Humic acids 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. 

Jince the time of Berzelius, chemists have proposed structures for the amorphous, black substance known as humic acid. In the past 150 years, much experimental work has appeared on the nature of humic acid, most of it based on classical chemical and microbiological studies. Very little information about the molecular structure of humic add has resulted from these studies however. Some of the problems plaguing investigators in this field have been (a) variation in the source of humic acid, (b) variation in the definition of humic fractions of soil and coal, (c) lack of crystallinity of the samples, (d) uncertainty of molecular weight measurements, (e) variation in extraction techniques, and (f) variation in elemental composition. The little unambiguous information that exists today is based on extensive degradation of the humic acid polymer and represents only a small fraction of the total molecule. 

EPR analysis of unextracted soil shows definitely that an organic radical is present in approximately die same concentration as in the isolated humic acid. Thus extraction procedures do not create the radical. 

Figure 2 shows the kind of data one can obtain from the apparatus described here. The absorption values from the flow colorimeter are plotted directly vs. time assuming Beer s law, the slope of the locus of this data is the rate of production of humic acids. This rate exhibited a definite induction time. For this experiment, between time zero when the coal was introduced and 10 hours, the rate of production of humic acids increased by a factor of 5. After the induction time of 10 hours the rate of production of humic acids remained constant. 

If it is also recalled that alkali soluble material (humic acid) builds up much more slowly than acidity (and always markedly dependent on T and [O]), and that the distinctly acidic parent coal is effectively insoluble in alkali, it becomes evident that acidity and alkali solubility are not necessarily covariant, and that accepted definitions of humic acid are, chemically speaking entirely arbitrary. Under the conditions of this study the oxidation appears to involve two simultaneous but seemingly unrelated reactions which result in the development of acidity and in molecular (skeletal) breakdown, respectively, and this suggests that alkali solubility is mainly a consequence of degradation which is only coincidentally connected with the formation of acidic functional groups. Figure 20 illustrates this concept qualitatively and leads to the inference that the wide spread in molecular weights of humic acids reported... 

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). 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

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 ... 

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. 

Data in Table 7 show that acetonitrile, dioxane, ethanol, and water are very poor solvents for humic acids. Humic acids are, by definition, insoluble in water, although traces of the H -exchanged substances are invariably dissolved in it. Similar trace amounts were dissolved in ethanol. Pyridine and formic acid (90%) were also poor solvents for humic substances. The extent of dissolution in pyridine was less than would be predicted from the data quoted for the H -exchanged soil in Table 4. However, the Glade humic... 

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-... 

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). 

The effect of concentration of suspended adsorbent on sorptive behavior in a seawater matrix was studied by determining adsorption isotherms for DPM and 94, 150, 200,400, and 1000 ppm humic acid. The corresponding values of Kads (Table III) show no definite trend with respect to Increased adsorbent concentration. This is not totally unexpected in view of O Connor and Connolly s observation that systems with low Kads values do not show dramatic changes of Kads with changes in suspended matter concentrations 9). Only systems with high Kads values typically show definite decreases in Kads with increases in adsorbent concentration. 

Clays are fine-grained, sedimentary rocks originated from the hydrothermal weathering volcanic volcanic ashes in akaline lakes and seas. As such, clays are classified based on then-stratigraphic position, location, and mineral content. Clays contain minerals of definite crystaline structure and elementary composition, some as main components, many as impurities, which usually include orj nic matter in the form of humic acids. Notwithstanding the fundamental difference between clay and day mineral, both terms are sometimes used as indistinctly, esjjecially in the frequent occasions in which the day has a single principal mineral component in this sense, the day is considered as the impure mineral and the mineral as the purified day (Utracki, 2004). 

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. 

---