796.2750—Sediment and soil adsorption isotherm.
(a) Introduction—
(1) Background and purpose.
The adsorption of chemicals to sediments and soils is an important process that affects a chemical's distribution in the environment. If a chemical is adsorbed to soil particles, it will remain on the soil surface and will not reach ground water. If a chemical is not adsorbed, it will leach through the soil profile and may reach ground waters and then surface waters. Similarly, if a chemical adsorbed to sediment, it will accumulate in the bed and suspended load of aquatic systems. If a chemical is not adsorbed to sediment, it will accumulate in the water column of aquatic systems. Information on the adsorption potential is needed under certain circumstances to assess the transport of chemicals in the environment. This section describes procedures that will enable sponsors to determine the adsorption isotherm of a chemical on sediments and soils.
(2) Definitions and units.
(i)
The “cation exchange capacity” (CEC) is the sum total of exchangeable cations that a sediment or soil can adsorb. The CEC is expressed in milliequivalents of negative charge per 100 grams (meq/100g) or milliequivalents of negative charge per gram (meq/g) of soil or sediment.
(ii)
“Clay mineral analysis” is the estimation or determination of the kinds of clay-size minerals and the amount present in a sediment or soil.
(iii)
“Organic matter” is the organic fraction of the sediment or soil; it includes plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by the microbial population.
(iv)
“Particle size analysis” is the determination of the various amounts of the different particle sizes in a sample (i.e., sand, silt, clay), usually by sedimentation, sieving, micrometry, or combinations of these methods. The names and diameter range commonly used in the United States are:
| Name | Diameter range |
|---|---|
| Very coarse sand | 2.0 to 1.0 mm |
| Coarse sand | 1.0 to 0.5 mm |
|
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|
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| Medium sand | 0.5 to 0.25 mm |
| Fine sand | 0.25 to 0.125 mm |
| Very fine sand | 0.125 to 0.062 mm |
| Silt | 0.062 to 0.002 mm |
| Clay | <0.002 mm |
(v)
The “pH” of a sediment or soil is the negative logarithm to the base ten of the hydrogen ion activity of the sediment or soil suspension. It is usually measured by a suitable sensing electrode coupled with a suitable reference electrode at a 1/1 solid/solution ratio by weight.
(vi)
The adsorption ratio, “Kd ,” is the amount of test chemical adsorbed by a sediment or soil (i.e., the solid phase) divided by the amount of test chemical in the solution phase, which is in equilibrium with the solid phase, at a fixed solid/solution ratio.
(vii)
“Sediment” is the unconsolidated inorganic and organic material that is suspended in and being transported by surface water, or has settled out and has deposited into beds.
(viii)
“Soil” is the unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. Its formation and properties are determined by various factors such as parent material, climate, macro- and microorganisms, topography, and time.
(ix)
“Soil aggregate” is the combination or arrangement of soil separates (sand, silt, clay) into secondary units. These units may be arranged in the soil profile in a distinctive characteristic pattern that can be classified according to size, shape, and degree of distinctness into classes, types, and grades.
(x)
“Soil classification” is the systematic arrangement of soils into groups or categories. Broad groupings are based on general soil characteristics while subdivisions are based on more detailed differences in specific properties. The soil classification system used in this standard and the one used today in the United States is the 7th Approximation-Comprehensive System. The ranking of subdivisions under this system is: Order, Suborder, Great group, family, and series.
(xi)
A “soil horizon” is a layer of soil approximately parallel to the land surface. Adjacent layers differ in physical, chemical, and biological properties such as color, structure, texture, consistency, kinds and numbers of organisms present, and degree of acidity or alkalinity.
(xii)
“Soil Order” is the broadest category of soil classification and is based on the general similarities of soil physical/chemical properties. The formation of soil by similar general genetic processes causes these similarities. The Soil Orders found in the United States are: Alfisol, Aridisol, Entisol, Histosol, Inceptisol, Mollisol, Oxisol, Spodosol, Ultisol, and Vertisol.
(xiii)
“Soil series” is the basic unit of soil classification and is a subdivision of a family. A series consists of soils that were developed under comparable climatic and vegetational conditions. The soils comprising a series are essentially alike in all major profile characteristics except for the texture of the “A” horizon (i.e., the surface layer of soil).
(xiv)
“Soil texture” is a classification of soils that is based on the relative proportions of the various soil separates present. The soil textural classes are: clay, sandy clay, silty clay, clay loam, silty clay loam, sandy clay loam, loam, silt loam, silt, sandy loam, loamy sand, and sand.
(3) Principle of the test method.
