The Role of One- and Two-Electron Transfer Reactions in Forming Thermodynamically Unstable Intermediates as Barriers in Multi-Electron Redox Reactions

In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O(2) to O(2) (-) with Fe(2+) and Mn(2+) is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO(3) (-), but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H(2)S and NH(4) (+) oxidation in a one-electron transfer step by O(2) is also not favorable unless activation of oxygen can occur. H(2)S oxidation by Cu(2+), Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H(2)S. NH(4) (+) oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N(2)O and N(2), respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH(4) (+) oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element-element transformations.