Degradation of Microcystin-LR in water: Hydrolysis of peptide bonds catalyzed by maghemite under visible light

Abstract

Maghemite, an abundant iron-oxide mineral, was investigated as a visible light catalyst for decomposing and mineralizing Microcystin-LR (MC-LR) in water using hydrogen peroxide (H2O2) as the initial oxidant. Interestingly, of the twelve intermediates identified by LC–MS, half were products of MC-LR peptide bond hydrolysis rather than the oxidation products typical of photocatalytic oxidation (PCO). The catalytic hydrolysis of MC-LR by maghemite and visible light has not been previously reported. Detailed examination of intermediates provided insight into the catalytic pathway. Electron spin resonance (ESR) spectra revealed that visible light irradiation does not increase hydroxyl radical (OH) production from H2O2 decomposition. Visible light appears to play a new role in this unique PCO system, generating Brønsted sites on the maghemite surface via a metal ligand charge transfer (MLCT), rather than producing OH from H2O2. Because opening the cyclic peptide structure detoxifies MC-LR, this catalytic system could be uniquely efficient in detoxifying MC-LR in the presence of relatively high levels of natural organic matter (NOM). This work introduces a new means of selectively and effectively detoxifying MCs in water using an inexpensive, environmentally friendly and bio-compatible mineral.