Sorption by Soil and Bed Sediment
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
Rapid 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.
mineral matter versus soil organic matter
Adsorption versus partition
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.
the distribution of contaminants
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.
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.
objective and approach
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.
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.
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
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.
Koc data may serve as an effective sensor
and environmental applications
-C. T. Chiou and D. E. Kile
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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.
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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.
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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
U.S. Geological Survey,