Evaluating the fate of agrochemical through adsorption and desorption studies of chlorfluazuron in selected agricultural soils

2.1 Soil analysis

Ten agricultural regions from province of Punjab, Balochistan, Khyber Pukhtoon Khw and Azad Kashmir Pakistan were selected for sorption studies of pesticide. Therefore 10 different soil samples were collected from 10 different districts of Pakistan, which were classified into four types of soil based on their texture. Details of sampling and soil samples are given in Table 1. Considering the significance of the sampling depth for studying the soil-pesticide interaction, in the present research, all soils were sampled at 0–15 cm depth following random and composite sampling. The soil samples were air dried at room temperature (20–25 °C). Disaggregated first manually and then using a mortar and pestle. Homogenized soils samples were sieved (2 mm sieve) to obtained final prepared soils. The prepared soil samples were then investigated for the physiochemical parameters which were considered to be mainly responsible for adsorptive capacities of soils (Table 1)

For sorption studies it is essential to find out texture of soils (OECD, 2005). In this research texture of selected soil samples was determined by Octagon digital sieve shaker and USDA textural triangle was used to delineate soil textural classes. Behavior of pesticides is greatly affected by soil pH which was determined by saturation extract methods (Wagh et al., 2013). 5 g of soil was mixed in 10 ml of re-distilled water (1:2 soil solution), after contact time of one hour pH of the slurry was measured by MM 40 + pH meter of Crison company (Wagh et al., 2013). For sorption studies most important component of soil is soil organic matter (SOM). To determine SOM Loss on Ignition (LOI) method was used by the loss of weight of the soil sample heated at a high temperature. Temperature of LOI method is enough to burn organic matter of soil but should not decompose carbonates present in the soil (Robertson et al., 2011). First soil samples were placed in oven at 105 °C for 24 h, and then the samples were placed in muffle furnace (of Ncycraft Company) for 2 h at 360 °C. %OM was calculated by comparing the weight of a sample before and after the soil has been ignited (Robertson et al., 2011; Onojake and Osuji, 2012; Hoogsteen et al., 2015). $$\%\text{OM}=\text{weight at}105°\text{C}(\text{g})-\text{weight at}360°\text{C}(\text{g})/\text{weight at}360°\text{C}(\text{g})*100$$

2.2 Adsorption–desorption studies

10.3 ppm stock solution was prepared by dissolving 10.3 mg of chlorfluazuron in 1000 ml of deionized water on a magnetic stirrer for 12–24 h. Before adding distilled water few drops of analytical grade 99.9% acetone was added in order to enhance solubility of pesticide in water. The stock solutions were prepared just before application to tested soil samples, However extra amount of stock solutions were kept and stored at 4 °C. Other stock of 0.1 M sodium chloride and 0.1 M CaCl₂ were also prepared.

When pesticide was completely dissolved in water, 16 concentrations (with duplicates) (0, 0.25, 0.5, 0.75, 1.0, 2.5, 5.0 and 7.5 ppm) of chlorfluazuron were made in 10 ml of 0.1 M NaCl. The volume of each concentration was raised up to 100 ml by distilled water. 10 ml of each concentration was added into 15 ml falcon valves having 0.5 g of selected soil samples. One control sample having chlorfluazuron in 0.1 M NaCl solution was subjected to the same batch equilibrium procedure to check the stability of the CF in NaCl solution. One blank sample (soil + NaCl solution) was also run in order to detect contamination of soils or interfering compounds which could cause matrix effects. All the falcon valves, including controls and blanks were then continuously agitated in overhead orbital shaker of Irmeco Gmbh Germany at room temperature for 24 h at 90 rpm velocity. After achieving equilibrium, the falcon valves were centrifuged by Sigma 2-6E centrifuge of high-speed at 3500 rpm for 20 min at room temperature. Supernatant was then collected and filtered through 0.2 µm nylon filters. Clear aliquots were now ready for analysis by UV–Visible spectrophotometer and High-performance liquid chromatography (HPLC).

