Environmental Chemistry

FeSS S2O2aq FeS2S SOeqn 616

Mid Ocean Ridge Volume

The sulphite (SO2-) is subsequently oxidized to SO4-. Sedimentary pyrite, formed as a byproduct of sulphate reduction in marine sediments, is a major sink for seawater SO4-. The presence of pyrite in ancient marine sediments shows that SO42- reduction has occurred for hundreds of millions of years. On a geological timescale, removal of SO4- from seawater by sedimentary pyrite formation is thought to be about equal to that removed by evaporite deposition (Section 6.4.2). Compilations of pyrite...

Order in the elements

Subgroup Periodic Table

Most of the chemistry in this book revolves around elements and isotopes (see Box 1.1). It is therefore helpful to understand how the atomic number (Z) of an element, and its electron energy levels allow an element to be classified. The electron is the component of the atom used in bonding (Section 2.3). During bonding, electrons are either donated from one atom to another, or shared in either case the electron is prised away from the atom. One way of ordering the elements is therefore to...

Contamination of groundwater

Landfill Aerobic Well

Although aspects of groundwater chemistry have been discussed elsewhere in this chapter, this section highlights issues relating to the contamination of ground-water. Groundwater is critically important to humans since it is a major source of drinking water. For example, in the USA over 50 of the population rely on groundwater as a source of drinking water. Groundwater quality is therefore very important and, in most developed countries, water must conform to certain standards for human...

Box 42 Electronegativity

Electronegativity is a measure of the tendency of an atom to attract an additional electron. It is used as an index of the covalent (see Section 2.3.1) or ionic (see Section 2.3.2) nature of bonding between two atoms. Atoms with identical electronegativity, or molecules such as nitrogen (N2) consisting of two identical atoms, share their bonding electrons equally and so form pure covalent bonds. When component atoms in a compound are dissimilar, the bonds may become progressively polar. For...

The global sulphur cycle and anthropogenic effects

Sulfur Cycle John Evans

We now turn to the cycling of the element sulphur, outlining the nature of the cycle prior to any major alteration by human industrial and urban activity and examining how these activities have impacted, in a very major way, on the contemporary sulphur cycle. Comparison of the global sulphur cycle as it is thought to have been prior to any major anthropogenic influence (Fig. 7.17a) with the cycle as it was in the mid 1980s (Fig. 7.17b) reveals some interesting apparent changes in the sizes of...

CO2aq CO2g H2Ol 2HCO3aqeqn

This reaction makes seawater about eight times more effective at absorbing CO2 than a solution of similar ionic strength but not containing CO2-. The discussion above assumes that equilibrium is achieved between the sea-water and the air with respect to CO2. This leads to the second factor which must be taken into account, since the slow mixing time of the oceans means that it takes hundreds, if not thousands, of years for equilibrium to be attained over the whole depth. In general, it is not...

Aluminium solubility and acidity

Oxidation Reaction Rocks

Aluminium is largely insoluble during weathering processes (Table 5.1), but becomes soluble when pH is both low and high. At the simplest level, three aluminium species are identified soluble Al3+, dominant under acid conditions, insoluble aluminium hydroxide (Al(OH)3), dominant under neutral conditions, and Al(OH)-, dominant under alkaline conditions. Aluminium solubility is therefore pH-dependent and aluminium is insoluble in the pH range 5-9, which includes most natural waters. The details...

Scavenged behaviour

Scavenged Nutrient Profile

Elements that are highly particle-reactive, characterized by large z r ratios (see Section 5.2), often have vertical profiles with surface maxima and decreasing concentrations with depth aluminium (Al) Table 6.9 Concentrations of nutrients and metals in deep (> 3000m) water in the North Atlantic and North Pacific, together with estimated oceanic residence times. Table 6.9 Concentrations of nutrients and metals in deep (> 3000m) water in the North Atlantic and North Pacific, together with...

Nutrientlike behaviour

Environmental Chemistry 1970

As in continental waters (see Section 5.5.1), NO-, DIP (dissolved inorganic phosphorus) and silicate are usually considered to be the limiting nutrients for biological production, although in some situations it has been suggested that trace elements, particularly iron, may be limiting (Section 6.6). Excepting high-latitude areas the oceans are so large and deep that they are effectively permanently stratified. The production of biological material removes nutrients from surface waters (Box...

