The formation of chlorinated organics during the removal of refractory organic matter in landfill leachate by the UV/H2O2 and UV/PDS processes

Landfill leachate contains high levels of refractory organic compounds that pose significant environmental risks. The treatment of landfill leachate through advanced oxidation processes (AOPs) is an effective method for removing these compounds. However, the presence of chloride ions (Cl − ) might lead to the formation of chlorinated organics. In this study, we compared the formation of chlorinated organics during the removal of refractory organic compounds from landfill leachate treated by a membrane bioreactor (MBR) through UV/H 2 O 2 and UV/PDS processes with different oxidant dosages. The relationship between the formation of chlorinated organics and the concentration of organics was explained via ultraviolet visible spectra, three-dimensional excitation and emission matrix spectra, and total organic carbon. The results revealed that the concentration of chlorinated organics increased with increasing oxidant dosage. Under the oxidant dosage of 40 mM and a reaction time of 90 min, the cumulative concentration of the 4 chlorinated organics in the UV/H 2 O 2 process reached 482.8 μg/L, and those of the 8 chlorinated organics reached 34,370.2 μg/L in the UV/PDS process. Concentrations of chloroacetaldehyde, chloroacetic acid and trichloroacetic acid produced in the UV/H 2 O 2 process and concentrations of chloroacetaldehyde, dichloroacetaldehyde and dichloroacetonitrile produced in the UV/PDS process were significantly and positively correlated with the concentrations of aromatic organic compounds. In the UV/H 2 O 2 and UV/PDS processes, the concentration of Cl − affects the conversion of reactive chlorine species, such as Cl • and Cl 2 •– , and the reactive species, such as HO • , SO 4 •– , Cl • and Cl 2 •– generated in the process react with organic compounds to form intermediates through hydrogen extraction, addition and electron transfer processes that gradually open ring structures to form small molecules. Cl • and Cl 2 •– further react with these small-molecule intermediates to form chlorinated organics through chlorine substitution. The toxicity evaluation of chlorinated organics revealed that haloacetaldehydes were the main source of chlorinated organic toxicity in both the UV/H 2 O 2 and UV/PDS processes. This study provides theoretical support for reducing the formation of chlorinated organics during refractory organic compound removal processes from landfill leachate via advanced oxidation processes such as UV/AOPs.