Humic Substances in Environmental Remediation
Humic substances, a mixture of complex organic compounds that are usually separated into three fractions: humic acids, fulvic acids and humans, are generally seen as important components of soil and natural water. They are formed during humification of organic matter by soil microorganisms. Hu-mification is the chemical-microbiological process of transforming debris from living organisms into a general class of refractory organic compounds.
Humic substances account for 50 to 80% of the organic carbon of soil, natural water, and bottom sediments. They are typically derived on an industrial scale from natural deposits like peat and coal.
Their peculiar feature is polyfunctionality, which enables them to interact in a variety of ways with both metal ions and organic chemicals. As a result, numerous studies have shown, humics are capable of altering both the chemical and the physical speciation of ecotoxicants and in turn affecting their bioavailability and toxicity. Hence humic substances hold great promise functioning as amendments to mitigate the adverse impacts of ecotoxicants and as active agents in remediation.
It has been found that humic substances can enhance biotic and abiotic degradation of phenols, polyaromatic hydrocarbons (PAH) and pesticides in the aquatic environment. They are generally recognized to be responsible for the binding of major parts of the available metal ions in water and soil.
High adsorption capacity, high ion exchange capacity and environmental compatibility makes humic substances an attractive material for environmen-tal remediation. The results of biochemical studies indicate that humates can detoxify organic and inorganic inhibitors of biological processes. Humates also enhance biodegradation of toxic organic substances (phenols, formaldehyde, mineral oil) thus making their treatment more efficient. The results of chemical studies demonstrate that humates can be successfully used for immobilization of heavy metals (copper, iron, manganese). Thus humates can potentially be used as a filling material for barrier walls to prevent transport and bioavailability of heavy metals in soil.
The ability of humic substances to act as chelating agents for metal ions is well-documented. The particular effect that humic substances have on che-latable metals in hazardous wastes depend upon the following factors:
· the nature of the humic substances, particularly on their fulvic and humic acid content
· the chemistry of soil or water environment with respect to acidity-alkalinity and oxidation-reduction
· the presence of competing species (e.g. cyanide that compete with humic ligands for metal ions)
There are very few reports on practical applications of humic substances in environmental remediation. Most of them utilized humates to remove met-als from water or immobilize heavy metals in soil. Pilot scale applications of humates for removal of petroleum products from groundwater have also been reported.
Recent studies have been initiated on the application of humates in environmental remediation. The studies included biochemical and chemical tests with various heavy metals and organic pollutants. The initial results of the studies indicate that humates can detoxify organic and inorganic inhibitors of biological processes and enhance biodegradation of toxic organic substances (phenols, formaldehyde) as well as detoxify and immobilize phenols, ammonia and heavy metals (copper, chromium, iron, lead, manganese, nickel and zinc). The removal of phenol, formaldehyde and phosphorus by humates was found substantially higher in biological treatment as compared to chemical treatment. The removal of heavy metals was also higher in biological system but the difference was not as dramatic as for organic pollutants.