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The sorption of selenite on various cement formulations
Waste Management 20 (2000) 509±516
www.elsevier.nl/locate/wasman
The sorption of selenite on various cement formulations Elizabeth A. Johnson, Mark J. Rudin *, Spencer M. Steinberg, William H. Johnson University of Nevada-Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-3037, USA Accepted 17 February 2000
Abstract Twenty-seven cementitious formulations containing three levels of water/solids ratio (0.45, 0.50, and 0.55), three concentrations of silica fume (0, 10, and 20%), and three concentrations of clay (0, 3, and 5%) were evaluated for their ability to eectively sorb selenite (SeO23 ) from an alkaline solution. A batch sorption procedure was utilized to determine distribution coecients (Kd) for selenite between water and each cement formulation. Experimental Kd values obtained ranged from 250 to 930 l kgÿ1. The results indicated that varying the water and clay content of the mixes had little eect on selenite sorption, while adding increasing amounts of silica fume in a cement mix tended to decrease selenite sorption. A sorption/desorption study using several concentrations of selenium ranging from 6.5 to 1510 ppb was also conducted on cement formulations at one water/solids ratio (0.50), no silica fume, and three concentrations of clay (0, 3, and 5%). Freundlich isotherms were ®tted to the sorption and desorption data. Results indicated that selenite sorption was irreversible under these conditions. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Selenite; Batch sorption; Freundlich isotherms; Cement
1. Introduction There is relatively little information available in the literature regarding the behavior of selenium in cement matrices. This is due in part to the fact that elevated levels of selenium are rarely encountered in waste streams that require treatment strategies such as solidi®cation prior to land disposal. However, there have been instances where waste streams have exhibited extremely high levels of selenium exceeding 300 mg lÿ1 [1,2]. Cases such as these provide the incentive for investigating the ability of various cement formulations to immobilize selenium to ensure ®nal waste forms meet appropriate regulatory requirements prior to disposal. Selenium is a non-metallic, trace element that exists in the following four oxidation states: selenide (2ÿ); selenium (0); selenite (4+); and selenate (6+). Selenite (SeO2ÿ 3 ) is expected to be the predominant form of selenium at pH levels ( 13.5) and redox potentials ( 80 mV) typically found in the pore spaces of ordinary Portland cement [3,4]. The selenate species (SeO2ÿ 4 ) may also be found if oxygensaturated conditions existed in the cement matrix. * Corresponding author Tel.: +1-702-895-4320; fax: +1-702-8954819. E-mail address: [email protected] (M.J. Rudin).
Selenium is generally assumed to behave similarly to other elements which form oxyanion species in aqueous solutions such as technetium and chromium, and exhibit limited sorption capacity in cement systems [5]. Unfortunately, there appears to be very little experimental data available in the literature to support this observation. Distribution coecients (Kd) for selenium have been determined in a number of European cements [5]. These authors report Kd values of selenium in cement systems that range between 0.4 and 5.0 l kgÿ1. However, there was no mention of which anionic species of selenium was used in these studies. Due to the scarcity of data, it is more common for authors to estimate Kd values for selenium in cement systems based on the behavior of a chemical analog. Using Kd values derived from thermodynamic models in risk and performance assessments may not provide an accurate representation of the behavior of selenium in actual ®eld conditions. The purpose of this research is to study the eects of water/solids (w/s) ratio, silica fume content, and clay content on the ability of cement to sorb selenite. A batch sorption procedure was used to determine distribution coecients (Kd) for each of the formulations at a solution pH value of 12.4. This pH value approximates conditions expected in the pore space of a hardened cement [4]. A second study was conducted on three of
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00024-6
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the 27 cement formulations to determine the if the selenite sorption was reversible. The data obtained in this work can be used to more accurately quantify the release of selenium from cement-encapsulated wastes. 2. Materials and methods 2.1. Materials The cement used in this work was a Type V Portland cement manufactured by Ash Grove Cement Co., Nephi, UT and obtained from WMK Materials in Las Vegas, NV. Type V Portland cement is commonly used in the arid southwest US because of its ability to resist sulfate attack. The chemical and physical characteristics of the cement are given in Table 1. The silica fume used in this study, Rheomac SF1001, was obtained from Master Builders, Inc. of Phoenix, AZ. The chemical and physical properties of this additive are presented in Table 2. An attapulgite clay, Attagel1, was obtained from E.T. Horn Co. of La Mirada, CA. The chemical and physical properties of the Attagel1 are presented in Table 3. The combination of relatively low w/s ratios Table 1 Chemical and physical characteristics of ash grove type V Portland cementa Chemical composition (%) SiO2 Al2O3 Fe2O3 CaO MgO SO3 Loss on ignition Insoluble residue Total alkalines (as Na2O)
22.7 3.3 3.8 63.9 2.3 2.0 0.1 0.16 0.46
Potential compound composition (%) Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite Gypsum and ``other materials''
52 26 3 12 7
Physical data Blaine ®neness (m2 kgÿ1) Normal consistency (%) Vicat set times (min) Initial Final False set (%) Autoclave expansion (%) Air entrainment Compressive strength (PSI) 1 day 3 days 7 days a
363 25.6 110 25.6 87.8 0.05 6.7 1730 3160 4050
Data provided by Ash Grove Cement Company, Nephi, UT.
