Novel Phosphoric Acid-Modified Biochar–Chitosan Nanocomposite for an Efficient and Cost-Effective Multimetal Removal from Wastewater

This study presented a novel and cost-effective adsorbent developed from phosphoric acid-modified biochar–chitosan nanocomposite for the efficient removal of Cu2+, Ni2+, and Zn2+ from wastewater. The biochar was synthesized at an optimized pyrolysis temperature of 550 °C for 2 h, followed by modification with phosphoric acid and composed of chitosan, resulting in a mesoporous PGB–CS composite (9.18 nm pore diameter) that exhibited a high surface area (167.98 m2/g), low crystallinity, good thermal stability, and abundant surface functional groups such as amine, carboxylic, and hydroxyl. The adsorption parameters were optimized using the Box–Behnken design of response surface methodology, obtaining maximum adsorption capacities of 221.56 mg/g for Cu2+, 175.47 mg/g for Ni2+, and 127.46 mg/g for Zn2+ under optimal conditions. The pH study further improved the adsorption capacities to 249.78 mg/g for Cu2+, 191.48 mg/g for Ni2+, and 145.91 mg/g for Zn2+. The adsorption process followed pseudo-second-order kinetics, indicating chemisorption, and confirmed the Langmuir isotherm, suggesting monolayer adsorption. Thermodynamic parameters confirmed the spontaneous and endothermic nature of the adsorption. Real industrial effluent from a battery manufacturing industry demonstrated removal efficiencies of 83.19% (Cu2+), 61.94% (Ni2+), and 52.34% (Zn2+). The adsorbent maintained stability and reusability over 8 regeneration cycles, with desorption efficiencies of 53.17%, 51.97%, and 51.07% for Cu2+, Ni2+, and Zn2+, respectively, using H2SO4, HNO3, and HCl. The synthesis cost was estimated as USD 8.13/g (Rs. 682.14/g), indicating strong economic potential. Adsorption mechanisms were attributed to surface complexation, ion exchange, and electrostatic attraction. The developed adsorbent provided a sustainable and efficient approach for treating heavy-metal-contaminated industrial wastewater.