2 FeS2 + 16 H2O + 15 Br2 → 30 (Br)- + 2 (Fe)3+ + 32 H+ + 4 SO42-
This is an oxidation-reduction (redox) reaction:
30 Br0 + 30 e- → 30 Br-I (reduction)
4 S-I - 28 e- → 4 SVI (oxidation)
2 FeII - 2 e- → 2 FeIII (oxidation)
Br2 is an oxidizing agent, FeS2 is a reducing agent, FeS2 is a reducing agent.