Abstract
Iodide reductively dissolves hematite in acidic solutions at 353 K. Initial dissolution rates were measured under conditions of negligible back-reaction. The rate is first order in [H+] and second order with respect to iodide concentration. These results are consistent with a dissolution mechanism that involves fast equilibrated formation of inner-sphere ⋮Fe(III)I surface complexes, fully equilibrated internal electron transfer, slow scavenging of surface coordinated I• by adsorbed I- (within the diffuse layer), and subsequent phase transfer of Fe(II) ions. The similarities between surface and solution redox chemistries are stressed, and their differences are discussed. The operation of a nonreductive parallel pathway during dissolution of iron(III) oxides by nonmetallic complexing two-electron reductants is discussed in terms of the established mechanism. The implications are tested by exploring the dissolution kinetics in iodide−thiocyanate-containing solutions. The synergism predicted by the postulated mechanism is indeed observed and can be described in terms of an additional path in which iodide scavenges chemisorbed SCN• generated by internal electron transfer within ⋮Fe(III)SCN surface complexes.
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