Box 54 EhpH 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 components. In this case Fe2+, Fe3+ and S = 10-6mol l-1 and dissolved inorganic carbon = 1 mol l-1. After Garrels and Christ (1965).

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 components. In this case Fe2+, Fe3+ and S = 10-6mol l-1 and dissolved inorganic carbon = 1 mol l-1. After Garrels and Christ (1965).

(continued)

the existence of water. Under very oxidizing conditions (Eh 0.6 to 1.2 V) water is broken into oxygen and hydrogen ions and under highly reducing conditions (Eh 0.0 to -0.6 V) water is reduced to hydrogen. Eh-pH diagrams are used to visualize the effects of changing acidity and/or redox conditions. The diagram for iron minerals is typical (Fig. 1). The lines represent conditions under which species on either side are present in equal concentrations. The exact position of the lines varies depending on the activities of the various species.

From the diagram it is clear that the mineral haematite (Fe2O3) is usually the stable iron species under oxidizing conditions with pH above 4. Soluble Fe3+ is only present under very acidic conditions because of its tendency to form insoluble hydroxides (Section 5.2). This tendency is only overcome under very acid conditions when hydroxide ion (OH-) concentrations are low. Fe2+ is less prone to form insoluble hydroxides because of its small z/r value (Section 5.2). Fe2+ is thus soluble at higher pH, but can only persist under low Eh conditions, which prevent its oxidation to Fe3+. The small stability field for the common iron sulphide, pyrite (FeS2), shows that this mineral only forms under reducing conditions, usually between pH 6 and 8. Iron carbonate, (siderite FeCO3) is typically stable at either slightly higher or slightly lower Eh than pyrite.

5.5)), DIP can be returned to the water column in association with iron(III) reduction to iron(II). This process of DIP release from sediments can confound efforts to control eutrophication in lakes that are based on reducing the direct DIP inputs.

Increased riverine NO- concentrations due to human activity (see below) mean that DIP is now the main limiting nutrient for plant growth in many freshwaters. The consequent relationship between DIP and chlorophyll levels (a measure of algal biomass) (Fig. 5.11) makes the management of phosphorus inputs to rivers and lakes very important. In some areas DIP inputs are consequently stringently

Fig. 5.10 Relationship between dissolved ion concentration and river discharge in the River Yare (Norfolk, UK). Direct discharges of phosphorus from sewage and products of weathering (HCO3- and Na+) decline in concentration (are diluted) as discharge increases. By contrast, heavy rainfall leaches NO3- from soil, causing NO3- concentrations to rise as discharge increases. After Edwards (1973).

Fig. 5.10 Relationship between dissolved ion concentration and river discharge in the River Yare (Norfolk, UK). Direct discharges of phosphorus from sewage and products of weathering (HCO3- and Na+) decline in concentration (are diluted) as discharge increases. By contrast, heavy rainfall leaches NO3- from soil, causing NO3- concentrations to rise as discharge increases. After Edwards (1973).

1000

. t t

S100

-

. t,

_o

° 1

— .. 1

•I-►

u

0.1

1

1

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0.3

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Total phosphorus (umol l-1, log scale)

Fig. 5.11 Relationship between summer particulate chlorophyll a levels (as a measure of phytoplankton abundance) and total phosphorus concentrations in various lakes, with data plotted on log scales. After Moss (1988).

Fig. 5.11 Relationship between summer particulate chlorophyll a levels (as a measure of phytoplankton abundance) and total phosphorus concentrations in various lakes, with data plotted on log scales. After Moss (1988).

Oxidation

Denitrification

N2(N2O)

Photosynthesis

Oxidation

Oxidation

Photosynthesis

Oxidation

■ Nitrogen fixation

Fig. 5.12 Nitrogen cycling in natural waters.

controlled to limit algal growth in the receiving waters, even though the need for this strategy arises from nitrate enrichment.

Nitrogen

Nitrogen (N) chemistry and cycling (Fig. 5.12) is complex because nitrogen exists in several oxidation states (see Box 4.3), of which N(0) nitrogen gas (N2), N(3-)

Fig. 5.13 Mean annual dissolved nitrate concentrations in the River Thames, UK, 1930-1980. WHO limit refers to the World Health Organization recommended maximum safe nitrate concentration in drinking water. Data courtesy of UK Department of the Environment National Water Council, Crown Copyright 1984.

Fig. 5.13 Mean annual dissolved nitrate concentrations in the River Thames, UK, 1930-1980. WHO limit refers to the World Health Organization recommended maximum safe nitrate concentration in drinking water. Data courtesy of UK Department of the Environment National Water Council, Crown Copyright 1984.

ammonium (NH+) and N(5+) nitrate (NO-) are the most important. Nitrogen gas dissolved in natural waters cannot be utilized as a nitrogen source by most plants and algae because they cannot break its strong triple bond (see Section 2.3.1). Specialized 'nitrogen-fixing' bacteria and fungi do exist to exploit N2, but it is not an energetically efficient way of obtaining nitrogen. Hence, these microorganisms are only abundant when N2 is the only available nitrogen source. Nevertheless, along with fixation of N2 by lightning, nitrogen-fixing microorganisms provide the major natural source of nitrogen for rivers. Increases in both the area and the intensity of agricultural activity are probably responsible for the increased NO- concentrations seen in British (Fig. 5.13), other European and North American rivers. Globally, human activity has doubled the natural riverine transport of reactive nitrogen, mostly as NO3-.

