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Contaminant Sorption by Soil and Bed Sediment
Is There a Difference ?

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Chiou, C.T., and Kile, D.E., 2000, Contaminant sorption by soil and bed sediment--Is there a difference?: U.S. Geological Survey Fact Sheet 087-00, 4 p.

Table of Contents

  Soil mineral matter versus soil organic matter
Calculating the distribution of contaminants
Study objective and approach
Study results
Implications and environmental applications

core sampling in soil and bed sedimentRapid industrialization since the mid-19th century has produced a large quantity and a wide variety of chemical wastes. Many relatively non-biodegradable chemicals originating from these wastes have spread throughout the environment, into water, soil and sediment. These chemicals may persist for indefinite periods, depending on their chemical properties and their interactions with the environment. Some of these chemicals are known to be harmful to humans and other species, either by direct exposure or by intake of contaminated water and food. Soils and sediments are important "sinks" for such contaminants because of their enormous quantities and their abilities to pick up, or sorb, large amounts of a wide variety of contaminants. It is essential to understand the mechanism by which the contaminant is sorbed to soil and sediment.

Sorption to soils and sediments is probably the most influential factor on the transport and fate of organic contaminants in the environment. The extent of the sorption to soil and sediment affects not only the contaminant level in an ecosystem, but the movement and fate of the contaminant as well. For example, in a hydrogeologic system, the increased sorption of a contaminant to soil and sediment reduces its level in the adjacent water column, and thus decreases its exposure and transport to other parts of the ecosystem, such as fish and plankton.

Soil mineral matter versus soil organic matter
The way in which contaminants are sorbed into soil or sediment varies with the nature of the contaminant and the makeup of the soil and sediment (Chiou and others, 1979; Karickhoff and others, 1979). The composition of soil and sediment includes both mineral matter and organic matter as the primary constituents. Under relatively dry conditions, the soil/sediment mineral matter acts as an adsorbent, where the sorbed organic compounds are held on the surface of the mineral grains. The soil/sediment organic matter (SOM) acts as an absorbent, or a partition medium, where the sorbed organic compounds dissolve (partition) into the matrix of the entire SOM. The soil or sediment, then, is characterized as a dual-function sorbent, in which the mineral matter sorbs the contaminant by adsorption while the SOM sorbs the contaminant by a partition process (Chiou and others, 1983; Chiou and Shoup, 1985; Chiou and others, 1985).

Adsorption versus partition process

absorption diagram

In the presence of water with many contaminants, water is adsorbed on the surface of mineral matter, whereas, contaminants are absorbed into the organic matter by a partition process.

Consider a natural water system with many organic contaminants present. Adsorption to soil/sediment mineral matter occurs as a consequence of the competition between all species, including water. In the presence of water, the soil/sediment mineral matter prefers to adsorb water because of their similar molecular polarities, while the soil organic matter prefers to absorb the contaminants (organic solutes) in water. This means that the (nonionic) organic contaminants are not significantly adsorbed to minerals, and that the partition of a contaminant is not affected by water or by other contaminants. So two processes are at work: (1) the organic contaminants are competitively prevented by water from adhering to the surface of the soil mineral matter, while at the same time, (2) the organic contaminants are able to partition independently into the SOM. Because so many environmental contaminants are transported by ground water and surface water, it is important to understand the unique function of the soil organic matter within these aquatic systems and how the partition processes affect the fate of common environmental contaminants.

Calculating the distribution of contaminants
In aquatic systems, a linear relationship exists between the concentration of a contaminant solute in soil/sediment and the concentration of the contaminant in water at equilibrium. In other words, when the concentration of a contaminant in water is increased, the concentration of that contaminant in the soil/sediment will also increase by a constant factor. This linear relationship is expressed mathematically via the solute distribution coefficient, which is the ratio of the solute concentration in soil (Cs) to the solute concentration in water (Cw).

Kd
 =  Cs
Cw

Kd is the solute distribution coefficient, Cs is the solute concentration in soil/sediment, and Cw is the solute concentration in water. Knowing the concentration of a contaminant in one compartment, either water or soil/sediment, thus allows scientists to predict the concentration of a contaminant in the other compartment. Because the contaminant sorption occurs predominantly by partition into the soil organic matter, it is more useful to express the distribution coefficient in terms of the SOM content.

