Effect of Mg-Ca-Sr on the sorption behavior of

Effect of Mg-Ca-Sr on the sorption behavior of uranium(VI) on silica Sreejesh Nair1 and Broder J Merkel1 1

Department of Hydrogeology, Technische Universität Bergakademie Freiberg, Gustav-Zeuner Str.12, 09599 Freiberg, Germany.

Abstract. The effect of alkaline earth uranyl carbonate complexes on the sorption behavior of U(VI) on quartz was studied by means of well defined batch experiments. In the pure U(VI) – silica system, the pH dependency was minor and about 90% of the uranium sorbed onto quartz. However, in the presence of Ca, Sr, or Mg the sorption of uranium decreased to 10, 30, and 50% respectively. The reduction in sorption behavior is due to the presence of zero valent and anionic complexes of alkaline earth uranyl carbonate species and show the importance to include these species in thermodynamic data bases. It is assumed that, in particular, the zero-valent species are responsible for less sorption in the presence of alkaline earth metals.

Introduction Uranium is a significant contaminant to the environment. The mobility of uranium in the aquatic system is mainly dependent on the speciation and the major controlling factors of uranium speciation are pH, ionic strength, redox potential, availability of organic and inorganic ligands etc. Ternary complexes of alkaline earth uranyl carbonates, M2UO2(CO3)30 (M = Ca, Sr) and MUO2(CO3)32- (M = Mg, Ca, Sr), may play a key role in the environmental chemistry of uranium under neutral to alkaline pH conditions. The formation of Ca2UO2(CO3)30 was first reported by Bernhard et al.(1996) and further studies on the alkaline earth uranyl carbonates revealed that they can play a significant role in the aqueous speciation of U(VI) at neutral to alkaline pH range (Kalmykov and Choppin 2000; Zheng et al. 2003). Thus it is important to know the major uranium species in order to predict the distribution and migration behavior. The migration of radionuclide in groundwater is often controlled by the sorption on minerals present along the flow path. Hence the sorption behavior in the subsurface is important to estimate the

Effect of Mg-Ca-Sr on the sorption behavior of uranium(VI) on silica

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suitability of geologic repositories for nuclear waste materials, the long-term behavior of uranium mining and milling sites, and migration of uranium in groundwater with slightly elevated uranium concentrations. The objective of this study was to understand the importance of alkaline earth uranyl carbonates in the sorption behavior of U(VI) on quartz as a rock forming mineral (e.g. granites, sandstones).

Materials and methods The quartz sand used for this work, termed as F32, was obtained from Quarzwerke Frechen, Germany. The properties of F32 and the purification process are explained in detail in (Nair and Merkel 2011). U(VI) stock solution (0.126x10-6 M) was prepared from UO2(NO3)2∙6H2O (Chemapol, Czech Republic). Solutions of 1x10-3 M magnesium chloride, calcium chloride, strontium chloride, and sodium hydrogen carbonate were prepared by dissolving MgCl2∙6H2O, CaCl2∙6H2O, SrCl2∙6H2O, and NaHCO3 in deionised and purified water. All chemicals used were of ACS reagent grade or better. Four sets of sorption experiments were conducted with uniform amount of uranium concentration, quartz (10 g) and solution (100 ml) under normal laboratory conditions (temp. 23 ± 1°C) in 150 ml teflon (PTFE) beakers: 1. 2. 3. 4.

Only uranium and quartz uranium, calcium (1x10-3 M ) and quartz uranium, strontium (1x10-3 M) and quartz uranium, magnesium (1x10-3 M) and quartz

All experiments were conducted at a pH ranging from 6.5 to 9. Saturation indices were modeled in advance using the geochemical speciation code PHREEQC (Parkhurst and Appelo 1999) and the Nuclear Energy Agency thermodynamic database NEA_2007(Grenthe et al. 2007) to avoid over saturation and thus precipitation of mineral phases. The experiments were performed for a period of 48 h and the samples were collected at a time interval of 1, 3, 6, 12, 24 and 48 h. Solution was analysed for uranium with differential pulse adsorptive cathodic stripping voltammetry (797 Va Computrace, Metrohm, Switzerland) using a hanging mercury drop electrode (HMDE) as reference electrode (Benedikt 2007). The percentage of uranium sorption on quartz was calculated using Eq. 1: Sorption (%) =

⎛ Ci − C f ⎜⎜ ⎝ Ci

⎞ ⎟⎟ × 100 ⎠

(1) where Ci and Cf are the initial and the final concentration of uranium in the solution.

