the following stoichiometric reaction as proposed byPruden and Ollis (15): In air, the reaction requires the simultaneous presence of TCE, oxygen, and water vapor. Under dry conditions, alternative stoichiometries control, producing such incom- plete oxidation products as dichloroacetyl chloride and phosgene (12). Berman and Dong (16) have shown that operation under reasonable humidity levels (e.g., 50% RH) and with sufficient residence time leads to essentially the complete oxidation stoichiometry listed above. The flow system is illustrated schematically in Figure 1. Trichloroethylene (495 ppm in nitrogen, Alphagaz), ultra- high-purity nitrogen (99.999%, Liquid Air) and oxygen (99.999% Liquid Air) were supplied from gas cylinders. All flows were controlled by mass flow controllers (Tylan). The relative humidity of the feed gas stream was controlled by varying the amount of nitrogen passing through a water bubbler. The relative humidity of the gas stream entering the reactor was measured usinga calibrated relative humidity meter (Vaisala). The concentration of TCE was varied by diluting it with nitrogen and oxygen. Nitrogen and oxygen flows were always maintained in the ratio of 79:21. Most of the fittings in the vacuum system were Cajon VCR fittings; the tubing was 316 stainless steel. All runs were conducted at ambient temperature (ca. 27 °C.). The reactor pressure wasmeasuredusinga0-50psia pressure transducer (Omega). The vacuum pump wasa dry(oil-free)type rotaryvane pump (Process Physics, Inc.). This type of pump was employed so that no VOCs would be internally generated from vacuum *Corresponding author telephone (602)965-2828; fax: (602) 965- 0037; e-mail: raupp@asu.edu. Cl 2 C ) CHCl + 3 2 O 2 + H 2 O 9 8 h ν 2CO 2 + 3HCl Environ. Sci. Technol. 1997, 31, 1898-1901 1898 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 7, 1997 S0013-936X(96)00541-X CCC: $14.00  1997 American Chemical Society