Abstract
Arsenic (As) is a common constituent of mining and geothermal systems. Literature has described the element as a toxin and human carcinogen particularly when it exists as As (III). Wairakei (North Island, New Zealand) geothermal effluents contain ---4 ppm As and the effluent is As (ill) dominated, whereas mining effluents in Otago (South Island, New Zealand) contain -160 ppm. Treatment and removal technologies for the element have been reported to be effective for As (V) but less effective for As (III), and the oxidation of As (III) would not occur, if not a very slow process in the absence of oxidizing agents.
In geothermal effluents, the oxidation of As (III) was studied by following the decrease in As (HI) concentrations in collected samples. The oxidation was found to be biotic and abiotic and influenced by bacteria, manganese, and temperature. The higher the concentration of manganese or the lower the As (III)/Mn molar ratio, the higher the rate of As (ill) oxidation. Furthermore, bacteria were found to increase the rates of oxidation by several order of magnitudes, whereas temperature affects these rates only in the presence of oxidizing agents.
In many hard-rock gold mine operations, As is released in high concentrations during gold extraction. The element is however, highly attenuated such that by the time the effluents are disposed in the tailings dam, only ~2 ppm is detected in porewaters. Attenuation of the element occurs through adsorption with sediments, calcium and iron. The concentration of As released during gold extraction also depends on operating conditions. A higher concentration was released when the cyanidation process was fully oxidized while an 83% reduction in this concentration was observed when oxidation was omitted, although the concentration of As (III) in the effluents increased.