Effect of coal fly ash's particle size on U adsorption in water samples and thermodynamic study on adsorption

Abstract

In the present study, the influence of coal fly ash's particle size on uranium (U) decontamination from water samples is investigated. Accordingly, fly ash is segregated into five different size fractions (i.e., 63–90 μm, 125 μm) using an electromagnetic sieve shaker. Adsorption experiments are carried out in triplicate samples (for equilibration time at pH 6) using a laboratory batch method on all the size fractions with known U concentration. The highest U adsorption is observed for the lowest size fraction (i.e., <20 μm). Sample t-test on adsorption data indicates that there exists a statistically significant difference between U adsorption values observed in each size fraction at 95% confidence level. The adsorbent dose and initial U concentrations were found to have positive effect on adsorption capacity and removal efficiency. The experimental data were well fitted with Freundlich and Dubinin–Radushkevich (D-R) isotherm models indicating a multilayer adsorption. The maximum adsorption capacity predicted by Freundlich and D–R isotherm models is 58.7 mg/g and 2.9 mg/g, respectively. Study results also suggest that FA < 20 μm size fraction can be useful in decontaminating the water samples with U initial concentration of 3658 μg/L to below the World Health Organization (WHO, 2011). The mean free energy for U adsorption on FA < 20 μm fraction is 3.9 kJ/mol. Thermodynamics of adsorption revealed that adsorption is an endothermic process with ΔH0 and ΔS0 values 20.6 kJ/mol and 83 J/mol.K, respectively. U adsorption on FA is spontaneous with ΔG0 values ranging from −3.3 to −7.2 kJ/mol over the temperature range of 283 °K to 333 °K.

Keywords

• Uranium
• Fly ash size fraction
• Batch method
• Adsorption
• Thermodynamics