Fate And Prediction Of Environmental Chemicals In Soils, Plants, And Aquatic Systems

Mohammed Mansour

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315892900
• eBook ISBN: 9781351088909
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Agriculture
• Index: Technology & Engineering

Fate and Prediction of Environmental Chemicals in Soils, Plants, and Aquatic Systems focuses on the chemical persistence and ecotoxicological behavior of pesticides in soil, water, and plants. The book examines recent developments in research on various substances and relays information regarding transport, adsorption, absorption, accumulation, degradation, biological effects, toxicity to aquatic organisms, air pollution, exposure, and risk estimation. Leading international scientists present their advances in analytical methodology and instrumentation in the fields of agrochemicals and environmental chemistry. This useful review of data, methods, and principles will benefit environmental researchers, managers, biologists, chemists, pharmacologists, and others interested in assessing the potential for contamination of soil, air, water, and plants.

CHAPTER 1

Transport and Transformation of Pesticides in Soil

Irene Scheunert

I. Introduction

In most cases, pesticides are xenobiotic organic substances which are applied intentionally to terrestrial ecosystems. However, when they have accomplished their intended effects, their residues which are left in the ecosystem must be regarded as undesirable environmental chemicals. The behavior of these residues in the soil-plant system is governed by the same rules as that of other organic xenobiotic pollutants for which input into the soil was not intended by man.

After the input of pesticides into the soil system, various physical and physicochemical processes, as well as chemical and biochemical reactions, determine their residue behavior. Physical and physicochemical processes govern the transport within and out of the soil system. Chemical and biochemical reactions effect transformations of the pesticide, resulting in molecular alterations and in degradation down to complete mineralization.

Alterations in the chemical structure of a pesticide, as performed by biotic or abiotic conversion reactions, lead to the formation of new xenobiotic compounds. Due to the changes of molecular substituents, these compounds may differ significantly from the parent compounds in physicochemical properties and also in their mobility and distribution within the system. These facts result in further complications in prediction of the fate of a pesticide in the soil-plant system and thus in evaluation of its ecological significance.

In this chapter, an overview of the information available today both on transport and on transformation and degradation of pesticides in soil is given, and possibilities of prediction are discussed.

II. Transport of Pesticides Within Soil and Transfer from Soil to Other Media

A. Adsorption-Desorption Processes in Soil

Adsorption-desorption processes in soil play a paramount part in all physical processes affecting the residue behavior of pesticides in the soil-plant system, such as volatilization from soil into the air, soil-water transfer resulting in mobility in soil and in leaching of pesticides to surface- or groundwater, and uptake by plants or soil fauna. Additionally, biological activity and degradation also are affected by adsorption.¹ In general, adsorption is defined² as the adhesion or attraction of one or more ionic or molecular layers to a surface. In soil, the sum of all kinds of fixation of ions or molecules on or within the solid phase is called adsorption.

Pesticides may be adsorbed by soil organic matter as well as by inorganic soil fractions, and the relation between adsorption at both sites depends on soil properties as well as chemical structure of the pesticide. Adsorption by inorganic and organic matter is not by just one mechanism of pesticide-soil interaction. Physical, physicochemical, and chemical processes are involved. There is a continuum of possible adsorption interactions, starting with fixed site adsorption and ending with partition between three-dimension phases such as aqueous solution and soil organic matter.³ Bonds vary between reversible ones and those resulting in so-called bound residues.

The adsorption of pesticides may be quantitatively characterized by the linear adsorption coefficient K_D⁴ or by the adsorption constants derived either from the Langmuir or the Freundlich adsorption equations.⁵ The linear adsorption coefficient K_D is the quotient of the concentration in the solid soil phase divided by that in the liquid soil phase. It is applicable to characterizing the adsorption from dilute aqueous solutions where the partition between solid and liquid soil phases is more or less independent on concentration.

Numerous publications have demonstrated that adsorption is significantly correlated with soil organic carbon, at least in the case of nonionic pesticides. Therefore, the use of adsorption coefficients related to soil organic carbon only may be appropriate. The adsorption coefficient related to soil organic carbon, K_OC, is K_D multiplied by 100 and divided by the percent of organic carbon in soil.

