Destruction of Decachlorobiphenyl Using ... - ACS Publications

Energy & Fuels 2004, 18, 1257-1265

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Destruction of Decachlorobiphenyl Using Supercritical Water Oxidation Zhen Fang,*,† Sikun Xu,† Ian S. Butler,‡ Richard L. Smith Jr.,§,| and Janusz A. Kozin´ski*,† McGill University, Energy and Environmental Research Group, Department of Mining, Metals and Materials Engineering, 3610 University Street, Wong Building, Room 2290, Montreal, QC, Canada H3A 2B2, Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, QC, Canada H3A 2K6, and Tohoku University, Research Center of Supercritical Fluid Technology, Department of Chemical Engineering, Tohoku University, Aoba-ku Aramaki Aza Aoba-04, Sendai 980-8579, Japan Received February 4, 2004. Revised Manuscript Received May 26, 2004

A microreactor (50-nL), batch reactors (6-mL), and a flow reactor (11.3-mL) were used to study the oxidation of decachlorobiphenyl (10-CB) in supercritical water. In the microreactor experiments, it was found that complete dissolution of 10-CB occurred at excess O2 (>93%) and that both hydrolysis and oxidation contributed to 10-CB destruction. In experiments performed with the batch reactors, at excess O2, 99.2% and 100% 10-CB could be destroyed in SCW without and with Na2CO3, respectively. Addition of the neutralization agent Na2CO3 promoted the destruction rate and reduced reactor corrosion significantly. A reaction mechanism for 10-CB destruction was proposed and this was examined further in flow experiments, where we found that 100% destruction could be achieved.

Introduction Polychlorinated biphenyls (PCBs; C12H10-mClm) are mixtures of synthetic chlorinated aromatic hydrocarbons with the same basic chemical structure and similar physical properties ranging from oily liquids (m ) 1-4, light, oily fluids; m ) 5, heavy, honey-like oils) to waxy solids (m > 5, greases and waxy substances; m ) 10, solid).1,2 Due to their nonflammability, chemical and thermal stability, high boiling point, and low electrical conductivity, PCBs have been used in hundreds of industrial and commercial applications, including electrical equipment (e.g., transformers and capacitors), plasticizers, hydraulics and lubricants, and carbonless copy paper. PCBs were first created in 1881, commercially produced beginning in 1929, and banned in 1979 in the United States, with similar phase-outs in Japan and Canada because of their potential carcinogenic and health effects.1-3 Despite the ban, more than 1.5 billion pounds of PCBs were manufactured in the United States.1 Over 180.4 million pounds are estimated to be in water, sediments, disposal sites, transformers, * Corresponding authors. E-mail: [email protected] (ZF); [email protected] (JAK). † McGill University, Energy and Environmental Research Group, Department of Mining, Metals and Materials Engineering. ‡ Department of Chemistry, McGill University. § Tohoku University. | E-mail: [email protected]. (1) PCB home page at EPA: http://www.epa.gov/opptintr/pcb/. (2) Economic and Policy Analysis Branch, Economic analysis of the final rule to modify reporting of persistent bioaccumulative toxic chemicals under EPCRA section 313, U.S. Environmental Protection Agency, Washington, DC, 1999. (3) Cogliano, J. PCBs: Cancer dose-response assessment and application to environmental mixtures, U.S. Environmental Protection Agency, Washington, DC, Rep. No. EPA/600/P-96/001F, 1996.

and other containers (1986);4 approximately 54.9 million pounds are in storage (1994).2 Therefore, destruction of PCBs is essential for public health and the ecological system. Technologies to destroy PCB wastes include chemical and biological treatment, incineration,5 and supercritical water oxidation (SCWO). Chemical processes are proven technologies, but are used to destroy lower concentrations of PCBs in contaminated oils. Biological treatment is still at an early stage of development and is not commercially available. Incineration, the thermal destruction of organics by combustion at high temperature (900-1300 °C) and often with excess air (as high as 100%), is one of the many techniques for the treatment of organic and hazardous wastes.6,7 However, during the burning of chlorinated wastes, low oxygen concentration in local space due to incomplete miscibility with oxygen and high temperatures can lead to formation of polyaromatic hydrocarbons (PAHs), dioxins, and NOx. These secondary pollutants have adverse health effects such as carcinogenicity and mutagenicity.8 These pollutants (4) Eisler, R. Polychlorinated biphenyl hazards to fish, wildlife, and invertebrates: a synoptic review, U.S. Fish Wildlife Serv. Biol. Rep. 1986, 85 (1.7), 72. (5) Environment Canada, PCB destruction technologies, http:// www.ec.gc.ca/pcb/fs4/eng/pcb41_e.htm, 2003. (6) Tillman, D. A.; Rossi, A. J.; Vick, K. M. Incineration of municipal and hazardous solid wastes; Academic Press: San Diego, California, 1989. (7) Tester, J. W.; Holgate, H. R.; Armellini, F. J.; Webley, P. A.; Killilea, W. R.; Hong, G. T.; Barner, H. E. Supercritical water oxidation technology: Process development and fundamental research. ACS Symp. Ser. 1993, 518, 35-76. (8) Niessen, W. R. Combustion and incineration processes, application in environmental engineering, 2nd ed.; Marcel Dekker: New York, 1995.

