Abstract
Mineral solubility is widely recognised as a control on environmental release of dissolved arsenic. Thermodynamic modelling indicates that most arsenic minerals are readily soluble when exposed to surface waters. However, field observations and actual measurements demonstrate that in some environments, these thermodynamically predicted solubilities are overestimated. This study compiles empirical observations made on several arsenic minerals across a range of sites in New Zealand. These observations were made on natural settings and at mine sites and cover a range of time scales, from months in the case of kinetic leach tests, decades in historic mine wastes, to millions of years in natural settings.
We have observed widespread arsenopyrite (FeAsS) in oxic settings, despite it being thermodynamically unstable in oxygenated environments. In each case, the arsenopyrite has a coating of As-bearing Fe-oxyhydroxide and/or scorodite (FeAsO4.2H(2)O). Further, scorodite is theoretically highly soluble under circumneutral-alkaline conditions, but secondary scorodite, kankite (FeAsO4.3.5H(2)O) and bukovskyite (Fe-2(AsO4)(SO4) (OH).7H(2)O) can remain essentially unaffected by rain and surface water runoff, with minor local mobilisation and recrystallization.
Arsenolite (As2O3) is generally regarded as highly soluble; solubility studies show that arsenolite dissolution can yield 12 g/L dissolved As. Our empirical observations have seen a maximum of "only" 0.3 mg/L dissolved As in saturated historic mine tailings and arsenolite has remained largely intact for 80 years. In a lab experiment to simulate arsenopyrite dissolution in waste rock with monthly water inundation, dissolved As levels reached 16mg/L and an arsenolite coating formed on the arsenopyrite surface which persisted after subsequent inundations.
We find that the processes and physical parameters that control the dissolution rates of arsenic minerals are complex; and grain size, surface area, secondary mineral armouring and subsequent encapsulation are important factors. Our observations provide some context for further research of dissolution rates of arsenic mineral solubilities.