The effects of elevated carbon dioxide levels on global temperature and other properties

Elevated Co2 Chemistry Panel

So far we have examined the global cycling of carbon without paying attention to the role CO2 plays in the Earth's climate. Although CO2 is a minor component of the atmosphere (see Section 3.2), it plays a vital role in the Earth's radiation balance and hence in controlling the climate. This is illustrated in Fig. 7.12a, which shows the wavelength emission spectrum of the Sun and the Earth, at their effective radiating temperatures of about 5700 C and -23 C respectively. Fig. 7.12 (a) Black...

Ocean circulation and its effects on trace element distribution

Idealized Global Circulation

The preceding discussion of trace elements in seawater has assumed that the oceans have a uniform, warm, nutrient-depleted surface mixed layer and a static deep zone. In fact, at high latitudes surface seawater is cold enough to destroy any density stratification, mixing the oceans to depths of up to 1000 m. This dense surface water sinks and flows slowly into the centre of the oceans as a layer of Fig. 6.26 Chlorophyll concentrations as indicators of phytoplankton growth, inside and outside an...

The sulphur cycle and atmospheric acidity

Trahre Jacques Sulphur Cycle 1991

If CO2 were the only atmospheric gas controlling the acidity of rain, then the pH of rainwater would be close to 5.6 (see Box 3.7). However, most pH measurements of rainwater fall below this value, indicating other sources of acidity. Much of this 'extra' acidity arises from the sulphur cycle, as shown in Fig. 7.18. Only two major routes give rise to the sulphur acidity. One is the burning of fossil fuels to produce the acidic gas SO2. The other is the production of the gas DMS by marine...

Box 65 Abiological precipitation of calcium carbonate

Sediment Covering Skeleton

Where a skeletal source cannot be identified, calcium carbonate (CaCO3) grains and finegrained muds may be of abiological origin. The most famous occurrences occur in shallow, warm, saline waters of the Bahamas and the Arabian Gulf. In these areas two distinctive morphologies are present, ooids and needle muds (Fig. 1). aragonite* crystals around a nucleus, usually a shell fragment or pellet. Successive layers of aragonite precipitation build up a concentric structure, which may vary in size...

Contaminated land

Benzene Converted Catechol

In exceptional cases, the rocks, minerals and soils of the land surface contain compounds that generate natural chemical hazards. Uranium (U) and potassium (K), common elements in granitic rocks, are inherently unstable because of their radioactivity (see Section 2.8) and radioactive decay of isotopes of uranium to form radon (Rn) gas can be a health hazard (Box 4.13). Some chemicals, such as herbicides and pesticides, are present in soils because we put them there intentionally. Other...

Origin and evolution of the Earth

Chemical Spread Environment

The planets of our solar system probably formed from a disc-shaped cloud of hot gases, the remnants of a stellar supernova. Condensing vapours formed solids that coalesced into small bodies (planetesimals), and accretion of these built the dense inner planets (Mercury to Mars). The larger outer planets, being more distant from the sun, are composed of lower-density gases, which condensed at much cooler temperatures. As the early Earth accreted to something like its present mass some 4.5 billion...

Soil structure and classification

Wet Spodosol With Argillic Horizon

As a result of the various factors and processes outlined in Sections 4.6.1-4.6.5, over time soils develop stable and diagnostic features, many of which are recog nizable in the field. These features, particularly specific layers called 'soil horizons', are the basis for soil classification. An idealized soil profile, i.e. a vertical section, is shown in Fig. 4.21. Soil horizons are described using an internationally agreed system of abbreviations that are shown on Fig. 4.21 and used in the...

The terrestrial environment crust and material cycling

Terrestrial Igneous Rock

Terrestrial environments consist of solid (rocks, sediments and soils), liquid (rivers, lakes and groundwater) and biological (plants and animals) components. The chemistry of terrestrial environments is dominated by reactions between the Earth's crust and fluids in the hydrosphere and atmosphere. The terrestrial environment is built on continental crust, a huge reservoir of igneous and metamorphic rock (mass of continental crust 23.6 X 1024g). This rock, often called crystalline basement,...

Wider controls on soil and clay mineral formation

Clay Minerals Formation

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 & 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This...

Water chemistry and weathering regimes

And Weathering

Comparison of dissolved major ion compositions in four large rivers draining very different crustal areas (Table 5.2) shows the dominance of calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K). Overall, however, the chemistry of each river is different and weathering regimes control most of these variations. The dissolved ion composition of freshwater depends upon 1 the varying composition of rainfall and atmospheric dry deposition 2 the modification of atmospheric inputs by...

