Element chemistry

The 20 largest rivers on Earth carry about 40% of the total continental runoff, with the Amazon alone accounting for about 15% of the total. These rivers give the best indication of global average riverwater chemical composition, which can be compared with average continental crust composition (Table 5.1). Three features stand out from this comparison:

1 Four metals dominate the dissolved chemistry of freshwater, all present as simple cations (Ca2+, Na+, K+ and Mg2+).

2 The low concentration of ions in freshwater.

3 The dissolved ionic composition of freshwater is radically different from continental crust, despite the fact that all of the cations in riverwater, with the exception of some of the sodium and chloride (see Section 5.3), are derived from weathering processes.

Although it is not meaningful to derive a single global average composition for groundwater—because of the marked differences in aquifer rocks—it is nonetheless true that most groundwaters share the three features listed above for riverwater (see Table 5.3).

The difference between crustal and dissolved riverwater composition is particularly marked for aluminium and iron relative to other metals (Table 5.1). This difference results from the way specific metal ions react with water.

Ionic compounds dissolve readily in polar solvents like water (see Box 4.1). Once in solution, however, different ions react with water in different ways (Fig. 5.1). Low-charge ions (1+, 2+, 1-, 2-) usually dissolve as simple cations or anions. These ions have little interaction with the water itself, except that each ion is surrounded by water molecules (Fig. 5.1a). In general, for elements with similar atomic number, the smaller the ionic radius the higher the charge on the ion. Small high-charge ions react with water, abstracting OH- to form uncharged and insoluble hydroxides, liberating hydrogen ions in the process (Fig. 5.1b), for example:

Cation

Cation log z/r < +0.48 Cation electronegativity < 1.2

Oxygen

Hydrogen

Cation

Cation

Oxygen

Hydrogen

Oxygen

Hydrogen

Cation log z/r between +0.48 and +1.08 Cation electronegativity between 1.2 and 1.9

Fig. 5.1 Relationship between cation properties and force of repulsion between a cation and the hydrogen ions in a water molecule. The log z/r data should be compared to Fig. 5.2. Electronegativity is explained in Box 4.2. After Raiswell et al. (1980). (a) Large, low-charge, electropositive ions (e.g. K+) are surrounded by water molecules such that the centre of the negative charge on the oxygen is aligned toward the cation. (b) Smaller, more highly charged, electronegative cations (e.g. Fe3+) interact more strongly with the water molecule forming a bond to the oxygen and displacing one H+ to form hydroxide. (c) Small, highly charged, strongly electronegative cations (e.g. P5+) react to displace H+ ions from the water, forming an oxyanion.

Fe3+

Co2".Ni2+

Log Izl /r

s Si4+(H4SiO4)

Fig. 5.2 Ratio of average elemental riverine particulate to dissolved concentrations plotted against the ratio of charge to ionic radius for the most abundant ions of those elements. In the case of dissolved oxyanions, the relevant dissolved species are shown in brackets. Concentration data from Martin and Whitfield (1983), other data from Krauskopf and Bird (1995).

Still smaller and more highly charged ions react with water to produce relatively large and stable anions (so called oxyanions), such as sulphate (SO4-), by abstracting oxygen ions from water and again liberating hydrogen ions (Fig. 5.1c), for example:

The net effect is to produce large anions, which dissolve readily since the charge is spread over a large ionic perimeter. Other important oxyanions are nitrate (NO-) and carbonate (CO2-).

The general pattern of element solubility can be rationalized in terms of charge and ionic radius (z/r) (Fig. 5.2). Ions with low z/rvalues are highly soluble, form simple ions in solution and are enriched in the dissolved phase of river-water compared with the particulate phase, for example Na+. Ions with intermediate z/r values are relatively insoluble and have a relatively high particulate : dissolved ratio in riverwater, for example Fe3+, Al3+. Ions with large z/r values form complex oxyanions and again are soluble, for example SO4-, NO-.

Some oxyanions exist in solution as weak acids and will ionize or dissociate (see Box 4.5) depending on the pH, as shown here for phosphorus.

At pH 8 the HPO44 species predominates. Understanding dissociation behaviour is very important in calculating the solubility of an ion. To do this it is important to know which species is the dominant one. A water chemist, for example, may wish to know if iron phosphate (FePO4) can form in a river, as this species helps regulate phosphorus concentrations in freshwaters. To know this it is necessary to calculate the PO44 concentration from the total dissolved inorganic phosphorus concentration that is measured analytically and contains all the species in equations 5.3-5.5. This can be done knowing the pH and dissociation constant (see Box 4.5) for equations 5.3-5.5. A worked example for the carbonate system is given in Section 5.3.1

Silicon is mobilized by the weathering of silicate minerals, mainly feldspars (see eqn. 4.14), and is transported in natural waters at near-neutral pH as undis-sociated silicic acid (H4SiO4), an oxyanion (Fig. 5.2). Silicate minerals weather slowly, such that rates of input—and concentrations—of silicon in most fresh-waters are quite low. Despite this, where silicates are the main component of bedrock or soil, H4SiO4 can be a significant dissolved component of freshwater.

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