The linear adsorption coefficient K_D is sometimes not sufficiently exact to describe the adsorption of a pesticide over a larger range of concentrations. The relations between the concentrations in the liquid and the solid phase can be described by isotherms. The Freundlich isotherm is derived on the assumption that a decrease in heat of adsorption with increasing surface concentration or coverage can occur due to surface heterogeneity. The Freundlich isotherm equation is written as follows:

$\frac{\text{x}}{\text{m}}={\text{K}}_{\text{F}}\times {\text{C}}_{\text{e}}^{1/\text{n}}$

$\begin{array}{ccc}\text{x/m}& =& \text{pesticide concentration in the solid phase}\\ {\text{C}}_{\text{e}}& =& \text{concentration in the liquid phase at equilibrium}\\ {\text{K}}_{\text{f}}& =& \text{adsorption constant}\\ 1/\text{n}& =& \text{another constant providing a rough estimate of the intensity of adsorption.}\end{array}$

The desorption of adsorbed pesticides by water or by electrolytes in most cases is not complete, i.e., adsorption is not fully reversible. The nondesorbable part of pesticides is due either to strong bonds which are broken only by solvent extraction, or to even stronger bonds which result in the formation of “bound residues.”

Adsorption coefficients and constants related to soil organic carbon have been shown to be correlated with simpler physicochemical parameters. Empirical equations have been obtained experimentally for nonionic chemicals relating soil adsorption to water solubility, n-octanol-water partition coefficients, parachor, and molecular connectivity indices. Both molecular size and structure are important in adsorption of a chemical compound by the soil. Dissociation constants play a role for ionizable pesticides.⁶ The n-octanol-water partition coefficient is the preferred physicochemical parameter to predict soil adsorption. However, it should be considered that this parameter is correlated only with that part of the adsorbed pesticide which is reversibly fixed by “partition” in the lipophilic organic soil fraction.

B. Mobility of Pesticides in Soil

Mobility of pesticides in soil is related to adsorption, on the one hand, and the mass flux of dissolved fractions, on the other hand. Adsorption retards the mass flux which consists of diffusion, convection, and dispersion and which also is a function of the removal of pesticides from the solution by biological and chemical reactions.⁷

Diffusion (diffusive mass flux) is a physical process by which molecules, atoms, and ions — due to their thermic mobility — move from sites of higher concentration to those of lower concentration. This transport mechanism is independent of water movement.

Convection (convective mass flux) is the passive movement of solutes with the moving water. It is a function of the volume flow velocity of the water.

Dispersion (dispersive mass flux) is the distribution or mixing of solutes in the moving pore water, which results from different flow velocities of individual water volumes. Like convection, it represents a current-dependent contribution to mass flux in soil water.

According to these processes affecting pesticide mobility in soil, relevant soil parameters (apart from adsorption coefficients and microflora performing biodegradation) are soil texture, porosity, density, moisture ratio in the layers, water flow velocities, and aggregate stability.¹

C. Transfer of Pesticides from Soil to Water

Surface run-off, tile drainage, and percolation into deeper groundwater are the main processes of water — and thus pesticides — leaving the root zone. Consequently, these processes are potential sources of contamination of rivers, lakes, and groundwater with pesticides.

The contamination of groundwater and potable water with pesticides has been a source of major concern for the last few years. Therefore, the prediction of leaching rates of pesticides from soil is becoming more important for assessing possible concentrations in groundwater and potable water for the future. In mathematical models under discussion, the most sensitive parameters are the adsorption coefficient K_OC, the Freundlich exponent 1/n, and the half-life of degradation (DT₅₀). The variability of K_OC between different soils is a source of uncertainty in groundwater contamination prediction. Thus, a variation of K_OC by a factor of 5, which is quite realistic, changes the predicted pesticide concentration in groundwater by at least one order of magnitude.^8,9 The relationship between K_OC and predicted pesticide concentration in groundwater is also affected by variations in the Freundlich exponent between 0.8 and 1.0. The DT₅₀ of biological degradation is also uncertain since degradation of pesticides in soil is neither linear, nor does it follow first-order kinetics in most cases; and degradation kinetics are widely unexplored. The reduction of microbial concentrations in the soil below the surface horizon also has to be taken into account.¹

