Fulvic acids molecular weight distribution

As stated above, De Haan and co-workers did not consider photolysis of fulvic acids by UV irradiation as an important determinant of fulvic acid molecular weight distribution pattern or chemical composition. Since cleavage of fulvic acid molecules and subsequent degradation to CO2 and H2O by UV irradiation can occur especially in summer, the observed fluctuations in Tjeukemeer may be attributed to phytolysis as well as degradation by bacteria. This is supported by findings of Strome and Miller (1978) and Gilbert (1980), who demonstrated an enhanced biodegradability of fulvic acids after UV irradiation. 

Beckett, R., Jue, Z. and Giddings, J. C. (1987). Determination of molecular weight distributions of fulvic and humic acids using flow field-flow fractionation, Environ. Sci. Technol., 21, 289-295. 

The humates present in soil are polyelectrolytes and bear some similarity to polyacrylic acid and polymethacrylic acid (49, 50). The molecular weight distribution for the humates is considerable fulvic acid fractions of 1,000 daltons have been isolated (51) while humic acid molecular weights obtained by gel chromatography are in the range 17,000 to 100,000 daltons according to the type of soil from which it was extracted (52). However, ultracentrifugation analysis indicates a molecular range of 2,000 to 1,500,000 daltons for humic acids (55). 

A problem for both humic- and yellow substances is that for these groups of experimentally defined components of different sources, each analysis will be ambiguous in terms of relative composition and molecular weight distribution. Additionally it appears that almost every scientist working in this field has developed his own extraction procedure (Weber and Wilson, 1975 Mantoura and Riley, 1975 a Schnitzer, 1976 Stuermer and Harvey, 1977). Different extraction times and -procedures result in different compositions of the organic constituents (Laane and Kramer, 1984). Soil humic-and fulvic acids, often used for studies on the interaction with trace elements, and those derived from water have certainly not the same composition and contain not the same distribution of functional groups. Therefore, results should be compared with care (Buffle, 1980 Buffle et al., 1984). 

Humic acid is composed of aromatic, aliphatic and carbohydrate carbon compounds. An average humic acid s elemental composition is 55.1% C, 5.0% H, 3.5% N, 35.6% O, and 1.8% S (Rice and MacCarthy, 1991). Its molecular weight distribution is typically broad, and it is a relatively high-molecular-weight material relative to the fulvic acid isolated from the same soil or sediment. It s predominantly functionalized by carboxylic acid and phenolic groups. At least some components of humic acid are surface-active, and these components have been shown to form micelles in concentrated, alkaline aqueous solutions (Piret et al., 1960 Visser, 1964 Wershaw et al., 1969 Tschapek and Wasowski, 1976 Chen et al., 1978 Rochus and Sipos, 1978 Hayano et al., 1982 Hayase and Tsubota, 1984 Guetzloff and Rice, 1994). 

Pershina (Perminova), I. V., Vermool, V. M., Polenova,T. V., and Ivanova, E. K. (1989). Study of molecular weight distribution and spectral parameters of the fulvic acids of natural water origin. I. Gel-permeation chromatography fractionation of fulvic acids. Bulletin of Moscow University [Vestnik MGU], Series 2 (Chemistry) 30,176-182. 

Fig. 8 illustrates the effect of the change in molecular weight distribution of HS in model water. Over 50% of HS2 (Fig. 2) can be taken for fulvic acid. The dose of 25 mg/L was determined by the same procedure used for Fig. 4. Fig. 8 is comparable to Fig. 6 with respect to optimal coagulant dose. The comparison of Fig. 6 with Fig. 8 reveals a lowered ability of separation of coloured substances... 

The seasonal variations observed in the composition of the fulvic acids occurred simultaneously with changes in their concentrations and molecular weight distribution. This suggests that the same processes (mainly hydrology and microbial activity) are involved both in the seasonal changes of the composition concentration and the molecular weight distribution of fulvic acids. 

TABLE 4. Apparent Molecular Weight Distribution of Humic Acids and Fulvic Acids from Lake Sediments as Determined by Sephadex Gel Permeation... 

MacFarlane, R. B. (1978). Molecular weight distribution of humic and fulvic acids of sediments from a north Florida estuary. Geochim. Cosmochim. Acta. 42, 1579-1582. 

Rashid, M. A. and King, L. H. (1969). Molecular weight distribution measurements on humic and fulvic acid fractions from marine clays on the Scotian Shelf. Geochim. Cosmochim. Acta 33, 147-151. 

GOH K.M. and REID M.R. 1975. Molecular weight distribution of soil organic matter as affected by acid pretreatment and fractionation into humic and fulvic acid. Journal of Soil Science, 26, 207-222. 

In spite of this variation in molecular weights and solubilities humic acid and fulvic acid have a very similar chemical composition. These acids consist of aromatic moieties such as phenols, benzenepolycarboxylic acids, hydroxybenzenepolycarbo-xylic acids, 1,2-dihydroxybenzene carboxylic acids, together with more complex condensed structures and polycylic compounds. It is conjectured that these various units are joined together by aliphatic chains (45, 54) the distribution of functional groups is presented in Table 5. 

FIGURE 1 DOM pie diagram based on distribution of DOC in a typical river with a DOC concentration of 5 mg C/L (adapted from Thurman, 1985). Fulvic acids typically constitute the largest percentage of DOC, followed by low-molecular-weight organic acids. 

Cabaniss, S., Zhou, Q., Maurice, P., Chin, Y.-R, and Aiken, G. (2000). A log-normal distribution model for the molecular weight of aquatic fulvic acids. Environ. Sci. Technol. 34, 1103-1109. 

Lakshman S., Mills R., Patterson H., and Cronan C. (1993) Apparent differences in binding site distributions and aluminum(lll) complexation for three molecular weight fractions of a coniferous soil fulvic acid. Anal. Chim. Acta 282, 101-108. 

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