Physically and chemically activated durian rind biochar as an adsorbent for indigo carmine dye

Biochar, a by-product of pyrolysis in liquid smoke production, was prepared as an adsorbent material for the removal of indigo carmine dye. It was chemically activated using 0.1 M NaOH, followed by microwave-assisted activation. The activated biochar was characterized using Fourier transform infrared spectrometry, scanning electron microscopy-energy dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller analysis. Batch adsorption of indigo carmine was performed under various conditions: pH 3–8, initial concentrations of 400–2100 mg/L, and contact times of 15–75 minutes. Optimal results were achieved at pH 5, an initial concentration of 2100 mg/L, and a contact time of 45 minutes, with a maximum adsorption capacity of 192.55 mg/g. The equilibrium and kinetic adsorption models followed the Langmuir isotherm and pseudo-second-order kinetics, respectively. A thermodynamic study indicated a non-spontaneous adsorption process with positive Gibbs free energy (ΔG). Negative enthalpy (ΔH) and entropy (ΔS) values suggested an exothermic adsorption process with decreased randomness during adsorption. The enhanced surface characteristics and high adsorption capacity obtained in this study demonstrated that the physicochemical activation of the biochar increased its capability to adsorb indigo carmine dye. Thus, it is a promising, cost-effective, and environmentally friendly adsorbent material.