Environmental soil chemistry 2nd ed donald l. sparks (elsevier academic press, 2003), Exercises of Chemistry

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Environmental Soil Chemistry

Second Edition

Environmental

Soil

Chemistry

Second Edition

Donald L. Sparks

University of Delaware

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Table of Contents

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Preface

Since the first edition of Environmental Soil Chemistry was published in 1995, a number of important developments have significantly advanced the soil and environmental sciences. These advancements were the primary motivation for publishing the second edition. The use of synchrotron-based spectroscopic and microscopic techniques, which employ intense light, has revolutionized the field of environmental soil chemistry and allied fields such as environmental chemistry, materials science, and geochemistry. The intense light enables one to study chemical reactions and processes at molecular and smaller scales and in situ. A new multidisciplinary field has evolved, molec- ular environmental science, in which soil chemists are actively involved. It can be defined as the study of the chemical and physical forms and distri- bution of contaminants in soils, sediments, waste materials, natural waters, and the atmosphere at the molecular level. Chapter 1 contains a major section on molecular environmental science with discussions on synchrotron radiation and important spectroscopic and microscopic techniques. The application of these techniques has greatly advanced our understanding of soil organic matter macromolecular structure (Chapter 3), mechanisms of metal and metalloid sorption on soil components and soils, and speciation of inorganic contaminants (Chapter 5). This second edition also contains new information on soil and water quality (Chapter 1), carbon sequestration (Chapter 3), and surface nucleation/precipitation (Chapter 5) and disso- lution (Chapter 7). Other material throughout the book has been updated. As with the first edition, the book provides extensive discussions on the chemistry of inorganic and organic soil components, soil solution–solid phase equilibria, sorption phenomena, kinetics of soil chemical processes, redox reactions, and soil acidity and salinity. Extensive supplementary readings are contained at the end of each chapter, and numerous boxes in the chapters contain sample problems and explanations of parameters and terms. These should be very useful to students taking their first course in soil chemistry. The second edition is a comprehensive and contemporary textbook for advanced undergraduate and graduate students in soil science and for students and professionals in environmental chemistry and engineering, marine studies, and geochemistry. Writing the second edition of Environmental Soil Chemistry has been extremely enjoyable and was made easier with the support and encourage- ment of a number of persons. I am most grateful to the administration at the University of Delaware for providing me with a truly wonderful environment

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1

Environmental Soil

Chemistry:

An Overview

S

oil chemistry is the branch of soil science that deals with the chemical composition, chemical properties, and chemical reactions of soils. Soils are heterogeneous mixtures of air, water, inorganic and organic solids, and microorganisms (both plant and animal in nature). Soil chemistry is concerned with the chemical reactions involving these phases. For example, carbon dioxide in the air combined with water acts to weather the inorganic solid phase. Chemical reactions between the soil solids and the soil solution influence both plant growth and water quality. Soil chemistry has traditionally focused on the chemical reactions in soils that affect plant growth and plant nutrition. However, beginning in the 1970s and certainly in the 1990s, as concerns increased about inorganic and organic contaminants in water and soil and their impact on plant, animal, and human health, the emphasis of soil chemistry is now on environmental soil chemistry. Environmental soil chemistry is the study of chemical reactions between soils and environmentally important plant nutrients, radionuclides, metals, metalloids, and organic chemicals. These water and soil contaminants will be discussed later in this chapter.

A knowledge of environmental soil chemistry is fundamental in pre- dicting the fate of contaminants in the surface and subsurface environ- ments. An understanding of the chemistry and mineralogy of inorganic and organic soil components is necessary to comprehend the array of chemical reactions that contaminants may undergo in the soil environment. These reactions, which may include equilibrium and kinetic processes such as dissolution, precipitation, polymerization, adsorption/desorption, and oxidation–reduction, affect the solubility, mobility, speciation (form), toxicity, and bioavailability of contaminants in soils and in surface waters and groundwaters. A knowledge of environmental soil chemistry is also useful in making sound and cost effective decisions about remediation of con- taminated soils.

Evolution of Soil Chemistry

Soil chemistry, as a subdiscipline of soil science, originated in the early 1850s with the research of J. Thomas Way, a consulting chemist to the Royal Agricultural Society in England. Way, who is considered the father of soil chemistry, carried out a remarkable group of experiments on the ability of soils to exchange ions. He found that soils could adsorb both cations and anions, and that these ions could be exchanged with other ions. He noted that ion exchange was rapid, that clay was an important soil component in the adsorption of cations, and that heating soils or treating them with strong acid decreased the ability of the soils to adsorb ions. The vast majority of Way’s observations were later proven correct, and his work laid the ground- work for many seminal studies on ion exchange and ion sorption that were later conducted by soil chemists. Way’s studies also had immense impact on other disciplines including chemical engineering and chemistry. Research on ion exchange has truly been one of the great hallmarks of soil chemistry (Sparks, 1994). The forefather of soil chemistry in the United States was Edmund Ruffin, a philosopher, rebel, politician, and farmer from Virginia. Ruffin fired the first Confederate shot at Fort Sumter, South Carolina. He committed suicide after Appomattox because he did not wish to live under the “perfidious Yankee race.” Ruffin was attempting to farm near Petersburg, Virginia, on soil that was unproductive. He astutely applied oyster shells to his land for the proper reason—to correct or ameliorate soil acidity. He also accurately described zinc deficiencies in his journals (Thomas, 1977). Much of the research in soil chemistry between 1850 and 1900 was an extension of Way’s work. During the early decades of the 20th century classic ion exchange studies by Gedroiz in Russia, Hissink in Holland, and Kelley and Vanselow in California extended the pioneering investigations and conclu- sions of Way. Numerous ion exchange equations were developed to explain and predict binary reactions (reactions involving two ions) on clay minerals

