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

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.