Reusable thermally modified natural clay for phenazopyridine removal via adsorption–thermolysis regeneration

Abstract

A sustainable adsorbent for water contaminated with drugs should be able to remove pollutants and regenerate them with minimum secondary waste. In this study, the effects of thermally and acidically activated natural clay (MC) on the adsorption of phenazopyridine hydrochloride (PHY) were assessed. The preparation of the MC involved heating the clay sample at 550°C and then acid activation. Characterization of MC included analysis via XRD, SEM‒EDS, FT‒IR, TGA, ICP‒OES, and pHpzc. Batch studies revealed the removal of 84% PHY in 45 min under working conditions at pH 3.5 and 25°C. The experimental results revealed that the adsorption equilibrium could be described using the Langmuir model, where q_max was 36.01 mg/g, K_L was 0.255 L/mg, R_L was 0.440, and R² = 0.9827. Adsorption kinetics obeyed the pseudo-second-order model. Thermodynamic parameters reveal that spontaneous and exothermic processes occur during adsorption, where ΔH = -39.49 kJ/mol and E_a = 28.83 kJ/mol. Fixed-bed studies have demonstrated that the pH and flow rate influence PHY removal, with the effect being more pronounced under acidic pH and lower flow rates. A temperature of 550°C was sufficient to restore the MC adsorption efficiency when it was applied for 120 min. FT-IR, TGA, and visible confirmation verified the considerable decomposition/removal of PHY from the adsorbent through thermal degradation, but mineralization could not be confirmed because of the absence of gas product analysis. This study demonstrates the suitability of the designed process for PHY elimination using a regeneration-friendly clay method, in which the use of chemicals and liquid wastes is minimized. Nevertheless, further study of gas product evaluation, energy consumption, and real wastewater application is needed.