The conservative mixing behavior of dissolved organic matter: Assessment of size distributions and optical properties

Dissolved organic matter (DOM) is ubiquitous and influences pollutant transport, photochemistry, and water treatment. As a heterogeneous mixture, current theories suggest that DOM forms supramolecular assemblies; yet, common statistical methods assume the principles of additivity and conservative mixing. This study evaluates whether mixtures of DOM sources follow conservative mixing principles as measured by size exclusion chromatography (SEC) coupled with organic carbon, absorbance, and fluorescence detectors. Blends of DOM isolates and wastewater effluent were evaluated using one-, two-, and three-dimensional analysis techniques. Predicted chromatograms based on the linear superposition of end-members showed strong agreement with measured data, suggesting that DOM mixing largely preserves size distributions and photophysical characteristics for SEC. However, a recurring rider peak had systematic residuals that are associated with a small molecular size fraction, commonly described as low molecular weight acids, causing deviations in organic carbon concentration, absorbance <300 nm, and fluorescence emission at 420 nm (excitation 276 nm). These deviations are likely due to column interactions rather than supramolecular reassembly. Discrepancies between whole-water and SEC fluorescence data suggest that fluorophores in wastewater may be preferentially impacted by column interactions. Except for the recurring rider peak, these findings broadly support the use of SEC-derived metrics in end-member mixing analyses and the use of statistical methods that assume additivity. More fundamentally, this study suggests that if DOM supramolecular assemblies are present, they are tightly knit at the blending conditions (i.e., pH 6.8, 3 mg_C L^-1) and SEC ionic strength (i.e., 0.1 M), and DOM does not readily reorganize with new DOM inputs.