Humic acid is the generic name of a family of organic compounds that are part of humic substances. They have various functional groups, including carboxylics, phenolics, sugar-like rings, quinones, and amino acid derivatives..
Humic substances, of which humic acids are a part, are distributed in soils, natural waters and sediments, as they are the result of the decomposition of plant, animal and natural residues.
Humic acids are part of humus and have the ability to improve plant growth and nutrition, as they allow nutrients to be retained for longer in the soil so that they are available to plants.
They are amphiphilic compounds, that is, they have parts that are related to water and parts that reject water, all within the same molecule.
Due to their -OH and -COOH groups they can form complexes with metal ions or cations.
Thanks to their hydrocarbon chains or aromatic portions, they can solubilize and lock into themselves polycyclic aromatic molecules that are toxic. In addition, they have potential use in cancer medicine and in the preparation of more effective pharmaceutical products..
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Humic acids contain different functional groups whose amounts depend on the geographical origin of the humic acid, age, environmental and biological conditions and climate in which the molecule was produced. For this reason, its precise characterization has become difficult.
Its main functional groups are phenolic, carboxylic, enolic, quinone, ether, sugars and peptides.
The functional groups that give it its main characteristics are the phenolic, carboxylic and quinone groups..
The large structure of a humic acid is made up of hydrophilic portions made up of -OH groups and hydrophobic portions made up of aliphatic chains and aromatic rings..
- Humic acids.
- HA or HAs Humic acids).
Amorphous solids.
Their molecular weights range from 2.0 to 1300 kDa.
One Da or Dalton is worth 1.66 x 10-24 grams.
Humic acids are the fraction of humic substances that is soluble in alkaline aqueous medium. They are partially soluble in water. Insoluble in acid medium.
Its dissolution in water is complex because humic acids are not an individual component but a mixture of components, where only some of them are soluble in water..
Its solubility can vary depending on its composition, the pH and the ionic strength of the solvent..
Humic acid molecules generally have a hydrophilic or water-like part and a hydrophobic part, which rejects water. That is why it is said that they are amphiphilic.
Due to their amphiphilic character, humic acids form, in a neutral or acidic medium, structures similar to those of micelles, called pseudo-micelles..
They are weak acids, which is caused by phenolic and carboxylic groups.
The quinone-type groups are responsible for the formation of reactive oxygen species, as they are reduced to semiquinones and then to hydroquinones that are very stable.
The presence of phenolic and carboxylic groups in humic acid molecules gives them the ability to improve plant growth and nutrition. Such groups can also favor the formation of complexes with heavy metals. And they also explain its antiviral and anti-inflammatory activity.
On the other hand, the presence of quinone, phenol and carboxylic groups is related to their antioxidant, fungicidal, bactericidal and antimutagenic or demutagenic capacities..
In alkaline medium the carboxylic and phenolic groups suffer the loss of protons H+, what leaves the molecule negatively charged in each of those groups.
Due to this, negative charges are repulsed and the molecule stretches..
As the pH decreases, the phenolic and carboxylic groups are again protonated and the repulsion effects cease, causing the molecule to adopt a compact structure, similar to that of micelles..
In this case the hydrophobic portions are tried to be located inside the molecule and the hydrophilic portions are in contact with the aqueous medium. These structures are said to be pseudo-micelles.
Due to this behavior, it is stated that humic acids have detergency characteristics..
Furthermore, they form intramolecular aggregates (within their own molecule), followed by intermolecular aggregation (between different molecules) and precipitation..
Humic acids can solubilize polycyclic aromatic hydrocarbons which are toxic and carcinogenic and are relatively insoluble in water..
These hydrocarbons are solubilized in the hydrophobic heart of the pseudo-micelles of humic acids..
Humic acids interact with metal ions in an alkaline medium where cations or positive ions act to neutralize the negative charges of the humic acid molecule..
The higher the charge of the cation, the greater its effectiveness in forming pseudo-micelles. The cations are located in thermodynamically preferred places in the structure.
This process generates humic acid-metal complexes that acquire a spherical shape..
