Mechanistic Insights into Humic Acid-Mediated Photosensitized Activation of Mn(VII) under UVA Irradiation

Abstract Ultraviolet A/permanganate [UVA/Mn(VII)] is a promising advanced oxidation process (AOP), while few studies have noticed that humic acid (HA) could enhance microcontaminant degradation in the process. For the first time, the promotive effect of HA and the underlying mechanisms were investigated in UVA/Mn(VII). HA significantly enhanced the generation of hydroxyl radical (HO•) and reactive manganese species (RMnS). Consequently, the pseudo-first-order reaction rate constants for microcontaminant degradation in UVA/Mn(VII)/HA were several to tens of times those in UVA/Mn(VII). The promotive effect of HA was attributed to the enhanced Mn(VII) activation by photogenerated reactive intermediates derived from irradiated HA. Among them, the excited triplet state of HA (3HA*) played a major role, which was also supported by benzophenone (BP), a model triplet state photosensitizer. The second-order reaction rate constants of Mn(VII) with 3HA* and 3BP* were determined to be 2.54(±0.31) × 108 and 2.88(±0.10) × 109 M–1 s–1, respectively. Spectroscopic analysis and theoretical calculations supported an electron transfer mechanism for the reactions of Mn(VII) with 3HA* and 3BP*. Mn(VII) accepted one electron from 3HA*/3BP* and was reduced to Mn(VI), which underwent disproportionation to form Mn(VII) and Mn(V), followed by HO• and other RMnS generation. These findings provide novel perspectives for the application of UVA/Mn(VII) in practical water treatment.