10.1021/ef0499630 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004

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caused by incomplete combustion can be totally avoided if wastes are oxidized at low temperatures in a solvent such as supercritical water (SCW; > 374 °C and 22.1 MPa). In the supercritical state, water becomes a weakly polar solvent having the ability to dissolve organics and oxygen and thus to provide homogeneous reaction conditions. In SCW oxidation (SCWO) processes (450 to 600 °C and ca. 30 MPa),7,9-11 organics, water, and oxygen are mixed and react in a homogeneous phase, and then organics (C, H, O) are oxidized completely to CO2 and H2O. Chlorine atoms in the wastes are oxidized to HCl, which can be neutralized and precipitated as salts by adding chemicals such as caustic to the feed.12-14 Many wastes, such as low-chlorinated PCBs, DDT, dioxins, explosives, chemical weapon agents, radioactive wastes, and municipal sludge, have been successfully destroyed using SCWO technology as reported in the review papers.7,9,10,15-18 Commercial SCWO is being practiced at Harlingen, Texas, for processing up to 9.8 dry tons per day of municipal sludge,19 and also in Japan where a pilot plant has been built. However, little work on SCWO of high-chlorinated PCBs (e.g., decachlorobiphenyl) has been reported. In this present work, oxidation of pure decachlorobiphenyl (10-CB; C12Cl10) was chosen for study since it is highly chlorinated and one of the most highly stable PCBs, and can be expected to occur as a practical waste or possibly as a reaction product of more complicated feedstock. The 10CB sample was oxidized in supercritical water to study the phase behavior, destruction rate, reaction mechanism, corrosion of reactors, and effects of neutralization agent (Na2CO3). Experimental Section Solid decachlorobiphenyl powder (10-CB; 100 to 500 µm, 99.0% purity) was used (Supelco-Aldrich, Bellefonte, PA). It has a melting point (Tm) of 302.5 °C and a density of 1200 kg/m3. Neutralizing agent sodium carbonate (Na2CO3) powder (9) Blaney, C. A.; Li, L.; Gloyna, E. F.; Hossain, S. U. Supercritical water oxidation of pulp and paper mill sludge as an alternative to incineration. ACS Symp. Ser. 1995, 608, 444-455. (10) Shaw, R. W.; Dahmen, N. Destruction of toxic organic materials using supercritical water oxidation: current state of the technology. NATO Science Series, Series E: Applied Sciences 2000, 366, 425-437. (11) Kritzer, P.; Dinjus, E. An assessment of supercritical water oxidation (SCWO); Existing problems, possible solutions and new reactor concepts. Chem. Eng. J. 2001, 83, 207-214. (12) Muthukumaran, P.; Gupta, R. B. Sodium-carbonate-assisted supercritical water oxidation of chlorinated waste. Ind. Eng. Chem. Res. 2000, 39 (12), 4555-4563. (13) Ross, D. E.; Jayaweera, I.; Leif, R. N. Method for hot and supercritical water oxidation of material with addition of specific reactants. U.S. Patent 5,837,149, 1998. (14) Mitton, D. B.; Yoon, J. H.; Cline, J. A.; Kim, H. S.; Eliaz, N.; Latanision, R. M. Corrosion behavior of nickel-based alloys in supercritical water oxidation systems. Ind. Eng. Chem. Res. 2000, 39 (12), 4689-4696. (15) Schmieder, H.; Abeln, J. Review - Supercritical water oxidation: state of art. Chem. Eng. Technol. 1999, 22, 903-908. (16) Smith, K. A.; Harris, J. G.; Howard, J. B.; Tester, J. W.; Griffith, P.; Herzog, H. J.; Peters, W. A.; Latanision, R. Supercritical water oxidation: principles and prospects. Official Proceedings - International Water Conference, 56th 1995, 468-478. (17) Thomason, T. B.; Hong, G. T.; Swallow, K. C.; Killilea, W. R.; Coden, I. The MODAR supercritical water oxidation process. Innovative Hazard. Waste Treat. Technol. Ser. 1990, 1, 31-42. (18) Ding, Z. Y.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic oxidation in supercritical water. Ind. Eng. Chem. Res. 1996, 35, 32573279. (19) Griffith, J. W.; Raymond, D. H. The first commercial supercritical water oxidation sludge processing plant. Waste Management 2002, 22, 453-459.

Fang et al. (99.8% purity) was obtained from Sigma-Aldrich (Seelze, Deutschland). Hydrogen peroxide (49 wt %) was used as the oxidant and was obtained from Fisher Scientific (certified A.C.S. grade, NJ). Low concentrations of H2O2 solution with Na2CO3 (used in batch and flow reactors) were made by diluting the 49-wt % H2O2 with double-distilled water and adding solid Na2CO3. An HPLC-grade methanol (Fisher, NJ) was used as a solvent for making a solution of {10-CB + H2O2 + CH3OH} for the experiments in a flow reactor. The concentrations of H2O2 and Na2CO3 needed were calculated according to eqs 1 and 2.

C12Cl10 + 19H2O2 (H2O + 1/2O2) f 12CO2 + 14H2O + 10HCl (1) 10HCl + 5Na2CO3 f 10NaClV + 5CO2 + 5H2O

(2)

