Dynamics of pH on the oxidation of HS-HS- with iron(III) chelates in anoxic conditions

A fundamental study about a redox process for the removal of hydrogen sulfide by means of iron(III) organometallic complexes was instigated to evaluate its prospective application for the abatement of total reduced sulfurs in the pulp and paper industry. More specifically, the oxidation kinetics of the hydrosulfide species (HS-HS-) in alkaline solutions (pH=9pH=9–11) with iron(III) trans  -1,2,-diaminocyclohexanetetraacetate (cdta) complexes was investigated at room temperature and near zero ionic strength. Simultaneous measurements of iron(III)-cdta (CFe(III)0: 50–300mmol/m3) and HS-HS- (CHS-0: 30–125mmol/m3) were used to clarify the reaction mechanism as well as the role of individual iron(III)-cdta species (Fe3+cdta4-,Fe3+OH-cdta4-(Fe3+cdta4-,Fe3+OH-cdta4-) and sulfur species (HS-HS-, Sn2-, S8S8) on the kinetics. Fe3+OH-cdta4-Fe3+OH-cdta4-, being the dominating iron(III)-cdta species at pH>10pH>10, was found to be less reactive than Fe3+cdta4-Fe3+cdta4- towards the polysulfides (Sn2-;n=2–8). Oligomerization of HS-HS- into polysulfides accelerates the HS-HS- conversion rate until elemental sulfur (S8S8) appears in the solution. It was determined that elevated alkaline pH decreases the initial HS-HS- conversion rate, stretches the induction period for S8S8 formation, and generally inhibits the overall reaction.