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.