Chemical substances abiotic reactions

Once a chemical is released to the environment, biotic and abiotic reactions may degrade or transform the substance. 

Matter is neither created nor destroyed one can quanhfy the fate and transport of a chemical substance in the environment by a mass balance that accounts for phase changes, convective transport, biological reactions and interactions, and abiotic reactions. 

Recall from Section 1.3.1 that the mass balance equations (Eqs. 1.1a and 1.1b) contain internal source and sink terms which are due to chemical reactions. Chemical reactions result in the transformation of one chemical substance into another. In environmental media, i.e., sinface waters, the subsurface, and the atmosphere, chemical reactions may occur abiotically or be biologically mediated by organisms. Examples of abiotic chemical reactions include dissolved limestone (CaCOs) precipitating from dripping water to form a solid-phase stalactite or stalagmite in a cave, the attenuation of nuclear fallout over time, the degradation into dust of... 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

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. 

Humic substances consist of heterogeneous mixtures of compounds with no single structural formula. These compounds are formed by dynamic alterations of resistant tannins and lignins by abiotic and biotic reactions, resulting in accumulation of humic substances. Humic substances are divided into three major groups on the basis of chemical characteristics and their solubility in acids and bases (Figure 5.14) ... 

All the substances under discussion are very stable to hydrolysis, to chemical oxidation, and to thermal decomposition. Degradation by purely abiotic chemical reactions is therefore considered as an unlikely environmental sink [65, 107]. 

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