and increased water demand from the silica fume and clay additives made it dicult to mix a number of the formulations thoroughly. It became necessary to add 11.3 g of a high range water-reducer (Rheobuild/MBSF) to each mix to increase its workability. Twenty-seven cementitious formulations containing three levels of w/s ratio (0.45, 0.50, and 0.55), three concentrations of silica fume (0, 10, and 20% by percent of total solid mass), and three concentrations of clay (0, 3, and 5% by percent of total solid mass) were evaluated in this study. A list of the formulations is provided in Table 4. All formulations were mixed in accordance with ASTM Standard C192, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory [6]. To simulate alkaline conditions, each cement mix was made using deionized water that was adjusted to a pH of 12.5 using NaOH. After the mixing of each formulation was complete, the cement paste was poured into three 5.1-cm diameter plastic cylinders to a height of 5 cm. The cylinders were sealed with a polyethylene ®lm and the cement was allowed to cure for 28 days at room temperature. Once cured, the cement specimens were demolded and fragmented into smaller
Table 2 Chemical and physical characteristics of Rheomac SF 1001a Chemical composition (%) Silica, fume Silicon dioxide
>99 <1
Physical data Light grey ®ne powder Average particle diameter (mm) Speci®c gravity Surface area (m2 kgÿ1) Bulk density (lbs ftÿ3)
0.1 2.2 20,000 30±40
a
Data provided by Master Builders, Inc. of Phoenix, AZ.
Table 3 Chemical and physical characteristics of attapulgite claya Chemical composition (%) Silicon (SiO2) Aluminium (Al2O3) Magnesium (MgO) Iron (Fe2O3) Calcium (CaO) Phosphorous (P2O5) Potassium (K2O) Titanium (TiO2) Trace elements Physical data Physical form Average particle diameter (mm) pH BET surface area (m2 gÿ1) a
Data provided by E. T. Horn Co., La Mirad, CA.
65.2 11.9 10.8 3.5 6.2 1.0 0.8 0.5 0.1 Powder 0.14 7.5±9.5 150
E.A. Johnson et al. / Waste Management 20 (2000) 509±516 Table 4 Cement formulations and associated experimental Kd values (l kgÿ1) Formulation (%water, %silica fume, %clay)
Kd values
45/0/0 45/10/0 45/20/0 45/0/3 45/10/3 45/20/3 45/0/5 45/10/5 45/20/5 50/0/0 50/10/0 50/20/0 50/0/3 50/10/3 50/20/3 50/0/5 50/10/5 50/20/5 55/0/0 55/10/0 55/20/0 55/0/3 55/10/3 55/20/3 55/0/5 55/10/5 55/20/5
890110 51060 440100 770130 62080 41060 630120 36030 250140 570160 510200 410110 93090 63030 4909 84060 68030 41060 930140 52050 35090 80060 76080 47080 7009 700110 42010
pieces using a hammer. The fragments were then ground using a mortar and pestle and sieved to obtain cement samples with particle sizes between 250 mm and 1 mm. 75 Se was selected as a radiotracer in this study due to its relatively long half-life and ease of detection. A NIST-traceable stock solution of 75SeO2ÿ 3 was diluted to a concentration of 340 Bq lÿ1 and a total selenite concentration of 53 mg lÿ1, which approximates the maximum contaminant level of 50 mg lÿ1 for community water systems in the US [7]. The solution pH was adjusted to a value of 12.4 using NaOH, as necessary. Sodium selenite (Na2SeO3) or deionized water was added to the standard solutions during the sorption/desorption phase of the project to obtain selenite solutions with concentrations of 6.5, 26, 53, 126, 784, and 1510 mg lÿ1. 2.2. Sorption protocol The batch sorption procedure used to determine Kd values for each formulation was adapted from Relyea [8]. Triplicate 1.000.01-g cement samples per formulation were placed in 50-ml centrifuge tubes. All samples were subjected to three overnight washes with 30 ml of deionized water adjusted to a pH of 12.4 to establish equilibrium between the solution pH and surfaces
