Reaction mechanism and stoichiometry of chemical sulfide oxidation
- Chemical oxidation of sulfide is a multi step process involving a series of chemical reactions catalyzed by chemicals present in the wastewater.
- The stoichiometry of this process is a complex one because of the formation of various intermediates and products during the course of the reactions as shown below.
- Elemental sulfur (S0 ), thiosulfate (S2O32-), sulfite (SO22-) and sulfate (SO22-) are the key intermediates and products.
- There is also a possibility of the formation of polysulfides, which originate from a reaction between elemental sulfur and hydrogen sulfide.
- Further chemical oxidation of the thiosulfate and sulfite proceeds at a significantly lower rate than that of sulfide oxidation. Depending upon the time scale considered, thiosulfate could be considered as a stable reaction product, especially when availability of oxygen is limited.
- For complete oxidation of sulfide to sulfate, each gram of sulfide S requires 2 grams of O2, while 0.5 gram of O2 per gram of sulfide (as S) is needed if thiosulfate is produced.
- Kinetics of sulfide oxidation is described by the rate equation of the following form:
where [ST ] is the total dissolved sulfide concentration in mg S/L, [DO] is dissolved oxygen concentration in mg/L, k is the reaction rate constant, is the reaction order with respect to sulfide and is the reaction order with respect to oxygen.
- The reaction rate constant, and the reaction order terms in the above reaction are shown to vary with type of wastewater, temperature, pH, and sulfide to oxygen ratio.
- The equation above does not adequately describe the oxidation of sulfide under high DO conditions, which is normally the case with the oxygen injection. The following kinetic expression is more appropriate under high DO conditions.
Where, rH2S is the oxidation is the sulfide oxidation rate in mg S/L-h, [ST ] is the total dissolved sulfide concentration in mg S/L, k is the reaction rate constant, is the reaction order with respect to sulfide, KO2 is the oxygen half saturation constant and [DO] is the DO concentration in mg/L.
Further information can be obtained as follows:
Reaction mechanism, kinetics and stoichiometry for low DO conditions
Paper: Determination of kinetics and stoichiometry of chemical sulfide oxidation in wastewater of sewer networks by Neilsen et al. (2003)
Kinetics and stoichiometry for high DO conditions
Report – ARC Linkage Project on Sewer Biotransformation
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