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Topics in Catalysis Vol. 33, Nos. 1–4, April 2005 ( 2005) DOI: 10.1007/s11244-005-2538-9

Degradation of halogenated organic compounds in ground water by heterogeneous catalytic oxidation with hydrogen peroxide Jo¨rg Hofmanna,*, Ute Freiera, Mike Wecksa, and Alexander Demundb a Institut fu¨r Nichtklassische Chemie e.V. an der Universita¨t Leipzig, Permoserstr. 15, D-04318 Leipzig, Germany Wilhelm Ostwald Institut fu¨r Physikalische und Theoretische Chemie der Universita¨t Leipzig, Linnestr. 2, D-04103 Leipzig, Germany

b

Catalytic oxidation with hydrogen peroxide is a reliable method for the treatment of polluted water. The conversion of a wide spectrum of components including aliphatic and aromatic hydrocarbons as well as halogenated organic compounds into non-toxic or minor-toxic substances and finally into carbon dioxide and water can be achieved. The degradation of chlorobenzene, 4chlorophenol, 4-chloroaniline and 1-nitro-4-chlorobenzene was investigated. Several catalysts can be implemented into the catalytic process. It has been demonstrated that the type of catalyst and oxidation agent as well as the reaction parameters influence the degradation rate and has to be adjusted to the concrete waste water problem to be solved. KEY WORDS: catalytic oxidation; ground water; heterogeneous catalyst; hydrogen peroxide; waste water

1. Introduction Over the past 20 years the development of improved methods for remediation of ground water contaminated by various organic compounds has emerged as a significant environmental priority. Selecting an appropriate treatment method for groundwater contaminated by organic pollutants depends on factors that vary from site to site. A disadvantage of ex situ methods is their ineffectiveness. As a result these technologies lead to very long remediation times. With growing awareness of limitations of pump-and-treat technologies, the priority is shifting to in-situ technologies. The contaminated ground water is treated with reactive wall technologies [1] or funnel and gate systems [2]. The so called SAFIRA project [3] (remediation research in regionally contaminated aquifers; Sanierungsforschung in regional kontaminierten Aquiferen) supports the development of several in situ technologies by testing them in a laboratory and in a pilot plant. The motivation for developing methods is the high and complex contamination of the ground water by the chemical plants and their dumps in the area of the former ‘‘Mitteldeutsches Chemiedreieck’’ (Bitterfeld/ Wolfen–Leuna/Buna–Bo¨hlen). The test site in Bitterfeld was chosen near a former chemical plant. In the ground water the inorganic contamination with heavy metals is very small, whereas the concentration of sulphate (up to 1000 mg/L) and chlorides (up to 1300 mg/L) is remarkable high. In contrast to the inorganic contamination the concentration of the organic compounds especially by halogenated organic compounds, namely chlorobenzene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, ben*To whom correspondence should be addressed. E-mail: [email protected]

zene, 1,2-dichloroethenes and various other organic compounds is very high. Wet oxidation is a common technology for the reduction of total organic carbon (TOC) and degradation of organic compounds in waste water. The high reaction temperatures required allow economic operation only at high concentrations of organic pollutants and autothermal operation. For lower concentrations, oxidation agents which are more effective than oxygen are necessary. Ozone and hydrogen peroxide are among the more preferable oxidants, because of higher oxidizing potential [4]. Ozonation is a known technology for water treatment [5], but for treating solutions with higher concentrations of organic pollutants the limited ozone solubility in water at atmospheric pressure and its short life time make the process expensive. In some cases the oxidizing potential of hydrogen peroxide and ozone is not high enough for the oxidative degradation of organic pollutants in water. In the ‘‘advanced oxidation processes’’ hydroxyl radicals, having a high degradation and oxidizing potential (Eo ¼ 3.06 V), are generated by different reaction paths [4]. These high reactive radicals initiate radical chain reactions which lead to degradation and total oxidation. The UV-radiation induced oxidation with ozone and/or hydrogen peroxide [6] and the photo fenton reaction [7] are carried out in homogeneous phase. The disadvantage of homogeneous catalytic processes is the necessity of the recover of the dissolved catalyst. In heterogeneous catalytic processes the active component is fixed at the surface of the catalyst and separation processes can be avoided. In the catalytic oxidation of organic compounds in ground water heterogeneous catalysts are used: (1) Full metal catalysts are wire gauze and wire nettings with a catalytic active surface [8]. 1022-5528/05/0400–0243/0  2005 Springer Science+Business Media, Inc.

