H2S-II + Fe2III(SO4)3 S0 + 2 FeIISO4 + H2SO4

This is an oxidation-reduction (redox) reaction:

S-II - 2 e- S0 (oxidation)

2 FeIII + 2 e- 2 FeII (reduction)

H2S is a reducing agent, Fe2(SO4)3 is an oxidizing agent.