Biological processes use nitrogen in the 3- oxidation state, particularly as amino functional groups (see Table 2.1) in proteins. This is the preferred oxidation state for algal uptake (although NO3- and NO2- can be taken up) and also the form in which nitrogen is released during organic matter decomposition, largely as NH4+. Once released into soils or water, NH4+ being cationic may be adsorbed on to negatively charged organic coatings on soil particles or clay mineral surfaces. Ammonium is also taken up by plants or algae, or oxidized to NO- and ultimately NO3-, a process that is usually catalysed by bacteria.

In contrast to NH+, NO-—the thermodynamically favoured species in oxygenated waters—is anionic, soluble and not retained in soils. Therefore, NO-from rainwater or fertilizers, or derived from the oxidation of soil organic matter

Fig. 5.14 Seasonal variations in dissolved phosphorus, silicon, nitrate and particulate chlorophyll a (as a measure of phytoplankton abundance) in the River Great Ouse (eastern England). Data from Fichez et al. (1992).

and animal wastes, will wash out of soils and into rivers. The seasonal variation of NO 3 concentrations in many temperate rivers is caused mainly by fluctuation in supply of NO- from soil. In summer, NO- concentrations are low because soil-water flushing by rainfall is low. In the autumn, soil moisture content increases, allowing nitrate to wash out of the soil into rivers (Fig. 5.14). In nitrate-rich rivers like the Great Ouse in eastern England (N:P about 30:1 in winter), biological production makes little initial impact on NO- levels until later in the season as the NO 3 supply decreases due to reduced runoff. A NO3 minimum is reached in summer as a result of the reduced supply from soils and increased biological uptake, before rising again in the autumn (Fig. 5.14). DIP concentrations, in contrast, show more erratic behaviour (Fig. 5.14), reflecting the influences of biological and dilution control working out of phase, but are generally higher during low-flow conditions in summer.

Apart from biological uptake, denitrification in low-oxygen environments is the most important way that NO- is removed from soil, rivers and groundwater. It has been estimated that, in the rivers of northwest Europe, half of the total nitrogen input to the catchment is lost by denitrification before the waters reach the sea. Thus, under low redox conditions, DIP is mobilized during iron(III) reduction and NO- is lost, again emphasizing the importance of redox processes in environmental chemistry.

Phosphorus and nitrogen in groundwater

The very different chemistry of DIP and NO- is illustrated by their behaviour in groundwater. On the limestone island of Bermuda there is little surface water because rainfall drains rapidly through the permeable rock to form groundwater. Almost all of the sewage waste on Bermuda is discharged to porous walled pits that allow effluent to gradually seep into the groundwater. The sewage has a N: P ratio of about 16:1, and yet Bermudian groundwater is characterized by very low DIP concentrations (average 3.5 |imoll-1) and very high NO- concentrations (average 750 |imoll-1). The N:P ratio of 215:1 for the groundwater implies removal of more than 90% of the DIP, by natural precipitation of calcium phosphate minerals. This process is now used as a sewage treatment stage for DIP removal from some waste waters.

High nitrate concentrations are characteristic of groundwater in areas of intensive agriculture and can compromise its use as a drinking water supply (Section 5.7). The main potential human health hazard is a condition called methaemoglobinaemia, where NO- combines with and oxidizes haemoglobin in the blood, robbing the cells of oxygen. This condition affects some adults with specific enzyme deficiencies, but also newborn babies, resulting in the name 'blue-baby syndrome'. To safeguard drinking water supplies in agricultural areas such as southeast England, farmers are required to control fertilizer inputs in areas of groundwater recharge.

Silicon

One other important nutrient, silicon, is used by diatoms (a group of phyto-plankton) to build their exoskeletons. Diatoms are capable of rapid and prolific growth in nutrient-rich conditions. In temperate rivers, diatom blooms occur early in the year. For example, in the River Great Ouse in eastern England, silicon levels fall in early spring as diatom growth begins and rise again in summer as diatoms are displaced by other algal groups (Fig. 5.14). Since silicon is derived entirely from weathering reactions, its naturally low concentrations may be drastically reduced by diatom blooms, such that further diatom growth is limited, particularly where NO3- and DIP have been enriched by human activity. Thus silicon availability limits species diversity but not total phytoplankton biomass.

The presence of lakes and dams in river catchments has an important effect on nutrient transport. Increased water residence times and improved light conditions promote algal (particularly diatom) blooms that are very effective at removing dissolved silicon (DSi). In Scandinavia, for example, catchments without lakes have DSi concentrations around 164 |imoll-1, almost four times higher than those in catchments where lakes and reservoirs cover more than 10% of the area (DSi = 46 |mmoll-1). A particularly striking example of this effect is seen in the Danube, the largest river draining to the Black Sea, which was dammed by the 'Iron Gates' on the Yugoslavian/Romanian border in the early 1970s. DSi concentrations fell three-fold as a result of the damming, and the resulting decrease in Si: N ratios in water draining to the Black Sea (exacerbated by increases in nitrate inputs) has resulted in large-scale changes in phytoplankton ecology in the Black Sea itself. Large numbers of the world's rivers are now extensively dammed for flood control and to provide hydroelectric power. These numbers are expected to increase in the near future and the resulting effects on riverine nutrient fluxes will also grow.

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