Effects of the addition of Fe(0) on the reduction of Fe(III) in submerged rice soil

The present study aimed at determining firstly whether Fe(0) is oxidized to Fe(II) in submerged rice soil, what is the acceptor of electrons released from Fe(0), and finally how the Eh values changed in the presence of Fe(0). Under laboratory conditions, hooded soil with the addition of Fe(0) was incubated anaerobically. Although the addition of Fe(0) may increase the amount of Fe(II), it is difficult to determine whether Fe(II) is derived from Fe(III) or Fe(0). For the current experiments, therefore, P-poor soil with the addition of ferric phosphate (Fe(III)PO4) and P-rich soil were used. If the electrons from Fe(0) hows to Fe(III)PO4 and/or the complex of Fe(III) oxides and phosphate in soil, Fe(III) should be reduced to soluble Fe(II), This reduction increases the Bray-P content because of the production of Fe(II)(3)(PO4)(2) that is more soluble than Fe(III)PO4. When P-poor soil with the addition of Fe(III)PO4 and Fe(0) was incubated, the amounts of both Fe(II) and P in soil increased. This finding indicates that Fe(0) was oxidized to Fe(II) and, subsequently, that the electrons released from Fe(0) flowed to Fe(III)PO4. When the P-rich soil with Fe(0) was incubated, the amount of Fe(II) increased, unlike the amount of P. These findings show that Fe(0) was oxidized to Fe(II). However, the electrons released from Fe(0) did not how to the complex of Fe(III) oxides and phosphate in soil. On the basis of these results, it can be concluded that the electrons from Fe(0) hows more easily to Fe(III)PO4 than to the complex of Fe(III) oxides and phosphate in soil. In the presence of Fe(0) and Fe(III)PO4, the changes in the Eh values were investigated. If Fe(0) alone contributes to the maintenance of the redox equilibria, then the Eh values should increase with the change of Fe(0) to Fe(II) because Fe(0) and Fe(II) are the reduced and oxidized forms, respectively. In the current study, however, the addition of Fe(0) did not increase the Eh values. The presence of Fe(0) decreased the Eh values and increased the pH and Fe(II) content during the incubation. These findings suggest that Fe(0) and Fe(III) oxides in soil contribute to the maintenance of the redox equilibria in soil.