Mechanisms Governing Methylene Blue and Xylene Cyanol Adsorption on Granular Activated Carbons: Electrostatic vs π–π Interactions

This study investigates the efficacy of granular activated carbons (GACs) for the removal of cationic Methylene Blue (MB) and anionic Xylene Cyanol (XC) from aqueous solutions. Batch adsorption experiments were conducted to evaluate the influence of key parameters, including solution pH, adsorbent type, and initial dye concentration. The equilibrium adsorption data were analyzed using the Langmuir and Freundlich isotherm models, with model fitness assessed via linear regression analysis. The results indicated that Norit GAC 300 exhibited superior adsorption performance compared to Fisherbrand activated carbon, achieving a loading capacity of 0.144 mg MB/mg C at pH 8, approximately 2.5 times higher than that of Fisherbrand. This enhanced performance is attributed to its more favorable particle size and accessible pore structure. Isotherm analysis revealed that the adsorption data for both dyes were best described by the Langmuir model (R² > 0.81), suggesting a monolayer adsorption mechanism onto a surface with a finite number of identical sites. In contrast, the Freundlich model provided a poor fit for most systems, particularly for XC. Adsorption was also highly dependent on solution pH. As expected, the uptake of cationic MB increased at higher pH due to enhanced electrostatic attraction to the negatively charged carbon surface. Conversely, the adsorption behavior of anionic XC contradicted theoretical predictions based solely on electrostatic interactions, as significant removal was observed at high pH where repulsive forces should dominate. This anomaly, coupled with the large aromatic structure of XC, provides strong evidence that non-electrostatic mechanisms, particularly π-π interactions, govern its adsorption, effectively overcoming electrostatic repulsion. The findings provide critical insights for selecting and designing activated carbon adsorbents for the treatment of dye-laden wastewater.