ORIGINAL ARTICLE Sorption of uranium(VI) at the clay mineral–water interface Samer Bachmaf • Broder J. Merkel Received: 11 November 2009 / Accepted: 26 September 2010 / Published online: 11 October 2010 Ó Springer-Verlag 2010 Abstract Batch experiments were conducted to study the sorption of uranium on selected clay minerals (KGa-1b and KGa-2 reference kaolinite, SWy-2 and STx-1b reference montmorillonite, and IBECO natural bentonite) as a func- tion of pH (4–9) and 0.001, 0.01, and 0.025 M NaCl in equilibrium with the CO 2 partial pressure of the atmosphere. Uranium concentrations were kept below 100 lgL -1 to avoid precipitation of amorphous Uranium- hydroxides. Solely PTFE containers and materials were used, because experiments showed significant sorption at higher pH on glass ware. All batch experiments were performed over a period of 24 h, since kinetic experiments proved that the common 10 or 15 min are in many cases by far not sufficient to reach equilibrium. Kaolinite showed much greater uranium sorption than the other clay minerals due to the more aluminol sites available. Sorption on the poorly crystallized KGa-2 was higher than on the well- crystallized KGa-1b. Uranium sorption on STx-1b and IBECO exhibited parabolic behavior with a sorption maximum around pH 6.5. Sorption of uranium on mont- morillonites showed a distinct dependency on sodium concentrations because of the effective competition between uranyl and sodium ions, whereas less significant differences in sorption were found for kaolinite. The presence of anatase as impurity in kaolinite enhanced the binding of uranyl-carbonate complexes with surface sites. The kinetic of uranium sorption behavior was primarily dependent on the clay minerals and pH. A multisite surface complexation model without assuming exchange is based on the binding of the most dominant uranium species to aluminol and silanol edge sites of montmorillonite, respectively to aluminol and titanol surface sites of kao- linite. For eight surface species, the log_k was determined from the experimental data using the parameter estimation code PEST together with PHREEQC. Keywords Montmorillonite  Kaolinite  Uranium(VI)  Sorption  Surface complexation model Introduction Uranium is of increasing environmental concern due to a grown awareness of its risk to soil and water. Elevated concentration of uranium can be related to uranium milling and mining sites (Morrison and Cahn 1991), nuclear fuel and nuclear weapons production sites (Riley et al. 1992), combustion of coal and oil, in particular, when no proper combustions gas cleaning systems are installed, and the application of phosphate fertilizers (Barisic et al. 1992; Zielinski et al. 2006). A provisional drinking water MCL for uranium of 15 lgL -1 has been established by the World Health Organization (WHO 2004). However, it is discussed worldwide that 10 or 5 lgL -1 would be more reasonable (BFR 2005; Hickox and Denton 2001; Kurttio et al. 2002; Raymond-Whish et al. 2007). Geochemical processes occurring naturally, including dissolution/pre- cipitation, redox reactions, and sorption/desorption reac- tions at the water–rock interface, control the mobility and transport of uranium in the subsurface system, such as aquifer sediments, soils, and groundwater. Sorption is an important removal mechanism that con- trols uranium concentration in groundwater (Prikryl et al. 2001). However, the dependence of sorption on aqueous S. Bachmaf (&)  B. J. Merkel Department of Hydrogeology, Technische Universita ¨t Bergakademie Freiberg, Gustav-Zeuner Str.12, 09599 Freiberg, Germany e-mail: samerbachmaf@yahoo.de 123 Environ Earth Sci (2011) 63:925–934 DOI 10.1007/s12665-010-0761-6