Modification of calcium-rich biochar by loading Si/Mn binary oxide after NaOH activation and its adsorption mechanisms for removal of Cu(II) from aqueous solution

A novel modified calcium-rich biochar by loading Si/Mn binary oxide after NaOH activation (A-Si/Mn-CRBC) was successfully synthesized at 800 °C to enhance the removal of Cu(II) from aqueous solution. The composition and structure of calcium-rich biochars before and after modification were characterized by SEM + EDS, FTIR, XRD and BET. The effect of adsorbent dosage and initial pH on the adsorption behavior of Cu(II) by A-Si/Mn-CRBC was investigated by batch adsorption experiments. The results showed that activation of NaOH before Si/Mn binary oxide loading greatly influenced properties of the materials. It was beneficial to promote smoother surface morphology and more uniform loading of Si/Mn oxide particles in crystalline state. The removal rate of A-Si/Mn-CRBC for Cu(II) with an initial concentration of 50 mg/L was nearly 100 % under the conditions of dosage of 1 g/L and pH = 3–6, and the acidic of the solution could be neutralized during the adsorption process. Kinetics and isotherms studies demonstrated that Cu(II) adsorption onto A-Si/Mn-CRBC was well described by pseudo-second order model and Langmuir model, indicating that monolayer chemisorption was dominant. Langmuir maximum adsorption capacity of A-Si/Mn-CRBC for Cu(II) was 141.76 mg/g. The Weber-Morris model indicated that intra-particle diffusion was the rate-limiting step for the adsorption of high Cu(II) concentration. Thermodynamic analysis confirmed that the adsorption process was spontaneous and endothermic. Adsorption mechanism was ascribed to surface complexation, precipitation, ion exchange and pore-filling. Above all, A-Si/Mn-CRBC is a promising adsorbent for the treatment of acidic heavy metals wastewater.