This paper investigated ultraviolet A light-emitting diode (UVA-LED) irradiation to activate Fe­(VI) for the degradation of micropollutants (e.g., sulfamethoxazole (SMX), enrofloxacin, and trimethoprim). UVA-LED/Fe­(VI) could significantly promote the degradation of micropollutants, with rates that were 2.6–7.2-fold faster than for Fe­(VI) alone. Comparatively, UVA-LED alone hardly degraded selected micropollutants. The degradation performance was further evaluated in SMX degradation via different wavelengths (365–405 nm), light intensity, and pH. Increased wavelengths led to linearly decreased SMX degradation rates because Fe­(VI) has a lower molar absorption coefficient at higher wavelengths. Higher light intensity caused faster SMX degradation, owing to the enhanced level of reactive species by stronger photolysis of Fe­(VI). Significantly, SMX degradation was gradually suppressed from pH 7.0 to 9.0 due to the changing speciation of Fe­(VI). Scavenging and probing experiments for identifying oxidative species indicated that high-valent iron species (Fe­(V)/Fe­(IV)) were responsible for the enhanced degradation. A kinetic model involving target compound (TC) degradation by Fe­(VI), Fe­(V), and Fe­(IV) was employed to fit the TC degradation kinetics by UVA-LED/Fe­(VI). The fitted results revealed that Fe­(IV) and Fe­(V) primarily contributed to TC degradation in this system. In addition, transformation products of SMX degradation by Fe­(VI) and UVA-LED/Fe­(VI) were identified and the possible pathways included hydroxylation, self-coupling, bond cleavage, and oxidation reactions. Removal of SMX in real water also showed remarkable promotion by UVA-LED/Fe­(VI). Overall, these findings could shed light on the understanding and application of UVA-LED/Fe­(VI) for eliminating micropollutants in water treatments.

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Permanganate [Mn­(VII)] has gained broad attention in water treatment. However, its limited reactivity toward some refractory micropollutants hinders its application for micropollutant degradation. Herein, we introduced UVA-LED photolysis of Mn­(VII) (UVA-LED/Mn­(VII)) to degrade micropollutants (diclofenac (DCF), 4-chlorophenol (4-CP), atrazine, and nitrobenzene) by selecting DCF and 4-CP as target micropollutants. The effects of operating conditions (e.g., light intensity, radiation wavelengths, pH, and water constituents) on DCF and 4-CP degradation as well as the underlying mechanisms were systematically studied. The degradation rates of DCF and 4-CP linearly decreased with increasing radiation wavelengths (from 365 to 405 nm), likely due to the decreased molar absorption coefficients and quantum yields of Mn­(VII). Reactive manganese species (RMnS), including Mn­(V), Mn­(III), and HO•, were generated in the UVA-LED/Mn­(VII) process. Mn­(V) and HO• were responsible for DCF degradation, while Mn­(III), HO•, and likely Mn­(V) accounted for 4-CP degradation. Competitive kinetic results revealed that contributions of RMnS and HO• decreased with increasing radiation wavelengths, wherein RMnS played the dominant role. Increasing pH displayed opposite effects on DCF and 4-CP degradation with higher degradation efficiency obtained at acidic pH for the former one but alkaline pH for the latter one. The presence of water background ions (e.g., Cl–, HCO3 –, and Ca2+) barely influenced DCF and 4-CP degradation. Finally, in comparison with Mn­(VII) alone, enhanced degradation of DCF and 4-CP by UVA-LED/Mn­(VII) was observed in real waters. This work advances the understanding of the photochemistry of manganese species in micropollutant degradation and facilitates Mn­(VII) oxidation in practical application.

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