Electrosorption of alkaline earth metal ions onto activated carbon as affected by pH and applied potential (pE)

The electrosorption of alkaline earth metal ions from aqueous solution was studied using a graphite supported activated carbon electrode (NSA@G). Zeta potential measurement revealed a low ${\mathrm{pH}}_{\mathrm{zpc}}$ of 3.0 for the NSA electrode, suggesting a negatively charged L-type carbon surface. The electrosorption behavior of Ca²⁺ ion followed the Langmuir adsorption isotherm and pseudo-first-order rate law. The initial Ca²⁺ ion concentration, solution pH, and applied potential affected the electrosorption capacity. Results showed that both reversible surface charge (regulated by the potential determining ions, i.e., H⁺ and OH⁻, or pH) and polarizable surface charge (controlled by the applied potential, or pE) contributed to the overall Ca²⁺ ion removal process. The contribution of reversible and polarizable surface charge varied with pH and pE, respectively. Specifically, the reversible surface charge played a more significant role in Ca²⁺ electrosorption at high pH value and low pE (at 60–83 % of the total Ca²⁺ uptake), while the polarizable surface charge dominated at low pH and high pE (at 60–62 % of the total Ca²⁺ uptake). Factors, such as ionic radius, hydration ratio, and hydration enthalpy, significantly affected the electrosorption capacity of divalent alkaline earth metals, i.e., Ca²⁺, Mg²⁺, Sr²⁺, and Ba²⁺, over NSA@G electrode.

Novelty

This work elucidated the mechanisms of electrode charging and Ca²⁺ ion uptake via electrosorption. Previous research often attributed ion electrosorption capacity solely to the surface charge derived from a polarizable electrode. In addition to polarizable surface charge, which is controlled by the applied potential (or pE), this study demonstrated that reversible surface charge, regulated by the potential determining ions, i.e., H⁺ and OH⁻ ions (or pH), also played a significant role in total Ca²⁺ ion removal. The novelty of this work lies in quantifying the contribution of reversible and polarizable surface charge to overall Ca²⁺ ion electrosorption. Furthermore, this study investigated the factors affecting the electrosorption behavior of alkaline earth metals, i.e., Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. A rational approach to predicting electrosorption performance is also proposed, using capacitance characterization from cyclic voltammetry measurements based on Lipmann's electrocapillarity theory.