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

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.

Automated summary

Cl2'- can be the dominant radical species in the treatment of high salinity wastewater. HO' accounted for the majority of DOM transformation in wastewater with low chlorine concentration, while Cl2'- accounted for a higher percentage in wastewater with high chlorine concentration. The reactivities of HO' and Cl2'- towards DOM were compared, and it was found that HO' preferentially reacted with compounds with low nitrogen content, while Cl2'- was more reactive towards compounds with higher nitrogen content.