Chitosan-mercaptoacetic acid functionalized pepper stem derived biochar for enhanced adsorption of Pb heavy metal ions

Heavy metal contamination, particularly from ions such as Pb²⁺ and Hg²⁺, poses a significant threat to soil fertility and agricultural productivity by impairing microbial activity and enzymatic functions. In this study, biochar (BC) was synthesized from pepper stem waste through pyrolysis at temperatures ranging from 400 °C to 700 °C. Among the samples, biochar pyrolyzed at 500 °C (BC@500) exhibited optimal physical and chemical properties. To enhance its adsorption capacity for Pb²⁺ ions, BC@500 was further functionalized with thiolated chitosan (TC) using varying concentrations (2.5 %, 5 %, 7.5 %, and 10 %) of mercaptoacetic acid (MA). The resulting TC-modified biochars (TCBCs), particularly those with 5 % and 7.5 % MA, demonstrated increased surface area and enriched surface functionalities. Adsorption studies revealed that 5 % and 7.5 % TCBCs followed a pseudo-second-order kinetic model, indicating that chemisorption is the dominant mechanism. Langmuir isotherm analysis confirmed a monolayer, exothermic adsorption behavior, with the 5 % TCBC achieving a high Pb²⁺ adsorption capacity of ∼158.41 mg/g. These findings highlight the potential of thiolated chitosan-functionalized pepper stem biochar as a sustainable and efficient adsorbent for Pb²⁺ remediation in contaminated environments.