Humin definition

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. 

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). 

Humic acids A group of naturally occurring organic compounds of biological origin that are common in the A horizons of soils and other natural environments. By definition, they are only soluble in water at pH > 2 (Drever, 1997), 113-114 (compare with humin and fulvic acid). 

The terms HAs, FAs, and humin are general terms that refer to broad differences between soil organic components. In order to provide more discrete definitions, careful considerations need to be given to additional fractionation procedures that can give more discrete and distinct fractions. 

Humin has been regarded as the most intractable component of SOM. It must be considered to be a very important component, however, because typically it represents more than 50% of the organic carbon in a soil (Kononova, 1966 Stevenson, 1982,1994) and more than 70% of the organic carbon in unlithified sediments (Durand and Nicaise, 1980 Rice, 2001). The definition of humin (Section 1.3.3) is similar to that of a protokerogen (Calvin and Philip, 1976 Rice, 2001), which is often used in petroleum geochemistry to describe insoluble organic matter in unlithified sediments. 

Researchers have devised numerous extraction and fractionation schemes to deal with the heterogeneous nature of humic substances. Traditionally, the operational definition of humic substances as used by the International Humic Substances Society (Hayes et al., 1989) is based on the solubility in a series of acids and bases. In this scheme, humic substances are classified into three chemical groupings (1) fulvic acid, soluble in both alkali and acid solutions, has the lowest molecular weight and is generally considered the most susceptible to microbial degradation (2) humic acid, soluble in alkali but not in acid, is intermediate in molecular weight and decomposability and (3) humin, insoluble in both alkali and acid solutions, is the most... 

Humic substances are those organic compounds found in the environment that cannot be classified as any other chemical class of compounds (e.g., polysaccharides, proteins, etc.). They are traditionally defined according to their solubilities. Fulvic acids are those organic materials that are soluble in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values (pH < 2) but are soluble at higher pH values. Humin is the fraction of natural organic materials that is insoluble in water at all pH values. These definitions reflect the traditional methods for separating the different fractions from the original mixture. 

Although the definitions of humic acid and of other fractions were initially based on extraction of soil with alkaline reagents, the same terminology is used when the extractants are neutral, acidic, or organic. In these cases the term humin means not extracted, humic acid means soluble in the ex-... 

Humin s insolubility obviates its presence in a dissolved organic matter aquatic sample. In aquatic systems, the operational definitions of humic and fulvic acids are stated somewhat differently [4]. Humic acid and fulvic acid are defined by the aqueous solution pH necessary to desorb them from XAD-8 resin fulvic acid can be desorbed with water humic acid is desorbed with an alkaline aqueous solution. However, the defining operational characteristic, solubility in an aqueous solution as a function of pH, is consistent in both operations. Thus, the use of the term fulvic acid in this paper refers to the humic substance fraction soluble in aqueous solution at any pH value, and the term humic acid refers to the humic substance fraction soluble in alkaline aqueous solution but insoluble under acidic conditions. 

---