Introduction

The environment is in a constant state of change. In order to describe this adequately, this chapter reviews environmental dynamics, the unification of fluid dynamics, chemical thermodynamics, and chemical kinetics.

An important goal of environmental chemistry is to model Nature, including predicting the fate of a given chemical species, whether of natural or anthropogenic origin. The purpose of this chapter is to establish the nomenclature used in the book and to present fundamental concepts and expressions. It is particularly important to derive the explicit time dependence of the relevant physical quantities. In separate sections of this introduction, we first describe and define a few basic concepts, and then introduce important concepts for the time dependence of concentration, field, and transport.

Basic concepts

System and phase

The concepts of system, surroundings, and phase used in thermodynamics are the cornerstones of any environmental investigation. A system is the part of the world that has been chosen as the subject of study; it may contain matter, in which case it also contains energy. Everything else is said to belong to the surroundings. The separating boundary may be real or imaginary, but must be defined before the study begins. Depending on the properties of the boundary, one can define three types of system. An open system is one that can exchange substance and energy (work, heat, radiation) with the surroundings. A closed system cannot exchange substance with the surroundings, and an isolated system can exchange neither substance nor energy with the surroundings. Except in the last case, the surroundings enter the physical description, because they must have certain properties as sources or sinks for the exchanged physical quantities.

It seems “natural” to describe the Earth in terms of spherical shells, given the shape of the planet and its layered structure. The atmosphere is a good example of such a shell. This chapter begins by introducing some basic ideas and continues with a more thorough discussion, including the chemical structure and composition of the spheres, along with some key features of the physics of each sphere and their interactions.

From a chemical perspective, the Earth has a distinct central region, called the barysphere, on which the lithosphere, the hydrosphere, and the atmosphere rest. The term sphere is used in a more abstract sense than the simple meaning of “ball”; for example, the hydrosphere, comprising all states of aggregation of water, is irregular in form and extent. In the context of the environment, it is convenient to discuss separately the pedosphere, which consists of the material in which plants grow, and to delimit the biosphere as follows later.

Suess (1875) was the first to attempt to unite the fields of geophysics and biology. His contribution was to recognize and characterize the biosphere as the region that gives rise to life. The concept was refined by Vernadsky (1925) to mean the top part of the lithosphere, part of the hydrosphere, and thetroposphere, the lowest layer of the atmosphere. According to his definition, the biosphere is the part of the Earth that sustains life, but not including biota, defined as living and dead organisms. Today, biosphere is understood to mean all organic material and that part of the inorganic surroundings in which life is found. It has been proposed that the word ecosphere be used with this meaning, in contrast to biosphere,whichwould then be used to mean all living and dead organisms and their degradation products; however, these definitions have not been widely accepted. The part of the Earth that is not the biosphere is called the geosphere.

Biogeochemical cycles

The Wilson cycles circulate elements are circulating through the crust over long time periods, as discussed in Section 1.4. Cycling of material within and between the atmosphere, hydrosphere, and biosphere is typically much faster.

Biogeochemical cycles are flows of matter within and between the spheres as defined in the preceding Chapters 3 to 6. These flows are driven by solar energy, and they begin and end in the crust, which acts as the main reservoir of nearly all elements. Motions within the crust are driven by energy from radioactive decay, and the circulations of the atmosphere and ocean are driven by the sun. In fact, biogeochemical cycles are processes that take place within an open physicochemical system as depicted in Figure 7.1. The spheres are marked by ellipses, and their mutual interactions are indicated by arrows. We refer to the discussion of open systems in Section 2.2b, in particular with respect to the significance of the surroundings. The system can be treated in its entirety, but usually the focus is on a few chemical elements and their flow within and through the subsystems. Since 1971, the International Council for Science (ICSU) committee SCOPE has published a series of reports on the global turnover of elements central to the environment. Because of their importance for life, the nonmetals C, N, P, and S have been taken up several times, but Cl, Pb, Hg, Cd, and As have also been investigated. In Sections 7.2 through 7.6 of this chapter, we will follow the approach of SCOPE, that is, little emphasis is given to the details of the biochemical processes involved.

A few properties of the hydrosphere, including its interaction with the other spheres, were described in Section 3.2. In this chapter we present fundamental aspects of the hydrosphere's chemistry that are critical for a thorough understanding of the environment. Pollution of water and its treatment are discussed in Section 9.5.

Chemistry in an aqueous solution always takes place with a great surplus of water. At room temperature, the amount concentration of water is caq ≈ 55.5 m, which may be compared with cNaCl ≈ 0.5 m in seawater (see Table 3.13). Seawater is a fairly concentrated solution, and even here each ion is surrounded by about 100 water molecules. Therefore, the physicochemical properties of water itself underlie all chemistry in the hydrosphere. We shall see that water exerts a leveling effect, not only on the apparent strengths of acids and bases, but also on the range of electrode potentials that are observed, allowing species to be stable in solution. In this chapter we discuss acid-base chemistry, coordination chemistry, redox chemistry, and the thermodynamics of species in aqueous solution.

The atmosphere interacts directly with the lithosphere, hydrosphere, biota, and society. Noble gases given off by radioactive decay in the core and crust have accumulated since the planet formed, with the exception of He, which has escaped to space. The exchange of water between the atmosphere and the oceans and the land is a key component of weather and the planetary heat engine (see Figure 3.3). The biosphere has dramatically changed the atmosphere's composition, including oxygen and the majority of trace gases. Human activities, such as agriculture, industry, and transportation, have left a clear mark on the composition and chemistry of the atmosphere. A summary of the gases found in the atmosphere is given in Section 3.3.