(i)
The extent of adsorption of a chemical onto sediment or soil is measured, using this test guideline, by equilibrating aqueous solutions containing different, but environmentally realistic, concentrations of the test chemical with a known quantity of sediment or soil. After equilibrium is reached, the distribution of the chemical between the water phase and the solid phase is quantitatively measured by a suitable analytical method. Then, sorption constants are calculated by using the Freundlich equation:
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where:
Ce=Equilibrium concentration of the chemical in the solution phase
Cs=Equilibrium concentration of the chemical in the solid phase
K=Freundlich adsorption coefficient
m=The mass of the solid in grams
l/n=Exponent where n is a constant
x=The mass in micrograms of the chemical adsorbed by m grams of solid.
Logarithmetic transformation of the Freundlich equation yields the following linear relationship:
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(ii)
In order to estimate the environmental movement of the test chemical, the values K and l/n are compared with the values of other chemicals whose behavior in soil and sediment systems is well-documented in scientific literature.
(iii)
The adsorption isotherm (AI) test has many desirable features. First, adsorption results are highly reproducible. The test provides excellent quantitative data readily amenable to statistical analyses. Also, it has relatively modest requirements for chemicals, soils, laboratory space, and equipment. It allows solution phase organic chemical determinations that are relatively uncomplicated. A chemical extraction-mass balance procedure to elicit information on chemical transformations occurring at colloid interfaces can be incorporated into this test. The ease of performing the isotherm test and mass balance will depend upon the physical/chemical properties of the test chemical and the availability of suitable analytical techniques to measure the chemical.
(iv)
The papers by Aharonson and Kafkafi (1975) under paragraph (d)(1) of this section, Harvey (1974) under paragraph (d)(3) of this section, Murray (1975) under paragraph (d)(4) of this section, Saltzman (1972) under paragraph (d)(5) of this section, Weber (1971) under paragraph (d)(6) of this section, and Wu (1975) under paragraph (d)(7) of this section served as the basis for this section. The soil and colloid chemistry literature and the analytical chemistry literature substantiate the experimental conditions and procedures specified in this guideline as accepted, standard procedures.
(4) Applicability and specificity.
The AI Test Guideline can be used to determine the soil and sediment adsorption potential of sparingly water soluble to infinitely soluble chemicals. In general, a chemical having a water solubility of less than 0.5 ppm need not be tested with soil as the solid phase, since the literature indicates that these chemicals are, in general, immobile in soils, see Goring and Hamaker (1972) under paragraph (d)(2) of this section. However, this does not preclude future soil adsorption/transformation testing of these chemicals if more refined data are needed for the assessment process.
(b) Test procedures—
(1) Test conditions—
(A) Equilibrating solutions that contain, besides the test chemical, 0.01M calcium nitrate dissolved in sterilized, distilled-deionized H2 O adjusted to neutral pH 7 by boiling to remove CO2.
(B)
Containers shall be composed of material that (1) adsorb negligible amounts of test chemical, and (2) withstand high speed centrifugation. The volume of the container is not a major consideration; however, it is extremely important that the amount of soil or sediment and the solid/solution ratio used in the study result in minimal container headspace. It is also extremely important that the containers be sterilized before use.
(G)
High speed temperature-controlled centrifuge capable of sedimenting particles greater than 0.5 micron from aqueous solution.
(iv) Soil pretreatment.
The following soil pretreatment steps shall be performed under the following conditions:
(C)
Eliminate microbial growth during the test period using a chemical or physical treatment that does not alter or minimally alters the soil surface properties.
(v) Sediment pretreatment.
The following sediment pretreatment steps shall be performed under the following conditions:
(A)
Decrease the H2 O content by air or oven-drying sediments at or below 50 °C. Sediments should not be dried completely and should remain moist at all times prior to testing and analysis.
(B)
Eliminate microbial growth during the test period by using a chemical and/or physical treatment that does not alter or minimally alters the colloid surface's properties.
(vi) Solid/solution ratio.
The solid/solution ratio shall be equal to or greater than 1/10. If possible, the ratios should be equal to or greater than 1/5. The sediment or soil dry weight after drying for a 24-hour minimum at 90 °C is recommended for use as the weight of the solid for ratio and data calculations. If an insufficient amount of chemical remains in the water phase for quantification, the solid/solution ratio should be adjusted so that measurable amounts of the test chemical remain in solution.
(vii) Equilibration time.