The theoretical oxygen (mol %) was the stoichiometric oxygen necessary for complete oxidation of 10-CB according to eq 1. Similarly, the theoretical Na2CO3 (mol %) was determined according to the neutralization reaction eq 2. Three types of reactors were used: (i) a microreactor (50nL), hydrothermal diamond anvil cell (DAC) for visual observation of the phase changes; (ii) larger batch reactors (6-mL) to determine decomposition products, 10-CB destruction rates and reactor corrosion characteristics; and (iii) a flow reactor (11.3-mL) to evaluate practical 10-CB destruction rates. Microreactor, Hydrothermal Diamond Anvil Cell. A microreactor, hydrothermal diamond anvil (DAC)20 was used for the study of the phase behavior of 10-CB in supercritical water. The reaction chamber (50 nL; 500 µm i.d., 250 µm thickness) was sealed by compression of two opposing anvils made of diamond and heated by two electric micro-heaters. The temperature of the two diamond anvils was measured in three locations and recorded by a data acquisition unit (Strawberry Tree, model DS-12-8-TC, Sunnyvale, CA). The initial density of water was adjusted by changing the size of air bubbles. It was calculated by visually knowing the temperature (Th) at which air bubbles disappeared during heating described by Bassett et al.20 When the sample and water were loaded into the DAC chamber, air bubbles would appear. Heating the chamber caused the liquid to expand and the air bubbles to shrink until they disappeared, at which point the chamber would be filled with the expanded liquid at Th. It was assumed that the bulk density of the water is that of the liquid water along the L-V curve at Th. The initial density can also be known by estimating air bubble size with digital image analysis. Little change of the chamber volume was observed during the reaction process.20 Usually, only the initial density of water was provided in batch experiments without pressure data.21-25 But, in this work, pressure was calculated as a reference with an equation of state of water (EOSW)26 by knowing its density and temperature, assuming the pressure (20) Bassett, W. A.; Shen, A. H.; Bucknum, M.; Chou, I.-M. A new diamond anvil cell for hydrothermal studies to 2.5 GPa and from -190 to 1200 °C. Rev. Sci. Instrum. 1993, 64 (8), 2340-2345. (21) Townsend, S. H.; Abraham, M. A.; Huppert, G. L.; Klein, M. T.; Paspek, S. C. Solvent effects during reactions in supercritical water. Ind. Eng. Chem. Res. 1988, 27, 143-149. (22) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Lowtemperature catalytic gasification of lignin and cellulose with a ruthenium catalyst in supercritical water. Energy Fuels 2004, 18, 327333. (23) Sato, T.; Sekiguchi G.; Adschiri T.; Arai, K. Control of reversible reactions in supercritical water: i. Alkylations. AIChE J. 2004, 50 (3), 665-672. (24) Sato, T.; Osada, M.; Watanabe, M.; Shirai, M.; Arai, K. Gasification of alkylphenols with supported noble metal catalysts in supercritical water. Ind. Eng. Chem. Res. 2003, 42, 4277-4282. (25) Jin, F. M.; Zhou, Z. Y.; Enomoto, H.; Moriya, T.; Higashijima, H. Conversion mechanism of cellulosic biomass to lactic acid in subcritical water and acid-base catalytic effect of subcritical water. Chem. Lett. 2004, 33 (2), 126-127.

Destruction of Decachlorobiphenyl was contributed only by water. Bassett et al.20 have compared the pressures by the calculation with the P-T boundary of the R-β transition in quartz. The difference was less than 4% for the water and quartz system. After loading the sample, the reaction chamber was heated and observations were made at 110× magnification with a stereomicroscope (Olympus SZ11), with the images being recorded by a Panasonic 3 CCD camera (AW-E300) and a video cassette recorder (Panasonic AG-5720). Digital imaging analysis of the sample area was calculated by using digital image analysis software (Scion Image, Frederick, MD). After the reactions were complete for each experiment, the residues deposited on the diamond faces were analyzed by FT-IR microscopy (UMA 500, Bio-Rad, Cambridge, MA). The decomposition of 10-CB was not monitored in-situ by FT-IR because of strong IR absorption by diamonds and water. The details of the experimental setup and procedures are described in the previously published works.27-30 Batch Reactors. Tubular 316 stainless steel (316-SS) batch reactors (6 mL; length ) 105 mm, o.d. ) 1/2 in. ) 12.7 mm) were used to study the oxidation of 10-CB in SCW. Both temperature and pressure were measured with a pressure transducer fitted with a J-type thermocouple (Dynisco E242). Pressure was also calculated by knowing density of water {(weight of water added)/(volume of batch reactor) ≈ 1/6 g/mL ) 167 kg/m3}, and temperature was measured as described above. In the experiments, approximately 10-mg of 10-CB, and 1-mL of H2O2 (2.5-4.2 wt %) were loaded into the reactor. After sealing and connection to the data acquisition system, the reactor was submerged in a fluidized sand bath (Omega FSB3). The reactor was heated at a rate of 3.5 °C/s to 450 °C and then held at this temperature for 20 min. After a given period, the reactor was quenched in cold water and the reaction mixture was emptied into a flask. The reactor was washed with water and benzene and emptied into the same flask. The solution in the flask was filtered through a membrane filter to yield a solid phase (ash) that was dried at room temperature. The benzene phase was decanted and the aqueous phase was obtained. The aqueous phase was analyzed using ion chromatography (Dionex DX-100) to determine anion concentrations of HCOO-, CH3COO-, and Cl-. For each ion, three concentrationssblank, 20-ppm and 50-ppmsprepared from standards were used for calibration. The benzene phase was analyzed by gas chromatography-mass spectrometry (GC-MS; GCQ Packages: Polaris MS, Trace 2000 GC, ThermoQuest, Austin, TX). A calibration coefficient for 10-CB analysis was obtained using 500-ppm 10-CB standard and tetradecane as internal standard. Other products were identified using library data without standard material calibration. Elemental compositions of ash (Cl, Fe, O, Cr, Si, Ni) were analyzed by energy-dispersive X-ray spectrometry (EDX, EDAX Phoenix System, NJ). The ash morphology was determined by a scanning electron microscope (SEM; JEOL 840A, Tokyo, Japan). EDX spectra were also obtained for selected single particles. Flow Reactor. Tubular flow reactors are widely applied to study reaction mechanism and kinetics for short reaction (26) Wagner, W.; Pruss, A. The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use. J. Phys. Chem. Ref. Data 2002, 31 (2), 387-535. (27) Fang, Z.; Smith, R. L., Jr.; Inomata, H.; Arai, K. Phase behavior and reaction of poly(ethylene terephthalate)-water systems at pressures up to 173 MPa and temperatures up to 490 °C. J. Supercrit. Fluids 1999, 15, 229-243. (28) Fang, Z.; Smith, R. L., Jr.; Inomata, H.; Arai, K. Phase behavior and reaction of polyethylene in supercritical water at pressures up to 2.6 GPa and temperatures up to 670 °C. J. Supercrit. Fluids 2000, 16, 207-216. (29) Smith, R. L., Jr.; Fang, Z.; Inomata, H.; Arai, K. Phase behavior and reaction of Nylon 6/6 in water at high temperatures and pressures. J. Appl. Polym. Sci. 2000, 76 (7), 1062-1073. (30) Fang, Z.; Kozinski, J. A. Phase behavior and combustion of hydrocarbon-contaminated sludge in supercritical water at pressures up to 822 MPa and temperatures up to 535 °C. Proc. Combust. Inst. 2000, 28, 2717-2725.

