Modeling Batch Kinetics of Cadmium Removal: by a Recycled Iron Adsorbent

The kinetics of cadmium adsorption onto a recycled iron-bearing material is investigated. Batch rate data are analyzed using a surface reaction rate model, solved analytically, and a dual rate diffusion model solved numerically. Experimental investigations examined the effects of sorbent particle size in the 0.06–0.5 mm diameter range and pH in the 4–7 range. While both rate models can describe time-concentration data for cadmium well, particularly for the larger granular sizes of sorbent, the analysis reveals a substantial variation in the relevant rate coefficients with particle radius, r, suggesting diffusion versus surface-controlled reaction kinetics. The second-order rate constant for the surface reaction model and the intraparticle diffusion coefficient of the dual rate diffusion model, represented as surface diffusion, were proportional to 1/r.