Visible light-induced catalytic activation of peroxymonosulfate using heterogeneous surface complexes of amino acids on TiO2

Abstract

Peroxymonosulfate (PMS) is being extensively investigated as an eco-friendly oxidant and various activation methods of PMS have been investigated. Here we demonstrated a new method of catalytic PMS activation, which employed amino acids as both a visible light sensitizer and a substrate to be degraded. Although PMS and amino acids do not absorb any visible light, the surface adsorption of amino acids on titania formed charge-transfer complexes that absorb visible light (λ > 420 nm). Serine and histidine were employed as main target amino acids and their surface complexes on TiO2 were characterized by various spectroscopic methods The ligand-to-metal charge transfer between amino acids and TiO2 enabled the absorption of visible light and the subsequent electron transfer catalytically activated PMS with generating sulfate radicals which were detected by electron paramagnetic resonance analysis. Based on various scavenger tests, amino acids seem to be degraded mainly by sulfate radical (radical pathway) and by a non-radical pathway (PMS serving primarily as an electron acceptor) to some extent. Amino acids were degraded with producing ammonium as a sole nitrogenous product in this process, whereas most advanced oxidation processes of amino acid generate not only ammonium but also nitrate and nitrite. The visible light-induced charge transfer characteristics of the amino acid-TiO2 complexes were demonstrated by the photoelectrochemical characterizations and the time-resolved laser spectroscopic analysis.