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Carbon Suboxide, a Highly Reactive Intermediate from the Abiotic Degradation of Aromatic Compounds in Soil

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Carbon Suboxide, a Highly Reactive Intermediate from the Abiotic Degradation of Aromatic Compounds in Soil Environ. Sci. Technol. 2007, 41, 7802-7806 Because of their possible emission into the atmosphere, Carbon Suboxide, a Highly Reactive the natural formation of volatile compounds has been the Intermediate from the Abiotic subject of numerous publications over the past years (6-9). Biotic processes through mammals, insects, fungi, plants, Degradation of Aromatic Compounds and bacteria leading to methane, chloromethane, and chloroform are well-known. However, abiotic processes have in Soil been attracting more and more attention. Many investigations were performed discussing the degradation of soil organic STEFAN G. HUBER,* matter (SOM) through different redox processes. As the GERHARD KILIAN, AND structure of SOM is very complex, it is common to use model HEINZ F. SCHOÈ LER compounds, such as substituted phenols or catechols, which represent fragments of the humic substances in soil (7). Institute of Environmental Geochemistry, University of Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Hydroxyl radicals play an important part in these redox Germany processes because they readily react with organic molecules initializing subsequent oxidation steps. Their release from the reaction of redox-active metals such as iron or manganese with members of the oxygen family (hydrogen peroxide, The formation of volatile compounds during abiotic superoxide, molecular oxygen, etc.) is explained by the Fenton degradation processes of aromatic compounds in soil has (10) and Fenton-like or Haber-Weiss (11) reactions. Small been the subject of many experimental studies but molecules such as chloroethyne, chloroethene, and carbon should be examined further. In this context, the present dioxide are formed during the degradation of catechol in the presence of Fe(III) and chloride (12-15). For their formation, work investigates the natural formation of carbon suboxide cleavage of the aromatic ring is an important step that is using the model compounds catechol and 3,5-dichloro- followed by further redox reactions. catechol and also a soil sample from a peat bog. The Chloroethyne and carbon dioxide merely comprise small measurements were performed with a purge and trap GC/ fragments of the aromatic ring; thus, we expected further MS system following various optimization steps. Under volatile compounds that are generated in the course of SOM certain conditions, we obtained 16.7 ng of carbon suboxide decomposition. Carbon monoxide and ethene are some from a 250 mg soil sample. We also found that the plausible candidates. On the basis of mechanistic models, formation of carbon suboxide requires a definite activation we postulated that carbon suboxide is a conceivable inter- energy and that it is rather short-lived in the natural mediate among the C3-fragment molecules during the environment. A subsequent reaction to malonic acid is cleavage of the aromatic ring. This finally led to the assumption that the natural formation of C3O2 in soil should expected in the presence of water. It is shown that iron- be possible. (III), hydrogen peroxide, and chloride are prerequisites for its To verify this hypothesis, we selected catechol as the formation. Experimental parameters for the highest yield starting model compound because it is a constituent of of carbon suboxide depend on the precise molecular structure natural SOM. Iron as a redox-sensitive element, promoting of the model compound or on the individual soil sample, the necessary electron transfer for the degradation reaction, respectively. The presented results point to a new degradation and hydrogen peroxide as a strong oxidant were chosen. process for aromatic compounds in soil. Additionally, chloride was used to enable halogenation processes during the degradation pathway, which may affect the formation of carbon suboxide. Preliminary results from Introduction the catechol reaction showed an elevated yield, if either Publication Date (Web): October 12, 2007 | doi: 10.1021/es071530z With four successive double bonds, carbon suboxide (Od chloride or chlorine covalently bound to the aromatic CdCdCdO) is a very unstable compound. This linear and structure are present. A chlorinated catechol was therefore Downloaded by UNIV HEIDELBERG on September 15, 2009 | http://pubs.acs.org symmetric molecule, known as doubly dehydrated malonic applied in a series of optimization steps, wherein the influence acid, is a toxic gas with an unpleasant odor. Its escharotic of different parameters on the production of C3O2 was property is principally due to a remarkable charge distribu- tested: the time dependence, the concentration of hydrogen tion. The central carbon atom is the most nucleophilic center peroxide, the concentration of chlorinated catechol, the pH (1). C3O2 is used in organic chemistry in various