Although Bauxsol™ 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 Bauxsol™ 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 Bauxsol™ 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.

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