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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.
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•2 M metal salt-modified biochars (MBs) was optimal for Congo red dye removal.•Dye removal by Al–Fe-MBs achieved 96.8%, while original biochar (BC) was 2.4%.•MBs were significantly different from BC that satisfied study hypothesis (P < 0.05).•Max adsorption capacity was highest for Fe-modified biochar with 130.0 mg g−1•Fe-modified biochar is the preferred biochar for Congo red dye treatment application.