Biochar derived from bean husk for the removal of lead and hexavalent chromium from aqueous solutions

Abstract

Heavy metals are among the most prevalent and difficult environmental pollutants to manage. This study explored the conversion of bean husks into biochar and evaluated its effectiveness as an inexpensive adsorbent to eliminate Pb(II) and Cr(VI) from water. Several analytical methods were utilized to ascertain the properties of the bean husk-based biochar (BHB), while sorption performance was evaluated under different conditions, including contact time, solution pH, biochar dosage, starting metal concentrations, and temperature. The findings revealed that BHB had a surface area of 24.28 m²/g and a pore volume of 0.5105 cm³/g, and achieved removal efficiencies of 94.7% for Pb(II) and 91.5% for Cr(VI) under optimal conditions. Several kinetic and equilibrium isotherm models were also employed to interpret the uptake behavior. The highest uptake capacities recorded for Pb(II) and Cr(VI) were 18.38 mg/g and 22.78 mg/g, respectively, with the isotherm closely matching the Freundlich model. Kinetic modeling revealed that Pb(II) uptake conformed more closely to the pseudo-second-order model, while Cr(VI) aligned with the pseudo-first-order model. Regeneration tests showed that the biochar retained more than 60% of its initial performance after three reuse cycles for both metal ions, indicating good reusability for a non-activated, agriculturally derived adsorbent. The sorption mechanism involved a combination of ion exchange, surface complexation, electrostatic interactions, and physical attraction via van der Waals forces. Graphical Abstract