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Interactions are complex between pesticides and soils and sediments. While other governmental agencies have documented their policies, state of knowledge, or both regarding unextracted residues, a global agreement on the issues has not been achieved; 11 therefore, in the absence of a more global approach, this guidance document provides a procedural approach for EFED scientists.
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Fate, Transport and Transformation Test Guidelines. OPPTS October Last accessed May 28, EPA C Evaluating unextracted residues involves best professional judgment.
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This procedure includes steps intended to maximize the robust and consistent characterization of unextracted residues in exposure and risk assessments. A compound's residues of concern should be clearly defined before using this procedure. If the parent compound is the only residue of concern and the pattern of formation of unextracted residues clearly does not correlate with the pattern of decline of the test compound, then the unextracted residues need not be considered.
If the parent compound is sufficiently persistent so that expected i. If the unextracted residues do not need to be considered, then they are not residues of concern, and the procedure stops here; otherwise, proceed to Step 4. Evaluate whether the extraction method is adequate. Adequate extractions of organic compounds involve use or exploration of multiple solvents with different chemical properties e.
If the extraction method is adequate, then the unextracted residues are considered to be strongly sorbed to the soil or sediment and are not considered residues of concern. For example, solvent systems for the extraction of ionic compounds should include polar solvents.
Solvent systems for the extraction of neutral organic compounds should include non-polar solvents. Combinations of solvents, including a weak acid or weak base, may enhance the extraction efficiency. Addition of ammonium carbonate, harsh acidic or alkaline extraction, and use of elevated temperature, elevated pressure, vigorous shaking, sonication, and Soxhlet extraction do not substitute for the exploration of alternate polar and non-polar solvents.
However, use of sonication or Soxhlet extraction with multiple solvents ranging in relative polarity is not discouraged. In general, extractions should not chemically alter the test compound or its transformation products e. Some example polar solvents with dielectric constants ranging from 18 to 80 at environmental temperatures include water, formic acid, methanol, ethanol, isopropanol, acetone, acetonitrile, and dimethyl sulfoxide Honeywell, ; LSU, Some example polar solvents with lower dielectric constants ranging from 6.
Example nonpolar solvents include hexane, benzene, toluene, 1,4- dioxane, chloroform, and diethyl ether dielectric constants range from 1. Judgment should be used in the choice of solvents since factors other than dielectric constant may be important.
For example, emulsions are useful for compounds such as antibiotics that do not dissolve well in other solvent systems. Generally, unless there is a reason for a different approach, at least one solvent from each of the three groups identified above by range of dielectric constant should be used when there are a high proportion of unextracted residues i.
Also, the solvent system pH should be adjusted to maximize recovery of compounds known to exhibit acid-base behavior. Solvent systems used in other laboratory studies on the compound should be reviewed for effectiveness, although results can vary across different soils and sediments. Also, enforcement methods that convert analytes to a common moiety are not likely to involve extraction solvents useful for laboratory studies.
In addition, if extraction recoveries are not quantitative or are highly variable at the zero time point, the extraction solvent system may not have been effective at dissolving the parent compound or the extraction procedure may not have been conducted in a reliable manner. If the extraction method or its development can be considered adequate, the unextracted residues are not likely to have been available for rapid desorption, and may have been bound into organic matter.
Therefore, when the extraction procedure is considered adequate, the unextracted residues are considered strongly sorbed and are not residues of concern from a regulatory perspective; those residues represent a sink from a degradation kinetics perspective. If this is the case, the procedure stops here. If the extraction method or its development cannot be considered adequate, or it is unclear, then proceed to Step 4. Consider the following additional lines of evidence, which may include the open literature, to determine whether unextracted residues may be considered weakly or strongly sorbed and, therefore, whether or not they may be considered potential residues of concern.
Substantial formation of unextracted residues in other laboratory studies on the compound in which solvents with different dielectric constants are used may indicate that the unextracted residues can be considered strongly sorbed, especially if the solvents are targeting different transformation products that form in different conditions. However, if studies using solvents with different dielectric constants result in substantially different amounts of unextracted residues, it may indicate that either.
Use of additional solvent systems at different intervals of a single study can be an effective practice to keep unextracted residues low. An effective solvent for the test compound may not be effective for degradates that form later in the study. However, solvent systems used at initial sampling intervals should not be discontinued during a study. Also, care should be taken that additional solvents do not alter the test compound residues.
If additional solvents result in unextracted residue amounts substantially lower than in previous sampling intervals, the study should be repeated with the initial and additional solvents used throughout the study, from the initial to final sampling events. A decreasing trend in unextracted residues may indicate that the extraction solvent system is not effective for all of the dissolved and weakly sorbed residues. Similar compounds with evidence of covalent bonding to soil or sediment can be used to conclude that unextracted residues are likely not a residue of concern.
For example, chlorinated anilines such as propanil CAS No. However, differences between similar compounds, such as degree of chlorination or the addition of a functional group such as an amide, may affect the relative levels of unextracted residues that occur Roberts et al. Also, unextracted residues of 4-chloroaniline CAS No.
Further, soil-bound aniline has been observed to partially desorb due to anaerobic microbial activity Kosson and Byrne, Whereas, brominated and non-brominated bisphenol-A, which bind to soil in anoxic conditions, desorbed when the redox environment returned to oxic conditions Liu et al. Therefore, this potentially useful line of evidence of covalent bonding should be used with caution, depending on the available evidence regarding irreversible binding to soil, structural similarity, and degree of chlorination of similar compounds.