Date of Award
Master of Science (MS)
Thiocyanate (SCN-) is formed from both natural and industrial processes, including the use of cyanide to process gold and silver ores rich in sulfide minerals. In mining operations however, thiocyanate also plays a role as an alternative lixiviant to cyanide for leaching gold and silver out of their ores. Thiocyanate may be less toxic than cyanide but it is more stable and has proven to be more difficult to destroy in mining effluents. In addition, just like cyanide, an environmental contaminant of concern – nitrate – is the final product of the oxidation of thiocyanate.
This study investigated the chemical behavior of thiocyanate in the presence of oxidized nitrogen in an anoxic environment as well as under controlled pH and temperature conditions; with the possibility of generating nitrogen gas. If possible, this reaction will be similar to the anaerobic, microbe-mediated oxidation of ammonia (anammox) in the presence of nitrite or nitrate – a technique currently in use for wastewater treatment. The results obtained from this investigation can have possible application in the treatment of mining effluents; particularly of abandoned gold or silver mines (post-production) where SCN- may have accumulated from the use of cyanide as a leaching agent.
Both low and high temperature experiments were conducted at different pH to examine the various mechanisms by which anaerobic thiocyanate oxidation (anSCNox) can occur. A 30 – 40% decrease in thiocyanate concentration was observed after thirty-one days in experiments performed at room temperature and ambient pH where thiocyanate was in association with nitrate or nitrite and gravel substrate obtained from a decommissioned mine tailings impoundment. It is believed that the microbes required to mediate anSCNox were present in the gravel substrate. Whether or not nitrogen gas was generated from this reaction is yet to be determined.
Hydrothermal experiments were conducted at pH between 2.0 –10.95 and temperatures between 60-300oC. The results from these experiments show that thiocyanate dissociation is both temperature and pH dependent. Thiocyanate is stable at temperatures of up to 150oC at ambient pH. However, thiocyanate will dissociate completely to form H2S, NH4+ and CO2 at pH 2.0 and temperature ≥ 150oC. The rates of thiocyanate dissociation were determined at different temperature and pH conditions and can be described by the following equation: k = -0.3430 + 0.001525T – 0.04835pH; where k and T represent reaction rate constant (per day) and temperature (Kelvin) respectively. The activation energies of thiocyanate dissociation at ambient pH (150 – 250oC) and pH 2.0 (25 – 200oC) were estimated to be 71.1kJ/mol and 32.7kJ/mol respectively.
Gure, Alero, "ANAEROBIC DISSOCIATION OF THIOCYANATE UNDER CONTROLLED PH AND TEMPERATURE" (2016). Graduate Theses & Non-Theses. 65.