Humic Acids and Humins

Humic substances in sediments and soils have commonly been, defined as heteropolycondensates of decomposing plant and animal detritus 46. For lack of a better structural definition, these macromolecular substances have been divided into three categories fulvic acids and humic acid and humin. Fulvic acids and humic acids are soluble in dilute alkaline solutions, whereas humin is insoluble. 

Fig. 6. Relationship between the log Koc for phenanthrene sorption and the polarity index of humic acids and humin, sequentially extracted from a soil. F-l, F-4, F-7, and F-9 are the first, fourth, seventh, and nineth extracted humic acids, respectively. 0.005, 0.05, and 0.5 ig mL 1 are selected liquid-phase equilbrium concentration of phenathrene (Kang and Xing 2005).

Kang S, Xing B (2005) Phenanthrene sorption to sequentially extracted soil humic acids and humins. Environ Sci Technol 39 130-140... 

The concentrations of HS fractions, i. e., fulvic acids, humic acids, and humin expressed as mol Cl kg of SPH s in terms of various functional groups such as carboxyl, phenolic OH, alcoholic OH, and carbonyl are shown in Fig. 10. 

Organic matter extracted from earth materials usually is fractionated on the basis of solubility characteristics. The fractions commonly obtained include humic acid (soluble in alkaline solution, insoluble in acidic solution), fulvic acid (soluble in aqueous media at any pH), hymatomelamic acid (alcohol-soluble part of humic acid), and humin (insoluble in alkaline solutions). This operational fractionation is based in part on the classical definition by Aiken et al. (1985). It should be noticed, however, that this fractionation of soil organic matter does not lead to a pure compound each named fraction consists of a very complicated, heterogeneous mixture of organic substances. Hayes and Malcom (2001) emphasize that biomolecules, which are not part of humic substances, also may precipitate at a pH of 1 or 2 with the humic acids. Furthermore, the more polar compounds may precipitate with fulvic acids. 

Chiou, C. T., D. E. Kile, D. W. Rutherford, G. Shung, and S. A. Boyd, Sorption of selected organic compounds from water to a peat soil and its humic-acid and humin fractions Potential sources of the sorption nonlinearity , Environ. Sci. Technol., 34,1254-1258 (2000). 

Fulvic acids A group of naturally occurring organic compounds of biological origin that are common in the A horizons of soils and other natural environments. While humic acids are only soluble in water at pH > 2, fulvic acids are water-soluble under all pH conditions (Drever, 1997), 113-114 (compare humic acid and humin). 

Hatcher, P. G. D., VanderHart, L., and Earl, W. L. (1980). Use of solid-state 13C NMR in structural studies of humic acids and humin from Holocene sediments. Org. Geochem. 2, 87-92. 

Guignard, C., Femee, F., and Ambles, A. (2005). Fipid constituents of peat humic acids and humin. Distinction from directly extractable bitumen components using TMAH and TEAAc thermochemolysis. Org. Geochem. 36, 287-297. 

Nearpass, D. C. (1976). Absorption of picloram by humic acids and humin. Soil Sci. 121, 272-277. 

Humic substances represent a large fraction of what is termed chromophoric dissolved organic matter (CDOM) in aquatic systems around the world. Aquatic humic substances can be further categorized as fulvic acids, humic acids, and humin based on theis solubility in acid and base solutions. 

Formation of trihalomethanes. Reactions of chlorine with organic compounds such as fulvic and humic acids and humin produce undesirable by-products. These by-products are known as disinfection by-products, DBFs. Examples of DBFs are chloroform and bromochloromethane these DBFs are suspected carcinogens. Snoeyink and Jenkins (1980) wrote a series of reactions that demonstrate the basic steps by which chloroform may be formed from an acetyl-group containing organic compounds. These reactions are shown in Figure 17.4. 

It is of interest to note that the particle size gap supplies a rational basis to the traditional German classification scheme of defining humic acid and humins on the basis of a particle size separation (filtration). 

Vacuum Pyrolysis. Fukushima (1982) applied vacuum pyrolysis at 500°C to characterize lake humic acid and humin and analyzed organic-solvent-soluble pyrolysates by GC-MS. The results showed that normal alkanes (C -C32) and normal alkenes (C14-C28) were present in the pyrolysates although their amounts were extremely small (0.001% of the initial weight for humic acid and 0.003% for humin). 

Gas chromatography was used to analyze amino acids in 6N HCl hydrolysates of fulvic acid, humic acid, and humin from lake sediments (Lakes Suwa, Nakanuma, Yunoko, Haruna, Shoji, Motosu, and Biwa) (Yamamoto, 1983). Table 8 gives an example of analytical results of amino acids (Lake Haruna). The total amino acids for the seven-lake sediments accounted for 3-16% of humin, 11-21% of humic acid, and 4-24% of fulvic acid. The percentage of amino nitrogen in the total nitrogen in each fraction was 20-44% for humin, 21-36% for humic acid, and 4-30% for fulvic acid. 

In the seven lakes studied by Yamamoto (1983), amino acid distribution of fulvic acid, humic acid, and humin resembled each other. However, after detailed examination of amino acid distribution, the following regularities were found to exist in almost all humic substances studied ... 

Several ideas have been presented in the literature on the formation process of humic substances in marine sediments. Abelson (1967) claimed that polymerization of unsaturated fatty acids in phytoplankters after their death accounts for the formation of kerogen in marine sediments. Abelson and Hare (1971), Hoering (1973), and Hedges (1976) studied reactions between carbohydrates and amino acids under laboratory conditions as a possible formation reaction of humic acid and humin in sediments. They prepared a number of artificial humic acids by reacting glucose with amino acids. The synthetic products resembled natural humic acid and humin. A comprehensive review was published by Abelson (1978). 

