Kinetics and isotherm analysis of 2,4-dichlorophenoxyl acetic acid adsorption onto soil components under oxic and anoxic conditions

Abstract

2,4-dichlorophenoxyl acetic acid (2,4-D, pKa = 2.8) is used extensively as a herbicide in agricultural practices. Its sorption behavior on both untreated and soils treated to significantly remove specific components (organic and iron and manganese [Fe-Mn] oxides and hydroxides phases) was investigated under oxic and anoxic conditions. The chemical and structural heterogeneity of the soil components were characterized by elemental analysis and X-ray diffraction (XRD). The coexistence of the various components seems to either mask sorption sites on the untreated soil surfaces or inhibit interlayer diffusion of 2,4-D. All sorption data conform to the Freundlich description and a pseudo-second-order kinetic model. There was a strong positive correlation between sorption capacity K_d, and surface area (r² ≤ 0.704), but a negative correlation was uncovered with both pH and organic carbon (r² ≤ −0.860). The results indicate that 2,4-D is preferably sorbed under oxic rather than anoxic conditions and it is greater on soils containing a high Fe content. There was incomplete 2,4-D sorption reversibility, with desorption occurring more rapidly under anoxic conditions. The study suggests that stimulation of Fe III reduction could be used for the bioremediation of a 2,4-D-contaminated site.

Keywords:

• 2
• 4-dichlorophenoxyl acetic acid
• herbicide
• soil components
• organic matter
• sorption
• desorption

Acknowledgment

The authors are especially thankful to Assistant Professor Ifedayo Victor Ogungbe of the Department of Chemistry and Biochemistry, Jackson State University, USA for valuable contributions.

Funding

This research was funded by the Adekunle Ajasin University Akungba Akoko (Nigeria) Tertiary Education Trust Fund (TETFUND 2013/2014).