Box 54 EhpH diagrams

Environmental Chemistry Diagrams

Acidity (pH) and redox potential (Eh) (see Box 4.3) may determine the chemical behaviour of elements or compounds in an environment. In theory, an infinite valley of Eh-pH combinations is possible, although the pH of most environments on Earth is between 0 and 14 and more usually between 3 and 10. Redox potential is constrained by Fig. 1 Simplified Eh-pH diagram showing stability fields of common iron minerals. The stability fields change position slightly depending on the activity of the...

Persistent organic pollutant mobility in the atmosphere

Many persistent organic pollutants (POPs) are semivolatile organic compounds (SVOCs) having vapour pressures (see Box 4.14) between 10 and 10-7Pa. At these vapour pressures SVOCs can evaporate (volatilize) from soil, water or vegetation into the atmosphere. However, as vapour pressure is temperature dependent (see Box 4.14), it follows that at lower temperatures (lower vapour pressures) Fig. 7.24 150000-year record of methane sulphonic acid (MSA) concentration, non-sea-salt-sulphate (nss-SO -)...

Two to one clay mineral structure

The other important structural arrangement is a 2 1 structure, comprising an octahedral layer, sandwiched between two tetrahedral sheets with apical oxygens pointing inward on each side of the octahedral sheet (Fig. 4.11). The mutual sharing of two layers of apical oxygens in the octahedral sheets implies a higher oxygen OH ratio in the structure of the 2 1 vs. the 1 1 octahedral sheets. All of the other clay mineral groups share this structure, the most important being the Fig. 4.8 Schematic...

Box 46 Isomorphous substitution

Clay Structure Chemistry

Isomorphism describes substances which have very similar structure. The carbonate mineral system is a good example, where some minerals differ only on the basis of the cation, for example, CaCO3 (calcite), MgCO3 (magnesite), FeCO3 (siderite). The basic similarity of structure allows interchangeability of cations between end-member minerals. For example, most natural calcite has a measurable amount of both Mg2+ and Fe2+ substituted for some of the Ca2+. The amount of isomorphous substitution is...

Reactivity of trace substances in the atmosphere

Gases with short residence times in the atmosphere are clearly those that can be removed easily. Some of these gases are removed by being absorbed by plants or solids or into water. However, chemical reactions are the usual reason for a gas having a short residence time. What makes gases react in the atmosphere It turns out that most of the trace gases listed in Table 3.3 are not very reactive with the major components of air. In fact, the most important reactive entity in the atmosphere is a...

Box 64 Ion interactions ion pairing ligands and chelation

When ionic salts dissolve in water, the salts dissociate to release the individual ions. The charged ions attract the polar water molecules such that a positively charged ion will be surrounded most closely by oxygen atoms of the water molecules (Fig. 1). Thus, ions are not free in solution, but interacting, or coordinating, with water molecules. The water molecules can be considered to be bound to the ion by so called coordinate bonds. For example, the hydrogen ion (H+) is hydrated to form...

London smogprimary pollution

Great Smog 1952 Definition

Urban pollution is largely the product of combustion processes. In ancient times cities such as Imperial Rome experienced pollution problems due to wood smoke. However, it was the transition to fossil fuel burning that caused the rapid development of air pollution problems. The inhabitants of London have burnt coal since the 13 th century. Concern and attempts to regulate coal burning began almost immediately, as there was a perceptible and rather strange smell associated with it. Medieval...

HgO Hg2 H2Oeqn 523

This ionic mercury (HgII) adheres to aerosols and thus has a short (days to weeks) residence time in the atmosphere rainfall delivers it to the local soils and rivers. Ionic mercury is readily methylated (eqn. 5.24) by both abiotic and biotic pathways. However, most scientists now agree that methylation by anaerobic sulphate reducing bacteria (SRB) is most important. In equation 5.24 the Hg2+ ion forms a covalent compound dimethylmercury (Hg(CH3)2) by bonding with the methyl anion (CH-). The...

Naaq Claq NaClSeqn

And, in addition, some magnesium (Mg) salts begin to crystallize if evaporation continues, highly soluble potassium (K) salts precipitate (Box 6.2). The problem with invoking evaporation as a removal mechanism for ions in seawater is that there are currently very few environments in which evaporite salts are accumulating to a significant extent. This is because enormous volumes of seawater need to be evaporated before the salts become concentrated enough to precipitate. Clearly this cannot...

106 mol NO11x 137 x 1021l 41 x 1015molNO eqn 620

Fig. 6.31 Estimated nitrogen fertilizer use in the USA and mean nitrate-nitrogen concentrations* in the Mississippi River at St Francisville near Baton Rouge, Louisiana (A on inset map) between 1955 and 1995. Inset map shows the Mississippi drainage basin and the region of low oxygen that forms offshore of the Mississippi delta in the Gulf of Mexico. Data courtesy of United States Geological Survey. * Note on units although we mainly use molar units in this book, in some applications results...

