Coal combustion ash sorbents for Cd and Zn capture in single-compound and binary systems

The work deals with the adsorption of cadmium and zinc on coal combustion ash (CCA) sorbents, in both single-compound and binary systems. Three sorbents were tested; one is CCA as received, the other two derived from the gasification of raw CCA carried out with either steam (SG sample) or carbon dioxide (DG sample). Experimental adsorption runs showed a significantly higher adsorption capacity of zinc with respect to cadmium, for all the investigated sorbents. This was ascribed to a higher affinity of sorbent active sites towards Zn(II) and, secondarily, to the smaller dimension of zinc cations (Zn2+), which resulted to be less affected by steric hindrance. Moreover, for both the analytes, a ranking DG > SG > CCA of adsorption capacity can be recognized. This is likely due to the increase in inorganic fraction of SG and DG sorbent with respect to CCA, due to the gasification treatments. Moreover, for DG sorbent, further improvement of the performances could be ascribed to an increase in the concentration of oxidized surface functional groups, which are considered the preeminent active sites for zinc and cadmium adsorption on the investigated sorbents. Binary adsorption data showed the same ranking among sorbents retrieved in single-compound tests and a higher zinc adsorption capacity.