A novel utilization of sugarcane bagasse-derived ash to reductively remove gold(III) to gold metal: Energetics, kinetics and mechanism studies

A new method utilizing sugarcane bagasse-derived ash (SB-dA) to remove and convert gold(III) to valuable noble gold metal has been developed. The SB-dA was purified with a mixed solution of 0.1 mol/L HCl and 0.3 mol/L HF, followed by 3 mol/L HNO₃, resulting in a silica-rich material with silanol (Si–OH) and siloxane (Si–O–Si) groups, along with an aromatic component. The gold(III) removal was endothermic (ΔH^o of 34.51 kJ/mol) and spontaneous (ΔG^o ranging from −27.72 to −29.81 kJ/mol as temperature increased from 30 to 50 °C). The activation energy (Ea) and standard entropy (ΔS^o) values were 35.15 and 0.2 kJ/mol, respectively, indicating increased interfacial irregularity during gold(III) removal. At an optimum pH of 4.2, the removal followed the Langmuir isotherm and the second-order kinetics models. The rate constant (k₂) enhanced from 3.32 to 7.56 × 10² L/mol·min, and Langmuir's removal capacity (b) rose from 0.18 to 0.25 × 10⁻⁴ mol/g as temperature increased from 30 to 50 °C. Silanol and siloxane groups played crucial role in gold(III) removal through adsorption, with silanol also active in reducing gold(III) to gold metal, a process that intensified by increasing temperatures.