Tuning iron loading and specific surface area in iron-containing clays for the oxidative photodegradation of 4-nitrophenol

The photocatalytic degradation of the persistent organic pollutant 4-nitrophenol (4-NP) was evaluated using a series of iron-containing clays, including montmorillonite K10 (M-K10), iron-exchanged M-K10 (Fe-MMT), natural mineral bentonite (raw bentonite), and acid-activated bentonite, prepared under various treatment conditions. The highest conversion of 4-NP under UV light and in the presence of H₂O₂ reached 96 %, 94 % and 90 % for M-K10, Fe-MMT and BAN1 (bentonite treated with HNO₃ for 1 h), respectively, after 90 min of reaction. The rate of 4-NP removal followed pseudo-first order kinetics with constants of 3.519 × 10⁻² min⁻¹, 3.325 × 10⁻² min⁻¹ and 2.803 × 10⁻² min⁻¹ for M-K10, Fe-MMT and BAN1, respectively. The high efficiency of the catalysts tested is attributed to the specific surface area and iron content of the clays. Free radical quenching experiments confirmed the involvement of hydroxyl (^.OH) and superoxide anion (^.O₂^-) radicals in the photocatalytic degradation process, indicating the occurrence of heterogeneous photo-Fenton-like reactions. Furthermore, M-K10 exhibited excellent stability and reusability over five consecutive reaction cycles without significant loss in activity. These findings demonstrate the potential of iron-containing clays as efficient, non-toxic, and low-cost catalysts for the photocatalytic degradation of recalcitrant pollutants in sustainable and green wastewater treatment processes.