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(23) Sposito, G. The Thermodynamics of Soil Solutions; Oxford University Press (Clarendon): New York, 1981; Chapter 5. (24) Sposito, G.; Holtzclaw, K. M.; Johnson, C. T.; LeVesqueMadore, C. S.Soil Sci. Soc. Am. J . 1981, 45, 1079-1084. (25) Kinniburgh, D. G.; Jackson, M. L. In Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson, M. A.; Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; Chapter 3, pp 91-160. (26) Schindler, P. W. In Adsorption o f Inorganics ut SolidLiquid Interfaces; Anderson, M. A.; Rubin, A. J., Eds.; Ann
Arbor Science: Ann Arbor, MI, 1981; Chapter 1, pp 1-49. (27) Kinniburgh, D. G. J . Soil Sei. 1983, 34, 759-769. (28) Kinniburgh, D. G.; Barker, J. A.; Whitfield, M. J . Colloid Interface Sei. 1983, 95, 370-384. (29) Schofield, R. K.; Taylor, A. W. Soil Sei. Soc. Am. h o c . 1955, 19, 164-167.
Received f o r review June 21, 1985. Accepted March 19, 1986. This research is published with the permission of the Director of the British Geological Survey (NERC).
Limitations in the Use of Commercial Humic Acids in Water and Soil Research Ronald L. Malcolm” US. Geological Survey, Box 25046, M.S. 408, Federal Center, Denver, Colorado 80225
Patrick MacCarthy Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401
Seven samples of commercial “humic acids”, purchased from five different suppliers, were studied, and their characteristics were compared with humic and fulvic acids isolated from streams, soils, peat, leonardite, and a dopplerite sample. Cross-polarization and magic-angle spinning 13C NMR spectroscopy clearly shows pronounced differences between the commercial materials and all other samples. Elemental and infrared spectroscopic data do not show such clear-cut differences but can be used as supportive evidence, with the 13CNMR data, to substantiate the above distinctions. As a result of these differences and due to the general lack of information relating to the source, method of isolation, or other pretreatment of the commercial materials, these commercial products are not considered to be appropriate for use as analogues of true soil and water humic substances, in experiments designed to evaluate the nature and reactivity of humic substances in natural waters and soils. W
Introduction Extraction and isolation of humic and fulvic acids from soils and sediments are time-consuming and laborious processes. Humic substances in natural waters are considerably more difficult to isolate, primarily due to the low concentration of humic substances in waters as compared to concentrations in soils and sediments. In addition to extensive labor and time requirements, the purchase of and familiarity with specialized equipment and procedures usually are necessary in order to isolate humic substances. Most researchers are anxious to conduct experiments that test various hypotheses concerning the diverse nature and reactivities of humic substances, but many researchers apparently decide that the initial process of extracting, isolating, and purifying humic substances from the waters, soils, or sediments that are to be investigated, is not necessary. Some researchers may consider the extraction process to be more of an annoyance, or a menial task, as well as causing an unnecessary delay of weeks or months before real and meritorious experimentation can begin. In reality, these steps usually constitute some of the most essential prerequisites to meaningful studies on humic substances and comprise an indispensable part of the experimental design. 904
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For reasons cited above and others, many researchers have used commerciallyavailable so-called “humic acids” in their studies. The use of these commercial humic acids has been mildly criticized by some researchers in the past, but these materials are still commonly used in research. The purposes of this paper are (1)to show that several common commercial humic acids are not representative of soil or aquatic humic or fulvic acids, (2) to demonstrate that use of commercial humic acids may be of limited value in terms of understanding the nature and reactivity of natural humic substances in waters and soils, and (3) to present evidence that many of the commercial humic acids purchased from different suppliers possibly come from the same stockpile. For this paper, humic acids from a variety of commercial sources were characterized and their properties are compared to humic substances from soil and stream sources. The humic acids from five commercial suppliers that were investigated for this paper (Table I) are the most common commercial humic products used in the United States during the last decade. The use of other commercial humic acids such as Ega Chemie sodium humate ( I ) , Merck humic acid (2,3),Light humic acid (3),Wako Ltd. humic acid (4))Riedel and de Haen humussaure (5-7), leonardite humic acid from American Humates (8), and Carbonox from a North Dakota lignite (9-12) also has been reported in the literature. Some of the commercial humic acids cited in the older literature above may no longer be available. Fulvic acid is available commercially from Contech, Inc., but was not evaluated in this study. The use of brand names in this r e p o r t is for identification purposes only and does not constitute endorsement by the US.Geological Survey or the Colorado School of Mines. One of the major limitations in working with commercial humic acids is the lack of information on their origin and method(s) of extraction. Without this information it is impossible to attach any geochemical or environmental significance to data obtained by using these materials. These problems are compounded by the evident lack of consistency from batch to batch even with samples from the same supplier. This undesirable situation is well illustrated by letters received from various suppliers of commercial humic acids in response to requests for in-
Not subject to US. Copyright. Published 1986 by the American Chemical Society
