Published: May 27, 2011 r2011 American Chemical Society 5536 dx.doi.org/10.1021/es201046x | Environ. Sci. Technol. 2011, 45, 5536–5542 ARTICLE pubs.acs.org/est Uranium Sorption on Various Forms of Titanium Dioxide À Influence of Surface Area, Surface Charge, and Impurities M. Josick Comarmond,* ,† Timothy E. Payne, † Jennifer J. Harrison, † Sangeeth Thiruvoth, † Henri K. Wong, † Robert D. Aughterson, † Gregory R. Lumpkin, † Katharina M€ uller, ‡ and Harald Foerstendorf ‡ † Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia ‡ Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Radiochemistry, P.O. Box 510119, D-01314 Dresden, Germany b S Supporting Information ’ INTRODUCTION The adsorption of radionuclides onto mineral surfaces has been extensively studied over the last few decades. Such studies have particular relevance to the mobility of radionuclides in surface and groundwaters and assessing the impacts of uranium mining and radioactive waste disposal. Sorption experiments have often involved solid materials typical of the field site under study, as well as model minerals having well-defined surface properties, such as clays or metal oxides, to derive mechanistic information. 1À3 Titanium oxide (TiO 2 ) is widely used as a model mineral for sorption studies, due to its high chemical stability, negligible solubility over a wide pH range, and near-neutral point of zero charge, in addition to its well-known structure and surface properties. There are three types of crystal structures (anatase, rutile, and brookite), 4 with the most common phases being anatase and rutile. The surface properties of TiO 2 have been extensively studied to understand the interactions occurring at the solid-solution interface, 5,6 and there has been much discus- sion on the isoelectric point (IEP) and point of zero charge (PZC) of TiO 2 materials. Kosmulski 7À9 compiled reported IEPs and PZCs for TiO 2 , concluding that there is no significant difference between the IEP of anatase and rutile and that both the IEP and PZC of TiO 2 are rather insensitive to the crystal- lographic structure or the experimental method. Discrepancies in IEP and PZC were attributed to impurities in the materials, or to other experimental factors, rather than to differences in the surface properties of the materials studied. This finding is particularly relevant, since a number of different forms of TiO 2 have been used in experimental sorption studies, including hydrous titanium oxide, anatase, rutile, and various commercially available samples that contain a mixture of anatase and rutile. Over the past decade, experimental studies of the sorption of various actinides onto TiO 2 have been reported. 10À13 The effects of contact time, ionic strength, concentration, and humic sub- stances on uranium sorption have been investigated 12 in addition to mechanistic studies. 14À19 In a study of uranium adsorption onto kaolinite, it was found that uranium was preferentially adsorbed by Ti-rich impurity phases present in the sample. 20 This is an important observation since TiO 2 , although generally low in abundance, is of environmental significance as it is widely found in association with other minerals. The objective of the present study is to investigate potential differences between the sorption of uranium on various com- mercially available TiO 2 surfaces and determine whether these are significantly related to the following factors: surface charge, surface area, presence of contaminants, and crystal form. Received: October 21, 2010 Accepted: May 19, 2011 Revised: May 17, 2011 ABSTRACT: Titanium dioxide (TiO 2 ) has often served as a model substrate for experimental sorption studies of environmental contaminants. However, various forms of Ti-oxide have been used, and the different sorption properties of these materials have not been thoroughly studied. We investigated uranium sorption on some thoroughly characterized TiO 2 surfaces with particular attention to the influence of surface area, surface charge, and impurities. The sorption of U(VI) differed significantly between samples. Aggressive pretreatment of one material to remove impurities significantly altered the isoelectric point, determined by an electroacoustic method, but did not significantly impact U sorption. Differences in sorption properties between the various TiO 2 materials were related to the crystallographic form, morphology, surface area, and grain size, rather than to surface impurities or surface charge. In-situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopic studies showed that the spectra of the surface species of the TiO 2 samples are not significantly different, suggesting the formation of similar surface complexes. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption.