Environ. Sci. Technol. 1990, 24, 1875-1877
general for dissimilar organic bases. Registry NO. 4-MA, 106-49-0; 2,4,5-TMA, 137-17-7;A, 62-53-3; 3,4-DMA, 95-64-7; KC1, 7447-40-7; water, 7732-18-5; octanol, 111-87-5.
Literature Cited (1) Zachara, J. M. Selection of Organic Chemicals for Subsurface Transport; DOE/ER-0206; U.S. Department of Energy, Office of Health and Environmental Research, Ecological Research Division; Washington, DC, 1984. (2) Westall, J. C. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987; pp 3-32. (3) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241-248. (4) Schwarzenbach,R. P.; Westall, J. C. Enuiron. Sci. Technol. 1981, 15, 1360-1367. (5) Westall, J. C.; Leuenberger, C.; Schwarzenbach,R. P. Environ. Sci. Technol. 1985, 19, 193-198. (6) Schwarzenbach,R. P.; Stierli, R.; Folsom, B. R.; Zeyer, J. Environ. Sci. Technol. 1988, 22, 83-92. (7) Jafvert, C. T.; Westall, J. C.; Grieder, E.; Schwarzenbach,
R. P. Environ. Sci. Technol., this issue. (8) Westall, J. C.; Johnson, C. A.; Zhang, W. Enuiron. Sci. Technol., this issue. (9) Brownawell, B. J.; Chen, H.; Collier, J. M.; Westall, J. C. Environ. Sci. Technol. 1990, 24, 1234-1241. (10) Zachara, J. M.; Ainsworth, C. C.; Felice, L. J.; Resch, C. T. Enuiron. Sci. Technol. 1986, 20, 620-627. (11) Zachara, J. M.; Felice, L. J.; Sauer, J. K. Soil Sci. 1984,138, 209-219. (12) Moreale, A.; Van Bladel, R. Soil Sci. 1979, 127, 1-9. (13) Cloos, P.; Moreale, A.; Broers, C.; Badot, C. Clay Miner. 1979, 14, 307. (14) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Wiley: New York, 1974; pp 136-238.
(15) Karickhoff, S. W.; Brown, D. S. J. Enuiron. Qual. 1978, 7, 246-252. (16) Brown, D. S.;Combs, G. J.Enuiron. Qual.1985,14,195-199. (17) Zierath, D.; Hassett, J. J.; Banwart, W. L.; Wood, S. G.; Means, J. C. Soil Sci. 1980, 129, 277-281. (18) Simmons, K. E.; Bollag, J. M. Enuiron. Sci. Technol. 1989, 23, 115-121. (19) Ezumi, K.; Kubota, T. Chem. Pharm. Bull. 1980,28,85-91. (20) Sherrer, R. A. In Pesticide Synthesis through Rational
Approaches;Magee, P., Kohn, G. K., Menn, J. J., Eds.; ACS (21) (22)
(23)
(24) (25)
Symposium Series 255; American Chemical Society: Washington, DC, 1984; Chapter 14. Mayer, J. M.; Testa, B.; van dewaterbeemd, H.; Bornand-Crausaz, A. Eur. J . Med. Chem. 1982,17,461-466. Karickhoff, S. W.; Brown, D. S. Determination of Octano11 Water Distribution Coefficients, Water Solubilities, and Sediment/ Water Partition Coefficients for Hydrophobic Organic Pollutants; EPA-600/4-79/032; U.S. Environmental Protection Agency. Environmental Research Laboratory: Athens, GA, 1979. Westall, J. FITEQL-A Computer Program for Determination of Chemical Equilibrium constants from Experimental Data. Version 2.0. Report 82-02; Department of Chemistry, Oregon State University; Corvallis, OR, 1982. Bevington, P. R. Data Reduction and Analysis f o r the Physical Sciences; McGraw-Hill: New York, 1969. Handbook of Chemical Property Estimation Methods; Lyman, W. J., Reehl, W. F., Rosenblatt, D. H., Eds.; McGraw-Hill: New York, 1982.