Steady state or equilibrium

Let us look at an individual trace gas in the atmosphere. We will take methane (CH4), not an especially reactive gas, as an illustration. It is present in the atmosphere at about 1.7 ppm (Box 3.1). Methane could be imagined to react with O2 in the following way CH4(g) + 2O2(g) CO2(g) + 2H2O(g) eqn. 3.2 The reaction can be represented as an equilibrium situation (Box 3.2) and described by the conventional equation which can be written in terms of pressure (Box 3.1) The equilibrium constant (K)...

Natural and anthropogenic sources and sinks

There are three main sources and sinks for atmospheric CO2 in near-surface environments the land biosphere (including freshwaters), the oceans, and anthropogenic emissions from the burning of fossil fuels and other industrial activities. In the natural state the land biosphere and the ocean reservoirs exchange CO2 with the atmosphere in an essentially balanced two-way transfer. These reservoirs are also sinks for anthropogenic CO2. Volcanic emissions (see Section 3.4.1) are not considered here...

Radioactivity of elements

Where the number of both protons and neutrons in an atom is known we are able to identify a specific isotope of a specific element and this is termed a nuclide. Some naturally occurring elements are radioactive and specific isotopes of these elements are called radionuclides. This term implies that their nuclei are unstable and spontaneously decay, transforming the nucleus into that of a different element. Radioactive decay is written in equations that look a little like those for chemical...

FeS2s14Feq8H20 a 15Feq 2S04aq 16Haq

At pH values much above 3 the iron(III) precipitates as the common iron(III) oxide, goethite (FeOOH) Fe(+q)+ 2H2O(l) FeOOH(s) + 3H+aq) eqn. 5.18 The precipitated goethite coats stream beds and brickwork as a distinctive yellow-orange crust (Plate 5.2, facing p. 138), a very visible manifestation of the problem. Bacteria use iron compounds to obtain energy for their metabolic needs (e.g. oxidation of ferrous to ferric iron). Since these bacteria derive energy from the oxidation of inorganic...

Box 44 Metastability reaction kinetics activation energy and catalysts

Some reduced compounds appear to be stable at Earth surface temperatures despite the presence of atmospheric oxygen. Graphite, for example, is a reduced form of carbon which we might expect to react with oxygen, i.e. Cgraphite O2 g CO2 g eqn. 1 Although the reaction of oxygen with graphite is energetically favoured, graphite exists because the reaction is kinetically very slow. Many natural materials are out of equilibrium with their ambient environment and are reacting imperceptibly slowly....

Global persistent organic polllutant equilibrium

The manufacture and use of many exotic organic compounds (see Section 1.4) has now been discontinued because of their persistence, potential health effects and global mobility. For example, PCBs were first manufactured and used in the 1930s and their usage increased until the early 1970s. Thereafter PCB usage was banned in many instances or subject to restrictions. However, PCBs did not immediately disappear from the environment. Even today PCBs are present in all of Earth's environmental...

Ion exchange and soil pH

Exchangeable ions are those that are held temporarily on materials by weak, electrostatic forces. If particles with one type of adsorbed ion are added to an electrolyte solution containing different ions, some of the particle-surface adsorbed ions are released into solution and replaced by those from the solution (Fig. 4.20). We have seen that the interlayer sites of clay minerals, particularly smectites, hold ions weakly, giving these minerals a capacity for ion exchange. Clay mineral ion...

Ca2q 2HCOaq CaCO3aq CO H2Oeqn

We have already noted the importance of this reaction in buffering the pH of continental waters (see eqn. 5.12), and it behaves in exactly the same way in seawater. The Le Chatelier Principle (see Box 3.2) predicts that any process that decreases the concentration of HCO- in equation 6.4, will encourage dissolution of CaCO3 to restore the amount of HCO- lost (Fig. 6.8). The oceans contain an effectively infinite amount of CaCO3 particles suspended in surface waters and in bed sediments (see...

Box 35 The pH scale

The acidity of aqueous solutions is frequently described in terms of the pH scale. Acids (Box 3.3) give rise to hydrogen ions (H+) in solution and the pH value of such a solution is defined We can write a similar relationship identifying pOH However, pH is related to pOH through the equilibrium describing the dissociation of water H2O H+ + OH-, i.e. Kw 10-14 aH+ . aOH- It is important to notice that this is a logarithmic scale, so it is not appropriate to average pH values of solutions...

