Adsorption modeling and thermodynamic characteristics of silver ions on to low-rank Pakistani coal

Abstract

Naturally occurring low-rank Pakistani coal was studied for the adsorptive removal of silver ions from aqueous solutions. A series of essential physico-chemical conditions were optimized as a function of concentration of electrolytes (${\mathrm{HNO}}_{3}, {\mathrm{H}}_{2}{\mathrm{SO}}_{4},\mathrm{ and }{\mathrm{HClO}}_{4}$), weight of coal, equilibration time, temperature, initial concentration of adsorbate, and the quantification of silver was made by atomic absorption spectrometric technique. Maximum sorption of silver ions i.e., 96% (percentage efficiency of coal) was achieved at 0.0001 mol/L ${\mathrm{HNO}}_{3}$ solution, with 0.75 g of coal for 10 mL of 9.271 × 10⁻⁵ mol/L of silver ion concentration using 30 min equilibration time. The sorption of silver followed pseudo-second order kinetics with overall intra-particle/film diffusion process. The determined rate constant ${\mathrm{K}}_{2}$ was 14.303 g/mg min. Sorption data obeyed Langmuir, Freundlich, and Dubinin–Radushkevich isotherms over the silver concentration range of 9.271 × 10⁻⁵ to 1.391 × 10⁻³ mol/L. The Langmuir constants Q = 9.366 × 10⁻³ m mol/g and b = 20.238 × 10³ ${\mathrm{dm}}^{3}/\mathrm{ mol}$ have been computed for the sorption system. The determined sorption energy from the Dubinin–Radushkevich isotherm was 12.91 kJ/mol indicating chemisorption phenomena. The sorption of silver was increased with the rise in temperature (278–333 K) indicating endothermic process. Thermodynamic parameters i.e., ΔG, ΔH and ΔS have also been computed and discussed for the system. The low-rank Pakistani coal was characterized using FTIR and SEM both before and after sorption of silver ions. The influence of various foreign ions on the sorption of silver in solution and desorption has also been studied.

Graphical abstract