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freshly exposed from grinding. The samples were agitated overnight on an orbital shaker at 60 rpm. The tubes were then centrifuged at 10,000 g (1 g=980 cm sÿ2) for 20 min. Finally, the wash solution was extracted with a vacuum pipette and fresh wash solution was reintroduced to the cement. The entire 30 ml of cold wash was removed from each tube after the third cycle in preparation for the introduction of the selenite solution. 30 ml of the selenite solution adjusted to a pH of 12.4 was added to each of the tubes containing the cement samples. After the tubes were oscillated on an orbital shaker at 60 rpm for 7 days at room temperature, they were centrifuged at 5700 g for 45 min to separate the solution and cement phases. 15-ml aliquots were removed using a plastic transfer pipette and placed into counting vials for radioanalysis of the 75Se. The remaining 15 ml was not removed to avoid disturbing the centrifugate. The extracted aliquots were counted on high purity germanium gamma-ray spectrometers to quantify radioactive emissions from the 75Se using the 264-keV gamma line. The aliquots were counted for either 300 or 900 s as necessary to achieve counting errors of less than 10%. All samples had a much higher concentration than the reported minimum detectable selenite for the analytical method of 0.2 mg lÿ1 at the 95% con®dence interval. The distribution coecient (in l kgÿ1) of each formulation was calculated using the following equation: Kd
A V m SA
1
where: A
m V SA
decay corrected 75Se activity sorbed on cement (Bq) calculated by taking the dierence between the activity present in 30 ml of 75Se solution per sample prior to contact with cement (determined by counting ``blank'' samples) and the activity present in the sample after contact with the cement (determined by counting the aliquot), mass of the cement (kg) present in each centrifuge tube prior to introducing the 75Se solution, amount of 75Se solution added to each centrifuge tube (liter), and 75 Se activity present in solution after contact with the cement (Bq)
2.3. Sorption/desorption protocol The reversibility of selenite sorption in cement was studied only in formulations containing 50% water and
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0, 3, or 5% clay. All samples were subjected to three overnight washes with 30 ml of 0.01 M-CaCl2 buer solution instead of deionized water to control ionic strength and maintain matrix stability. CaCl2 prevented the dispersion of small particulates and assured clean separation of dissolved and sorbed selenite. 20 ml of the selenite solution with a concentration of 6.5, 26, 53, 126, 784, or 1510 mg lÿ1, was then added to each tube containing triplicate 1.000.01-g samples of each of the formulations. After the tubes were oscillated on an orbital shaker at 60 rpm for 7 days at room temperature, they were centrifuged at 5700 g for 45 min to separate out the liquid phase. 15 ml of solution was removed from each centrifuge tube and analyzed by gamma-ray spectroscopy as described above. 15 ml of 0.01 M-CaCl2 solution was then added to each centrifuge tube, which also contained 5 ml of residual selenite solution, resulting in a total aqueous volume of 20 ml. The test tubes were allowed to equilibrate for 13 days at room temperature under gentle agitation (orbital shaker at 60 rpm). The tubes were then centrifuged at 10,000 g for 20 min. 15 ml of the solution was again extracted and analyzed. This process was repeated a total of four times with equilibrium times of 13, 13, 11 and 11 days, respectively. The aliquots were analyzed for times long enough to achieve an error less than 10% at the 95% con®dence level. The concentration of selenite sorbed by the cement particles following the original sorption wash and each subsequent desorption wash was then calculated. 2.4. Surface area measurements Surface area measurements of all 50% water-fraction cement formulations were measured using the BET nitrogen adsorption method with a Micrometrics1 Gemini 2370 surface area analyzer before and after the 0.01 M-CaCl2 or deionized water washes. This allowed an evaluation of any changes in surface area caused by the experimental procedure. Samples were dried at 200 C under nitrogen prior to measurements.
