Abiotic Chemical Transformations

Abiotic chemical transformation is the reduction of chemical concentrations by degrading the chemicals into other products. The most important chemical transformations are hydrolysis and oxidation/reduction reactions. 

Most of the abiotic chemical transformation products reported in this book are limited to only three processes hydrolysis, photooxidation, and chemical oxidation-reduction. These processes are the most widely studied and reported in the literature. Detailed information describing the above technologies, their availability/limitation and company sources is available (U.S. EPA, 1987). 

ABIOTIC CHEMICAL TRANSFORMATIONS TABLE 2.11 Half Lives for Disappearance via Direct Photolysis 187 in Aqueous Media ... 

Chemical, or abiotic, transformations are an important fate of many pesticides. Such transformations are ubiquitous, occurring in either aqueous solution or sorbed to surfaces. Rates can vary dramatically depending on the reaction mechanism, chemical stmcture, and relative concentrations of such catalysts as protons, hydroxyl ions, transition metals, and clay particles. Chemical transformations can be genetically classified as hydrolytic, photolytic, or redox reactions (transfer of electrons). 

More likely, there are ephemeral intermediate species with short residence times, and the reaction proceeds in several steps with several intermediates. In such a reaction pathway, changes in the relative rates of the reaction steps can result in changes in the fractionation. Furthermore, there may be multiple pathways by which a chemical transformation can occur. For example, transformation of Se(IV) to Se(0) could proceed via simple abiotic reaction, or via uptake of FlSeOj by a plant, reduction to Se(-ll) within the plant, incorporation into amino acids, death and decay of the plant, release of the Se(-II), and oxidation to Se". The overall transformation, from Se(lV) to Se(0), is the same, but because the two reaction pathways differ greatly, the overall isotopic fractionation may be greatly different. 

For many elements, the concentration of a species in a given body of water can be predicted by abiotic chemical reactions such as protolysis, precipitation, complexation, redox, and sorption. Aquatic organisms can influence the concentration of compounds directly by metabolic uptake, transformation, storage, and release. Aquatic organisms may also cause chemical reactions by changing the concentrations of solutes which are important in abiotic equilibria. 

Abiotic degradation transforms organic compounds by chemical reactions such as oxidation, reduction, hydrolysis, and photodegradation. Abiotic degradation processes do not usually achieve a complete breakdown of the chemical (mineralization). 

We have chosen to follow Watts [24] and discuss chemical and biological transformation processes in the same section. Watts notes that, although this approach is somewhat nontraditional, it is advantageous in that understanding of the abiotic chemical reactions serves as a conceptual basis for understanding the biochemical reactions (which are essentially the same except for the fact that the biochemical reactions are mediated by microorganisms). Where a reaction is predominantly abiotic or biotic, it will be noted in the discussion. In this section, the fundamentals of each chemical or biological reaction will be discussed, and model formulations for the reaction kinetics presented. 

Chemical transformations of biotic and abiotic nature, using air and water as their mobile phases and equilibrating media... 

Arsenic speciation in anaerobic sediments is controlled by both microbially mediated transformations of species and by abiotic chemical processes including adsorption. The two... 

Abiotic degradation comprises chemical transformation and photochemical transformation. Usually abiotic transformations will yield other organic compounds but will not cause a full mineralization (Schwarzenbach et al, 1993). Chemical transformation is defined as transformation that happens without light and without the mediation of organisms whereas photochemical transformations require light. 

Examples of relevant chemical transformation processes in aqueous environment are hydrolysis, nucleophilic substitution, elimination, oxidation and reduction reactions (Schwarzenbach et al, 1993). Of these, hydrolysis is often considered the most important and it is the only chemical transformation process for which international test guidelines are generally available. The tests for abiotic degradation of chemicals are generally in the form of determination of transformation rates under standardized conditions. 

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. 

Oxidation reactions that take place in aquatic environments can be mediated by direct or indirect photolysis reactions, which depend on the organic chemicals and substrates present. Nonphotolytic oxidation of organic chemicals can occur directly by reactions involving ozone, or via catalytic pathways with certain metals. Abiotic reduction reactions that influence organic chemical transformation in wetlands include Fe and Mn species and sulfides. 

Abiotic reactions include chemical transformation, which results in the formation of more simple organic compounds but not in total mineralization. An important role in these reactions is played by nucleophils -electron-excessive chemical reagents (anions or molecule), which have an undivided pair of electrons on the outer electron level (OH, Cl, Br, CN, H O, CHjOH, NHj etc.). Most important abiotic reactions are hydrolysis and nucleophile substitution. 

Solar radiation drives a number of chemical transformations of mercury. These include (i) atmospheric speciation and deposition, (ii) oxidation-reduction in both freshwater and seawater, and (iii) methyl mercury degradation. Both biotic and abiotic redox reactions are influenced. While microbes have been thought to dominate methyl mercury production, abiotic formation cannot be... 

The chemical era of abiotic photochemical transformation of the primordial atmosphere and oceanic water to form the organic molecules from which life could spring. 

Transformation reactions concern chemicals in the environment by abiotic (chemical and photochemical) and biotic (microbial) pathways. 

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