Kinetic effect of electrolytes on the oligomerization of hydrosulfide into polysulfides and colloidal sulfur with iron(III) trans-1,2-diaminocyclohexanetetraacetic acid in anoxic aqueous solutions

The kinetic electrolyte effect of dissolved NaCl, LiCl and Na2SO4 on the oligomerization of the hydrosulfide ion (HS-)(HS-) into polysulfides then in colloidal sulfur assisted by iron(III) trans  -1,2,-diaminocyclohexanetetraacetate (iron(III)-cdta) complexes was studied in anoxic alkaline solutions at 25±1∘C. The three electrolytes have been confirmed to boost the HS-HS- oligomerization reaction considerably. For example, the HS-HS- half-life decreased from 16.7 min in demineralized water (pH=10.5)(pH=10.5) to 12.4 min for the corresponding 0.01 mol/L NaCl solution. It further decreased to 5.1 min for a 0.1 mol/L NaCl solution. The kinetic enhancement factor is attributed to the cationic species (i.e. Na+Na+, Li+Li+) both affecting the reactive species’ activity coefficients within experimental and calculation accuracy. It improves the initial HS-HS- and iron(III)-cdta reaction rates, reduces the induction period for elemental sulfur formation which in turn enhance the overall HS-HS- consumption rate. The HS-HS- and iron(III)-cdta concentration profiles were also fitted with corresponding rate laws derived from a simplified HS-HS- oligomerization mechanism and application of kinetic electrolyte constants on the activity coefficient Hückel equation.