formation on the source and nature of their products. The response from one company stated that the humic acid is “... a natural source derived from dead plants. There is no analysis available. For further information, please refer to the Merck Index ....” Perhaps the most useful information provided in that reference was that humic acid is used in mud baths! Another company listed ”Soil” as the source of their humic acid. No indication of the location or nature of the soil or the method of extraction was provided. The element,al composition of that sample was given as 38.6% carbon, 3.6% hydrogen, and 0.3% nitrogen. Ash content was not provided, and it was not specified whether or not the stated compositon was on an ash-free basis or moisture-free basis. Two other suppliers of humic acid gave very similar responses and reported an identical source (namely, soil) and analytical data for their product,: “...our humic acid is obtained by open-pit mining in Oberhessen (Kassel) region of Germany. The elemental analysis ... varies from batch to batch, with percent carbon being in the range 45.87 to 50.50, percent hydrogen in the range 3.25 to 3.83, and percent nitrogen in the range 0.57 to 0.60 ... also contains approximately 9.4% oxides of iron. Our humic acid is typically a dark brown powder having a melting point of greater than 300O.” In all of the above cases, no indication was given as to the method of extraction or other pretreatment (if any) of the samples. While it may be reassuring to know that some commercial humic acids were obtained from dead, rather than living plants, it is clear that when bne uses commercial humic acids, the difficulties are compounded by many additional unknown factors besides the unavoidable unknowns which normally confront the humic substances researcher. The implication in the responses from two of the companies, and a corresponding statement in the catalog of one distributor, that humic acids actually have a melting point is a clear indication that some of the suppliers of these products have been poorly informed as to the nature of humic substances. The molecular mass of commercial humic acid is given as 600-1000 Da in one chemical catalog; this value is most likely erroneous and is more typical of fulvic acid. The ash contents determined by us for commerical humic acids are considerably greater than those stated in the chemical catalogs. Commercial humic acids are variously designated as practical grade (90-95%) or technical grade (>go%) in the trade literature. The assignment of such grades to these operationally defined materials is misleading, a t best. While these drawbacks to the use of commercial humic acids may be evident to the experienced researcher in this area, they may not be quite so clear to newcomers to this field, or to those who work with humic substances only on an occasional basis; it is precisely these latter groups that may be most inclined to avoid the time-consuming task of extracting humic substances from known source materials and to purchase commercial humic acids. The need for pointing out the limitations of the commercial humic acids is more urgent a t this time, because the use of these products appears to be increasing in recent years and a number of authors are advocating their use. An examination of the literature reveals that commercial humic acids are being used rather extensively in environmental, water, and analytical chemistry studies; there is very little evidence of their use in soil science except for the occasional use of leonardite. In addition to our own prior criticisms relating to the use of commercial humic acids (13), dissatisfaction of working with these commercial products has also been expressed to us by other scientists. However, the authors
of this paper are not aware of any prior, extensive evaluation or criticism of the use of these materials in the literature. Literature Review. The widespread use of commercial humic acids is documented by the references that follow. In many of t,hese investigations, the commercial materials were the sole “humic” substances used and the investigators assumed that the commercial humic acids were representative of soil and water humic substances; in other studies the commercial humic products were used in association with humic substances isolated from soil, sediment, or water. However, in no case has a thorough comparison of both classes of materials been conducted. Several researchers have used commercial humic substances to infer enhancement of the apparent water solubility of hydrophobic pollutants in streams (14, 15). The apparent dissimilarity of Aldrich humic acid to natural fulvic acids in water was not discussed; however, in those studies samples of natural water and of sediment-derived humic substances also were used in association with the commercial products. Perdue and Wolfe (16) used an extract of Aldrich humic acid in studying the hydrolysis kinetics of 2,4-DOE in waters; however, they did not point out that the sorptive characteristics of Aldrich humic acid toward 2,4-DOE probably is more pronounced than that of the naturally occurring lower molecular weight fulvic acids found in stream waters; these investigators also used stream-sediment humic substances in their studies. Callaway et al. (17) used Aldrich humic acid as an analogue of soil humic acid in studying the partitioning of lowmolecular-weight halocarbons between the vapor phase and an aqueous sodium humate solution. Haas and Kaplan (78) used Aldrich humic acid as an analogue of aquatic material in studying the sorption of toluene by humic substances. Very recently, Gabbita et al. (19) reported on the association of low-molecular-weight halocarbons by Aldrich sodium “humate”, Hassett and Milicic (20) described the binding of 2,2’,5,5’-tetrachlorobiphenylto Aldrich sodium humate, McCarthy and Jimenez (21) studied the interaction between polycyclic aromatic hydrocarbons and Aldrich humic acid, and Garbarini and Lion (22) studied the binding of nonionic pollutants by humic acid from ICN Pharmaceuticals. Because humic substances are one of the known precursors of trihalomethanes formed during chlorination in water-treatment processes (23,24), several investigations have been conducted to decrease this potential through removal of humic substances by activated carbon or other sorbents. Commercial humic materials were used in many of these investigations, and it was assumed that these materials were representative of stream humic substances. Large differences in the trihalomethane potential of Pfaltz & Bauer humic material and soil humic acid were attributed to source of the material (2Ei);however, the suitability of neither the commercial humic acid nor the soil humic acid a$analogues of water humic substances was evaluated. Major differences between the sorption behavior of wellwater organic matter and Pfaltz & Rauer humic acid on the same resin at different pHs led Roening et al. (26) to conclude that the variability of humic substances in water made it difficult to predict the nature of the pH effect. Glaser and Edzwald (27) examined the flocculation of commercial Aldrich humic material by poly(ethy1enimine) to evaluate the removal of humic substances from water with sand filters used in ordinary water treatment. Farrah et al. (28)advocated the use of epoxy-fiberglass to remove what they called humic acid from tapwater even though only 50% of Aldrich humic acid added to tapwater was Environ. Sci. Technol., Vol. 20, No. 9, 1986
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Table I. Elemental Contents of Commercial Humic Acids and Selected Humic Acids from Dopplerite, Peat, and Leonardite (Expressed on a Percent by Weight Ash-Free and Moisture-Free Basis) atomic H/C 0.98
101.3 113.0
9.28 32.82
86 108
1.02
