An Assessment of the Current EPA Guidance on Additional Extractions in Environmental Fate Studies

December 04, 2017

An Assessment of the Current EPA Guidance on Additional Extractions in Environmental Fate Studies
Kalumbu Malekani, MSc, Ph.D., North American Technical Director, Smithers Viscient

In September of 2014, the Environmental Fate and Effects Division (EFED) of the United States Environmental Protection Agency (US EPA) published a guidance for addressing unextracted residues in laboratory studies involving soils and sediments.  Since then, several registrants have conducted several soil and aquatic sediment laboratory studies with pesticides and pharmaceuticals following the recommendations in this guidance, i.e., using “optimized” extraction solvents.  The results generated in some of these studies were recently compiled and presented at the annual ACS meetings in Philadelphia (August 2016) and Washington, DC (August 2017).  These results all demonstrated that the additional extractions with less polar and non-polar solvents did not remove significant radioactivity (only a combined average of ≤5% of unextracted residues).

The EPA recommended an approach aimed at removing weakly sorbed residues using solvents of different polarities since laboratory degradation or photolysis studies primarily utilize polar extraction solvents such as acetonitrile, methanol, water, and an acid such as hydrochloric acid.  EFED’s objective is to extract all residues available for degradation so that dissipation can be accurately accounted for in exposure models.  They indicated in their guidance document that if a substantial component of the applied test substance remained in the soil or sediment following extraction (e.g., 10% or more), additional extractions with less polar and non-polar solvents should be used in addition to the initial polar solvents.   These solvents should be selected from each of three groups with a range of dielectric constants, namely, non-polar solvents (e.g., hexane, diethyl ether and others with dielectric constants from 1.9 to 4.8), less polar solvents (e.g., dichloromethane, ethyl acetate, THF and others with dielectric constants from 6.0 to 9.1) and more polar solvents (e.g., methanol, acetone, acetonitrile, water, formic acid and others with dielectric constants from 18 to 80).  However, Smithers Viscient and other laboratories found that if the initial extraction procedure involved polar solvents such as acetonitrile, water, and hydrochloric acid, the additional less polar and non-polar solvents did not remove appreciable soil/sediment residues.

Materials and Methods
Studies conducted since EFED promulgation of the new guidance consisted of 14 pesticides and four pharmaceutical API’s.  These compounds were studied in 47 aerobic soils, eight anaerobic soils, 13 aerobic aquatic sediments, 12 anaerobic sediments and two soil photolysis, for a total of 82 matrices.  The characteristics of the soils and sediments used varied with % organic carbon ranging from 0.4 to 14.2% and including the following USDA textural classes: Sandy Loam (13), Clay (4), Clay Loam (7), Loamy Sand (14), Loam (7), Silt Loam (15), Silty Clay Loam (4), Silty Clay (3), Sandy Clay Loam (4), Sandy Silt Loam (1) and Sand (10). 

The extraction procedure was generally performed as described below.  Initial solvent extractions included acetonitrile/water or acetonitrile/water/dil. acid or methanol/water/acid or acetone.  This was followed by less polar solvents, including dichloromethane (DCM) or tetrahydrofuran (TFH) or ethyl acetate.  The last set of extractions was with non-polar solvent chloroform, hexane, or toluene and, in certain cases, calcium chloride solution or a surfactant (Tween 80) were used.

These studies were performed in accordance with appropriate OPPTS and OECD guidelines using radiolabeled test compounds at application rates chosen to permit the determination of degradation kinetics and to readily monitor the formation and decline of major transformation products where possible.

Non-extractable residues (NER) occurring at ≥10% of applied radioactivity (% AR) were further assessed per the USEPA EFED guidance.

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 Example water-sediment test set-up
 
Results and Discussion
Results obtained in most of these studies were compiled and the following observations were made.  Additional extractions with less polar and non-polar solvents collectively removed ≤5% of applied radioactivity following polar solvent extractions.  As long as initial polar solvents could solubilize or remove test substances from the soils and sediments, the additional less polar and non-polar solvents did not remove appreciable amounts of radioactive residues.  This is because under aerobic conditions, microbial degradation would proceed via oxidative mechanisms to produce degradates that are more polar than parent compound.  Polar solvents are better able to remove polar compounds from soil and sediment, i.e., “like dissolves like.”  Even under anaerobic conditions, degradates are often more polar even though oxygen is not the terminal electron acceptor.  However, degradates can also be represented by reduction products which could be less polar than parent compound and more readily extracted with less polar solvents. 
 
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Conclusions and Recommendation

  • In this dataset (18 compounds in 82 soil/sediments studied in five kinds of environmental fate studies, both aerobic and anaerobic and soil photolysis), additional less polar and non-polar solvents did not remove appreciably more residues (<5% AR on average for the total extractions) following extractions using polar solvents.
  • Extractions using polar solvents, e.g., acetonitrile or methanol with water and acids, appear sufficiently capable of removing the large majority of weakly bound residues from soil or sediment.
  • The additional extractions currently prescribed by EFED only increase the scope of fate studies without improving the quality of the data generated.
  • Now that several studies have been performed and a significant database has been generated, the guidance may need to be evaluated and revised to provide some flexibility.  Instead of a prescribed approach, we recommend that use of less polar and non-polar extraction solvents be left to the discretion of the experimentalist.
 
 
References
US EPA-EFED 2014. Guidance for Addressing Unextracted Residues in Laboratory Studies.
 

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About The Author | Dr. Kalumbu Malekani

Dr. Kalumbu Malekani is the North American Technical Director. He holds an M.Sc. and Ph.D. in Environmental Soil Science and Applied Analytical Chemistry. He is a member of many scientific and professional associations and has authored numerous publications, technical reports, position papers, and dossiers for global registration submission. He served for five years as Smithers Viscient's Director of Environmental Fate and Metabolism, has over 20 years of experience in CRO, academia, and regulatory pesticide fate and transport research, and is recognized by clients and regulatory professionals as an industry spokesperson, strategist, and a skilled hands-on scientist.

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