Analytical challenges in the research of structure and functionality of humic substances from surface water

The study of natural organic carbon structure and functionality faces great analytical challenges due both to the substantial temporal and spatial variations found in nature, and the complexity of the humic matter itself. The most significant features of humic substances are their size and the lack of a well-defined chemical structure. Any study of these compounds can at best provide information on the average and span of properties assigned to this complex mixture. This poses challenges on all analytical levels in humic research, from instrument performance to the ultimate interpretation of the structure and functionality of natural organic carbon. In this thesis, covering four research articles, different analytical challenges in the reseach of humic matter structure and functionality on various levels are presented: • An intercalibration study covering the merits and possible artefacts of the high temperature combustion method for determination of organic carbon compounds, revealed that the merits of these methods when used on marine samples could not be directly adapted to cover samples of terrestrial/aquatic origin. Samples of high concentrations and with refractory materials proved to be the most challenging and were likely to be underestimated. A substantial overconfidence regarding the limit of detection and analytical performance was revealed, likely to be the result of using syntetic and more combustible standard materials for method calibration. • In a critical challenge of the NOM XAD-8 fractionation technique, it was confirmed that this method is of a highly operational and chromatographic nature. The boundary between the hydrophobic and hydrophilic fractions was revealed to be weak and dependent on concentration. The hydrophilic fraction changed gradually to a more aromatic content during the fractionation, and refractionation of hydrophilic and hydrophobic fractions revealed the establishment of a new secondary cut-off between hydrophobic and hydrophilic matter. • In a study on the use of Chitosan and Chitosan/iron as coagulants for the removal of natural organic matter in drinking water, the physio-chemical properties of the residual organic matter after the treatment were studied by XAD-8 fractionation, fluorescence and UV-VIS spectroscopy scans. It was demonstated by combining these methods that the treatment preferentially removed the larger, more aromatic and hydrophobic entities of the organic matter and that the combination of Chitosan and iron proved to be the most efficient treatment with synergistic effects between the coagulants. The XAD-8 fractionation also revealed that the residual organic matter regained some of its previous hydrophobic character under storage. This effect was assigned to a secondary aggregation effect of organic matter commonly observed in fractionation and filtration studies. • In a study covering the effect of liming on natural organic matter structure and mobility, data from two large research projects – the Gjerstad liming experiment and the NOM-typing project – were combined with great success. Through the combination of these data sets, increased charge density as a result of an increase in pH was seen to be the major mechanism behind the increased mobilization of organic matter observed in the stream. This increased mobilisation due to higher pH was also