The popular intraparticle diffusion equation, originally introduced for pure diffusion, does not account for the effect of adsorption.

A diffusion-adsorption model is proposed to replace the intraparticle diffusion equation.

For short times, the formula for the uptake has the same form vs. time as the intraparticle diffusion equation.

However, it predicts a decrease of the fractional uptake with the initial bulk concentration, as observed in the literature.

The model is used for a discussion of the intraparticle diffusion equation and for a description of experimental results.