Magnetic tuned sorghum husk biosorbent for effective removal of cationic dyes from aqueous solution: isotherm, kinetics, thermodynamics and optimization studies

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Abstract Dyes containing effluents constitute hazards to the environments and endanger human and aquatic lives. Although activated charcoal has been adjudged the best for adsorption treatment of wastewater, its regeneration and high cost have limited their applications, hence the quest for alternative adsorbent. Magnetic tuned biosorbent was prepared from sorghum husks by in situ co-precipitation of Fe3O4. It was characterized using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy. The biosorbent was then used for the removal of crystal violet (CV) and methylene blue (MB) dyes from aqueous solutions in a batch process. The effects of temperature, initial dye concentration, dosage, contact time as well as pH were investigated, and data obtained were analysed with appropriate kinetic and isotherm models. Response surface method was used for the optimization study of the adsorption using Box–Behnken experimental design. Pseudo-second-order kinetic model was the most appropriate model for both dyes with correlation coefficient (R2) > 0.9 and low % standard error values. The equilibrium data were best fitted with Langmuir isotherm with maximum adsorption capacity (Qmax) of 18.87 and 30.00 mg g−1for CV and MB, respectively. The thermodynamic parameters for the adsorption processes showed that it was spontaneous, endothermic and random systems with free energy changes less than zero, enthalpy changes (∆H) of + 49.81 and + 51.18 kJ mol−1, entropy changes (∆S) of + 178.39 and + 177.34 J mol−1 K−1for MB and CV dyes, respectively. Optimization studies revealed that 95% of the dyes are removable at 1.0 g adsorbent dosage and pH of 4.05 at 50 °C with initial dye concentration of 50 mg L−1. The prepared adsorbent is cheap, easily recycled and highly effective for the treatment of dye-contaminated water.

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