Zeolites from coal fly ash as efficient sorbents for cadmium ions

The aim of present study was to verify the applicability of fly ash from the combustion of brown-coal in power plant ENO Nováky (Slovak Republic) for synthesis of zeolitic materials ZM1 and ZM3 by hydrothermal alternation with 1 M NaOH and 3 M NaOH, respectively. For characterization of fly ash and zeolitic material, instrumental methods such as XRF analysis and SEM-EDX analysis were used. Obtained zeolitic materials were applied as sorbents to remove Cd²⁺ ions as a model of toxic heavy metals from water solutions. It was shown that cadmium removal is a time dependent process significantly influenced by solution pH. Using the Akaike's information criteria, we found that the sorption of cadmium by both types of zeolites obeys Langmuir adsorption isotherm model. The maximum sorption capacities Q _max at pH 6.0 calculated from Langmuir isotherm were 696 ± 22 μmol Cd²⁺ g⁻¹ of ZM1 and 1,160 ± 44 μmol Cd²⁺ g⁻¹ of ZM3. Box–Behnken design under the response surface methodology was used for investigation of interaction and competitive effects in binary metal system Cd²⁺–Cs⁺. Three independent variables (initial concentration of cadmium ions, initial concentration of cesium ions and solution pH) were correlate to response (cadmium removal) using a second-order polynomial model. The adequacy of model was confirmed by analysis of variance, coefficient of determination (R ²), and adjusted R ². Maximum sorption capacities of ZM3 in binary system Cd²⁺–Cs⁺ were 1,025 μmol Cd²⁺ g⁻¹ and 1,231 μmol Cs⁺ g⁻¹, indicating higher affinity for Cs⁺ comparing with Cd²⁺ ions.