Adsorption of methylene blue dye from aqueous solution by agricultural waste: Equilibrium, thermodynamics, kinetics, mechanism and process design

The adsorption of methylene blue (MB) dye from aqueous solution onto a cashew nut shell (CNS) was investigated as a function of parameters such as solution pH, CNS dose, contact time, initial MB dye concentration and temperature. The CNS was shown to be effective for the quantitative removal of MB dye, and the equilibrium was reached in 60 min. The experimental data were analysed by two-parameter isotherms (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models) using nonlinear regression analysis. The characteristic parameters for each isotherm and the related correlation coefficients were determined. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° were also evaluated, the sorption process was found to be spontaneous and exothermic. Pseudo-first-order, pseudo-second-order and Elovich kinetic models were used to analyze the adsorption process. The results of the kinetic study suggest that the adsorption of MB dye matches the pseudo-second-order equation, suggesting that the adsorption process is presumably chemisorption. The adsorption process was found to be controlled by both surface and pore diffusion. Analysis of adsorption data using a Boyd kinetic plot confirmed that the external mass transfer is a rate determining step in the sorption process. A single-stage batch adsorber was designed for different CNS doses to effluent volume ratios using the Freundlich equation. The results indicated that the CNS could be used effectively to adsorb MB dye from aqueous solutions.