| Aluminium is one of the main elements in most mining-affected environments, where it may
influence the mobility of other elements and play a key role on pH buffering. Moreover, high
concentrations of Al can have severe effects on ecosystems and humans; Al intake, for
example, has been implicated in neurological pathologies (e.g., Alzheimer’s disease; Flaten,
2001).
The behaviour of Al in mining-affected environments is commonly determined, at least
partially, by the dissolution of Al sulphate minerals and particularly by the dissolution of
alunite (KAl3(SO4)2(OH)6), which is one of the most important and ubiquitous Al sulphates
in mining-affected environments (Nordstrom, 2011). The presence of alunite has
been described in other acid sulphate environments, including some soils (Prietzel
& Hirsch, 1998) and on the surface of Mars (Swayze et al., 2008). Despite the
important role of alunite, its dissolution rates and products, and their controlling
factors under conditions similar to those found in these environments, remain largely
unknown.
In this work, batch dissolution experiments have been carried out in order to shed light on
the rates, products and controlling factors of alunite dissolution under different pH conditions
(between 3 and 8) and temperatures (between 279 and 313K) similar to those encountered in
natural systems.
The obtained initial dissolution rates using synthetic alunite, based on the evolution of K
concentrations, are between 10-9.7 and 10-10.9 mol/
m-2/
s-1, with the lowest rates obtained
at around pH 4.8, and increases in the rates recorded with both increases and decreases in pH.
Increases of temperature in the studied range also cause increases in the dissolution
rates.
The dissolution of alunite dissolution is incongruent, as has been reported for jarosite
(isostructural with alunite) by Welch et al. (2008). Compared with the stoichiometric ratio in
the bulk alunite (Al/K=3), K tends to be released to the solution preferentially over Al,
leading to dissolved Al/K ratios between 0.5 and 2.5. This depletion of Al in the solution is
especially clear for the experiments at pH 4.5-4.8 and 8 and it is consistent with the results of
elemental quantifications of the same proportions in the reacted alunite surfaces using X-ray
Photoelectron Spectroscopy (XPS).
REFERENCES
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Nordstrom, D.K. (2011): Hydrogeochemical processes governing the origin, transport and
fate of major and trace elements from mine wastes and mineralized rock to surface waters.
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soil-added synthesized aluminium hydroxy sulphate minerals. European journal of soil
science, 49(4), 669-681.
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