Desorption was conducted on the same soil pesticide solutions. Once the adsorption was done, the supernatant was discarded. The falcon valves with the contents the valves were reweighed and 9 ml of freshly prepared 0.1 M CaCl₂ solution was added to the remaining soil in the falcon valves. The samples were again shaken, centrifuged and analyzed at same condition as for adsorption.

2.3 Analytical method

The analytical methods which were used for analysis of the pesticide in the present research were:

4.1 Linear adsorption and desorption of chlorfluazuron

Chlorfluazuron sorption was studied by HPLC and UV Visible spectrophotometer. Linear isotherms of CF are as follow (Figs. 2 and 3).

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Fig. 2

Linear adsorption isotherms of chlorfluazuron by selected soils.

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Fig. 3

Linear desorption isotherms of chlorfluazuron by selected soils.

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The K_{d ads} values of chlorfluazuron varied between 12 and 4.7 µg ml⁻¹ while the regression coefficient R² value ranging from 0.81 to 0.97 with standard error (S.E.) ranged between 0.059 and 2.5. The competitive effect of chlorfluazuron adsorption by each soil normally decreased with increasing initial insecticidal concentrations. This is because of the saturation of sorption sites in soils with increasing concentration of pesticides (Doretto et al., 2014). The desorption experiments were conducted with a nonionic surfactant, the comparative results of linear isotherms are shown in Table 2. According to this data K_{d des} varied between 4.09 and 9.2 µg ml⁻¹ with R² and S.E ranging between 0.96 to 0.80 and 2.04 to 0.065 respectively. Usually values of K_OM below 500 specify minimum or no adsorption of a pesticide, this would indicates high probability of runoff or leaching (Onojake and Osuji, 2012). Pesticides have been categorized into three classes based on K_OM i.e. very high mobility group having K_OM values less than 50, high mobility group having K_OM values 150–500 and low mobility group having K_OM values more than 500 (Ahmad et al., 2014). According to data in Table 2 chlorfluazuron lies in high mobility class of pesticides as its K_OM values are ranged from 272.13 to 403.82. chlorfluazuron exhibited less adsorption potential through all selected soil sample. From data in Table 2 it is predicted that chlorfluazuron has significant leaching potential for type C and D as these samples demonstrates less adsorption and more desorption. The calculated values of hysteresis for chlorfluazuron on the selected soil samples ranged from 0.24 to 1.549. H indicates greater or lesser irreversibility of adsorption in all samples, the highest values were corresponding for samples having highest adsorption constant i.e. sample-6, sample 5, sample 2, sample 9 and sample 10.

4.2 Freundlich isotherms of chlorfluazuron

Chlorfluazuron for all soil samples were then analyzed for Freundlich kinetics. Figs. 4 and 5 are Freundlich adsorption and desorption isotherms of chlorfluazuron on all studies soils.

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Fig. 4

Freundlich adsorption isotherms of chlorfluazuron by selected soils.

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Fig. 5

Freundlich Desorption isotherms of chlorfluazuron by selected soils.

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Highest value of K_{f ads} found for sample-5 (9.05 µg ml⁻¹) and lowest value was observed for sample-7 (3.9 µg ml⁻¹). Values of R² were ranging between 0.81 and 0.99 with associated S.E. between 0.04 and 0.59. These results intensely proved that Freundlich is good fitted model for adsorption studies of chlorfluazuron. K_f was calculated from the regression equation demonstrated that Freundlich adsorption model effectively describes isotherms for the chlorfluazuron in all cases. Desorption isotherms of the chlorfluazuron were fitted to the liberalized form of the Freundlich equation. Table 3 is also depicting 1.23–8.69 µg ml⁻¹ variations in K_{f des} values. Minimum value of R² for desorption is 0.87 with associated maximum and minimum S.E. values are 0.88 and 0.22 respectively. According to K_f analysis chlorfluazuron illustrated comparatively enhanced adsorption for soil samples of type A while enhanced desorption Sandy loam and Sandy clay loam soils respectively.

