Efficient Capture of Phosphate and Cadmium Using Biochar with Multifunctional Amino and Carboxylic Moieties: Kinetics and Mechanism

The presence of phosphorus in water is a major cause behind the eutrophication of the aquatic environment. The growing trend focuses on the use of agriculture waste to fabricate biosorbents with higher removal capabilities for phosphate present in wastewater. Herein, a novel adsorbent named multifunctional biochar (MFBC) was applied for the decontamination of both phosphate and cadmium through two-stage adsorption. The as-prepared MFBC was characterized by TEM, BET, XPS, FTIR, and Raman spectroscopy techniques to confirm the successful grafting and presence of multiple functionalities with both amino and carboxylic functional groups on biochar (BC) surface after chemical modification route was applied. The saturated uptake capacity of phosphate that reached 57.50 mg P g⁻¹ for MFBC was noticed within 75 min at pH 5.0 and 20 °C. Based on the results obtained from adsorption kinetics and isotherm studies, as well as XPS analysis, it was interpreted that the phosphate removal was due to physical electrostatic interaction developed between protonated amino groups (–NH₃⁺) and anionic phosphate. The as-obtained multifunctional biochar loaded with phosphate (MFBC-P) demonstrated efficient cadmium ion (Cd²⁺) uptake up to 61.40 mg g⁻¹ when it was further applied for second-stage adsorption in aqueous solution. Both residual carboxylic group and phosphate loaded on MFBC-P surface were responsible for Cd²⁺ sorption. Further XRD analysis revealed that cadmium was immobilized in the form of Cd (H₂PO₄)₂ and CdCO₃. In the binary solution system, the synergistic effects between phosphate and Cd²⁺ ions were monitored such as phosphate removal increases from 91.25 to 95.40% in the presence of Cd²⁺ ions as well as the remarkable enhancement from 36.11 to 83.76% in Cd²⁺ ion uptake value noticed for coexisting phosphate ions when MFBC was tested. The study shows that BC incorporated with multiple functionalities can provide attractive applications for environmental purification.