Formation of soils

So far we have discussed the mechanisms and solid products of chemical weathering without precise consideration of the environment in which these reactions occur. While chemical attack of exposed bedrock surfaces can happen, most weathering reactions occur in (or under) soils. We have already noted that the oxidation of soil organic matter causes acidity of natural waters (Section 4.4.2), promoting chemical weathering. This acknowledges the important role of soils in environmental chemistry. So...

The global budget of natural and anthropogenic carbon dioxide

We now synthesize much of the knowledge outlined in previous sections on the global budget of CO2. Firstly, the relative sizes of the natural reservoirs are considered and then the natural flows between them, followed by how anthropogenic CO2 partitions between the boxes. Finally, likely future levels of atmospheric CO2 are discussed in terms of possible scenarios of fossil fuel consumption. A simplified version of the carbon cycle is given in Fig. 7.9. By far the largest reservoir is in marine...

Natural sources

Since the atmosphere can be treated, on a large scale, as if it were in steady state, we have a model that views the atmosphere as having sources, a reservoir (i.e. the atmosphere itself) and removal processes, all in delicate balance. The sources need to be quite stable over the long term. If they are not, then the balance will shift. In terms of our earlier analogy, the level in the leaking bucket will change. The best-known, and most worrying, example of such a shift is the increasing...

The role of iron as a nutrient in the oceans

Although abundant in the Earth's crust (see Fig. 1.3), iron is present in seawater at very low concentrations (about 1 nmol l-1or less) because the thermodynami-cally stable Fe(III) species is both insoluble (see Fig. 5.2) and particle reactive, being a highly charged small ion (Section 6.5.5). Despite this, iron is an essential component for a number of life-supporting enzyme systems including those involved in photosynthesis and nitrogen fixation. It may appear surprising that phytoplankton...

Minor chemical components in seawater

Gases dissolve from the atmosphere into the oceans according to the Henry's law constant (see Box 3.4). In the absence of biological processes the ocean surface waters would therefore be saturated with all atmospheric gases. Some of these gases, such as argon (Ar) and helium (He), are chemically inert, while others such as nitrogen gas (N2) are available as a nutrient source to only a few specialized nitrogen-fixing organisms and are hence effectively inert in seawater, and thus very close to...

Box 38 Removal of sulphur dioxide from an air parcel

A parcel of air over a rural area of an industrial continent would typically be expected to contain sulphur dioxide (SO2) at a concentration of 5 x 10-9atm. This means that a cubic metre of air contains 5 x 10-9m3 of SO2. We can convert this to moles quite easily because a mole of gas occupies 0.0245 m3 at 15 C and atmospheric pressure. Thus our cubic metre of air contains 5 x 10-9 0.0245 2.04 x 10-7mol of SO2. In a rain-laden cloud we can expect one cubic metre to contain about 1 g of liquid...

Box 36 Reactions in photochemical smog

Reactions involving nitrogen oxides (NO and NO2) and ozone (O3) lie at the heart of photochemical smog. where M represents a 'third body' (Section 3.10.1) It is conventional to imagine these processes that destroy and produce nitrogen dioxide (NO2) as in a kind of equilibrium, which is represented by a notional equilibrium constant relating the partial pressures of the two nitrogen oxides and O3 If we were to increase NO2 concentrations (in a way that did not use O3), then the equilibrium could...

Box 33 Acids and bases

Acids and bases are an important class of chemical compounds, because they exert special control over reactions in water. Traditionally acids have been seen as compounds that dissociate to yield hydrogen ions in water The definition of an acid has, however, been extended to cover a wider range of substances by considering electron transfer. For example, boric acid (H3BO3), which helps control the acidity* of seawater, gains electrons from the hydroxide (OH-) ion For most applications the simple...

Composition of the atmosphere

Bulk composition of the atmosphere is quite similar all over the Earth because of the high degree of mixing within the atmosphere. This mixing is driven in a horizontal sense by the rotation of the Earth. Vertical mixing is largely the product of heating of the surface of the Earth by incoming solar radiation. The oceans have a much slower mixing rate, but even this is sufficient to ensure a relatively constant bulk composition in much the same way as the atmosphere. However, some parts of the...

Organic moleculesstructure and chemistry

Organic matter and organic compounds are integral components of all environmental reservoirs it is therefore important to understand some of the basic facts about their structure and chemistry. Organic molecules contain carbon, hydrogen and often some other non-metallic elements such as oxygen, nitrogen, sulphur or halogens such as chlorine. Organic molecules are often complex structures, typically a skeleton of carbon atoms arranged in chains, branched chains or rings. It is more convenient to...