Electro-sorption of ammonium ion onto nickel foam supported highly microporous activated carbon prepared from agricultural residues (dried Luffa cylindrica)

An electrode made of loofah sponge derived activated carbon supported on nickel foam (AC/Ni) was successfully fabricated and used to remove ammonium ion (NH₄⁺) from aqueous solution. A multilayer adsorption isotherm was used to describe ammonium electro-sorption on AC/Ni electrodes at different temperature, initial NH₄⁺ concentration, and electrical field. The cyclic voltammetry (CV) results suggested that the electrical capacitance of AC/Ni electrodes, with the AC being prepared without preheating (OAC) or with low temperature heating (i.e., 300 AC), were higher than those prepared at high preheating temperature (i.e., 400 AC and 500 AC). Increasing the electro-sorption temperature from 10 to 50 °C decreased the monolayer NH₄⁺ adsorption capacity from 5 to ca. 2-3 mg-N g^-1, respectively. Background electrolyte, namely, sodium sulfate, exhibited significant competitive effect on the adsorption of ammonium ion at sodium ion concentration > 10^-2 M. The activation energy and heat of adsorption were 9-23.2 kJ mol^-1 and -3.7--10.7 kJ mol^-1, respectively, indicating a physisorption and exothermic adsorption characteristics. Based on the kinetics and thermodynamics analysis, there was slight increase in the activation energy with elevating preheating temperature, which increased the quantity of micro-pores and surface heterogeneity of the AC materials. Overall, results clearly demonstrated that carbon pyrolysis played a role on the capacitive charging behaviors of electrodes and the efficiency of NH₄⁺ electro-sorption on the AC/Ni electrodes.

Binary electrolyte solutions

A full theoretical account of the differential capacitance of the diffuse part of the electric double layer at electrode-electrolyte solution interfaces is presented. It builds upon the standard electrokinetic model adding all the additional effects related to the finite ionic size. This includes steric interactions among ions by means of either the Bikerman or Carnahan-Starling expressions and all the permittivity related effects that arise when ions are represented as dielectric spheres. These include the solution permittivity dependence on the local ionic concentration, calculated by means of the Maxwell mixture formula, and two additional forces acting on the ions, namely the Born and the dielectrophoretic forces that depend on the permittivity and the electric field gradients, respectively. The obtained results show that the diffuse double layer behavior is sufficient to qualitatively account for the observed differential capacitance dependence on the electrode voltage. Moreover, when combined with an inner layer capacitance and using the Carnahan-Starling expression, a remarkably good quantitative agreement is achieved.