| The reduction potential of the Fe(II)/Fe(III) redox couple overlaps those of naturally occurring forms of C, O, N and S, and electron transfer between Fe and these elements is often kinetically facile. The chief intermediary in this process appears to be reactive oxygen species (ROS) generated by the concurrent partial reduction of dioxygen, although some reduced S species are known to react with Fe(III) directly. These overlapping potentials couple with the recurring oxidation and biogenic reduction of Fe species in coastal environments to allow ROS to catalytically link the redox cycles of those elements and others. This coupling occurs over long (days) and short (seconds) time scales and represents a critically important mechanism for kinetically rapid equilibration between atmospheric oxygen and biogenically reduced materials. In this work, we show that S reduces Fe(III) even in the presence of oxygen, thereby acting as a co-catalyst for supporting extended periods of ROS production in natural waters. We report an investigation of the mechanism by which the Fe and S cycles interact in the presence of O on a short time scale, with particular focus on the kinetic controls on the reduction of Fe(III) and concurrent reactive oxygen species (ROS) formation. |
