Humic substances

Humic acid is a principal component of humic substances, which are the major organic constituents of soil (humus), peat, and coal. It is also a key organic constituent of many upland streams, dystrophic lakes, and ocean water. It can be extracted from these natural sources using a variety of methods, and has numerous applications in agriculture, environmental remediation, and industrial processes.

Composition and properties

Humic acid is a complex mixture of many different acids containing carboxyl and phenolate groups. The precise properties and structure of a given sample depend on the water or soil source it was extracted from, and the specific methods used for extraction and purification.

Despite this variability, all humic acids have several common characteristics. They are large, complex molecules that are soluble in water under alkaline conditions, and have a dark brown to black color. They can form complexes with ions that are commonly found in the environment, creating humic colloids that have a high cation exchange capacity.

Extraction and purification

Humic acid can be extracted from any material containing a high concentration of humic substances, such as soil, peat, or coal. The most common extraction method involves treating the material with a strong base, such as sodium hydroxide or potassium hydroxide, which dissolves the humic substances. The solution is then filtered to remove any insoluble material, and the humic acid is precipitated out by adjusting the pH to a more neutral value.

Applications

Humic acid has a wide range of applications, due to its ability to chelate metal ions, its high cation exchange capacity, and its effect on the physical properties of soils.

In agriculture, it is often used as a soil conditioner, to improve the soil's ability to retain water and nutrients. It can also enhance the growth of crops by stimulating the activity of beneficial soil microorganisms, and by directly influencing the metabolism of plants.

In environmental remediation, humic acid can be used to immobilize or remove toxic metals from contaminated soils or water. It can also enhance the biodegradation of organic pollutants, by acting as a source of nutrients for microorganisms.

In industrial processes, humic acid can be used as a flocculant, to help remove suspended particles from water. It can also be used as a dispersant, to prevent the aggregation of particles in suspensions.

See also

• Fulvic acid
• Humus
• Soil science
• Environmental chemistry