Mendeleev Communications Electronic Version, Issue 4, 1998 (pp. 129–168) Dechlorination of carbon tetrachloride in water on an activated zinc surface Tatiana N. Boronina,*a Kenneth J. Klabundeb and Gleb B. Sergeeva a Department of Chemistry, M. V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 939 0283; e-mail: tbor@cryo.chem.msu.su b Department of Chemistry, Kansas State University, KS 66506 Manhattan, USA.Fax: +1 913 532 6666; e-mail: kenjk@ksu.edu Bimetallic enhancement with Pd, Ag and Au, or mechanical and cryochemical treatment, are shown to increase Zn(0) surface reactivity towards carbon tetrachloride in water and to promote both dechlorination and conversion into methane and other hydrocarbons. The use of zero-valent metals for in situ conversion of onerous water contaminants, such as carbon tetrachloride (CCl4), trichloroethylene (TCE), etc., into hydrocarbon and other non-toxic products has emerged lately as an important area of environmental chemistry.1–4 Chlorocarbon dechlorination in water by zero-valent metals is a heterogeneous processes in which the surface plays an important role.1,2,5–8 Commercial Zn(0) dust converts CCl4 via chloroform and methylene dichloride into methyl chloride and methane, though methylene dichloride degrades about two orders of magnitude slower than CCl4.9,10 Mechanical grinding or striking,11 preparation of metal particles by cryo-condensation,12 or doping with a second metal13,14 are known to create a chemically active surface.Herein we employed Zn(0) activated by cryochemical or mechanical treatment or doping with Pd, Ag and Au in an attempt to enhance the surface performance toward CCl4 in water and to promote dechlorination and conversion into methane and other hydrocarbons.† Activated zinc surface reactivity towards CCl4 in water.The observed first-order kinetic constant kI obs for CCl4 degradation and the CCl4 :methane ratio were used to compare zinc particle reactivity in reactions with identical initial conditions (Table 1).The kI obs value is known to increase with the amount of metal, specific surface area and active site concentration.2,4 CCl4 was shown to degrade in water in the presence of Zn dust.9,10 The methane concentration was gradually increased over time.10 Methane evolution was dramatically enhanced on the activated metal particles during the first few hours.The CCl4 degradation rate was also significantly raised by doping with Pd or Ag and cryo treatment. Cryo zinc exhibited a lower reactivity † Experimental. Distilled, argon-purged water, carbon tetrachloride of spectranalytical grade and Zn(0) dust of certified grade (Fisher) were used.Mechanically activated zinc was prepared by pressing zinc dust at 2000 psi into pellets 5 mm in diameter and 0.025 g in weight. Zinc cryoparticle (cryo) was obtained by co-deposition of zinc and pentane vapours at 77 K, followed by warming to room temperature and solvent evaporation.12 Bimetallic enhancement was performed by deposition of silver, palladium or gold from the salts AgNO3, 99.9% (Fisher), K2PdCl4 and AuCl3 99.99% (Aldrich) on a zinc surface via the red-ox reaction in water.14 Kinetics of CCl4 dechlorination by zinc systems in water were studied using a 1–1.5 fold excess of metal.Experiments were carried out in 40 ml glass amber vials, capped with Teflon Mininert valves. Each vial contained 28 ml of 0.9–1.0 mM CCl4 water solution. The initial pH (no buffer) ranged from 6.10 to 6.20.Zinc dust, 0.1–2.2 g (cryo or pellets, 0.11 g) was added to the cooled solution. To achieve the required loading of silver or palladium, ca. 2.14 and 1.1 mol%, zinc dust was followed by an aliquot of aqueous AgNO3 or K2PdCl4. Vials were mixed at 60 rpm at room temperature and sampled at certain intervals of time. The reaction time varied from 10 to 120 h.Chlorocarbons were analysed with a direct water or headspace injection on a Perkin-Elmer Auto System Gas Chromatograph/Q-Mass 910 Mass Spectrometer and methane on a GOW-MAC GC.6 Hydrocarbon identification was performed on a Headspace Analyser Varian Star 3600CX GC (FID) Automatic System and a PLOT capillary column.10 Experiments were carried out in 22 ml vials for the Headspace analysis, containing 0.5 g of zinc dust or cryo in 5 ml of 0.5 mM CCl4 water solution at 32 °C without mixing.Deposition of ca. 0.1 mol% of Ag, Pd or Au was carried out before injecting CCl4 stock solution. which one would expect, given its large specific surface area: 5.23 m2 g–1 compare to 0.243 m2 g–1 for dust. We put the surface active site deactivation down to oxide formation, and/or a parallel reaction with water.10 Pellets showed a lower kI obs value than dust, although it is not clear how to account for a pellet mass which actually participates in the reaction.Products and possible reaction