Energy & Fuels, Vol. 18, No. 5, 2004 1259 times (0.05-10 s31,32) and rapid heating rates.33 On the other hand, continuous flow reactors are important for the commercial applications of the SCWO technology, where various designs have been proposed for the case where salts are present, e.g., tank, transpiring wall, and tubular pipes.10 Reactors with transpiring walls allow clean water to maintain a boundary layer free of corrosive species and prevent the deposition of solids on the inner reactor surface. In this work, we used a flow reactor to check practical aspects and operation. The flow reactor used here was made of a 3/8 in. (9.19 mm) 316-SS tube (11.3 mL; length ) 400 mm, o.d. ) 9.19 mm, wall thickness ) 1.59 mm), which was heated by an electric tube furnace (F21100, Fisher, NJ) to 450 °C. One high-pressure liquid chromatography pump (Waters Associates, Inc., Milford, MA, model 510; flow rate 0.1-9.9 mL/min, pressure up to 41.4 MPa) was used to feed the prepared 1-ppm 10-CB solution (3.1wt % methanol, 14.9-wt % H2O2 and Na2CO3 with 0-150 mol % concentration) in a 500-mL beaker stirred by a magnetic bar to prevent 10-CB from precipitation. The solution was preheated to 350 °C at the end of a 1/16 in. 316-SS tube (2.19 mL, length ) 5 m, o.d. ) 1.53 mm, thickness ) 0.39 mm) by Samox heavy insulated heating tape (Omega, Stamford, CT; STH051-80) and subsequently entered into the reactor. The reaction temperature (450 °C) was measured by a K-type thermocouple (Omega; TJ36-Cain-116G-12) inserted into the center of the reactor tube. The exit temperature at the end of the reactor and the preheated temperature outside the 1/16 in. (1.53 mm) tube were also measured by the same type of thermocouples. The pressure was controlled by a back pressure regulator (Tescom, MN, model: 54-2162D26; pressure up to 40 MPa) at 30 MPa. After leaving the reactor, effluent was rapidly quenched with a cooling water jacket to terminate the reaction. After reaction, the aqueous sample was collected and extracted with benzene. The benzene phase was analyzed with GC-MS as described before. However, the calibration coefficients in the analyses were obtained by using two standard solutions {10-CB (1 and 5.3 ppm) + benzene}. The reaction time (τ; min) was calculated by dividing the reactor volume (V; mL) by the flow rate at the reaction temperature (Qr; mL/min):12

τ ) V/Qr

(3)

or τ ) V(Fr/F0)/Q0

(4)

where Fr represents the solution density at reaction temperature (g/mL), F0 is the initial solution density (g/mL), and Q0 is the initial flow rate (mL/min). At 450 °C and 30 MPa, water density is 0.1486 g/mL,26 while at 25 °C it is 0.9971 g/mL for a reactor volume V ) 11.3 mL. If the fluid mixture density is assumed to be that of water,34 eq 4 becomes

τ ) 1.68/Q0 (min)

(5)

Therefore, the reaction time should be between 10.2 s and 16.8 min for the flow rates of 0.1-9.9 mL/min. The preheating time to 350 °C (τ ) 1.80/Q0; average water density ) 0.8208 g/mL from 25 to 350 °C, V ) 2.19 mL) was calculated as being between 10.9 s and 18.0 min, which was about 107% of the (31) Maharrey, S. P.; Miller, D. R. Quartz capillary microreactor for studies of oxidation in supercritical water. AIChE J. 2001, 47 (5), 1203-1211. (32) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T.; Arai, K. Dissolution and hydrolysis of cellulose in subcritical and supercritical water. Ind. Eng. Chem. Res. 2000, 39, 2883-2890. (33) Kabyemela, B. M.; Takigawa, M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Mechanism and kinetics of cellobiose decomposition in sub- and supercritical water. Ind. Eng. Chem. Res. 1998, 37, 357361. (34) Anitescu, G.; Tavlarides, L. L. Oxidation of Aroclor 1248 in supercritical water: a global kinetic study. Ind. Eng. Chem. Res. 2000, 39 (3), 583-591.

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Figure 1. Temperature (solid curves) and pressure (dots) profiles vs time. (a) DAC: temperatures measured (solid curves 1, 2, and 3 at average heating rates of 4.2, 3.0, and 2.5 °C/s) and pressures calculated (dots b, 0, and O) by EOSW26 for the experiments with pure water (water density ) 867 kg/ m3), partial O2 (water density ) 917 kg/m3), and excess O2 (water density ) 832 kg/m3). (b) batch reactors: temperatures measured (solid curve at an average heating rate of 3.5 °C/s) and pressures calculated (dots b) by EOSW26 for 1-mL pure water (water density ) 167 kg/m3), (dots O) for pressures measured for {1-mL H2O + 14 mol % O2}. probable reaction time but at lower temperatures with little reactions. The high stability of 10-CB was confirmed from the experiments in DAC and batch reactors. One way to lessen the preheating required for the sample stream is to combine another stream of pure water that is preheated to a temperature greater than 450 °C. Then the two streams (25 °C sample solution and 450 °C pure water) can be combined by two HPLC pumps and mixed just before the reactor.

