Effects of Dissolved Organic Matter on Sorption of Metribuzin Herbicide by Two Different Soils

The environment is contaminated through intensive or inappropriate use of herbicides. Quantifying the fate of applied herbicides in the soil is essential for minimizing their mobility in the soil and environmental pollution. The adsorption behavior of the soil-applied herbicides is one of the most important factors governing its environmental impacts such as degradation, transition and leaching. To date, No studiy has been conducted to investigate the effects of DOM on the sorption of metribuzin by soils. The objective of this study was to investigate the impacts of DOM on metribuzin sorption by two defferent soils.
In this research, DOM (0, 10, 20, 40, 80 and 160 mg of OC/L) adsorption in two different soil samples was assayed under laboratory conditions at constant temperature. The effect of pH and DOM concentrations (0, 10, 40 and 160 mg/L) on metribuzin (1.5, 2, 3, 4, 5 and 6 mg/kg) adsorption was also studied. Soil samples were selected and collected from surface layers (0–20 cm). The soil samples were air-dried and passed through a 2-mm sieve. The DOM sorption in both soils was performed by adding 10 ml DOM solution with a series of initial concentrations in each 15 ml glass tube containing 1.00 g soil. All the DOM solutions contained 0.01 mol/L CaCl2 and 0.01 mol/L thymol, and the pH of the solutions were adjusted to 9.0 (about the pH of the initial extracted DOM solution) with 0.1 mol/L HCl or 0.1 mol/L Ca(OH)2. The tubes were shaken at 140 rpm for 24 h at 22°C. After centrifugation at 4,500 rpm for 15 min, the DOM concentrations in solutions (presented as OC) were measured using a total organic carbon analyzer. Sorbed organic carbon was calculated from the difference between the OC content of the DOM solution, which was initially added, and that found in equilibrium solution with the soil, of which the amount of native DOM released from the air-dried soil samples was subtracted. DOM solutions (10 ml) with different concentrations were added to the soils in 15 ml glass tubes with PTFE lined screw caps. The solid-to-solution ratios were adjusted to attain 20–80 percentage of the initially added metribuzin adsorption by the soils. All the DOM solutions contained 0.01 mol/L CaCl2 to maintain a constant ionic strength and 0.01 mol/L thymol to inhibit potential microbial activities, and the pH values of the solutions were adjusted to 9.0 with 0.1 mol/L HCl or 0.1 mol/L Ca(OH)2. Metribuzin was mixed at high concentration in acetonitrile before being added to the DOM solutions. Acetonitrile concentrations were always less than 0.1 percentage of the total solution volume to avoid the cosolvent effect. The tubes were shaken at 140 rpm for 24 h at 25°C. Preliminary studies showed that sorption equilibrium was approached within this time period. After mixing, the tubes were centrifuged at 4,500 rpm for 15 min, and 1.0 ml of the supernatant was removed into a sampling vial for analysis. All sorption samples were conducted in triplicate. The sorption experiments were conducted at different pH values in the absence of DOM by addition of HCl and Ca(OH)2 as required to solutions containing 0.01 mol/L CaCl2 and 0.01 mol/L thymol. The investigated pH values ranged from 4 to 9.5. The initially added concentration of metribuzin was 5 mg/L. After shaking and centrifugation, the pH values of the supernatants were measured using a pH meter. The samples were analyzed by gas chromatography equipped with a mass (6890N, Agilent, USA). Metribuzin sorption was calculated from the difference between the total amount of metribuzin initially added to the solution and the amount remained in the solution at equilibrium.
Dissolved organic matter (DOM) was adsorbed on the soils and the experimental data was better fitted to the Freundlich isotherm (R>0.999). The KF parameter amount of DOM sorbed on the soils were 3.82 and 0.95 L/kg for the soils of 1 and 2, respectively which suggested that the soils have low affinity to metribuzin . In the presence of DOM, the sorption behaviors of metribuzin by the soils were different. The effects of DOM on metribuzin sorption were dependent on the characteristics of soils and the concentrations of present DOM. Metribuzin sorption by soil 1 and soil 2 was inhibited in the presence of DOM. Metribuzin sorption was quantified by comparing the apparent distribution coefficient of metribuzin in the presence of DOM (Kd*) and the absence of DOM (Kd). The sorption was promoted when the ratio of Kd*/Kd was more than 1, and was inhibited when the ratio of Kd*/Kd was less than 1. The aqueous solubility, sorption, and bioavailability of metribuzin are pH dependent. The effects of pH on the metribuzin sorption by soils showed when pH increased from 4 to 9, metribuzin sorbtion by soils was decreased. When the pH was higher than 6.0, it was clear that the amount of sorbed metribuzin decreased as the solution pH increased in each soil in the absence of DOM. Metribuzin was present in both molecular and ionic forms owing to the ionization in the pH range examined in the study. As the solution pH increases, the protonic form decreases. Therefore, the sorption of metribuzin protonic form and the negatively charged surface of soils become more pronounced and the amount of metribuzin partitioned to soil decreases. In addition, an increasing pH may enhance the release of native organic matter from the soils into the solution that results in the decrease of metribuzin sorption. At the same pH, the amount of metribuzin sorbed by soil 2 was lower than soil 1 which was probably owing to the different organic matter content of the soils.
The effects of DOM on metribuzin sorption were dependent on the characteristics of soils and the concentrations of present DOM. In general, metribuzin sorption by the soils was inhibited in the presence of DOM. Metribuzin sorption by the soils also decreased with increasing the solution pH in the absence and present of DOM. The results of the study will be useful for the better understanding of the behavior of metribuzin in soils and its ecological risks.