Microwave biochar produced with activated carbon catalyst: Characterization and adsorption of heavy metals

Novel microwave biochar derived from wheat straw (WS) using a range of power levels, with activated carbon catalyst as microwave absorber, was produced, characterized and tested as adsorbent of three heavy metals (Pb²⁺, Cd²⁺, and Cu²⁺). The microwave biochar with the greatest specific surface area (156.09 m² g^-1) and total pore volume (0.0790 cm³ g^-1) were produced at 600 W (WS600) and 500 W (WS500) power level, respectively. Maximum adsorption capacities of WS500 to Pb²⁺, Cd²⁺ and Cu²⁺ were 139.44 mg g^-1, 52.92 mg g^-1, and 31.25 mg g^-1, respectively. Optimal pH value for heavy metal removal was at range of 5-6, and Pb²⁺ showed the strongest affinity in competitive adsorption experiments. The adsorption data were fitted better by pseudo-second-order model and Langmuir isotherm, indicating that adsorption process was mainly explained by monolayer adsorption, and chemical adsorption occupied important role. The predominant adsorption mechanisms of heavy metals on microwave pyrolysis biochar included complexation with oxygen-containing functional groups (i.e., carboxylic acid CO and -OH) and precipitation with carbonate. In addition, reused WS600 maintained 76.17% and 96.07% of their initial adsorption capacity for Cu²⁺ and Cd²⁺, respectively. These results suggest that microwave biochar produced with activated carbon catalyst has excellent potential for efficient use in the removal of heavy metals from waste water.