Evaluating various conditions for the production of biochar from cocoa pod hull for Cd2+ adsorption
摘要
This study evaluates the influence of pyrolysis conditions and CO₂ physical activation on the Cd2⁺ adsorption performance of biochars derived from cocoa pod hulls. Biochars were produced at 400, 650, and 900 °C, and a fraction of the biochar obtained at 900 °C was physically activated under CO₂ at 800 °C for 1 h. A commercial activated carbon (B-Pure® 10-NB, DESOTEC) was used as a benchmark material. Adsorption kinetics and isotherms were analyzed using standard models with corresponding statistical indicators (R2). The results show that Cd2⁺ removal performance strongly depends on surface chemical properties rather than textural properties such as surface area. The biochar produced at 400 °C exhibited the highest performance (Langmuir adsorption capacity = 23.58 mg.g⁻1, R2 = 0.99, Qmax/SBET = 7.93 mg.m⁻2), mainly governed by complexation and ion exchange mechanisms. In comparison, lower performances were obtained for biochars produced at 650 °C (9.78 mg.g⁻1, R2 = 0.98, Qmax/SBET = 1.96 mg.m⁻2) and 900 °C (10.28 mg.g⁻1, R2 = 0.98, Qmax/SBET = 3.43 mg.m⁻2), the activated biochar (10.19 mg.g⁻1, R2 = 0.99, Qmax/SBET = 0.057 mg.m⁻2), and the commercial activated carbon (2.28 mg.g⁻1, R2 = 0.97, Qmax/SBET = 0.0028 mg.m⁻2), where ion exchange predominates. Despite its high surface area, commercial activated carbon showed the lowest normalized adsorption capacity, confirming that adsorption efficiency is not directly proportional to surface area. Under the specific experimental conditions (a single CO₂ activation condition at 800 °C for 1 h; adsorption: 50 mg adsorbent, 50 mL Cd2⁺ solution at 2–10 mg.L−1, pH = 6.0 ± 0.05, 150 rpm, 48 h, 22 ± 1 °C), physical activation did not improve Cd2⁺ adsorption performance. These findings highlight the key role of surface chemistry and mineral content and demonstrate that low-temperature biochar can be an efficient and low-cost adsorbent.
Graphical abstract