Kinetic study of the oxidation of hydrogen sulfide by Fe(III)-Tes=trapolyphosphate (TPP) and Fe(II)-TPP by O2

- Fe(III)-tetrapolyphosphate(TTP) is a highly stable catalyst for the air/O2 oxidation of HS− in buffered/non-buffered solutions.
- The complex catalyzes e−-transfer from HS− to O2, which depends on the E1/2 (varies from 0.061 to −0.14 V vs. SHE as pH varied from 4-9) of Fe(III)-TPP.
- The Fe(III)TPP catalytic oxidation of HS− generates Fe(II)TPP, which is oxidized by O2 according to the well know four-step reaction mechanism.
- The second-order rate constant of the reaction between Fe(III)TPP and HS− at pH 9 was found to be about 30.5 ± 21.0 M−1 s−1.
- This study elucidates the kinetics of Fe(II)TPP oxidation by O2 and the capability of Fe(III)TPP as a catalyst for the air/O2 oxidation of HS−.

Tetrapolyphosphate (TPP) forms complex with iron(III) (Fe(III)-TTP), which is a highly stable catalyst for the air/O2 oxidation of HS− in both the buffered and non-buffered solutions. The first step in this process is formation of the Fe(III)(HS−)TPP complex followed by an electrontransfer from HS− to Fe(III) and formation of Fe(II)TPP and HS
- . The electron transfer process depends on the half-wave potential (E1/2) of the complex. The E1/2 of Fe(III)/(II)TPP was measured electrochemically and it was found to decrease from 0.061 to −0.14 V vs. SHE (standard hydrogen electrode) with increasing solution pH from 4 to 9. The oxidation of Fe(II)TPP by O2 is shown to follow a four-step reaction mechanism, and the reaction kinetic parameters were calculated using experimental data. The second-order rate constant of the reaction between Fe(III)TPP and HS− at pH 9 was found − within 95% confidence limit − to be about 30.5 ± 21.0 M−1s−1. This study provided valuable information about the oxidation characteristics of Fe(II)TPP by O2 and the capability of Fe(III)TPP as a catalyst for the air/O2 oxidation of HS−.

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