Effect of Biochar on Adsorption Behavior of Nonylphenol onto Loess Soil in Northwest China

Abstract

In the present study, nonylphenol (NP) was selected as the target pollutant to investigate the effect of biochar produced from wheat residue at different temperatures on loess soil based on the batch experiments. The research basically included adsorption kinetic, thermodynamic and some influencing factors such as biochar with different pyrolysis temperature, particle size and pH value. The results showed that the adsorption reaction of NP onto loess soil without biochar was 10 h during fast reaction, and after the addition of biochar into loess soil, the fast reaction time of NP adsorption was shortened. Meanwhile, in the fast stage the adsorption reaction of NP onto loess soil with biochar was significantly higher than loess soil without biochar, while the difference of adsorption capacity was small at different carbonization temperatures. The adsorption reaction of NP onto loess soil by adding biochar could be well described by the pseudo-second-order kinetics model and reached equilibrium in 16 h. The kinetic data showed that the adsorption of NP accorded well with the Freundlich isotherm model. The saturated adsorption capacity was improved as temperature increased with or without biochar. Thermodynamic parameter analysis indicated Gibbs free energy ΔGθ<0, entropy ΔHθ>0 and enthalpy ΔSθ>0, demonstrating it was a spontaneous, endothermic and chaos-increasing adsorption process. At the same temperature, the adsorption capacity of NP in loess soils increased dramatically with the increase of carbonization temperature. The smaller particle size of the loess with the addition of biochar, the better the adsorption of NP. When the pH value was 4 to 7, the adsorption capacity of NP onto loess soil by adding biochar showed an increasing trend; in the pH range of 7 to 10, the adsorption saturation capacity decreased with the increase of pH value. Therefore, the adsorption of NP on loess with the addition of biochar had the best adsorption effect in the neutral range. Acid and alkalinity were not conducive to the adsorption of NP.