Deeper insights into fractal concepts applied to liquid-phase adsorption dynamics

In this work, the superior accuracy of “fractal-like kinetic models” with respect to their classical counterparts in interpreting cadmium adsorption dynamics on both raw and beneficiated (by means of mechanical sieving and CO2/steam gasification) combustion fly ashes has been validated. Results showed that classical and “fractal-like kinetic models” produce comparable outputs only for low fractional adsorption degrees (generally lower than 0.5); this was ascribed to a likely more homogeneous pollutant concentration in the particle outer shells. On the other hand, for longer adsorption times a less random pollutant intraparticle distribution should be responsible for a fractal-like behaviour. Additionally, for beneficiated ashes, the generally higher values of fractal pseudo-first order model rate coefficients (the model that produced best fitting results) with respect to what was observed for the parent sorbent, were linked to both the reduction of the particle diameter and the porosity development induced by the activation treatments, determining faster diffusion paths for the pollutant.