Biosorption of methylene blue by the buckwheat hull—kinetics and optimization of adsorption parameters using response surface methodology

A low cost agricultural waste buckwheat hull after extracted natural pigment was used as bisorobent for removal of a cationic dye methylene blue from aqueous solution. The sorbent was characterized by FTIR, scanning electron microscopy (SEM), and potentiometric mass titration. The results showed that buckwheat hull is porous material with functional groups such as hydroxyl and carboxyl, and the pH_zcp is found to be at pH 4.12. The kinetics of adsorption of methylene blue onto buckwheat hull at five different initial concentrations was determined by batch experiments. The obtained data were analyzed by pseudo-first-order, pseudo-second-order, and intraparticle diffusion equations. Sorption processes were found to follow pseudo-second order kinetics. Intraparticle diffusion was found to take part in sorption processes but it could not be accepted as the primary rate determining step. Response surface methodology based on Box-Behnken design was used to further evaluate the effects of important parameters, namely, pH, initial dye concentration and temperature on the percentage removal of dye. A quadratic model for predicting the optimum point was established. The optimum conditions for removal of dye from water were found to be as follows: adsorbent concentration of 0.3 g per 50 mL of dye solution, pH 6.0, temperature of 40°C, and the initial dye concentration of 100 mg/L. At the optimized condition the maximum removal was found to be 99.54%, which was in good agreement with the experimental value. This study further implies that buckwheat hull is a promising sorbent for removal of dye from water.