G. Dios

Photodegradation of mecoprop and dichlorprop on dry, moist and amended soil surfaces exposed to sunlight

The effects of environmental conditions on the photodecomposition of two phenoxy-alkanoic acid herbicides were investigated on different soil surfaces conditions under sunlight exposition. A technique has been developed to study this process on moist nonsterile soil surfaces. A slow rate of disappeareance occurs in the three soils exposed to sunlight in absence of water. Photolysis of these herbicides is controlled by soil texture and its adsorption capacity. The organic matter does not have a sensitizing effect on the photodecomposition of these herbicides on dry soil surfaces. On nonsterile moist soil surfaces exposed to sunlight, the photolytic process prevails in the two first days of exposure and the transformation kinetics fits the Hoerl function better than the first-order exponential equation.

Adsorption of mecoprop and dichlorprop on calcareous and organic matter amended soils: Comparative adsorption of racemic and pure enantiomeric forms

Abstract The adsorption isotherms of mecoprop [R,S‐2‐(4‐chloro‐2‐ methylphenoxy) propanoic acid] and dichlorprop [R,S‐2‐(2,4‐dichlorophenoxy)propanoic acid] on twelve calcareous soils were studied. All the isotherms fit the Freundlich equation. The distribution coefficients (Kf) were low and were only significantly related with the organic matter content of the soils. The main adsorption mechanism at the pH of these soils was found to be by cation bridging between the anionic forms of the two herbicides, and the external surfaces of the organic matter and/or negatively charged clay. The Koc values for mecoprop (R,S‐ MCPP) and dichlorprop (R,S‐ DCPP) adsorption predicted a high mobility of these herbicides in the soils. Their corresponding enantiomeric forms, R (biologically active) and S (biologically inactive) were adsorbed at the same ratio on these soils. The organic matter amended soils showed a significant increase on the retention capacity of both molecules.

Bentazone Leaching in Spanish Soils

Adsorption, incubation and soil-column experiments with bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] were carried out in ten different soils from the marches surrounding the Donana National Park (Huelva, SW Spain). Adsorption isotherms for the different soils showed a good fit with the Freundlich equation. Bentazone was poorly adsorbed in all the soils studied, with no significant relationship between the K f values and soil characteristics. A significant correlation was obtained between the soil organic matter content and the distribution constant values (K d ) calculated at an equilibrium concentration of 200 μg cm -3 . The low adsorption and non-degradation of bentazone on these soils suggest that the herbicide readily percolates through soils to reach the surface and ground waters. The mobility of bentazone through three soil columns was also studied. The mass balances carried out showed that bentazone was totally eluted from the soil columns. The theoretical model applied to explain bentazone leaching under our experimental conditions seems to be suitable for soil columns with a uniform water-flow rate.

Simultaneous determination and enantiomeric resolution of mecoprop and dichlorprop in soil samples by high-performance liquid chromatography and gas chromatography-mass spectrometry

Abstract A rapid, precise and sensitive method was developed for the quantitative simultaneous determination of mecoprop and dichlorprop in soil samples by reversed-phase high-performance liquid chromatography (HPLC), thus making it very appropriate for degradation studies where many samples have to be analysed. This method was supplemented with a simple gas chromatography–mass spectrometry (GC–MS) ion trap method developed using a chiral permethylated-β-cyclodextrin stationary phase in order to determine the relative amounts of the R and S enantiomeric forms of these herbicides. Recovery rates for natural soils were around 80% at concentrations of 0.5 and 1.0 μg/g. Lower recoveries were achieved for the 10% peat-modified soil samples. The analytical sensitivity of the HPLC technique was 0.11 and 0.07 ng/μl, for 20 μl injections, for mecoprop and dichlorprop, respectively.

Effect of Storage on the Recovery of Different Types of Pesticides Using a Solid-phase Extraction Method

Recoveries of different pesticide groups after storage either on C 18 cartridges or as dried residues from organic solutions, and their analysis by gas chromatography with electron capture and flame photometric detection, were studied. Two storage temperatures, 4 and -18 °C, and three storage periods, 3, 7 and 30 d, were considered. The effect of storage temperature and storage time on the recovery of 27 pesticides in water was investigated. In general, the pesticide recoveries were ≥70% after 30 d of storage at -18 °C on C 18 cartridges. Exceptions included captan and folpet. The storage of the dried residues generally did not affect the pesticide recovery when kept at -18 °C for up to 30 d.

Simultaneous determination of various pesticides in water by solid-phase extraction/HPLC with photodiode array detection

Abstract A solid-phase extraction/high-performance liquid chromatographic method with photodiode array detection has been developed for the simultaneous determination of carbendazim, thiophanate-methyl, lenacil, methabenzthiazuron, diuron, phenmedipham, captan and diflubenzuron in water at the low ng/ml level. Recoveries (82–102%) and precision (R.S.D. 0.5–2.8%) are quite good and the detection limits achieved are up to 2.5 times lower than the maximum limit permitted by the E.C.

Leaching of mecoprop and dichlorprop in calcareous soil. Effect of the exogen organic matter addition in this process

Leaching studies of mecorprop (R,S)-2-(4-chloro-2-methylphenoxy)propanoic acid, and dichlorprop, (R,S)-2-(4-chloro-2,4-dichlorophenoxy) propanoic acid, under saturated conditions were conducted in unamended and amended soil columns. The purpose of the study was to investigate the leaching of these herbicides in three type of soils and the exogen organic matter effect on this process. The leaching patterns could be related to variation in the soil texture and diffusion processes of the herbicides into micropores within the walls of conducting pore. The leaching rate in the amended soil columns decreased with the addition of organic matter. The breakthrough curves (BTC) of these herbicides in the leachates of the amended soil columns were wider and more diffused than the BTC obtained for the corresponding unamended soil. The theoretical BTC overestimated the pore volume required for the displacement of these pesticides from the soil column. This may be due to the differences in the adsorption process between the bacth and soil columns methods.

Determination of Mecoprop and Dichlorprop in Aqueous Soil Solutions by HPLC with DAD

Abstract A reverse phase high performance liquid chromatographic method was developed for the direct determination of mecoprop (MCPP) and dichlorprop (DCPP) in the presence of soil and peat constituents. Spiked aqueous soil or soil/peat samples were directly injected after centrifugation and filtration. The analytical sensitivity, 0.35 and 0.24 ng L-1, detection limit, 1.02 and 0.70 ng L-1, and precision, 1.28 and 0.80%, for MCPP and DCPP respectively, are very appropriate. The concentration ranges studied 0.0–94.2 ng L-1 for MCPP and 0.0–103.1 ng L-1 for DCPP are suitable for adsorption/desorption and mobility studies of these herbicides in soil. The two tests used to demonstrate the purity or otherwise of every chromatographic peak give valuable information.

Liquid chromatographic determination of methabenzthiazuron in soil aqueous solutions with photodiode-array detection

Abstract A reversed-phase high-performance liquid chromatographic method has been developed for the determination of methabenzthiazuron in aqueous solutions in the presence of soil constituents. Spiked aqueous soil samples were injected after centrifugation and filtration. Quantitative recoveries were observed and high precision was obtained. The concentration range studied, 2.93–46.92 mg/l, is very suitable for adsorption-desorption studies of methabenzthiazuron in soil.