Competitive adsorption of 2,4-dichlorophenoxyacetic acid herbicide and humic acid onto activated carbon for agricultural water management

Adsorptive removal of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and humic acid (HA) from their single- and binary solutions was investigated using powdered activated carbon. Both pore- and film diffusion coefficients for 2,4-D calculated based on two resistance diffusion kinetics were higher than those found for HA both single- and binary component systems. Similar trend was also observed for the rate constants calculated by applying pseudo-second-order kinetic model. The L-shaped isotherm curves converted into H shaped with an increase in the temperature from 288 to 318K indicating the complete removal of both solutes in dilute solutions. A synergetic effect was observed in low-concentration region in binary systems; however, the combined action of 2,4-D and HA was found to be antagonistic in concentrated solutions. The experimental equilibrium results in single-component systems were better predicted by the Freundlich than the Langmuir isotherm equation. The extended Freundlich isotherm model fit also quite well to the data obtained from binary solutions. The effects of medium acidity and salinity on 2,4-D and HA adsorption process were also examined. Analysis of the pH-dependent adsorption results using Kurbatov approximation suggested that 2,4-D and HA were mainly adsorbed in molecular form on carbon surfaces rather than in their anionic forms.