Results Experiments on the phase behavior of 10-CB in the DAC were conducted under conditions of only pure water solvent, at partial (insufficient) oxygen and excess oxygen concentrations according to eq 1. Figure 1a gives temperature and pressure profiles for DAC. Figure 2 shows IR spectra of the solid residues obtained from 10CB decomposition in SCW using pure water and partial oxygen. Figure 3 shows visual observation of 10-CB phase transition in SCW when excess oxygen was used. Oxidation of 10-CB in the batch reactors was performed at 450 °C, 32 MPa, and a 20-min reaction time without (tests 1-3) and with (tests 4-5) Na2CO3 at 93.1%, 159.5%, and 225% excess oxygen concentrations in SCW. Figure 1b shows temperature and pressure profiles during heating of batch reactors. Table 1 gives the experimental conditions and the destruction rate of

Fang et al.

10-CB in SCW. For the solid phase, SEM micrographs (test 1), EDX spectra (tests 1, 3), and the images of EDX maps (test 1) are shown in Figures 4, 5, and 6, respectively. Nine experiments (runs 1-9) for the destruction of 10-CB in the flow reactor were conducted at 450 °C, 30 MPa, and 159.5-mol % excess oxygen. Table 2 summarizes the experimental conditions and results. The theoretical Na2CO3 concentrations (mol %) were 0 for runs 1-3, 100 for runs 4-6, and 150 for runs 7-8. The flow rates (mL/min) used were 8, 6, and 4, corresponding to reaction times (s) of 12.6, 16.8, and 25.2 (heating rate ) 15.9, 11.9, 7.9 °C/s from 250 to 450 °C/s), and preheated times (s) of 13.5, 18.0, and 27.0 (heating rate ) 24.0, 18.0, 12.0 °C/s from 25 to 350 °C), respectively. Visual Observations in the DAC. Many DAC experiments were performed; however, only 3 experimental results are presented here. (i) Phase Behavior in Pure Water. When a mixture of {48-wt % 10-CB and pure water} with an air bubble in DAC was rapidly heated at 4.2 °C/s to a maximum temperature of 599 °C (temperature profile: Figure 1a, solid curve 1), the air bubble disappeared at 198 °C at which the saturated water density was calculated to be 867 kg/m3. DAC experiments follow an isochoric heating path and so the temperature at which the gas bubbles disappear gives a close approximation to the true loaded water density, and pressure was estimated as showed in Figure 1a (dots b).20 The 10-CB particles started dissolving at 475 °C. However, solubility was very low at the maximum temperature of 599 °C, and thus the majority of particles still remained undissolved at these conditions. The undissolved residues on the diamond after the reaction were analyzed with FT-IR. Figure 2 (middle curve) showed that the 10-CB had been hydrolyzed according to the C-H bond stretch at 3060 cm-1 and an O-H bond stretch at 3533 cm-1. The bands from 2800 to 3700 cm-1 are for the Y-H (Y ) C, O, N) stretch vibrations; characteristic stretch bands for aromatic C-H and O-H are at 3000-3100 and 3400-3700 cm-1, respectively.35 (ii) Phase Behavior with Partial O2 in Water. In the partial oxidation experiments, 43.9% of the theoretical oxygen (25-wt % H2O2, water density ) 917 kg/m3) was used to oxidize a 34-wt % 10-CB mixture by heating to 616 °C at 3.0 °C/s (Figure 1a, curve 2). The visual observations were similar to those in pure supercritical water but yielded a yellow-colored solution. A majority of the 10-CB sample remained heterogeneous and did not dissolve into solution. The IR spectrum (Figure 2; the top curve) showed that nondissolved residue had an additional O-H band at 3451 cm-1 as compared with the spectrum obtained for the above experiment in pure SCW. The additional band, which was not found in pure supercritical water and pyrolysis experiments, was likely from oxidation. (iii) Phase Behavior with Excess O2 in Water. In Figure 3, 49-wt % H2O2 was mixed with 14-wt % 10-CB (225% excess O2, water density ) 832 kg/m3) and heated to 603 °C at 2.5 °C/s (Figure 1a, curve 3). The 10-CB sample started dissolving along with solution color changes at 473 °C (Figure 3b). As the temperature (35) Shouchang, Xu. Organic Chemistry, 2nd ed.; Higher Education Press: Beijing, 1994.

Destruction of Decachlorobiphenyl

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Figure 2. IR spectra of the residues from the decomposition of 10-CB in supercritical water at 599 °C without O2 (water density ) 867 kg/m3) and at 616 °C with partial O2 (43.9% theoretical O2 and water density ) 917 kg/m3) in the DAC.

Figure 3. Visual observation of complete dissolution of {14.1wt % 10-CB + 49-wt % H2O2} at 548 °C as DAC was rapidly heated to 603 °C (225% excess O2 and water density ) 832 kg/m3; Figure 1a: temperature and pressure profiles).

increased, the sample gradually dissolved and completely disappeared at 548 °C (Figure 3d). The solution color had changed from light yellow at 473 °C (Figure 3b), to yellow at 516 °C (Figure 3c), to light orange at 548 °C (Figure 3d), and finally to orange at a maximum temperature of 603 °C (Figure 3e). When 93.1% excess oxygen (49-wt % H2O2, water density ) 842 kg/m3) was used to oxidize 21-wt % 10-CB, the complete dissolution was also achieved at 450 °C after holding 596 s. Oxidation in the Batch Reactors. Since a homogeneous phase was achieved with excess oxygen, batch experiments were performed at 450 °C, 32 MPa, and 20-min reaction time at excess oxygen concentrations of 93.1%, 159.5%, and 225%. Figure 1b shows temperature and pressure profiles. Pressure was estimated by EOSW26 from 1-mL water (Figure 1b; dots b), which is close to that of 1-mL H2O2 with concentration e4.2 wt %. Figure 1b (dots b) also shows the pressure measured for {1-mL H2O + 14-mol % O2} at temperature from 374 to 400 °C. In tests 1-3, without Na2CO3 (Table 1), 10CB destruction rate increased from 86.5%, to 95.9% and

