FeIIS2-I + 14 (FeIII)3+ + 8 H2O 15 (FeII)2+ + 16 H+ + 2 SVIO42-

This is an oxidation-reduction (redox) reaction:

2 S-I - 14 e- 2 SVI (oxidation)

14 FeIII + 14 e- 14 FeII (reduction)

Change the oxidation states