Natures Acid Drainage Laboratory: Lessons from the Rotlbach (Zillertaler Alps / Austria)

摘要

The small perennial alpine stream we have investigated drains pyritic schist. It is located in the High Alps and is famous for being the first finding of naturally formed schwertmannite. Changing pH-conditions due to alkaline tributaries lead to the precipitation of a complete succession of secondary iron minerals which can be sampled easily on the barren bed rock. The mineral series starts with iron(III) sulphates at the springs, and encompasses ferrihydrite and goethite as well as the already mentioned sulfoxyhydroxide schwertmannite. The pH of the main stream stays at relatively low values around 3 - 3.5 which is only slightly higher than the springs, despite the temporarily high inflows of the alkaline tributaries. This is obviously the result of the hydrolysis of iron(III). At the springs the acid drainage also contains up to several hundred micrograms per-litre of heavy metals and metalloids. However, metal concentrations are reduced to drinking water standard by the very high adsorption capacities of the precipitated iron minerals. Even at very low metal concentrations the degree of adsorption (especially for arsenic) to the precipitated iron oxides can lead to concentrations of up to 6,000 mu g g~(-1). This can be as much as 150 times the concentration found in the original source, i.e. the pyrite grains. These high metal loads should be of concern whenever and wherever iron (and/or manganese) precipitates form during the life cycle of a mining project, even when water quality is excellent.