The global sulphur cycle and anthropogenic effects

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...

Contaminated land

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...

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...

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...

Human effects on biogeochemical cycles

In discussing the chemistry of near-surface environments on Earth it is important to distinguish between different types of alteration to Earth systems caused by humans. Two main categories can be distinguished 1 Addition to the environment of exotic chemicals as a result of new substances synthesized and manufactured by industry. 2 Change to natural cycles by the addition or subtraction of existing chemicals by normal cyclical and or human-induced effects. The first category of chemical change...

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...

Order in the elements

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...

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...

Origin and evolution of the Earth

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...

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...

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...

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...

Water chemistry and weathering regimes

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...

Wider controls on soil and clay mineral 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...

FeSS S2O2aq FeS2S SOeqn 616

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...

Contamination of groundwater

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...

The sulphur cycle and atmospheric acidity

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...

London smogprimary pollution

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...

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...

Box 65 Abiological precipitation of calcium carbonate

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...

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...

Soil structure and classification

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...

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....