Koc
 =  Kd
foc

In this equation, Koc is the partition coefficient normalized to the organic carbon of the soil/sediment, and foc is the organic carbon fraction of the soil (or sediment). By normalizing the partition coefficient to the soil organic matter (organic carbon) content, scientists can then compare the relative partition properties of the organic matter from different geographic sources. Any variation between soils (or sediments) from different geographic sources can then be attributed to the variation in SOM properties.

Study objective and approach
To what extent do the soil organic matter properties differ among soils and sediments of diverse geographic origins? If it is found that the contaminant sorption to organic material in different soils or sediments varies widely, then it would be necessary to study each soil individually. This would be a time-consuming process. On the other hand, if the contaminant sorption to the organic material in different soils or sediments shows only minor variation, then contaminant sorption could be quantified collectively, at a great savings of time and effort. A study was undertaken by the U. S. Geological Survey to provide the answer by comparing the organic-carbon normalized partition coefficients (Koc)of selected organic contaminants on a large number of soils and sediments (Kile and others, 1995).

Koc
Soil Samples
%OC
CT
DCB
1. Oliver Co., N. Dakota (USEPA*reference soil 3)
1. 43
53
277
2. West-central Iowa (USEPA reference soil 10)
2. 04
58
248
3. Manchester, Ohio (USEPA reference soil 12)
2. 25
57
230
4. Anoka, Minnesota
1. 08
61
261
5. Spinks soil, East Lansing, Michigan
1. 03
65
318
6. Woodburn soil, Corvallis, Oregon
1. 26
65
296
7. Anda, Heilongjiang, China
2. 83
67
288
8. Nanjing, Jiangsu, China
1. 08
61
236
9. Gangcha, Qinghai, China
1. 12
62
295
10. Luochuan, Shanxi, China
0. 46
66
315
11. Xuwen, Guangdong, China
0. 64
55
257
12. Qiongzhong, Hainan, China
0. 34
62
304


Bed-sediment samples
%OC
CT
DCB
1. Isaacs Creek at Ohio River, Ripley, Ohio
(USEPA reference sediment 11)
1. 50
66
301
2. Illinois River, near Lacon, Illinois
(USEPA reference sediment 22)
2. 20
116
572
3. Mississippi River, Helena, Arkansas
1. 60
109
534
4. Mississippi River, St. Francisville, Louisiana
0. 40
119
549
5. Lake Charles, adjacent to the Calcasieu
River, Lake Charles, Louisiana
1. 97
112
536
6. Marine sediment from Suisin Bay, northern
San Francisco Bay, California, site 416
1. 48
107
532
7. Tumen River, Helong, Jiling, China
1. 99
93
420
8. Xuanwu Lake, Nanjing, Jiangsu, China
4. 12
103
557
9. Zhujiang River, Guangzhou, Guangdong, China
3. 37
95
545
10. Yellow River, Zhengzhou, Henan, China
0. 11
112
589
11. Sangonghe River, Fukang, Xinjiang, China
0. 38
103
499
12. Lake Pumo, Langkazi, Tibet, China
1. 94
101
539
*USEPA -- United States Environmental Protection Agency
This table is a partial list of the Koc values calculated from the soil/sediment samples. They are plotted on the figure below
%OC = Percent of soil/sediment organic carbon content
Koc = Measured partition coefficient
CT = carbon tetrachloride
DCB = 1, 2-dichlorobenzene

Sampling

The organic carbon partition coefficient (Koc)values were measured for carbon tetrachloride (CT) and 1, 2-dichlorobenzene (DCB), on 32 soil samples from widely diverse geographic regions in the United States and People 's Republic of China, and 36 bed-sediment samples from widely diverse aquatic systems within both countries. All U. S. soils and some Chinese soils were taken from the soil layer (or zone) that lies near the surface and is characterized to be the zone of maximum water leaching. All other Chinese soils were taken from depths about 1 meter below the land surface to minimize the impact of agricultural practice. The bed-sediment samples were taken from the top 0 -20 centimeters of the sediment surface. Sediment samples include those from rivers, freshwater lakes, and marine bays or harbors. Soil and sediment samples were dried, ground, and homogenized to pass a 35-mesh sieve (U. S. samples) or a 200-mesh sieve (Chinese samples).

