Humic acid

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Humic acid is one of the major components of humic substances^[1] which are dark brown and major constituents of soil organic matter humus that contributes to soil chemical and physical quality and are also precursors of some fossil fuels. They can also be found in peat, coal, many upland streams, dystrophic lakes and ocean water.

Definition

Humic substances are degraded bio-molecules made up a large portion of the dark matter in humus and consist of heterogeneous mixtures of small-size molecules which arise from the biological transformation of dead cells and mutually associate in a supramolecular structure ^[2], that can be separated in their small molecular components by chemical fractionation ^[3]. Humic molecules are held together in supramolecular conformations by weak hydrophobic bonds at neutral and alkaline pH and also by Hydrogen-bonds at low pH.

Since the end of the 19th century, humic substances have been designated as humic acid, fulvic acid or humin. These fractions are defined strictly on their solubility in either acid or alkali, describing the materials by operation only, thus imparting little chemical information about the extracted materials. The term ‘humic substances’ is used in a generic sense to distinguish the naturally occurring material from the chemical extractions named humic acid and fulvic acid, which are defined “operationally” by their solubility in alkali or acid solutions. It is important to note, however, that no sharp divisions exist between humic acids, fulvic acids and humins. They are all part of an extremely heterogeneous supramolecular system and the differences between the subdivisions are due to variations in acidity, degree of hydrophobicity (content of aromatic and long-chain alkyl molecules) and entropy-driven self-associations of molecules. When humic substances are characterized for their molecular structure, a chromatographic and/or chemical separation of their large number of different bioorganic molecules is required ^[4].

Some of the earliest work by Carl Sprengel on the fractionation of organic matter still forms the basis of methods currently in use. These methods utilize dilute sodium hydroxide (2 percent) to separate humus as a colloidal sol from alkali-insoluble plant residues. From this humus sol, the humic fraction is precipitated by acid which leaves a straw-yellow supernatant, the fulvic fraction. The alcohol soluble portion of the humic fraction is generally named ulmic acid. Gray humic acids (GHA) are soluble in low ionic strength alkaline media, brown humic acids (BHA) are soluble in alkaline conditions independent of ionic strength, and fulvic acids (FA) are soluble independent of pH and I.^[5]

Formation of humic substances in the environment

Humic substances are the most stable fraction of organic matter in soils and can persist for up to thousands of years ^[6]. They arise by the microbial degradation of plant (and possibly animal) biomolecules (for example aromatic lignin polymers) dispersed in the environment after the death of living cells. Humic material is a supramolecular structure of relatively small bio-organic molecules (having molecular mass <1000 Da) self-assembled mainly by weak dispersive forces such as Van der Waals force, π-π, and CH-π bonds into only apparently large molecular sizes (Piccolo, 2002). Their dark color is due partially to quinone structures formed at the environmental redox conditions and partially to enhanced light absorption by the strictly associated chromophores (Piccolo, 2002).

Chemical characteristics of humic substances

Recent studies using pyrolysis-FIMS and -GC/MS, multidimensional NMR and synchrotron-based spectroscopy have shown that humic substances possess both aromatic and aliphatic characteristics. The dominant functional groups which contribute to surface charge and reactivity of humic substances are phenolic and carboxylic groups ^[6].

Humic substances may chelate multivalent cations such as Mg²⁺, Ca²⁺, and Fe²⁺. By chelating the ions, they increase the availability of these cations to organisms, including plants.

Determination of humic acids in water samples

The presence of humic acid in water intended for potable or industrial use can have a significant impact on the treatability of that water and the success of chemical disinfection processes. Accurate methods of establishing humic acid concentrations are therefore essential in maintaining water supplies, especially from upland peaty catchments in temperate climates.

As a lot of different bio-organic molecules in very diverse physical associations are mixed together in natural environments, it is cumbersome to measure their exact concentrations in the humic superstructure. For this reason, concentrations of humic acid are traditionally estimated out of concentrations of organic matter (typically from concentrations of total organic carbon (TOC) or dissolved organic carbon DOC).

Extraction procedures are bound to alter some of the chemical linkages present in the soil humic substances (mainly ester bonds in biopolyesters such as cutins and suberins). The humic extracts are composed by large numbers of different bio-organic molecules which have not yet totally separated and identified. However, single classes of residual biomolecules have been identified by selective extractions and chemical fractionation and are represented by alkanoic and hydroxy alkanoic acids, resins, waxes, lignin residues,sugars, and peptides.

