High-efficient co-removal of copper and zinc by modified biochar derived from tea stalk: Characteristics, adsorption behaviors, and mechanisms

Tieguanyin tea stem was used as a raw material, and potassium hydroxide (KOH), hydrochloric acid (HCl), potassium ferrate (K2FeO4), and other modifiers were used to synthesize three modified biochar (HCl-BC, KOH-BC, Fe-BCs), for competitive adsorption of copper (Cu2+) and zinc (Zn2+). Among them, Fe-BCs had the highest adsorption capacity. Fe-BC650 (K2FeO4-modified biochar heated at 650 °C) could reach adsorption equilibrium in 1 min with a high adsorption capacity. The maximum adsorption capacity of Fe-BC650 for Cu (II) and Zn (II) in the single-heavy metal solution was 303.5996 mg/g and 254.0711 mg/g, respectively. However, there were antagonistic impacts between Cu2+ and Zn2+ after analysis by the extended Langmuir model. The Brunauer-Emmett-Teller (BET) results indicated the specific surface area of Fe-BC650 was multiplied by 245 than the raw biochar (BC650), and the iron species was present in the pores of biochar through FeO bonding rather than on the surface. The results of dissolved organic carbon showed that iron species promoted the formation of aromatic structure but decreased the abundance of aromatic species in the composition. Moreover, K2FeO4 provided a large amount of K+ that could undergo cation exchange reactions with Cu2+ and Zn2+. The heavy metals became flocculent precipitate(MCO3) and deposited at the bottom of the centrifuge tube by flocculation. These characteristics constitute the excellent adsorption performance of Fe-BC650.