Kinetic and Mechanistic Study of Reaction between Sulfide and Sulfite in Aqueous Solution

The reaction between sulfide and sulfite in neutral to weak alkaline aqueous solutions was studied by following thiosulfate and sulfite concentrations using ion chromatography. The thiosulfate formation rate from the reaction 2HS - + 4HSO 3 - → 3S 2 O 3 2- + 3H 2 O at pH 8 to 9 was found to be d[S 2 O 3 2- ]/d t = k A [HS - ][HSO 3 - ] 2, where k A = 1.1 × 10 12 exp(−48000/ RT ) M -2 s -1 . A mechanism for this reaction has been proposed with disulfite (S 2 O 5 2- ) and HSO 2 - intermediates. The measured rate of sulfite disappearance was higher than that calculated from the stoichiometry of the above reaction. This phenomenon is attributed to other reactions, that consume sulfite and form other sulfur compounds such as polythionates, polysulfides, and elemental sulfur. These reactions were treated as a single reaction, whose rate was found to be (−d[HSO 3 - ]/d t ) B = k B [H + ] -0.6 [HS - ] 0.7 [HSO 3 - ] 1.5, where k B = 5 × 10 -5 M -0.6 s -1 at 20 °C. A kinetic model was established on the basis of the kinetic data obtained in this and a previous work. The experimental data at pH 7 agreed with the model prediction in a satisfactory manner. The biphasic behavior of thiosulfate is considered to be critical in developing a new sulfur-producing flue gas desulfurization (SP-FGD) process based on sulfur dioxide absorption using sodium sulfide solution.