Thermodynamics and kinetics of adsorption of ammonium ions by graphene laminate electrodes in capacitive deionization

Electrodes made of graphene laminates (GL) with favourable electrochemical properties were used to remove sodium and ammonium ions by membrane capacitive deionization (MCDI). First, the electrosorption mechanisms of the GL-based electrodes were investigated by adsorption isotherms, kinetic models and thermodynamics under different concentrations of aqueous sodium chloride (NaCl). The ion sorption followed the Langmuir isotherm corresponding to monolayer adsorption and indicated a physisorption process according to thermodynamic parameters. The GL-based electrodes were then used to remove ammonium chloride (NH4Cl). With an initial concentration of 400 mg/L, the NH4Cl removal capacity was 15.3 mg/g at 20 °C with a removal efficiency of 99%, indicating similar electrosorption characteristics to NaCl. A higher removal rate of ammonium ions was demonstrated compared to that of sodium, confirming that the GL-based capacitive deionization (CDI) process can effectively remove ammonium ions from aqueous solutions.