Journal of Hazardous Materials 190 (2011) 863–868 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Modeling uranium transport in acidic contaminated groundwater with base addition Fan Zhang a,∗ , Wensui Luo b , Jack C. Parker c , Scott C. Brooks d , David B. Watson d , Philip M. Jardine e , Baohua Gu d a Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Beijing, 100085, China b Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, China c Institute for a Secure and Sustainable Environment, Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, USA d Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA e Biosystems Engineering and Soil Science Department, University of Tennessee, Knoxville, TN 37996, USA article info Article history: Received 3 September 2010 Received in revised form 1 April 2011 Accepted 3 April 2011 Available online 8 April 2011 Keywords: Uranium Sequestration Transport pH Modeling abstract This study investigates reactive transport modeling in a column of uranium(VI)-contaminated sediments with base additions in the circulating influent. The groundwater and sediment exhibit oxic conditions with low pH, high concentrations of NO 3 - , SO 4 2- , U and various metal cations. Preliminary batch exper- iments indicate that additions of strong base induce rapid immobilization of U for this material. In the column experiment that is the focus of the present study, effluent groundwater was titrated with NaOH solution in an inflow reservoir before reinjection to gradually increase the solution pH in the column. An equilibrium hydrolysis, precipitation and ion exchange reaction model developed through simulation of the preliminary batch titration experiments predicted faster reduction of aqueous Al than observed in the column experiment. The model was therefore modified to consider reaction kinetics for the precip- itation and dissolution processes which are the major mechanism for Al immobilization. The combined kinetic and equilibrium reaction model adequately described variations in pH, aqueous concentrations of metal cations (Al, Ca, Mg, Sr, Mn, Ni, Co), sulfate and U(VI). The experimental and modeling results indicate that U(VI) can be effectively sequestered with controlled base addition due to sorption by slowly precipitated Al with pH-dependent surface charge. The model may prove useful to predict field-scale U(VI) sequestration and remediation effectiveness. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Disposal of highly radioactive wastes during the cold war era created substantial subsurface contamination that poses challeng- ing environmental problems at U.S. Department of Energy weapon complex sites associated with nuclear material production and pro- cessing [1,2]. For instance, at the Oak Ridge Reservation (ORR) in east Tennessee, massive amounts of acidic radioactive wastes were released into four unlined disposal ponds designated the S-3 Ponds. Leakage from the S-3 Ponds produced a contamination plume in the groundwater that extends more than 2 km along the geologic strike [3]. The contaminated groundwater is characterized by low pH and high concentrations of cationic aluminum, calcium, magne- sium and manganese, anionic nitrate and sulfate, and radioactive uranium and technetium [4]. ∗ Corresponding author. Tel.: +86 10 62849383; fax: +86 10 62849886. E-mail address: zhangfan@itpcas.ac.cn (F. Zhang). In contaminated groundwater and sediments, U is the most fre- quently detected and Tc is the eighth most frequently detected radionuclide [5]. U is more reactive with soil and sediment miner- als and thus exhibits less mobility than Tc [6]. The dominant forms of U and Tc in groundwater and sediments under oxidizing con- ditions are U(VI) as uranyl (UO 2 2+ ) and Tc(VII) as pertechnetate (TcO 4 - ) [7,8]. However, U(VI) speciation is also complicated by for- mation of negatively-charged carbonate complexes UO 2 (CO 3 ) 2 2- and UO 2 (CO 3 ) 3 4- under neutral or alkaline pH conditions [9]. Previous studies have shown that Fe and Al minerals sorb or copre- cipitate with dissolved sulfate [10] and U [11,12]. Under high Al, near neutral pH conditions, the sequestered U may be quite sta- ble [13]. Sorption of U(VI) onto Fe hydroxides has been extensively modeled [14,15]. However, few reliable thermodynamic data are available to simulate sorption of U(VI) by Al hydroxides. An anion exchange reaction model under simultaneous consideration of var- ious aqueous phase and precipitation/dissolution reactions was newly developed to simulate the sorption of uranium onto Al and Fe hydroxides in competition with sulfate over a wide range in pH [4]. 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.04.022