Addressing the Challenges of Polycyclic Aromatic Hydrocarbons Photodegradation Using Affordable and Easily Synthesized Titania- and Zinc-Based Nanocatalysts

Equimolar (1:1) mixtures of TiO 2 /ZnO, TiO 2 /MoO 3, and ZnO/ZrO 2 were prepared by simple mortar blending; selected samples were further processed by ultrasonic treatment (TiO 2 /MoO 3 UsT) or high-energy milling (TiO 2 /ZnO McT). All composites retained the two parent phases but showed smaller crystallite sizes (11–50 nm) and marginally narrowed band gaps (3.04–3.30 eV). The photocatalytic degradation of a four-component polycyclic aromatic hydrocarbon (PAH) cocktail─acenaphthylene, fluorene, phenanthrene, and fluoranthene (∼2.6 mg/L each)─was evaluated under 254 nm irradiation with varied catalyst dosage, pH, and exposure time. After 120 min, complete mineralization of acenaphthylene and fluorene was achieved with every catalyst; after 180 min, phenanthrene degradation reached 95% with TiO 2 /ZnO McT whereas fluoranthene removal ranged from 60 to 88%. To benchmark the photocatalytic gain, photolysis-only experiments showed at least a 2.5 times lower effect than a photocatalyst showing a monotonic decrease as a function of the enhancement factor with ring size: acenaphthylene < fluorene < phenanthrene < fluoranthene. Our data show that mortar-blended rutile TiO 2 /ZnO and TiO 2 /MoO 3 materials rapidly degrade mixed PAHs, even with competitive adsorption, and their efficiencies rival known anatase photocatalysts tested on single pollutants at near-UV or visible wavelengths. Kinetics fits revealed pseudo-first-order rate constants of up to 43 × 10 –3 min –1 (acenaphthylene, TiO 2 /MoO 3 ) and an apparent second-order regime for fluoranthene on pristine TiO 2 /ZnO, attributed to Langmuir–Hinshelwood surface coverage effects and competitive oxidation. GC–MS snapshots (15 min vs 120 min) identified benzoate and phthalate esters as universal intermediates, with long-chain fatty acid esters appearing only on TiO 2 /ZnO, evidencing deeper β-scission. The combined adsorption, kinetic, and product data show that (i) MoO 3 sites accelerate early attack on two-ring PAHs, (ii) ZnO interfaces maximize electron–hole separation and sustain radical flux for four-ring PAHs, and (iii) postsynthetic ultrasonic or mechanochemical treatment modulates performance by tuning surface area vs recombination centers. Further optimizations with TiO 2 /ZnO McT showed that pH ≈ 6 ensured the highest overall degradation efficiency, with complete removal of three PAHs and 79% fluoranthene conversion. A catalyst dosage of 5 mg was found to be optimal, as both lower and higher loadings decreased activity due to a limited active surface or light scattering, respectively. Reusability tests confirmed stability, with over 90% degradation efficiency maintained after four irradiation cycles. These findings establish mechanically mixed oxide heterojunctions as low-cost, high-efficiency candidates for the UV-assisted remediation of mixed aromatic contaminants.