FeS2 + 3 O2 + 2 H2O → Fe + 2 H2SO4
This is an oxidation-reduction (redox) reaction:
2 S-I - 14 e- → 2 SVI (oxidation)
FeII + 2 e- → Fe0 (reduction)
6 O0 + 12 e- → 6 O-II (reduction)
FeS2 is a reducing agent, FeS2 is an oxidizing agent, O2 is an oxidizing agent.