By Ozulumba, Tochukwu Jennifer
The adsorption of Crystal violet (CV) and Malachite green (MG) onto peanut and coconut husk powders was investigated using batch experiments. The experiments were carried out by shaking the adsorption mixtures at 120 rpm at various intervals after which the supernatants were filtered and analyzed for dye content using a UV/Vis spectrophotometer. The residual dye concentrations were determined by extrapolation from a standard curve. The experiments were carried out by varying the parameters- pH, adsorbent dose, initial dye concentration and exposure time so as to obtain removal kinetic data. The efficiency of the adsorbents (coconut and peanut husk powders) were investigated under different parameters and optimal conditions of these adsorbents determined. The optimum pH of adsorption were 2 and 6 for Crystal violet and Malachite green respectively. Atthe optimum pH, the percentage removal efficiency was found to increase with increasing adsorbent dose and exposure time- at 60 minutes, more than 90% of the dye concentration had been adsorbed onto the adsorbents. The linear regression coefficient (R2) was used to elucidate the best fitting adsorption isotherm model. Of the three isotherm models, Langmuir, Freundlich and Elovich, applied to the equilibrium adsorption data, the results indicated that the Freundlich isotherm provided the best correlation of the experimental data (R2 = 0.866 to 0.978 and n = 0.50 to 0.88). This showed that the adsorption of CV and MG occurred by multilayer sorption. The monolayer (maximum) adsorption capacities (qm) of the dyes by the husk powders were found to be between 11.76 to 12.33 mg/g. The adsorption kinetic data was modelled using the Lagergen pseudo-first order and pseudo-second order kinetic equations and the pseudo second order model was found to better describe the kinetics of the adsorption process. The calculated values of qe (adsorptive capacity) obtained from the pseudo-second order model were found to be in good agreement with the values of qe (adsorptive capacity) obtained from the experiments. Also, the correlation coefficients (R2) for the second order kinetic model had very high values for both adsorbents (mean R2 ≈ 0.999). This indicates that adsorption possibly occurs via chemisorption which in turn influences the rate-determining step of the dye removal process, the knowledge that is important in designing sorption systems such as batch and stirred tank flow reactors for the treatment of wastewater produced by textile and pulp industries which make use of dyes in their operations. Comparing the percentage removal efficiencies and adsorptive capacities of both adsorbents, coconut husk powder was found to be a better adsorbent than peanut husk powder. Considering that the adsorbents used were in a semi-natural state (having only been washed, dried and ground), it is believed that carbonization and heat activation will increase their removal efficiencies and shorten the time required for maximum dye removal.