SummaryPlutonium(VI) sorption on the surface of well-characterized synthetic manganese-substituted goethite minerals (Fe 1-x Mn x OOH) was studied using X-ray absorption spectroscopy. We chose to study the influence of manganese as a minor component in goethite, because goethite rarely exists as a pure phase in nature. Manganese X-ray absorption near-edge structure measurements indicated that essentially all the Mn in the goethite existed as Mn(III), even though Mn was added during mineral synthesis as Mn(II). Importantly, energy dispersive X-ray analysis demonstrated that Mn did not exist as discrete phases and that it was homogeneously mixed into the goethite to within the limit of detection of the method. Furthermore, Mössbauer spectra demonstrated that all Fe existed as Fe(III), with no Fe(II) present. Plutonium(VI) sorption experiments were conducted open to air and no attempt was made to exclude carbonate. The use of X-ray absorption spectroscopy allows us to directly and unambiguously measure the oxidation state of plutonium in situ at the mineral surface. Plutonium X-ray absorption near-edge structure measurements carried out on these samples showed that Pu(VI) was reduced to Pu(IV) upon contact with the mineral. This reduction appears to be strongly correlated with mineral solution pH, coinciding with pH transitions across the point of zero charge of the mineral. Furthermore, extended X-ray absorption fine structure measurements show evidence of direct plutonium binding to the metal surface as an inner-sphere complex. This combination of extensive mineral characterization and advanced spectroscopy suggests that sorption of the plutonium onto the surface of the mineral was followed by reduction of the plutonium at the surface of the mineral to form an inner-sphere complex. Because manganese is often found in the environment as a minor component associated with major mineral components, such as goethite, understanding the molecular-level interactions of plutonium with such substituted-mineral phases is important for risk assessment purposes at radioactively contaminated sites and long-term underground radioactive waste repositories. Previous work by Kaplan et al.[4] using laboratory and lysimeter studies showed that > 95% of plutonium migrating from a purposefully placed solid Pu IV (NO 3 ) 4 source remained within 1.25 cm of the source over an 11 year period under natural environmental conditions at the Department of Energy (DOE) Savannah River Site (SRS).Such work on large-scale environmental systems is an important part in uncovering essential components of the soil and mineral composition that controls plutonium redox chemistry. However, individual sorption and redox processes must also be tested on well-characterized model mineral phases that properly represent individual environmental In nature, manganese is rarely found as a pure mineral phase. It is often found as a minor component associated with major mineral phases, such as goethite. As a model to these environmental minerals, we have synthesiz...