2 FeIIS2-I + 16 H2O + 15 Br20 30 (Br-I)- + 2 (FeIII)3+ + 32 H+ + 4 SVIO42-

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)

Change the oxidation states