Effects of chloride ions on the transformation of dissolved organic matter in biologically treated waste leachate by a Fenton process

The Fenton process is widely used in the advanced treatment of municipal wastewater (such as waste leachate) with complex organic compositions because of its high efficiency in removing dissolved organic matter (DOM). Actual wastewater, however, contains high concentrations of Cl−, the effect of which on the Fenton process mechanism is unclear. We investigate the effect of Cl− on the formation mechanism of chlorinated DOM (Cl-DOM) during advanced treatment of biologically treated waste leachate (BTL) by the Fenton process via Fourier transform ion cyclotron resonance mass spectrometry. We found that increased Cl− concentration decreases the effectiveness of DOM mineralization and produces many species of Cl-DOM, both known and unknown. Elevated Cl− decreases the amount of •OH, the reactive oxygen species in the Fenton process, producing radical chlorine species instead. This weakens demethylation, (de)carboxylation, and (de)hydroxylation pathways for DOM degradation while enhancing chlorine substitution and addition reactions, which generate Cl-DOM. Increasing the amount of H2O2 to provide more available •OH increases the vigor of the degradation reactions, but Cl-DOM is still primarily transformed from macromolecules to smaller ones. This study provides theoretical support for the prevention and control of Cl-DOM in Fenton processes on complex DOM in BTL with high concentrations of Cl−.