Fulvic acid defined

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. 

A problem for both humic- and yellow substances is that for these groups of experimentally defined components of different sources, each analysis will be ambiguous in terms of relative composition and molecular weight distribution. Additionally it appears that almost every scientist working in this field has developed his own extraction procedure (Weber and Wilson, 1975 Mantoura and Riley, 1975 a Schnitzer, 1976 Stuermer and Harvey, 1977). Different extraction times and -procedures result in different compositions of the organic constituents (Laane and Kramer, 1984). Soil humic-and fulvic acids, often used for studies on the interaction with trace elements, and those derived from water have certainly not the same composition and contain not the same distribution of functional groups. Therefore, results should be compared with care (Buffle, 1980 Buffle et al., 1984). 

A conceptual DOM pie diagram (Fig. 1) represents the chemical view of the DOM pool of a typical riverine sample. The specific chemical classes or wedges can be separated based upon several different operational fractionation schemes (Thurman and Malcolm, 1981 Aiken, 1985 Aiken et al., 1992 Benner, 1998). For typical natural waters, the dominant fraction comprising DOM is fulvic acid, which is defined as the yellow, moderate molecular weight organic acid fraction of aquatic humic substances that is soluble at all pH values (Aiken et al., 1985). In surface waters receiving... 

We found consistent differences in the fluorescence properties of fulvic acids from streams where fulvic acids are terrestrially derived and from lakes where fulvic acids are microbially derived (McKnight et al., 2001). The upper maximum in microbially derived samples is more sharply defined and... 

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-... 

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). 

Humic acids are those humic substances that are precipitated at pH = 1. Fulvic acids stay in solution at pH = 1 they contain more —COOH and —OH groups than humic acids. Dissolved organic acids, which are not retained by a nonionic XAD resin, are hydrophilic acids, they are not very well defined They contain, in addition to single aliphatic acids, uronic and polyuronic acids. 

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. 

The presence of at least two fluorophores, and possibly a third, associated with metal ion binding in fulvic acid strongly suggests the need for multiple binding site models. Existing linear and nonlinear models will be reviewed for both fluorescence quenching and enhancement. A new modified 1 1 Stem - Volmer model will be introduced as well as two site and multiple site models. Application of the models to Cu binding by fulvic acid and certain well defined model systems are discussed. 

As their name implies, humic and fulvic acids have a significant impact on the pH of natural waters. The graphic approach described earlier for determining the pH of water as a function of Aik and CT has been extended to include DOC effects, as shown in Fig. 2-16. The defining equation is again electroneu-... 

Soil DOM comprises a wide variety of organic substances. Specific microbial or plant exudates are released as a consequence of metabolic activity of soil biota. Transformations of biogenic compounds, through partial degradation and polymerization, result in the formation of a structurally ill-defined mixture of humic substances, a term used to include both humic and fulvic acids (20, 21). Nonuniform effects of DOM components on metal mobilization may be anticipated because of the heterogeneity of their chemical structures and properties. 

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). 

The classic definitions of soil humic and fulvic acids are based on solubility (Schnitzer and Khan, 1972). Thus, humic acid is the alkali-soluble material in soil, which is precipitated at pH 1. The material which remains soluble in the extract at pH 1 is fulvic acid. A more recent definition for aquatic humic substances is given by Thurman and Malcolm (1981). Here the material which adsorbs on an XAD column from an acid aqueous solution is defined as aquatic humus. That part of the adsorbed material which is soluble in acid and base is fulvic acid the portion insoluble in acid is humic acid. Another definition of an aquatic humic substance is based on adsorption by DEAE-cellulose columns (Miles etal., 1983). 

I cability of fluorescence to the direct determination of functionality in humic substances is apparent from those reviews. Figure 14 (from Plechanov et al., 1983) shows the fluorescence spectra of several aquatic humic and fulvic acids. The relatively well-defined structure typical of the spectra of simple molecules such as anthracene (Fig. 15) is absent from the humic acid spectra. Ewald et al. (1983) report corrected fluorescence spectra for humic substances, but these spectra provide no further insight into the functionality of these materials. 

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