Application of isotherm, kinetic and thermodynamic models for the adsorption of nitrate ions on graphene from aqueous solution

Graphene was prepared by a liquid phase exfoliation and was characterized by Raman spectroscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy and zeta potential measurements. A systematic study of the adsorption process was performed by varying pH, ionic strength, and temperature. The experimental results showed that graphene is an excellent nitrate (NO₃^-) adsorbent with an adsorption capacity of up to 89.97mg/g at an initial NO₃^- concentration of 500mg/L and temperature of 303K. The adsorption kinetics was modeled by first- and second-order rate models. The rate constants for all these kinetic models were calculated, and the results indicate that the second order kinetics model was well suitable to model the kinetic adsorption of NO₃^-. The Langmuir and Freundlich models were used to describe the equilibrium isotherms and the isotherm constants were determined. Equilibrium data were well described by the typical Langmuir adsorption isotherm. Thermodynamic studies revealed that the adsorption reaction was spontaneous and was an endothermic process.