Effect of Wheat Residue-Derived Biochar on Naphthalene Adsorption in Loess Soil in Northwest China

Abstract

Research on the environmental behavior of polycyclic aromatic hydrocarbons (PAHs) in soil is limited, particularly regarding the influence of biochar on naphthalene (NAP) adsorption on the loess soil of Northwest China. In this study, a batch equilibrium experiment was used to analyze the sorption kinetics, sorption isotherms, and influencing factors of NAP adsorption by biochar derived from wheat residue at various pyrolysis temperatures on loess soil. The results indicated that NAP adsorption onto biochar-modified soil was rapid, within 6 h, and reached equilibrium after 20 h. The sorption kinetics was accurately described by a pseudo-second-order model. Additionally, the sorption isotherms were best described by the Freundlich model, indicating a multilayer adsorption mechanism. The average value of KF decreased as follows: BC-600 (2.03) > BC-400 (1.52) > BC-200 (1.25) > soil (0.91), indicating that biochar addition was beneficial for the adsorption of NAP on loess soil. The Gibbs free energy (ΔGθ) of NAP was less than zero, and the enthalpy (ΔHθ) and entropy (ΔSθ) values were greater than zero, suggesting that the adsorption occurred spontaneously through an endothermic reaction. Furthermore, the initial concentration of NAP influenced its adsorption amount. Pyrolysis temperatures of 400 °C and 600 °C resulted in higher NAP adsorption capacities, highlighting the beneficial effect of biochar addition on enhancing NAP adsorption in loess soil. However, at a pyrolysis temperature of 200 °C, the process of carbonization became incomplete, resulting in a reduction in the adsorption amount.