Humic substances Because of known and perceived threats to safe water, interest in the fate and transport of humics and their reactions with metals, anions, and organics, as well as chlorine, is increasing sharply
Sometimes in the course of scientific progress a relatively obscure field is suddenly brought into prominence. A recent example involves the chemistry and biochemistry of terrestrial and aquatic humic substances. Since 1970 certain developments have sharply increased attention given to these substances: • Trihalomethanes (THMs) were found in drinking water supplies. • THMs such as chloroform, dibromochloromethane, and several others, came under suspicion of being carcinogenic. • THMs are formed when chlorine reacts with humic substances in raw water to disinfect it for municipal drinking water supplies. • In addition, humic substances react with metals, anions, and organics, apart from chlorine, and thus may present further threats to health and the environment. Humic substances are formed when living matter, especially (though not exclusively) plants, die and decay. Many of these substances are formed in soils and eventually find their way to lacustrine (lake), riverine, and estuarine waters. Other humic substances are formed directly within aquatic systems. This is the dominant source of humics in ocean waters, for example. Their overall molecular structures "are a mystery," Russell Christman, chairman of the University of North Carolina's Department of Environmental Science and Engineering and editor of ES&T, observed. "The mechanism by which they react chemically is also a mystery," he added. A vacuum cleaner
During the past decade, chlorination products of humic substances and their treatment have been increasingly discussed at conferences on chlorination, such as one recently held in California. 20A
Environ. Sci. Technol., Vol. 16, No. 1, 1982
The symposium's major objective was "to develop a commonality of ideas on humic substances' [and their reaction products'] structure, composition, identification, quantification, and control," Gjessing explained to ES& T. He noted that a symposium of this type had been held in Holland in 1971; while it was a "first" for the U.S., more importantly, it was a "first ever" since the acceleration of interest in humic substances. Expressing his personal view of Gjessing: "interest came none too soon" humic substance chemistry and biology, Gjessing said the increased inThey have also been subjects of reguterest and study "came none too soon. latory activity. However, those meetOne important aspect of these subings were not devoted to humic substances is that they act as a 'vacuum stances per se; rather, they covered a cleaner1—their molecules can take up single aspect—a significant one, to be and increase the environmental dissure—of this subject. There was a need tribution of inorganic and organic for discussions by experts of not only micropollutants. These can include chlorination products, but many other metals, and persistent organics such as facets of humic substances. PCBs, DDT, and polynuclear aromatic To address this need, Christman and hydrocarbons [PAHs]. In Norway, we Egil Gjessing of the Norwegian Instihave been observing PAHs in associtute for Water Research (Oslo), ation with humic substances. through their respective organizations, "PCBs, DDT, and PAHs can be cohosted the Symposium on Terrescarried to soils and waters by precipitrial and Aquatic Humic Materials tation or on aerosols," Gjessing conheld at Chapel Hill, N.C., in Novemtinued. "They can attach themselves to ber. Water and soil chemists and bihumic substance molecules in soil or ologists from the U.S., Canada, and water, especially to the water-soluble several other countries, attended and portion of such molecules, and so be presented papers. transported to, and in, water," he explained. Echoing this concern about humic substances, ES&T advisor Humics: some definitions William Glaze of the University of Texas at Dallas (Richardson, Tex.) Humic substances = substances added that what has been learned containing humic and fulvic acids plus about humic chlorination products and other components. their suspected health effects— Humic acids = the acid components "proven with experimental aniof humic substances, which are solumals"—may be but a signpost of much ble In bases, but precipitate in acids. broader toxicological and epidemioFulvic acids = the acid components logical data to come. of humic substances that are soluble in both bases and acids. Obtaining these data may prove to be a formidable task, indeed. "While Source: Charles O'Melia of the Johns Hopkins we know that humic substances can University accumulate heavy metals, we know 0013-936X/82/0916-020A$01.25/0 © 1981 American Chemical Society
'near zero' about animal and human health effects," Joseph Cotruvo of EPA's Office of Drinking Water re minded the symposium. A witches' brew In order to ascertain more fully the mechanisms and products of humic substance reactions, an important first step is to determine their molecular structures. It will be a difficult step: as E. Michael Thurman of the U.S. Geological Survey (USGS, Denver) said, "These substances are a witches' brew made up of reaction products of fatty acids, sugars, hydroxy acids, polysaccharides, glycosides, and many other compounds. Identification alone will be a real challenge." Moreover, humic substances of aquatic origin, and those from soil seem to be different types of "witches' brews" that differ from marine humic substances. But certain clues are becoming ap parent to researchers. First of all, humic substances seem to be highly polar. Secondly, soil humic substances, under basic hydrolysis, release aro matic (phenolic) substances charac teristic of lignin breakdown products, while those of deep-sea origin do not. Researchers indicate that it may be possible to classify humic substances in categories of high and low molecular weight. Those humic substances heavier than 1000 amu would be "high," while those lighter would be termed "low." Moreover, lighter humics may have a much lower frac tion of hydrophobic groups than the heavier ones. Various means are at the scientist's disposal to break and separate humic substance components. Precipitation, dialysis, electrophoresis, resin ad sorption, size exclusion chromatogra phy (SEC), positive and negative ion exchange, and preparative high-per formance liquid chromatography (HPLC) were listed by Thurman. Hydrophobic fractions can be ad sorbed on resins and eluted with a weak base. Modern analytical techniques have presented problems, especially where heavier humics were concerned. However, Thurman said that carbon13 nuclear magnetic resonance spec troscopy ( 1 3 C-NMR) and gas chromatography/mass spectrometry ( G C / M S ) show promise. Secondary ion MS (SIMS), M S ionization, and fast-atom bombardment (FAB) have also been used successfully. One ap proach is to "tag" a humic substance by methylation with l 3 C-diazomethane together with N a O H and other necessary reagents, and then proceed with l 3 G N M R .
Water samples from the Okefenokee Swamp, Ga., from which the Suwanee River flows, were analyzed by this technique. The Suwanee Riv er's water with a pH of 3 stemmed from humic acids, not acid rain, Thurman said. Dating with l 4 C as signed them an age of less than 30 years. Their general composition seemed to be 52% C, 43% O, 4.3% H, 0.56% Ν (Ν content is normally quite low),
