Synergizing Monte Carlo simulations and experimental insights for efficient cationic dye removal using natural fluorapatite

Context

Natural fluorapatite (FAP) has been investigated as an adsorbent for the removal of dyes such as methylene blue (MB) and crystal violet (CV) from aqueous solutions. Effective dye removal is crucial for water treatment, particularly for industrial wastewater containing toxic dyes. FAP, a naturally abundant material, was characterized using XRD, FTIR, and SEM analysis. The maximum adsorption efficiency achieved was 97% (23 mg/g) for CV and 95% (13 mg/g) for MB under optimal conditions within an equilibrium time of 50 min. The adsorption capacity increased with the ionic strength of the dye solution, reaching 35 mg/g for CV and 28 mg/g for MB. The kinetic study showed that the adsorption of CV and MB is well described by the pseudo-second-order kinetic model (R² = 0.999) and fits the Freundlich model significantly, with an R² = 0.99 for both studied molecules. The thermodynamic analysis (ΔH° = 22.647 and 14.907 kJ.mol⁻¹, ΔS° = 88.627 and 47.330 J.mol⁻¹.K⁻¹ for CV and MB, respectively) revealed that the adsorption process is spontaneous and endothermic, with significant randomness at the adsorbent-adsorbate interface. However, desorption and regeneration tests showed that the efficiency of FAP decreases upon reuse. Despite this, the abundance of natural FAP balances its drawbacks. MD simulations confirmed that adsorption is exothermic and spontaneous, especially in basic conditions, where Van der Waals interactions dominate. These findings suggest that natural FAP has significant potential for dye removal in wastewater treatment applications.

Methods

The effects of various parameters, including dye concentration, temperature, adsorbent mass, and pH, on the adsorption capacity of FAP were studied. Experimental conditions included an initial dye concentration of 20 mg/L, adsorbent mass of 1 g/L, pH of 12, and temperature of 298 K. The Freundlich model was used to describe the adsorption process, while MD simulations provided insights into the adsorption mechanism.