Received for review April 19,1990. Revised manuscript received July 11, 1990. Accepted July 17, 1990. This research was supported by the Ecological Research Division, Office of Health and Environmental Research (OHER), US.Department of Energy (DOE) under Contract DE-ACOG-76RLO 1830 as part of OHER’s Subsurface Science Program.
COMMUNICATIONS A Model of Humin James A. Rice’ and Patrick MacCarthy Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401
Introduction
Humin is defined as the fraction of humic materials that is insoluble in an aqueous solution at any pH value (1,2). The nature of humin has remained something of an enigma. Despite the fact that it typically comprises 50% or more of the organic carbon in soil (3), sediment ( 4 ) ,and peat (5), it has been the subject of comparatively little research interest. For example, only 409 (4%)of the 10315 citations under the keywords humic acid, fulvic acid, and humin in all volumes of Chemical Abstracts through Vol. 1 0 3 (1985) pertain to humin (6). Furthermore, a substantial portion of these 409 citations do not refer to humin as it has been traditionally defined, and in many cases the term, as used, does not even refer to an extract of a soil, sediment, or similar substrate. The comparative lack of *Present address: South Dakota State University, Department of Chemistry, Brookings, SD 57007-0896. 0013-936X/90/0924-1875$02.50/0
interest in the study of humin may be the result of its insolubility and consequent difficulty of its separation from nonhumic materials. However, in recent years there has been a growing interest in the nature of humin (2,4, 7,8). By definition, humin is obtained as the solid residue that remains after centrifugation of the alkali extract of a humus sample (e.g., ref 1). T o separate the organic components of humin from the inorganic components, the humin is generally subjected to extensive digestion with a mixture of concentrated hydrofluoric and hydrochloric acids (I). As a result of this treatment the inorganic material is decomposed, but the organic constituents are also likely to undergo significant changes. Despite the previously limited interest in the study of humin, several models have been proposed to describe its nature. It has been described as humic and/or fulvic acid complexed to inorganic colloids or clay minerals (9-13), a “high molecular weight Polymer” (I), ya 1ignoProtein” (14), “a melanin” (8,15), or “plant and fungal residues in
0 1990 American Chemical Society
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24, No. 12, 1990
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1 SAMPLE IN NaOH
Table I. Compositions of Humin Samples from Different Sources”
sample
bitumen
bound lipids
soil peat stream sediment
24.3
0.7
56.2 26.3
1.8 1.7
I 2 Add MlBK and HCI
Discharge aqueous: fulvic acid
3
NaOH
MIBK contains: Humin-salt form
A c
7
I
add HCI
Humln-hydrogen
form
1
aM H20
1 c
f
Aqueous oontalns
Bound-humlc 1516
MIBK c o n t s l n i Llplds
Figure 1. Flow chart for the isohtion and disaggregation of humin with the MIBK method. Reprinted wkh permission from ref 18. Copyright 1989 American Chemical Society.
varying stages of decomposition” (16). The purpose of this research communication is to report on insights into the nature of humin gained from an alternative approach to its isolation and fractionation.
The MIBK Method Recently, we described a method in which humin is obtained by a direct isolation step rather than being simply collected as the residue that remains after extracting humic and fulvic acids from a sample (17). In this method, the humic components of a soil, sediment, or peat sample are quantitatively partitioned between water and methyl isobutyl ketone (MIBK) as a function of the pH of the aqueous phase. By use of the MIBK method, the humin in a sample can be readily separated from fulvic acid and humic acid (Figure 1, steps 1-5A). Most of the inorganic material and insoluble, nonhumic organic materials present in a sample settle out and are removed when fulvic acid is discharged (Figure 1, step 2). More importantly for the purposes of this communication, an extension of the MIBK method causes the humin to disaggregate; the disaggregated components can then be quantitatively separated from each other and assigned to other, recognized classes of humic and nonhumic substances (Figure 1,steps 1-5B). The Components of Humin With the MIBK method, every humin sample that we have studied, including humins from soils, sediments, peats, lignites, and dopplerite has been found to consist of the following fractions: (1)a lipid fraction obtained by Soxhlet extraction with organic solvents (referred to as “bitumen”); (2) a lipid fraction that is not removed by organic solvents during exhaustive extraction in a Soxhlet apparatus but can be isolated by the MIBK method (the “bound-lipid’’fraction); (3) a fraction isolated by the MIBK method that is brown in color, soluble in alkaline, aqueous solutions, and precipitates under acidic conditions (referred 1876
16.2 16.3 11.0
67.8 18.2 70.0
a Data are from ref 3. All values are weight percents of the dry humin.
