ASTM E1195-01(2008)

Sorption data are useful for evaluating the migratory tendency of chemicals into the air, water, and soil compartments of our environment. They can be used in the prediction or estimation of volatility from water and soil, concentration in water, leaching through the soil profile, run-off from land surfaces into natural waters, and biological availability. Additional information concerning testing to determine sorption coefficients can be found in OECD Test Guideline 106 (7).

This test method assumes that sorption of at least nonpolar organic chemicals is mainly influenced by the organic matter of the soil or sediment solids. There is ample evidence in the literature to support this assumption, and the user of this test method should refer to Ref. (2) for more information on this subject. Organic carbon content is chosen as the basis for sorption instead of organic matter content. This is because organic carbon values generally are measured directly by analytical methods. Organic matter may be estimated by multiplication of the organic carbon values by a somewhat arbitrary constant of 1.7 (3). This test method is based on the assumption that all of the material sorbed to the solids is reversibly bound. The analyses described herein assume equilibrium between the liquid and solid concentrations of the test compound. In some cases, there may be a fraction of the compound that is irreversibly bound to the solids. For these cases, the measurements made by the test may not reflect a true “equilibrium”. The irreversible sorption phenomena has been extensively documented and the reader is referred to (9), (10) and (8) for more discussion on this topic.

A sorption constant is obtained and is essentially independent of soil properties other than organic carbon. This value is useful because, once it is determined, the sorption distribution characteristics for any solid can be estimated based on its organic carbon content.

This test method is designed to evaluate sorption at environmentally relevant concentrations as a function of organic carbon content of different soil and sediment solids. Therefore, the number of different solids is emphasized in the procedure rather than the number of chemical concentrations studied with each solid. In general, one concentration is employed since the test method assumes that at low solution concentrations, sorption isotherms approximate linearity and sorbed concentrations do not exceed typical environmental loading. Errors arising from concentration effects at low environmental concentrations usually are less than the variation existing between different solids, when dealing with sorption trends in a general manner. Therefore, the initial concentration of the test chemical in solution should not exceed 0.5 of its water solubility.

As an option, a procedure is given for determining concentration effects on sorption. This is because high concentrations may be present in certain environmental situations; such as landfills and spills. This procedure should be done at four concentrations over a hundred fold concentration range (for example, 0.1, 0.5, 2, and 10 ppm initial solution concentration). If low solubility presents analytical difficulties, solution concentrations should range over at least one order of magnitude. The Freundlich equation is an appropriate expression of these effects:

A log plot of the Freundlich equation yields the following linear relationship:

1.1 This test method describes a procedure for determining the partitioning of organic chemicals between water and soil or sediment. The goal is to obtain a single value which can be used to predict partitioning under a variety of environmental conditions from the measurement of sorption coefficients for specific solids.

1.2 Sorption represents the binding process of chemicals to surfaces of soils or sediments through chemical, or physical, or both interactions.

1.3 The sorption of nonpolar organic chemicals, and to some extent polar organic chemicals, is correlated with the organic carbon content of the sorbing solid. Charged inorganic and organic molecules may behave differently, and some other property, such as, cation exchange capacity, clay content, or total surface area of sorbing solids, may influence sorption. Hydrous metal oxides of iron and aluminum may significantly affect sorption in sediments. In order to provide a sorption coefficient that is useful for a wide range of soils and sediments, the coefficient is based on organic carbon content. This approach, however, will not apply to all chemicals or all soils and sediments. In cases where it does not apply, the investigator may need to seek other methods of relating sorption to the properties of the chemical, soil, or sediment.

1.4 It is possible that, in addition to organic carbon, sorption is correlated with the total surface area of sorbing solids. This may be particularly important with solids having organic carbon contents so low that sorption to inorganic surfaces is significant in comparison to sorption by organic material. In such a case, inclusion of the total surface area into the sorption calculation may be useful. For further information on this subject see Ref (1).

1.5 Equilibrium sorption coefficients are determined. It is recognized that equilibrium conditions do not always exist in environmental situations, but sorption equilibria values are necessary for making generalizations about environmental partitioning.

1.6 Studies are conducted preferably with an analytical or technical-grade chemical. Mixtures are used only if analytical methods allow measurement of individual components of interest in the mixture. Good laboratory procedures must be followed to ensure validity of the data.

1.7 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.