Utilization of alkali-activated dolomite waste toward the fabrication of an effective adsorbent: Experimental study and statistical physics formalism for the removal of methylene blue and crystal violet

This study reports a potential valorization strategy for carbonate-rich dolomite waste (DW), which is mainly composed of calcite (CaCO3) and dolomite CaMg(CO3)2 minerals, to prepare an alkali-activated adsorbent (AADW). The activation process was easily performed by mixing sodium silicate (7.5 wt% Na2O) with DW at room temperature. The developed adsorbent was characterized using XRD, FTIR, TGA, DTG, FESEM, EDX, and zeta potential methods and employed to remove methylene blue (MB) and crystal violet (CV). The formation of a highly-crystalline calcium silicate hydrate phase with effective adsorption sites was mainly attributed to calcite. The pseudo-first-order (PFO) and the Freundlich equations were the best-fitted models used to explore the uptake of CV and MB. Theoretical calculations were considered to support the set-up/management of the CV and MB adsorption processes performed at 25, 40, and 50 °C. A double layer model with two energies was the best option to analyze these adsorption systems and it was found that the removal of CV and MB was controlled by parallel and non-parallel orientations. The adsorption capacities (Qsat) ranged from 343.0 to 442.1 mg/g for MB and 396.8–466.0 mg/g for CV, and the density of the AADW functional groups affected this parameter. Finally, the adsorption energies were <40 kJ/mol where physical interactions could be involved.