A Comparison Between a Low-Cost Sorbent and an Activated Carbon for the Adsorption of Heavy Metals from Water

In this work, a coal combustion ash (CCA) has been tested as an alternative low-cost sorbent to commercial activated carbons (GAC) for cadmium and zinc removal from polluted water. To this aim, the effect of pH and metal concentration on CCA adsorption capacity has been investigated, and a comparative analysis with GAC has been carried out in the same experimental conditions. Furthermore, in order to improve CCA adsorption capacity, two different activation treatments of raw CCA have been tested. In particular, the CCA was subjected to a gasification process conducted with steam and to different acidic treatments, conducted either with hydrochloric acid or nitric acid at different acid concentrations. Experimental results showed that all the acid treatments determined a substantial reduction of both cadmium and zinc adsorption capacity. Differently, the steam gasification determined a substantial increase in adsorption capacity with respect to raw CCA, in particular for zinc as its adsorption capacity resulted even higher than the correspondent of GAC. Finally, a thorough analysis of sorbent physical and chemical properties and of adsorption data allowed the individuation of the main cadmium/zinc adsorption mechanism both on CCA and activated carbon, adequately described by the Freundlich model.