Interfacial and molecular characteristics associated with persistent organic matter in mature landfill leachate

Mature landfill leachate remains a long-term management challenge because the humic-rich dissolved organic matter (DOM) can persist through conventional biological and phase-separation processes, yet the interfacial and molecular characteristics associated with this persistence remain insufficiently resolved. Here, a single bulk leachate sample from one landfill source was sequentially fractionated using 0.8, 0.45, and 0.22 μm membranes to obtain operationally defined aqueous permeates (L0, L1, L2, L3) and membrane-retained materials (Z1, Z2, Z3). Water-quality and electrokinetic measurements were based on independently processed aliquots, whereas FT-ICR MS compared pooled composite samples of the final < 0.22 μm permeate (L3) and material retained at the corresponding 0.22 μm filtration step (Z3). EEM-FRI analysis showed consistently humic-dominated fluorescence, with the humic acid-like region accounting for 80.0-84.3 % of corrected regional fluorescence across L0-L3. Negative zeta potentials over the neutral-to-alkaline range supported substantial electrostatic stabilization, while FTIR and semi-quantitative XPS indicated abundant oxygen-containing surface functionality. FT-ICR MS identified 6,623 formulas common to L3 and Z3, representing 77.58 % and 71.87 % of their formula counts and 93.65 % and 90.19 % of cumulative relative ion intensity, respectively (Jaccard similarity = 0.595). These results indicate substantial formula-level compositional continuity without implying molecular identity. Overall, the observations support complementary interfacial and molecular characteristics associated with persistent DOM in the investigated leachate, while their broader relevance requires validation across additional sites and operating conditions.