Thiosalt behaviour in aqueous media

Vongporm, Yaovaluk (2008) Thiosalt behaviour in aqueous media. Masters thesis, Memorial University of Newfoundland.

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Abstract

Understanding the behaviour of thiosalts is important in acid mine drainage study as thiosalt oxidation results in pH depression. This research focuses on a study of thiosalt kinetics under acidic, neutral and basic conditions in the temperature range typical of receiving ponds. The thiosalt studies performed in this research included a HSC equilibrium study, a study of freeze-thaw stability, and a thiosalt kinetic reactions study. -- The purpose of the HSC equilibrium study was to identify the major factors involved in thiosalt behaviour and to use this data to aid in later experimental design. The study of thiosalt equilibrium composition was performed using HSC Chemistry software and these results were statistically analysed by Design-Expert software. According to the analysis, the formation of thiosulphate depends upon its initial concentration, temperature, and pH. The formation of polythionate is also influenced by temperature, pH, and initial polythionate concentrations. Though these results were not unexpected, it was not anticipated that under equilibrium conditions the behaviour of thiosulphate and polythionate would be relatively independent of each other. -- As the freeze-thaw cycle has been shown to influence thiosalt speciation, a stability study was conducted to determine the stability of thiosalt species at different pHs. The thiosalts were found to be reasonably stable using a fast-freeze approach, where at pH 2, 4, and 7, most thiosalt concentrations changed within the range of ±6% after fast freezing in liquid nitrogen, storage in a freezer, and thawing in a water bath at 20°C just prior to analysis. -- Kinetic experiments were designed and conducted with the purpose of investigating thiosalt reaction kinetics under acidic, neutral, and basic conditions at variable temperatures. According to the results of this study, thiosulphate decomposed to trithionate, sulphate, and tetrathionate at pH 2. At pH 2 and 9, trithionate decomposed to sulphate, thiosulphate and tetrathionate only at 30°C; while at pH 4 and 7, trithionate decomposed to sulphate, thiosulphate and tetrathionate at temperatures of 15°C or greater. At pH 9, tetrathionate decomposed to thiosulphate, trithionate, and sulphate. In a mixture of thiosalt, thiosulphate and trithionate reacted as in the single species experiments, but tetrathionate was more reactive. Generally, the relative rates of decomposition of thiosulphate, trithionate, and tetrathionate were second or higher order. The decomposition rates increased with increasing initial thiosalt concentration and temperature (except for thiosulphate decomposition at pH 9 and 30°C).

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