MIBK:
HUMIN a HUMIC ACID
4
bound insoluble humic acid residue
Environ. Sci. Technol., Vol. 24, No. 12, 1990
to as “bound humic acid”); and (4) an “insoluble residue”, which is usually inorganic in nature (3). The lipid fraction in Figure 1contains both bitumen and bound lipids unless the bitumen has been removed by Soxhlet extraction of the sample or the humin prior to disaggregation of the humin with the MIBK method. The insoluble residue settles out during the disaggregation step (step 5B) but is not shown in Figure 1. Table I shows the contributions of each of these four fractions to the overall composition of humin for several types of samples. All values in the table were measured directly, and deviations of the mass balances from 100% are due to experimental error. Each of these fractions exhibits characteristics that make it distinct from the other components of humin, or from humic and fulvic acids isolated from the same sample by the MIBK method (3,18). In all three of these samples the bitumen represents the largest fraction of the organic components of humin. Analysis by gas chromatography-mass spectrometry has shown that the bound-lipid fraction is distinctly different from the bitumen fraction based on the fatty acid and n-alkane distributions (3, 18). Similarly, the fraction referred to as bound humic acid displays some characteristics different from those of the regular humic acid that is isolated from the sample by the MIBK method. For example, the 13C NMR spectrum of the bound humic acid exhibits a substantially smaller fraction of aliphatic and carbohydrate carbon (3,18). X-ray diffraction has shown that the insoluble residues from a soil and a stream sediment consist primarily of clay minerals, quartz, and feldspars, while X-ray diffraction and optical microscopy have shown the insoluble residue from a peat to consist of quartz, illite, various feldspars, and readily identifiable, cellular remains of plant tissue (3). A Model for Humin It is proposed that humin consists of an association of bitumen, bound humic acid, bound lipids, and some insoluble material. None of these fractions, by itself,conforms to the operational definition of humin. This model describes humin as an aggregate of four previously recognized classes of materials, and the MIBK method provides a tool for disaggregating the humin into these constituents under mild conditions. The precise manner in which these components are bound together in humin has not yet been established, but is presently under investigation. Preliminary experiments have shown that remixing of the separated bitumen, bound lipids, bound humic acid, and insoluble residue in the presence of MIBK and water followed by evaporation of the liquid phases leads to the formation of an acid- and alkali-insoluble material similar to humin. Humin that is obtained by the traditional alkali extraction method also yields these four components when the isolated humin is subjected to Soxhlet extraction with organic solvents and is then fractionated by the MIBK procedure. There is no guarantee that humin isolated by the MIBK method is identical with humin isolated by the traditional method or with humin as it occurs in the en-
(4) Vandenbrouke, M.; Pelet, R.; Debyser, Y. In Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation, and Characterization; Aiken, G. R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.; Wiley-Interscience: New York, 1985; pp 249-273. (5) Zelazny, L. W.; Carlisle, V. W. In Histosols: Their Characteristics, Classification, and Use;Aandahl, A. R., Buol, S. W., Hill, D. E., Bailey, H. H., Eds.; Soil Science Society of America: Madison, WI, 1974; pp 63-78. (6) Rice, J. A.; MacCarthy, P. Geoderma 1989, 43, 65-73. (7) Preston, C. M.; Schnitzer, M.; Ripmeester, J. A. Soil Sci. SOC. Am. J. 1989,53, 1442-1447. (8) Anderson, D. W.; Paul, E. A,; St. Arnard, H. A. Can. J. Soil Sci. 1974, 54, 