Applied Catalysis B: Environmental 218 (2017): 186-191.
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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