Five river-suspended solids were also collected for the sorption experiments. Suspended solids were collected in June 1989 from the Illinois River at Hardin, Illinois, during a low-to-normal river flow and from the Missouri River at Herman, Missouri, during a moderately high flow. Suspended solids from the Mississippi River at Thebes, Illinois, and St. Louis, Missouri, were collected during a high river flow in June 1990. The collected water was processed by filtration through a 63-micrometer sieve to remove the sand fraction, followed by continuous-flow centrifugation.

The suspended solid from the Yellow River, near Zhengzhou in Henan Province, People's Republic of China, was collected in August 1991 during the high-flow season from a depth of 0. 5 meters below the water surface. The water samples were pooled, and the suspended solids were separated by gravitational settling over night.

Koc plots

These plots show the Koc values of CT and DCB on the studied soils and bed sediments.

Koc values of both DCB and CT from most soils do not vary much between regions, which suggests similar properties for relatively uncontaminated soils.

Higher Koc values of DCB and CT in bed sediments suggests that the process that turns soils into bed sediments results in a change of the organic properties.

The variation in Koc within bed sediments reflects the extent of conversion of soils to bed sediments -the older sediments have higher Koc values.

Study results

isokinetic sampler

A noncontaminating Isokinetic sampler (made entirely of Teflon.) was used to collect samples of bed sediment, water, and suspended sediment.

The finding that Koc's for DCB are generally about five times the Koc's for CT on all soil and bed-sediment samples is consistent with the difference in water solubility of CT (800 milligrams per liter)and DCB (154 milligrams per liter), which is also approximately a five-fold difference, and with the similarity of their solubilities in soil organic matter (Rutherford and others, 1992).

The high degree of invariance of the Koc values of CT and DCB between most soils or between most bed sediments is striking, since these samples came from widely dispersed locations in the United States and the People 's Republic of China. This invariance suggests that the properties of the soil or sediment organic matter that control nonpolar solute solubility are quite similar for a wide variety of uncontaminated shallow soils and also likely for relatively pristine surfocial bed sediments. It appears that there may not be much variability in the soil organic matter polarity and composition between soils of relatively shallow depths from diverse geographic locations. This speculation will be further tested.

The fact that most soil Koc's are distinct from bed sediment Koc's suggests that the process that turns eroded soils into bed sediments brings about a noticeable change in the property of the organic constituent. A possible cause for this change is that the sedimentation process fractionates soil organic constituents such that the more polar and more water-soluble organic components in soil organic matter are separated out to form dissolved organic matter and colloids in water, and hence the less polar organic constituents in soils are preserved in the bed sediment. The time scale required to bring about a complete soil-to-sediment conversion should depend, among other factors, on river depth and flow dynamics.

Koc
Suspended-soil samples
%OC
CT
DCB
1. Mississippi River, Thebes, Illinois
1. 82
60
296
2. Mississippi River, St. Louis, Missouri 1. 78 58 283
3. Illinois River, Hardin, Illinois 2. 60 89 423
4. Missouri River, Herman, Missouri 2. 87 49 231
5. Yellow River, Zhengzhou,
Henan, China
0. 38 63 300
This table is a list of the Koc values for river- suspended solids. Koc values can serve as an indicator of the source of suspended soils. For example, Koc values for CT and DCB for the rivers at high water, such as the Yellow and Mississippi, are typical of those for soils.

Part of the variation in Koc within bed sediments may reflect the extent of conversion of the eroded soils to bed sediments. Recently eroded soil retains most of its soil organic composition and has significantly lower Koc values. The difference between soil and bed-sediment Koc values as detected by relatively nonpolar solutes provides a basis for identifying the source of suspended solids in rivers. For instance, the Koc values for CT and DCB in the suspended solids for the rivers sampled at high water are typical of those for soils, whereas the Koc values for CT and DCB in the suspended solids for the one river sampled at low-to-normal flow are more representative of bed sediments. The assumption can be made that the suspended solids from high-water flows consist mainly of newly eroded soil, and the suspended solids from low-to-normal flows consist largely of resuspended bed sediment. Thus, sorption data serves as a simple indicator of the source and time history of the suspended solids.

