The soil sorption coefficient Kd and the soil organic carbon sorption coefficient KOC of pesticides are basic parameters used by environmental scientists and regulatory agencies worldwide in describing the environmental fate and behavior of pesticides. They are a measure of the strength of sorption of pesticides to soils and other geosorbent surfaces at the water/solid interface, and are thus directly related to both environmental mobility and persistence. KOC is regarded as a 'universal' parameter related to the hydrophobicity of the pesticide molecule, which applies to a given pesticide in all soils. This assumption is known to be inexact, but it is used in this way in modeling and estimating risk for pesticide leaching and runoff. In this report we examine the theory, uses, measurement or estimation, limitations and reliability of these parameters and provide some 'rules of thumb' for the use of these parameters in describing the behavior and fate of pesticides in the environment, especially in analysis by modeling.

Pesticide soil sorption parameters: theory, measurement, uses, limitations and reliability.

Phenanthrene sorption and desorption equilibria were measured for 10 natural sorbents having significantly different geological ages and organic matter compositions. Three geologically young peats, one humic acid, three geologically old shales, and samples of kerogen isolated from each of the shales were examined. Elemental analyses and solid-state 13C-NMR spectra reveal that the oxygen/carbon (O/C) atomic ratios of the soil organic matter (SOM) as sociated with the samples decrease with increased age and, thus, apparently with diagenetic alteration. The sorption affinities of these materials for phenanthrene as well as their respective isotherm nonlinearities and hysteretic behaviors were found to correlate inversely with the O/C atomic ratio; samples containing more physically condensed and chemically reduced SOM matrices exhibited greater solute affinity, more nonlinear sorption equilibria, and more pronounced hysteresis. Observed relationships between the chemical and structural characteristics of ass...

A Distributed Reactivity Model for Sorption by Soils and Sediments. 10. Relationships between Desorption, Hysteresis, and the Chemical Characteristics of Organic Domains.

Sorption Dynamics of Organic Compounds in Soils and Sediments

Retention is one of the key processes affecting the fate of organic chemicals in the soil-water environment. Retention refers to the ability of the soil to hold a pesticide or other organic molecule and to prevent the molecule from moving either within or outside of the soil matrix. As such, retention refers primarily to the adsorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. Retention controls, and is subsequently controlled, by chemical and biological transformation processes. Retention strongly influences chemical transport to the atmosphere, groundwater, and surface waters. Not surprisingly, retention is a primary factor influencing the efficacy of soil-applied pesticides. The literature abounds with references on the retention of pesticides in soils (e.g., Bailey & White, 1964; Hamaker & Thompson, 1972; Green, 1974; Weed & Weber, 1974; Calvet, 1980). Adsorption is defined as the accumulation of a pesticide or other organic molecule at either the soil-water or the soil-air interface. Adsorption is often used to refer to a reversible process involving the attraction of a chemical to the soil particle surface and retention of the chemical on the surface for a time that depends on the affinity of the chemical for the surface. The distinction between true adsorption in which molecular layers form on a soil particle surface, precipitation in which either a separate solid phase forms on solid surfaces or covalent bonding with the soil particle surface occurs, and absorption into soil particles and organisms is difficult. In practice, adsorption is usually determined only by chemical loss from solution, thus adsorption is often replaced by the more general term, sorption. Sorption refers to a general retention process with no distinction between the specific processes of adsorption, absorption, and precipitation. The individual retention processes are highly complex. This complexity is primarily the result of soil heterogeneity and the soil's contiguity with biological, atmospheric, and water systems. Therefore, one of the keys to understanding the mechanisms of the retention process is the composition of the soil matrix.

The Retention Process: Mechanisms

Since the publication of the original compilation of the pesticide parameter database as the entire Volume 123 of Reviews of Environmental Contamination and Toxicology (199), considerable interest, both domestic and foreign, has been shown in this unique compilation. This is likely the result of increased incidence of detections of pesticides in groundwater. While the original intent was for use in screening procedures that could lead to improved pesticide stewardship in the agricultural arena, the database has taken on a much wider role, including remediation assessments, environmental audits, and more in-depth assessments of the uncertainty of environmental risks of pesticide contamination of ground and surface water. These uses have prompted questions about pesticide products that are no longer on the market but are still found in soil and aqueous environments.

The SCS/ARS/CES Pesticide Properties Database for Environmental Decision-Making. II. Additional Compounds

The adsorption/desorption potential of several phenylurea herbicides was studied on representative prairie soils, using slurry-type adsorption experiments. The herbicides showed the following order of increasing tendency to be adsorbed: fenuron < monuron monolinuron < metobromuron < diuron linuron chlorbromuron. The relative adsorption of each herbicide on various soil types was significantly correlated with the soil organic matter content, but not with the clay content. The relationship between the k value and the soil organic matter content was found to be of the exponential type. The extent of adsorption was also inversely related to the order of their water solubilities. The urea herbicides were readily desorbed by water from the low to medium organic matter content sandy loam and heavy clay soils, but not from a loam with very high organic matter content. It is suggested that the relative adsorption/desorption potential of a herbicide may provide a mechanism by which soil applied herbicides can be bi...

R. Grover

Papers

Adsorption and desorption of urea herbicides on soils