In spite of the outstanding importance of adsorption properties and degradation rates for the leaching of pesticides from soil, soil texture may be decisive in special cases. Organic matter in soil affects the mobility and leaching of pesticides not only negatively by adsorption, but also positively by creating a higher aggregate stability. Furthermore, organic matter has a positive effect on pesticide mobility and leaching by the formation of water-soluble adducts between poorly water-soluble pesticides and readily water-soluble humic substances.¹⁰

Temperature also affects leaching of pesticides in soil. Interactions between different organic chemicals concerning mobility and leaching in soil may occur through mutual influences on their water solubility or by competition for adsorption sites in the soil solid phase.

The interaction of all parameters discussed above and of further unexplored ones may result in the appearance of pesticide residues in groundwater. Trace amounts of pesticides detected in groundwater after agricultural use¹¹ include those for which penetration into groundwater was not predicted by theoretical and experimental models. In former years, chlorinated pesticides such as aldrin and dichlorodiphenyltrichloroethane (DDT) were the pesticides preferably found in potable water.¹² Their presence was not predicted, due to their low water solubility; however, by their biological and chemical persistence they were able to reach groundwater. Today, pesticides detected in groundwater after a...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Preface
• The Editor
• Contents
• Chapter 1 Transport and Transformation of Pesticides in Soil
• Chapter 2 Prediction of Uptake of Some Aromatics and Pesticides by Soil
• Chapter 3 Accelerated Degradation of Soil Insecticides: Comparison of Field Performance and Laboratory Behavior
• Chapter 4 Adsorption of 2,4-D on Organoclays
• Chapter 5 Competitive Adsorption of 2,4-D and Phosphate in Soils
• Chapter 6 Adsorption of Two Weak Acids on Goethite
• Chapter 7 Adsorption of Maleic Hydrazide on Mineral Surfaces
• Chapter 8 The Euro-Soil Concept as a Basis for Chemicals Testing and Pesticide Research
• Chapter 9 Effect of Polymers on Adsorption of Flumequine on Kaolinite
• Chapter 10 Groundwater Contamination by Pesticides: Field Experiments in Shallow and Deeper Groundwater
• Chapter 11 Potential of Fluorescence Spectroscopy in the Study of Interactions of Pesticides with Natural Organic Matter
• Chapter 12 Retention of Atrazine by Humic Substances of Different Natures
• Chapter 13 The Binding of Pesticide Residues to Natural Organic Matter, Their Movement, and Their Bioavailability
• Chapter 14 Conversion of Lindane to HCH Isomers and HCB in the Agricultural Field Conditions
• Chapter 15 Ecological Test Procedures for Organic Xenobiotics in Terrestrial Systems
• Chapter 16 Role of Microbial Competition on Activity of 2,4-D Degrading Alcaligenes xylosoxidans Strain Introduced into Fumigated Soil
• Chapter 17 Thermodynamic Properties of Halogenated Dibenzo-p-Dioxins, Dibenzofurans, and Pesticides
• Chapter 18 New Results and Considerations on the Bioconcentration of the Superlipophilic Persistent Chemicals Octachlorodibenzo-p-Dioxin (OCDD) and Mirex in Aquatic Organisms
• Chapter 19 Behavior of Soil Microflora in Pesticide Degradation
• Chapter 20 Modeling the Uptake of Organic Compounds into Plants
• Chapter 21 Influence of Soil-Water Ratio on Adsorption-Desorption Kinetics of Isoxaben in Soil
• Chapter 22 Toxicity and Metabolism of Cypermethrin in Earthworms
• Chapter 23 Determination of the Rate Constant KOH (Air) Using Freon 113 as an Inert Solvent
• Chapter 24 Abiotic Degradation Pathways of Selected Pesticides in the Presence of Oxygen Species in Aqueous Solutions
• Contributor
• Index