3. Results 3.1. Sorption phase Average Kd values determined for each cement formulation are presented in Table 4. The mean Kd value obtained for all formulations studied was 593 l kgÿ1. An Analysis of Variance (ANOVA) test was used to analyze the data from the sorption experiment and allow a comparison between data. The ANOVA test indicated that there was a signi®cant decrease (F=112.4, P=0.001) in selenite sorption as the silica fume content of the cement formulations increased. This eect is illustrated in a box and whisker diagram shown in Fig. 1. The lower and upper edge each box represents the 25th and 75th percentiles of the Kd distribution, respectively. The lower and upper whiskers (error bars) represent the tenth and ninetieth percentiles. The solid line and dotted line within each box represent the median and mean values for the pooled Kd data, respectively. As depicted in Figs. 2 and 3, there were no signi®cant dierences at the 95% con®dence level in selenite sorption as the water or clay contents of the mixes were varied. 3.2. Sorption/desorption phase Sorption data for cement systems is typically expressed as a Freundlich isotherm [9]. The equation describing this model [10] is: log Cs log k n log Cw
2
where CS is the concentration of selenium sorbed on the cement and CW is the concentration of selenium in
2.5. Dissolved silica measurements A LaMotte Smart colorimeter using the silica molybdate complex colorimetric method was used to detect dissolved silica in the wash solutions from the second phase of the study. The wash solutions were also analyzed by Inductively Coupled Plasma±Atomic Emissions Spectroscopy (ICP±AES) for total silica, calcium, and aluminum that comprise the primary constituents of cement and clay. Their presence indicates the degree of dissolution of the cement matrix. From this data, the stability of various cement formulations was examined as a function of the type of wash solution.
Fig. 1. Eect of silica fume on selenite sorption across all formulations.
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Fig. 2. Eect of water content on selenite sorption across all formulations.
Fig. 4. Sorption/desorption isotherms for 50/0/0 formulation.
Fig. 3. Eect of clay content on selenite sorption across all formulations.
Fig. 5. Sorption/desorption isotherms for 50/0/3 formulation.
solution in contact with the cement. Figs. 4±6 present sorption isotherms for the three formulations examined in this phase of the study. In addition to the sorption isotherms, desorption isotherms originating from each of the initial equilibration points are also shown on each plot. The experimental constants (k and n) determined for each formulation by regression analyses are presented in Tables 5 and 6. The value k represents a Kd if the isotherm is linear. The value n represents the deviation from linearity. If n=1, as is the case in this study, a linear relationship exists in which the use of a Kd value would be valid. The Kd values obtained for the three formulations in this phase were similar to the values
determined for these same formulations in the sorption phase of the study. Any discrepancies in the Kd values would mostly likely be a result of using CaCl2 as a ¯occulation promoter in the sorption/desorption phase and not in the sorption phase. Without CaCl2, small particles may remain suspended in solution during washes, which would tend to result in lower Kd values. Desorption results indicate irreversible selenite sorption or very slow desorption kinetics. This is indicated by values of n approaching zero for the desorption data and is shown by the horizontal pattern created by desorption isotherms for each selenium concentration studied. Reversible desorption would have followed the sorption isotherm.
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E.A. Johnson et al. / Waste Management 20 (2000) 509±516 Table 6 Freundlich constants determined for the desorption isotherms Formulation (%water, %silica fume, %clay)
Initial selenium concentration (mm lÿ1)
k
n
R2
50/0/0
1510 784 126 53 26 6.5
24,000 12,000 2300 1000 530 140
0.062 0.080 0.065 0.081 0.080 0.075
0.64 0.81 0.73 0.81 0.74 0.70
50/0/3
1510 784 126 53 26 6.5
24,000 14,000 2400 1000 520 140
0.057 0.049 0.007 0.046 0.035 0.039
0.68 0.78 0.010 0.46 0.32 0.29
50/0/5
1510 784 126 53 26 6.5
24,000 13,000 2300 1000 530 140
0.067 0.046 0.073 0.048 0.048 0.042
0.70 0.66 0.77 0.85 0.37 0.38
Fig. 6. Sorption/desorption isotherms for 50/0/5 formulation.
