Adsorption of methylene blue onto clay: experiments and statistical physics interpretation

Abstract This study explored the adsorption of methylene blue (MB) from aqueous solutions using natural clay as an adsorbent. The effects of contact time, initial dye concentration, adsorbent dosage, and pH were systematically evaluated. Analysis of correlation coefficients (R2) for the kinetic and isotherm models indicated that the adsorption kinetics follow a pseudo-first-order model, while the Langmuir isotherm provided the best fit to the experimental data at 303, 313, and 323 K. To further elucidate the adsorption mechanism, a generalized model based on statistical physics was applied. Steric analysis examined the number of dye molecules bound per active site, the estimated number of layers formed, receptor site density, and overall adsorption capacity. This model also yielded two adsorption energies associated with MB-clay and MB-MB interactions, ranging from 29.845 to 31.191 kJ/mol and from 21.981 to 24.692 kJ/mol, respectively. Thermodynamic parameters confirmed that MB adsorption is spontaneous under all studied conditions. Overall, these results highlight natural clay as a cost-effective and efficient adsorbent for removing dyes from aqueous solutions.