Mass transfer processes in the adsorption of basic dyes by peanut hulls

Waste peanut hulls, a low-cost and abundantly available material in parts of the world, are shown to act as an adsorbent for basic dyes. The extent of dye uptake was determined in standardized batch adsorbers. The effect of process variables such as the initial dye concentration, the mass of adsorbent, and the agitation speed on uptake during the batch adsorption was evaluated. Increased initial dye concentration resulted in an increase in the amount of dye adsorbed per gram of adsorbent for all sorbent/dye combinations.The initial uptake of dye was found to be more rapid from solutions with lower concentrations. The rate of dye adsorption increased with increasing adsorbent mass and agitation speed, due to the reduction in the boundary layer resistance. The applicability of three single resistance mass transfer models was evaluated. Good model fits were obtained. However the single resistance external mass transfer failed at longer contact times due to the retardation of the adsorption process by an internal diffusion process. Results suggest the internal diffusion step could be the rate-limiting step in the adsorption process and the overall rate of adsorption was controlled by intraparticle diffusion, adsorption inside the adsorbent surface, or both.