NATURAL ORGANIC MATTER INTEREST GROUP

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What is NOM?

Natural organic material (NOM) is found in varying concentrations in all natural water sources. It is a complex mixture of compounds formed as a result of the breakdown of animal and plant material in the environment. The composition of the mixture is strongly dependent on the environmental source, and generalisations regarding chemical character can be prone to misinterpretation as the character of the compounds present in the mixture is extremely diverse. However, if the complexity of the NOM is kept in mind, some broad understanding of the character is possible. NOM consists mainly of carbon, oxygen and hydrogen. Nitrogen and sulphur can also be present, their prevalence will depend on the source of the NOM. A range of compounds, from small hydrophilic acids, proteins and amino acids to larger humic and fulvic acids, are constituents of most NOM. The organic compounds can range from largely aliphatic to highly coloured and aromatic, from apparent molecular weights around 10,000 down to 200, and from highly charged to uncharged compounds. Most characterisation studies indicate that NOM in natural water (unaffected by chemical or physical water treatment processes) on average has a significant charge due to carboxylic acid groups, and some aromatic/hydrophobic character (always with the stipulation that compounds of many types will be present in the mixture).

Most characterisation techniques provide information on average structural and spectroscopic properties, while resin adsorption methods can be used to isolate compounds with similar chemical properties for further characterisation. Figure 1 shows the NMR spectra of a reservoir NOM and an ultrafiltration fraction of the same NOM. The whole water NOM has a high carboxyl, aromatic and aliphatic carbon content, whereas the higher molecular weight fraction of the same NOM displays a relatively low carboxyl, aromatic and aliphatic character, and high O-alkyl character, indicative of carbohydrates. This is a good illustration of the diversity of the compounds present in NOM, and the difficulty of describing the mixture adequately.

Detailed descriptions of types of compounds present in NOM and characterisation techniques can be found in [1-5].

a	b

Figure 1. NMR spectra of a reservoir NOM a)
and a high molecular weight ultrafiltration fraction of the same NOM b).

1. Humic Substances in Soil, Sediment and Water: Geochemistry, Isolation and Characterisation; Aitken, G., McKnight D., Wershaw, R., MacCarthy, P., Eds.; John Wiley and Sons: New York, 1985; 53-85.
2. Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants Suffet I. H. and McCarthy P. Eds.; American Chem. Society, Washington DC.
3. Choudry, G. Humic Substances - Structural, Photophysical, Photochemical and Free Radical Aspects of Interactions with Environmental Chemicals; Gordon and Breach: New York, 1984.
4. Frimmel, F.; Christman, R. Humic Substances and their Role in the Environment; John Wiley and Sons: New York, 1988.
5. Croue J.-P.; Korshin G.; Benjamin M. Characterisation of Natural Organic Matter in Drinking Water; AWWARF Report No. 90780; AWWA Research Foundation: Denver, Co., USA, 2000.