Hierarchical Porosity Redirects Fe/ZSM-5 from Adsorption-Assisted Removal Toward Fenton/Photo-Fenton Oxidation

Hierarchical porosity and iron speciation govern the balance between adsorption and catalytic oxidation in Fe-modified ZSM-5. This study compares a microporous low-iron material, Z0–Fe0.1 (0.9 wt.% Fe), with a hierarchically porous high-iron material, Z0.4–Fe1.0 (25 wt.% Fe). Structural, textural, optical, X-ray photoelectron, and Mössbauer characterization was combined with methylene blue removal experiments under adsorption, photocatalytic, Fenton-like, and photo-Fenton-like conditions. Both materials retained the MFI structure. UV–Vis analysis indicated a greater relative contribution of oligomeric FexOy species in Z0.4–Fe1.0. Mössbauer spectroscopy identified predominantly Fe3+ environments and revealed a magnetically ordered component in the high-iron material at 78 K. Despite an almost 28-fold difference in iron loading, both materials achieved nearly identical final methylene blue removal (~99%) under photo-Fenton-like conditions, yet this similar efficiency was reached through fundamentally different dominant pathways: predominantly adsorption for the microporous Z0–Fe0.1 (dark adsorption ~82%) and predominantly catalytic oxidation for the hierarchical Z0.4–Fe1.0 (dark adsorption only ~38%, rising to ~87% under dark Fenton-like conditions). These results indicate a shift in the balance between adsorption and H2O2-dependent removal accompanying changes in porosity and iron loading. The findings highlight the need to consider pore architecture, adsorption, and iron speciation jointly when interpreting the performance of Fe-containing zeolites for water treatment.