The equilibration time will depend upon the length of time needed for the parent chemical to attain an equilibrium distribution between the solid phase and the aqueous solution phase. The equilibration time shall be determined by the following procedure:
(A)
Equilibrate one solution containing a known concentration of the test chemical with the sediment or soil in a solid/solution ratio equal to or greater than 1/10 and preferably equal to or greater than 1/5. It is important that the concentration of the test chemical in the equilibrating solution (1) does not exceed one-half of its solubility and (2) should be 10 ppm or less at the end of the equilibration period.
(B)
Measure the concentration of the chemical in the solution phase at frequent intervals during the equilibration period.
(C)
Determine the equilibration time by plotting the measured concentration versus time of sampling; the equilibration time is the minimum period of time needed to establish a rate of change of solution concentration of 5 percent or less per 24 hours.
(viii) Centrifugation time.
Calculate the centrifugation time, tc, necessary to remove particles from solution greater than approximately 0.5 µm (5×10−5 µm) equivalent diameter (which represents all particles except the fine clay fraction) using the following equation:
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where:
tc=centrifuge time in minutes
R2=distance from centrifuge spindle to deposition surface of centrifuge
R1=distance from spindle to surface of the sample
N=number of revolutions of the centrifuge per minute.
(ix) Storage of solutions.
If the chemical analysis is delayed during the course of the experiment, store all solutions between 0 and 5 °C.
(x) Solvents for extraction.
It is important that the solvent used to extract the chemical from the sediment or soil is reagent grade or better. Solvents shall contain no impurities which could interfere with the determination of the test compound.
(2) Test procedure—
(i) Equilibration.
Add six solutions containing different concentrations of the test chemical to at least one gram of each solid. The initial concentration of the test chemical in these solutions will depend on the affinity the chemical has for the sediment or soil. Therefore, after equilibrium is attained, it is extremely important that the highest concentration of the test chemical in the equilibrating solution does not exceed 10 ppm, is at least one order of magnitude greater than the lowest concentration reported, and does not exceed one half of its solubility.
(A)
Immediately after the solutions are added to the solids, tightly cap the containers and vigorously agitate them for several minutes with a vortex mixture or similar device.
(B)
Shake the containers throughout the equilibration period at a rate that suspends all solids in the solution phase.
(ii) Centrifugation.
When the equilibration time has expired, centrifuge the containers for tc minutes.
(iii) Chemical extraction.
(A)
After centrifugation, remove the supernatant aqueous phase from the solid-solution mixture.
(iv) Chemical analysis.
Determine the amount of parent test chemical in the aqueous equilibrating solution and organic solvent extractions. Use any method or combination of methods suitable for the identification and quantitative detection of the parent test chemical.
(2)
Detailed description of the analytical technique(s) used in the chemical extraction, recovery, and quantitative analysis of the parent chemical.
(4)
Extent of adsorption by containers and the approach used to correct the data for adsorption by containers.
(5)
The individual observations, the mean values, and graphical plots of x/m as a function of Ce for each sediment or soil for (i) the equilibration time determination and (ii) the isotherm determination.
(7)
Soil information: Soil Order, series, texture, sampling location, horizon, general clay fraction mineralogy.
(9)
Sediment and soil physical-chemical properties: percent sand, silt, and clay (particle size analysis); percent organic matter; pH (1/1 solids/H2 O); and cation exchange capacity.
(10)
The procedures used to determine the physical/chemical properties listed under paragraphs (c) (7) through (9) of this section.
(d) References.
For additional background information on this test guideline the following references should be consulted:
(1)
Aharonson, N., Kafkafi, U. “Adsorption, mobility and persistence of thiabendazole and methyl 2-benzimidasole carbamate in soils,” Journal of Agricultural and Food Chemistry, 23:720-724 (1975).
(2)
Goring, C.A.I., Hamaker, J.W., (eds). Organic Chemicals in the Soil Environment. Vol. I & II (New York: Marcel Dekker, Inc., 1972).
(3)
Harvey, R.G. et al. “Soil adsorption and volatility of dinitroaniline herbicides,” Weed Science, 22:120-124 (1974).
(4)
Murray, D.S. et al. “Comparative adsorption, desorption, and mobility of dipropetryn and prometryn in soil,” Journal of Agricultural and Food Chemistry, 23:578-581 (1973).
(5)
Saltzman, S.L. et al. “Adsorption, desorption of parathion as affected by soil organic matter,” Journal of Agricultural and Food Chemistry, 20:1224-1226 (1972).
(6)
Weber, J.B. “Model soil system, herbicide leaching, and sorption,” Weed Science, 19:145-160 (1971).
(7)
Wu, C.H., et al. “Napropamide adsorption, desorption, and movement in soils,” Weed Science, 23:454-457 (1975).