Studies on castor seed shell as a sorbent in basic dye contaminated wastewater remediation

The potentialities of castor seed shell (CSS), a waste agricultural by-product, in the remediation of water, contaminated with Methylene Blue (MB), a basic dye, were investigated in the present study. The CSS was ground and washed, thoroughly, to remove any water extractable constituents. The dried CSS was reground, sieved and used in series of agitated batch adsorption experiments. The experiments were conducted to assess the effect of two process variables i.e. initial MB concentration and CSS dosage on the sorption process. The equilibrium sorption isotherm was studied using the two widely used isotherm models (i.e. Freundlich and Langmuir isotherm models). The results from the isotherm studies showed that the process of sorption of MB occurred on a heterogeneous surface of the CSS. The sorption capacity of the CSS, as obtained from the Langmuir plot was 158 mg/g. The mass transfer property of the sorption process was studied using Lagergren pseudo-first-order and chemisorptions pseudo-secondorder kinetic models. The sorption process obeyed the pseudo-second-order kinetic model more than the pseudo-first order; hence the mechanism of the sorption process was analysed further using this kinetic model. The application of the intraparticle diffusion model to determine the rate limiting step showed that intraparticle diffusion is not the singular rate limiting step in the sorption of MB onto CSS. The role of chemisorptions in the mechanism of sorption was established by an empirical relationship between the pseudo-second-order rate constant, K, and the initial MB concentration. The results of this relationship showed that pseudo-second-order chemisorptions are important in the sorption process.

The results from the equilibrium adsorption isotherm studies showed that the capacity of CSS for MB was 158 mg/g and that the sorption process took place on heterogeneous surface of the CSS. The mechanism of adsorption of MB onto CSS was better described by the chemisorption pseudo-second-order kinetic model. The pseudo-second-order rate constant, k, the initial sorption rate, h0, and the equilibrium sorption capacity, qe, were a function of initial MB concentration and the CSS dose. Analysis of the results with the intraparticle diffusion model showed that intraparticle/pore diffusion is not the rate limiting step, but the results obtained from the linearization of the plot of the pseudo-secondorder rate constant, k, versus initial MB concentration, C0, showed that pseudo-second-order chemisorption is important in the sorption of MB onto CSS.