Molecular composition of hydroxyl radical-resistant organics in municipal solid waste leachate

Although hydroxyl radicals (^•OH) degrade organic pollutants nonselectively, their mineralization rate during the treatment of waste leachate biological treatment effluent (BTL) using Fenton or Fenton-like systems is not high, and the reason is unknown. In this study, we investigated three typical Fenton-like systems that act on dissolved organic matter (DOM) in BTL. We analyzed the molecular composition of DOM resistant to ^•OH, using ultrahigh resolution mass spectrometry. We find that DOM resistant to ^•OH is more oxidized, less unsaturated/aromatic, has higher molecular weights, and contains more unsaturated oxygen-containing functional groups than does DOM reactive to ^•OH. Resistant-DOM is further categorized into DOM derived by the action of ^•OH (DOM_derived) and DOM initially present (DOM_intrinsic), whose quantities account for approximately 20 % and 80 %, respectively. The DOM_derived is gradually removed under extended reaction time, while DOM_intrinsic is relatively unreactive with ^•OH and is always present in the treated effluent. Based on the molecular composition of resistant-DOM, we propose a method to increase the mineralization rate (up to 95 % TOC removal with only 5 mM persulfate). This study provides direct evidence for the first time that the presence of resistant-DOM (mainly stemming from DOM_intrinsic) in BTL is an important reason for the unideal mineralization rate in the advanced treatment of Fenton or Fenton-like systems.