Repurposed marble dust as a promising adsorbent for modelling the removal of methylene blue from aqueous solutions†

Marble dust (MD) is a significant landfill waste generated as a byproduct of mining and construction industries. Methylene blue (MB) is a widely used hazardous dye responsible for serious ecological and health risks, and its treatment has become increasingly alarming. This investigation scrutinizes the facile preparation of a non-complex, low-cost, sustainable, and industrially feasible adsorbent along with conducting its mechanistic studies, including XRD, TEM, WD-XRF, FE-SEM, FTIR, BET, TGA, and XPS, followed by its implementation in the removal of MB dye. To examine the relative influence of different variables, namely, time, temperature, pH, activated marble dust (AMD) amount and MB concentration, a central composite design (CCD) model of response surface methodology (RSM) was employed with approved R² = 0.9914, supporting the credibility of the model. The additional verification was provided by ANOVA results, including the lack of fit and p-values, endorsing a quadratic model. The 3D response plots clarified the influence of variables on the removal yield; the pH had a dominant influence on the system at its higher value, while at lower pH values, the concentration played a more significant role. The removal process followed a pseudo-second-order kinetics (R² = 0.999) and adhered to the Langmuir isotherm model (R² = 0.9735), representing monolayer adsorption with q_max = 1.16 mg g⁻¹. The thermodynamic study of the process fell under Henry's law region and unveiled that the removal of MB is exothermic, spontaneous, and feasible and has appreciable reproducibility up to five cycles. The overall process of adsorption followed physisorption, which was confirmed by the adhesion probability and activation energy calculations. The adsorption process followed pore diffusion and bond formation mechanisms.