**5. Uranium oxidation and reduction**

The reduction of U(VI) to U(IV) may be presented as [53]:

$$\rm{UO\_2^{2+} + 4H^+ + 2e^- = U^4 + 2H\_2O} \quad E^\circ = 0.329 \text{volts} \tag{1}$$

At Eh values less than 0.2 volts, U(VI) reduction to uraninite (UO2 ) is favored. Stewart et al. [60] observed that U(VI) reduction to U(IV) is inhibited in the presence of ferrihydrite. Yajima et al. [61] also observed that U(VI) reduction to U(IV) limited mobility. Goldhaber et al. [62] observed that coffinite formed via reduction processes in sedimentary rocks. Fendorf et al. [63] reviewed the biotic and abiotic pathways for U(VI) reduction in anaerobic soils, and they noted that U(IV) has more limited mobility and binds more preferentially to substrates than U(VI). Uranyl reduction is facilitated by bacterially mediated reactions [64]; however, noncrystalline ferric oxides and nitrate may be effective terminal electron acceptors. Similarly, Burgos et al. [63] observed that soil humic acid partially inhibits U(VI) reduction.
