Pesticide degradation
Pesticide degradation is the process by which a
Persistence
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
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
Information on pesticide degradation is available from the required test data. This includes laboratory tests on aqueous hydrolysis,
Biotic transformation
Biodegradation is generally recognized as the biggest contributor to degradation. Whereas plants, animals and fungi (
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]
Abiotic Transformation
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
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,
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]
Prediction
Available strategies to identify in situ pesticide transformation include measuring remnant or transformation product concentrations and estimating of a given environment's theoretical transformation potential. Measurements are only usable on the micro- or mesocosm scale.[1]
Transformation product detection may calibrate degradation. Target analysis is straightforward when products and standards are understood, while suspect/nontarget analysis can be attempted otherwise. High-resolution mass spectrometry facilitated the development of multi-component analytical methods for 150 pesticide transformation products and for screening for suspected transformation products. In combination with transformation product structure models, screening allows a more comprehensive assessment of transformation products, independent of field degradation studies.[1]
Isotopic analysis may complement product measurements because it can measure degradation in the absence of metabolites and has the potential to cover sufficiently long time scales to assess transformation in groundwater. Isotope ratios (e.g.,13
C/12
C, 15
N/14
N) can reveal history in the absence of any label. Because kinetic isotope effects typically favor the transformation of light isotopes (e.g., 12
C), heavy isotopes (13C) become enriched in residues. An increased 13
C/12
C isotope ratio in a parent compound thus provides direct evidence of degradation. Repeated pesticide analyses, in groundwater over time, or direct measurements in combination with groundwater dating that show increasing 13
C/12
C isotope ratios in a parent pesticide, provides direct evidence of degradation, even if the pesticide was released long before. Multiple transformation pathways were revealed for atrazine by measuring the isotope effects of multiple elements. In such a case, transformation mechanisms are identifiable from plots of 13
C/12
C versus 15
N/14
N parent compound data, reflecting different underlying carbon- and nitrogen-isotope effects. The approach requires a relatively high amount of substance for
Geochemical analysis including pH, redox potential and dissolved ions is routinely applied to assess the potential for biotic and abiotic transformations, complicated by any lack of specificity in the targets. Selective probe compounds must be used to detect individual reactive species when a mixture of reactive species is present. Combining probe compounds and scavengers or quenchers increases accuracy. E.g., N, N-dimethylaniline, used as a probe for the carbonate radical reacts very quickly with DOM-excited triplet states and its oxidation is hampered by DOM.[1]
13C-labeled parent pesticides were used in the nontarget analysis of degraders by stable isotope probing (SIP) to demonstrate biotransformation potential in soil and sediment samples. A complimentary, potentially more quantitative technique is to directly enumerate the biodegradative gene(s) via quantitative
Transformation products
Even though their undesirable effects are typically lowered, transformation products may remain problematic.
The issue is specifically addressed in major regulatory frameworks. In Europe, for instance, "nonrelevant" metabolites are distinguished from metabolites that are "relevant for groundwater resources" or even "ecotoxicologically relevant". The latter are those whose risk to soil or aquatic biota is comparable to or higher than the parent and must meet the same standards as their parent. Groundwater-relevant metabolites are those likely to reach groundwater in concentrations above 0.1 μg/liter and to display the same toxicity as the parent compound. In the past toxicology issues typically emerged only decades after market introduction. Examples are the detection of chloridazon products (first marketed in 1964) in surface and groundwater, or tolylfluanid (first marketed in 1971). That these substances were overlooked for so long may partially be attributable to prior limits on analytical capabilities. However, labeling some metabolites as nonrelevant may have resulted in directing attention away from them.[1] The decision to tolerate up to 10 μg/liter of "nonrelevant" metabolites in groundwater and drinking water is politically highly contentious in Europe. Some consider the higher limit acceptable as no imminent health risk can be proven, whereas others regard it as a fundamental deviation from the precautionary principle.[8]