Fulvic acid 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 inherent chemical complexity of DOM presents many challenges to understanding the role of DOM in C and N cycling and other processes in aquatic ecosystems. The measurement of trace organic moieties in major fractions of DOM, such as fulvic acids, can provide valuable data for understanding sources and biogeochemical pathways. In field studies, multiple lines of evidence can be critical for definitive interpretation of results. The tracer approaches outlined in this chapter should be used in conjunction with mass balance and flux measurements, for example. 

Humic and fulvic acids (HA and FA, respectively) are differentiated on the basis of operational definitions Both fractions include not only the aromatic components but also a variety of plant components. Thus, in reality, HA and FA are extremely heterogeneous fractions. 

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. 

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). 

Obviously, a large number of compounds would seem to fit these two definitions. (Salicylic acid and galacturonic acid, for example, would fit the solubility parameters for soil humic and fulvic acids, respectively.) However, the added restrictions of elemental analysis (see Table 1)—dark color (e.g., absorption spectra) titration data, and molecular weight—would narrow the possible chemical structures. 

The classical definition of fulvic acids is not very specific. Many biochemical substances such as proteins, sugars, and fatty acids would fall under this classification. These substances, in many instances, can hardly be considered "humic" in nature. But, if one uses the classical definition of fulvic acids these substances are included. No doubt the observations made above that polysaccharidelike substances constitute the major components of sedimentary fulvic acids is partly attributable to the fact that the operational definition classes polyuronides as fulvic acids. It is not the intent of this paper to discuss the merits of using the classical operational definition for fulvic acids as opposed to one s perception of what true fulvic acids are. He know far too little about the composition of humic isolates and about their origin to begin discussions of whether they are humified or not. He therefore chose to use operational definition with recognition that well defined structural entities can sometimes be a part of what is isolated. 

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. 

A further division into humic acids (HA) and fulvic acids (FA) can be made. According to the working definition, HAs are only soluble in an alkaline environment, whereas FAs are soluble in both acidic and alkaline media. To differentiate between the two, add HCl to the alkaline eluate in the XAD column until pH 2.0 0.1 is reached. Filter off the precipitated HA... 

Due to the complexities of the substances involved and the preliminary nature of the calculations discussed here, we do not make any definitive claims regarding fulvic or humic acid chemistry. The complexity and range of compositions found in fulvic and humic acids makes definitive conclusions based on a few simulations impossible. The value of this chapter hopefully lies in the hypotheses that we have suggested above and in the methodology developed for further simulations. The key elements of our methodology follow ... 

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