FORMATION OF SCHWERTMANNITE- AND SCORODITE-LIKE MINERAL COATINGS ON
PYRITE AND ITS IMPLICATIONS IN ACID MINE DRAINAGE CONTROL
ABSTRACT
Acid mine drainage (AMD), the very acidic and heavy metals-contaminated leachate produced when pyritebearing rocks are exposed to oxidizing conditions, is a serious environmental problem encountered after the
closure of mines and mineral processing operations. Although numerous researches have been published
related to pyrite oxidation and AMD treatment, an economically sustainable solution to this problem is still
out of reach. Moreover, the majority of these studies were done with freshly cleaved or polished pyrite rather
than the weathered ones typically found in the environment. Long-term exposure of pyrite to air and moisture
enhances the formation of secondary mineral phases like soluble salts and metallic oxyhydroxides that could
play significant roles in pyrite oxidation dynamics and AMD generation.
In this study, we investigated the effects on pyrite oxidation of soluble phases and metallic oxides commonly
found in strongly weathered pyritic wastes. The pyrite sample used in the study came from Furikusa mine,
Japan, contains minor amounts of arsenopyrite, and had been exposed to air and moisture for a very long
time. This long-term exposure resulted in the formation of considerably thick salt-like mineral coatings
composed mainly of iron, sulfur and oxygen with trace amounts of calcium, potassium, silicon and arsenic
(Fig. 1(a)). Our results showed that soluble salts could either enhance or retard the oxidation of pyrite because
of their strong influence on the leachate pH as well as in the formation of secondary mineral phases like
scorodite, jarosite or schwertmannite. When the amounts of soluble salts decreased, a mineral-like coating
formed on the pyrite surface that suppressed pyrite oxidation. Moreover, formation of this coating was
apparently promoted after the addition of hematite into the system (Fig. 1(b)). This coating material,
composed of iron oxyhydroxysulfates and arsenates, was mechanically strong enough to withstand the
constant shaking motion of the experiments, and could be potentially used as precursor materials to passivate
pyrite with iron oxyhydroxides that are very stable under surface oxidizing conditions.
Figure 1 – (a) a 3D photomicrograph of the salt-like coating formed on pyrite, and (b) the schwertmanniteand scorodite-like mineral coating formed after 21 days
KEYWORDS
Acid mine drainage, Pyrite, Arsenopyrite, Pyrite oxidation, Pyrite passivation