1. Introduction
Diuron and Linuron are phenylurea herbicides widely used in the world to control annual weeds [1]. The amount used in Gaza Strip Palestine, progressively increased in the past ten years [2]. This situation may have led to groundwater contamination. However, recent studies indicate toxicity to cyanobacteria [3], and Fish [4]. Several trials have been made to reduce environmental contamination [5-7]. These studies are based on adsorbing the herbicides to organo-clay complexes. This process restricts the leaching potential of pesticides. However, it is still important to understand the adsorption behavior of herbicides in soil. Chiristen et al. [8] and Nkedi-Kizza et al. [9] studied the adsorption behavior of phenylurea herbicides in different soils and reported that organic carbon was the main factor affecting urea sorption. Chaplain et al. [10] reported that soil hydrophobicity was an important factor for diuron adsorption in soils. Whereas, several researchers [11-13] reported that the adsorption of linuron was significantly correlated with soil organic matter and clay content. Haouari et al. [12] found that the adsorption of diuron and linuron in clayey soils in Morocco fitted the Freundlich equation. Furthermore, Yihua Liu et al. [14] studied the adsorption-desorption behaviors of diuron investigated in six cultivated soils of China and reported that diuron adsorption on soil was at a rather high level under low pH value conditions and decreased with increasing pH value. Moreover, Jianhua et al. [15] studied the adsorption kinetics of diuron from aqueous solutions onto activated carbon fiber, and showed the formation of hydrogen bonding between diuron and water, and temperature variations may possibly affect the adsorption process. This study investigated the adsorption behavior of linuron and diuron in selected soils from Gaza Governorates.
2. Materials and Methods
Technical Diuron and Linuron (purity = 99%) were purchased from Sigma-Aldrich company in Germany. Physicochemical properties of the tested compounds are shown in Table 1. Technical Diuron and Linuron (purity = 99%) were purchased from Sigma-Aldrich company in Germany.
2.1. Soil Sampling
The soil samples were collected from top soils (0 - 30 cm) of three agricultural locations in Gaza governorates (Khanyounis Governorate, Middle Governorate and North Gaza Governorate). The samples were transferred to a plastic bag, air-dried, sieved through 2 mm, and stored in well-closed plastic bottles in the laboratory. Site description, identification data and coordinates were recorded as shown in the Table 2.
2.2. Determination of Soil Texture
The soil texture expresses the proportions of the various size classes (clay < 0.002 mm, silt 0.002 - 0.02 mm and sand 0.02 - 2.0 mm particle size). The proportions of these fractions were determined by Hydrometer method using ASTM 152-H hydrometer [16].
2.3. Determination of Soil pH
Soil pH values were measured potentiometrically in a 1: 2.5 soil-water suspension. Twenty grams of a driedsieved soil were transferred into 200 ml beaker. 50 ml of distilled water were added while stirring for one hour using Electric magnetic stirrer. The pH meter was calibrated using pH buffer 4.0, 7.0 and 9.0, and then the pH of suspension was measured according to previous report [17].
2.4. Electrical Conductivity (EC)
Electrical conductivity (EC) of soil was measured potentiometrically in a 1: 2.5 soil-water suspensions. Twenty grams of an oven dried-sieved soil were weighed, transferred into 200 ml beaker. 50 ml of distilled water were added while stirring for one hour using Electric magnetic stirrer. The Electrical conductivity meter was calibrated using Standard potassium chloride (KCl) solutions 0.01 and 0.1 M and then the EC of filtrate were measured, Unit of measurement dS/m [18].
2.5. Determination of the Organic Matter
The organic carbon content in the soils used were analyzed in Ministry of Agriculture in Gaza using Walkley-Black method [19]. The organic carbon in the sample is oxidized with potassium dichromate and sulfuric acid. The excess potassium dichromate is titrated against ferrous ammonium sulfate. One gram soil was weighed and transferred into 500 mL conical flask. 10 mL of 1 N K2Cr2O7 and 20 mL of conc. H2SO4 were added. Swirled carefully then let to stand for 30 minutes. 200 mL distilled water and 10 mL H3PO4 were slowly added. Then 1 mL of diphenylamine indicator was added and the resulted suspension was titrated against 0.5 N ferrous ammonium sulfate solution until green color started to appear indicating the end point. Blank must run simultaneously.
