Impact of anion adsorption on the determination of the basic site concentration of activated carbons by a reaction with different acids

The basicity of activated carbon materials is derived from basic sites. The concentration of basic sites is fundamental to understanding their properties and can usually be determined by simply reacting with acids. Herein, we measured the basic site concentrations of steam-, carbon dioxide- and ammonia-activated basic carbon materials by reacting with acidic HCl, NaHSO₄ and H₂SO₄ solutions. It was found that the values determined using HCl solution were unexpectedly much lower than those using H₂SO₄ or NaHSO₄, with the H₂SO₄ and NaHSO₄ results being similar, independent of the species of activated carbon. Adsorption experiments verified that this huge discrepancy was caused by apparent adsorption of anions by the activated carbon due to the establishment of an electric double layer between the carbon surface and solution. The further analysis revealed that the adsorption of Cl⁻ is governed by electrostatic and molecular interaction forces, while adsorption of SO₄²⁻ or HSO₄⁻ is mainly dependent on electrostatic forces, leading to apparent adsorption of Cl⁻ under neutral conditions. The introduction of nitrogen-containing functionality favors adsorption of anions from neutral and acidic solutions. These findings provided new insight into the quantitative analysis of basic activated carbon materials and their electrochemical, environmental and catalytic applications.