Effect of Mg-Ca-Sr oon the sorption n behavior of uraanium(VI) on ssilica

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The aqueous a speciiation calculattion of anionic or zero-valeent complexess of Mg, Ca, andd Sr were don ne using PHR REEQC with NEA_2007 N daatabase addingg log K values from Dong and Brooks (2006) for alkaline a earth h uranyl carnnonates. Differennt results wou uld show by using the datta from Bernh hard et al.(20001) and Geipel et e al.(2008), because b these authors also stated s zero vallent species foor Ca as well as Sr, and discu ussed elsewherre (Merkel an nd Nair 2011). Aqueous sppeciation and log K values of alkaline a earth uuranyl carbon nates are summ marized in tablle 1. Table 1. Aqueous speciiation of U(VI) with alkaline earth e metals and d stability consttants Aqueouss Reactions 1. 2Ca2+ + UO22+ + 3CO O32- = Ca2UO2(C CO3)30 2. Ca2+ + UO22+ + 3CO32- = CaUO2(CO O3)322+ 2+ 23. Mg + UO2 + 3CO O3 = MgUO2(C CO3)324. Sr2+ + UO22+ + 3CO32- = SrUO2(CO O3)325. 2Sr2+ + UO22+ + 3CO O32- = Sr2UO2(C CO3)30 a b Dong annd Brooks (2006), Bernhard eet al. (2001), cGeipel G et al. (200 08)

log K 30.7aa 30.799b 27.188a 26.111a 26.866a 29.733c

Resullts and Dis scussion Aqueous Speciatio on U with reespect to Figures 1 to 4 showss the aqueous speciation caalculation of U(VI) pH in the t presence as a well as in tthe absence off Mg, Ca and Sr. In the abssence of alkalinee earth elem ments, uranyll carbonate complexes – (UO2)2CO33(OH)3-, 2UO2(CO O3)2 and UO O2(CO3)34-– arre the most dominant d speccies under neeutral to alkalinee pH condition ns (Fig. 1).


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Fig. 1. Calculated U(V VI) species diistribution (Phrreeqc with NE EA_2007 data base) in °° relation to t pH (0.126x1 10-6 M U, 1.5xx10-3 M NaCl, 1x10 1 -3 M NaHCO3, temp: 23 C, pCO2: -3.5 10 hPaa).

The effect e of Mg on o the speciatiion of U(VI) is i illustrated in n fig.2. Here tthe most dominannt species in alkaline pH range is MgU UO2(CO3)32-. Figure 3 deppicts the importaance of Ca in n the aqueouss speciation of o U(VI). Th he most predoominant species under alkaline pH conditioons are Ca2UO O2(CO3)30(aq.)) and CaUO2((CO3)32-. The chaange in U(VI)) speciation bby Sr is show wn in fig. 4. The T SrUO2(CO O3)32- is significaantly dominan nt in the alkaaline pH rang ge. Ternary co omplexes of alkaline earth urranyl carbonates dominate in the alkalin ne pH conditions and otherr uranyl carbonaate complexes are less impoortant within th he given expeerimental condditions. Due to the uncertaainty of the eexistence of Mg M 2UO2(CO3)30 species, it was not taken innto account forr the speciatioon calculation.

Fig. 2 Aqueous speciatiion calculation of Magnesium uranyl carbonaate species in reelation to e al. (2006). (00.126x10pH usingg Phreeqc with NEA_2007 datta base and log K from Dong et 6 -3 -3 -3 M U, 1x10 M Mg, 1x10 M NaHC CO3, 1.5x10 M NaCl, temp: 230C, pCO2: 10-3..5 hPa).


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Fig. 3. Aqueous A speciaation calculationn of Calcium uranyl u carbonatte species in reelation to pH usingg Phreeqc with NEA_2007 datta base and log K from Dong et e al. (2006). (00.126x106 -3 -3 -3 M U, 1x10 M Ca, 1x x10 M NaHCO O3, 1.5x10 M NaCl, temp: 23 30C, pCO2: 10-3.55 hPa)

Fig. 4. Aqueous A speciation calculationn of Strontium uranyl carbonaate species in reelation to pH usingg Phreeqc with NEA_2007 datta base and log K from Dong et e al. (2006). (00.126x106 -3 -3 -3 M U, 1x10 M Sr, 1x x10 M NaHCO O3, 1.5x10 M NaCl, N temp: 230C, pCO2: 10-3.5 hPa).


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Sorptio on U(VI) sorption s on quartz q in the ppure U-CO3 system is ind dependent of pH and about 90% of uranium sorbed ontto quartz. Thiis high sorptio on behavior iss due to the veryy low uranium m concentratioon and the hiigher solid to solution ratioo in the current study. Similaar sorption beehavior was reported r for uranium-silica u a system under allkaline pH ran nge (Stamberrg et al. 2003)). In the preseence of alkalinne earth elementts, U(VI) sorption is significcantly less wiith an average of 50% for M Mg, 30% for Sr and a 10% for Ca C in the soluttion (fig. 5, 6 and 7). The sorption of U U(VI) on quartz was w decreased d from 65% att pH 6.5 to 32 2% at pH 9 in n the presencee of Mg and the sorption rate was variable throughout th he pH range. (fig. 5). The ssorption retardatiion is due to o the formatioon of MgUO O2(CO3)32- (fig g. 2). Moreovver, the decreasee in U(VI) so orption, in thee presence of Mg, on anion n exchange reesin was reportedd elsewhere (D Dong and Broooks 2008). In nstead of Mg, the introducction pH range. Competitive C sorption of U U(VI) on quaartz with Mg, Ca and Sr can be excludeed as proved by sorptionn experimentss with solutions containinng only alkalinee earth metals in the absencee of uranium (data ( not show wn).

