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Major, D. W.; Mayfield, C. I.; Barker, J. F. Ground Water 1988, 26, 8-14.
Barker, J. F.; Patrick, G. C.; Major, D. Ground Water Monit. Rev. 1987, 7, 64-71. Vogel, T. M.; Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1987, 21, 722-136. Barrio-Lage, G.; Parsons, F. Z.; Nassam, R. S.; Lorenzo, P. A. Enuiron. Sci. Technol. 1986, 20, 96-99. Wolf, K.; Holland, R.; Rajaratnam, A. J. Hazard. Mater. 1987,15, 163-184. Roy, W. R.; Griffin, R. A. Enuiron. Geol. Water Sci. 1985, 7, 241-247. Lawidsen, S.,Du Pont Canada, Maitland, Ontario, personal communication, August 1988. Srinivasan, P.; Patwardhan, V. R.; Devotta, S.; Watson, F. A. Chem. Eng. Res. Des. 1984,62, 266-268. Monig, J.; Asmus, K.-D. J . Chem. SOC.,Perkin Trans. 2 1984,-12, 2057-2063. Poyer, J. L. Oklahoma Medical Research Foundation, Oklahoma City, presentation at the 2nd International Symposium on Spin Trapping and Aminoxyl Radical Chemistry, Guelph, Ontario, Canada, July 2-7, 1989. Lew, D.; Bouwer, E. J., Johns Hopkins University, Baltimore, MD, personal communication, 1989.
(38) Lesage, S.; Riemann, P. G.; McBride, R. A. In Proceedings of The Ontario Ministry of the Environment Technology Transfer Conference;Toronto, Ontario, Canada, 1989; Vol. 2, pp 88-97. (39) Esau, R. R.; Chesterman, D. J. In Hazardous Waste Site Management: Water Quality Issues; National Academy Press: Washington, DC, 1988; pp 120-136. (40) Mahurin, R. G.; Bernstein, R. L. Environ. Res. 1988, 45, 101-107. (41) Longstaff, E.; Robinson, M.; Bradbrook, C.; Styles, J. A.; Purchase, I. F. H. Toxicol. Appl. Pharmacol. 1984, 72, 15-31. (42) Sax, N. I. Dangerous Properties of Industrial Materials, 6th ed.; Van Nostrand Reinhold Co.: New York, 1984. (43) Schwarzenbach,R.; Giger, W.; Schaffner, C.; Wanner, 0. Environ. Sci. Technol. 1985, 19, 322-327. (44) Vogel, T. M.; Criddle, C. S.; McCarty, P. L. Enuiron. Sci. Technol. 1987,21, 722-736. (45) Headley, J. V. Biomed. Enuiron. Mass Spectrom. 1987,14, 275-280.
Received for review March 8,1989. Revised manuscript received August 30, 1989. Accepted December 11, 1989.
Acid-Catalyzed Oxidation of 2,4-Dichlorophenoxyacetic Acid by Ammonium Nitrate in Aqueous Solution David D. Leavtttt and Martin A. Abraham*
Department of Chemical Engineering, University of Tulsa, Tulsa, Oklahoma 74 104
2,4-Dichlorophenoxyaceticacid (2,4-D) was oxidized to C 0 2and water by homogeneous, liquid-phase reaction with ammonium nitrate a t temperatures between 250 and 450 O F and pressures below 100 psi. N2 and NzO were produced from the thermal decomposition of the ammonium nitrate oxidant. An unexpected maximum in conversion was observed at an intermediate reaction temperature, which was consistent with rapid thermal decomposition of the NHINOBoxidant. Postulated reaction pathways consisting of simultaneous oxidation of 2,4-D and decomposition of the oxidant allowed estimation of kinetic constants from best-fit analysis of the data. The proposed reaction model provided a mathematical description of 2,4-D conversion, which allowed extrapolation of the results to reaction conditions and reactor configurations that were not experimentally investigated. Introduction
Oxidation of organic materials to C 0 2 and HzO provides a suitable (although certainly not the only) means of reducing the toxicity of many waste products. These wastes may be present in water, in solids, or in air, typically as complicated mixtures of many components within a solvent stream. Depending on the concentration and the phase of the undesired organic components, a variety of destruction processes may be used, including thermal treatment techniques such as incineration or wet air oxidation ( I ) , pure chemical processes such as ozonation and chlorination (2), and biological oxidation processes ( 3 ) . Physical treatment may also be accomplished to adjust the concentration of the pollutant prior to further processing. Particularly, in cases involving cleanup of contaminated Current address: Allied-Signal, P.O. Box 580970, Tulsa, OK 74158. 566
Environ. Sci. Technol., Vol.