addition value, and the temperature. To confirm a possible natural reactions with nucleophilic substances, such as alcohols or formation of carbon suboxide, studies with soil samples were phenols, producing the corresponding esters (2). In addition, carried out, which are discussed in the last section of this carbon suboxide can polymerize to the chemically and paper. thermodynamically stable (C3O2)n (3), which has been considered a constituent of the atmosphere of Venus, and Materials and Methods it also has been proposed to be responsible for the red color Chemicals. The following chemicals were used as received: of the surface of Mars (1, 4). Therefore, a natural occurrence catechol (99%; Lancaster Synth), 4-chlorocatechol (97%; of C3O2 is conceivable. Aldrich), 3,5-dichlorocatechol (97%; Aldrich), 4,5-dichloro- In recent research (5), the use of polymerized carbon catechol (97%; Aldrich), isoprene (99%; Aldrich), hydrogen suboxide with an average oxidation state of 1.33 is discussed peroxide (30%; Merck), iron(III) sulfate (Fe 21-23%; Riedel- as an alternative end product of fossil fuel burning instead de-HaeÈn), potassium chloride (99%+, Merck), and sodium of volatile carbon dioxide. The aim is to obtain a still profitable hydroxide (99%, Aldrich). Doubly distilled deionized water energy yield and yet reduce the release of the greenhouse (18 MΩ cm), from an ELGASTAT UHQ PS water treatment gas, CO2. system, was employed in all experiments. All pH measure- * Corresponding author phone: ++49 6221 544818; fax: ++496221 ments were made using a Mettler Toledo 320 pH meter, 545228; e-mail: [email protected]. calibrated on the free hydrogen scale with 4.0 and 7.0 buffers. 7802 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 22, 2007 10.1021/es071530z CCC: $37.00 2007 American Chemical Society Published on Web 10/12/2007 Instrumentation. GC/MS analyses were performed on a Varian STAR 3400 Cx gas chromatographic system connected TABLE 1. Properties of Rotwasser Soil to a Saturn 2000 ion trap mass spectrometer. Sampling, soil type peatland preconcentration, and injection of the volatile analytes were controlled by a Tekmar purge and trap system. The GC was geographic position 49°,36′,39′′ N, 8°,53′,11′′ E a equipped with a BP624 (25 m; 0.53 mm i.d.; 3.0 µm film organic carbon (Corg) 1.64% inorganic carbonb <0.05% thickness) and a BPX5 (60 m; 0.32 mm i.d.; 1.0 µm film Cl- concentrationc 4.5 ppm thickness) capillary column, both connected in series. Fe concentrationd 1.8% Experimental Procedure. The experiments were carried pH value 4.0 out in airtight closed 20 mL headspace glass vials, filled with a b c 10 mL of aqueous solution. They were shaken for a CS measurement. Via a carbonate bomb (16). IC measurement. d µ-XRF measurement. determined time on a rotary board at 500 rpm at a precise temperature. After introduction of an inlet and an outlet stainless steel needle through the septum, the volatile compounds were purged for 4 min with a stream of helium at 40 mL/min and concentrated on a cooled trap (below -60 °C), filled with Tenax. Moisture was removed on a magnesium perchlorate water trap from the sample stream. The loaded preconcentration trap was switched into the GC carrier gas line (He, 2.0 mL/min) and heated to 180 °C. The desorbed compounds were refocused at the beginning of the capillary column, which was cooled with liquid nitrogen to -90 °C. After 4 min, the GC oven temperature program started automatically: -90 to 0 °Cat40°C/min, hold 1 min, 0 to 150 °Cat5°C/min, hold 5 min, 150 to 210 °Cat30°C/min, hold 15 min. Preliminary Tests. The formation of carbon suboxide was examined using catechol as well as chlorinated catechols. In these assays, solutions of 10 mL of water with initial FIGURE 1. Chromatogram section, showing the formation of C3O2 concentrations of 0.50 mM organic compound, 2.50 mM Fe2- in preliminary tests with catechol, 4-chlorocatechol, 3,5-dichlo- (SO4)3, 5.00 mM H2O2, and 5.00 mM KCl were prepared and rocatechol, 4,5-dichlorocatechol, and tetrachlorocatechol using Fe - ° 2 shaken for 20 s at 40 C. (SO4)3,H2O2, and KCl. Process Optimization. On the basis of the preliminary results, 3,5-dichlorocatechol was chosen for the next series of experiments. The yield of carbon suboxide was to be this, solutions of 10 mL of water with initial concentrations maximized by systematic sequential variation of the following of 0.50 mM catechol, 2.50 mM Fe2(SO4)3, 5.00 mM H2O2, and reaction parameters. 5.00 mM KCl were prepared and shaken for 1 min at 40 °C. Time Dependence. The first optimization step was to Soil Sampling. The next logical step was to apply the elaborate the time dependence of C3O2 formation and knowledge gained with the model compounds to natural degradation. Solutions of 10 mL of water with initial samples. The employed peatland soil was sampled in March concentrations of 0.50 mM 3,5-dichlorocatechol, 2.50 mM 1996 from the natural reserve Rotwasser, Odenwald/Germany Fe2(SO4)3, and 5.00 mM H2O2 were prepared and shaken at (49°,36′,39′′ N, 8°,53′,11′′ E).
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