317-323. (9) Shah, R. K.; Choski, M. R.; Joshi, B. C. Chem. Era 1975, 6, 1-3. (10) Shah, R. K.; Choski, M. R.; Joshi, B. C. Chem. Era 1975, 4, 31-34. (11) Banerjee, S. K. J . Indian Chem. SOC.1979,56,1094-1097. (12) Theng, B. K. G., Formation and Properties of ClayPolymer Complexes; Elsevier: Amsterdam, 1979. (13) Cloos, P.; Badot, C.; Herbillon, A. Nature 1981, 289, 391-393. (14) Somani, L. L.; Saxena, S. N. Agrochimica 1982,26,95-103. (15) Kononova, M. M. Soil Organic Matter; Nowakowski, T. A., Newman, A. C. D., Transl.; Pergamon Press: Oxford, 1966. (16) Russell, J. D.; Vaughan, D.; Jones, D.; Fraser, A. R. Geoderma 1983,29, 1-12. (17) Rice, J. A.; MacCarthy, P. Sci. Total Enuiron. 1989,81/82, 61-69. (18) Rice, J. A.; MacCarthy, P. I n Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants; Suffet, I. H., MacCarthy, P., Eds.; Advances in Chemistry Series 219; American Chemical Society: Washington, DC 1989; pp 41-54.
vironment. It is possible that the humin actually disaggregates partially or completely during the MIBK extraction procedure, and that it aggregates into a “reconstituted” humin during evaporation of the MIBK. This model has features in common with some of the previous models of humin but, nevertheless, is quite different from all of them. It presents a simple picture of the composition of humin and provides a basis for understanding its properties in terms of the four constituents that can be isolated by the MIBK method. The bitumen and bound-lipid fractions would contribute hydrophobic character to the humin, such as the ability to retain nonpolar compounds. The bound humic acid fraction would add some humic characteristics, such as acidity and metal-complexing properties, to the humin. The clay minerals in the humin samples from soil and stream sediment would contribute cation-exchange capacity to those humin samples in addition to that provided by the bound humic acid. Accordingly, this model provides a basis for accounting for many of the known properties of different humin samples and for predicting their behavior in other situations. The MIBK method allows the components of humin to be disaggregated in a mild manner, apparently without any significant chemical decomposition, so that all four major constituents can be studied separately and their modes of interaction investigated.
Literature Cited (1) Stevenson, F. J. Humus Chemistry: Genesis, Composition, Reactions; Wiley-Interscience: New York, 1982. (2) Hatcher, P. G.; Breger, I. A,; Maciel, G. E.; Szeverenyi, N. In Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation, and Characterization; Aiken, G. R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.; Wiley-Interscience: New York, 1985; pp 275-302. (3) Rice, J. A. Ph.D. Dissertation no. T-3204, Colorado School of Mines, Golden, CO, 1987.
Received for review June 15, 1990. Revised manuscript received September 10, 1990. Accepted September 17, 1990.
1990 Volume 24, Numbers 1-12 Refer to the list below to determine in which issue an entry appears. January pp. 1-146 February pp. 147-268 March pp. 269-390 April pp. 391-590
May pp. 591-760 June pp. 761-930 July pp. 93 1-1 108 August pp. 1109-1270
September pp. 1271-1432 October pp. 1433-1602 November pp. 1603-1756 December pp. 1757-1894
Front Section index Critical Reviews Determining chemical toxicity to aquatic species, 284 (Diane J . W . Blum and Richard E . Speece) Features (condensed titles) Acid air and health, 946 (John D . Spengler, Michael Brauer, and Petros Koutrakis)
Air pollution; Clearing the air, 442 (Anthony D . Cortese) Bioconcentration, 1612 (Mace G. Barron) Data, treatment of less-than values, 1766 (Dennis R. Helsel) Emissions from motor vehicles, controlling, 1128 (Lester B. Lave, William E. Wecker, Winthrop S. Reis, and Duncan A . Ross) Environmental protection, 1442 (Renate D . Kimhrough) Environ. Sci. Technol., Vol. 24, No. 12, 1990
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