Molecular-level transformation characteristics of refractory organics in landfill leachate during ozonation treatment

Macromolecular refractory organics in landfill leachate are extremely complex compounds. This study examined the molecular-level transformation characteristics of refractory organics in biologically-treated landfill leachate (i.e., membrane bioreactor (MBR) leachate) during ozonation treatment. Results indicated that higher ozone dosage and longer reaction time enhanced organics removal. When ozone dosage was 32.16 mg/min and reaction time was 30 min, the efficiencies of removing color number, chemical oxygen demand (COD), and dissolved organic carbon (DOC) were 95.16%, 51.13%, and 26.40%, respectively. Furthermore COD / DOC decreased from 3.38 to 2.24, and the content of aromatic substances and macromolecular humic substances (e.g., humic- and fulvic-like substances) substantially decreased. The MBR-treated leachate mainly contained phenolic compounds (82%, aromatic index ≤ 0.50 and H/C < 1.5), and the major elements within the dissolved organic matter in the MBR-treated leachate were C, H, O, N, and S. The CHOS and CHONS compounds in the leachate indicated that it would have a much greater biorefractory property than the natural organic matter (i.e., technical grade humic acid). After the MBR-treated leachate was treated by ozonation for 10 min, the CHO, CHON, CHOS and CHONS compounds were greatly degraded and removed, and the oxidation degree of dissolved organic matter was significantly increased owing to the strong oxidation ability of ozone. At 30 min of ozonation, CHON, CHOS and CHONS compounds were further degraded, and CHOS and CHONS compounds (as the biorefractory substances) were almost completely removed. It was noteworthy that some CHO compounds that mainly contained phenolic compounds (m/z = 250-300, carbon number > 20, and double bond equivalent < 6) with a higher bioavailability and higher saturation degree accumulated. This study provides beneficial references for practical application of landfill leachate treatment using the ozonation process.