Chemical and photolytic decomposition

The abiotic chemical processes, however, do not include reactions catalysed with enzymes, which occur in certain amounts in the free soil, outside of cells. These enzymes, mostly bound to soil particles, are excreted actively by metabolizing cells or released during the decay of dead organisms. Reactions catalysed in this way should, however, be considered as processes dependent on biological activity. 

Even in purely chemical reactions, toxic substances may be converted to less harmful ones, which can then more readily undergo further, possibly biological conversions. Certain substances are chemically degraded more easily when being adsorbed on soil particles. 

In the case of photochemical reactions, photodecomposition is characteristic for certain complex organic molecules which are exposed to solar radiation. Besides those which are directly suspended in the atmosphere, there are also substances attached to the surface of plants, soil, etc. 

Many pesticides, such as 2,4-D, chloramben, picloram, bromoxynil, ami-trol, triflualin and paraquat also tend to participate in photochemical reactions. 

In the case of water-soluble substances, the extent of photodecomposition is restricted to a certain extent, since they are easily washed out from 

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