Humic Substances, Part 8: Environmental Applications

ENVIRONMENTAL APPLICATIONS


Natural organic colloids (humic and fulvic acids) are important because they form water-soluble complexes
with many metals including radionuclides (Lubal et al. 1998, Lubal et al. 2000, Pacheco and Havel 2001, Ghabbour et al. 2001). These organics may therefore be important as radionuclide transport agents through the environment. It is known that the presence of humic substances in natural waters can influence the uptake of radionuclides by natural solids and thus their migration to surface and ground waters (Bondietti 1982, Samanidou et al. 1991). The main task of humic substances in environmental chemistry is to remove toxic metals, anthropogenic organic chemicals and other pollutants from water. Ion-exchange materials based on calcium humate were found suitable for the removal of such heavy metals as iron, nickel, mercury, cadmium and copper from water and also to remove radioactive elements from water discharges from nuclear power plants. Their selective binding capabilities are also exploited for the destruction of munitions and chemical warfare agents (Ghabbour and Davies 1999). Humus-based filters have been developed for sewage purification, with many applications. The filters are useful to clean chromate smelter wastewater, to remove oil and dyes from wastewaters and aquatic systems (Versraete and Devliegher 1997), to filter urban and industrial wastewaters, to remove pesticides from sewage and to remove phenol from water.

Humus-containing materials have been also utilized for sorbing gases, e.g. the removal of waste gases from an animal-carcass rendering plant. Slightly modified humates can be applied to remove hydrogen sulfide and mercaptans from municipal gas supplies, and sulfur dioxide from stack gases (Green and Manahan 1981).

Different groups of compounds such as herbicides, fungicides, insecticides, nematicides, dioxins and also some pharmaceutical products like estrogenic compounds were determined as possible environmental endocrine disruptors. Thanks to their ability to adsorb organic pollutants from the environment, humic substances were found to be useful to remove those contaminants from water, soil and sewage sludges (Shin et al. 1999, Lofredo et al. 2000). The complex nature of the interaction between HS and xenobiotics and their influence in the environmental quality (water, soil, and atmosphere) has been studied by different authors. The study of the acido-basic and complexation properties of HAs with several inorganic and organic compounds has attracted increased attention due to their influence on many aspects of soil and water quality, and industrial processes. Pacheco et al. (2003) found that some inorganic and/or organic pollutants were strongly complexed (bound) only with some of the HAs components. As a consequence of such interaction quite stable entities of the supramolecular kind were formed.

The interactions between humic materials and microorganisms have been intensively studied for the past 30 years. It was found that fermenting bacteria could reduce humic substances. This fact has significant implications for the autecology of anaerobic bacteria in soils and sediments. The cumulative production of acetate during this process seems to be energetically advantageous for fermenting bacteria (Benz et al. 1998). Utilization of humics as the energy supply for specific bacteria, fungi and higher microorganisms was
studied by many investigators (Bhardwaj and Gaur 1971) but it was noted that they can not exploit humic materials as a food source.