Comparison with contaminated sites

soil sample from Bimidje

This soil sample was taken from Bimidji, Minnesota

Marine bed sediment samples were collected from Fort Point Channel of Boston Harbor, which is known to be severely contaminated by hydrocarbons. Bed sediments were collected from the Bayou d 'Inde, which drains industrial wastewaters into the Calcasieu River downstream from Lake Charles, Louisiana, and is contaminated by chlorinated hydrocarbons. From Bemidji, Minnesota, soil was collected from an oil-spill site. In comparison with In comparison with the Koc values with normal soils and sediments, the Bayou d 'Inde sediments yield noticeably higher Koc values, and the sediment from Fort Point Channel of Boston Harbor and the soil from Bemidji exhibit exceptionally high Koc values, which are 5 -10 times the values for uncontaminated soils and sediments. A similar effect was reported by Sun and Boyd (1990), who noted that nonpolar solutes exhibit unusually high Koc values on soils contaminated by petroleum and/or polychlorinated biphenyl (PCB) oils. The sorption data may serve as an effective sensor for relatively high levels of contamination in soils and sediments.

The Koc data may serve as an effective sensor
for contamination in soils and sediments

Implications and environmental applications
The Koc data of both CT and DCB on most normal soils from shallow depths are shown to be quite invariant, suggesting that soil organic matter at such depths from diverse geographic sources maintains a comparable polar-to-nonpolar balance and possibly a comparable composition. However, soil samples are genuinely different from bed-sediment samples in terms of their Koc values. The average Koc values for nonpolar solutes on bed sediments are about twice those on soils, suggesting that sediment organic matter is in general less polar in nature than soil organic matter. This unique geochemical disparity has not been documented until now. The observed difference in Koc between the soil and bed sediment samples must be taken into account in future studies. Because it has now been shown that Koc values for soils are largely consistent worldwide, just as Koc values for sediments are largely consistent worldwide, Koc values can be used to assess the sorption of nonpolar contaminants to the organic matter of different soils/ sediments. This finding should result in considerable savings in cost and time for contamination studies.

-C. T. Chiou and D. E. Kile


REFERENCES

Chiou, C.T., Peters, L.J., and Freed, V.H., 1979, A physical concept of soil-water equilibria for nonionic organic compounds: Science, v.206, p.831-832.

Chiou, C.T., Porter, P.E., and Schmedding, D.W., 1983, Partition equilibria of nonionic organic com- pounds between soil organic matter and water: Environmental Science and Technology, v.17, p.227-231.

Chiou, C.T., and Shoup, T.D., 1985, Soil sorption of organic vapors and effects of humidity on sorption mechanism and capacity:Environmental Science and Technology, v.19, p.1196-1200.

Chiou, C.T., Shoup, T.D., and Porter, P.E., 1985, Mechanistic roles of soil humus and minerals in the sorption of nonionic organic compounds from aqueous and organic solutions: Organic Geochemistry, v.8, p.9-14.

Karickhoff, S.W., Brown, D.S., and Scott, T.A., 1979, Sorption of hydrophobic organic pollutants on natural sediments: Water Research, v.13, p.241-248.

Kile, D.E., Chiou, C.T., Zhou, H., Li, H., and Xu, O., 1995, Partition of nonpolar organic pollutants from water to soil and sediment organic matters: Environmental Science and Technology, v.29, p.1401-1406.

Rutherford, D.W., Chiou, C.T., and Kile, D.E., 1992, In .uence of soil organic matter composition on the partition of organic compounds: Environmental Science and Technology, v.26, p.336-340.

Sun, S., and Boyd, S.A., 1990, Residual petroleum and polychlorobiphenyl oils as sorptive phases for organic contaminants in soils: Environmental Science and Technology, v.24,p.142.

For further information contact:
Cary T.Chiou or Daniel E.Kile

U.S. Geological Survey,
Box 25046, MS 408
Denver Federal Center
Denver, CO 80225

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