Humic acids soil/terrestrial

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. 

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). 

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. 

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. 

Humus is a group of organic compounds in terrestrial ecosystems that is not readily decomposed and therefore makes up a carbon reservoir with a long turnover time. There are also significant structural differences between the marine and terrestrial substances (Stuermer and Payne, 1976). The soil organic matter of humus is often separated into three groups similar in structural characteristics but with differing solubility behavior in water solutions. Humic acids, fulvic acids, and humin are... 

Humic substances, which are biopolymers widely and abundantly present in natural waters and soils, also have a high complexing ability with various heavy metal ions. These compounds are formed by the random condensation of breakdown products of terrestrial and aquatic plants and extracellular metabolites of phytoplankton. Concentrations of metals in marine and fresh waters are often higher than predicted from the solubility products of corresponding hydroxide and carbonate compounds. The complexation of metal ions with dissolved humic substances is responsible for the apparent supersaturation of metals in natural waters [9-21], Water-soluble humic substances are usually divided into two fractions, humic acid (HA) and Mvic acid (FA). HA is defined in operational terms as the fraction of humic substance soluble in alkaline solutions and insoluble in acidic solutions, while FA is the fraction soluble in both alkaline and acidic solutions. A general method for the fractionation of humic substances is illustrated in Fig. 1. HA is easily obtained as a precipitate in acidic solution (pH < 1.5). Although HA appears to be an attractive adsorbent for the recovery of heavy metal ions, there is little information on its practical application as adsorbent. It is difficult to use humic acid as the adsorbent because of its high solubility in water. 

FIGURE 10.15 Acid-base titration curves of 14 humic substances, from IHSS standards Terrestrial FA samples are standard Elliot soil, standard Pahokee Peat and reference Pahokee Peat aquatic FA include standard Suwannee River, reference Suwannee River and reference Nordic Lake NOM is Suwannee River aquatic HA are standard Suwannee River and reference Nordic Lake and terrestrial HA include standard Elliot soil, standard Pahokee Peat standard Leonardite, reference Pahokee Peat and reference Summit Hill Soil (Reprinted frotri Geochimica et Cosmochimica Acta, 67, no. 1, Ritchie, J. D and Perdue, E Protonbindins study of standard and reference fulvic acids, humic acids, and natural organic matter 85-93 Copyright 2003, with permission from Elsevier.)... 

The term humus originally meant all organic compounds of plant origin in the soil the term humic acids is used for the brown-black, polymeric, alkali-soluble acids found in soils, plants, sea-grasses, fungi, sediments and terrestrial and marine waters. 

Breakdown of organic matter in an oxidising environment does not lead to the preservation of fossil fuels. In the terrestrial environment, anoxic conditions and increasing temperature lead to the transformation of woody plant materials, cellulose to lignins releasing humic acids and methane. Humic and the related fulvic acids are found in soils, coals and peats, as well as various water bodies throughout the environment. 

Allochthonous DON sources from terrestrial runoff, plant detritus leaching, soil leaching, sediments, and atmospheric deposition may also represent important inputs to estuaries (Berman and Bronk, 2003). DON typically represents about 60 to 69% of the TDN in rivers and estuaries (Berman and Bronk, 2003). The major components of DON include urea, dissolved combined amino acids (DCAA), DFAA, proteins, nucleic acids, amino sugars, and humic substances (Berman and Bronk, 2003). However, less than 20% of DON is chemically characterized. 

Aquatic humic substances may be found in groundwater, river water, lakes, marshes, bogs, swamps, and seawater. The source of the humates may be autochthonous or allochthonous that is, the humates may be formed from phytoplankton in the water or they may be leached into the aquatic environment from terrestrial plants, leaf litter, soil, or subsurface deposits. Relatively undisturbed marine environments have humic and fulvic acids formed almost entirely from native phytoplankton inland surface waters contain major contributions from allochthonous sources. Mixing of the two types of materials may occur, as in the estuaries of rivers. 

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