At pilot- or full-scale treatment, cometabolic and direct dechlorination may be indistinguishable, and both processes may contribute to contaminant removal. Cometabolic dechlorination does not appear to produce energy for the organism. In this case, contaminants are degraded by microbial enzymes that are metabolizing other organic substrates. Anaerobic dechlorination also may occur via cometabolism where the dechlorination is incidental to the metabolic activities of the organisms. In anaerobic metabolism, nitrate, sulfate, carbon dioxide, oxidized metals, or organic compounds may replace oxygen as the electron acceptor. Depending on the contaminant of concern, a subset of these activities may be cultivated. EPA 2006 Engineering Issue: In Situ and Ex Situ Biodegradation Technologies for Remediation of Contaminated SitesĪnaerobic: Direct anaerobic metabolism involves microbial reactions occurring in the absence of oxygen and encompasses many processes, including fermentation, methanogenesis, reductive dechlorination, sulfate-reducing activities, and denitrification. These conditions may be present naturally but often in the presence of a source area oxygen and a substrate such as methane or propane will need to be introduced. For aerobic cometabolism to occur there must be sufficient oxygen and a suitable substrate which allows the microbe to produce the appropriate enzyme. This can sometimes be reversed by adding oxygen in the form of air (air sparging, bioventing), ozone, or slow oxygen release compound (e.g., ORC(r)).Īerobic dechlorination may also occur via cometabolism where the dechlorination is incidental to the metabolic activities of the organisms. Degradation of contaminants that are susceptible to aerobic degradation but not anaerobic often ceases in the vicinity of the source zone because of oxygen depletion. The bacteria uses a carbon substrate as the electron donor and oxygen as the electron acceptor. Technical and Technology Support ServicesĪerobic: Direct aerobic metabolism involves microbial reactions that require oxygen to go forward.Measurement and Monitoring Technologies for the 21st Century (21M 2). Federal Remediation Technologies Roundtable.Triad: A Smarter Solution to Site Cleanup.High-Resolution Site Characterization (HRSC).Global Efforts to Advance Remediation at Contaminated Sites.Polycyclic Aromatic Hydrocarbons (PAHs).Per- and Polyfluoroalkyl Substances (PFAS). Dense Nonaqueous Phase Liquids (DNAPLs).Their salts have great practical significance – chromates and dichromates accordingly. They can only exist in solutions and are practically not used. 2 acids are examined in this case as hydroxyls – chrome HCrO₄ and dechrome H₂Cr₂O₇. Chromium in the oxidation state of +6Ĭompounds of chromium in which it displays an oxidation state of +6 are strong oxidizers. Salts where chromium at an oxidation state of +3 acts as a cation display all typical properties of salts (most of them are soluble in water and hydrolyze – they decompose in water with the formation of chromium hydroxide Cr(OH)₃):Ĭhromium salts with an oxidation state of +3 can take part in oxidation-reduction reactions, for example in the following:ĢCrCl₃ + 3Zn + 4HCl = 2CrCl₂ + 3ZnCl₂ + 2H₂ (in the reaction between hydrochloric acid and zinc, atomic hydrogen is released, which reduces the chromium cation to the chromium cation). Chromium (II) oxideĬhromium (II) oxide is formed in the decomposition of chromium carbonyl (with heating):Ĭr(CO)₆ = CrO + 5CO + C. The reductive ability of Cr²⁺ salts is very high (in some cases these salts can even displace hydrogen from water). Hydrogen released in the course of reaction reduces Cr³⁺ to Cr²⁺. They are usually obtained by oxidation-reduction reactions from chromium (III). Salts of chromium (II) have a bluish color. The compounds are colored – chromium (II) oxide is black, and the hydroxide is yellow. Chromium in the oxidation state of +2Ĭhromium (II) oxide and hydroxide CrO and Cr(OH)₂ are compounds which display typical base properties. In compounds (which are brightly colored in the majority of cases), chromium displays several possible oxidation states - +2, +3, +4 (encountered quite rarely, chromium oxide CrO₂ is known), +6.
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