10 NO3- + 3 As2S3 + 10 H+ + 4 H2O → 9 S + 6 H3AsO4 + 10 NO
This is an oxidation-reduction (redox) reaction:
6 AsIII - 12 e- → 6 AsV (oxidation)
9 S-II - 18 e- → 9 S0 (oxidation)
10 NV + 30 e- → 10 NII (reduction)
As2S3 is a reducing agent, As2S3 is a reducing agent, NO3- is an oxidizing agent.