Valorization of Popocatépetl volcanic ash as a sustainable photocatalyst for ciprofloxacin removal

<title>Abstract</title> <p> The widespread occurrence of fluoroquinolone antibiotics in surface waters and wastewater effluents is recognized as one of the environmental drivers of antimicrobial resistance (AMR). Ciprofloxacin (CPF), one of the most prescribed fluoroquinolones, is poorly removed by conventional treatment trains and persists in aquatic systems at concentrations ranging from ng L ^− 1 to µg L ^− 1 . Heterogeneous photocatalysis is among the most studied advanced oxidation processes for CPF removal, but the dominant benchmark commercial TiO ₂ P25 is produced by an energy-intensive route whose cost is a barrier to community-scale water remediation in low- and middle-income regions. In this work, we evaluate Popocatépetl volcanic ash, an abundant and recurrent atmospheric residue in central Mexico, as a sustainable iron-bearing photocatalyst for CPF degradation. The material was used as collected (Ash 298) and after thermal treatment at 623 K and 823 K (Ash 623, Ash 823) and was characterized by X-ray diffraction, Raman spectroscopy, UV–Vis diffuse reflectance and SEM/EDS. The three samples consist of an aluminosilicate matrix (quartz, cristobalite, labradorite, muscovite) hosting 40–44 wt % of iron-bearing phases, with hematite (α-Fe ₂ O ₃ , 14–17 wt %) unambiguously identified and a secondary iron-bearing phase (SIBP, 25–28 wt%) tentatively present. Effective bandgaps of 3.51–3.52 eV match the emission of commercial UV-A lamps (350–365 nm). Under identical conditions (pH 2, [CPF] ₀ = 20 mg L ^− 1 , 1.25 g L ^− 1 catalyst, 180 min), the three ashes removed 53–56% of CPF, with pseudo-first-order constants of 0.0048–0.0052 min ^− 1 , outperforming commercial TiO ₂ P25 (47%, 0.0039 min ^− 1 ). After subtracting photolytic (8%) and dark-adsorption (12–15%) contributions, the net photocatalytic removal is 30–35%. Iron-normalized rate constants are 27–39 times higher for the ash than the corresponding titanium-normalized value for P25, indicating an efficient use of the active sites. The untreated Ash 298 sample shows the highest rate constant, which supports its use without thermal pre-treatment and reinforces the sustainability argument. The results support the valorization of Popocatépetl volcanic ash as a low-cost photocatalyst for antibiotic-contaminated water in regions of active volcanism. </p>