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(2) Mixed metal oxide catalysts are metal oxide pellets made by pressing the oxides with a special binding agent [9]. (3) Heterogeneous Fenton-type catalysts consist of an inert porous basis material for example aluminium oxide [10,11]. The active components are fixed at the surface of the basis material. Hydrogen peroxide is used as oxidizing agent, because it is easy to handle and low-priced. The aim of this work is to study the treatment of synthetic aqueous solutions containing organic pollutants from the test site in Bitterfeld, namely chlorobenzene, 4-chlorophenol, 4-chloroaniline and 1-nitro-4chlorobenzene, by means of heterogeneous catalytic oxidation to demonstrate the application area of this technology and to get further information on the mechanism. The adaptation of the heterogeneous catalytic oxidation follows in three consecutive steps: from the labscale batch experiments (10 or 500 mL) over continuous bench scale tests (2 L) to a pilot plant. In this paper the investigation in the laboratory scale are described only. 2. Experimental and analytical The lab scale experiments were performed in 20 mL headspace vials (screening tests) and in a 500 mL batch system in a temperature range of 30–80 C. Hydrogen peroxide was used as oxidizing agent. About 50 catalysts were tested at all. In this paper the results with four catalysts are described, which distinguish by the highest degradation rates. 2.1. Catalysts In the catalytic oxidation of organic compounds in water three types of catalysts are used: mixed metal oxide catalysts (SG 2157), full metal catalyst (MOLoxW 2101), and heterogeneous Fenton-type catalysts (Fe-03 and DELTA). The catalysts are industrial products. The detailed composition and the production procedure are know how of the producer. The mixed metal oxide catalysts SG 2157 consists of copper oxide, manganese oxide and a bonding agent. The diameter of the pellets is 2.5–3.0 mm. The full metal catalyst MOLoxW 2101 is a wire gauze which was tempered at 600 C in an oxygen atmosphere to generate a catalytic active surface. The basis material consists of nickel, copper and iron. The diameter of the wire is about 0.5 mm. Heterogeneous Fenton-type catalysts consist of an inert porous basis material like aluminium oxide (Fe-03) [10,11] or activated carbon (DELTA catalyst) [12]. Iron as active component is fixed at the surface of the basis material by special techniques. Fe-03 spheres has a diameter of 5.0 mm, the cylindrical pellets of the DELTA catalyst are 2–3 mm long with a diameter of 0.7 mm.

2.2. Screening tests in headspace vials A modified headspace gas chromatography equipment of DANI (HSS 86.55, DANI, HP 5890 series II) was used for the experiments. To 100 ml of freshly prepared aqueous solution of pollutant (20–25 mg/L) defined quantities hydrogen peroxide were added. For a test series four headspace vials are used—two with about 0.1 g catalyst and two without catalyst (reference probes). Into these vials 10 mL of the contaminated water were added, the vials closed and introduced into the oven of the headspace autosampler. The reaction parameters are determined by the equilibration time and the oven temperature of the autosampler (reaction time ¼ equilibration time minus 5 min warm up time). After the shaking phase automatically probes from the gas phase in the four headspace vials are taken and analyzed with the gas chromatography (SE-30 capillary column, 50 m · 0.25 mm · 0.2 lm, oven program: 100 C isotherm, carrier gas: nitrogen, FID). The conversion degree of chlorobenzene was calculated by means of the corresponding signal areas. The concentration of hydrogen peroxide was determined by titration with potassium permanganate. The formation of hydrochloric, formic and oxalic acid was followed by ion chromatography analysis (DX 100, Dionex GmbH, column: AS12A, column head pressure: 15 MPa, flow rate: 1.5 mL/min, eluant: 2.7 mM Na2CO3, 0.3 mM NaHCO3, conductivity cell with suppression) [13]. 2.3. Batch system The experiments were carried out in a 500 mL conical glass flask equipped with a magnetic stirrer. The reaction temperature was adjusted by a water bath with thermostat. In a typical run, 5 g catalyst were placed in the batch reactor and filled with 500 mL model water containing the pollutant in concentrations between 10 and 50 mg/L. The initial concentration of the pollutants corresponds with amounts in typical ground water in the industrial area of Bitterfeld. The model waters were prepared from distilled water or ground water and the model pollutants chlorobenzene, 4-chlorophenol, 4-chloroaniline and 1-nitro-4-chlorobenzene. For the test the corresponding model water was heated to the designated temperature and the degradation reaction started by the addition of Perhydrol as oxidizing agent (Perhydrol is a solution of 30% hydrogen peroxide in water). During the running oxidation at fixed time intervals samples of 10 mL were taken for analysis. The organic pollutants were analyzed by gas chromatography or high pressure liquid chromatography. Low molecular mass organic acids and inorganic acids were identified and evaluated by means of ion chromatography using a conductivity detector. The content of hydrogen peroxide was determined by titration.

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Control experiments were carried out by corresponding tests without hydrogen peroxide and/or without the respective catalyst.

