What are Humic Substances?
Humic substances (HS) are major components of the natural organic matter (NOM) in soil and water as well as in geological organic deposits such as lake sediments, peats, brown coals and shales. They make up much of the characteristic brown color of decaying plant debris and contribute to the brown or black color in surface soils. They are major components of NOM in surface waters and at higher concentrations can impart a dark color, especially in brown fresh water ponds, lakes, and streams. In leaf litter or composts, the color may be yellowish-brown to black, depending on the degree of decay and concentration.
Humic substances are very important components of soil that affect physical and chemical properties and improve soil fertility. In aqueous systems, like rivers, about 50% of the dissolved organic materials are HS that affect pH and alkalinity. In terrestrial and aquatic systems HS affect the chemistry, cycling and bioavailability of chemical elements, as well as transport and degradation of xenobiotic and natural organic chemicals. They affect biological productivity in aquatic ecosystems, as well as the formation of disinfection by-products during water treatment.
Humic substances are complex and heterogeneous mixtures of polydispersed materials formed by biochemical and chemical reactions during the decay and transformation of plant and microbial remains (a process called humification). Plant lignin and its transformation products, as well as polysaccharides, melanin, cutin, proteins, lipids, nucleic acids, fine char particles, etc., are important components taking part in this process.
Humic substances in soils and sediments can be divided into three main fractions: humic acids (HA or HAs), fulvic acids (FA or FAs) and humin. The HA and FA are extracted from soil and other solid phase sources using a strong base (NaOH or KOH). Humic acids are insoluble at low pH, and they are precipitated by adding strong acid (adjust to pH 1 with HCl). Humin cannot be extracted with either a strong base or a strong acid.
Aquatic HS contain only HA and FA and these components are generally removed from water by lowering the pH to 2 and adsorbing both components on a suitable resin column. The HA and FA are extracted from the resin with strong base followed by lowering the pH to 1 to precipitate the HA. The resin column separation is also used to separate FA from the non-humic materials (amino acids, peptides, sugars, etc.) extracted from soils. At low pH the FA adsorbs on the resin, but non-humic materials pass through the column.
Humic substances are highly chemically reactive yet recalcitrant with respect to biodegradation. Most of the data on HA, FA and humin refer to average properties and structure of a large ensemble of components of diverse structure and molecular weight. The precise properties and structure of a given HS sample depends on the water or soil source and the specific conditions of extraction. Nevertheless, the average properties of HA, FA and humin from different sources are remarkably similar.