Table 5 Freundlich constants determined for the sorption isotherms Formulation (%water, %silica fume, %clay)
k
n
R2
50/0/0 50/0/3 50/0/5
820 1100 1100
0.98 1.00 0.98
1.00 1.00 1.00
3.3. Surface area measurements Results of the surface area measurements indicate that the cement particles undergo little change because of the washes and centrifugation. The dierences between pre-wash and post-wash surface areas ranged from 0 to 8.7%. Slight reductions in pre-wash and postwash surface areas can most likely be attributed to partial dissolution of the amorphous phases. The addition of clay increases the surface area of the unwashed cement from 9.860.01 m2 gÿ1 (50/0/0) to 18.40.01 m2 gÿ1 (50/0/3) and 17.10.01 m2 gÿ1 (50/0/5). The surface area of Attagel 501 is 150 m2 gÿ1 (moisture free value) and likely contributes to the higher surface area measurements obtained for cement formulations containing clay. 3.4. Dissolved silica measurements Total silica determined using colorimetric and ICP± AES methods were compared in order to determine whether the silica present in the wash solutions existed as dissolved silica or in colloidal form. The results
indicated that SiO2 was monomeric (reactive) silica as indicated by the absence of particulates in solution. The presence of colloids in solution would have been detected by ICP±AES, but not by the colorimetric method [11]. The signi®cance of this ®nding is that centrifugation resulted in complete separation of cement particles from solution. In addition, it also implies that the Kd values determined are considered true Kd values. If colloids existed, sorption by these colloids would decrease the calculated Kd values and imply decreased sorption by the cement. 4. Discussion As previously mentioned, selenite (SeO2ÿ 3 ) in an alkaline solution was prepared for use in both the sorption and sorption/desorption phases of the study. No special eort was made to prevent the introduction of oxygen into the cement or solutions during preparation. Under equilibrium conditions, the pH and redox conditions used in this work appear to favor the presence of the selenate species (SeO24 ) [3]. The thermodynamic models, MINEQL+ [12] and MINTEQA2 [13] also predict that selenate is the dominant in these conditions. Although little information on the behavior of selenate and selenite in cement matrices exists in the literature, several studies have investigated the sorptive capacity of these species in soils and clays [14±16]. In general, these studies reported that selenate typically sorbs to soil particles via weakly bonded, outer-sphere surface complexes, whereas selenite sorbs by strongly
E.A. Johnson et al. / Waste Management 20 (2000) 509±516
bonded inner-sphere surface complexes. Compared to selenate, selenite is immobile, and more likely to become irreversibly sorbed. Although selenate is the anticipated solution species according to thermodynamic models, the results of this study suggest that the strong sorption behavior observed can best be described by the presence of selenite. This indicates that little or no transformation of selenite to selenate occurred throughout the duration of the study. The relatively slow oxidation of species such as selenite to selenate has been shown in previous work [17,18]. Kd values ranged from 350 to 930 l kgÿ1 in the sorption phase and 820 to 1100 l kgÿ1 in the sorption/desorption phase of the study. Kd values estimated in the literature typically range from 410ÿ4 to 510ÿ3 l kgÿ1 [5,9,19]. The discrepancy between these Kd ranges is rather large. It should be re-emphasized that the only experimental Kd values for selenium found in the literature were determined for European cements. The results obtained in this study are exceptional only in that they are not consistent with currently accepted Kd values for selenium in cement systems. The results do, however, approximate mean Kd values reported in the literature for selenium sorption in sand, loam, clay, and organic materials of 150, 500, 740, and 1800 l kgÿ1, respectively [20]. Increasing the water content of a cement mix is known to increase the amount of calcium silicate hydrates in the hardened paste early (i.e. 28 days) in the curing process [21]. The increased water content promotes the ¯occulation of the raw cement particles, thereby creating a favorable curing environment. Since calcium silicate hydrates have been shown to be the most important sorbing material in cement mixes [9], it is expected that these mixes would exhibit increased sorption capacity for selenite. However, varying water content did not appear to in¯uence the sorption capacity of the formulations. The range of w/s ratios used in this study (0.45 to 0.55) may not have been large enough to aect a change in selenite sorption. In addition, the use of a water reducer during the preparation of all formulations may have minimized any impact varying the w/s ratios may have had on selenite sorption. Withholding silica fume resulted in cement formulations with greater sorption capacity than those with either 10 or 20% silica fume. Silica fume is known to react with a hydration by-product, calcium hydroxide, to form additional calcium silicate hydrate [22]. However, the reaction is quite slow and may not result in increased levels of calcium silicate hydrate after a standard 28-day cure. Subsequently, the 10 and 20% mixes would most likely contain relatively large amounts of unreacted silica fume at the completion of their cure. This would deleteriously eect the ability of these mixes to sorb selenite because silica fume is known to provide very little sorption capacity for anionic species [23].
