Trace element uptake in fresh and aged aluminium oxyhydroxysulfates and hydroxides: Implications for mine drainage-affected environments

Abstract

Aluminium oxyhydroxysulfates and hydroxides are known to precipitate from mining-generated acidic, neutral and basic pH drainage waters that are enriched in potentially toxic metal(loid)s. Their role as medium-term (1 year) sinks of Al, As, Cu and Ni at pH values of 4, 7 and 10 was assessed using batch experiments. X-ray diffraction analysis showed that basaluminite initially formed at pH 4 and 7, whereas at pH 10, bayerite or nordstrandite (both Al(OH)3) formed. After 12 months of ageing at 20 °C, some of the pH 4 basaluminite recrystallised to form alunite, the pH 7 basaluminite recrystallised to form gibbsite, and the bayerite/nordstrandite was unchanged. At pH 4, the basaluminite took up As but did not take up Ni or Cu. By the end of the 12 months, some As was released from the basaluminite or alunite, and some Cu was taken up by these phases. At the beginning of the pH 7 experiments, As, Cu and Ni were taken up in the basaluminite, and these were retained in the gibbsite that replaced the basaluminite after 12 months. In the pH 10 experiments, only As was expelled from the bayerite/nordstrandite after 12 months. The differences in uptake of As, Cu and Ni are attributed to their aqueous speciation relative to the net surface charge of the Al oxyhydroxsulfates and hydroxides. Release of As from the pH 4 and pH 10 phases at 12 months was likely due to incorporation within the poorly crystalline original phases, followed by progressive release as the phases transformed to more crystalline minerals (at pH 4) or become more crystalline (at pH 10) with ageing. The results have significant implications for the mineralogy of Al oxyhydroxysulfates and hydroxides, and the cycling of As, Cu and Ni, in mine drainage systems.