Aluminium solubility and acidity

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 of aluminium solubility are also complicated by the formation of partially dissociated Al(OH)3 species and complexing between aluminium and organic matter (see Box 6.4). Although aluminium is soluble at high pH, alkaline waters are uncommon because they absorb acid gases, for example CO2 and SO2, from the atmosphere. However, alkaline rivers with aluminium mobility are known. For example, the industrial process for abstracting aluminium from bauxite involves leaching the ore with strong sodium hydroxide (NaOH) solutions. In Jamaica, discharge of wastes from bauxite processing produces freshwater streams with high pH (>8) in addition to high sodium and aluminium concentrations. As these streamwaters evolve, the pH falls to about 8 and the dissolved aluminium: sodium ratio declines as the aluminium precipitates.

It is, however, acidification of freshwaters that commonly results in aluminium mobility resulting in ecological damage. This acidification is typically caused by two anthropogenic processes, acid rain and acid mine drainage.

Al(OH)3(S)+ OH(aq) ^ Al(OH); Al(OH)3(s) ^ Al(a+q)+ 3OH-aq)

5.4.1 Acidification from atmospheric inputs

Acidification of soilwater occurs if the rate of displacement of soil cations by H+ exceeds the rate of cation supply from weathering. Ion-exchange reactions in soils (e.g. eqn. 4.18) help to buffer pH in the short term (see Section 4.8), but over longer periods cation supply to soils from the underlying bedrock is very slow. Rainwater is naturally acidic (see Box 3.7) and soilwaters are further acidified by the production of H+ from the decomposition of organic matter (see eqns 4.6-4.10). Thus, acidification can be a natural process, although acid rain (see Section 3.9) has greatly increased the rate of these processes in many areas of the world.

Acidification of freshwater is most marked in upland areas with high rainfall (hence high acid flux), steep slopes (resulting in a short residence time for water in the soil) and crystalline rocks (which weather, and supply cations, slowly). Thus, while acid rain is a widespread phenomenon, acidified freshwaters are less common and are controlled both by rates of atmospheric input and by rock types (Fig. 5.7). All weathering processes, except sulphide oxidation (see Sections 4.4.2 & 5.4.2), consume hydrogen ions, driving pH toward neutrality. Hence, mature rivers, which drain deeper, cation-rich lowland soils, have higher pH and lower aluminium concentrations.

The effects of upland acidification of freshwaters can be dramatic. Between 1930 and 1975 the median pH of lakes in the Adirondack Mountains of northeastern USA decreased from 6.7 to 5.1, caused by progressively lower pH in rainwater (Fig. 5.7). The acidified lakewater killed fish and other animals by several mechanisms. The problem for fish is that the dissolved Al3+ in the acidic water precipitates as an insoluble Al(OH)3 gel on the less acidic gill tissues, preventing normal uptake of oxygen and suffocating the animal. Similar problems have occurred in Scandinavia and Scotland. In addition to problems in freshwaters, the loss of forests in high-altitude areas has been linked to acid leaching, which leads to impoverishment of soils coupled with direct loss of cations from plant leaves.

5.4.2 Acid mine drainage

Acidification of surface and groundwaters in mining regions is a worldwide problem. In a mid-1980s survey, 10% of streams fed by groundwater springs in the northern Appalachians (USA) were found to be acidified by mine drainage. The acidity is caused by oxidation of sulphide minerals (see eqn. 4.4), common in sedimentary mudrocks, mineral veins and coal deposits. When mineral and coal deposits are mined, sulphide is left behind in the waste rock, which is piled in heaps. These waste heaps have large surface areas exposed to the atmosphere (Plate 5.1, facing p. 138), allowing extensive and rapid oxidation of the sulphide. The problem is long-lived, and often intensifies once mining has finished, because abandoned mines are rapidly flooded by groundwater once the pumps are switched off.

While the oxidation reaction is a natural one (see Section 4.4.2), the mining activities increase the scale and rate of the reaction (see eqn. 4.4). The resulting

Fig. 5.7 Rates of acid deposition (contours of |imolH+1-1) and areas most sensitive to acidification (shaded) based on their rock type: (a) Europe, (b) North America. Modified from Likens et al. (1979), with kind permission of A.M. Tomko III.

H2SO4 makes drainage from abandoned mines strongly acidic (pH as low as 1 or 2). This acidity increases the solubility of aluminium and other metals, causing toxicity in aquatic ecosystems. Microorganisms are closely involved in sulphide oxidation, which can be modelled by a series of reactions:

Fig. 5.7 Cont.

2FeS2(S) +2H2Ofn +70

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