pH-dependent phenanthrene degradation by Fe(III)-sodium tripolyphosphate-activated H2O2: dominant reactive oxygen species and roles of ligands

Polycyclic aromatic hydrocarbons (PAHs) persist as environmentally recalcitrant contaminants across terrestrial, aquatic, and atmospheric matrices, posing substantial carcinogenic and mutagenic threats to human health. Conventional Fenton systems face limitations in alkaline wastewater treatment due to pH constraints, iron sludge generation, and poor oxidant efficiency. This study demonstrates a sodium tripolyphosphate (STPP)-enhanced Fe(III)/H₂O₂ system achieving effective phenanthrene degradation (95.4 % removal) under mild alkaline conditions (pH 9.0). Mechanistic investigations reveal that STPP coordinates Fe(III) through P=O bonding, forming stable [Fe(H_mP₃O₁₀)₂]ⁿ⁻ complexes that prevent iron precipitation and sustain redox cycling. The optimized Fe(III)/STPP/H₂O₂ molar ratio (1/1/20) delivered an apparent kinetic constant (k_obs) of 2.8 × 10⁻² min⁻¹, representing a 31-fold enhancement over conventional Fenton systems. The system maintained efficacy across pH 7.0–9.0, showing resistance to common anions while exhibiting sensitivity to elevated Ca²⁺/Mg²⁺ concentrations (500 mg/L). Reactive oxygen species (ROS) analysis revealed pH-dependent mechanisms: hydroxyl radicals (^•OH) predominated under acidic conditions, while singlet oxygen (¹O₂) became primary under alkalinity through H₂O₂ → O₂^•−→¹O₂ conversion. Density functional theory calculations confirmed that alkaline OH⁻ promotes H₂O₂ activation to O₂^•−, facilitating ¹O₂ generation. Three degradation pathways involving hydroxylation and ring cleavage produced less toxic intermediates. This STPP-modified Fenton process enables ¹O₂-driven PAH oxidation in alkaline wastewater, overcoming conventional Fenton limitations. The developed technology demonstrates significant potential for sustainable remediation of PAH-contaminated wastewater through ligand-enhanced ROS generation.