Pesticide degradation

In principle, pesticides are registered for use only after they are demonstrated not to persist in the environment considerably beyond their intended period of use. Typically, documented soil half-lives are in the range of days to weeks. However, pesticide residues are found ubiquitously in the environment in ng/liter to low μg/liter concentrations. For instance, surveys of groundwater and not-yet-treated potable water in industrialized countries typically detect 10 to 20 substances in recurrent findings above 0.01 μg/dL (3.6×10⁻¹² lb/cu in) the maximum accepted drinking water concentration for pesticides in many countries. About half of the detected substances are no longer in use and another 10 to 20% are stable transformation products.^[1]

Pesticide residues have been found in other realms. Transport from groundwater may lead to a low-level presence in surface waters. Pesticides have been detected in high-altitude regions, demonstrating sufficient persistence to survive transport across hundreds of kilometers in the atmosphere.^[1]

Degradation involves both biotic and abiotic transformation processes. Biotic transformation is mediated by microorganisms, while abiotic transformation involves processes such as chemical and photochemical reactions. The specific degradation processes for a given pesticide are determined by its structure and by the environmental conditions it experiences. Redox gradients in soils, sediments or aquifers often determine which transformations can occur. Similarly, photochemical transformations require sunlight, available only in the topmost meter(s) of lakes or rivers, plant surfaces or submillimeter soil layers. Atmospheric phototransformation is another potential remediating influence.^[1]

Information on pesticide degradation is available from the required test data. This includes laboratory tests on aqueous hydrolysis, photolysis in water and air, biodegradability in soils and water-sediment systems under aerobic and anaerobic conditions and fate in soil lysimeters. These studies provide little insight into how individual transformation processes contribute to observed degradation in situ. Therefore, they do not offer a rigorous understanding of how specific environmental conditions (e.g., the presence of certain reactants) affect degradation. Such studies further fail to cover unusual environmental conditions such as strongly sulfidic environments such as estuaries or prairie potholes, nor do they reveal transformations at low residual concentrations at which biodegradation may stop. Thus, although molecular structure generally predicts intrinsic reactivity, quantitative predictions are limited.^[1]

Biodegradation is generally recognized as the biggest contributor to degradation. Whereas plants, animals and fungi (Eukaryota) typically transform pesticides for detoxification through metabolism by broad-spectrum enzymes, bacteria (Prokaryota) more commonly metabolize them. This dichotomy is likely due to a wider range of sensitive targets in Eukaryota. For example, organophosphate esters that interfere with nerve signal transmission in insects do not affect microbial processes and offer nourishment for microorganisms whose enzymes can hydrolyze phosphotriesters. Bacteria are more likely to contain such enzymes because of their strong selection for new enzymes and metabolic pathways that supply essential nutrients.^[2] In addition, genes move horizontally within microbial populations, spreading newly evolved degradation pathways.^[1]

Some transformations, particularly substitutions, can proceed both biotically and abiotically, although enzyme-catalyzed reactions typically reach higher rates. For example, the hydrolytic dechlorination of atrazine to hydroxyatrazine in soil by atrazine-dechlorinating bacterial enzymes reached a second-order rate constant of 105/mole/second, likely dominating in the environment. In other cases, enzymes facilitate reactions with no abiotic counterpart, as with the herbicide glyphosate, which contains a C-P bond that is stable with respect to light, reflux in strong acid or base, and other abiotic conditions. Microbes that cleave the C-P bond are widespread in the environment, and some can metabolize glyphosate. The C-P lyase enzyme system is encoded by a complicated 14-gene operon.^[1]

Biodegradation transformation intermediates may accumulate when the enzymes that produce the intermediate operate more slowly than those that consume it. In atrazine metabolism, for example, a substantial steady-state level of hydroxyatrazine accumulates from such a process. In other situations (e.g., in agricultural wastewater treatment), microorganisms mostly grow on other, more readily assimilable carbon substrates, whereas pesticides present at trace concentrations are transformed through fortuitous metabolism, producing potentially recalcitrant intermediates.^[1]

Pesticides persist over decades in groundwater, although bacteria are in principle abundant and potentially able to degrade them for unknown reasons. This may be related to the observation that microbial degradation appears to stall at low pesticide concentrations in low-nutrient environments such as groundwater. As yet, very little is known about pesticide biodegradation under such conditions. Methods have been lacking to follow biodegradation in groundwater over the relevant long time scales and to isolate relevant degraders from such environments.^[1]

In surface waters, phototransformation can substantially contribute to degradation. In “direct” phototransformation, photons are absorbed by the contaminant, while in “indirect” phototransformation, reactive species are formed through photon absorption by other substances. Pesticide electronic absorption spectra typically show little overlap with sunlight, such that only a few (e.g., trifluralin) are affected by direct phototransformation.^[3] Various photochemically active light absorbers are present in surface waters, enhancing indirect phototransformation. The most prominent is dissolved organic matter (DOM), which is the precursor of excited triplet states, molecular oxygen, superoxide radical anions, and other radicals. Nitrate and nitrite ions produce hydroxyl radicals under irradiation. Indirect phototransformation is thus the result of parallel reactions with all available reactive species.^[4] The transformation rate depends on the concentrations of all relevant reactive species, together with their corresponding second-order rate constants for a given pesticide. These constants are known for hydroxyl radical and molecular oxygen.^[5] In the absence of such rate constants, quantitative structure–activity relationships(QSARs) may allow their estimation for a specific pesticide from its chemical structure.^[6]

The relevance of "dark" (aphotic) abiotic transformations varies by pesticide. The presence of functional groups supports textbook predictions for some compounds. For example, aqueous abiotic hydrolysis degrades organophosphates, carboxylic acid esters, carbamates, carbonates, some halides (methyl bromide, propargyl) and many more. Other pesticides are less amenable. Conditions such as high pH or low-redox environments combined with in situ catalyst formation including (poly)sulfides, surface-bound Fe(II) or MnO
₂. Microorganisms often mediate the latter, blurring the boundary between abiotic and biotic transformations. Chemical reactions may also prevail in compartments such as groundwater or lake hypolimnion, which have hydraulic retention times on the order of years and where biomass densities are lower due to the almost complete absence of assimilable organic carbon.^[1]