Equilibrium, kinetics and mechanistic study of PM567 laser dye adsorption from ethanol

The Pyrromethene 567 (PM567) dye, dissolved in ethanol, is used in various industries, generating liquid waste. This study investigates the adsorption mechanism of PM567 onto activated carbon (AC) from ethanol. Equilibrium adsorption data are analysed using different isotherm models, including Langmuir, Temkin, Freundlich, Sips, Redlich-Peterson, and Toth, with statistical analysis via the Akaike Weight (AW) criterion to select the best-fit model. The Langmuir isotherm provides the best fit, with a capacity of (153.5 ± 14.5) mg g⁻¹. The adsorption kinetics follow pseudo-second-order model. The temperature-independent adsorption behaviour suggests a physisorption mechanism. Time-dependent adsorption data fitted to the Homogeneous Surface Diffusion Model (HSDM) shows that the surface diffusion coefficient (<i>D_s</i>) in ethanol (2 × 10⁻¹³ m² s⁻¹) is an order of magnitude higher than in aqueous system while the mass transfer coefficient (<i>K</i>_s) is similar. This increased <i>Ds</i> is attributed to the higher diffusivity of the dye in AC, likely due to the lower solvation of the dye in ethanol. Co-adsorption of ethanol along with PM567 dye on AC also influences the adsorption kinetics and capacity. Industrial dye adsorber design has to hence account for solvent’s properties, including polarity, dye solubility, and interactions with the adsorbent.