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[C6H5Cl] or [C6H5OCl] or [Cl ] / [C6H5Cl]0 in % 100 90 80 70

3. Results and discussion

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For catalytic oxidation with full metal catalysts percarbonate, peracetic acid and hydrogen peroxide can be used as oxidizing agents. For economical reasons and due to easy metering hydrogen peroxide is applied above all. For our investigations chlorobenzene, 4chlorophenol, 4-chloroaniline and 1-nitro-4-chlorobenzene were used as model substance. Chlorinated aromatic compounds are pollutants repeatedly detected in the waste water of chemical industry and in the ground water in the area of Bitterfeld/Wolfen. In water phase the catalytic oxidation of pollutants depends on pH-value. At high pH-values an alkaline catalyzed decomposition of hydrogen peroxide takes place and in consequence this competition reaction leads to the necessity of a high excess of oxidizing agent. At extreme low pH-values the effective catalytic surface layer of the catalyst is attacked chemically. Therefore experiments were carried out in the pH-range from 6 to 7. Figure 1 shows the dependence of the degradation of chlorobenzene by catalytic oxidation with MOLoxW 2101 and hydrogen peroxide in distilled water on temperature. Chlorobenzene with aromatic bonded chlorine is converted total at 60 C within 50 min. At higher temperatures thermal decomposition of hydrogen peroxides yields to the degradation of the pollutant in the absence of the catalyst already. The degradation of chlorobenzene was investigated at 20 C. The low reaction rate at this temperature allows a detailed product study. According to figure 2 the primary oxidation products formed from chlorobenzene are chlorophenols and unsubstituted phenol. The min[C6H5Cl]/[C6H5Cl]o in% 100

temperature: 80 ˚C 70 ˚C 60 ˚C 50 ˚C 40 ˚C 30 ˚C

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Figure 1. Catalytic oxidation of chlorobenzene—influence of temperature (screening system, headspace vials; volume: 10 mL, catalyst: 85 mg MOLoxW 2101, oxidizing agent: 0.5 g/L Perhydrol, [C6H5Cl]o ¼ 25 mg/L).

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Figure 2. Catalytic oxidation of chlorobenzene—formation of intermediates and chlorine balance (500 mL batch reactor, catalyst: 5 g MOLoxW 2101, oxidizing agent: 0.5 g/L Perhydrol, [C6H5Cl]o ¼ 20 mg/L, temperature: 20 C).

eralization yields to hydrochloric acid. In trace amounts dihydroxybenzenes and maleic acid were formed intermediately. The concentration of formic acid and oxalic acid in the maximum of the concentration time function account about 20% of the initial concentration of chlorobenzene. Hydrochloric acid was formed during the oxidative catalytic decomposition of chlorobenzene by dehalogenation reaction. Such decomposition and mineralization products are typical for advanced oxidation processes based on hydroxyl radical chain reactions [14,15]. The function of the sum of chlorine containing compounds demonstrates that chlorinated intermediates (like chlorohydroquinones) are formed and degraded during the oxidation process. At the end of the reaction the chlorine balance is 100% indicating a complete dehalogenation without formation of chlorine. From the product studies, a detailed degradation mechanism has been constructed for the OH-radicalinitiated oxidation of chlorobenzene (figure 3). Two possible ways for the formation of hydroxyl radicals at the surface of the catalysts can be postulated: (1) homolytic cleavage of the oxygen–oxygen bond of hydrogen peroxide (catalytical process with adsorption of hydrogen peroxide, destabilization of the OAO bond, and homolytic cleavage of the OAO bond and immediately reaction with organic compound; the formation and the reaction of the hydroxyl radicals occur at the surface of the catalyst) and (2) in a reaction similar to the homolytic Fenton reaction. The hydroxyl radical initiates the oxidation process in the immediate environs of the catalyst. During this radical chain process various oxidizing intermediates (hydroperoxy radicals etc.) are formed [16,17]. Ground water containing chlorobenzene was treated by catalytic oxidation with the purpose to convert the organic bonded chlorine ‡95% to inorganic hydrochloric

J. Hofmann et al./Catalytic oxidation in ground water treatment

Figure 3. Reaction scheme of the degradation of chlorobenzene by catalytic oxidation.

