Kinetic Study of the Oxidation of Hydrogen Sulfide by Fe (III)-Tetrapolyphosphate (TPP) and Fe (II)-TPP by O2

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 (E_1/2) of the complex. The E_1/2 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⁻.