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Additionally, silica that does not react during hydration is potentially free to compete with selenite for residual surface charges on the cement. Silica has been shown to exhibit a stronger sorption capacity than selenite in some materials [24]. An increase in selenite sorption would be expected at curing times beyond that examined in this study. Clays do not readily react with cement hydration products, and their surfaces have little sorption capacity for anionic species such as selenite or selenate [25]. The amount of clay used in each of the mixes (0±5%) may not have been substantial enough to aect change in the selenite sorption in each of the hardened cement pastes studied. A decrease in selenite sorption would be expected in cements with > 5% clay due to the replacement of calcium silicate hydrates with a relatively nonreactive clay additive. 5. Conclusion No eort was made to determine an optimal formulation for immobilizing selenite in cement. However, the study was designed to identify trends in selenite sorption as w/s ratio, silica fume content, and clay content of a cement mix was varied. Results from this study suggest that Kd values for selenium in cement currently used in performance assessments may substantially underestimate the sorption capacity of the ®nal cement waste form. Selenium sorption in cement systems appears to be greatly in¯uenced by redox conditions which should be considered when predicting its behavior in the environment. Acknowledgements Support with statistical analyses was provided by Susan Franson from the US Environmental Protection Agency, Las Vegas, NV. Guidance with chemical analysis was provided by Steven George of the Department of Chemistry of the University of Nevada Las Vegas. References [1] US Department of Energy, Fernald Oce. Treatability study work plan for Operable Unit 4. Fernald, OH: Fernald Environmental Management Project, 1992. [2] Reynolds Electrical and Engineering Company. Cotter concentrate treatability study plan: revision 1. Las Vegas (NV), 1995. [3] Wedepohl KH, editor. Handbook of Geochemistry: Volume II/3. New York: Springer-Verlag, 1978. [4] Angus MJ, Glasser FP. The chemical environment in cement matrices (Scienti®c Basis for Nuclear Waste Management 50). Pittsburgh (PA): Materials Research Society, 1985. [5] McKinley IG, Scholtis A. A comparison of radionuclide sorption databases used in recent performance assessments. J Contam Hydrol 1993;13:347±63.
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[6] American Society for Testing and Materials (ASTM). Practice for making and curing concrete test specimens in the laboratory (ASTM Standard C192-90a. In: 1994 Annual book of ASTM standards, Vol. 4.02. Philadelphia (PA): ASTM, 1990. [7] US Environmental Protection Agency. Code of Federal Regulations. 40CFR Part 141.62. Washington (DC): US Government Printing Oce, 1998. [8] Relyea JF, Serne RJ, Rai D. Methods for determining radionuclide retardation factors: status report (PNL-3349). Richland (WA): Paci®c Northwest Laboratory, 1980. [9] Bradbury MH, Sarott FA. Sorption databases for the cementitious near-®eld of a L/ILW repository for performance assessment (PSI Bericht. nr. 95-06). Wurenlingen/Villigen: Paul Scherrer Institut, 1995. [10] Stumm W, Morgan JJ. Aquatic chemistry: chemical equilibria and rates in natural waters. 3rd Ed. New York: John Wiley & Sons, Inc, 1996. [11] Iler RK. The chemistry of silica: solubility, polymerization colloid and surface properties, and biochemistry. New York: John Wiley & Sons, Inc, 1979. [12] Schecher WD, McAvoy DC. MINEQL+, a chemical equilibrium modeling system: Version 4.0 for Windows users manual. Hallowell, ME: Environmental Research Software, 1998. [13] Allison JD, Brown DS, Gradec KJ. MINTEQA2/PRODEFA2, a geochemical assessment model for environmental systems: version 3.0 users manual (EPA/600/3-91/021). Washington (DC): US Government Printing Oce, 1991.
[14] Sposito G. The surface chemistry of soils. New York: Oxford University Press, 1984. [15] Goldberg S, Glaubig RA. Anion sorption on a calcareous, montmorillonitic soil Ð selenium. Soil Sci Soc of Am J 1988;52:954±8. [16] Zhang P, Sparks DL. Kinetics of selenate and selenite adsorption/desorption at the goethite/water interface. Environ Sci Tech 1990;24:1848±56. [17] Cary EE, Wieczorek GA, Allaway WH. Reactions of seleniteselenium added to soils that produce low selenium forages. Soil Sci Soc of Am J Proc 1967;31:21±6. [18] Ylaranta T. Sorption of selenite and selenate in the soil. Ann Agric Fenn 1983;22:29±39. [19] Nancarrow DJ, Summerling TJ, Ashton J. Preliminary radiological assessments of low-level waste repositories. DOE/RW/88.084. London, UK: Department of the Environment, UK, 1988. [20] Sheppard MI, Thibault DH. Default soil solid/liquid partition coecients, Kds for four major soil types: a compendium. Health Phys 1990;59:471±82. [21] Mindess S. Concrete. Englewood Clis (NJ): Prentice Hall, 1981. [22] Kosmatka SH, Panarese WC. Design and Control of Concrete Mixtures. 13th Ed. Skokie IL: Portland Cement Association, 1988. [23] Komarneni S, Roy DM, Kumar A. Cation exchange properties of hydrate cements. Nucl Waste Manage 1983;8:441±7. [24] Balistrieri LS, Chao TT. Selenium adsorption by goethite. Soil Sci Soc of Am J 1987;51:1145±51. [25] Swartzen-Allen SL, Matijevic E. Surface and colloid chemistry of clays. Chem Rev 1974;74:285±400.
