Reusable heat-treated montmorillonite for sustainable phenazopyridine removal from water via adsorption thermolysis regeneration with mechanistic insight

A sustainable approach for phenazopyridine (Phz) removal from aqueous solutions using adsorption–thermolysis with heat-treated montmorillonite (MOT) was investigated. This approach enables efficient pollutant removal coupled with subsequent regeneration of the exhausted adsorbent through thermolysis. Such a method can be considered an improvement over classical single-use adsorption systems, providing simultaneous removal, regeneration and reusability in a single operation cycle. Rapid and effective Phz removal (up to 95.42% at pH 5) was achieved by employing heat-treated montmorillonite. The equilibrium data were well fitted by both the Freundlich and Langmuir isotherms, with the Freundlich model providing a slightly better fit (R 2 = 0.98314) and the Langmuir model yielding a maximum adsorption capacity q max = 61.53 mg g −1 . The pseudo-second-order kinetic model appeared to be the most suitable, with q e , cal = 29.58 mg g −1 (q e , exp = 27.95 mg g −1 ), (R 2 = 0.99998), and the lowest root mean square error (RMSE) and chi-square (χ 2 ) values among the tested kinetic models. Analysis of the thermodynamics of the adsorption process demonstrated its exothermic nature (ΔH° = − 134.77 kJ mol −1 ). An activation energy of E a = 30.37 kJ mol −1 was consistent with a physisorption-dominated adsorption process. The observed adsorption behavior resulted from a combination of factors such as Phz speciation, MOT surface charge, hydrogen bonding, π–π interactions, van der Waals forces, and electrostatic forces. Thermolysis of the spent MOT was carried out at 700 °C, after which it was reused for adsorption with an efficiency greater than 91%. Gas-emission monitoring was conducted to provide a preliminary assessment of emissions under the investigated thermolysis conditions. Therefore, MOT could be considered an effective and potentially sustainable adsorbent, although further optimization is needed to reduce the energy demand of thermal regeneration.