The Prediction Of The Rate Limiting Step For A Sorption Process

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The prediction of the rate limiting step for a sorption process is an important factor. The process consists of four consecutive steps: transport of the solute from the bulk solution through a liquid film to the exterior surface of the adsorbent (film diffusion), binding of adsorbate molecules to the active sites of the surface of the sorbent, transport of the solute within the pores of the sorbent (intra-particle diffusion) and binding of the solute molecules on the active sites distributed within the sorbent’s particles. Generally, the steps involving binding are rapid and can be neglected when evaluating the rate-determining step of the sorption process.(Kumar &Gaur 2011) The Morris-Weber model (Weber &Morris 1963) is widely used to identify the mechanism involved and to distinguish between film or intra-particle diffusion. It is described by Eq. (7): q=kd • t1/2 + C (7) where, kd is the intra-particle diffusion rate constant (mg g-1 min-1/2) and C (mg g-1) is the intercept of the vertical axis. The intercept reflects the boundary layer effect and the linearity of the plot of q versus t1/2 denotes the presence of intra-particle diffusion. If C = 0, then intraparticle diffusion is the rate-controlling step, whereas if C ≠ 0, intra-particle diffusion is not the rate-controlling step, but film diffusion plays an important role in the metal sorption process. (Al-Degs et al. 2006). As may be seen in Fig. 4, there is a linear dependence of q on t1/2 at all temperatures

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