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Title: Investigating the environmental degradation and toxicity of organophosphorus pesticides
Author: Ukpebor, Justina
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2011
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Abstract:
Organophosphorus insecticides (OPs) are some of the most widely used pesticides in the world and are present in a variety of diverse environments including remote areas well away from known usage areas. Unlike their older organochlorine counterparts, OPs are generally more polar, exhibit higher solubility in water and experience shorter environmental half-lives, but their ongoing use and concerns over their toxicity, particularly through the rise of transformation products, to non-target organisms requires research into factors which infuence their environmental transformation and toxicity. This thesis focuses on the environmental degradation, fate and toxicity of selected OPs under novel or non- standard environmental conditions. "Non-standard" is defined as conditions which deviate from current chemical fate testing environments provided by the US-EPA, EU Technical Guidance Documents and OECD Fate testing guidelines, used for pesticide/chemical risk assessment purposes. An analytical method was developed utilising both GC-MS (El) and LC- MS (ESI-Ion Trap) techniques to qualify/quantify thirteen OPs commonly reported in environmental media. A solid-phase extraction technique was developed and optimised for aquatic samples that yielded excellent recoveries (~70-110%) for OP fortified field water and the method was successfully applied to examine OPs artificially doped into a melting snowpack in northern Sweden as part of wider programme examining chemical fate in northern catchments. The rationale for examining OPs in snow was due to recent evidence that shows their accumulation in ice-cores taken in the high Arctic. OP chemicals showed rapid percolation (within 24 h) from a surface snow layer to the deeper pack and were also present in meltwater collected at the base of the snowpack, implying that a significant proportion of accumulated OPs were likely to be available within meltwater runoff during spring. Failure to recover the mass of OPs initially spiked into the snow implies that volatilisation may also be a loss process during snow ageing over relatively short time periods. The LC-MS(Ion trap) method, while selective. - particularly in MS/MS mode (with product ions used to qualify OPs) - was not as sensitive as GC-MS, with method detection limits in the umolar range compared to nmolar quantities for the GC. The LC-MS(Ion trap) was therefore unsuitable for trace-level determination of OPs in environmental samples. Using these analytical methods, laboratory-based studies were undertaken to understand the degradation/loss of selected OPs (fenitrothion and diazinon) when subject to simulated sunlight (using an Atlas Suntest chamber fitted with a UV-filtered Xenon arc lamp) in various aqueous solutions (to resemble field water) and on crop/lettuce (lactuca sativa) surfaces under different environmental conditions. For the aqueous solutions, photodegradation was examined in the presence of nitrate (N03-), carbonate (COl-), and dissolved organic carbon (DOC - as fulvic acid) at concentrations typically found in natural waters. Fenitrothion absorbs light within the solar spectrum, whereas diazinon does not (I."max ~247 nm). Both chemicals were found to show enhanced photodegradation in the presence of N03-, and to a lesser extent col, presumably due to the light- induced formation of aqueous hydroxyl radicals (OH) which attack organic solutes. For fenitrothion, rates of photochemical degradation (conforming to pseudo-first order kinetics) increased by >2-fold in the presence of N03- compared to the direct photolysis rate measured in pure (MilliQ) water. Similarly, for diazinon, degradation was observed in the constituent-based solutions but not in MilliQ water. The rate constant observed in the presence ofN03- was 0.084 hr" 1, which was not statistically different than the photolysis rate of fenitrothion in MilliQ water (0.072 hr-1), highlighting the importance of indirect photochemical processes on chemical loss, even for diazinon. It is recommended that use of dissolved constituents be incorporated into aquatic photochemical fate testing guidelines for pesticides, as current testing procedures, while recommending the use of pH buffered solutions, will currently overestimate pesticide longevity in most sunlit natural waters. The photolysis of fenitrothion was also examined on tetfuie leaf surfaces and a pesticide application and extraction method was developed that provided a high degree of reproducibility (%RSD = 2.03) necessary for leaf photolysis experiments. Fenitrothion photolysis was carefully examined in aqueous solutions under discrete wavelength band-pass filters and quantum yields of degradation, ɸ, were calculated for each filter (~10nm wavelength bands) across the DV region (270-380 nm). Values of ɸ closely matched literature studies validating the light exposure techniques and light intensity measurements used in this study. As accurate values of ɸ are difficult to obtain on plant surfaces due to light scattering and absorption then a novel 'pesticide action spectrum' (PAS) - a light weighted effective degradation rate - was developed to predict photolysis rates on salad- leaf surfaces with knowledge on the ambient light conditions. The PAS was tested for fenitrothion decay irradiated under a range of agricultural plastics commonly used as cladding for plastic-greenhouses. Light transmission is markedly reduced under these plastics, particularly in the DV actinic region, resulting in reductions in fenitrothion decay (k = 0.144 ± 0.021 hr-I for 'standard' plastic, compared to 0.258±0.028 hr-I in the absence of plastic film). Derived degradation rates for fenitrothion using the PAS, for the various spectral irradiances measured under the plastics, compared favourably to measured photolysis rates. The issue of pesticide degradation and fate in light-altered micro-environments like plastic- protected enclosures has implications for pesticide longevity on crop surfaces and is currently not accounted for in pesticide risk management regimes. The toxicity of OPs is commonly associated with inhibition of acetylcholinesterase (AChE) activity and hence disruptions of nerve signal transmission in a wide range of organisms from insects to mammals. Concentrations of OPs in natural waters, away from regions of direct pesticide use, are typically lower than threshold concentrations likely to disrupt AChE activity. Therefore, low-dose (10-12 to 10-8 M) toxicity assays were performed using fenitrothion, diazinon and 2-isopropyl-6-methyl-4-pyrimidinol (IMP) (a hydrolysis degradate of diazinon) on human breast cancer cells (MCF-7) to assess the genotoxicity of these chemicals. Exposure to OPs at low concentrations (10-12 to 10-8 M) revealed chromosomal damage akin to exposure to the known genotoxin, benzo[a]pyrene (B[a]P), as well as a fenitrothion-induced 2-fold increase in CYP-IAl and BCL-2 gene expressions, involved in enzyme formation and cell-death processes, respectively. Furthermore, the utilisation of ATR-FTIR spectroscopy revealed that each of the OP chemicals altered the biochemical 'signature' within exposed cells relative to control cells, notably in the lipid, protein, and DNA/RNA absorbance regions, providing strong evidence that exposure to OPs, notably for aquatic organisms, may be deleterious even at trace concentrations and that toxic effects may also arise due to exposure to OP transformation products (e.g. IMP) also identified in the various degradation experiments conducted in this thesis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.587503  DOI: Not available
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