Fractionation humic substances

Esteves, V. I., Otero, M., Santos, E. B. H., and Duarte, A. C. (2007). Stable carbon isotope ratios of tandem fractionated humic substances from different water bodies. Org. Geochem. 38, 957-966. 

Determination of the Flory-Huggins interaction parameter (x) for solvent-polymer pairs again requires careful fractionation of the humic macromolecules. In the view of this author, much can be learned about the ways in which humic substances are associated through determination of Flory-Huggins and solubility parameters of carefully fractionated humic substances, and through applications of empirical equations such as those of Flory-Huggins [Equation (6)] and Hildebrand-Scatchard [Equation (7)]. 

Organic solvents have long been used for extraction and sequential extraction, which is fractionation of a sort (Flaig et al., 1975 Schnitzer, 1978). While the direct use of organic solvents in fractionation has not been widespread, nonetheless, the technique has received some attention. For instance, the separation of hymatomelanic acid from precipitated humic acid is obtained by extraction with ethanol (Oden, 1919). Ethanol has been used to bring about fractional precipitation by addition to alkaline solutions of humic acid (Kyuma, 1964 Kumada and Kawamura, 1968). There is no reason why other water-miscible solvents such as acetone and methanol should not be used in this way. Solvents that are highly immiscible with water (e.g., hexane and benzene) do not appear to remove any substantial fraction of humic substances. These are perhaps best used to remove nonhumic substances (such as fats and waxes) prior to extraction. However, recent work by Allen and MacCarthy (personal communication) has shown that more polar water-immiscible solvents, such as methyl isobutyl ketone and diethyl ether, can be used successfully to purify and fractionate humic substances. 

Newly developing multidimensional NMR might become a powerful tool in the future for obtaining valuable information on the chemical structure of humic substances. Two-dimensional phase-sensitive correlation spectra has also been successfully applied for substructure elucidation in size-fractionated humic substances. 

S. Cesco, V. Rdmheld, Z. Varanini, and R. Pinton, Solubilization of iron by a water extractable humic substances fraction. J. Plant Nutr. Soil Sci. 763 285 (2000). 

R. Pinton, S. Cesco, M. De Nobili, S. Santi, and Z. Varanini, Water- and pyrophosphate-extractable humic substances fractions as a source of iron for Fe-dettcient cucumber plants. Biol. Fert. Soils 26 23 (1998). 

G. Concheri, S. Nardi, A. Piccolo, G. Dell Agnola, and N. Rascio, Effects of humic fractions on moiphological changes related to inverta.se and peroxidase activities in wheat seedling roots. Humic Substances in the Global Environment and Implications on Human Health (N. Senesi, and T. M. Miano, eds.), Elsevier. Amsterdam, 1994, p. 257. 

Malcolm RE, Vaughan D (1979) Effects of humic acid fraction on invertase activities in plant tissues. Soil Biol Biochem 11 65-72 Malcolm RL (1989) Application of solid-state 13C NMR spectroscopy to geochemical studies of humic substances. In Hayes MHB, MacCarthy P, Malcolm RL, Swift RS (eds) Humic substances II. In search of structure. Wiley, Chichester, UK, pp 339-372... 

Humic acids (HA) and fulvic acids (FA) are the main components of humic substances (HS), which are the most chemically and biochemically active and widely spread fractions of nonliving natural organic matter in all terrestrial and aquatic environments. They comprise a chemically and physically heterogeneous group of substances with colloidal, polydis-persed, polyelectrolyte characteristics and mixed aliphatic and aromatic nature (Senesi and Loffredo 1999). 

These hydrolytic profiles have been obtained mostly in homogeneous buffered aqueous solutions, but the natural water body contains many kinds of dissolved and suspended matters such as humic substances, metal oxides, and clay particles. The distribution of nonionic pyrethroid molecules in these matters can be explained in terms of a partition model. The association with these matters reduces the fraction of... 

The microorganisms in sewer biofilms are embedded in a matrix of EPS that consists mostly of polysaccharides produced by the bacteria (Characklis and Marshall, 1990). The EPS fraction is the largest organic fraction in the biofilm, i.e., up to about 90% of the total organic content. Only limited studies of the total composition of sewer biofilms in terms of carbohydrates, proteins and humic substances have been undertaken (Figure 3.12). Corresponding information on the composition of anaerobic biofilms in pressure mains does not yet exist. 

Fujitake N, Kusumoto A, Tsukamoto M, Kawahigashi M, Suzuki T, Otsuka H. Properties of soil humic substances in fractions obtained by sequential extraction with pyrophosphate solutions at different pHs I. Yield and particle size distribution. Soil Sci. Plant Nutr. 1998 44 253-260. 

The LFER that results when correlating partitioning in the octanol-water system and the humic substances-water system Implies that the thermodynamics of these two systems are related. Hence, much can be learned about humic substances-water partitioning by first considering partitioning In the simpler octanol-water system. The thermodynamic derivation that follows is based largely on the approach developed by Chlou and coworkers (18-20), Miller et al. (21), and of Karickhoff (J, 22). In the subsequent discussion, we will adopt the pure liquid as the standard state and, therefore, use the Lewls-Randall convention for activity coefficients, l.e., y = 1 if the mole fraction x 1. 

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

The most striking characteristic of the dissolved humic substances is their chromophoric nature. As part of the DOM, they impart a yellow-brown cast to marine and freshwaters and, hence, are part of the CDOM pool. Terrestrial hiunic substances compose a significant fraction of the riverine DOM entering the ocean. In seawater, humic substances compose 5 to 15% of the HMW DOM. Differences exist in the bulk properties of marine and terrestrial humic substances. These are summarized in Table 23.6. They have been used to trace the fate of terrestrial organic matter in the ocean. 

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