Environ. Scl. Technol. W82, 16, 448-453
Destruction of Pollutants in Water with Ozone in Combination with Ultraviolet Radiation. 1, General Principles and Oxidation of Tetrachloroethylene Gary R. Peyton,? Francis Y. Hung,$ Jlmmle L. Burleson,§and Wllllam H. Glaze”
Department of Chemistry and Institute of Applied Sciences, North Texas State University, Denton, Texas 76203 Oxidation of organic micropollutants in water is significantly faster with ozone in combination with ultraviolet radiation than one would predict on the basis of the individual processes involved. A formalism for the analysis of 03/UV kinetics is presented in which substrate decay is represented as a linear combination of terms representing purging, ozonation, photolysis, and photolytic ozonation (03/UV). For the substrate tetrachloroethylene (TCE) the process is overall first order in TCE. With a continuously sparged stirred tank reactor, times for the elimination of 63% of the substrate (rl) have values of 100, 26,20, and 7 min for purging, ozonation only, photolysis only, and photolytic ozonation, respectively. Significant retardation of 03/UV kinetics is observed in a lake-water matrix as opposed to purified water, possibly due to a radical intermediate involved in the 03/UV process.
Introduction This paper is the first of a series that describes an investigation of the ozone-ultraviolet radiation (03/UV) process for the treatment of water and hazardous wastes (1). The 03/UV process, which involves the simultaneous transfer of ozone and UV photons into water for the purpose of oxidizing micropollutants, was developed by Prengle and co-workers (2-5)and has since been applied by other groups (6-8). While promising, the 03/UV process is as yet unoptimized, poorly understood, and largely untried. At fiist glance the process appears to be entirely too energy intensive and expensive to be competitive with alternative treatment processes. In the final analysis that may be true; but for the present, one should not summarily dismiss 03/UV or any of the other “catalytic oxidation” techniques. These processes, which include 03/UV, H202/UV(9, IO),and 03/H202(11)metal-catalyzed ozonations (12) and peroxidations (13), may represent viable alternatives to granular activated-carbon adsorption for the destruction of toxic organic substances. The present work describes a laboratory scale investigation of the application of the 03/UV process for destruction of several organic micropollutants in water. In this particular paper the procedures used in the study will be described, and some general aspects of the 03/UV process discussed. The kinetic analysis procedure will be illustrated for one micropollutant of interest, tetrachloroethylene. Future papers will describe similar studies with other organohalides and trihalomethane precursors. Photolytic Ozonation, the 03/UV Process. Ozone in the gas phase and in solution absorbs ultraviolet radiation with a maximum at 254 nm (14). In the water-rich gas phase, the process involves dissociation into an oxygen molecule and an oxygen atom in the lD state (15,16).The latter may react with a water molecule to produce two *To whom correspondence should be addressed at Graduate Program in Environmental Sciences, The University of Texas at Dallas, Richardson, TX 75080. ‘Present address: SumX Corporation, 1300 E. Braker Ln., Austin, TX 78763. Raba-Kistner Consultants, Inc., 10526 Gulfdale, San Antonio, TX 78216. 8 Zoecon Industries, 12200 Denton Drive, Dallas, TX 75234.
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Enviran. Sci. Technoi., Voi. 16, No. 8, 1982
hydroxyl radicals (eq 1and 2). In the aqueous phase, the
radicals apparently combine in a solvent cage to yield hydrogen peroxide (17),which may also photolyze (9,10) or combine with ozone through a complex radical mechanism ( I 7). In the presence of other substrates the radicals may react by a variety of reaction pathways. Figure 1 shows that the net result may be quite dramatic, in this case a very rapid oxidation of chloroform. As Prengle has the 03/UV system may also involve direct noted (2-5,18), excitation of the substrate, with subsequent reaction, or direct ozonation. However, this work will show a synergism betwen ozone and UV photon transfer that cannot be accounted for on the basis of an additive effect. The magnitude of this synergism varies from substrate to substrate, and the process is subject to substantial matrix effects. In future papers we shall report on these matrix effects in more detail and on a pilot scale application of the process for the purpose of water purification.
Experimental Section Apparatus and Materials. The reactor used in these studies was a quartz bell jar, fitted with an insertable borosilicate sparger and baffle assembly, as shown in Figure 2. The borosilicate top was fitted with sampling ports and a stirring motor. The contents were stirred by a six-bladed paddle-type impeller, the speed of which was measured by a phototachometer. Stainless steel fittings and PTFE tubing were used for all connections. Ozone was generated from oxygen by a Grace Model LG-2-L2 ozone generator (W. R. Grace & Co., Columbia, MD), and the ozone concentration in the gas stream was measured at 254 nm by using an ISCO Model UA-5 absorbance monitor. Pure oxygen was used in the reference side of the monitor. A Teflon and stainless steel six-port valve enabled the operator to select either the inlet or outlet gas stream for ozone monitoring (Figure 3). For ozone/UV experiments the quartz reactor could be lowered into a Rayonet photochemical chamber in which a maximum of 16 low-pressure mercury lamps could be used. The lamps emit principally at 2537 A and are rated at 14 W apiece. In the course of this work, either one, two or four lamps were used. Substrate disappearance was followed gas chromatographically by using either a Tracor 560 or a HewlettPackard 5840 instrument, each fitted with an electroncapture detector. Analysis of tetrachloroethylene was by direct aqueous injection onto a 6 f t X 2 mm i.d. Chromosorb 101 (100/120 mesh) column at 160 OC. Ozone levels in the liquid were measured iodimetrically by the borate-buffered potassium iodide (BKI) method of Flamm (19). Purified water was prepared by running commercial deionized distilled water through a 1-in. diameter column of Filtrasorb 400 activated carbon (22 in.), XAD-4 macroreticular resin (Rohm & Hass) (12 in.), Bondapak C 18/corasil (1in,), and a 0.45-pm filter (Millipore). This water usually still contained small amounts of organic impurities (see discussion section). Thus, for all kinetic
0013-336X/82/0916-0448$01.25/0
0 1982 American Chemical Society
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Figure 1. Example of the effect of UV Irradiation on the oxidation of chlorofon In purified water matrix: (0)pH 7, N2purge at 25 mL/mln; (0) pH 7, O3dose rate 1.3 mg/(L min); (A)pH 6.5, 0, 1.4 mg/(L min) with 0.42 W/L 254-nm UV irradiation (transferred). STIRRER
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Flgure 2. Schematic dlagram of quartz bell jar reactor for ozone/UV studies.
runs in purified water the water was photolytidy oxidized for ca. 1h before use. Lake water was obtained from Lake Lewisville, TX, filtered twice through Whatman no. 1filter paper, and used immediately. The water was checked for the presence of substrates that would interfere with chromatographic analysis of the substrates. Standard water quality analysis of the lake water gave the following: TOC (4.3 mg/L); COD (10 mg/L); pH 8.1; NH3/NH4+as N (120 pg/L); C032-/HC0< (1mmol/L); TDS (120 mg/L); suspended solids (
