The safe disposal of arsenic (As) is important for the metal extraction industry
worldwide. Previous work has shown that crystalline ferric arsenate (FeAsO4·2H2O),
which is the synthetic version of the natural mineral scorodite, fulfils most of the
criteria for safe disposal. Presently, this requires the use of expensive pressurised
equipment in industrial applications. There is increasing evidence that
microorganisms play an important part in the geochemical cycle of As. In
environmental samples, bacteria are often closely associated with mineral precipitates,
as well as in bioleaching samples.
Strains of acidophilic bacteria (Acidithiobacillus spp., Leptospirillum spp.,
Sulfobacillus spp. and Thiomonas spp.) have been enriched and isolated in liquid and
on solid media from rock samples from former mine sites in the Czech Republic and
the UK, and experiments carried out to determine whether the bacteria can catalyse
the formation of ferric arsenate.
Crystalline ferric arsenate has been identified in material precipitated by an
environmental strain of Acidithiobacillus spp. in liquid media within 150 h-1 at 35°C
and pH 3.0. Acidithiobacillus spp. also precipitated poorly crystalline ferric arsenate
at pH 2.4 25°C. An environmental strain of Leptospirillum spp. also showed slightly
crystalline scorodite at pH 1.5 25° and at pH 3.0 35°C. There is no evidence that the
selected bacteria can precipitate ferric arsenates on solid media, however, despite
varying concentrations of As added as As3+ or As5+. Material formed has been x-ray
amorphous due to rapid formation, and FTIR analyses have shown no increase in
crystallinity with age over the months in which the precipitates have been studied.