Treatment of tannery effluent by adsorption onto fly ash released from thermal power stations: Characterisation, optimization, kinetics, and isotherms.

Fly ash is a significant pollutant in thermal power stations. Although this waste harms the environment and humans, it is badly removed and managed, and only a few studies are interested in this waste. For that, this study aims to valorise fly ash into potential adsorbents to treat tannery effluents for the first time. The physicochemical characterisation showed that fly ash has a pHpzc of 9.78, a very porous structure, a high specific surface area of 3127.2 m/g with a total pore volume of 3.27 cm/g, and a high silica and aluminium percentage. SEM showed that the fly ash studied has a small particle size ranging between 32 nm and 100 μm. Batch adsorption experiments were done, and the effects of adsorption parameters were investigated. The kinetics and isotherms models indicate that the equilibriums were achieved in 30 min, where the maximum uptake capacity was 2496, 223.7 and 106.8 mg/g for Chemical Oxygen Demand (COD), chromium (VI) and sulfide ions, respectively. The kinetic data were well fitted to the pseudo-second-order model and showed that adsorption onto fly ash may be chemical and physical simultaneously. Freundlich's model gave a better fit for the experimental adsorption equilibrium data and displayed multilayer adsorption. The thermodynamic isotherm showed that the adsorption onto fly ash is thermodynamically spontaneous (ΔG° < 0) and endothermic (ΔH° > 0). In conclusion, fly ash, which is a free material, has a more robust adsorption capacity than other expensive materials. Thus, it can be a promising, eco-friendly, attractive adsorbent for industrial wastewater treatment.