Struvite as a Multiphase Catalyst in Radical-Driven Aqueous Transformations: Evidence from Guaiacyl Acetone Photochemistry

Abstract

Struvite (NH₄MgPO₄·6H₂O), a wastewater-derived mineral proposed as a slow-release fertilizer, may also influence aqueous photochemical transformations of organic matter. This study investigates the role of struvite in radical-driven reactions using a model system containing guaiacyl acetone (GA), a lignin-derived phenolic compound, and 3,4-dimethoxybenzaldehyde (DMB), a triplet-state photosensitizer. Photochemical experiments using Solar Simulator combined with electron paramagnetic resonance (EPR) spectroscopy and high-performance liquid chromatography – photodiode array detector – electrospray ionization – high-resolution mass spectrometry (HPLC-PDA-ESI HRMS) analysis reveal that struvite microcrystals alter both the radical pool and molecular product distributions. In the presence of struvite, the signal of the 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-trapped hydroxyl radical (DMPO–OH) decreases, while signals corresponding to DMPOX and the DMPO dimer (associated with oxidation and coupling pathways) increase, indicating a redistribution of the radical pool driven by heterogeneous surface interactions. Struvite enhances dimeric product formation, demonstrating its ability to promote coupling reactions. UV–vis optical measurements show a transient ∼5-fold increase in solar-flux-weighted mass absorption coefficients for the GA+DMB+struvite mixture during early irradiation stages, driven primarily by increased dimer formation, followed by a decline to values comparable to the struvite-free system after 120 min of irradiation. HPLC-PDA-ESI HRMS molecular analysis confirms enhanced oligomerization and distinct reaction pathways in the presence of struvite, with evidence for surface-mediated radical coupling and potential contributions from ammonium-derived chemistry. These results demonstrate that struvite acts as an active participant in aqueous photochemistry, modifying radical pathways, promoting formation of light-absorbing products, and transiently altering optical properties. The findings suggest that struvite may play a broader role in controlling the chemical evolution and light absorption of organic matter in natural and engineered aquatic systems.