Comparison of Aqueous-Phase S IV Oxidation Paths

We see that under these conditions oxidation by dissolved H202 is the predominant pathway for sulfate formation at pH values less than roughly 4-5. At pH 5 oxidation by O3 starts dominating and at pH 6 it is 10 times faster than that by H202. Also, oxidation of S(IV) by 02 catalyzed by Fe and Mn may be important at high pH, but uncertainties in the rate expressions at high pH preclude a definite conclusion. Oxidation of S(IV) by N02 is unimportant at all pH for the concentration levels above. 

The oxidation rate of S(IV) by OH cannot be calculated using the simple approach outlined above. Since the overall rate depends on the propagation and termination rates of the radical chain, it depends, in addition to the S(IV) and OH concentrations, on those of H02, HCOOH, HCHO, and so on, and its determination requires a dynamic chemical model. 

The inhibition of most oxidation mechanisms at low pH results mainly from the lower overall solubility of S02 with increasing acidity. H202 is the only identified oxidant for which the rate is virtually independent of pH. 

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