The formation reaction of humic substances in lakes proceeds rapidly and is almost completed in decaying phytoplankton and/or in surface sediments. The major characteristics of humic acid and humin in sediments are determined by this formation stage, reflecting primarily organic constituents of phytoplankton. 

The distribution pattern of a,w-dicarboxylic acids for lipids resembled those for humic acid and humin (Fig. 3). This fact clearly indicates the common origin for the polymethylene chains in lipids, humic acid, and humin, which means that phytoplankton-derived lipids actively took part in the formation of humic acid and humin. The relative abundance of polymeth-ylene chains in lipids and humic substances was estimated on the assumption that the yield of production of aliphatic acids from polymethylene chains by alkaline permanganate oxidation was the same for these organic fractions. The following estimations resulted 42% (% of the total amount of polymethylene chains in the sediment) for humin, 38% for lipids, 19% for humic acid, and 1% for fulvic acid. 

The abundance of polymethylene chains in humic acid and humin cannot be explained by the formation of these humic substances by polymerization of fulvic acid which has an extremely small amount of polymethylene chains. It is therefore concluded that, as far as polymethylene chains are concerned, the mechanism of sequential formation of humic substances proposed by Nissenbaum and Kaplan (1972) is not valid for humic acid and humin. 

Ishiwatari and Machihara (1983) estimated roughly the degree of contribution of lipids to humic acid and humin by assuming that all polymethylene chains (C4-C14) in these fractions were derived from sedimentary lipids. Surprisingly, by this calculation 43% of the humic acid carbon and 74% of the humin carbon were derived from lipids. These extremely high values are in conflict with 8 C calculations. Using S C data, the degree of contribution of lipid to humic acid and humin was estimated on the assumption that (1) humic acid and humin were formed from lipids and nonlipid materials and (2) S C of humic acid and humin were the simple sum of 8 C of lipid (-30.56... 

TABLE 9. Humic Acids and Humin Isolated from Lake Biwa Sediments"... 

Ishiwatari (1977) isolated humic acids and humin from samples at various depths (11-130 m) of Lake Biwa sediment. As shown in Table 9, a small amount of humic acid was extracted from sediments of 11 m depth, but no humic acid was obtained from sediments in deeper layers (45-130 m) although alkali extracts were yellow-colored. Humin isolated from sediment samples increased with depth from 6.2% of the total organic matter to 64%, and at 130 m in depth accounted for 80% of the nonbiochemicals. Ishiwatari and Kawamura (1981) again measured approximate amounts of alkali-ex-traetable humie substances in the long sediment core sample of Lake Biwa by colorimetry (at 400 nm). The ratios of alkali-extractable humic substances to the total organic matter decreased gradually with depth, as shown in Table 10. 

The chemical characteristics of humic substances in productive lake sediments are determined primarily by organic composition of aquatic organisms (e.g., phytoplankton) as precursors. Humic substances (humic acid and humin) remain in sediments because they are relatively hydrophobic, causing them to be more rapidly deposited than other hydrophilic organic materials and to be less easily attacked by microorganisms (Vallentyne, 1962). 

The ratio of fulvic acid to humic acid decreases with the burial depth of a sediment (Brown et al., 1972 Ishiwatari, 1975 Hue and Durand, 1974). Some authors (Nissenbaum and Kaplan, 1972) believe this diagenetic decrease is a result of progressive condensation into humic acid and then humin. Others think, however, that the progression from fulvic acid to humic acid and humin is not the only possible mechanism to explain the decrease in fulvic acid content (Ishiwatari, 1975b Jocteur-Monrozier, 1981 Pelet, 1983 Poutanen and Morris, 1983). 

At the present, we are unaware of NMR data in the literature on humin of aerobic soils although such data have been presented for whole soils (Wilson et al., 1981b) and for histosols (Preston and Ripmeester, 1982). We recently collected samples of an aerobic soil from southern Georgia. The sandy soil was essentially forming in a fallow agricultural field and the primary contributors to the soil were various grasses. Samples were collected from the upper 5 cm and from 5-10 cm and were later extracted to isolate humic acids and humin. 

Comparison of this region of the spectrum with that of the respective humic acids shows that the humin contains significantly less carboxyl (peak at 175 ppm). The peaks at 250 and 0 ppm are spinning sidebands of the aromatic carbon peak. In summary, the primary difference between the spectra of humic acids and humin is the presence of a paraffinic carbon peak at 30 ppm in the spectrum of humin. There is little indication that lignin-like structures exist in either spectrum due to the lack of significant peaks at 55 and 150 ppm for methoxyl and phenolic carbons, respectively. 

The problem of foremost interest in future studies should be to ascertain whether operationally defined humin contains significant amounts of clay-humic acid complexes and to determine the genetic relationship between humic acids and humin. Another problem would be to define structurally the paraffinic components of humin and to ascertain whether they are truly biomolecules or are rapidly formed products of decomposition of vegetal matter. Finally, the hypothesis of selective preservation should be tested in other environments. 

Humic acids and humin contain between 2 and 6% nitrogen, while the nitrogen content of fulvic acids ranges from <1 to 3% (Schnitzer, 1976). Nitrogen... 

The most striking features of the amino acid composition of the three humic preparations (Table 6) are (1) the high concentrations of acidic amino acids in the fulvic acids (2) the relatively low concentrations of basic and some neutral amino acids (phenylalanine, tyrosine, leucine, and isoleucine) in the fulvic acids and (3) the accumulation of cysteic acid in the fulvic acids. The distributions of amino acids in the humic acids and humin are quite similar. 

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