[Code of Federal Regulations]
[Title 40, Volume 31]
[Revised as of July 1, 2006]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR796.2750]
[Page 87-92]
TITLE 40--PROTECTION OF ENVIRONMENT
CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
PART 796_CHEMICAL FATE TESTING GUIDELINES--Table of Contents
Subpart C_Transport Processes
Sec. 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
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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
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
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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:
Equation 1
x/m=Cs=KCel/n
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:
Equation 2
log Cs=log K+(l/n) log Ce
(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--(i) Special laboratory
equipment. (A) Equilibrating solutions that contain, besides the test
chemical, 0.01M calcium nitrate dissolved in sterilized, distilled-
deionized H2O 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.
(C) A 150 micron (100 mesh) stainless-steel or brass sieve.
(D) Drying oven, with circulating air, that can attain 100 [deg]C.
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(E) Vortex mixer or a comparable device.
(F) Rotary shaker or a comparable device.
(G) High speed temperature-controlled centrifuge capable of
sedimenting particles greater than 0.5 micron from aqueous solution.
(ii) Temperature. The test procedure shall be performed at 23 5 [deg]C.
(iii) Replications. Three replications of the experimental
treatments shall be used.
(iv) Soil pretreatment. The following soil pretreatment steps shall
be performed under the following conditions:
(A) Decrease the water content, air or oven-dry soils at or below 50
[deg]C.
(B) Reduce aggregate size before and during sieving, crush and grind
dried soil very gently.
(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.
(D) Sieve soils with a 100 mesh stainless-steel or brass sieve.
(E) Store all solutions and soils at temperatures between 0 and 5
[deg]C.
(v) Sediment pretreatment. The following sediment pretreatment steps
shall be performed under the following conditions:
(A) Decrease the H2O content by air or oven-drying
sediments at or below 50 [deg]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.
(C) Store at temperatures between 0 and 5[deg]C.
(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 [deg]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 [micro]m (5x10-5 [micro]m) equivalent
diameter (which represents all particles except the fine clay fraction)
using the following equation:
Equation 3
tc(min)=1.41x10\9\ [log(R2/R1)]/N\2\
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
[deg]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
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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.
(B) Extract the chemical adsorbed on the sediment or soil colloid
surfaces with solvent.
(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.
(c) Reporting. Report the following information:
(1) Temperature at which the test was conducted.
(2) Detailed description of the analytical technique(s) used in the
chemical extraction, recovery, and quantitative analysis of the parent
chemical.
(3) Amount of parent test chemical applied, the amount recovered,
and the percent recovered.
(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.
(6) The quantities K, n, and l/n.
(7) Soil information: Soil Order, series, texture, sampling
location, horizon, general clay fraction mineralogy.
(8) Sediment information: sampling location, 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/H2O); 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).
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(7) Wu, C.H., et al. ``Napropamide adsorption, desorption, and
movement in soils,'' Weed Science, 23:454-457 (1975).
[50 FR 39252, Sept. 27, 1985, as amended at 52 FR 19058, May 20, 1987;
54 FR 29715, July 14, 1989]