Treatment of Municipal Landfill Leachate by Electrochemical Oxidation: Assessing Operating Variables and Oxidation Products

In this study, the electrochemical oxidation treatment of landfill leachate on a boron-doped diamond (BDD) electrode was examined. A factorial design method was applied to evaluate the experimental variables: treatment time, initial pH, current intensity, and chloride concentration. The processes of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen removal, and nitrate (NO3--N) formation were all assessed. Results demonstrated that the interaction between pH value and chloride concentration had a significant effect on COD removal. NH4+-N was mainly affected by the interaction between pH value and chloride concentration. In addition, the products of NH4+-N oxidation were investigated under varying operating conditions. Results showed that nitrogen (N-2) was the main transformation product of NH4+-N and was strongly affected by the current density. Specifically, when the current density increased from 50 to 100 mA/cm(2), the proportion of N-2 increased from 52.2% to 82.06% after 4 h of electrolysis. The alkaline condition was not conducive to the accumulation of NO3--N, while the addition of chloride ion contributed to an increase in both NO3--N and nitrite (NO2--N) production. After 4 h, the percentage of the formed NO3--N increased from 14.49% to 20.72%, for chloride concentrations of 2,498 and 7,000 mg/L, respectively. At the same time, the NO2--N percentage increased from 0.19% to 1.74%. Furthermore, fluorescence and gas chromatography-mass spectrometry analysis showed that the toxic and biorefractory substance levels decreased following electrochemical oxidation treatment. This work is able to provide valuable information and sound evidence for the application of electrochemical oxidation technology in landfill leachate, and thus it has a practical significance in environmental pollution control.