Synthesis and Characterization of Microporous and Mesoporous Zeolites from Flyash for Heavy Metal Removal from Wastewater

Abstract

Class F fly ash was hydrothermally modified in one- and two-step processes to prepare zeolites with enhanced adsorption characteristics. Highly crystalline X type zeolite and hydroxy sodalite were targeted as microporous and mesoporous zeolite and were successfully synthesized using fly ash precursor by adjusting Si/Al molar ratio in reaction mixture. The heavy metal removal performance of the obtained fly ash-based zeolites was investigated in batch Pb2+ adsorption experiments. Lead (Pb2+) is a positively charged toxic pollutant that can be present in surface water and industrial wastewater and may cause harmful physiological effects to human. As a result, standards for water quality are becoming increasingly stringent. This has accelerated the development of innovative, cost effective adsorbent during recent years. Since zeolites have an inherent negative charge in their framework, they are capable of removing heavy metals from water. This study explores economical alternatives of removing positively charged contaminants using fly ash-based zeolites. Zeolites are a class of porous aluminosilicates with high cation exchange capacity. Fly ash has large amounts of silica and alumina in its composition and is a suitable precursor for synthesis of a wide variety of zeolites with enhanced adsorptive characteristics. The hydrothermally synthesized zeolites were subject to instrumental characterization. The ammonium acetate saturation method followed by atomic absorption spectroscopy was employed for cation exchange capacity measurements. The presence of porosity, as well as high specific surface area (as high as 424 m2/g) and exchange capacity (as high as 320 meq/g) suggest potential applications for the hydrothermally synthesized zeolites in adsorption and ion exchange applications. The removal of Pb2+ with hydroxy sodalite and Na-X zeolite was investigated under various conditions. The adsorption capacity of hydroxy sodalite increased with an increase in contact time, and a 98.1% removal efficiency was seen for a solid/liquid ratio of 3.0 g/L, pH=5.5 and an initial lead concentration of 100 mg/dm3. The Langmuir model represented lead adsorption using sodalite better than the Freundlich model and the maximum adsorption capacity of Pb2+ onto hydroxy sodalite was 153.8 mg/g. The results indicate that Pb2+ can be adsorbed successfully in significant amounts on the modified fly ash. Kinetic and isotherm studies were conducted to investigate the potential of Na-X for the removal of lead from aqueous solution. The effect of initial pH on adsorption process was investigated and kinetic equations were used to model experimental data. Based on the kinetic experiments, the equilibrium time required for the adsorption of lead from aqueous solution by Na-X under different pH conditions was evaluated. The pseudo second order model represented the adsorption kinetics better than the pseudo first order model. The reaction rate constants, as well as sorption capacity increased with increasing pH in the range of 3 to 6. Optimum pH for the removal of lead was found to be 6.0.