Competitive adsorption of Dibutyl phthalate (DBP) and Di(2-ethylhexyl) phthalate (DEHP) onto fresh and oxidized corncob biochar

Abstract

Phthalates (PAEs) often exist simultaneously in contaminated soil and wastewater systems, and their adsorption to biochar might impact their behavior in the environment. So far, the competitive adsorption of PAEs to biochar has not been reported. In this study, the competitive adsorption of Dibutyl phthalate (DBP) and Di(2-ethylhexyl) phthalate (DEHP) on corncob biochar (fresh and oxidized) was investigated, and experiments of kinetics, isotherms, and thermodynamics were conducted. Langmuir and Freundlich models, pseudo-first-order and second-order kinetic models were used to simulate the experimental data. In the mono PAEs component systems, the biochar showed significantly greater adsorption capacity for DEHP (11.8–16.16 mg g−1) than for DBP (9.86–13.2 mg g−1). The oxidized biochar has higher adsorption capacities than the fresh one. Moreover, a fast adsorption rate for DBP was observed, which can be attributed to the smaller size and shorter carbon chains in the DBP molecule, resulting in faster diffusion into the biochar pores. In the binary PAEs component systems, competition between DEHP and DBP in their adsorption to the biochars was observed, and DEHP (11.7–15.0 mg g−1) was preferred over DBP (3.4–7.9 mg g−1). The stronger adsorption of DEHP can be explained by stronger hydrophobic interaction with biochar. Compared to DBP, DEHP has a high octanol-water partition coefficient (logKow) and low water solubility. The positive entropy (ΔS0) and enthalpy(ΔH0) values for the adsorption of both DEHP and DBP further indicated that hydrophobic interaction played an important role, even though H-bonds and π-π interactions could also be involved.