99.2% at excess oxygen of 93.1%, 159.5%, and 225.0%. The corresponding conversion rates of Cl in 10-CB to Cl-1 in the aqueous phase (e.g., HCl) were from 66.0% to 88.1% and 95.9%. The HCOO-1 (0.5-3.4 ppm) and CH3COO-1 (8.6-13 ppm) were also detected in the aqueous phase. Analysis of the reaction products in the benzene extract phase after the reaction by GC-MS showed that there were twelve other products consisting mainly of lower chlorinated PCBs (9-CBs, 8-CBs), chlorobenzenes (pentachlorobenzene, tetrachlorobenzene, and chlorinated furans (hexachlorodibenzofurans, dinaphthofurans). It was found that batch reactors made of 316-SS became severely corroded, resulting in the formation of a large amount of solid material containing hexagonal-, needle-, and rhombic-shaped crystals (Figure 4). The solid phase was definitely from the reactor wall corrosion since analysis by EDX revealed that the majority of elements were O and Fe, and minor elements of Si (traces of Cr for test 3) as shown in Figure 5. EDX mapping (Figure 6, parts a and b) showed the presence of O and Fe distributed over all areas of the sample, which is the evidence that the main component in the solid phase were FeOx. Si and Cr were concentrated in some parts of the sample area (Figure 6, parts c and d), presumably in the form of oxides. When 100% theoretical Na2CO3 was added, 99.7% 10-CB was oxidized at 93.1% excess oxygen in test 4 as compared with 86.5% in test 1 under similar conditions. When excess oxygen increased to 159.5%, all 10-CB was destroyed (test 5). In both tests, little solid phase and no HCOO-1 or CH3COO-1 was found. Oxidation in the Flow Reactor. Since 100% 10CB was destroyed at 159.5-mol % excess oxygen in batch reactors, all nine experiments (Table 2) were carried out at 159.5-mol % excess oxygen, 30 MPa, and 450 °C with different reaction times and Na2CO3 concentrations. When no Na2CO3 was added, 98.4, 97.45, and 99.95-wt % of 10-CB was oxidized in 12.6, 16.8, and 25.2 s (Table 2, runs 1-3) as compared to only 99.2% destruction rate in the batch reactors at higher excess O2 concentration (225%; >1.4 times) and longer reaction time (1200 s;

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Table 1. Destruction Rate of 10-CB at 450 °C, 32 MPa, and 20-min Reaction Time in Batch Reactorsa Oxidation in SCW Test 1 Neutralization agent H2O2 concentration (wt %) Excess O2 (mol %)

no Na2CO3 2.50 93.1

Conversion rate (wt %) (benzene phase) Cl-1 (wt %, Cl in 10-CB) (aqueous phase)

86.5 66.0

a

Test 2 Conditions no Na2CO3 3.33 159.5

Analysis and Results 95.9 88.1

Test 3

Test 4

Test 5

no Na2CO3 4.16 225.0

with Na2CO3 2.50 93.1

with Na2CO3 3.33 159.5

99.2 95.9

99.7 99.7

100.0 103.5

Na2CO3 was added in the amount needed to completely neutralize HCl according to eq 2. Table 2. Experimental Summary of Conditions and Results Obtained in a Flow Reactor at 30 MPa and 450 °Ca,b Run 1

Na2CO3 concentr. (mol %) Preheated time at 350 °C (s) Reaction time (s) Flow rate (mL/min)

0 13.5 12.6 8

GC-MS analysis Area Conversion rate (wt %)

11103.0 98.40

Run 2 0 18.0 16.8 6

Run 3

Run 4

Run 5

Run 6

Run 7

Run 8

Run 9

0 27.0 25.2 4

100 13.5 12.6 8

100 18.0 16.8 6

100 27.0 25.2 4

150 13.5 12.6 8

150 18.0 16.8 6

150 27.0 25.2 4

standard 10-CB: area ) 713167 and 3571705 for 1 and 5.3 ppm, respectively 17642.0 369.0 ND ND ND ND 516 97.45 99.95 ND ND ND ND 99.93

1.1 100.00

a ND ) Nondetectable. b Feeding 1-ppm 10-CB aqueous solution with 14.9-wt % H O and 3.1-wt % (4.3-vol %) methanol (159.5-mol % 2 2 excess oxygen).

Figure 4. SEM micrographs of the solid phase from batch test 1 (Table 1). (a) hexagonal and needlelike crystals; (b) cubic crystals.

Figure 6. EDX maps (O, Fe, Si, and Cr) of the solid phase from the batch test 1 (Table 1).

Figure 5. EDX spectra of the single crystals (in Figure 4) and random areas of the solid phase from batch tests 1 and 3 (Table 1).

>47 times) (Table 1; test 3). All 10-CB was oxidized when Na2CO3 was used (runs 4-9) and when reactions were performed at similar conditions of temperature and pressure as those used in the batch reactions at longer reaction times (1200 s, Table 1, test 5). Discussion Besides oxidation, decachlorobiphenyl can be also decomposed by hydrolysis at low temperatures (599 °C

and 450 °C), as confirmed by FT-IR (Figure 2) from the DAC tests and GC-MS (hydrolyzed products: 9-CBs and 8-CBs) from the batch SCW experiments as compared with its high stability up to 800 °C at pyrolysis conditions.36 Using hydrolysis, Sako et al.37 have successfully destroyed 97.5% 3-PCB in pure water at 450 °C and 31.8 MPa at 1200-s reaction time. It is believed that ionic and radical intermediates both exist during the 10-CB destruction in SCW. In ambient solutions, solvents stabilize ions through the solventsolvation mechanisms, intermediates are formed via ionic dissociation, while radicals are not stable. However, in the gas phase, ionic intermediates are unstable and therefore radicals tend to form by evenly breaking chemical bonds. A threshold value of water density for (36) Bleise, A.; Kleist, E.; Gunther, K.; Schwuger, M. J. Formation of octachloroacenaphthylene in the pyrolysis of decachlorobiphenyl. Chemosphere 1997, 35 (4), 655-666. (37) Sako, T.; Sugeta, T.; Otake, K.; Kamizawa, C.; Okano, M.; Negishi, A.; Tsurumi, C. Dechlorination of PCBs with supercritical water hydrolysis. J. Chem. Eng. Jpn. 1999, 32 (6), 830-832.

