Adsorption of dye from aqueous solution by cashew nut shell: Studies on equilibrium isotherm, kinetics and thermodynamics of interactions

Cashew nut shell (CNS) — a novel, low cost adsorbent prepared from agricultural waste has been utilized as the adsorbent for the removal of Congo red (CR) dye from an aqueous solution. The effect of pH, adsorbent dose, initial dye concentration, time and temperature on adsorption was studied. The results indicate that CNS can be employed as a low cost alternative compared to other commercial adsorbents in the removal of dyes from wastewater. The experimental data were analyzed by Langmuir, Freundlich, Redlich–Peterson, Koble–Corrigan, Sips, Toth, Temkin and Dubinin–Radushkevich adsorption isotherms. The characteristic parameters for each isotherm and related correlation coefficients have been determined using MATLAB 7.1. Thermodynamic parameters such as ∆Go, ∆Ho and ∆So have also been evaluated and it has been found that the sorption process was feasible, spontaneous and exothermic in nature. Pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used to fit the experimental data. Kinetic parameters, rate constants, equilibrium sorption capacities and related correlation coefficients, for each kinetic model were calculated and discussed. It was shown that the adsorption of CR could be described by the pseudo-second-order equation, suggesting that the adsorption process is a presumably chemisorption.