Abiotic and biotic transformation

Vogel TM, McCarty PL. 1987. Abiotic and biotic transformations of 1,1,1-trichloroethane under methanogenic conditions. Environ Sci Technol 21 1208-1213. 

The second case refers to hormones, pharmaceuticals, and other compounds that are ingested, metabolized, and excreted by mammals (Table 1). Usually a hormone or pharmaceutical is extensively metabohzed in the body and is excreted by mammals as a mixture of different metabolites. Although the general belief is that metabolism renders a drug more water soluble and consequently less hazardous for the aquatic environment, there are exceptions for pro-drugs and specifically acting metabolites. The third case refers to environmental transformation products of pesticides and other environmental pollutants (Table 1), which are formed both by abiotic and biotic transformation processes. 

Figure 2.20. Transformation of catechol by laccase (0.4 units mT1), tyrosinase (0.4 units ml-1) and birnessite (600ugml 1) after repeated addition of substrate. Reprinted from Pal, S., Bollag, J.-M., and Huang, P. M. (1994). Role of abiotic and biotic catalysts in the transformation of phenolic compounds through oxidative coupling reactions. Soil Biol. Biochem. 26, 813-820, with permission from Elsevier.

The pollutants discharged into the environment, may, in the best case, undergo abiotic or biotic transformations, resulting in innocuous compounds that may not represent an immediate health hazard. However, many of the xenobiotic compounds are toxic and may undergo partial transformations that result in even more toxic derivatives. This process is known as activation. 

The global expansion of industrial and consumer-oriented societies is linked to large-scale industrial production and consumerism that utilize a vast array of numerous chemical compounds. The listings of such chemicals are too vast to present in this paper but some examples will be discussed here. Environmental contaminants in nature typically involve complex mixtures, partitioning factors, chemical transformations, and abiotic and biotic interactions. The biological and environmental effects are complex and may be additive, synergistic and even antagonistic in nature. 

By no means is this book intended to provide a detailed account of all progress made in the past 25 years by aquatic chemists. Rather, chapters provide examples of recent developments in the field and contribute toward a better understanding of the mechanisms regulating the chemical composition of natural waters. Also, the transformation and transport of species (abiotic and biotic or soluble and insoluble) in aquatic systems (lakes, rivers, estuaries,... 

Solid-water interfaces provide sites for important abiotic and biotic reactions that involve the uptake and transformation of pollutants. The availability of solid surfaces and the physical and chemical characteristics of solid-water interfaces in aquatic environments can be determined by particle aggregation and deposition reactions. The kinetics of these colloid chemical reactions play an important role in the transport, reactivity, fate, and impact of pollutants and other particle-reactive substances in natural waters. 

The alterations in chemical composition of naturally weathered spilled oils are generally resulted from the combined effects of abiotic and biotic weathering, as Figure 27.8 and Figure 27.9 shows. The transformations of oil hydrocarbons by biodegradation are likely to occur stepwise, producing alcohols, phenols, aldehydes, and carboxylic acids in sequence. 

In the absence of oxygen, humic substances are resistant to decomposition and represent a significant carbon and nutrient storage in wetlands. Under drained conditions, humic substances are rapidly degraded, which releases nutrients to the bioavailable pool, thereby affecting downstream water quality. Humus is generally defined as the organic material that has been transformed by abiotic and biotic processes into stable form (see Section 5.4.3) and consists of two major types of compounds (Stevenson, 1986) ... 

There has been considerable interest in the reductive transformation of the polynitroaromatics, 2,4-dinitrotoluene (2,4-DNT), and 2,4,6-trinitrotoluene (TNT) in anaerobic environments. These chemicals are high volume munitions, which because of their improper disposal and storage, have contaminated soils and aquifers at numerous sites (Kaplan and Kaplan, 1982). Enhancing the reduction of these contaminants in situ using abiotic and biotic processes has been proposed as a remediation technique for contaminated soils and sediments (Kaplan, 1990 Ou et al., 1992). The reduction of TNT and DNT, as well as other polynitroaromatics, has been studied in some detail in anaerobic microbial enzyme systems (McCormick et al.,... 

As shown in Figure 4, 1,1,1 TCA is transformed by parallel abiotic and biotic pathways. Dehydrohalogenation yields 1,1-DCE [45]. As shown in pathway 1, 1,1-DCE is susceptible to reductive dehalogenation to vinyl chloride. Abiotic or biotic hydrolysis of TCA is illustrated by pathway 3 of Figure 4 [11]. Reductive dechlorination of TCA to 1,1-DCA [27,31,34,69] and chloroethane (CA) is illustrated in pathway 2 [31,34, 69,70]. CA apparently undergoes hydrolysis to ethanol (a nonvolatile product), followed by oxidation of the ethanol to CO [31]. In many systems, the above pathways operate simultaneously. For example, Clostridium sp. strain TCAIIB, transformed TCA to 1,1-DCA ( 30-40%), acetic acid (7%), and unidentified products (45%) [71]. 

Figure 4. Abiotic and biotic pathways for transformation of 1,1,1-TCA (a=abiotic, b=biotic, p=pure culture, M=mixed culture, mg=methanogens).

The disappearance of a solvent fi om solution can also be the result of a number of abiotic and biotic processes that transform or degrade the compound into daughter compounds that may have different physicochemical properties from the parent solvent. Hydrolysis, a chemical reaction where an organic solvent reacts with water, is not one reaction, but a family of reactions that can be the most important processes that determine the fate of many organic compounds. Photodegradation is another family of chemical reactions where the solvent in solution may react directly under solar radiation, or with dissolved constituents that have been made reactive by solar radiation. For example, the photolysis of water yields a hydroxyl radical ... 

The disappearance of a plasticizer from water can be the result of a number of abiotic and biotic processes that can transform or degrade the compound into daughter compounds that have different physicochemical properties from the parent compound. Hydrolysis is a family of chemical reactions where a plasticizer reacts with water. Phthalate esters may hydrolyze to form monoesters and then dicarboxylic acid. It has been predicted that di-(2-ethylhexyl) sebacate will form 2-ethylhexanol and decanedioic acid. Wolfe et al experimentally measured second-order alkaline hydrolysis rate constants for dimethyl, diethyl, di-n-butyl, and di-(2-ethylhexyl) phthalates, and it appears that hydrolysis may be too slow to have a major impact on the fate of most dissolved plasticizers. The estimated hydrolysis half-lives at pH 7 for 20 plasticizers were longer than 100 days. No information was located for diallyl, ditridecyl and diundecyl phthalates. Under alkaline conditions, hydrolysis may be important for tricresyl phosphate and tri-(2-ethylhexyl) trimellitate at pH 8 their predicted half-lives are 3.2 and 12 days respectively. 

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