Geology Reference
In-Depth Information
3.3.1.2 Aluminium. The effect of such acidification upon the mobili-
zation, behaviour, and subsequent biological impact of aluminium in
freshwaters has been spectacular. Constituting 8.13% of the Earth's
crust, aluminium is the third most abundant element and is largely
associated with crystalline aluminosilicate minerals of low solubility. In
the absence of strong acid inputs, processes of soil development nor-
mally lead to only a small fraction of aluminium becoming available to
participate in biogeochemical reactions. Thus, in the Northern Tempe-
rate Region, mobilization of aluminium from upper to lower mineral
horizons by organic acids (or H
2
CO
3
) leached from foliage or forest
floors often leads to the formation of Al(OH)
3
in a process known as
podzolization. Under elevated partial pressures of CO
2
, however, dis-
sociation of H
2
CO
3
produces H
1
, which may solubilize aluminium,
while HCO
3
serves as a counterion for transport of cationic Al
III
(aq)
through soil. Upon discharge to a surface water, CO
2
degasses, resulting
in aluminium hydrolysis and precipitation as Al(OH)
3
. Thus, in most
natural waters, concentrations of dissolved aluminium are generally low
due to the relatively low solubility of natural aluminium minerals under
circumneutral pH values. In the case of strong acid (H
2
SO
4
, HNO
3
)
inputs, however, especially to sensitive regions with small pools of basic
cations (Ca
21
,Mg
21
,Na
1
,K
1
) and an inability to retain inputs of
strong acid anions (SO
4
2
,NO
3
,Cl
), acidic cations (H
1
,Al
III
) are
transported along with acid anions from soil to surface water.
43
The speciation of dissolved aluminium in surface waters is highly
pH-dependent (see Section 3.2.3.2), with, in a simple system (considering
only monomeric aluminium species) in equilibrium with solid phase
Al(OH)
3
, the aquated ion Al
31
(aq)
predominating at pH
o
3, progressive
hydrolysis to Al(OH)
21
and Al(OH)
2
in the pH range 4.5-6.5, followed
by Al(OH)
3
0
from pH 6.5 to 7, and then Al(OH)
4
predominating as
the pH increases beyond 7. From pH 4.5-7.5, the solubility of alumi-
nium is low and in this range it is often precipitated as Al(OH)
3
. Below
pH 4.5 and above pH 7.5 the concentration of aluminium in solution
increases rapidly. In a part experimental fractionation, part speciation
modelling study of aluminium in acidified Adirondack surface waters in
the north-eastern USA, Driscoll and Schecher
43
found that, in the
absence of organic ligands, Al
31
(aq)
was significant at pH
o
5, and that
the inorganic monomeric complexes of aluminium predominated at
pH45. In particular, Al-F complexes were the dominant form at pH
5-6 whereas, at pH46, Al-OH species became the major form. In the
presence of organic ligands, however, they found that, over a broad pH
range of 4.3-7.0, alumino-organic complexes were a major component
of the monomeric aluminium. It is the chemically labile inorganic
