Mn-Modified Biochar Composite for Efficient Adsorption of Pb(II) and Cr(VI) from Water: Synthesis, Characterization, and Mechanistic Insights

Heavy metal pollution in water threatens ecosystems and human health, necessitating efficient, low-cost, and sustainable remediation technologies. A manganese-modified bamboo biochar (Mn-BC) was synthesized via impregnation of raw biochar in KMnO₄ followed by pyrolysis at 500 °C, and its adsorption ability was systematically evaluated for Pb(II) and Cr(VI) removal through batch adsorption experiments investigating the effects of solution pH (2–9), adsorbent dosage (0.1–0.9 g in 20 mL), contact time (0–50 min), initial metal concentration (20–100 mg L⁻¹), and temperature (25–50 °C). SEM/TEM-EDS and XRD confirmed successful Mn incorporation as MnO_x phases, while textural analysis showed improved porosity after modification, with the BET surface area and total pore volume increasing from 77.28 m² g⁻¹ to 123.51 m² g⁻¹ and from 0.041 cm³ g⁻¹ to 0.063 cm³ g⁻¹, respectively. Batch adsorption experiments demonstrated strong pH dependence, with optimum removal at pH 8 for Pb(II) (91.87%) and pH 5 for Cr(VI) (88.2%). Adsorption was rapid within the first 30 min and reached equilibrium. A pseudo-second-order (PSO) model provided the best kinetic description (R² = 0.99) with calculated q_e values of 19.98 mg g⁻¹ for Pb(II) and 19.13 mg g⁻¹ for Cr(VI). Isotherm analysis yielded Langmuir monolayer capacities of 37.24 mg g⁻¹ (Pb(II)) and 16.39 mg g⁻¹ (Cr(VI)), with Pb(II) better described by Freundlich behavior and Cr(VI) closely fitting Langmuir assumptions. Thermodynamic results indicated endothermic adsorption (ΔH° = 41.98 and 29.67 kJ mol⁻¹ for Pb(II) and Cr(VI)) and increased interfacial randomness (ΔS°), with adsorption becoming more favorable at higher temperature (maximum removal at 50 °C: 93.21% Pb(II), 87.37% Cr(VI)). Mn-BC maintained >60% efficiency after five regeneration cycles. Mechanistically, Pb(II) removal was primarily governed by ion exchange and surface complexation, whereas Cr(VI) removal involved electrostatic attraction, partial reduction to Cr(III), and subsequent complexation on oxygenated and Mn–O sites. Overall, these findings demonstrate that Mn-BC is a practical, reusable, and competitive adsorbent for the efficient removal of Pb(II) and Cr(VI) from wastewater, supporting sustainable water treatment strategies.