Humic acid characteristics

Bayer, C., Martin-Neto, L., Mielniczuk, J., Saab, S. C., Milori, D. M. B. P, and Bagnato, V. S. (2002a). Tillage and cropping system effects on soil humic acid characteristics as determined by electron spin resonance and fluorescence spectroscopies. Geoderma 105(1-2), 81-92. 

Restricted access phases are another approach to exploiting the differences in characteristics of analytes. Large analytes are excluded from an internal surface on which an adsorptive stationary phase is present. A herbicide analysis for Metsulfuron methyl, Bentazone, Bromoxynil, methylchlorophenoxy acid, and Mecoprop in the presence of humic acid was performed on restricted access reversed phase media.52 The cytostatic compound epirubicin and its metabolites were separated from plasma using a Pinkerton GFF II column.53 Gradient separations of polymers on reversed phase and on normal phase represent an alternative to gel permeation chromatography. Polyesters of noncrystalline materials were separated on a variety of such phases.54... 

Humic acids (HA) and fulvic acids (FA) are the main components of humic substances (HS), which are the most chemically and biochemically active and widely spread fractions of nonliving natural organic matter in all terrestrial and aquatic environments. They comprise a chemically and physically heterogeneous group of substances with colloidal, polydis-persed, polyelectrolyte characteristics and mixed aliphatic and aromatic nature (Senesi and Loffredo 1999). 

A fractionation procedure has been established and widely applied to studies of humic materials [42-44]. The procedure begins with natural OM (i.e., humus) and uses an aqueous basic solution (e.g., 0.1-0.5 mol/1 NaOH and Na2C03) to solubilize a fraction of the OM. The basic extract is then acidified which causes a precipitate to form, i.e., humic acids (HA). The fraction, which remains in solution, is called fulvic acids (FA). Humin is the name given to the insoluble organic fraction that remains after extraction of humic and fulvic acids. At nearneutral pH (pH 5 - 8), which is characteristic of most natural water, the FA are the most water soluble of these three fractions. HA are somewhat less soluble, with their solubility increasing as the pH increases. Humin is insoluble at all pH values. 

Soil humic compounds generally possess higher degrees of total acidity the acidic characteristics arise from the participation of carboxyl as well as phenolic hydroxyl groups... 

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. 

No attempt was made to reduce the ash content of the humic acid samples, since drastic purification methods can cause abnormal changes in humate characteristics, and it was believed that most organic acids in their natural environment would be in salt form and associated with other colloidal matter. Though competition from displaced cations may have contributed to the smaller uptake by humic acid (HA II) in the adsorption stage (as shown in Table 2.1), any residual counterions should have had little effect on the metal-ion extraction step. 

Humic Materials. Humic materials are divided into three fractions based on their solubility in aqueous solutions as a function of pH humic acid, which is soluble in an alkaline aqueous solution fulvic acid, which is soluble in an aqueous solution regardless of pH and humin, which is insoluble in water at any pH value (and contains the OMN in soil organic matter). The chemical characteristics of humic acid and fulvic acid (e.g., Stevenson, 1994 Orlov, 1985 Rashid, 1985 Aiken et al., 1987 Hayes et al., 1987) and humin (Hatcher et al., 1985 Rice, 2001) are described in numerous reviews. 

Figure 3.7. Phenanthrene sorption isotherms on (A) the whole Amherst peat soil humic acid, (B) montmorillonite and a montmorillonite-humic acid complex (5 1 ratio), and (C) kaolin-ite and kaolinite-humic acid complex (5 1 ratio). Insets in parts B and C are the respective isotherms presented on a linear scale. Reprinted from Wang, K., and Xing, B. (2005). Structural and sorption characteristics of adsorbed humic acid on clay minerals. J. Environ. Qual. 34, 342-349, with permission from the Soil Science Society of America.

Chen, Y., Senesi, N., and Schnitzer, M. (1978). Chemical and physical characteristics of humic acid and fulvic acid extracted from soils of the Mediterranean region. Geoderma 26, 87-104. 

Wang, K., and Xing, B. (2005). Structural and sorption characteristics of adsorbed humic acid on clay minerals. J. Environ. Qual. 34, 342-349. 

Campitelli, P. A., Velasco, M. I., and Ceppi, S. B. (2006). Chemical and physicochemical characteristics of humic acids extracted from compost, soil and amended soil. Talanta 69, 1234-1239. 

Gonzalez-Vila, F. J., and Martin, F. (1985). Cehmical structural characteristics of humic acids extracted from composted municipal refuse. Agric. Ecosyst Environ. 14, 267-278. 

Figure 5.3. A humic acid macromolecule interacting with a surface of a clay mineral. The proposed macromolecular structure of the soil humic acid (HA) is based on the following common average characteristics of humic acids MW 6386 Da elemental analysis (%) C, 53.9 N, 5.0 H, 5.8 0,35.1 S, 0.5 C/N, 10.7 NMR information (%) aliphatic C, 18.1 aromatic C, 20.9 carbohydrate C, 23.7 metoxy C, 4.9 carboxylic C, 8.4 ketone C, 4.5 phenolic C, 4.2 functional groups (cmol/g) carboxyl, 376 phenol, 188 total acidity, 564. The structure was created using the ACD/ChemSketch program. [HA-clay complex Chen s group, unpublished (2008). Individual HA molecule Grinhut et al., 2007.]...

Chemical characteristics of humic acids isolated from rhizospheric and bulk soil were studied as a function of the plant genotype grown in the soil (Tartaro et al., 1999). Specifically, it was shown that the presence of a strongly acidifying species, such as rape, and a nodulated species, such as held bean, determined greater differences among rhizospheric and non-rhizospheric humic acids, as compared to wheat plants, with respect to total acidity and aromaticity. 

---