Metal salt-modified biochars derived from agro-waste for effective congo red dye removal

Anionic Congo red dye (CR) is not effectively removed by conventional adsorbents. Three novel biochars derived from agro-waste (Acacia auriculiformis), modified with metal salts of FeCl₃, AlCl₃, and CaCl₂ at 500 °C pyrolysis have been developed to enhance CR treatment. These biochars revealed significant differences in effluents compared to BC, which satisfied initial research expectations (P < 0.05). The salt concentration of 2 M realized optimal biochars with the highest CR removal of 96.8%, for AlCl₃-biochar and FeCl₃-biochar and 70.8% for CaCl₂-biochar. The modified biochars were low in the specific surface area (137.25–380.78 m² g⁻¹) compared normal biochar (393.15 m² g⁻¹), had more heterogeneous particles and successfully integrated metal oxides on the surface. The CR removal increased with a decrease in pH and increase in biochar dosage, which established an optimal point at an initial loading of 25 mg g⁻¹. Maximum adsorption capacity achieved 130.0, 44.86, and 30.80 mg g⁻¹ for BFe, BCa, and BAl, respectively. As magnetic biochar, which is easily separated from the solution and achieves a high adsorption capacity, FeCl₃-biochar is the preferred biochar for CR treatment application.