Fig. 5. U(VI) U sorption onto o quartz as a function of pH H, with and with hout Mg. (0.1266x10-6 M -3.5 U, 1x10-3 M Mg, 1.5x1 10-3 M NaCl, 11x10-3 M NaH HCO3, period: 48 4 hr, 23°C, pCO CO2: 10 hPa).


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Fig. 6. U(VI) U sorption onto o quartz as a function of pH H, with and witthout Ca. (0.1266x10-6 M

U, 1x10-3 M Ca, 1.5x10-3 M NaCl, 1x1 0-3 M NaHCO3, period: 48 hr,, 23°C, pCO2: 100-3.5 hPa). Fig. 7. U(VI) U sorption onto o quartz as a function of pH H, with and witthout Sr. (0.1266x10-6 M -3 -3 -3 U, 1x10 M Sr, 1.5x10 M NaCl, 1x100 M NaHCO3,3 period: 48 hr, 230C, pCO2: 100--3.5 hPa).

Concllusion The preesence of alkaaline earth eleements has a significant im mpact on the ssorption behavioor of U(VI) on n quartz. In tthe presence of o Mg, Ca an nd Sr, the sorpption of


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U(VI) on quartz was decreased considerably. This sorption retardation is due to the formation of ternary complexes of alkaline earth uranyl carbonates. The U(VI) speciation calculation using PHREEQC with NEA_2007 database enhanced with log K values from Dong and Brooks (2006), Bernhard et al. (2001) and Geipel et al. (2008) revealed that the alkaline earth uranyl carbonate complexes are dominant in the presence of Mg, Ca and Sr at neutral to alkaline pH conditions. Moreover, the log K values from the literature are sensitive to the speciation calculation. From this study, it is evident that Mg-, Ca-, and Sr–Uranyl-Carbonato complexes play a major role in the sorption behavior of U(VI) at neutral to alkaline pH conditions. Therefore these log-K values have to be added by the user to existing data bases if they do not contain the alkaline earth uranyl carbonate species. Further studies are recommended to evaluate the formation and existence of alkaline earth uranyl carbonate species, especially Mg2UO2(CO3)30 and Sr2UO2(CO3)30.

References Benedikt G (2007) 797 VA Computrace – voltammetric trace determination of uranium(VI) in drinking and mineral water. Metrohm Information Issue 2/2007. Metrohm Ltd.,CH9101 Herisau, Switzerland. 36 Bernhard G, Geipel G, Brendler V, Nitsche H (1996) Speciation of uranium in seepage waters of a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Radiochimica Acta 74:87-91 Bernhard G, Geipel G, Reich T, Brendler V, Amayri S, Nitsche H (2001) Uranyl(VI) carbonate complex formation: Validation of the Ca2UO2(CO3)(3)(aq.) species. Radiochimica Acta 89:511-518 Dong WM, Brooks SC (2006) Determination of the formation constants of ternary complexes of uranyl and carbonate with alkaline earth metals (Mg2+, Ca2+, Sr2+, and Ba2+) using anion exchange method. Environmental Science & Technology 40:46894695 Dong WM, Brooks SC (2008) Formation of aqueous MgUO2(CO3)32- complex and uranium anion exchange mechanism onto an exchange resin. Environmental Science & Technology 42:1979-1983 Fox PM, Davis JA, Zachara JM (2006) The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz. Geochimica Et Cosmochimica Acta 70:1379-1387 Geipel G, Amayri S, Bernhard G (2008) Mixed complexes of alkaline earth uranyl carbonates: A laser-induced time-resolved fluorescence spectroscopic study. Spectrochimica Acta Part a-Molecular and Biomolecular Spectroscopy 71:53-58 Grenthe I, Fuger J, Konings R, Lemire RJ, Muller AB, Wanner J (2007) The Chemical Thermodynamics of Uranium. Elsevier: New York Kalmykov SN, Choppin GR (2000) Mixed Ca2+/UO22+/CO32- complex formation at different ionic strengths. Radiochimica Acta 88:603-606 Merkel BJ, Nair S (2011) Impact of speciation and Sorption on migration of uranium in groundwater. Nachhaltigkeit und Langzeitaspekte bei der Sanierung von Uranbergbau


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und Aufbereitungsstandorten. Proceedings des Internationalen Bergbausmposiums WISSYM_2011, Ronneburg, Germany, p 269-274 Nair S, Merkel BJ (2011) Impact of Alkaline Earth Metals on Aqueous Speciation of Uranium(VI) and Sorption on Quartz. Aquatic Geochemistry 17:209-219 Parkhurst DL, Appelo CA (1999) User's Guide to PHREEQC (version 2). A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculation. U.S.G.S., Water Resources Investigation Report 99 - 4259 Stamberg K, Venkatesan KA, Rao PRV (2003) Surface complexation modeling of uranyl ion sorption on mesoporous silica. Colloids and Surfaces a-Physicochemical and Engineering Aspects 221:149-162 Zheng ZP, Tokunaga TK, Wan JM (2003) Influence of calcium carbonate on U(VI) sorption to soils. Environmental Science & Technology 37:5603-560


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