The state of the atmosphere affects us all. Crop yields depend on climate, rain, and nutrients present in rain, and on trace gases such as ozone and carbon dioxide. There are also direct effects: WHO estimates that between 2.5 and 11% of human deaths are due to exposure to air pollution. The thickness of the ozone layer at midlatitudes has decreased dramatically over the past generation, not to mention the Antarctic ozone hole. Climate change has important implications for the global ecosystem and public.

The pedosphere consists of soil and constitutes the interface between the lithosphere, the hydrosphere, and the atmosphere. Instead of attempting a short definition of soil, we shall look at the main disciplines of soil science: edaphology and pedology, both of which were founded in the 19th century. Edaphology deals with the conditions necessary for the growth of plants (Liebig, around 1840), that is, their nutrition, in particular the types of chemical species they consume and produce. Pedology is the study of soil genesis, morphology, and classification (Dokuchaev, 1883).

The upper layer of the Earth's crust consists of fragments of rocks, mainly silicates, combined with water, air, and organic material. The largest pieces of rock, boulders and cobbles, have sizes ranging from meters to decimeters, whereas small fragments are coarse gravel in the centimeter range, followed in order of decreasing size by fine gravel, sand, silt, and clay (maximum size 4 μm), as shown later in Table 6.3. This heterogeneous, solid material extending from the outer surface, in contact with the atmosphere or ocean, downward to the solid bedrock, is called the regolith. In some cases the regolith results from weathering of bedrock at a given location, but more often it has been transported to a given site; in a few cases there is no regolith. Material at a given site may have been deposited by ice, water, or wind, individually or in combination. The regolith is often stratified; for example, Figure 6.1 shows layers formed by water and air in material deposited by glaciers.

Introduction

The chemicals industry manufactures commodities on a large scale. Table 8.1 shows the commodities that, on a worldwide basis, are produced in the greatest amounts. Some industries use significant amounts of recycled material; this is the reason why annual production of pig iron is much less than that of steel. Production on this scale is very sensitive to the costs of raw materials, transport, and energy, and even small changes in synthetic efficiency can shift production patterns and have large socioeconomic impacts.

Environmental protection accounts for an increasing part of industrial costs, and we present here a survey of the chemicals industry because an understanding of why and how our society runs such a huge enterprise is a prerequisite for decision making on environmental issues.

Most production relies on energy whose origin is fossil carbon in a low oxidation state (see Table 8.3). Conversion of this fuel to energy requires a reduction of atmospheric O2 to water;d other reductions are also of industrial significance, such as the reduction of FeIII in Fe2O3 to Fe.

The environment is a fascinating subject for a chemist, with a seemingly endless variety of conditions combining dramatic changes in temperature, pressure, phase, and composition. It is also an extremely useful object of study from the practical viewpoints of economy and health. Environmental chemistry is the science in which the methods and results of chemistry are applied to processes involving chemical species in the environment.

Overview

More than 2,000 years ago in the Middle East, it was established that the Earth was a sphere, and the physical and chemical laws discovered since then allow the present description of the Earth as a physicochemical system. In a strictly chemical sense, the “elements” of the environment are the elements of the periodic table. However, it is convenient to add other kinds of structural units, for example, the spheres, which together with special branches of physics, for example, the laws of fluid dynamics, are needed to understand environmental systems.

Human activity

The agricultural and industrial revolutions (beginning around 1750) have allowed the human population to increase exponentially (Figure 1). In the recent past, the human population has been doubling about every 40 years, and 20 % of all humans born in the past 6,000 years are alive today. Population growth and advances in quality of life have been made possible by knowledge concerning how to use Earth’s resources. Specific examples include the production of fertilizer and cement, the invention of refrigeration, medical advances, and the invention and mass production of consumer goods ranging from clothes to personal electronic devices. A key challenge facing human society today is dealing with the consequences of our success.

Chemical species made by humans affect many naturally occurring processes and organisms. The observation of an anthropogenic substance in Nature raises a series of questions: Where did it come from? How and why was it produced and released? How does it move around within the environment? What is its chemistry, including the reaction rate, mechanism, and products, and how does it influence living organisms and the .EnvironmentalEarth system? More generally, and perhaps not within the focus of scientific chemistry, there are questions such as: Who is entitled to make use of Nature and to what extent? Are there limits to growth? A rational discussion of these questions involves the scientific method and results from the physical sciences, as well as law, economy, and the humanities.

Turning to chemistry: there is no doubt that success in the field of environmental chemistry requires mastering fundamental disciplines such as analytical chemistry, thermodynamics, and modern experimental and theoretical chemistry. The important role of environmental chemistry as a field in its own right is recognized internationally: the International Union of Pure and Applied Chemistry, IUPAC, has organized scientific investigations of the environmental impact of chemistry for many years; examples include its series of reports on pesticides, starting long before environmental issues were of political and public interest, and the White Book on chlorine. Recognition of the importance of the subject has driven IUPAC to rename its Applied Chemistry Division the Chemistry and the Environment Division. The significance of this change is underlined by the fact that the word Applied is part of the very name of the Union.