Like K_f 1/n was also calculated from same regression equation. Values of 1/n _ads were following the range of 0.71 to 3.6 and 1/n _des from 0.425 to 2.97. n_ads follow the range of 0.33 to 0.745. n_des shown range of values varied between 0.714 and 2.36. Vales of n_ads and n_des are indicating the greater or lesser irreversibility of adsorption in all samples, the highest values corresponded for soil which is showing highest adsorption constant i.e. .type A soils. ΔG of chlorfluazuron for selected soils followed the range from −14.8 to −13.8 Kj mol⁻¹. In present research ΔG demonstrated week physical adsorption as well as exothermic interaction of chlorfluazuron with selected soil samples.

Like K_om, K_fom also identified chlorfluazuron as highly mobile insecticide as K_fom values ranged from 265.96 to 402.06.

4.3 Effects of soil parameters on sorption behavior of chlorfluazuron

By comparing Tables 1–3 it was revealed that except sample 3, all samples of type A (Clay soil) depicted higher adsorption of CF while type B, C and D demonstrated higher desorption of chlorfluazuron. This is because of chemical properties of soils e.g. Clay content and SOM facilitate adsorption of pesticide (Krishna et al., 2008; Bansal, 2010; Shariff et al., 2012), therefore adsorption is higher for type A soil samples, similarly Type B, Type C and Type D soils have more sand content which results in higher desorption and enhance mobility of CF.

According to data in Table 1, highest values of SOM, Clay content, and CEC were found for Sample-5, thus Sample-5 depicted higher values of all adsorption coefficients. When adsorption coefficients were compared with pH of selected soil samples it was revealed that pH is negatively correlated with selected soil samples. Clay soils (sample 1 to sample 5) have comparatively lower pH and their adsorption coefficients (K_{d ads} and K_{f ads}) are higher as compared to rest of tested soils. From data in Tables 1–3 it was verified that SOM has positive correlation with K_{d ads} and K_{f ads}. SOM have greater potential of binding particles together and retaining pesticides in soils by providing adsorption sites for pesticides to adsorb. In present research chlorfluazuron illustrated relatively higher affinity for sample for Clay soils because these samples have comparatively higher SOM. Clay being smaller particle, facilitates adsorption of chlorfluazuron by providing greater surface area and significant adsorption sites (Onojake and Osuji, 2012; Khairatul et al., 2013). On other hand chlorfluazuron indicated less mobility and less desorption through samples having more clay content and SOM. These two soil parameters are strongly restricted the mobility and desorption of pesticide. For desorption and mobility there must be more sand content and less clay, SOM and CEC (Chen, 2015; Ahmad et al., 2014; Shariff, 2012; OECD, 2005). In Table 1 Sandy clay loam (sample-8, 9 and 10) and Sandy loam (sample-7) exhibited more sand content, therefore revealed higher values of desorption coefficients as well as higher mobility of chlorfluazuron (predicted from data in Tables 2 and 3). Sandy soil like sample-7 and sample- 10 has fewer pores or adsorption sites which result in minimum adsorption and enhanced mobility of pesticide (Gebremariam et al., 2012; Ahmad et al., 2014). Because of enhanced mobility and leaching, chlorfluazuron tend to contaminate ground water bodies of sandy clay loam (sample-8, 9 and 10) and Sandy loam (sample-7) while for clay soils risk of contamination of water bodies is negligible. However there is slight leaching of chlorfluazuron was assessed for clay soils.

However soil sample-3 depicted different behavior from other clay type soils (sample 1, 2, 4, 5). Sample-3 although a clay soil, yet it indicated less adsorption because of lower (1.78%) SOM content than other clay type soils. Sample-1, sample-2, sample-4, and sample-5 have high clay as well as SOM which is responsible for higher adsorption while sample-3 have higher clay content but lower SOM thus sample-3 revealed lower adsorption potential.