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

Abdulrhman Fahmi Alali

Abstract


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, a finding attributed to its favorable particle size and pore structure, which enhance surface area accessibility. 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 suggests that non-electrostatic mechanisms, such as hydrophobic or π-π interactions, may govern the adsorption of XC. The findings provide critical insights for selecting and designing activated carbon adsorbents for the treatment of dye-laden wastewaters.


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