Catalytic Dechlorination of Chlorobenzene in Water by Pd/Fe System

700Chinese Chemical Letters Vol. 14, No.7, pp 700- 703, 2003http://www.imm.ac.cn/journal/ccl.htmlCatalytic Dechlorination of Chlorobenzene in Water by PdFe SystemXin Hua XU*, Hong Yi ZHOU, Da Hui WANGDepartment of Environmental Engineering, Zhejiang University, Hangzhou 310027was dechlorinated by Pd/Fe bimetallic system in water through catalyticreduction. The dechlorination rate increases with increase of bulk loading of Pd due to theincrease of both the surface loading of the Pd and the total surface area. For conditions with0.005% Pd/Fe, 45% dechlorination efficiency was achieved within 5 h. The dechlorinatedreaction is believed to take place on the bimetal surface in a pseudo-first- order reaction, with therate constant being 0.0043 min'.Key words: Reductive dechlorination, catalytic, Pd/Fe, chlorobenzene.Chlorinated organic compounds have been used on a large scale in the chemical andpetrochemical industries. They are carcinogenic and stable, and also very difficult to betreated by biodegradation. Incinerating is feasible, but leads to the release of even moretoxic compounds, also cannot always be destroyed by advanced oxidation processes.Chlorinated organic compounds can be adsorbed by macromolecule resin, but cannot bedecomposed, need to be desorbed and treated by other method.Reductivedechlorination method promoted by zero-valent metals has been a very active researcharea since Gillham’. Iron can be used in this capacity in field tests, which has recentlybeen used to rapidly dechlorinate organic compounds of wide range, includingtrichloroethene (TCE), perchloroethene (PCE), polychlorinated biphenyls (PCBs), etc.Ni, Zn, Mg also can be used as catalysts for dechlorination, but often leads to secondarypollution+s. Reduction of nitro aromatic compounds and aromatic azo compounds byiron has also been reported-. Recently, it has been found that the addition of Pd as acatalyst can speed up the dechlorinated. The dechlorination reaction between Pd/Fe andlow-molecule hydrocarbons was found to be so fast that it is suitable for treatingwastewaterl2-14.ExperimentalPalladium/iron (Pd/Fe) powder was prepared by wet impregnation of the iron powderwith an aqueous solution of potassium hexachloropalladate. The iron powder waswashed with 0. 1mol/L HSO4 then acetone and rinsed with ditilled water to removesurface oxide layers and undesired organic compounds prior palladization.An aqueous"E-mail: xxh@ mail.hz.zj.cn中国煤化工MHCNMH G.Xin Hua XU et al.701solution of potassium hexachloropalladate was prepared and added to a bottle containingiron powder. The solution in the bottle was continuously stirred until the dark orangesolution turnedthe palladized iron was rinsed twice withdeionized water and used for reaction without drying.Surface area (BET area) of iron and Pd/Fe were measured using the nitrogenadsorption method with a ST-03 surface analyzer (Beijing).Batch experiments for chlorobenzene dechlorination were conducted in 30mLbottles, each bottle was placed in an incubator shaker (200 trpm, 25+19C). At varyingtimes, samples were withdrawn and filtered with a piece of 0.45 μum filter film, then usedfor analysis. Chlorobenzene and benzene were determined by HPLC system, chlorideanalysis was performed by ion chromatography (792 Basic IC, Metrohm).Results and DiscussionCharacterization results of the catalystThe surface morphology of both the Fe and Pd/Fe is shown inPd/Fe is rough with palladium deposited on the iron surface, and the more Pd loading,the more white dots. The total surface area of the Fe and Pd/Fe (0.005%) measured byBET-N2 was 0.49 nf/g and 0.62 m/g, respectively. The increased surface indicatedthat the Pd is dispersed on the surface of iron rather than clustered. That is consistentwith the information revealed in the SEM image of the catalyst. Similar results werereported by Liu Y.H.et al4.Figure 1 SEM image of (a) Fe powder, (b) PdFe (0.005%), (c) Pd/Fe (1%)052839 2eKV 2.山152848 28KV k28.ekHl:eu52886 20KV X2K 19iw(a)(b)(c)Dechlorination of chlorobenzeneIn all experiments, initial MCB concentration was 100 mg/L; metal to solution ratio was2 g/30 mL. As seen from the Figure 2, the aqueous concentration of the MCB declinedrapidly at the first 10 minutes, and then almost complete degradation was achievedwithin 5 hours. No byproducts were observed by GC-MS," 中国煤化工TYHCNMHG.702Catalytic Dechlorination of Chlorobenzene in Water by Pd/Fe Systemproduct of MCB dechlorination. The concentration of benzene and free chloride weregradually increased.Although close to 95% degradation was observed at the end of theexperiment, the free chloride formation observed was just 48% of the maximumattainable. It is supposed that most of the MCB remained adsorbed to the iron surfaceuntil complete dechlorination is achieved. This need further study to confirm it.Figure 3 showed the comparison of the dechlorination rates with Fe, Pd/Fe for MCB.The dechlorination efficiency for Fe was less than 6% during the 3 days experiment,while 45% for Pd/Fe within5 h. It is obvious that Pd as the catalyst accelerated thereductive dechlorination reaction.Figure2 Reduction of chlorobenzene (MCB) and respective formation of benzene1.(Temperature 25CFigure30.8Pd loading 0.005%HH-MCB.Comparison of theShaking at 200rpm|+ Benzenedechlorination rates04of Fe and Pd/Fe0.0.00 50100150200250 30(Reaction tim/min-◆Fe一t Pd/FeReaction time/minCatalytic reduction mechanismEvidence strongly suggests that, within a bimetallic system, one metal likely serves ascatalyst (Pd) while the other as electron donor (Fe) as illustrated in Figure4. Iron is avery effective reductant.中国煤化工MHCNMH G.Xin Hua XU et al.703Figure 4 Scheme of dechlorination of chlorobenzene with the bimetallic particle catalystChlorobenze刀Benzene +CIChlorobenzeneH]PAcknowledgmentThis work is supported by the Returnee Foundation of China Ministry of Education.References1. E. Lipczynska-Kochany, s. Harms, R. Milburn, Chemosphere, 1994, 29, 1477.2. C. Namasivayam, K. 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