Dichlorvos (DDVP) is an organophosphorus pesticide that has been classified as highly hazardous chemical by the World Health organization. In this study, the fate of the pesticide DDVP in natural water compartments was examined under simulated sunlight. Moreover, the effect of UV-254 irradiation on DDVP depletion was also studied. In deionized water, DDVP was photodegraded only in the presence of dissolved molecular oxygen. The photodegradation during the first 6 h of sunlight irradiation occurred with pseudo first-order kinetics, and the rate constants were 0.040 h at pH 7 and 0.064 h at pH 3. A reaction mechanism for the generation of reactive oxygen species (ROS) via DDVP photoabsorption was proposed. Humic acids (HA) played a double role as photosensitizer and inhibitor, observing an enhancement on DDVP photodegradation at low HA concentration (TOC = 2 mg L). The depletion of DDVP under 254 nm UV irradiation was ascribed to direct photodegradation and oxygen mediated photoinduced reactions. Direct photodegradation of DDVP decreased with 254 nm irradiation reduction, highlighting the importance of radical mediated mechanisms at low irradiation doses. Based on LC/MS data, the main photoproducts under simulated solar light and UV-C irradiation were identified and potential reaction pathways were postulated. The three main identified products were o-methyl 2,2-dichlorovinyl phosphate, dichloroacetaldehyde and dimethylphosphate. Moreover, the toxicity of samples was evaluated along the irradiation exposure time using Microtox® assays. This study brings new insights into the role of oxygen in the photodegradation of DDVP and the induced and inhibition mechanisms involved in the presence of the humic acids in natural waters.
During the last years, dichlorvos has had an extremely high occurrence in monitoring campaigns, showing concentrations that would result in severe environmental impact. Environmental quality concentrations (EQC) for dichlorvos in surface waters vary by more than two orders of magnitude among different regulations. Regulatory EQC-values are based on toxicity data where species sensitivity distributions (SSDs) are mostly used as a basis. Hence, what makes the difference between the different national and regional EQC-values? And will they all protect the aquatic fauna? To investigate these questions, we constructed an SSD based on our laboratory data and compared this with SSDs based on literature data for technical and formulated products and species of different taxa. Finally, we compared the SSD-derived EQCs with a meta-study on monitored environmental concentrations to determined the EQC-exceedance. The following hypotheses were tested: (i) formulated dichlorvos will result in lower EQCs, as formulated products may include synergists; (ii) different taxon will have different sensitivities with arthropods being the most sensitive group; (iii) environmental concentration of dichlorvos represents a risk for aquatic organisms. As a result, experimental SSD defined a protective concentration of 6.5 ng L-1 for 5% of the species, which is in line with the European limit values, but one to two-fold lower than the limit values of the USA, China, and Argentina. The EQCs of three of the four regions included in this study are therefore not enough to protect the aquatic fauna.
Dichlorvos is an organophosphorus insecticide frequently detected in surface waters all around the world. From an evaluation of the environmental quality concentrations (EQC) for dichlorvos in surface waters adopted by different countries, it was observed a wide variability among them. This is despite regulatory EQC-values are typically based on toxicity data and species sensitivity distribution (SSD) in all the investigated regulatory frameworks, and therefore should be similar. Hence, what is the cause of the differences between national and regional EQC-values? And, which ones will protect the aquatic fauna? These hypotheses were proposed to explain differences among SSDs based on the choice of toxicity data: (i) EQC values obtained from technical presentation (pure dichlorvos) will be higher than the estimated from dichlorvos formulation (containing other substances to improve the efficiency of the active principle), as they may include synergists; (ii) different taxa will have different sensitivities; (iii) data produced under different experimental conditions will severely affect the SSD. Regarding their capacity to protect the aquatic fauna the hypotheses were; iv) environmental concentration of dichlorvos represents a risk for aquatic organisms; and v) not all EQC-values are protective for the aquatic fauna. These were tested through a meta-analysis of toxicity data enabling the construction of SSD’s across technical and formulated dichlorvos and species of several taxa, and across literature and experimental data produced under analogous conditions. Finally, the EQC elaborated were compared with a meta-study on monitored environmental concentrations. The study suggested that technical dichlorvos increased toxicity compared to formulated products up to two-fold for arthropods. Species phylogeny affected sensitivity, but the SSD derived values used for setting regulatory concentrations were remarkably robust to the inclusion/exclusion of less sensitive species. The SSD results from the literature and experimental data were similar in the case of technical dichlorvos results. The regional differences in EQC values therefore most likely stem from political considerations on how to use SSDs to derive EQCs rather than from differences in SSDs. The experimental SSD defined a protective concentration of 6.5 ng L− 1 for 5% of the species, which is according to the European EQC, but one to two-fold lower than the limit values of the US, China, and Argentina.
Soils are the principal environmental fate of pesticides in agricultural areas. Thus, the kinetics, extension, and strength of the adsorption process become critical. Dichlorvos (DDVP) is an organophosphorous pesticide that is used both in agriculture and livestock production. Sorption/desorption assays of DDVP in two agricultural soils (with different textural characteristics) from Pampa Plain (Argentina) were performed in both batch and column systems. From batch studies, kinetics and sorption/desorption equilibrium parameters were estimated. Our results showed that the maxima adsorption is reached after 30 h of time of contact and followed a pseudo-first-order rate. Adsorption/desorption data were well fitted to the Freundlich model obtaining high adsorption constants of 90 mg(1-1/n) mL(1/n) g-1 and 21 mg(1-1/n) mL(1/n) g-1 for the clay loam and sandy loam soil, respectively. The isotherms were non-linear in both cases and the desorption process was unfavourable. Also, positive hysteresis was present for the sandy loam soil. From column studies, breakthrough curves were used to evaluate the mobility of DDVP in the soils at 1, 10, and 50 mg L-1 of DDVP. Eluted profiles were asymmetrical as well they presented retardation effects that were in connection with the results in batch conditions. Non-equilibrium sorption was stated for the DDVP movement through columns. Thus, high mobility was observed for DDVP in both soils despite their textural differences.
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