Abstract
This study was conducted to determine whether the physio-chemical properties of dissolved natural organic matter (DNOM) of similar origin are affected by the dissolved organic carbon (DOC) sample concentration and time in storage. Six reverse-osmosis (RO) isolated DNOM samples were prepared in dilution series (1, 2, 3, 5, 10 mg C/L) at three different pH levels (2, 4.5 and 7), and in triplicate. The UV-VIS- and fluorescent optical properties of the materials were measured after 0-, 2- and 4 months in storage (dark at 4 deg. C), and after 0.45 um filtration at the end of the experiment. Additionally, size-exclusion chromatograms were obtained for some samples after 4 months in storage.
The UV-VIS absorbance of the six DNOM samples were found to conform to Beer's law in a near perfect manner in the range 1-10 mg C/L. A deviation from linearity was found in the fluorescence emission intensity of the samples at increasing DOC concentrations. This asymmetry was interpreted as being partly due to inner-filtering effects and partly due to an increased degree of molecular aggregation in the more concentrated samples. During the storage period the samples underwent significant changes as a result of molecular aggregation and precipitation of the hig-Mw fraction of the DNOM. The loss of this fraction resulted in (i) lower DOC concentrations, (ii) a decreased specific UV absorbance (sUVa), and (iii) an increased specific fluorescence intensity (sFi).
It was concluded that, depending on the instrumentation, it may be necessary to perform both mathematical corrections for inner-filtering effects and normalization with respect to DOC concentrations when comparing the fluorescence intensity of DNOM materials, regardless of origin. Additionally, it was concluded that care should be taken when using the term dissolved NOM based on the operational definition of 0.45 um filtration as the high-Mw fraction is regenerated through molecular aggregation subsequent to filtration. Finally, it was concluded that storage effects of the nature reported herein must be accounted for when conducting experiments with RO-isolated DNOM involving any period of storage.