Molecular insights into the transformation of refractory organic matter in landfill leachate nanofiltration concentrates during a flocculation and O/HO treatment.

During leachate treatment, molecular information regarding the completely removed, partially removed, less-reactive, increased, and produced parts of dissolved organic matter (DOM) remains unknown. This study applied ESI FT-ICR MS to investigate the transformation characteristics of leachate nanofiltration concentrate (NFC) DOM during a combined flocculation-O/HO process. The NFC contained 5069 compounds in four main classes (CHO, CHON, CHOS, and CHONS compounds). The DOM number decreased to 4489 during flocculation and to 2903 after the O/HO process. During flocculation, the completely and partially removed DOM was mainly low-oxygen unsaturated and phenolic compounds. Saturated DOM was produced and remained in the flocculated effluent. During the O/HO process, the completely and partially removed DOM were mainly low-oxygen unsaturated and phenolic compounds that were mainly in a reduced state. Flocculation can remove many (condensed) aromatic compounds, and methylation and hydrogenation reactions occurred during flocculation. In the O/HO process, dearomatization, demethylation, carboxylation, and carbonylation reactions further achieved the degradation of DOM that was resistant to flocculation. Overall, the combined flocculation-O/HO process collectively eliminated a broader range of DOM than the single processes could achieve. The results of this study provide an in-depth understanding of DOM transformation in an NFC treatment.