Abiotic degradation

Natural attenuation refers to processes that naturally transform contaminants to less harmful forms or hnmobihze contaminants so that they are less of a threat to the environment see Natural Attenuation for Groundwater Remediation by the National Research Council (2000). This includes dispersion/dilution, sorphon, volatihzahon, and degradation (abiotic, biotic). 

Once in the water column, substances may be degraded abiotically via photodegradation and/or hydrolysis or biot-ically by aerobic or anaerobic organisms. Highly sorptive substances may partition to the bed sediment. A significant amount of information is available on the fate and behavior of many veterinary antibiotics in sediment due to their use as aquaculture treatments. " While many compounds degrade very quickly (e.g., chloramphenicol, florfenicol, ormethoprim), others persist in the sediment for months to years (e.g., oxolinic acid, oxytetracycline, sarafloxacin, sulfadiazine, trimethoprim). 

Presently enzymes can hardly be used to degrade artificial synthetic polymers unless it is under special conditions. It is worth noting that compounds like poly(vinyl alcohol), PVA, bacterial polymers and poly(e-caprolatone), PCL, that are biodegradable under outdoor conditions are degraded abiotically and thus very slowly in an animal body where they are not biodegradable. Despite this difficulty the number of artificial polymers proposed as biodegradable biomaterial candidates to replace biopolymers or biostable polymers is increasing. 

Degradation or Transformation. Degradation or transformation of a herbicide by soil microbes or by abiotic means has a significant influence not only on the herbicide s fate in the environment but also on the compound s efficacy. Herbicides that are readily degraded by soil microbes or other means may have a reduced environmental impact but may not be efficacious. Consider the phenomenon of herbicide-resistant soils. In these cases, repeated application of a given herbicide has led to a microbial population with an enhanced ability to degrade that herbicide (252,253). This results in a decrease or total loss of the ability of the herbicide to control the weed species in question in a cost-effective manner. 

Degradation of a herbicide by abiotic means may be divided into chemical and photochemical pathways. Herbicides are subject to a wide array of chemical hydrolysis reactions with sorption often playing a key role in the process. Chloro-j -triazines are readily degraded by hydrolysis (256). The degradation of many other herbicide classes has been reviewed (257,258). 

BASE SET Mutagenicity Toxicity to reproduction Toxicity to algae Acute daphnia and fish toxicity Abiotic and readily biotic degradability Additional physico-chemical properties 1 t/annum or 5 t cumulative... 

Upper LEVEL 1 Chronic toxicity Toxicity in soil and plants Additional mutagenicity Long-term toxicity Bioaccumulation Inherent biodegradability Additional abiotic degradability 100 t/annum or 500 t cumulative... 

Additional chronic toxicity Additional environmentally dangerous properties Toxicity to birds Long-term toxicity in water and soil Degradability simulation tests Additional abiotic degradability Mobility in water, soil and air cumulative... 

Endosulfan in aqueous solutions is also expected to undergo biodegradation. In laboratory tests at pH 7 and 20 , Pseudomonas bacteria degraded endosulfan (isomers not specified) under aerobic conditions with a half-life of about 1 week (Greve and Wit 1971). Biotic and abiotic transformations of endosulfan in seawater/sediment microcosms have been reported (Gotham and Bidleman 1989). In biotic tests, half-lives for the a- and P-isomers in seawater-only microcosms (pH 8) were about 5 and 2 days, respectively. In seawater-only microcosms under sterile conditions at a pH of 8 or higher, the half-life for the a-isomer was 2-3 days, whereas the half-life for the p-isomer was 1-2 days. Half-lives were longer in seawater/sediment microcosms, possibly because of the lower pHs (7.3-7.7) in these test systems half-lives were 22 and 8.3 days for the a- and P-isomers, respectively. Endosulfan diol was the main metabolite identified. 

Walker WW, Cripe CR, Pritchard PH, et al. 1988. Biological and abiotic degradation of xenobiotic compounds in in vitro estuarine water and sediment/water systems. Chemosphere 17 2255-2270. 

Ecotoxological properties Acute toxicity to fish Acute toxicity to Daphnia Biodegradability Hydrolysis (abiotic degradability)... 

Abiotic hydrolysis generally accomplishes only a single step in the ultimate degradation of the compounds that have been used for illustration. The intervention of snbseqnent biotic reactions is therefore almost invariably necessary for their complete mineralization. 

Klupinski TP, Y-P Chin (2003) Abiotic degradation of trifluralin by Fe(II) kinetics and transformation pathways. Environ Sci Technol 37 1311-1318. 

Nitrotoluene is degraded by a strain of Mycobacterium sp. via the corresponding 4-amino-3-hydroxytoluene (Spiess et al. 1998) this is dimerized abiotically to form a dihydrophenox-azinone, and after extradiol cleavage to 5-methylpyridine -2-carboxylate (Figure 2.2d). 

RDX and its partial reduction product hexahydro-l-nitroso-3,5-dinitro-l,3,5-triazine were degraded by K. pneumoniae to methylenedinitramine, and then to CHjO and methanol, while abiotic reactions produced NjO (Zhao et al. 2002). 

Nitrophenols are phytotoxic, and dinoseb (6-iec-butyl-2,4-dinitrophenol) has been used as a herbicide, while nitrophenols have been detected in rainwater and plausible mechanisms for their abiotic formation have been proposed (Kohler and Heeb 2003 Vione et al. 2005). The pathway for the degradation of phenols with a single nitro group depends on the position of the substituents, while... 

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