During the oxidation process, the sulfur atom passes through several oxidation states. In the first step the HS- ions are oxidized to elemental sulfur consuming one equivalent of H2O2, as show in Eq. (8). The produced elemental sulfur can react with two equivalents of H2O2 to form sulfite (SO3-2), which can subsequently be oxidized to sulfate (SO4-2), consuming another equivalent of H2O2 (Eqs. (9) and (10)).