Humic acids terrestrial

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). 

Fig. 1.9 Terrestrial humic acid model (a) tetramer open form and (b) trimer trapping an additional monomer (Schulten, 2001)...

More than 100 organochlorines have been identified and structurally characterized in the laboratory chlorination of terrestrial humic acid, although the major products are chloroform and trichloroacetic acid, followed by dichloroacetic acid and chlorinated C-4 dicarboxylic acids (324). In addition, other products that form in the chlorination of both humic acid and the model compound 3,4-dihydroxybenzoic acid are shown in Scheme 3.11. A more recently discovered source of natural organically bound chlorine is peat, reaching to 0.2% of the dry weight, and estimated to have accumulated globally to the extent of 280-1,000 million tons (169). 

NOM is common in sediments, soils, and near ambient (<50 °C) water. The materials result from the partial decomposition of organisms. They contain a wide variety of organic compounds, including carboxylic acids, carbohydrates, phenols, amino acids, and humic substances (Drever, 1997, 107-119 Wang and Mulligan, 2006, 202). Humic substances are especially important in interacting with arsenic. They result from the partial microbial decomposition of aquatic and terrestrial plants. The major components of humic substances are humin, humic acids, and fulvic acids. By definition, humin is insoluble in water. While fulvic acids are water-soluble under all pH conditions, humic acids are only soluble in water at pH >2 (Drever, 1997, 113-114). 

Only 0.05% (about 4X1019 g) of the Earth s carbon is not locked up in sedimentary rocks and only about 9% of that is organic.538 The organic carbon is divided roughly among seawater (45%), soil (40%), and terrestrial plants (15%). Humic substances are traditionally considered to comprise three main fractions humic acids that are soluble in alkali, but insoluble in acid fulvic acids soluble both in alkali and in acid and humin insoluble both in alkali and in acid. 

Sea water contains a much lower concentration of dissolved organic matter than river water. More than half of this dissolved organic load is of a humic nature. These dissolved organic acids tend to flocculate as the salinity increases (10). Hair and Bassett (11) have observed an increase in the particulate humic acid load of an estuary as one approaches the sea. Although no studies of the distribution of humic materials throughout an estuarine system have been performed, it would appear that estuaries and their sediments in particular, act as a major sink for the dissolved and particulate humic materials. Nissenbaum and Kaplan (12) have observed that terrestrial humic materials are not deposited at great distances from shore in the marine system. A study of the flux of particulate carbon through the Chesapeake Bay comes to a similar conclusion (13). 

Oxidation with alkaline CuO gave large amounts of meta-hydroxy derivatives of benzoic acid and benzene dicarboxylic acids (Hayatsu et al., 1980a). This suggests the presence of phenol ethers in the polymer structure. Interestingly, terrestrial polymers such as lignin, humic acid, and coal yield mainly para-rather than meta-hydroxy derivatives by this method. 

In conclusion, toxic chlorinated phenol intermediates formed during the chemical, photochemical and/or enzymatic degradation of chlorophenoxyalkanoic compounds would temporarily be detoxified when they are incorporated into the humic acid, since their bioavailability and movement into terrestrial and aquatic ecosystems would be greatly reduced. However, the knowledge of the potential toxicity problems which these bound-residues could give rise to in the environment is still very limited. Xenobiotic chemicals incorporated into humic polymers are not really removed from the ecosystem and they may maintain their identity and toxic properties for unknown time spans, eventually causing time-delayed pollution problems, if and when they will be released from humic substances. 

Relative to soil humic substances, humic substances from Lake Celyn, Wales, and fulvic acids from lakes near Mt. St. Helens contain larger amounts of reactive acidic functional groups (especially carboxyl groups). The reason for this is not known. In Lake Celyn, 24% of the humic acid carbon is carboxyl and 40% is aromatic, suggesting that the Lake Celyn humic acids are largely of terrestrial origin (M. A. Wilson et al., 1981a). 

FIGURE 7. Acidic functional groups in marine and terrestrial humic acids.. After Hue et al. (1974). 

FIGURE 8. Pyrolysis-gas chromatography of fulvic acids, humic acids, and stable residues from marine sediments containing terrestrial organic input (Mahakam Delta) and planktonic organic input (Black Sea). 

FIGURE 9. Average elemental composition of humic acids and stable residues from marine and terrestrial organic matter, compared to average elemental composition of some biopolymers. 

Elemental analyses of humic acid from samples at increasing burial depths >how a general decrease in the O/C and N/C ratios (Ishiwatari, 1975a Hue und Durand, 1977). As Table 8 shows, this decrease is also seen in terrestrial... 

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. 

H/C ratios are clustered around 1.0 for most soil and aquatic humates and fulvates. Lake and marine sedimentary humic substances have somewhat higher H/C ratios than their soil or water counterparts (Ishiwatari, 1975a). A plot of EJE(, ratios versus H/C ratios shows a direct correlation for terrestrial humic acids (Ertel and Hedges, 1983). The magnitude of the EjEe ratio is inversely proportional to the degree of condensation or the molecular weight (Chen et al., 1977). Ratios above 1.3 indicate that the material may be a nonhumic substance. 

Saito and Hayano (1981) have also interpreted the presence of a band between 3.3 and 4.6 ppm to indicate that there are polysaccharide ether structures in some of their samples. They found that this band was stronger in fulvic acids from marine sediments than the corresponding humic acids. The marine sediment fulvic acids were higher in oxygen than marine sediment humic acids. Aldrich humic, which is presumably terrestrial in origin, has a still lower oxygen content but does not have a band in this region. These data led Saito and Hayano to conclude that their marine sediment fulvic acids have a polysaccharide character. ... 

Hatcher et al. (1981) pointed out that the aliphatic region of terrestrial humic acids is very similar to that of marine humic acids and that the only difference is the presence of aromatic bands in the terrestrial humic acid spectra. In previous work, Hatcher (1980) and Hatcher et al. (1980b) concluded from the H/C ratio of 1.5 and presence of a strong terminal methyl band at 0.9 ppm that marine humic acids have highly branched and cross-linked paraffinic carbon atoms. These structures appear to arise from algal and microbial lipids. The similarity in the aliphatic region in terrestrial humic acids suggests that soil microbial lipids may be the source of the aliphatic structures in terrestrial humic acids. 

Ertel, J. R. and Hedges, J. I. (1983). Bulk chemical and spectroscopic properties of marine and terrestrial humic acids, melanoidins, and catechol-based synthetic polymers. In Aquatic and Terrestrial Humic Materials (R. F. Christman and E. T. Gjessing, eds.). Ann Arbor Science, Ann Arbor, MI, pp. 143-162. 

Leversee, G. J. (1981). Effects of humic acids on bioaccumulation of six polycyclic aromatic hydrocarbons. Presented at the Symposium on Aquatic and Terrestrial Humic Materials, University of North Carolina, Chapel Hill, November 5, 1981. 

As one examines humic acids from sediments where large terrestrial or vascular plant inputs are expected, the CPMAS NMR spectra show higher proportions of aromatic carbons and notable peaks for lignin-like contributions at 55 and 150 ppm. Such distinctions could possibly be used to estimate the relative contribution of vascular plant residues to the sediments. 

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