Stibnite precipitation in geothermal power station fluids

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WAI-O-TAPU

The Wai-O-Tapu system is the ultimate product of eruptions 160 000 years ago and the subsequent seismic and hydrothermal activity (Hedenquist and Henley, 1985). The central feature of Wai-O-Tapu is Champagne Pool, a 70 m diameter and 60 m deep, slightly acidic (5.7 pH), chloride-sulfate spring, which is shown in Figure 4.2 (Lloyd, 1959). Hydrogen sulfide (H2S) and carbon dioxide (CO2) gases continually evolve from the 75 oC waters (mostly CO2), and their tiny bubbles give the spring the champagne-like appearance for which the pool is named (Weissberg, 1969). The pool is rimmed with silica sinter, upon which an orange precipitate has formed. After silica and native sulfur, Sb and As sulfides are the next most abundant constituents of this precipitate, which also has elevated gold (Au), silver (Ag) and tellurium (Te) concentrations (Pope et al, 2005).

WAIMANGU

Waimangu is a younger system, forming after the Tarawera eruption in 1886, and shown in Figure 4.4 (Simmons et al, 1994). One of the central features of Waimangu is Frying Pan Lake (also known as Echo Lake), which formed as the result of hydrothermal eruptions in 1917 (Browne and Lawless, 2001). Ritchie (1961) reported similar levels of Sb in Frying Pan lake as found in Champagne Pool (~10 µg/L), and Sb- and As-sulfide minerals are present in Frying Pan Lake sediments (Seward and Sheppard, 1986). The lake is also characterised by the presence of large (up to 3 m long, 1.5 m wide) stromatolites (Jones et al, 2005). The lake covers 3.8 ha and discharges hot (50 oC) sulfate-chloride water from its northeast corner.

SAMPLING PROTOCOLS

The Wai-O-Tapu and Waimangu geothermal fields are “Protected Geothermal Systems”, which meant that a sampling permit was required from the Department of Conservation, and there were restrictions on the time of sampling and the type of sample that could be collected. Primarily for health and safety reasons, sampling could only be conducted between 8:30 a.m. and 4:30 p.m., so no sampling could occur overnight. Also, only aqueous samples and suspended particulate material in the water column (SPM) could be collected, which meant that bed sediment or precipitate samples could not be collected. However, because algae growing in the discharge were in the water column, algal samples from Waimangu were collected.

SITES AND FREQUENCY

The Wai-O-Tapu system was sampled on January 6th and on June 28th, 2007. These dates were chosen as being as close as practicably possible to the longest and shortest days of the year (by daylight hours). In the summer survey, six sites from between Champagne Pool and the discharge from Alum Lake were each sampled five times over the course of the eight hours (sites are shown in Figure 4.5). During winter, the sites sampled were restricted to concentrate on the undiluted discharge above Bridal Veil Falls, and the discharge from Lake Whangioterangi (Site WT 9). The extra winter sites are shown in blue on Figure 4.5. Each site was sampled six times. A list of sites and a brief description of each is presented in Table 4.1.

FIELD CONDITIONS AT WAI-O-TAPU

During summer sampling, the weather was fine (maximum brightness > 120 kLux) and the air temperatures warm (20-35 oC) throughout the day. In winter sampling, conditions were cold (air temperature 1-11 oC), with fog until 11:00 a.m., and overcast for the entire day (maximum brightness 26 kLux). Regardless of season, water temperatures peaked early afternoon (Figure 4.7a), typically later than the peaks in light intensity, which occurred about noon. The pH of the discharge from Champagne Pool steadily declined with time during the day in both seasons (Figure 4.7b), and was circum-neutral (pH ~7) in summer and acidic (pH ~4) in winter.

ARSENIC, SULFIDES AND SULFATE

The strongest correlations between Sb and any other measured parameter occurred within the winter dataset, where Sb concentrations correlated slightly with As (R2 = 0.55, Figure 4.12a) and reasonably well with SO4 concentrations (R2 0.73, Figure 4.12b). For As, the R2 value for the correlation is relatively low, but this in part can be explained by the problems inherent in comparing values of such different magnitudes. The correlation between As and SO4 concentrations for the winter data set was weaker (R2 = 0.40, not shown). [text]

TABLE OF CONTENTS :

• Abstract
• Acknowledgements
• Table of contents
• Nomenclature and list of abbreviations
• 1 Introduction
  • 1.1 Background and rationale
    • 1.1.1 History
    • 1.1.2 Chemistry
    • 1.1.3 Antimony in the environment
    • 1.1.4 Antimony in miningaffected environments
    • 1.1.5 Antimony in geothermally influenced systems
    • 1.1.6 Arsenic
  • 1.2 Research objectives
  • 1.3 Thesis outline
• 2 Methods and protocols
  • 2.1 Sample Collection
    • 2.1.1 Nonpowerstation aqueous samples
    • 2.1.2 Power station fluids
    • 2.1.3 Preservation
    • 2.1.4 Sediments from the Waikato River
    • 2.1.5 Biota
  • 2.2 Chemical analyses of waters
    • 2.2.1 In situ analyses
    • 2.2.2 Antimony and arsenic
    • 2.2.3 Other elements
  • 2.3 Solid Analyses
    • 2.3.1 Sediment digestions
    • 2.3.2 Sequential Extractions
    • 2.3.3 Plant digests
  • 2.4 Adsorption Experiments
  • 2.5 Modelling methods
    • 2.5.1 Geothermal fluid modelling
    • 2.5.2 Natural springs and receiving waters modelling
• 3 Stibnite precipitation in geothermal power station fluids
  • 3.1 Background and study obectives
    • 3.1.1 Mineral scales in geothermal power stations
    • 3.1.2 The Rotokawa and Ngawha sites
    • 3.1.3 Study objectives
  • 3.2 Sampling and analytical methodology
  • 3.3 Thermodynamic modelling
    • 3.3.1 Data selection
    • 3.3.2 Modelling methodology
  • 3.4 Power station fluid chemistry
    • 3.4.1 Rotokawa
    • 3.4.2 Ngawha
  • 3.5 Thermodynamic modelling results
    • 3.5.1 Rotokawa model
    • 3.5.2 Ngawha model
  • 3.6 Antimony transport in geothermal power stations
    • 3.6.1 Factors controlling stibnite precipitation
  • 3.7 Conclusions and potential further research
• 4 The fate of antimony released from surface geothermal features
  • 4.1 Background and study objectives
    • 4.1.1 WaiOTapu
    • 4.1.2 Waimangu
  • 4.2 Sampling protocols
    • 4.2.1 Sites and frequency
  • 4.3 The behaviour of Sb at WaiOTapu
    • 4.3.1 Field conditions at WaiOTapu
    • 4.3.2 Metalloid concentrations below Champagne Pool
    • 4.3.3 Correlations between variables at WaiOTapu
    • 4.3.4 Analysis of suspended material from WaiOTapu
    • 4.3.5 Diurnal fluctuations in Sb concentrations
    • 4.3.6 The fate of Sb produced from Champagne Pool
  • 4.4 The behaviour of Sb at Waimangu
    • 4.4.1 Field Conditions at Waimangu
    • 4.4.2 Metalloid distribution downstream of Frying Pan Lake
    • 4.4.3 Aqueous relationships at Waimangu
    • 4.4.4 Suspended sediments and algae
    • 4.4.5 The fate of Sb produced from Frying Pan Lake
  • 4.5 Conclusions
• 5 The behaviour of antimony in receiving environments
  • 5.1 Background and study objectives
    • 5.1.1 Study goals
  • 5.2 Waiotapu Stream
    • 5.2.1 Results
    • 5.2.2 Analysis and discussion of Sb in the Waiotapu Stream
  • 5.3 The Waikato River
    • 5.3.1 Antimony along the length of the Waikato River
  • 6 Synthesis and conclusions
  • 6.1 Insights into geothermal Sb behaviour
  • 6.2 Synthesis
  • 6.3 Limitations
  • 6.4 Future research
    • 6.4.1 Thermodynamics
    • 6.4.2 Speciation
    • 6.4.3 Bacteria and kinetics
    • 6.4.4 Adsorption and redox chemistry
    • 6.4.5 Sites and Sampling
  • 6.5 Final conclusions
• References

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The behaviour of antimony in geothermal systems and their receiving environments