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The sorption of selenite on various cement formulations Elizabeth A. Johnson, Mark J. Rudin *, Spencer M. Steinberg, William H. Johnson University of Nevada-Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-3037, USA Accepted 17 February 2000
Abstract Twenty-seven cementitious formulations containing three levels of water/solids ratio (0.45, 0.50, and 0.55), three concentrations of silica fume (0, 10, and 20%), and three concentrations of clay (0, 3, and 5%) were evaluated for their ability to eectively sorb selenite (SeO23 ) from an alkaline solution. A batch sorption procedure was utilized to determine distribution coecients (Kd) for selenite between water and each cement formulation. Experimental Kd values obtained ranged from 250 to 930 l kgÿ1. The results indicated that varying the water and clay content of the mixes had little eect on selenite sorption, while adding increasing amounts of silica fume in a cement mix tended to decrease selenite sorption. A sorption/desorption study using several concentrations of selenium ranging from 6.5 to 1510 ppb was also conducted on cement formulations at one water/solids ratio (0.50), no silica fume, and three concentrations of clay (0, 3, and 5%). Freundlich isotherms were ®tted to the sorption and desorption data. Results indicated that selenite sorption was irreversible under these conditions. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Selenite; Batch sorption; Freundlich isotherms; Cement
1. Introduction There is relatively little information available in the literature regarding the behavior of selenium in cement matrices. This is due in part to the fact that elevated levels of selenium are rarely encountered in waste streams that require treatment strategies such as solidi®cation prior to land disposal. However, there have been instances where waste streams have exhibited extremely high levels of selenium exceeding 300 mg lÿ1 [1,2]. Cases such as these provide the incentive for investigating the ability of various cement formulations to immobilize selenium to ensure ®nal waste forms meet appropriate regulatory requirements prior to disposal. Selenium is a non-metallic, trace element that exists in the following four oxidation states: selenide (2ÿ); selenium (0); selenite (4+); and selenate (6+). Selenite (SeO2ÿ 3 ) is expected to be the predominant form of selenium at pH levels ( 13.5) and redox potentials ( 80 mV) typically found in the pore spaces of ordinary Portland cement [3,4]. The selenate species (SeO2ÿ 4 ) may also be found if oxygensaturated conditions existed in the cement matrix. * Corresponding author Tel.: +1-702-895-4320; fax: +1-702-8954819. E-mail address: [email protected] (M.J. Rudin).
Selenium is generally assumed to behave similarly to other elements which form oxyanion species in aqueous solutions such as technetium and chromium, and exhibit limited sorption capacity in cement systems [5]. Unfortunately, there appears to be very little experimental data available in the literature to support this observation. Distribution coecients (Kd) for selenium have been determined in a number of European cements [5]. These authors report Kd values of selenium in cement systems that range between 0.4 and 5.0 l kgÿ1. However, there was no mention of which anionic species of selenium was used in these studies. Due to the scarcity of data, it is more common for authors to estimate Kd values for selenium in cement systems based on the behavior of a chemical analog. Using Kd values derived from thermodynamic models in risk and performance assessments may not provide an accurate representation of the behavior of selenium in actual ®eld conditions. The purpose of this research is to study the eects of water/solids (w/s) ratio, silica fume content, and clay content on the ability of cement to sorb selenite. A batch sorption procedure was used to determine distribution coecients (Kd) for each of the formulations at a solution pH value of 12.4. This pH value approximates conditions expected in the pore space of a hardened cement [4]. A second study was conducted on three of
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00024-6
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the 27 cement formulations to determine the if the selenite sorption was reversible. The data obtained in this work can be used to more accurately quantify the release of selenium from cement-encapsulated wastes. 2. Materials and methods 2.1. Materials The cement used in this work was a Type V Portland cement manufactured by Ash Grove Cement Co., Nephi, UT and obtained from WMK Materials in Las Vegas, NV. Type V Portland cement is commonly used in the arid southwest US because of its ability to resist sulfate attack. The chemical and physical characteristics of the cement are given in Table 1. The silica fume used in this study, Rheomac SF1001, was obtained from Master Builders, Inc. of Phoenix, AZ. The chemical and physical properties of this additive are presented in Table 2. An attapulgite clay, Attagel1, was obtained from E.T. Horn Co. of La Mirada, CA. The chemical and physical properties of the Attagel1 are presented in Table 3. The combination of relatively low w/s ratios Table 1 Chemical and physical characteristics of ash grove type V Portland cementa Chemical composition (%) SiO2 Al2O3 Fe2O3 CaO MgO SO3 Loss on ignition Insoluble residue Total alkalines (as Na2O)
22.7 3.3 3.8 63.9 2.3 2.0 0.1 0.16 0.46
Potential compound composition (%) Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite Gypsum and ``other materials''
52 26 3 12 7
Physical data Blaine ®neness (m2 kgÿ1) Normal consistency (%) Vicat set times (min) Initial Final False set (%) Autoclave expansion (%) Air entrainment Compressive strength (PSI) 1 day 3 days 7 days a
363 25.6 110 25.6 87.8 0.05 6.7 1730 3160 4050
Data provided by Ash Grove Cement Company, Nephi, UT.
