Adsorption of Zinc on Cassava Peels Activated Carbon and Kaolin Clay: Kinetics, Thermodynamics and Optimization Studies.

Abstract

Zinc in industrial, agricultural and domestic effluents gets into the human system and becomes toxic upon accumulation. The kinetics, thermodynamics and optimization of the process parameters for the adsorptions of zinc from zinc chloride (ZnCl₂) simulated aqueous effluent using cassava peels activated carbon and kaolin clay were investigated.  Proximate and oxide analyses carried out on the raw peels and the clay showed that the peels had 45% fixed carbon and the clay had more than 2% oxides of calcium, magnesium and iron in addition to alumina and silica. Whereas activated carbon was produced from the peels by carbonization at 700°C for 2hours in a muffle furnace, and subsequent treatment with phosphoric acid at 800°C for 2hours, the clay was not modified. The adsorption data generated at different levels of contact time were modeled and the pseudo-second-order kinetics model was adequate for the kinetics of the system at room temperature with regression coefficient (R²) of 0.998. The adsorption data also fitted into Langmuir, Freundlich and Temkin isotherms with a monolayer adsorption capacity of 4.8mg/g and 3.1mg/g for the activated carbon and clay respectively. The thermodynamic showed that the adsorption processes were endothermic and the magnitude of the standard enthalpy change showed that the adsorptions on the clay were more of physisorption while the adsorptions on the activated carbon were more of chemisorptions. Equally, the negative standard Gibb's free energy change showed that the processes were spontaneous. Optimization of the process parameters was carried out using the Box-Behnken design of Response Surface Methodology (RSM) and it was analyzed using Design Expert. Linear model was selected with R-squared, Adjusted R-squared and predicted R-squared values of 0.90, 0.89 and 0.88 respectively.  The Analysis of Variance (ANOVA) of the model parameters showed that the effects of the five numeric factors, which are time, temperature, pH dosage and initial concentration, were statistically significant with p-values less than 0.01. It also showed that the effect of the one categorical factor, which is the type of adsorbent (clay or activated carbon), was statistically significant with the p-value less than 0.01. The numerical optimum solutions of the process parameters were obtained with theoretical percentage removal of 88% and 81% of zinc on cassava peels activated carbon and kaolin clay respectively.