Adsorption of 2,4-Dichlorophenoxyacetic Acid onto Volcanic Ash Soils:
Effects of pH and Soil Organic Matter

Ei Ei Mon ^a, Taiki Hirata ^a, Ken Kawamoto ^a, Syuntaro Hiradate ^b, Toshiko Komatsu ^a and Per Moldrup ^c

^a Graduate School of Science and Engineering, Saitama University, 225 Shimo-okubo, Sakura-ku, Saitama, 338-8570, Japan.

^b Biodiversity Division, National Institute for Agro-Environmental Sciences(NIAES),3-1-3 Kannondai, Tsukuba, Ibaraki,305-8604, Japan.

^c Environmental Engineering Section, Dept. of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark.

Received 15 September 2008; Accepted 6 October 2008

Abstract
     The quantification of the linear adsorption coefficient (K_d) for soils plays a vital role to predict fate and transport of pesticides in the soil-water environment. In this study, we measured K_d values for 2,4-Dichlorophenoxyacetic acid (2,4-D) adsorption onto Japanese volcanic ash soils with different amount of soil organic matter (SOM) in batch experiments under different pH conditions. All measurements followed well both linear and Freundlich adsorption isotherms. Strong correlations were found between measured K_d values and pH as well as SOM. The 2,4-D adsorption increased with decreasing pH and with increasing SOM. Based on the data, a predictive K_d equation for volcanic ash soils, log (K_d) = 2.04 - 0.37 pH + 0.91 log (SOM), was obtained by the multiple regression analysis. The predictive K_d equation was tested against measured 2,4-D sorption data for other volcanic ash soils and normal mineral soils from literature. The proposed K_d equation well predicted K_d values for other volcanic ash soils and slightly over- or under-predicted K_d values for normal mineral soils. The proposed K_d equation performed well against volcanic ash soils from different sites and countries, and is therefore recommended for predicting K_d values at different pH and SOM conditions for volcanic ash soils when calculating and predicting 2,4-D mobility and fate in soil and groundwater.

Keywords: 2,4-D; adsorption; volcanic ash soil; pH; SOM

* corresponding author : Ms. Ei Ei Mon

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