Effects of different modifiers on the sorption and structural properties of biochar derived from wheat stalk

Nitrobenzene is a widespread contaminant in water. Biochar (BC) is a promising material for removing organic pollutants, but the adsorption capacity of pristine BC is low. Chemical modification is often used to improve the adsorption performance, but information on the sorption of nitrobenzene by modified BC is rare. In this study, BCs pyrolyzed at 300, 500, and 700 °C were modified by hydrochloric acid (HCl), sulfuric acid (H2SO4), sodium hydroxide (NaOH), hydrogen peroxide (H2O2), and nitric acid (HNO3), respectively. The properties, nitrobenzene sorption behaviors, and sorption mechanisms of different BCs were analyzed. The results showed that chemical modification decreased the sorption of nitrobenzene on BCs pyrolyzed at 300 °C, possibly due to the loss of the partition phase and the increase in polarity after modification. Regarding BCs pyrolyzed at 500 and 700 °C, the NaOH and HCl modifications significantly increased the sorption capacity by 19% and 60%, 18%, and 41%, respectively, possibly due to the increase in surface area, available pores, and aromaticity, while HNO3 modification decreased the sorption capacity by 41% and 31%. Two reasons were probably responsible for the decrease: one was the decrease in surface area after HNO3 modification due to the destruction of pore walls and the continuity of holes; the other was the strong repulsion between the nitro groups formed on the surface of BC and the nitro groups of nitrobenzene that drove nitrobenzene molecules away from the surface. A principal component-based comprehensive evaluation of the BC properties, which were significantly correlated with the sorption isotherm parameters, was used to evaluate the nitrobenzene sorption performance of the modified BC. Overall, BC pyrolyzed at 700 °C modified with NaOH or HCl were proposed as effective sorption materials for the removal of nitrobenzene in environment, which also provided a chemical modified method of biochar derived from agricultural waste.