2.6. Adsorption Experiments
Stock solution of diuron/linuron was prepared by dissolving 30 mg active ingredient in 2 - 3 mL methanol and diluting to 1 L with deionised water. The low concentration of methanol in the adsorption experiments had no influence on herbicide adsorption [20]. The adsorption of diuron/ linuron on soil was measured at room temp. (25˚C ± 2˚C). Appropriate aliquots of the aqueous stock solution of a diuron/linuron was diluted with distilled water to 25 mL and added to 50 mg soil in 30-mL centrifuge tubes. The concentration of diuron/linuron ranged between 1.2 mg/L and 31 mg/L. The final concentration of soil was 1 g/L. The dispersions were kept under continuous agitation during 48 hours. The supernatant was
Table 1. Physicochemical properties of the tested compounds.
Table 2. Sampling sites identification information.
separated by centrifugation at 20,000 g for 0.5 h.
The concentration of herbicide in the supernatants was determined by UV-spectrophotometer, (CT-220 Spectrophotometer), the wavelengths of the absorbance of diuron are 247 nm and for linuron is 246 nm according to previous report [21].
Linear regression was used to determine the equilibrium concentration of herbicide solutions. The regression showed R2 value close to unity (0.9992). The amount of linuron, diuron adsorbed was calculated from the depletion of the linuron concentration by adsorption according to Equation (3.2) El-Nahhal and Safi [22].
(3.1)
(3.2)
where Ci is the initial concentration of herbicide, and Ce is the remaining concentration of the herbicide in the solution in mg/L, V is the volume of the solution in litter, S is the concentration of the herbicide in the solid phase mg/g (the adsorbed amount), and M is the mass of soil in gram.
For each isotherm, a reference solution with an intermediate concentration was stirred without soil to evaluate adsorption on the glass or other losses. All adsorption experiments were made in duplicate samples with a control.
2.7. Standard Curve
Stock solution of diuron and linuron was diluted in 1ml of grade methanol and then diluted in water to a concentration of 38 ppm diuron and 35.4 ppm of linuron as a working standard. A series of linuron standards of, 0.00, 0.56, 2.26, 7.08, 14.16 and 21.24 ppm were prepared. The absorbance was measured spectrophotometrically at 246 nm. A series of diuron standards of 0.00, 0.506, 1.518, 3.036, 12.144, and 24.288 ppm, were prepared. The absorbance measured spectrophotometrically at 247 nm.
2.8. Statistical Analysis
The growth inhibition data was analyzed for variance, and main effects and interactions was tested for significance using repeated measures ANOVA. Univariate comparisons of mean growth inhibition at different depths were performed by T-test (a = 0.05), the statistical analysis was performed by using Microsoft Excel software.
3. Results and Discussion
3.1. Soil Properties
Properties of soil used in this study are shown in Table 3. It can be seen that pH values of the tested soils range between 7.32 and 8. Furthermore the EC values range from 1.81 and 2.63 dS/m, the total organic matter range from 0.801% and 0.254%. This indicates that the soil is nearly poor with organic matters. The clay fraction of soils range between 10% and 47%. The Characteristics of used soil are shown in Table 3.
3.2. Adsorption Isotherm of Linuron
The relationship between the Optical Density (OD) and low concentration of Diuron and linuron showed linear relationship (Data not shown) with R2 values of 0.9997 and 0.997 diuron and linuron respectively. These results indicate strong positive association. Accordingly, the regression equations of linear relationship of Diuron (Y = 0.082X) and linuron (Y = 0.0827X) were used to determine the remaining concentration of the corresponding compound in the aqueous solution in the adsorption experiment. Y and X represent OD and the remaining concentration respectively. Results of the control of adsorption experiments indicate no changes in the initial concentration of linuron and diuron in the glass tube that did not contain soil in the adsorption. Explanation of these results suggests that the glass tube used for adsorption experiment has no capacity to adsorb the herbicides. Accordingly, the disappearance of linuron or diuron in the experiment is due to the adsorption in the soil fraction. EL-Nahhal and Safi, [22] found similar results for other cases.
3.3. Adsorption of Diuron
Figure 1 presented the plot of adsorption of diuron in