pathways of CCl4 degradation in water by activated zinc. The reaction products are summarized in Table 2. Chloroform and methylene dichloride were intermediate products in all reactions studied.This, together with methyl chloride and methane as the final products, pointed towards stepwise CCl4 dechlorination. This pathway was proposed to be similar to the reductive hydrogenolysis initiated by electron transfer.2,15 However, the following experimental facts suggested a multiple parallel– sequential reaction pathway: i, methane evolution overtook methyl chloride; ii, acetylene formed on the dust; iii, traces of DCEs (0.5 mol% of the initial CCl4 concentration) appeared on the dust resulting from a parallel methylene dichloride degradation pathway; iv, TCE, 1,1-DCE and 1,2-DCE formed on cryo zinc and pellets (up to 5%), where DCEs might be the products from either TCE sequential dechlorination and/or from another reaction pathway; v, ethane, ethylene, acetylene C4–C6 hydrocarbons (1–3%), and TCE and DCEs (5%) were observed on bimetallic systems.The results reported here have shown that bimetallic enhancement and cryo and mechanical treatment affect the zinc surface reactivity, increase the reduction rate and alter the priority of the reaction pathways in a favour of those leading to C2 compounds and methane.The methods of surface activation used in the present study facilitate the initial electron transfer step which we believe is rate determining. As regards the pathways, C–Zn bond formation and a possible catalytic effect of Pd, Au and Ag might be considered, since the high reactivity of cryo metals in organometallic synthesis, and successful chlorocarbon dehydrohalogenation using a Pd/H2 catalyst,12,15 have previously been reported.Direct chlorocarbon hydrogenolysis on a zinc surface with metal or metal/H2 serving as a reactant or, in bimetallic systems, as a catalyst, could yield methane and C2 hydrocarbons, or carbide formation followed by hydrolysis would yield acetylene. The latter on Zn/Pd appeared to be an intermediate product and was subsequently reduced to ethylene and ethane or underwent coupling reactions yielding C4–C6 hydrocarbons. Table 1 Zinc system reactivity towards carbon tetrachloride in water. Zinc particle kind and mass CCl4 degradation rate, kI obs /h–1 CCl4 conversion into methane (%) dust 0.1 g 0.026±0.003 14.5 (120 h) 1.2 (2.5 h) cryo 0.1 g 0.084±0.006 27.3 (120 h) 18.0 (2.5 h) pellets 0.1 g 0.015±0.001 31.3 (120 h) dust 2.2 g 0.538±0.081 2.5 (4 h) dust 2.2 g/0.1 mol% Pd 4.2±0.3 12.0 (4 h) dust 2.2 g/2.1 mol% Ag 2.5±0.2 24.0 (4 h)Mendeleev Communications Electronic Version, Issue 4, 1998 (pp. 129-168) The support of the Hazardous Substance Research Centre at Kansas State University is acknowledged. References 1 R. W. Gillham and S. F. O’Hannesin, Ground Water, 1994, 32 (6), 958. 2 L. J. Matheson and P. G. Tratnyek, Environ. Sci. Technol., 1994, 28, 2045. 3 E. L. Appleton, Environ. Sci. Technol./News, 1996, 30, 536A. 4 T. L. Jonson, M. M. Scherer and P. G. Tratnyek, Environ. Sci. Technol., 1996, 30, 2634. 5 D. R. Burris, T. J. Campbell and V. S. Manoranjan, Environ. Sci. Technol., 1995, 29, 2850. 6 W. S. Orth and R. W. Gillham, Environ. Sci.Technol., 1996, 30, 66. 7 R. M. Allen King, R. M. Halket and D. R. Burris, Environ. Toxicol. Chem., 1997, 16, 424. 8 T. J. Campbell, D. R. Burris, A. L. Roberts and J. R. Wells, Environ. Toxicol. Chem., 1997, 16, 625. 9 T. N. Boronina, K. J. Klabunde and G. B. Sergeev, Environ. Sci. Technol., 1995, 29, 1511. 10 T. N. Boronina, I. Lagadic and K. J. Klabunde, Environ. Sci. Technol., 1998, in press. 11 V. A. Radzig, Kinet. Katal., 1978, 19, 713 [Kinet. Catal. (Engl. Transl.), 1978, 563]. 12 K. J. Klabunde, Free Atoms, Clusters, and Nanoscale Particles, Academic Press, New York, 1994. 13 C. G. Schreier and M. Reinhard, 209th Natl Meet.-Am. Chem. Soc., Div. Environ. Chem., Anaheim, CA, April 1995, 35 (1), 745. 14 R. Muftikian, K. Nebesny, Q. Fernando and N. Korte, Environ. Sci. Technol., 1996, 30, 3593. 15 L. T. Bryndzia, Environ. Sci. Technol., 1996, 30, 3642. a1,2-Dichloroethylene. bEthane, ethylene, acetylene and solid products were not analysed. c1,1-Dichloroethylene. Table 2 Products identified upon CCl4 dechlorination in water by zinc (in all reactions CHCl3 and CH2Cl2 were intermediate products). Zinc metal system Products identified Dust CH4, C2H2, CH3Cl, ZnCl2, Zn(OH)2, traces of trans-, cis- 1,2-DCEa Cryo the same + H2, TCE Pelletsb the same + H2, TCE, 1,1-DCEc Dust/Pd H2, CH4, C2H6, C2H4, C2H2, C4, CH3Cl, ZnO Dust/Ag CO2, CH4, C2H6, C2H4, C2H2, CH3Cl, 1,1-DCE, TCE, ZnO Dust/Aub CH4, C2H6, C2H4, C2H2, CH3Cl Pellets/Pdb H2, CH4, CH3Cl Pellets/Agb CO2, CH4, traces of 1,1-DCE, CH3Cl Received: Moscow, 24th March 1998 Cambridge, 1st May 1998; Com. 8/02399I