FACTORS CONTROLLING THE
FATE OF ANTIMONY FROM A
NATURAL GEOTHERMAL SPRING
FROM WAI-O-TAPU TO THE TASMAN SEA
NEW ZEALAND
Introduction
• Especially outside Yellowstone National Park, behaviour of Sb
in surface geothermal systems is rarely commented upon
• Most studies report concentrations of Sb, but little discussion
of the metalloid’s fate
• New Zealand, which has a relative wealth of easily accessible
geothermal systems, makes for an excellent study site to
investigate the behaviour of Sb produced from surface
features
• While most Sb does not make it to the surface (see Wilson et
al 2007), there is still sufficient Sb produced by surface systems
in New Zealand to noticeably elevate Sb concentrations in
New Zealand’s largest river, the Waikato River
Introduction
Conservative
Behaviour
Oxidising environments
pH?
Reducing environments
Non-conservative
Behaviour
Study Area
TAUPO
VOLCANIC
ZONE
Wai-O-Tapu
Wai-O-Tapu – Reducing conditions
• Champagne Pool: 75 oC alkali-chloride
pool (Cl 60 mM; pH 5.3 at surface)
• Very high As concentrations (>5 µM)
• High sulfide concentrations (190 µM)
• Low Sb in surface waters (Deep waters
Sb > 2.5 µM)
• Precipitate weight % Sb and As (with
Au, Hg, Te high too)
Wilson & Planer-Friedrich, In prep
Wai-O-Tapu – Oxidising conditions
• Pool drains in two directions,
wind significant influence
• Based on data from northwest
drainage, thioantimonate species
do not persist in drainage
• Southern drainage exhibits diurnal
behaviour
• Alum Lake sink for Sb
Wilson et al, Under review
N
Behaviour at Wai-O-Tapu
Oxidising environments
SbV dominant
Neutral pH
Sulfide mineral dissolution
Conservative
Behaviour
Acid pH
Enhanced Sb
adsorption
Acid pH
Reducing environments
SbIII dominant
Precipitation of Sbsulfide minerals
Non-conservative
Behaviour
Behaviour in freshwaters
• Aqueous Sb concentrations in Waikato River
relatively stable once dilution by tributaries etc. is
accounted for
• Distribution between aqueous Sb and Sb in
suspended particulate material (SPM)relatively stable
in winter
• Changes in Kd observed in summer a function of
increased algal growth in river
Wilson & Webster-Brown, 2009
Adsorption in the Waikato River
Wilson & Webster-Brown, 2009; Webster-Brown et al, In Prep
Neutral pH, changing redox conditions
• Work by Aggett et al. in 1980s produced evidence
for the release of As and Fe in the bottom of
stratified lakes along the Waikato River
• We expected similar results for Sb, which otherwise
behaves unremarkably in the Waikato lakes
• Instead, in slightly reducing conditions and
detectable sulfide concentrations (>2µM), Sb
apparently removed
Lake Arapuni
Lake Ohakuri
Wilson & Webster-Brown, 2009
Behaviour in the Waikato River System
Neutral pH
Diminished SbV
adsorption
Oxidising environments
SbV dominant
Conservative
Behaviour
Acid pH
Enhanced Sb
adsorption
Neutral pH
Acid pH
Reducing environments
SbIII dominant
Non-conservative
Behaviour
Salinity and Eventual Fate
• Behaviour of Sb in estuaries is still not well
understood
• At Port Waikato, which forms the mouth of the
Waikato River, Sb behaves conservatively in both
field studies and laboratory experiments
Port Waikato
Field
Laboratory
Wilson & Webster-Brown, 2009
Summary
Neutral pH
Oxidising environments
SbV dominant
Diminished SbV
adsorption
Sulfide mineral dissolution
Conservative
Behaviour
Acid pH
Enhanced Sb
adsorption
Neutral pH
Acid pH
Reducing environments
SbIII dominant
Precipitation of Sbsulfide minerals
Non-conservative
Behaviour
Post-Script
• When an element behaves conservatively, physical processes are
much more important than chemical processes
• In other words, to rehabilitate an Sb contaminated oxic and
neutral system, the source must be removed (or one must just
wait it out)
• Case in point - Wai-O-Tapu:
• Favourable winds = Sb transport
• Unfavourable winds = No Sb
The speaker‘s research and attendance was supported by:
The Alexander von Humboldt Foundation;
The Department of Environmental Geochemistry - University of Bayreuth, Germany;
The University of Auckland, New Zealand
Mighty River Power and the TEC
http://www.humboldt-foundation.de
http://www.umweltgeochemie.uni-bayreuth.de/
And thanks to all involved!