Schwertmannite. Formation, stability and applications / Швертманнит. Формирование, стабильность и применение

Atmospheric exposure of iron sulphide minerals, such as pyrite (FeS2) and arsenopyrite (FeAsS), around mine dumps and waste piles releases large amounts of SO4 2− and Fe2+ into nearby aquatic environments, such as streams and water channels. This leads to severe acidification of the affected water bodies, with pH levels as low as 1.0. Such acidity is a major global concern because it causes the dissolution of nearly all mineral phases. In addition to iron sulphides, similar acidic conditions can develop around other sulphide mines, such as those containing lead (PbS), zinc (ZnS), copper (CuS), and nickel (NiS). Toxic metals, including As, Pb, Cd, Zn, Cu, and Ni, are mobilized due to the dissolution of gangue materials in these extremely acidic environments, resulting in their enrichment in the surrounding ecosystem.

The Fe2+ released during the oxidative dissolution of FeS2  (Eq. 1.1) is simultaneously oxidized to Fe3+ by atmospheric O2 (Eq.  1.2) and/or by iron‑oxidizing bacteria (discussed below). The Fe3+ produced in this process accelerates FeS2 oxidation in some cases and promotes ferric hydroxide precipitation through hydrolysis in others (Eq. 1.3). The reaction kinetics show that the generation of acidity is eight orders of magnitude greater when sulphides are oxidized by Fe3+ (Eq. 1.4) compared to oxidation by atmospheric O2 (Eq. 1.1). Since oxidation by atmospheric O2 is extremely 
slow, Fe2+ generation is considered the rate‑determining step in the dissolution of iron sulphides (Evangelou and Zhang, 1995). <...>