Humic substance
Humic substances are relatively recalcitrant colored organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown leading to black. The term comes from humus, which in turn comes from the Latin word humus, meaning "soil, earth". Humic substances constitute the majority of organic matter in soil, peat, coal, and sediments, and are important components of dissolved natural organic matter in lakes, rivers, and sea water. Humic substances account for 50 – 90% of cation exchange capacity in soils.
"Humic substances" is an umbrella term covering humic acid, fulvic acid, and humin, which differ in solubility. By definition, humic acid is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. Fulvic acid is soluble in water at any pH. Humin is not soluble in water at any pH.
This definition of humic substances is largely operational. It is rooted in the history of soil science and, more precisely, in the tradition of alkaline extraction, which dates back to 1786, when Franz Karl Achard treated peat with a solution of potassium hydroxide and, after subsequent addition of an acid, obtained an amorphous dark precipitate. Aquatic humic substances were isolated for the first time in 1806, from spring water by Jöns Jakob Berzelius.
In terms of chemistry, FA, HA, and humin share more similarities than differences and represent a continuum of humic molecules. All of them are constructed from similar aromatic, polyaromatic, aliphatic, and carbohydrate units and contain the same functional groups, albeit in varying proportions.
Water solubility of humic substances is primarily governed by the interplay of two factors: the amount of ionizable functional groups and molecular weight. In general, fulvic acid has a higher amount of carboxylic groups and lower average molecular weight than does humic acid. Measured average molecular weights vary with source; however, molecular weight distributions of HA and FA overlap significantly.
Age and origin of the source material determine the chemical structure of humic substances. In general, humic substances derived from soil and peat have higher molecular weight, higher amounts of O and N, more carbohydrate units, and fewer polyaromatic units than humic substances derived from coal and leonardite.
HS can be isolated by the adsorption onto a resin of an alkaline extraction from solid sources of NOM. A newer view of humic substances is that they are not mostly high-molecular-weight macropolymers. Rather, they represent a heterogeneous mixture of relatively small molecular components of the soil organic matter auto-assembled in supramolecular associations and are composed of a variety of compounds of biological origin and synthesized by abiotic and biotic reactions in soil and surface waters. It is the large molecular complexity of the soil humeome that confers to humic matter its bioactivity in, its stability in ecosystems, soil and its role as plant growth promoter.
The academic definition of humic substances is under debate. Some researchers argue against the traditional concept of humification, proposing that alkali extraction does not provide a fair view of HS due to the use of highly alkaline extracts instead of water.
Concepts of humic substances
The formation of HS in nature is one of the least understood aspects of humus chemistry and one of the most intriguing. Historically, there have been three main theories to explain it: the lignin theory of Waksman, the polyphenol theory, and the sugar-amine condensation theory of Maillard. Humic substances are formed by the microbial degradation of dead biotic matter, such as lignin, cellulose, ligno-cellulose and charcoal. Humic substances in the lab are resistant to further biodegradation. Their structure, elemental composition and content of functional groups of a given sample depend on the water or soil source and the specific procedures and conditions of extraction. Nevertheless, the average properties of lab extractes HS from different sources are remarkably similar.Fractionation
Historically, scientists have used variations of similar methods for extracting HS from NOM and separating the extracts into HA and FA. The International Humic Substances Society advocates using standard laboratory methods to prepare humic and fulvic acids. Humic substances are extracted from soil and other solid sources using 0.1 M NaOH, under a nitrogen atmosphere, to prevent abiotic oxidation of some of the components of HS. The HA is then precipitated at pH 1. The soluble fraction is treated on a resin column to separate fulvic acid components from other acid soluble compounds. The fraction of NOM not extracted by 0.1 NaOH is humin. Humic and fulvic acid are extracted from natural waters using a resin column after microfiltration and acidification to pH 2. The humic materials are eluted from the column with NaOH, and humic acid is precipitated at pH 1. After adjusting the pH to 2, fulvic acid is separated from other acid soluble compounds, using a resin column as with solid phase sources. An analytical method for quantifying humic acid and fulvic acid in commercial ores and humic products, has been developed based on the IHSS humic acid and fulvic acid preparation methods.Scientists associated with the IHSS have also isolated the entire NOM from blackwater rivers using reverse osmosis. The retentate from this process contains both humic and fulvic acids, predominately humic acid. The NOM from hard water streams has been isolated using reverse osmosis and electrodialysis in tandem.
Extracted humic acid is not a single acid; instead, it is a complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Commercial humic acid used to amend soil is manufactured using these well-established procedures. Humic acids can form complexes with ions that are commonly found in the environment creating humic colloids.
A sequential chemical fractionation can isolate more homogeneous humic fractions and determine their molecular structures by advanced spectroscopic and chromatographic methods. Substances identified in humic extracts and directly in soil include mono-, di-, and tri-hydroxycarboxylic acids, fatty acids, dicarboxylic acids, linear alcohols, phenolic acids, terpenoids, carbohydrates, and amino acids. This suggests humic molecules may form a supramolecular structures held together by non-covalent forces, such as van der Waals force, π-π, and CH-π bonds.
Chemical characteristics
Since the dawn of modern chemistry, humic substances are among the most studied among natural materials. Despite long study, their molecular structure remains debatable. The traditional view has been that humic substances are hetero- poly-condensates, in varying associations with clay. A more recent view is that relatively small molecules also play major a role.A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together; The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups. Humic substances commonly behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups in HA. Fulvic acids are more acidic than HA. There is considerable overall similarity among individual humic acids. For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density.File:Humic acid.svg|left|thumb|upright=1.4|Stevenson model structure of a humic acid, with a variety of components including quinone, phenol, catechol, and sugar moieties
The more recent determinations of molecular weights of HS show that the molecular weights are not as great as once thought. Reported number average molecular weights of soil HA are < 6000 but they are highly poly disperse with some components with much larger measure molecular weights and much lower. Measured number average molecular weights of aquatic HS with HA ≤ 1700 and FA < 900. The aquatic HA and FA are also highly poly disperse. The number of individually distinct components in HS, as measured by mass spectroscopy is in the thousands. The average composition of HA and FA can be represented by model structures.
The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+, and Fe3+ creating humic colloids. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes. The formation of complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions.