Antimony (Sb) is an element of increasing concern as an environmental contaminant. Geothermal systems are a potential source of Sb in the fresh waters of New Zealand’s North Island, but little is known about the element’s behaviour within geothermal fluids, and even less about the eventual fate of geothermally produced Sb. The purpose of this thesis was to determine the factors controlling geothermal Sb behaviour in a range of environments, in order to begin to develop an understanding of the eventual fate of Sb produced from geothermal systems. Factors controlling Sb precipitation at two New Zealand geothermal power stations, were determined using field measurements and geochemical model predictions. Approximately 75 % of the incoming Sb, which ranged from 960 – 1650 μg/kg in the incoming fluids was removed from solution within the Ngawha and Rotokawa power stations. It was found that changes in pH and temperature were the most important factors controlling stibnite (Sb2S3) precipitation. Thermodynamic databases could be used to model this precipitation process, once updated with recent published Sb2S3 solubility data. The mobility of Sb from Sb2S3 precipitates in geothermal features at Wai-O-Tapu and Waimangu, two New Zealand geothermal fields, were investigated. At Wai-O-Tapu, daytime variations in aqueous Sb concentrations from the discharge of Champagne Pool may be due to to changing sulfide-sulfate equilibria coupled with photosynthetic bacterial processes. While daytime concentrations of Sb approached 200 μg/kg, most of the Sb remobilised by such mechanisms appears to be removed by adsorption onto suspended particulate material (SPM) or reprecipitation (as Sb2S3) in an anoxic, low pH lake feature downstream. Concentrations of Sb in the discharge from Alum Lake were below the analytical detection limit (<0.2 μg/kg). At Waimangu, these daytime fluctuations were not observed in the discharge of Frying Pan Lake, and concentrations of Sb were ~13 μg/kg. In the absence of any downstream acidic waters, no precipitation was observed and only minor adsorption onto SPM was observed. Most of the Sb produced from Frying Pan Lake is therefore transported into Lake Rotomahana, the system’s receiving environment. Natural Sb removal processes in receiving (non-geothermal) environments were also assessed. In the Waikato River, Sb concentrations were low (~1 μg/kg), compared to those observed in geothermal environments studied. The most important process was adsorption to SPM, which is enhanced at low (< 5) pH conditions, or in the anoxic base of stratified lakes. In Lake Ohakuri, which was stratified during the summer of 2007, there was also the potential for the removal of Sb as Sb2S3 in the presence of sulfide species that form in the anoxic layer. There was evidence that the adsorption of Sb changes with changing Fe concentrations in suspended particulate material, and therefore Sb adsorption was higher in winter than in summer. The behaviour of Sb was conservative in the Port Waikato estuary at the mouth of the river. Throughout the research, Sb was compared to arsenic (As), a metalloid previously thought to exhibit behaviour similar to Sb in aquatic environments. It was found that while any removal processes shown to affect Sb will also affect As, the inverse did not necessarily apply. Arsenic will adsorb more readily to SPM than Sb and, while there was evidence for bioaccumulation of As by geothermal algae and freshwater macrophytes, there was no such evidence for Sb. Therefore, if geothermally-derived Sb and As did ever significantly contaminate a downstream environment, it should not be assumed that the processes mitigating As contamination will necessarily also apply to Sb.
| Identifer | oai:union.ndltd.org:ADTP/247832 |
| Date | January 2009 |
| Creators | Wilson, Nathaniel James |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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