Fulvic acids elemental composition

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). 

Soil- and sediment-derived fulvic acid is also composed of aromatic, aliphatic, and carbohydrate carbon components, though it is generally believed to be more aromatic than the humic acid from that same environment. A typical fulvic acid s elemental composition is 46.2% C, 4.9% H, 2.5% N, 45.6% O, and 1.2% S (Rice and MacCarthy, 1991). The carboxyl group is the predominant functional group in... 

Grant, T.D., R.G. Wuilloud, J.C. WuiUoud, et al. 2004. Investigation of the elemental composition and chemical association of several elements in fulvic acids dietary supplements by size-exclusion chromatography UV inductively coupled plasma mass spectrometric. J. Chmmatogr. A 1054 313-319. 

The above scheme provides further fractionation of humic acids allowing the separation of a fraction known as hymatomelanic acid and of fulvic acids in several fractions (B, D, etc.). Humin is commonly further purified of inorganic components. The elemental composition of the fractions obtained with this separation scheme from an uncultivated prairie brown soil (TypicXerochrept) is given in Table 14.1.1. 

Table I. Elemental composition and peak areas for quantitative NMR spectra of fulvic and humic acids.

Elemental composition, functional group analyses, spectral properties, and characterization of acid hydrolysates have shown that peat humic acids tend to be similar to those from mineral soils. NMR spectroscopy has revealed that peat fulvic acids are largely carbohydrate in nature while the residue of alkali extraction is not all humin. 

Elemental composition is one of the most essential characteristics of humic substances. Average elemental composition of humic substances was calculated and results are presented in Table 3. As shown, data for fulvic acid and humin are rare compared to those for humic acid. Interestingly, the average... 

The ratios of humic to fulvic acids in estuarine and coastal sediments range from 0.4 to 3.4, the higher values being associated with areas or sediments having a terrestrial influence (Palacas et al., 1968 Brown et al., 1972 Hue and Durand, 1973 Pelet and Debyser, 1977 MacFarlane, 1978). These values are also consistent with those from other marine and terrestrial environments (Ishiwatari, 1966 Kononova, 1975 Stuermer et al., 1978 Cronin and Morris, 1982). Other parameters measured on coastal humic substances, such as elemental composition, spectral properties, organic components, stable isotope ratios, or C ages (Pelet and Debyser, 1977 Stuermer et al., 1978 Benoit et al., 1979 Nissenbaum, 1979) are consistent with terrestrial or marine humic compounds, or a mixture of these two endmembers. 

TABLE 1. Ash-Free Elemental Composition of a Marine Fulvic Acid and a Soil... 

Humin is commonly defined as the class of sedimentary humic matter that remains insoluble when sediments are treated with dilute alkali to extract the soluble humic and fulvic acids. Because of its insolubility and macromolecu-lar nature, humin has been the least studied of all humic fractions. The classification of humin as a separate class of humic substances was initially proposed at the turn of the century by Oden (1919), and this classification has been in use since then. Because of the many similar analytical characteristics (e.g., elemental compositions, functional group compositions, and infrared spectra) between humin and humic acids, and because of the known association of humin with inorganic clays, Khan (1945) and later Kononova (1966) regarded humin as being no more than a clay-humic acid complex. Consequently, Stevenson (1982) has recently questioned whether humin should be considered a separate class of humic substances. Treatment of humin with HF to destroy clays in many instances renders humin soluble in alkali (Stevenson, 1982). 

Elemental composition is an important chemical property which can be used to establish the nature and source of humic substances. When the percent composition data are displayed as the atom ratios H C, 01C, and NIC, some general characteristics become visible. Soil, coal, marine, and aquatic humates may be distinguished, one from the other. Structural trends may be identified in specific environments, such as lake sediments and soil profiles. Nonhumate contaminants can be detected. Atom ratios may also aid the investigator in proposing hypothetical structures for humic and fulvic acids and serve as a guide in the synthesis of artificial humates. 

Such is the accepted dogma (or methodology) of establishing chemical structures. What does elemental composition contribute to the elucidation of the structures of humic substances Here, one faces a range of analytical values instead of precise percentages for each element. For soil humic and fulvic acids, the most commonly accepted percentages are shown in Table 1 (Schnitzer and Khan, 1978). 

The absurdity of such empirical formulas is illustrated by just two examples the same empirical formula for fulvic acid could also be written for whole wood (a complex mixture) or an oligomer of maleic anhydride, a formula already proposed for humic acid (Anderson and Russell, 1976). The average elemental percentage composition would accommodate both substances. 

Humic and fulvic acids are presumed to arise by two classical natural processes. Terrestrial humates are found in the following pathway plants soil humates peat — coal. Aquatic humates start with soil leachates or marine phytoplankton and go through a sequence sediments kerogen petroleum. There are conditions which mix the two processes as well. As a result, there are a host of names and symbols applied to these compounds, such as peat humic acid, coal fulvic acid, soil humic acid, and so on. Depending on their oxidation state, they may be heavily bound to metal ions. Within each class of humic acid, there are subclassifications, such as Podzol Bj, humic acid, lignite fulvic acid. Other types are classified by geological age, depth in a sediment, and type of aquatic environment. The following discussion will attempt to relate elemental composition to these broad classes of humates. 

The composition of humic substances in lake sediments appears to be a function of depth. Sampling of cores from recent lake sediments in Japan (Koyama, 1966) shows that total carbon increased while total oxygen decreased with depth. Ishiwatari (1975a) isolated humic and fulvic acids from lake sediments and found a similar trend in the elemental composition. He summarized the changes in humic acid composition which occur with increased depth as follows ... 

Leenheer and Malcolm (1973) separated a soil fulvic acid into four fractions by electrophoresis. They found a dramatic change in elemental composition, which correlated with carbohydrate content. Thus, the high polysaccharide fraction had high H/C and 0/C ratios, while the low carbohydrate fractions had low H/C and 0/C ratios. All fractions, however, fitted into the accepted... 

Nevertheless, it is surprising how often one refers to elemental analysis to confirm the presence of a humic or fulvic acid. It is useful in determining whether a brown, macromolecular acidic material is from a coal, soil, marine sediment, or kerogen. It is most useful in characterizing structural trends in a specific environment, such as in sediments or soil profiles. Atomic ratios, especially 0/C ratios, are the simplest way to display elemental composition of humates. They also help one devise hypothetical structures for humates. As a guide in the synthesis of artificial humic substances, they are invaluable. In addition, atomic ratios help the investigator identify nonhumate contaminants. 

Competitive sorption of trace elements to organic soil components has also been studied. Kinniburgh et al. (1996) demonstrated that Cd sorption on a humic acid was reduced by Ca, but in contrast, Cu sorption was poorly reduced. Mandel et al. (2000) showed clear competitive effects of Ca and Mg on Ni sorption to a soil fulvic acid. Many studies have showed evidence that there may be differences in competition between selected trace elements depending on the functional group composition of the humic substances (Kretzschmar and Voegelin, 2001). 

The humic and fulvic acid fractions of a soil, analyzed for elemental composition, are found to contain the following (data from Hayes and Swift, 1978) ... 

Table 4.1 Typical elemental compositional ranges for soil humic and fulvic acids (after Schnitzer 1978)...

Fulvic acids and subsequently humic acids decrease in their concentrations over time as a result of progressive combination reactions with increasing diagenesis. This process of kerogen formation concurrently involves the elimination of small molecules like water, carbon dioxide, ammonia or hydrogen sulfide (Hue and Dmand 1977). As a consequence, the degree of condensation of the macromolecular kerogen increases. In terms of elemental composition it becomes... 

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