Molecular level insights into HO• and Cl2•−-Mediated transformation of dissolved organic matter in landfill leachate concentrates during the Fenton process

Abstract

Cl2•− can be the dominant radical species when high salinity wastewater is treated by advanced oxidation processes (AOPs). In this study, the steady-state concentrations of HO•, Cl• and Cl2•− were determined during the Fenton treatment of two landfill leachate concentrates with significantly different Cl− concentrations (XPNF: 1406 mg/L; SNNF: 8036 mg/L). Based on the steady-state concentrations of the radical species and measured second-order rate constants with dissolved organic matters (DOM), HO• accounted for 76.2% of DOM transformation in XPNF, while Cl2•− accounted for 96.5% in SNNF. The DOM transformation dominated by HO• or Cl2•− was then thoroughly compared to investigate the reactivities of HO• and Cl2•− with DOM at molecular level using high-resolution mass spectrometry. The results showed that formulas with low oxidation degree (O/C < 0.5) were preferentially removed by HO• and HO• was more reactive towards compounds with high saturation degree (H/C > 1.5) compared with Cl2•−. In contrast, Cl2•− was more reactive to the compounds with lower saturation level (H/C < 1.5) and the reaction of Cl2•− with DOM was less dependent on DOM oxidation degree. In addition, HO• preferentially reacted with the CHON compounds with low N content, while Cl2•− was more reactive to CHON compounds with higher N content compared with HO•. To our knowledge, this is the first study to distinguish the difference in the reactivity of HO• and Cl2•− towards DOM at molecular level.