Destruction of Decachlorobiphenyl

completely disabling ionic dissociation is calculated as about 30∼80 kg/m3 using a model compound of t-BuCl (tert-butyl chloride).38 Above the threshold density, ionic and radical products may coexist. During the 10-CB oxidation in SCW, water density (kg/m3) was 150 (flow reactor), 170 (batch reactors), and 800-900 (in the DAC). The solvolysis of Cl- still stabilizes ionic intermediates in ambient solutions. A number of PCBs and chlorinated alkanes were studied for their destruction in supercritical water.39-41 Ionic reactions have been proposed in addition to possible radical reactions.42 Most results show that considerable hydroxylation of PCBs occurs in supercritical water. In SCWO of 10-CB, radicals are partly inhibited because the hydrolyzed product HCl forms a strong ionic environment. Therefore, the SCWO of the 10-CB reaction pathway most likely consists of both ionic and radical reactions. The mechanisms of 10-CB oxidation and hydrolysis in SCW are considered below on the basis of analysis of the intermediates that formed from SCWO of 10-CB. In supercritical water, the hydroxylation of 10-CB is a single-molecule nucleophilic reaction (SN1),38,41 which includes two reactions eqs 6 and 7:

Energy & Fuels, Vol. 18, No. 5, 2004 1263

among the intermediates:

When H2O2 was used, both oxidation and hydroxylation were probably involved in the destruction of 10-CB. Evidence for this is the detection of several PCB isomers identified in the products of 10-CB oxidation. The formation of 2,2′,3,3′,4,4′,5′,6,6′-nonachloro-1,1′-biphenyl (9-CB) was probably due to substituting Cl for H in 10CB. Since H2O is the only source of hydrogen, substitution of Cl with H most likely took place through radical reaction, while the ionic hydroxylation probably gave phenolic species. First, the collision between molecules probably forms an activation complex:

Then, the above complex decomposes to a 9-CB radical and a chlorine radical:

The reactive position is at one of position 2, 2′, 6, 6′ since they are affected by their neighbors ring. The dissociation of 10-CB probably produces Cl- and a cation of 9-CB. Cl- forms HCl by reacting with H+. The 9-CB cation then attracts OH- to form 2-hydroxyl2′,3,3′,4,4′,5,5′,6,6′-chloro-1,1′-biphenyl: The HO2 radical, being a relatively stable species at high pressures, probably reacts with a 9-CB radical to form 2,2′,3,3′,4,4′,5′,6,6′-nonachloro-1,1′-biphenyl, which was a detected intermediate:

However, 2-hydroxyl-2′,3,3′,4,4′,5,5′,6, 6′-chloro-1,1′-biphenyl can condense to polychlorinated dibenzofuran (PCDF), which is found in relatively high amounts (38) Westacott, R. E.; Johnston, K. P.; Rossky, P. J. Simulation of an SN1 reaction in supercritical water. J. Am. Chem. Soc. 2001, 123 (5), 1006-1007. (39) Yang, H. H.; Eckert, C. A. Homogeneous catalysis in the oxidation of p-chlorophenol in supercritical water. Ind. Eng. Chem. Res. 1988, 27, 2009-2014. (40) Marrone, P. A.; Lachance, R. P.; DiNaro, J. L.; Phenix, B. D.; Meyer, J. C.; Tester, J. W.; Peters, W. A.; Swallow, K. C. Methylene chloride oxidation and hydrolyosis in supercritical water. ACS Symp. Ser. 1995, 608, 197-215. (41) Weber, R.; Yoshida, S.; Miwa, K. PCB destruction in subcritical and supercritical water - Evaluation of PCDF formation and initial steps of degradation mechanisms. Environ. Sci. Technol. 2002, 36 (8), 1839-1844. (42) Lin, K. S.; Wang, H. P. Supercritical water oxidation of 2-chlorophenol catalyzed by Cu2+ cations and copper oxide clusters. Environ. Sci. Technol. 2000, 34 (22), 4849-4854.

The Cl* finally combines with OH* that is formed when HO2* is formed:

Equations 9-12 provide a possible way in which substitution of Cl for H occurs and HCl forms through extraction of an H atom from H2O. However, reverse reactions of eqs 10-12 exist for the case where gaseous HCl is mixed with supercritical water and oxygen. These are probably responsible for the slow conversion rates in the SCWO of 10-CB for cases when Na2CO3 was not used. The radical products given in eq 10 may also react with O2 to form quinone:

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Fang et al.

Further, oxidation and breaking in eq 13 would produce chlorophenol. Besides the reactions of eq 13, hydroxylation products of 10-CB may break down according to the following reaction:

Pentachlorobenzene (5-CBz) is an intermediate with just half of the 10-CB structure . Oxidation of the product in eq 13 does not favor 5-CBz formation. Hence, 5-CBz is probably formed after the dissociation of the C-C bond between two rings of 10CB. A few possibilities are

alloy with an approximate chemical composition of 65% Fe, 17% Cr, 12% Ni, 2.5% Mo, 2.0% Mn, 1.0% Si, and other trace elements.47 At 450 °C for 20-min reaction time, soluble cations of 99.7% Fe, 87.5% Ni, 69.5% Cr, 91.1% Mn, and 98.3% Mo were converted to waterinsoluble compounds.44 Therefore, little Cr, no Ni, Mo, or Mn could be detected in the solid phase. Si detected was apparently from the corrosion of 316-SS reactors. When 100% theoretical Na2CO3 was added to the SCWO system of 10-CB, 99% Cl-1 was precipitated as solid NaCl in SCW because 10-mg 10-CB produced 11.6-mg or 11600 ppm (1-mL water) NaCl according to the stoichiometry of eqs 1 and 2, but its solubility is ∼150 ppm at 450 °C and 30 MPa.48 Similarly, the 100% Na2CO3 needed was calculated as 10.6 mg or 1.06 wt %, which is much higher than its solubility in SCW (e.g., 0.06 ppm at 400 °C and 28 MPa12). Thus, at the beginning of the reaction, most Na2CO3 probably precipitated as fine particles on the wall of the reactors that reduced corrosion.12 As the reaction proceeded, more Na2CO3 particles would dissolve to neutralize HCl to yield NaCl. On the other hand, parts of Na2CO3 would react with water to form NaOH (eq 20),

