Anaerobic oxidation, chlorinated

Aerobic, Anaerobic, and Combined Systems. The vast majority of in situ bioremediations ate conducted under aerobic conditions because most organics can be degraded aerobically and more rapidly than under anaerobic conditions. Some synthetic chemicals are highly resistant to aerobic biodegradation, such as highly oxidized, chlorinated hydrocarbons and polynuclear aromatic hydrocarbons (PAHs). Examples of such compounds are tetrachloroethylene, TCE, benzo(a)pyrene [50-32-8] PCBs, and pesticides. 

Anaerobic oxidation has also been demonstrated as a pathway for chlorinated ethene degradation. Microorganisms have been shown to oxidize VC to C02 under ferric iron-reducing conditions [43] and DCE has been shown to oxidize directly to C02 under Mn(IV)-reducing conditions [44]. In addition, Bradley [33] reports that VC can be oxidized under methanogenic conditions. Suarez and Rifai [31] report first-order rate constants for the anaerobic oxidation of VC. 

Biooxidation is decomposition of organic matter with oxidizing of its carbon. Organic matter in these reactions is donor of electrons, and the acceptors are elements or compounds outside it O, NO3. NO T Fe, iron hydroxide Fe(OH)3>, CO, some chlorinated solvents, etc. There may be aerobic and anaerobic oxidizing. In the former case acceptor of electrons is directly molecular oxygen O, in the latter oxidized forms of nitrogen (NOj", NO3 ), manganese (Mn ), iron (Fe +), sulphur (SO ), etc. 

Chlorinated ethenes are subject to a variety of microbial degradation processes that include reductive dechlorination (Vogel et al., 1987 Maymo-Gatell et al., 1997), aerobic oxidation, anaerobic oxidation (Bradley and Chapelle, 1996), and anaerobic cometabolism (McCarty and Semprini, 1994). Both, laboratory studies (Bradley and Chapelle, 1998), and field studies (Chapelle and Bradley, 2000) show that the efficiency of chlorinated ethene biodegradation depends on ambient redox conditions. Therefore, reliable tools to measure the redox conditions are crucial to imderstand and even predict chlorinated ethene degradation. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Certain anaerobic bacteria can reductively dechlorinate PCBs in sediments (EHC 140). Higher chlorinated PCBs are degraded more rapidly than lower chlorinated ones, which is in contrast to the trend for oxidative metabolism described earlier. Genetically engineered strains of bacteria have been developed to degrade PCBs in bioremediation programs. 

A facultatively anaerobic organism designated Anaeromyxobacter dehalogenans (Sanford et al. 2002) was capable of dechlorinating ortho-chlorinated phenols using acetate as electron donor—2-chlorophenol was reduced to phenol and 2,6-dichlorophenol to 2-chloro-phenol (Cole et al. 1994). A strain of Desulfovibrio dechloracetivorans was also able to couple the dechlorination of ortho-substituted chlorophenols to the oxidation of acetate, fumarate, lactate, and propionate (Sun et al. 2000). 

Anaerobic demetalation of Mg[pz(/V-Mc2)8 gives only the free base pz, H2[pz(7V-Me2)8] 101 (TFA, 21% and AcOH, 69% yield, respectively). Slow oxidation of aminoporphyrazines also occurs when solutions are left standing for prolonged times in chlorinated solvents CH2C12 and CHC13 while no oxidation is observed in polar solvents such as DMF and pyridine. 

Chemical/Physical. Matheson and Tratnyek (1994) studied the reaction of fine-grained iron metal in an anaerobic aqueous solution (15 °C) containing chloroform (107 pM). Initially, chloroform underwent rapid dehydrochlorination forming methylene chloride and chloride ions. As the concentration of methylene chloride increased, the rate of reaction appeared to decrease. After 140 h, no additional products were identified. The authors reported that reductive dehalogenation of chloroform and other chlorinated hydrocarbons used in this study appears to take place in conjunction with the oxidative dissolution or corrosion of the iron metal through a diffusion-limited surface reaction. 

Biological. A strain of Alcaligenes eutrophus degraded 35% of the congeners by dechlorination under anaerobic conditions (Bedard et ah, 1987). Indigenous microbes in the Center Hill Reservoir, TN oxidized 2-chlorobiphenyl (a congener present in trace quantities) into chlorobenzoic acid and chlorobenzoylformic acid. Biooxidation of the PCB mixture containing 54 wt % chlorine was not observed (Shiaris and Sayler, 1982). 

Chemical/Physical. Decomposes at elevated temperatures forming methyl isocyanate (Windholz et al., 1983) and nitrogen oxides (Lewis, 1990). Hydrolyzes in water to 1-naphthol and 2-iso-propoxyphenol (Miles et al., 1988). At pH 6.9, half-lives of 78 and 124 d were reported under aerobic and anaerobic conditions, respectively (Kanazawa, 1987). Miles et al. (1988) studied the rate of hydrolysis of propoxur in phosphate-buffered water (0.01 M) at 26 °C with and without a chlorinating agent (10 mg/L hypochlorite solution). The hydrolysis half-lives at pH 7 and 8 with and without chlorine were 3.5 and 10.3 d and 0.05 and 1.2 d, respectively. The reported hydrolysis half-lives of propoxur in water at pH 8, 9, and 10 were 16.0 d, 1.6 d, and 4.2 h, respectively (Aly and ELDib, 1971). In a 0.50 N sodium hydroxide solution at 20 °C, the hydrolysis half-life was reported to be 3.0 d (ELDib and Aly, 1976). 

For many years, niclosamide (Niclocide) was widely used to treat infestations of cestodes. Niclosamide is a chlorinated salicylamide that inhibits the production of energy derived from anaerobic metabolism. It may also have adenosine triphosphatase (ATPase) stimulating properties. Inhibition of anaerobic incorporation of inorganic phosphate into ATP is detrimental to the parasite. Niclosamide can uncouple oxidative phosphorylation in mammalian mitochondria, but this action requires dosages that are higher than those commonly used in treating worm infections. 

The chemistry of chlorine is such that it generally succeeds in oxidizing and breaking down only the surface of slime masses. It is often poor at penetrating and dispersing heavy anaerobic infestations and is completely indifferent to what it will oxidize, attack, or corrode, so there is a fundamental need for chlorine alternatives in the service industry s microbiological control armory. 

The reductive reaction is slow under anaerobic conditions, because iron may be oxidized by oxygen. Chlorinated contaminants possess an oxidizing... 

More oxidized compounds, such as chlorinated benzenes, are susceptible to biologically mediated reduction in environments under anaerobic conditions, such as in lake and river sediments. It is known that highly polychlorinated biphenyl (PCB) congeners, for example, are susceptible to reductive dehalo-genation, the result of the interaction of syn-trophic microbial communities that are active under methanogenic and sulfate-reducing... 

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