This interaction depends on the metal and the origin, molecular weight and concentration of the humic acid..
Humic acids can be obtained from organic matter in the soil. However, the structures of the various humic acid molecules vary according to the location of the soil, its age and climatic conditions..
There are various methods of obtaining. One of these is described below.
The soil is treated with an aqueous solution of 0.5 N NaOH (0.5 equivalents per liter) under a nitrogen atmosphere for 24 hours at room temperature. The whole is filtered.
The alkaline extract is acidified with 2N HCl to pH 2 and allowed to stand for 24 hours at room temperature. The coagulated material (humic acids) is separated from the supernatant by centrifugation.
The use of humic acids in agriculture has been known practically since the beginning of agricultural activity because they are part of humus.
Humic acids improve plant growth and nutrition. They also act as soil bactericides and fungicides, protecting plants. Humic acids inhibit phytopathogenic fungi and some of their functional groups have been correlated with this activity.
The presence of sulfur (S) and C = O carbonyl groups in the humic acid molecule favor fungistatic activity. On the contrary, a high content of oxygen, aromatic C-O groups and certain carbon atoms belonging to sugars inhibit the fungistatic force of humic acid..
Recently (2019), the effect of the application of slow-release nitrogen fertilizers on the stabilization of humic acids present in soils and their influence on crops has been studied..
It was found that the biocarbon-coated urea improves the structure and stability of the humic acids present in the soil, favoring the retention of nitrogen and carbon by the soil and improving the yield of the crops..
Due to its ability to form pseudo-micelles in a neutral or acid medium, its usefulness in removing pollutants from wastewater and soils has been explored for many years.
Metals are one of the pollutants that can be removed by humic acids.
Some studies show that the metal sorption efficiency of humic acids in aqueous solution tends to increase with increasing pH and humic acid concentration and with decreasing metal concentration..
It was also determined that metal ions compete for the active sites of the humic acid molecule, which are generally the phenolic -COOH and -OH groups in this.
Tried to increase the water solubility of hydrophobic drugs.
Silver (Ag) nanoparticles coated with humic acids were prepared, which managed to remain stable for at least one year..
These nanoparticles of silver and humic acid, together with the antibacterial properties of the latter, have a high potential for the preparation of drugs.
Likewise, complexes of carbamazepine, an antiepileptic medicine, with humic acids have been tried to increase its solubility, and it has turned out that the drug becomes much more soluble and effective.
The same effect of improving solubility and bioavailability was achieved with complexes of humic acids and β-carotene, a precursor of vitamin A.
Humic acids are strong allies in the treatment of various diseases.
Antiviral activity of humic acids has been observed against cytomegalovirus and the human immunodeficiency viruses HIV-1 and HIV-2, among others..
Humic acid molecules can inhibit virus replication by binding their negative charge in alkaline medium to certain cationic sites on the virus, which are necessary for the virus to bind to the cell surface..
Humic acids have been found to exert carcinogenic lesion healing properties. This is attributed to the presence of quinones in its structure.
Quinones generate reactive oxygen species which produce oxidative stress and induce apoptosis of cancer cells through the fragmentation of their DNA.
Humic acids are inhibitors of mutagenesis inside and outside the cell. Mutagenesis is the stable alteration of the genetic material of a cell that can be transmitted to daughter cells.
The ability to inhibit mutagenesis has been found to vary with the composition of humic acids and their concentration.
On the other hand, they exert a desmutagenic effect on mutagenic substances such as benzopyrene (polyaromatic hydrocarbon present in some foods), 2-nitrofluorene (polyaromatic hydrocarbon product of combustion) and 2-aminoanthracene.
The mechanism of this effect resides in the adsorption of mutagen, which is why humic acids with larger structures are the most effective. Mutagen is adsorbed by humic acid and loses its mutagenic activity.
This is believed to be important for protection against carcinogenesis..
Due to their ability to absorb UV and visible rays, humic acids have been proposed for use in sun blockers, anti-aging creams, and skin care products..
They can also be used as preservatives in cosmetic products.
Due to its antioxidant properties, its use has been suggested as food preservatives and as nutritional supplements..
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