Thermally regenerable montmorillonite/sand composite for the adsorptive removal of phenazopyridine from aqueous solutions

The presence of pharmaceuticals in aquatic ecosystems implies the need for efficient and sustainable treatment processes for their removal. This paper reports an innovative adsorption–regeneration process of a heat-resistant montmorillonite/sand composite (Mnt/sand) used to remove phenazopyridine (PHY) from aqueous media. Unlike other adsorption–thermolysis processes, which mostly use fine clay particles, this composite entraps the adsorptive Mnt phase within a fine sand matrix to improve the performance of the adsorbent. A maximum adsorption of 8.98 mg g⁻¹ was achieved using the total weight of the Mnt/sand composite, whereas 44.90 mg g⁻¹ was calculated on the basis of the weight of the active Mnt phase only. Kinetic studies revealed that the adsorption process occurs via pseudo-second-order kinetics (R² = 0.997), suggesting that the rate-limiting step in the process is due to surface interactions. The equilibrium data followed the Langmuir isotherm model, indicating that monolayer adsorption occurred on homogeneous sites, whereas thermodynamic calculations revealed that the process was spontaneous and endothermic (ΔG° < 0, ΔH° > 0). Furthermore, the Mnt/sand composite was tested under continuous flow conditions using a fixed-bed column. In the case of the breakthrough run with a nominal C₀ = 20 mg L⁻¹, Q = 2 mL min⁻¹ and m = 3.0 g, breakthrough was observed at 33.75 min, with a breakthrough capacity of 0.439 mg g⁻¹. The kinetic parameters obtained from the Thomas model are k_Th = 5.14 × 10⁻³ L mg⁻¹ min⁻¹, q_Th = 0.758 mg g⁻¹, and R² = 0.963, representing an efficiency of the bed usage of 57.9% compared with the theoretical saturation capacity. The experiments on thermal regeneration revealed highly efficient restoration of the adsorption capacity of the material at approximately 550 °C, with stable performance throughout multiple cycles (87–90%). From the point of view of computational research, the adsorption of PHY molecules on Mnt is energetically favorable (E_ads = −0.1273 hartree = −334.25 kJ mol⁻¹ = −3.46 eV), accompanied by structural modification of PHY, which could be helpful for the thermal degradation of PHY.