Humin defined

Three different compounds make up HSs Humin (defined as insoluble), humic acid (HA, insoluble at a pH of 1) and fulvic acid (FA, soluble at any pH). Other names used to describe HS are ulmic, h5rmatomelanic, gray humic, brown humic, crenic, apocrenic or 5fellow acid (Swift (1985)),... 

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. 

Operationally, it is common to define HS in terms of the methods used to extract or isolate them from soils, sediments, and natural waters. The classic soil extraction procedure yields three main fractions humic acid [also defined as high-molecular-weight (HMW) or high-molecular-size (HMS) fraction], fulvic acid (FA) [also defined as low-molecular-weight (LMW) or low-molecular-size (LMS) fraction], and humin. These fractions are defined in terms of their solubility in aqueous media as a function of pH or in terms of their extractability from soils or sediments as a function of the pH of the extracting medium. Humic acid is the fraction of HS that is not soluble in water under acidic conditions, but becomes soluble (or extractable) at higher pH values. Fulvic acid is the fraction that is soluble in aqueous media at all pH values. Humin represents the fraction that is not soluble in an aqueous medium (or is not extractable with an aqueous medium) at any pH value. Actually, humin consists of an aggregate of humic and nonhumic materials (Rice and Mac-... 

A number of poorly defined terms have been used for distinguishing various types of colorant that may occur in sugar materials. It appears probable that precise spectrophotometry may eventually help to show whether the distinctions which have been made between the various colorants (such as caramels, humin, humic acids, melanoidins, etc.) are justified from this point of view. 

Humic substances are a broad class of organic compounds operationaUy defined by their solubility at different pHs and retention on hydrophobic resins (Aiken, 1988 Thurman, 1985). There are three operational sub-categories of humic substances humic acids, which are soluble at a higher pH but become insoluble at a pH < 2 (isolated using XAD-8 resin) fulvic acids, which are hydrophilic acids soluble under aU pH conditions (isolated using XAD-4 resin), and humin, which is insoluble at any pH (Ishiwatari, 1992). For a review of humic substances in aquatic systems, see Hessen and Tranvik (1998), Benner (2002), and Chapter 3 by Aluwihare and Meador, this volume. 

Based on detailed analyses of the chemical nature of SOM, Hatcher and Spiker (1988) have extended this humification model to include other resistant biopolymers, including plant cutin and suberin, and microbial melanins and paraffinic macromolecules. During decomposition, these biopolymers are selectively preserved and modified to become part of what can be operationally defined as humin (acid and alkali insoluble component of humus) (Hatcher and Spiker, 1988 Rice, 2001). The humin becomes progressively enriched in acidic groups leading to the formation of first humic acids and then fulvic acids, which under this degradative scheme of SOM formation would be regarded as the most humified of humic substances (Stevenson, 1994). 

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). 

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. 

Humin is commonly defined as the class of sedimentary humic matter that remains insoluble when sediments are treated with dilute alkali to extract the soluble humic and fulvic acids. Because of its insolubility and macromolecu-lar nature, humin has been the least studied of all humic fractions. The classification of humin as a separate class of humic substances was initially proposed at the turn of the century by Oden (1919), and this classification has been in use since then. Because of the many similar analytical characteristics (e.g., elemental compositions, functional group compositions, and infrared spectra) between humin and humic acids, and because of the known association of humin with inorganic clays, Khan (1945) and later Kononova (1966) regarded humin as being no more than a clay-humic acid complex. Consequently, Stevenson (1982) has recently questioned whether humin should be considered a separate class of humic substances. Treatment of humin with HF to destroy clays in many instances renders humin soluble in alkali (Stevenson, 1982). 

Some attempts have been made to define more explicitly the chemical structure of humin of aquatic sediments. Philip et al. (1978) suggest that it... 

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. 

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. 

Other groups include, for example, hymatomelanic acid, the alcohol-soluble fraction (Senesi and Loffredo 1998). Clearly, these fractions are loosely defined and can include many different compounds. Usually, HSs are obtained from the soil through alkaline extraction, which separates FA and HA but not humin, albeit other procedures have also been proposed (Swift 1996). The structure and properties of HSs are considered in Chapter 10. 

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