Hydrothermal Chemical Modification of Biochar for Improving Cd and Pb Adsorption: Performance and Mechanism

Pyrolytic carbon (PC) shows potential as an adsorbent, but its adsorption capacity is limited due to its specific surface area (SSA) and low functional group abundance. The adsorption capacity of PC can be enhanced by increasing its SSA and the abundance of functionalities. Herein, novel acid/alkali hydrothermal modification of PC (H300ac and H300al) with H3PO4 and NaOH was employed to remove Cd and Pb from contaminated water. The hydrothermal modifications with acid/alkali not only increased the SSA and pore structure of the biochar but also introduced O- and P-containing functional groups (OFGs and PFGs, e.g., ─ COOH, ─ OH, and ─PO43−). The results of adsorption kinetics and isotherm revealed that the adsorption of Cd and Pb on H300ac and H300al was characterized by chemisorption monolayer formation, fitting well with the pseudo-second-order model and Langmuir model. The maximum adsorption capacities were 210.3 mg/g for Pb on H300ac, 21.7 mg/g for Cd, and 170.3 mg/g for Pb on H300al, which were 13–20 times higher than those of the pristine biochar. Characterization analysis confirmed that the primary mechanisms involved in the adsorption of Cd and Pb by H300ac and H300al are surface complexation, precipitation, and ion exchange between Cd/Pb ions and the OFGs and PFGs on the biochar surface. This study provides an efficient and eco-friendly adsorbent synthesized from agricultural waste for the remediation of Cd and Pb from contaminated water.