Humic acid

Humic acid is one of the major components of humic substances 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 and ocean water.

Humic substances make up a large portion of the dark matter in humus and consist of heterogenous mixtures of transformed biomolecules exhibiting a supramolecular structure (http://www.suprahumic.unina.it), that can be separated in their small molecular components by sequential chemical fractionation (Piccolo, 2002; Fiorentino et al., 2006). Since the end of the 18th century, humic substances have been designated as either 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 no 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 chemical composition, acidity, degree of hydrophobicity and self-associations of molecules. When humic substances are characterized, especially when functionality is studied, there is always the problem that one usually has to separate the huge number of different bioorganic molecules into homogenous fractions.

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Formation of humic substances in the environment

Humic substances arise by the microbial degradation of plant and animal tissues and ultimately biomolecules (lipids, proteins, carbohydrates, lignin) 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). It is well known that humic substances are the most stable fraction of organic matter in soils and can persist for tens, hundreds or even thousands of years (Stevenson, 1994). Their dark color is due to quinone structures formed in the oxidative soil conditions which remain trapped in the humic hydrophobic domains.

Chemical characteristics of humic substances

Recent studies using pyrolysis-FIMS and -GC/MS, multidimensional NMR and synchrotron-based spectroscopy have shown that humic substances posses both aromatic and aliphatic characteristics. The dominant functional groups which contribute to surface charge and reactivity of humic substances are phenolic and carboxylic groups (Stevenson, 1994).

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 is therefore essential in maintaining water supplies, especially from upland peaty catchments in temperate climates.

As a lot of different humic molecules in very diverse physical associations are mixed together in natural environments it is difficult to measure their exact concentrations and allocate them to a certain class of bio-organic molecules. For this reason concentrations of humic acid classes can be 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 bioorganic molecules which have not yet totally separated and identified. However, single classes of biomolecules have been identified in the past by selective extractions and treatments and are represented amino acids, proteins, sugars, fatty acids, resins and waxes.

The International Humic Substances Society (IHSS) has established extraction procedures for humic acid and fulvic acids and provides standard reference materials. The methodology for humic extraction is published by the Soil Science Society of America, Madison, Wisconsin which states that the IHSS method is “a standard method for comparisons between and within laboratories.”

Ecological effects

Ray von Wandruszka at the University of Idaho researched the effects of humic substances in water ecology and stated that, “this group of substances is a major part of the humus in soil and water, i.e. the material that results from the decay of organic material and gives the soil its brown color. Derived from both plant and animal matter, it is widely distributed in natural matrices and has a major influence on their properties. These include the retention of man-made pollutants by soils, and the ability of surface and ground water to transport them. The value of regular additions of organic matter to the soil has been recognized by growers since prehistoric times. 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 most 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.

Over the last 150 years much has been learned about the chemistry of organic matter. Some of the earliest work by 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 sot 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.

Professor Ronald A. Newcomb (SDSU Center for Advanced Water Technologies) researched humates for a patent with his son, Jeremiah Lee Newcomb (Pat. Pend.) on a process for making humates, “various fungi act on lignin in plant residues breaking and recombining the organic compounds into tannins, lignins, ulmic acid, fulvic acid, and so forth. These are the elements in a pond after a heavy leaf fall, and the very reason the algae die during that time.”

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 (Mg++, Ca++, Fe++, most other "trace elements" of value to plants, as well as other ions that have no positive biological role, such as Cd++ and Pb++.) 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.

Humic acid as a detergent

One of the most important properties of HA is its detergent character, i.e. its ability to solubilize such hydrophobic materials. This is a major cause of their dispersal through soil and water, as illustrated by the familiar spread of pollutant plumes from leaking underground storage tanks and other point sources. Plumes consisting of e.g. gasoline or jet fuel would have little tendency to be carried through the dry upper layers of the soil (the vadose zone) by percolating water, were it not for the HA dissolved in it. The behavior of HA in aqueous solution is therefore of considerable interest, especially because it exists in many varieties, depending on age and origin, and that its detergent character is strongly influenced by other substances present in the environment.

We have shown that both mechanisms of HA aggregation are promoted by the presence of positive ions in the solution, and by increased temperature. The former means that HA is a better detergent in the presence of dissolved salts. This is especially true in cases where the salts contain multivalent metal ions such as Mg2+ and Sn3+. Monovalent ions such as Na+ and H+ also have an enhancing effect, but to a somewhat lesser degree. In the case of H+, this implies that lowering the pH boosts the detergent character of HA. In all instances, however, a point is reached where the presence of the ions causes macroscopic HA aggregation, leading to its precipitation from solution.

The nature of HA itself (and there are many different types) also has a profound influence on its detergent qualities. Molecular size and flexibility are important variables in this regard. HAs containing larger and/or more flexible polymers form pseudomicelles more effectively and are better detergents. In keeping with this, we have shown that fulvic acid (HA's smaller "cousin") has little tendency to aggregate and is a relatively poor detergent

References

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• "Effects of pH and metals on the surface tension of aqueous humic materials", L.M. Yates and R. von Wandruszka, Soil Sci. Soc. Amer. J., in press.
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