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) and DOM initially present (DOM), whose quantities account for approximately 20 % and 80 %, respectively. The DOM is gradually removed under extended reaction time, while DOM 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) in BTL is an important reason for the unideal mineralization rate in the advanced treatment of Fenton or Fenton-like systems.