2 1 Environmental Soil Chemistry: An Overview

The modern environmental movement began over 30 years ago when the emphasis was on reducing pollution from smokestacks and sewer pipes. In the late 1970s a second movement that focused on toxic compounds was initiated. During the past few decades, several important laws that have had a profound influence on environmental policy in the United States were enacted. These are the Clean Air Act of 1970, the Clean Water Act of 1972, the Endangered Species Act, the Superfund Law of 1980 for reme- diating contaminated toxic waste sites, and the amended Resource Conser- vation and Recovery Act (RCRA) of 1984, which deals with the disposal of toxic wastes. The third environmental wave, beginning in the late 1980s and orches- trated by farmers, businesses, homeowners, and others, is questioning the regulations and the often expensive measures that must be taken to satisfy these regulations. Some of the environmental laws contain regulations that some pollutants cannot be contained in the air, water, and soil at levels greater than a few parts per billion. Such low concentrations can be measured only with very sophisticated analytical equipment that was not available until only recently. Critics are charging that the laws are too rigid, impose exorbitant cost burdens on the industry or business that must rectify the pollution problem, and were enacted based on emotion and not on sound scientific data. Moreover, the critics charge that because these laws were passed without the benefit of careful and thoughtful scientific studies that considered toxicological and especially epidemiological data, the risks were often greatly exaggerated and unfounded, and cost–benefit analyses were not conducted. Despite the questions that have ensued concerning the strictness and perhaps the inappropriateness of some of the regulations contained in environ- mental laws, the fact remains that the public is very concerned about the quality of the environment. They have expressed an overwhelming willingness to spend substantial tax dollars to ensure a clean and safe environment.

Contaminants in Waters and Soils

There are a number of inorganic and organic contaminants that are important in water and soil. These include plant nutrients such as nitrate and phosphate; heavy metals such as cadmium, chromium, and lead; oxyanions such as arsenite, arsenate, and selenite; organic chemicals; inorganic acids; and radionuclides. The sources of these contaminants include fertilizers, pesticides, acidic deposi- tion, agricultural and industrial waste materials, and radioactive fallout. Discussions on these contaminants and their sources are provided below. Later chapters will discuss the soil chemical reactions that these contaminants undergo and how a knowledge of these reactions is critical in predicting their effects on the environment.

4 1 Environmental Soil Chemistry: An Overview

Water Quality

Pollution of surface water and groundwater is a major concern throughout the world. There are two basic types of pollution— point and nonpoint. Point pollution is contamination that can be traced to a particular source such as an industrial site, septic tank, or wastewater treatment plant. Nonpoint pollution results from large areas and not from any single source and includes both natural and human activities. Sources of nonpoint pollution include agricultural, human, forestry, urban, construction, and mining activities and atmospheric deposition. There are also naturally occurring nonpoint source pollutants that are important. These include geologic erosion, saline seeps, and breakdown of minerals and soils that may contain large quantities of nutrients. Natural concentrations of an array of inorganic species in ground- water are shown in Table 1.1. To assess contamination of ground and surface waters with plant nutrients such as N and P, pesticides, and other pollutants a myriad of interconnections including geology, topography, soils, climate and atmospheric inputs, and human activities related to land use and land management practices must be considered (Fig. 1.1). Perhaps the two plant nutrients of greatest concern in surface and ground- water are N and P. The impacts of excessive N and P on water quality, which can affect both human and animal health, have received increasing attention. The U.S. EPA has established a maximum contaminant level (MCL) of 10 mg liter–1^ nitrate as N for groundwater. It also established a goal that total phosphate not exceed 0.05 mg liter–1^ in a stream where it enters a lake or reservoir and that total P in streams that do not discharge directly to lakes or reservoirs not exceed 0.1 mg liter–1^ (EPA, 1987). Excessive N and P can cause eutrophication of water bodies, creating excessive growth of algae and other problematic aquatic plants. These plants can clog water pipes and filters and impact recreational endeavors such as fishing, swimming, and boating. When algae decays, foul odors, obnoxious tastes, and low levels of dissolved oxygen in water (hypoxia) can result. Excessive nutrient concentrations have been linked to hypoxia conditions in the Gulf of Mexico, causing harm to fish and shellfish, and to the growth of the dinoflagellate Pfisteria , which has been found in Atlantic Coastal Plain waters. Recent outbreaks of Pfisteria have been related to fish kills and toxicities to humans (USGS, 1999). Excessive N, in the form of nitrates, has been linked to methemoglobinemia or blue baby syndrome, abortions in women (Centers for Disease Control and Prevention, 1996), and increased risk of non-Hodgkin’s lymphoma (Ward et al., 1996). Phosphorus, as phosphate, is usually not a concern in groundwater, since it is tenaciously held by soils through both electrostatic and nonelectrostatic mechanisms (see Chapter 5 for definitions and discussions) and usually does not leach in most soils. However, in sandy soils that contain little clay, Al or Fe oxides, or organic matter, phosphate can leach through the soil and impact groundwater quality. Perhaps the greatest concern with phosphorus is con-

Contaminants in Waters and Soils 5