Ecological effects

The value of regular additions of organic matter to the soil has been recognized by growers since prehistoric times.^{[citation needed]} However, the chemistry and function of the organic matter have been a subject of controversy since men began their postulating about it in the 18th century. Until the time of Liebig, it was supposed that humus was used directly by plants, but, after Liebig had shown that plant growth depended upon inorganic compounds, many soil scientists held the view that organic matter was useful for fertility only as it was broken down with the release of its constituent nutrient elements into inorganic forms. At the present time soil scientists hold a more holistic view and at least recognize that humus influences soil fertility through its effect on the water-holding capacity of the soil. Also, since plants have been shown to absorb and translocate the complex organic molecules of systemic insecticides, they can no longer discredit the idea that plants may be able to absorb the soluble forms of humus; this may in fact be an essential process for the uptake of otherwise insoluble iron oxides.

Humic acid as a chelator

A substantial fraction of the mass of the humic acids is in carboxylic acid functional groups, which endow these molecules with the ability to chelate (bind) (precipitate in some media, make solution in other media) positively charged multivalent ions (Mg²⁺, Ca²⁺, Fe²⁺, Fe³⁺, most other "trace elements" of value to plants, as well as other ions that have no positive biological role, such as Cd²⁺ and Pb²⁺.) This chelation of ions is probably the most important role of humic acids with respect to living systems. By chelating the ions, they facilitate the uptake of these ions by several mechanisms, one of which is preventing their precipitation, another seems to be a direct and positive influence on their bioavailability.

Ancient Masonry

In Ancient Egypt, according to archaeology as well as the account provided in the book of Exodus chapter 5, straw was mixed with mud in order to produce building bricks. Modern investigations have found that the humic acid is released from straw when the straw is mixed with mud and that this strengthens the material, which produces stronger bricks that are less likely to break or lose their shape.

See also

Leonardite

References

• "Stabilization of organic matter in temperate soils: mechanism and their relevance under different soil conditions - a review", M. von Lutzow, I. Koegel-Knabner, E. Eckschmitt, E. Matzner, G. Guggenberger, B. Marschner and H. Flessa, Eur. J. Soil Sci., 57, 426-445 (2006).
• "Soil mineral-organic matter-microbe interactions: Impact on biogeochemical processes and biodiversity in soils", P.M. Huang, M.K. Wang and C.H. Chiu, Pedobiologia, 49, 609-635 (2005).
• "The Micellar Model of Humic Acid: Evidence from Pyrene Fluorescence Measurements", Ray von Wandruszka, Soil Sci., 163(12), 921-930 (1998).
• "Characterization of humic acid size fractions by SEC and MALS", Ray von Wandruszka, Martin Schimpf, Michael Hill, and Regginal Engebretson, Org. Geochem., (30)4, 229-235 (1999).

1. ^ Description by the International Humic Substances Society
2. ^ Suprahumic research group, at the Università di Napoli Federico II
3. ^ A. Piccolo (2002). "The Supramolecular structure of humic substances. A novel understanding of humus chemistry and implications in soil science". Advances in Agronomy 75: 57–134. doi:10.1016/S0065-2113(02)75003-7. 
4. ^ Fiorentino G., Spaccini R., Piccolo A (2006). "Separation of molecular constituents from a humic acid by solid-phase extraction following a transesterification reaction". Talanta 68 (4): 1135–1142. doi:10.1016/j.talanta.2005.07.037. PMID 18970442. 
5. ^ Baigorri R, Fuentes M, González-Gaitano G, García-Mina JM, Almendros G, González-Vila FJ. (2009). "Complementary Multianalytical Approach To Study the Distinctive Structural Features of the Main Humic Fractions in Solution: Gray Humic Acid, Brown Humic Acid, and Fulvic Acid.". J Agric Food Chem. 57 (8): 3266–72. doi:10.1021/jf8035353. PMID 19281175. 
6. ^ ^{a b} F.J. Stevenson (1994). Humus Chemistry: Genesis, Composition, Reactions. John Wiley & Sons, New York. 

External links

• International Humic Substance Society
• Suprahumic Research Group