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In order to make accurate predictions about the fate of actinides released into the environment and to implement effective remediation strategies, fundamental studies are needed to understand the complex geochemical interactions occurring between actinides and mineral surfaces. Minerals having strong redox potentials, such as Fe and Mn oxides and oxyhydroxides, are of special interest because they are likely to influence the complex redox chemistry of Pu under groundwater conditions. Thus, the interactions of Pu(V) and Pu(VI) with the following redox active synthetic mineral phases were studied: 100% δ-MnO[sub2], 50% δ-MnO[sub2];50% α-FeOOH, and 10% δ-MnO[sub2];90% α-FeOOH (w/w). Electron microscopy data show that the δ-MnO[sub2] phases effectively coat the α-FeOOH. The binding of Pu(V) and P(VI) to sediments from the Upper Ringold Formation near the DOE Hanford Site were also investigated. Solution concentrations in the range of 1 x 10[sup-5] M to 4 x 10[sub-4] M Pu were used to minimize hydrolysis, and the pH of the final mineral suspensions was adjusted between 0.5 and 10. [ABSTRACT FROM AUTHOR]