Environ. Sci. Technol. 2003, 37, 5757-5762
Subcritical (Hot/Liquid) Water Dechlorination of PCBs (Aroclor 1254) with Metal Additives and in Waste Paint A L E N A K U B AÄ T O V AÄ , * , † J A M I E H E R M A N , ‡ TAMARA S. STECKLER,† MARLEEN DE VEIJ,§ DAVID J. MILLER,† EDGAR B. KLUNDER,| CHIEN M. WAI,‡ AND STEVEN B. HAWTHORNE† Energy and Environmental Research Center, Campus Box 9018, University of North Dakota, Grand Forks, North Dakota 58202, Department of Chemistry, University of Idaho, Moscow, Idaho 83844, and National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
No disposal option exists for “mixed wastes” such as paint scrapings that are co-contaminated with polychlorinated biphenyls (PCBs) and radioactive metals. Either removal or destruction of the PCBs is required prior to disposal. Comparison of subcritical water dechlorination (350 °C, 1 h) of Aroclor 1254 in paint scrapings (180 ppm) and of standard Aroclor 1254 showed significantly enhanced dechlorination in the presence of paint. While no significant degradation was observed for standard Aroclor (no paint), the dechlorination of PCBs in paint was 99, 99, and 80% for the hepta-, hexa-, and pentachlorinated congeners, respectively, indicating that metals in the paint enhanced the dechlorination reactions. Adding metals to the standard Aroclor (no paint) reactions enhanced PCB dechlorination in subcritical water in descending order of activity: Pb ≈ Cu > Al > Zn > Fe. In the presence of both zerovalent and divalent lead and zerovalent copper in subcritical water (350 °C, 1 h), 99% of the Aroclor 1254 mixture (tetrato heptachlorinated biphenyls) was dechlorinated. High dechlorination (ca. 95%) was also achieved with zerovalent aluminum. In contrast to other metals, lead retained its degradation ability at a lower temperature of 250 °C after 18 h. The high degradation efficiency achieved using metal additives in water at reasonable temperatures and pressures demonstrates the potential for subcritical water dechlorination of PCBs in paint scrapings and, potentially, in other solid and liquid wastes.
Introduction No straightforward disposal option exists for “mixed wastes”, defined as wastes containing both hazardous waste and radioactive components (1). At present, mixed waste is jointly * Corresponding author phone: (701) 777-2498; fax: (701) 7775181; e-mail:
[email protected]. † University of North Dakota. ‡ University of Idaho. § Present address: Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands. | U.S. Department of Energy. 10.1021/es030437h CCC: $25.00 Published on Web 11/04/2003
2003 American Chemical Society
regulated under the Resource Conservation and Recovery Act (RCRA) and the Atomic Energy Act (AEA) (1). Under those acts either removal or destruction of the hazardous waste components is required prior to disposal of radioactive material. Paint scrapings containing polychlorinated biphenyls (PCBs) and contaminated with radioactivity are one example of the mixed waste currently produced from the decommissioning of nuclear facilities (2). Several different technologies have been employed to treat PCBs including bioremediation (3), as well as chemical and physical methods such as poly(ethylene glycol)/potassium hydroxide treatment (4), ultrasonic degradation (5, 6), irradiation methods (7-9), peroxidation (10), and catalytic oxidation (11). However, only two approaches have been evaluated for the treatment of PCB-contaminated radioactive materials: a drastic chemical method involving heating in the presence of toluene and KOH (12) and the use of incineration (2). Sub- and supercritical water has recently been employed as a reaction media for PCB degradation (13-21). Reactions were performed either under oxidative conditions in the presence of hydrogen peroxide or oxygen (13-15) or under hydrolytic conditions in the presence of NaOH (15-17). Recently, pure water at subcritical conditions (defined as hot water with sufficient pressure to maintain the liquid state) has been used to extract and degrade high explosives and pesticides from highly contaminated soils (22, 23). Subcritical water degradation studies on aliphatic chlorinated compounds (24) and polychlorinated dibenzodioxins (PCDDs) (25) showed that dechlorination of unsaturated and aromatic species requires high temperatures (e.g., >300 °C). Therefore, to enhance subcritical water degradation of PCBs, zerovalent iron and nickel have been employed (18-20) for reductive dechlorination. In this work, we investigated subcritical water degradation of PCBs present in the waste paint from the Department of Energy/Department of Defense (DOE/DOD) complex. Since the PCB degradation in the paint was more enhanced than for an Aroclor standard, different metals based on the paint composition were selected and evaluated for their ability to enhance PCB degradation.
