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1 Environmental and Engineering Geochemistry Research Group, Environmental Science and Management, Southern Cross University , PO Box 157, Lismore, NSW 2480, Australia
2 Hunter Enviro-Mining Pty Ltd , 10 James Street, Waitara, NSW 2077, Australia
* Corresponding author (e-mail: malcolm.clark{at}scu.edu.au)
Although BauxsolTM technology is gaining acceptance for treatment of acid rock drainage and industrial effluents, most leaching studies of spent reagent have used material that has been stored in well-oxygenated conditions for no more than a few weeks. Consequently, few long-term stability data are available to predict potential metal release under various geochemical conditions. This study investigates the simulated aging of a metal-laden BauxsolTM reagent under oxic, anoxic and anoxic-reducing conditions. Long-term stability under oxic conditions was simulated by sealing samples in 50 ml centrifuge tubes with a small quantity of water (to facilitate metal transfer and mineral recrystallization reactions), and allowing the contents to age at 65°C over about 3 months. Anoxic conditions were maintained for 6 months in 200-l drums of seawater, using nitrogen to displace any dissolved oxygen; Eh remained between +50 and +100 mV and dissolved oxygen was maintained at <0.1 mg/l. Anoxic-reducing conditions were maintained for 6 months as for the anoxic drums with the addition of rotting oysters, mangrove forest sediment and a small quantity of Na2S; the dissolved oxygen content remained <0.1 mg/l, the dissolved sulphide content >0.3 mg/l and the water Eh less than –350 mV.
The proportion of most metals extractable with pH 2.88 buffered acetic acid extractant (used to indicate relative leachability) decreased by between 25 and >75% during aging under all tested conditions (i.e. the longer the samples were left, the less metal could be extracted) although minor short-term (2–4 weeks) increases in metal mobility were detected with anoxic and anoxic-reducing samples. The data show that metals bound by BauxsolTM reagents become increasingly resistant to leaching over time. The precipitation of insoluble sulphides, low solubility carbonates, hydroxycarbonates or hydroxysulphates, and solid-state diffusion process may explain the observed decreases. A measured increase of c. 16% in the mean volume of hematite crystallites over 3 months in oxic conditions also admits the possibility that metals may be incorporated as impurities in growing oxide and oxyhydroxide crystals.
Key Words: Bauxsol TM • acid rock drainage • long-term stability • pH • redox
