Batch adsorption and consecutive desorption experiments were performed under different pH conditions to assess the pH dependency of 2,4-dichlorophenoxyacetic acid (2,4-D) adsorption and desorption in Andosol (having pH-dependent surface charge characteristics) and kaolinite. The adsorption-desorption isotherms for both Andosol and kaolinite were well fitted by the Freundlich isotherm model. Adsorption of 2,4-D onto Andosol increased markedly with decreasing pH and was higher than for kaolinite under the same pH condition. Hysteresis, expressing the nonsingularity of the desorption compared with the adsorption isotherm, was found at all concentrations and each pH for both Andosol and kaolinite. For Andosol, the fitted Freundlich parameters for desorption, Kf,d and nd, showed a negative, linear and a positive, exponential relationship, respectively, with pH. Higher hysteresis index (ω), expressing the degree of partial irreversibility of 2,4-D adsorption, was obtained under lower pH, indicating that less adsorbed 2,4-D was released back from the Andosol under acidic condition. The effect of pH on 2,4-D desorption from kaolinite was less pronounced as compared with Andosol. Combining the measured data in this study with previously reported data on 2,4-D adsorption-desorption in several types of soil including Andosols, the possible mobility of adsorbed 2,4-D in soil was evaluated. Results show that adsorbed 2,4-D in soils exhibiting a total adsorption percentage of less than 55% of the applied pesticide typically exhibited a greater potential to be mobile in the soil-water system. Opposite, a total adsorption percentage greater than 55% implied higher residual adsorption and a prolonged existence of adsorbed 2,4-D in the soils.
1Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, Japan. Mr. Ei Ei Mon is corresponding author. E-mail: firstname.lastname@example.org
2Institute for Environmental Science and Technology, Saitama University, Saitama, Japan.
3Biodiversity Division, National Institute for Agro-Environmental Sciences (NIAES), 3-1-3 Kannondai, Tsukuba, Ibaraki, Japan.
4Department of Biotechnology, Chemistry, and Environmental Engineering, Aalborg University, Aalborg, Denmark.
Received March 2, 2011.
Accepted for publication September 13, 2011.
Financial Disclosures/Conflicts of Interest: This work was partly funded by a grant from the Research Management Bureau, Saitama University. This work was also partially supported by the grant-in-aid for Young Scientists (A) (No. 18686039) from the Japan Society for the Promotion of Science. The authors acknowledge the support from the project, “Soil Infrastructure, Interfaces, and Translocation Process in Inner Space” (“Soil-it-is”), from the Danish Research Council for Technology and Production Sciences.