Experimental Section Material. For our work, a representative sample of PCBcontaminated paint without radioactivity was used. Waste paint chips contaminated with PCBs were supplied by CMS Energy, in Charlevoix, MI. The paint sample was homogenized by freezing in liquid nitrogen and grinding with a mortar and a pestle. Commercial PCB mixtures (Aroclors), PCB mixtures of individual congeners, and 2,2′,4,4′,6,6′hexachlorobiphenyl (PCB 155), were obtained from AccuStandard, Inc. (New Haven, CT). HPLC-grade water was obtained from Fisher Scientific (Pittsburgh, PA). Zerovalent metals and metal salts of 45 µm particle size were purchased from Aldrich (Milwaukee, WI) and Atlantic Equipment Engineers (Bergenfield, NJ). Prior to use, all zerovalent metals were washed in hexane and acetone to remove organic contamination (18). Degradation Experiments. All reactions were performed using a static (no flow) 4.4 mL cell constructed from a stainless steel or Inconel 600 (26) threaded (npt) pipe fitting with end caps (63.2 mm long, 7.1 mm i.d., Parker Hannifin Corp., Columbus, OH). Paint samples of 100 mg were used for the degradation experiments. To test the effect of metal additives, Aroclor 1254 (20 µL of 1.25 µg/µL in acetone) was spiked into a cell containing 100 mg of metals or metal salts. The acetone was allowed to evaporate (ca. 10 min), and then 2.4 mL of VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Comparison of GC/ECD chromatograms of PCBs (a) in Aroclor 1254 and (b) in paint extract. SS represents a surrogate standard (PCB 155); IS is an internal standard (1,2,4-trichlorobenzene).
TABLE 1. Degradation of 25 µg of Aroclor 1254 in Subcritical Water in the Presence of 100 mg of Metal Additives (45 µm Particle Size) after 1 h at 350 °C, Quantified as Aroclor 1254 (EPA Method 8082A) metal additive
remaining Aroclor 1254 ( SDa, wt %
metal additive
remaining Aroclor 1254 ( SDa, wt %
none Pb° PbCO3 (PbCO3)2‚Pb(OH)2
75 ( 15 1(1 1(1 8(3
Cu° Al° Zn° Fe°
1(1 6(4 25 ( 2 61 ( 9
a Standard deviations were based on triplicate experiments with each metal.
FIGURE 2. Comparison of the degradation of PCBs in subcritical water after 1 h at 350 °C using the following: (a) pure Aroclor 1254 (no additives); (b) PCB-contaminated paint. The mass of PCBs corresponds to the mass in each experimental cell per 2.4 mL of water: (a) spike of 25 µg of Aroclor 1254; (b) PCBs present in 100 mg of paint. Error bars indicate (1 standard deviation based on triplicate degradation experiments. water previously purged with nitrogen was placed in the cell. The cell was then capped and placed in a GC oven (HewlettPackard model 5890) for heating. The internal cell pressure was governed by the steam/water equilibrium. Thus, pres5758
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sures were ca. 40 and 170 bar for reactions performed at 250 and 350 °C temperatures, respectively (27). All pressures were substantially below the 420 bar rating of the reaction cells. (Safety note: It is imperative to have sufficient headspace in the vessel to ensure that interior pressure is maintained by the steam/water equilibrium to avoid the excessive pressures that can occur with a completely full cell (28). Therefore, the proportion of water added should be
