Tetrapolyphosphate (TPP) forms complex with iron(III) (Fe(III)-TTP), which is a highly stable catalyst for the air/O-2 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 center dot. 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-0.14V vs. SHE (standard hydrogen electrode) with increasing solution pH from 4 to 9. The oxidation of Fe(II)TPP by O-2 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(-1)s(-1). This study provided valuable information about the oxidation characteristics of Fe(II)TPP by O-2 and the capability of Fe(III)TPP as a catalyst for the air/O-2 oxidation of HS-. (C) 2017 Elsevier B.V. All rights reserved.