Removal characteristics of ammonia nitrogen and refractory organic matter in waste leachate by Co2+/peroxymonosulfate process at high chloride ion concentration

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Landfill leachate contains high concentrations of refractory organic matter (ROM), ammonia nitrogen (NH4+), and chlorine ion (Cl–). With efficient ROM removal being the focus of advanced oxidation processes, there is an urgent need to clarify the role of high Cl– concentration on reactive species and preferential reactivity towards coexistent ROM and NH4+. This study investigated the removal characteristics of ROM and NH4+ using a Co2+/peroxymonosulfate (Co2+/PMS) process with high Cl– concentration. The results showed that the system produced large amount of hypochlorous acid (HOCl), with a maximum accumulated HOCl concentration reaching 2.78 mM at Co2+, PMS, and Cl– concentrations of 105 μM, 10 mM, and 17 mM, respectively. The differences in Cl–-to-PMS molar ratios altered HOCl formation pathway, and both Co3+ and SO4•− played important roles in HOCl generation. Although the initial concentrations of PMS, Co2+, and Cl– were proportional to the cumulative HOCl concentration, increasing pH significantly inhibited HOCl formation. The coexisting anions commonly found in waste leachate (i.e., CO32−, HCO3−, and NO3−) promoted HOCl production to some extent, but both NH4+ and dissolved organic matter (humic acid and amino acid complex) severely depleted the formed HOCl. Analysis of the removal characteristics of NH4+ and ROM in co-occurrence in actual waste leachate at high Cl– concentrations revealed that NH4+ and ROM competed for HOCl, and preferential removal of humic-like substances led to retention of fulvic-like substances. These results provide theoretical support for the design and optimization of methods for NH4+ and ROM removal from waste leachate containing high Cl– content.