Na2CO3 + H2O f 2NaOH + CO2

(20)

which is soluble in SCW and neutralized HCl (eq 21),

NaOH + HCl f NaClV + H2O

These intermediates should further decompose to light acids (e.g., acetic and formic acids)43 and HCl, and finally to CO2, H2O, and HCl. Because 10-CB at high concentrations (>14%) can be completely dissolved in SCW as shown in the DAC experiments, the destruction of low concentrations of 10-CB in batch reactors (1%) or in flow reactor (1 ppm) seems likely to occur under homogeneous conditions. Homogeneous conditions should promote degradation and unification of the products. The change of solution color (light yellow, yellow, and orange) in the DAC indicated the degree of the oxidation to HCl, which is one of the main products having a yellow color. The corrosion of the 316-SS reactors was due to the severe acidic HCl environment formed during SCWO of 10-CB according to eq 1. Metal (M) was corroded to MCly according to eq 17, and crystal metal oxides (MOy/2) were formed according to eqs 18 and 19:44-46

M + yHCl f MCly + y/2H2

(17)

MCly + yH2O f M(OH)y + yHCl

(18)

M(OH)y f MOy/2V + y/2H2O

(19)

The reactor material, 316-stainless steel, is an Fe-based

(21)

Due to the formation and subsequent precipitation of NaCl, the reverse reactions of eqs 6, 7 and so eqs 9-12 probably became greatly inhibited. Hence, the destruction rate of 10-CB was enhanced according to the overall eq 1. Therefore, when Na2CO3 was added, probably very few of the possible corrosion reactions (eqs 17-19) proceeded. The high conversion rate of 10-CB in the flow reactor (e.g., 99.95% in 25.2 s for run 3 without Na2CO3; 100% in 12.6 s for run 4 with Na2CO3) was probably due to oxidation being promoted by methanol and low 10-CB concentration. Another reason is that the flowing conditions promote mixing and heat transfer of the solution. Even though the 10-CB solution was preheated to 350 °C, at which H2O2 decomposes and subsequent oxidation of 10-CB occurred, the main reactions were still proceeding in the reactor. Compared with the batch reactors, reaction time of 20 min, the total preheated and reaction time for the flow reactor was shorter than 53 s (43) Jin, F.; Moriya, T.; Enomoto, H. Oxidation reaction of high molecular weight carboxylic acids in supercritical water. Environ. Sci. Technol. 2003, 37, 3220-3231. (44) Smith, R. L., Jr.; Atmaji, P.; Hakuda, Y.; Kawaguchi, Y.; Adschiri, T.; Arai, K. Recovery of metals from simulated high-level liquid waste with hydrothermal crystallization, J. Supercrit. Fluids 1997, 11 (1, 2), 103-114. (45) Adschiri, T.; Kanazawa, K.; Arai, K. Rapid and continuous hydrothermal crystallization of metal oxide particles in supercritical water. J. Am. Ceram. Soc. 1992, 75 (4), 1019-1022. (46) Fang, Z.; Xu, S. K.; Kozinski, J. A. Behavior of metals during combustion of industrial organic wastes in supercritical water. Ind. Eng. Chem. Res. 2000, 39 (12), 4536-4542. (47) Hayward, T. M.; Svishchev, I. M.; Makhija, R. C. Stainless steel flow reactor for supercritical water oxidation: corrosion tests. J. Supercrit. Fluids 2003, 27 (3), 275-281. (48) Cui, S. T.; Harris, J. G. The structure and phase equilibria of saltwater solution at supercritical conditions. Int. J. Thermophys. 1995, 16 (2), 493-502.

Destruction of Decachlorobiphenyl

(Table 2, run 3), which is just less than half of the heating time to 450 °C (2 min) in the batch tests. Conclusions Decachlorobiphenyl (10-CB) decomposes by both hydrolysis and oxidation in supercritical water. The complete dissolution in SCW occurs for temperatures greater than 450 °C and at excess O2 concentrations greater than 93.1%. The oxidation of 10-CB probably occurs under homogeneous conditions in the batch and flow reactors of this work when excess oxygen was used. In the batch tests at 450 °C, 32 MPa, and 20-min reaction time, the destruction rate increased with excess O2 concentration; at 225% excess oxygen, 99.2% of 10-CB was destroyed. However, severe corrosion of the 316SS batch reactors was found. Addition of the neutralizing agent, Na2CO3, greatly reduced the corrosion and promoted the destruction rate. The 10-CB could be 100%

Energy & Fuels, Vol. 18, No. 5, 2004 1265

destroyed at 159.5% excess oxygen. A sequence of reaction equations were proposed to explain mechanisms for the destruction. Finally, 100% 10-CB was successfully destroyed in the flow reactor at residence times as short as 25 s. Supercritical water oxidation at proper conditions is an effective technique to destroy PCB wastes, especially those stable species such as decachlorobiphenyl. Acknowledgment. The authors acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada through individual, strategic, and equipment grants (NSERC Grants OGP170464, STP-235055, EQPEQ-218842). We also thank Mr. E. Siliaskas for conducting GC-MS and IC analyses, and Ms. H. Campbell for conducting SEM and EDX analyses. EF0499630