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

Cl 2 •− 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 Cl 2 •− 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 Cl 2 •− accounted for 96.5% in SNNF. The DOM transformation dominated by HO • or Cl 2 •− was then thoroughly compared to investigate the reactivities of HO • and Cl 2 •− 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 Cl 2 •− . In contrast, Cl 2 •− was more reactive to the compounds with lower saturation level (H/C < 1.5) and the reaction of Cl 2 •− with DOM was less dependent on DOM oxidation degree. In addition, HO • preferentially reacted with the CHON compounds with low N content, while Cl 2 •− 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 Cl 2 •− towards DOM at molecular level.