Adsorption properties of fly ash–clay composites from Central European localities: Case study

In this study, composite adsorbents were prepared as mixtures of aluminosilicates from Central European deposits (clinoptilolite and bentonite) and two types of fly ash from Czech operations that differed in fuel type and combustion temperature. The composites were tested for the adsorption of individual toxic cations (cadmium (Cd2+) and caesium (Cs+)) and anions (arsenate (AsO43−) and chromate (CrO42−)) and for cation–anion co-adsorption in appropriate model solutions under laboratory conditions. The clay–fly ash combinations were more selective for cation adsorption, with an efficiency of about 80% or higher, and unsuitable for anion adsorption, with a maximum efficiency of 30% for AsO43− and <20% for CrO42−. In the co-adsorption systems, the co-adsorption of all tested ions was more efficient than the adsorption of individual ions—by >20%, on average, for Cd2+, Cs+ and AsO43− and by about 6% for Cr. Because the composite sorbents combined the properties of both components, they exhibited variability in active adsorption sites for effective cation and anion binding. The co-adsorption efficiency was up to 50% higher than that of the adsorption of any single kind of ion.