Humic materials humin

Many researchers have attempted to unravel the mystery of the structure of humus. One approach has been to isolate fractions by extracting humus using various extraction procedures. These procedures result in the isolation of three or more fractions humic acid, fulvic acid, and humin. Humic material is isolated from soil by treating it with alkali. The insoluble material remaining after this treatment is called humin. The alkali solution is acidified to a pH of 1.0 and the precipitate is called humic acid, while the soluble... 

A fractionation procedure has been established and widely applied to studies of humic materials [42-44]. The procedure begins with natural OM (i.e., humus) and uses an aqueous basic solution (e.g., 0.1-0.5 mol/1 NaOH and Na2C03) to solubilize a fraction of the OM. The basic extract is then acidified which causes a precipitate to form, i.e., humic acids (HA). The fraction, which remains in solution, is called fulvic acids (FA). Humin is the name given to the insoluble organic fraction that remains after extraction of humic and fulvic acids. At nearneutral pH (pH 5 - 8), which is characteristic of most natural water, the FA are the most water soluble of these three fractions. HA are somewhat less soluble, with their solubility increasing as the pH increases. Humin is insoluble at all pH values. 

Humic Materials. Humic materials are divided into three fractions based on their solubility in aqueous solutions as a function of pH humic acid, which is soluble in an alkaline aqueous solution fulvic acid, which is soluble in an aqueous solution regardless of pH and humin, which is insoluble in water at any pH value (and contains the OMN in soil organic matter). The chemical characteristics of humic acid and fulvic acid (e.g., Stevenson, 1994 Orlov, 1985 Rashid, 1985 Aiken et al., 1987 Hayes et al., 1987) and humin (Hatcher et al., 1985 Rice, 2001) are described in numerous reviews. 

Waksman (1936) recommended abandonment of the whole nomenclature of humic acids, beginning with humins and ulmins, through the whole series of humus, hymatomelanic, crenic, apocrenic, and numerous other acids, and ending with the fulvic acid and humal acids . Notwithstanding, terms such as humic acid, humin, fulvic acids, and others have survived and will undoubtedly continue to be used in the future. Most studies on humus chemistry involve preliminary separations on the basis of solubility characteristics, and abandonment of these terms would cause even greater confusion than their continued use. For example, reference to the alkali-soluble, acid-insoluble material as humic acid is considerably less cumbersome than repeated reference to the alkali-soluble, acid-insoluble fraction. ... 

Bourbonniere and Meyers (unpublished) hydrolyzed humic substances from Lake Huron sediments with 5N NaOH at 170°C for 12 hours under a nitrogen atmosphere and found the following organic acids -Ci6 and n-Cig monocarboxylic acids lactic acid, 2-hydroxybutanoic acid, 3,4-dihydroxy-butanoic acid, oxalic acid, and succinic acid. It was proposed that the smaller organic acids were derived from cellulose-related materials. 2-Hy-droxybenzoic acid, 4-hydroxybenzoic acid, 2,5-dihydroxy-3-pentenoic acid, and vanillic acid were also observed. It was believed that 4-hydroxybenzoic acid and vanillic acid originated from lignin and that the ratio of 3,4-dihy-droxybutanoic acid to vanillic acid indicates the proportion of cellulose to lignin. The proportion was in the order of fulvic acid > humic acid > humin. 

In this chapter, we consider humin to be the residue after successive extraction of sediments by benzene/methanol to remove lipids, dilute acid IN HCl), and 0.5N NaOH. In marine sediments, further treatment with concentrated HF/HCl (1 1 v/v) is required to concentrate the organic matter by removal of mineral matter. This treatment will partially or totally hydrolyze polysaccharides and proteins while probably having little effect on the humic material (Hatcher et al., 1983a). 

Humin isolates from sediments of the Mew York Bight and Potomac River estuary have spectra that are notably different in that aromatic carbons are the dominant components. The spectra resemble that of humin isolated in the same manner from an aerobic soil from southern Georgia (Figure 5). However, unlike the humin from soil which shows a significant peak for carboxyl carbon (175 ppm), spectra of humin from the New York Bight and the Potomac River do not display a discreet peak at 175 ppm and appear to be depleted of carboxy 1/amide groups. Elemental data for these humins (19) are consistent with the NMR results. Atomic H/C ratios of less than 0.8 are not typical of humic material but more like those of highly aromatic coal or coal-like products. The NMR spectra also resemble... 

Although this scheme of separation appears to be somewhat crude and arbitrary, it does achieve a degree of segregation of polymeric materials as gauged by their more important chemical properties. Table 2.5 summarizes some of these properties, showing that the fulvic acid-humic acid-humin sequence represents a continuum of... 

Humins - the fraction of humic substances that is not soluble in water at any pH value. On this basis, humins include any humic-type material that is dissolved in non-aqueous solvents after the soil has been exhaustively extracted with basic aqueous solvents. Humins are often considered to consist mainly of humic materials strongly associated with the soil inorganic colloids. 

Humic materials substances consist of a heterogeneous mixture of compounds for which no single structural formula will suffice. In fact, extracted humic materials are different from place to place. Therefore, no unique exact value could be represented the molecular weight of the extracted humics. Some researcher (Stevenson F.J., 1982) proposed that the molecular weight of humics are ranged from 2000 (fulvic acids) to 300000 (humins). Based on this point, a medium value of 150000 was used as the molecular weight in our quenching calculation. 

The term humic substances describes three groups of material fulvic acid, humic acid and humin. This distinction is based on the traditional fractionation of soil humic material. Treatment of bulk humic material with dilute alkali dissolves the fulvic and humic acids,... 

Studies of humic materials employing fractal geometry have generally focused on solution-state samples, and most of these studies have been directed towards humic acid (Table 7.1). Exceptions to this generalization are studies using turbidimetry that characterize aqueous suspensions of humic acid particles [33-38] and the application of fractals to describe the surface morphology of humin [23-25]. 

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. 

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). 

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