Reductive Dissolution of Oxides by Organic Reductants

The reductive dissolution of metal oxides such as Mn(III/IV) oxides by organic reductants occurs by the following sequential steps (Stone, 1986) (1) diffusion of reductant molecules to the oxide surface, (2) surface chemical reaction, and (3) diffusion of reaction products from the oxide surface. Steps (1) and (3), which are transport steps, are influenced by both the interfacial concentration gradient and the electrical potential gradient due to the net charge of the oxide surface. 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant 

The oxidant strength of transition metal oxides usually decreases in the order Ni304 Mn02 MnOOH CoOOH FeOOH. The rate of reductive dissolution in sediments and natural waters follows a similar order. 

The kinetics of the reductive dissolution mechanisms shown in Fig. 8.1 can be derived using the principle of mass action. The kinetic expression for precursor complex formation by way of an inner-sphere mechanism (Stone, 1986) is 

For steady-state precursor complex concentration and assuming negligible back reactions for B and C, the rate of Me2+(aq) formation is (Stone, 1986)  

---

Reductive Dissolution of Oxides by Organic Reductants 164 Reaction Scheme and Mechanism 164 Specific Studies 166... 

---