acid. Most chlorine free organic intermediates can be degraded biologically with good results. In consequence the whole process can be carried out in two consecutive steps. In the first step a catalytic oxidation to defined intermediates, namely chlorine free organic molecules, can occur. In the second step for instance a biological degradation technique can be applied. The influence of the hydrogen peroxide concentration on the degradation rate of organic pollutants was investigated. In the range below 0.5 g/L hydrogen peroxide, a linear correlation between the hydrogen peroxide concentration and degradation rate was observed. However, if the concentration of the oxidizing agent is increased above this value, the reaction rate deviates from the linear dependence. This is traced back to a significant increase of the OH-radical concentration, which themselves promotes a reconversion of the OHradicals and takes partly away the just formed OHradicals which are actually needed for the oxidative degradation process. Since at higher concentrations of the hydrogen peroxide the combination of the just formed OH-radicals obviously proceeds faster than the desired oxidation of the chlorobenzene, the use of increasing amounts of hydrogen peroxide causes two different effects: an increase of the degradation rate and therefore shorter treatment times as well as increasing costs for the oxidizing agent. From this follows conclusively that the amount of the used oxidizing agent has to be adopted thoroughly to the concrete problem to be solved. To get more information on the influence of the type of substituent at the aromatic ring the degradation of

4-chlorophenol, 4-chloroaniline and 1-nitro-4-chlorobenzene was carried out. The results are given in figure 4. The degradation rate decreases in the order AOH< ANH2 ANO2. The phenol and the aniline are transferred in the first oxidation step into dihydroxybenzene and aminohydroxybenzene, respectively. These intermediates can be easily oxidized by hydrogen peroxide without any catalyst in the water phase. For this reason the degradation rate of this two compounds is high. In the degradation process of 4-chlorophenole, 4chloroaniline and 1-nitro-4-chlorobenzene the mineralization product hydrochloric acid was detected by ion chromatography. From the aniline ammonium hydroxide was formed. The nitrobenzene yields to nitrate and nitrite. The ratio of nitrate to nitrite depends on the type of catalyst and on the conversion degree. With SG 2157 (mixed metal oxide catalyst) NO2) and NO3) are formed in the ration 10:1. By way of contrast with MOLoxW 2101 (full metal catalyst) NO3) is formed only. Control experiments were carried out with NO2). The reaction rate of the oxidation of NO2) and NO3) is very low and the formation of NO3) via NO2) can be neglected under experimental conditions. The differences in the distribution of the nitrogen containing mineralization products demonstrated clearly, that the oxidation processes with SG 2157 and MOLoxW 2101 are not the same and depend on the metal composition at the surface of the catalyst. For the degradation of organic compounds in water about 50 catalysts have been checked by screening tests in headspace vials. Based on the result in the degradation of chlorobenzene four catalysts were selected for further tests. Using the full metal catalyst MOLoxW 2101, the mixed metal oxide catalyst SG 2157, the heterogeneous FENTON catalyst Fe-03, and the DELTA catalyst high conversion rates of chlorobenzene has been observed. In contrast to the other catalysts with the 1,0

4-chlorophenol 4-chloroaniline 1-nitro-4-chlorobenzene

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Figure 4. Degradation of 4-chlorophenol, 4-chloroaniline and 1-nitro4-chlorobenzene (500 mL batch reactor, catalyst: 5 g SG 2157, oxidizing agent: 2 g/L Perhydrol, temperature: 40 C, initial concentration: 20 mg/L).

J. Hofmann et al./Catalytic oxidation in ground water treatment

DELTA catalyst hydrochloric acid is formed in trace amounts only. The basis material of this catalyst is activated carbon, which is an excellent adsorber of chlorobenzene but not for hydrochloric acid. The detected high ‘‘conversion’’ of chlorobenzene was caused by the adsorption at the surface of the catalyst. The degradation was going with very low reaction rate demonstrated by slow formation of hydrochloric acid. With Fe-03, SG 2157, and MOLoxW 2101 the degradation rate of chlorobenzene and the formation rate of hydrochloric acid are in the same range. The formation of chlorine was not observed. With these catalysts adsorption effects can be excluded. The catalyst SG 2157 contains manganese oxide, which effects in a partial degradation of hydrogen peroxide to oxygen and water. Due to this undesirable side reaction hydrogen peroxide gets lost. The highest degradation rates of chlorobenzene, 4-chlorophenol, 4-chloroaniline and 1-nitro-4-chlorobenzene were determined with the full metal catalyst MOLoxW 2101 and the mixed metal oxide catalyst SG 2157. The rates with Fe-03 are acceptable. The comparison of the rates with different pollutants in the model water demonstrates the necessity for adaptation of the catalyst. Further studies for scaling up of the reaction and for the improvement of the composition of the catalysts are planned.

4. Conclusions A large number of organic pollutants in water can be degraded by heterogeneous catalytic oxidation. In order to generate a high effective degradation process, the process parameters like temperature, dosage of the oxidizing agent and amount of catalyst have to be adjusted to the specific ground or waste water problem.

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Acknowledgment The authors are grateful to BMBF (grant 02WT9942/ 4) and to Max Buchner Forschungsstipendium (grant 2462) for financial support. Thanks for the good collaboration to the environmental research center (Umweltforschungszentrum Leipzig-Halle).

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