Study on the removal of Cr(Ⅵ) by acid-modified distillers’ grains biochar

The acid modified biochar (H-BC) was prepared with distillers’ grains, which was activated by KHCO3 and modified by dipping in acid solution. By optimizing the preparation parameters of H-BC (pyrolysis temperature, activator ratio, type and concentration of modified acid), it was found that H-BC prepared at a pyrolysis temperature of 700 ℃ and a mass ratio of distillers’ grains biochar to activator (KHCO3) of 1∶1.5, and modified with 0.5 mol/L nitric acid had the best adsorption performance for Cr(Ⅵ). The effects of initial pH (2.0-6.0), initial Cr(Ⅵ) mass concentration (100-600 mg/L), adsorption time (0-27 h), adsorption temperature (20-60 ℃), background ions on the adsorption efficiency of H-BC for Cr(Ⅵ) were investigated. The results showed that the best adsorption performance was achieved at pH 2.0, with the adsorption capacity of 171.05 mg/g and the removal rate of 86%. When the initial mass concentration of Cr(Ⅵ) was 100-400 mg/L and the dosage of biochar was 2.5 g/L, the adsorption capacity of H-BC could reach 151.36 mg/g, with removal rate of 94.54%, demonstrating good adsorption performance within this concentration range. The adsorption process could be divided into rapid adsorption stage (0-1 h), slow adsorption stage (1-20 h), and equilibrium stage (20-27 h). The adsorption of Cr(Ⅵ) by H-BC was an endothermic process, and the adsorption amount increased with the increase of adsorption temperature. The concentration of background electrolytes(NaCl, KCl) had a significant impact on the adsorption of Cr(Ⅵ). The adsorption process was more in line with the Langmuir isotherm model and quasi second order kinetic model. The adsorption mechanism of H-BC for Cr(Ⅵ) involved electrostatic attraction and redox reactions.