Ecological risk assessment of thiosalts

Fahd, Faisal (2014) Ecological risk assessment of thiosalts. Masters thesis, Memorial University of Newfoundland.

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Abstract

Thiosalts are sulphur compounds generated in the processing of sulphide ores, which concentrate in the mining wastewater. The most common thiosalt species are thiosulphate, trithionate, and tetrathionate. While thiosalts are not typically toxic, thiosalts can decompose resulting in pH depression. Current industry practice of periodic checking of the water quality downstream, to assess aquatic risk clearly points to the lack of a comprehensive risk based approach in managing thiosalts. Assessing the aquatic risk to organisms requires predicting thiosalt natural degradation in pond/stream conditions and toxicity data of thiosalt species. Due to the complex reaction pathways and pH dependence of the various thiosalt degradation reactions, assessing the risk to the environment is challenging. A novel methodology is developed for an aquatic community ‘No Observed Effect Concentration’ (NOEC) based on the limited toxicity data that is available for thiosalts. To analyze the indirect effect of thiosalts on pH, a new exposure model is developed to estimate the residual concentration of thiosalts and pH in the water body. The developed exposure assessment model is based on the understanding of the relationship between acid producing (oxidation) and acid consuming (disproportionation) pathways of thiosalts and their reaction kinetics. The results from this model are incorporated into the thiosalts risk assessment and a case study is used to illustrate the model. In this study, the exposure model predicts that trithionate and tetrathionate will degrade to sulphate ions, hydrogen sulphite ions, sulphite ions and elemental sulfur. The concentration of thiosulfate, trithionate and tetrathionate, initially at 25 mg/L, 40 mg/L and 6 mg/L respectively, decreased over the course of the study. Over the duration of 77 hours, thiosulfate degraded completely, while the estimated residual trithionate and tetrathionate concentrations were 13 mg/L and 5.77 mg/L, respectively. The pH of the undiluted effluent was estimated to decrease from pH 9.2 to pH 5.6 within an hour of the effluent discharge and decreased further to pH 4 over a period of the next 3 days. A framework and methodology developed in this thesis can be utilized to estimate the potential direct and indirect risks of thiosalts exposure to ecological entities.

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