Efficient Adsorption Removal of an Anionic Azo Dye by Lignocellulosic Waste Material and Sludge Recycling into Combustible Briquettes

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Abstract
In recent years, the removal of dyes has emerged as a significant problem that attracted several researchers. The search for green and eco-friendly adsorbents has been a never-ending task in environmental protection to overcome this issue. Herein, almond shells (AS) were used as an adsorbent to remove methyl orange (MO) from aqueous solutions. The AS was characterized using several techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR). Adsorption experiments were carried out under different pH, temperature, and AS particle size conditions. Kinetic and isothermal studies revealed that MO adsorption on the AS reached equilibrium at 90 min, following the pseudo-second-order (PSO) kinetic model. The Langmuir adsorption isotherm was found the suitable adsorption model for MO adsorption on AS, showing a maximum adsorption capacity of 15.63 mg/g. Thermodynamic parameters such as the change in standard enthalpy (ΔH°), the change in standard entropy (ΔS°), and the change in standard free energy (ΔG°) indicated that the MO dye adsorption process is non-spontaneous, endothermic, and physical, which was further confirmed from FTIR analysis of AS samples after adsorption. The contaminated sludge was converted into biochar by slow pyrolysis at a temperature of 400 °C for 2 h. Biochar has been exploited for the manufacture of combustible briquettes.

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