About Humic Substances
Humic substances are ubiquitous in the environment. Their importance in agriculture and soil sciences has been acknowledged for over 150 years. Aquatic scientists have been slower in appreciating their importance, but now realize that they may constitute as much as 95% of the total dissolved organic matter in aquatic systems and often are equal to or greater than the concentrations of inorganic ions present. In many cases they act as the major buffering system, which has serious implications for acidification of lakes and rivers.
SEM image (approx 2000x) of a solid humic acid.
While important for microbial processes that drive many ecosystems in our world, the true interest to the chemist is their interactions with other elements and compounds. Humic substances have been documented to interact in some manner with over 50 elements from the periodic table.
These include nutrients, toxic metals, radionuclides (including the transuranium series) and the halogens. The latter can interact with humic substances in drinking water treatment to produce halogenated carcinogens such as chloroform and bromoform, which are then directly introduced into the public drinking water with obvious health consequences. Toxic metals and micronutrients can be made either more available to organisms, or actually sequestered so as to reduce their toxicity or beneficial value. Furthermore, humic substances contain long-lived (almost stable populations) of free radicals, which are capable of reducing inorganic species such as Hg(II), Cr(VI), and Pu(VI) to name a few. They are also capable of interacting with anthropogenic organic compounds such as polycyclic aromatic hydrocarbons, including the known carcinogen benzo(a)pyrene, again altering the chemical reactivity of these important chemicals.
The mechanisms of many of these interactions are unclear. That is a result of our lack of knowledge of the structural components of humic substances. While it is true that we understand certain gross structural characteristics, i.e. many toxic metals are believed to be complexed through carboxyl groups, and the stabilization of free radicals appears to be through quinone/semi-quinone structures, we have no structural knowledge that allows us to predict the extent of these reactions under given physico-chemical regimes. Furthermore, without structural knowledge, measurement and prediction of the kinetics of reactions is impossible.
A further riddle concerns the nature of organic compounds produced by biological and geochemical processes that contain structures that can complex metals, sequester anthropogenic organic compounds, oxidize and reduce elements to and from toxic forms, photosensitize chemical reactions, and enhance or retard the uptake of toxic compounds or micronutrients to plant and microbial organisms? Without structural knowledge of humic substances, we will continue to rattle around in the black box of ignorance when asked to predict and forecast the impacts of chemical and biological actions on our environment. Those of us who have studied these compounds know their importance to chemical (another facet of humic substances is their interference in industrial processes such as Al processing), agricultural, environmental and even health issues (humic substances are widely used in the treatment of many animal maladies and their potential for use in human health is being explored).
While the study and eventual elucidation of the structure of these complex mixtures might at first glance seem esoteric, they are important components in processes that touch an extremely broad suite of scientific disciplines.
G. R. Aiken, P. MacCarthy, R. L. Malcolm and R. S. Swift, ‘Humic Substances in Soil, Sediment, and Water’, Wiley, New York, 1985.
M. H. B. Hayes, P. MacCarthy, R. L. Malcolm and R. S. Swift, ‘Humic Substances II: In Search for structure’, Wiley, Chichester, 1989.
P. MacCarthy, C. E. Clapp, R. L. Malcolm and R. R. Bloom, ‘Humic Substances in Soil and Crop Sciences: Selected Readings’, American Society of Agronomy, Madison, Wisconsin, 1990.
W. Ziechmann, ‘Humic Substances’, BI Wissenschaftsverlag, Mannheim, 1993.
N. Senesi and T.M. Miano, Humic Substances in the Global Environment: Implications for Human Health, Elsevier, Amsterdam, 1994.
F.J. Stevenson, Humus Chemistry: Genesis, Composition, Reactions, 2nd edn., Wiley, New York, 1994.
J.S. Gaffney, N. A. Marley and S. B. Clark, Humic and Fulvic Acids: Isolation, Structure and Environmental Role, American Chemical Society Symposium Series 651, 1996.
M.H.B. Hayes and W.S. Wilson, Humic Substances, Peats and Sludges: Health and Environmental Aspects, Royal Society of Chemistry, Cambridge, 1997.
G. Davies and E.A. Ghabbour, Eds., Humic Substances: Structures, Properties and Uses. Royal Society of Chemistry, Cambridge, 1998 (ISBN 0 85404 704 2).
E.A. Ghabbour and G. Davies, Eds., Understanding Humic Substances: Advanced Methods, Properties and Uses. Royal Society of Chemistry, Cambridge, 1999, ISBN: 0 85404 799 9.
E.A. Ghabbour and G. Davies, Eds., Humic Substances Versatile Components of Plants, Soil and Water. Royal Society of Chemistry, Cambridge, 2000, ISBN: 0 85404 855 3.
E.A. Ghabbour and G. Davies, Eds., Humic Substances: Structures, Models and Functions. Royal Society of Chemistry, Cambridge, 2001, ISBN: 0-85404-811-1.
E.A. Ghabbour and G. Davies, Eds., Humic Substances: Nature’s Most Versatile Materials. Taylor & Francis, New York, 2004, ISBN: 1-59169-015-3.
E.A. Ghabbour and G. Davies, Eds., Humic Substances: Molecular Details and Applications in Land and Water Conservation. Taylor & Francis, New York, 2005, ISBN: 1-59169-031-5.