24, No. 4, 1990
solids, physical removal of the organic contaminant from the solid is often accomplished through solvent extraction prior to destructive oxidation of the waste ( 4 , 5 ) through one of the treatment methods mentioned above. Regardless of the specific wastes within a particular waste stream or the specific oxidation technique that is being considered, an understanding of the oxidation reaction chemistry is beneficial for process development. In addition, knowledge of the reaction pathways and kinetics permits one to predict, a priori, the conversion of specific wastes within a waste stream. Through kinetic measurements, it is possible to extend experimental results beyond the specific conditions that have been investigated and optimize specific destruction processes. Since organic compounds may be grouped into broad classes of materials that undergo similar reactions, this fundamental kinetic information may be generally applied within these chemical classes. Unfortunately, the interaction between the many and varied materials present within an industrial waste stream hinders interpretation of results from experiments with actual wastes. For this reason, we chose to accomplish experiments with well-defined systems, wherein known concentrations of selected organic compounds were oxidized. In the current study, pure compounds were used (in place of actual industrial waste) to provide preliminary information on the feasibility of oxidizing chlorinated organic wastes within an aqueous acid. Although the current experiments with single compounds did not approach the complexity of an actual waste stream, fundamental reaction information necessary for the interpretation of future experiments involving multiple components was obtained. The current study is based on technology developed for the purification of crude phosphoric acid. In that operation, high molecular weight organic components are con-
0013-936X/90/0924-0566$02.50/0
0 1990 American Chemical Society
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verted to C02and H,O through oxidation by ammonium nitrate in concentrated phosphoric acid at temperatures above 250 O F (6). For the purposes of waste treatment, the aqueous waste stream would be mixed with the oxidant-containing acid solution and heated to the desired temperature. The inorganic acid permits solution of reasonably high concentrations of the undesirable organic material within the aqueous stream and, more importantly, acts as a catalyst for the reaction. Since all reactants (organic contaminant, oxidant, and catalyst) are dissolved within a single liquid phase, mass-transfer limitations are minimized and the rate of conversion is maximized. Further catalytic enhancement, through the addition of heterogeneous catalysts, may also be achieved. In situ production of such heterogeneous catalysts may occur through the interaction of the inorganic acid with metallic species that may be present in an industrial waste stream. This treatment could potentially be accomplished in a continuous-stirred tank reactor, as indicated in Figure 1. The gases that are produced as products of the oxidation should be suitable for discharge to the atmosphere, after some additional gas cleaning. Following reaction for the desired residence time, the effluent stream would be neutralized in a second unit, metal-containing species separated through standard techniques, and the liquid water discharged. Posttreatment steps might be required depending upon the purity of the water that is obtained. Although one pmible process flow sheet is shown in Figure 1, the current work was aimed at obtaining preliminary reaction rate information necessary for the design of the reactor vessel; thus, no analysis for pre- or pmttreatment steps was completed. In our previous report (3,we demonstrated the feasibility of oxidizing several pure-component organic species, including atrazine, 2,4-dichlorophenoxyaceticacid (2,4-D), and biphenyl by liquid ammonium nitrate dissolved in highly concentrated phosphoric acid. Destruction efficiencies greater than 95% were achieved under a wide range of conditions of concentration of the organic species, temperature, reaction pressure, and concentration of ammonium nitrate. In addition, the destruction efficiency was increased to greater than 99% through the addition of MnSO, as a heterogeneous oxidation catalyst. This increase in destruction efficiency through the addition of a metal-containingcatalyst gave further evidence that this process may be suitable for detoxification of waste streams containing both organic components and heavy metals. In the current work, we report on our investigation of the reaction pathways and kinetics for the oxidation of 2,4-D by ammonium nitrate in concentrated phosphoric acid. This development of the reaction pathways and
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318' cap
Flgure 2. Description of microbatch reactor constructed from swagelok tubing parts. All parts 316 stainless steel and sizes given as nominal outer diameter.
kinetics allows one to predict the effect of temperature, concentration of organic species, concentration of oxidant, and reaction time on the overall destruction efficiency. Further, since the reaction pathways remain essentially unchanged, these results are applicable to a large class of organic species that may undergo homogeneous oxidation by ammonium nitrate; only the kinetic parameters vary. Thus, this study provides insight into a chemical process for the destruction of organic materials that may be present in an aqueous stream and provides preliminary information required for process design considerations. Experimental Section Reactions were accomplished in separate batch reactors constructed of standard 316 stainless steel swagelok tubing parts, as described in Figure 2. A gas-sampling line consisting of 1/16-in. tubing and a Hoke ball valve was included to permit analysis of gaseous products. The internal volume of each reactor was estimated at 1.26 mL, based on part dimensions provided by the manufacturer. Reactions were run with the gas-sampling valve closed to achieve constant-volume conditions. Although 316 stainless steel may be corroded by concentrated acid to form measurable quantities of catalytic metals, all reactions were accomplished in identical reactors and were thus subject to identical reaction conditions. A typical reaction procedure was as follows. The desired amount of ammonium nitrate (Allied Chemical) oxidant and 2,4-dichlorophenoxyaceticacid (Aldrich) reactant (2,4-D) was mixed with 0.2 mL of polyphosphoric acid (Albright & Wilson, Ltd.) in the reactor. All materials were commercially available and used as received. The reactor was then sealed and pressurized with helium (to simplify the gaseous products analysis) to approximately 60 psi, which provided an initial reaction pressure, assuming ideal gas behavior, below 100 psi for all runs. The reactor valve was closed and the reactor disconnected from the helium source and placed in a holder within the fluidized sand bath, which had been preheated to the desired temperature. The reactor was agitated by a mechanical agitation arm, which operated at a preselected rate. After the desired reaction time, the reactor was removed from the sand bath and placed in cold water to quench the reaction. Experimental conditions are summarized in Table I. Gaseous products were analyzed by gas chromatograph (GC). A Hewlett-Packard 5890A GC equipped with gassampling valve, 30-ft Hayesep D packed column, and thermal conductivity detector was utilized in temperaEnviron. Sci. Technoi.. VoI. 24. NO. 4. 1990 567
1.o
Table I. Experimental Conditions
0.9
2,4-D concentration NH,N03 concentration temperature reaction time reaction pressure
5000 ppm = 0.0452 mol/L 0.226 mol/L 250-450 "F 0-15 min
