The Degradation of Pesticides
Although some pesticides such as DDT are very long-lived in the environment, most undergo chemical or biochemical reactions within a few days or months, producing other compounds. Based upon their typical half-lives in the environment, the U.S. EPA classifies pesticides as being:
• nonpersistent, if they last less than 30 days;
• moderately persistent for those lasting 30-100 days; and
• persistent for lifetimes greater than 100 days.
Like most organic compounds, pesticides in the environment—whether present in air, water, or soil—degrade to other compounds, which in turn decompose further. The complete eventual breakdown of organic compounds to C02, H20, and stable inorganic forms of its other elements is called mineralization.
In air, the degradation process usually begins either with attack on the organic molecule by the hydroxyl radical, OH, or with a photochemical reaction if the substance absorbs light with wavelength greater than about 285 nm, in accordance with the principles discussed in Chapter 5.
Photochemical decomposition is possible also for pesticides present in water or adsorbed onto soil resident at the Earth's surface. In some instances, adsorption onto soil particles increases the maximum wavelength of light the substance absorbs into the range in sunlight—an example is the herbicide paraquat, which undergoes photolysis more rapidly when adsorbed on clay than it does in solution. Complexation of organic molecules by metal ions also usually increases their maximum wavelength of absorption, thereby activating them for photochemical decomposition by sunlight in some cases.
As we already have discussed several times in this chapter, pesticides in water and in soil can undergo hydrolysis reactions, especially when the water is somewhat acidic or somewhat basic, since catalysis by H+ or OLT can then speed up the processes significantly. Organophosphate insecticides, for example, hydrolyze in alkaline water and soil owing to attack by OH on the P—O—C link. Even in quite dry soils, hydrated aluminum ions produce hydrog en ions that in the existing moisture can catalyze hydrolysis.
A1(H20)63+->A1(H20)50H2+ + H+
For example, in triazine herbicides, hydrolysis can convert their C—CI bonds to C—OH ones, thereby eliminating their herbicidal activity. Organic compounds, including pesticides, can also be transformed in water or soil by oxidation or reduction reactions. Although dissolved 02 itself can oxidize, its reactions are often accelerated by the presence of dissolved or adsorbed transition metal ions, which oxidize the pesticide and whose reduced form is subsequently re-oxidized by 02, For example, Fe3+ is a good oxidizing agent for many organic compounds; the Fe"+ state to which it is reduced in the process is subsequently oxidized by oxygen back to Fe3+, thereby completing the cycle.
Reducing agents are commonly found in anaerobic waters and soil; they include Fe2+ and sulfide ion, S2~. For example, pesticides that contain a C CI unit are dechlorinated by iron when it abstracts an electron from the C—CI bond, thereby releasing CP and forming Fe3+ and a reactive carbon-based free radical.
Even more important than the chemical processes described above are degradation reactions facilitated by microbial action in water and soil. Chemheterotropks are microorganisms that derive the energy they require from redox reactions and their carbon from organic compounds. The metabolic reactions proceed in stepwise fashion, the individual steps usually being oxidation, reduction, or hydrolysis. However, the rates of degradation vary over a very wide range, depending on the molecular structure of the pesticide and the properties of the soil. Compounds containing functional groups such as —OH, —N02, —NH2, and carboxylate degrade most readily in soils since they contain a site for enzymatic attack and are relatively soluble in water, whereas highly chlorinated hydrocarbons are much more resistant since there is no reactive site and their water solubility is very low.
A common example of a microbial oxidation step is enzyme-catalyzed epoxidation, a process in which an oxygen atom from an Oz molecule is added to a C=C bond, even one contained within an aromatic benzene ring system:
Following epoxidation, the adduct can undergo further reactions, for example,
• rearrangement to a hydroxylated compound, thereby reestablishing the highly stable aromatic ring; or
• hydrolysis to produce an ortho-dihydroxyl compound; or
• the addition of further oxygen and water to other double bonds within the aromatic system.
Subsequent reactions at an adjacent pair of carbons having —OH groups often lead to ring cleavage at that site, yielding a dicarboxylic acid.
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