and increased water demand from the silica fume and clay additives made it dicult to mix a number of the formulations thoroughly. It became necessary to add 11.3 g of a high range water-reducer (Rheobuild/MBSF) to each mix to increase its workability. Twenty-seven cementitious formulations containing three levels of w/s ratio (0.45, 0.50, and 0.55), three concentrations of silica fume (0, 10, and 20% by percent of total solid mass), and three concentrations of clay (0, 3, and 5% by percent of total solid mass) were evaluated in this study. A list of the formulations is provided in Table 4. All formulations were mixed in accordance with ASTM Standard C192, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory [6]. To simulate alkaline conditions, each cement mix was made using deionized water that was adjusted to a pH of 12.5 using NaOH. After the mixing of each formulation was complete, the cement paste was poured into three 5.1-cm diameter plastic cylinders to a height of 5 cm. The cylinders were sealed with a polyethylene ®lm and the cement was allowed to cure for 28 days at room temperature. Once cured, the cement specimens were demolded and fragmented into smaller
Table 2 Chemical and physical characteristics of Rheomac SF 1001a Chemical composition (%) Silica, fume Silicon dioxide
>99 <1
Physical data Light grey ®ne powder Average particle diameter (mm) Speci®c gravity Surface area (m2 kgÿ1) Bulk density (lbs ftÿ3)
0.1 2.2 20,000 30±40
a
Data provided by Master Builders, Inc. of Phoenix, AZ.
Table 3 Chemical and physical characteristics of attapulgite claya Chemical composition (%) Silicon (SiO2) Aluminium (Al2O3) Magnesium (MgO) Iron (Fe2O3) Calcium (CaO) Phosphorous (P2O5) Potassium (K2O) Titanium (TiO2) Trace elements Physical data Physical form Average particle diameter (mm) pH BET surface area (m2 gÿ1) a
Data provided by E. T. Horn Co., La Mirad, CA.
65.2 11.9 10.8 3.5 6.2 1.0 0.8 0.5 0.1 Powder 0.14 7.5±9.5 150
E.A. Johnson et al. / Waste Management 20 (2000) 509±516 Table 4 Cement formulations and associated experimental Kd values (l kgÿ1) Formulation (%water, %silica fume, %clay)
Kd values
45/0/0 45/10/0 45/20/0 45/0/3 45/10/3 45/20/3 45/0/5 45/10/5 45/20/5 50/0/0 50/10/0 50/20/0 50/0/3 50/10/3 50/20/3 50/0/5 50/10/5 50/20/5 55/0/0 55/10/0 55/20/0 55/0/3 55/10/3 55/20/3 55/0/5 55/10/5 55/20/5
890110 51060 440100 770130 62080 41060 630120 36030 250140 570160 510200 410110 93090 63030 4909 84060 68030 41060 930140 52050 35090 80060 76080 47080 7009 700110 42010
pieces using a hammer. The fragments were then ground using a mortar and pestle and sieved to obtain cement samples with particle sizes between 250 mm and 1 mm. 75 Se was selected as a radiotracer in this study due to its relatively long half-life and ease of detection. A NIST-traceable stock solution of 75SeO2ÿ 3 was diluted to a concentration of 340 Bq lÿ1 and a total selenite concentration of 53 mg lÿ1, which approximates the maximum contaminant level of 50 mg lÿ1 for community water systems in the US [7]. The solution pH was adjusted to a value of 12.4 using NaOH, as necessary. Sodium selenite (Na2SeO3) or deionized water was added to the standard solutions during the sorption/desorption phase of the project to obtain selenite solutions with concentrations of 6.5, 26, 53, 126, 784, and 1510 mg lÿ1. 2.2. Sorption protocol The batch sorption procedure used to determine Kd values for each formulation was adapted from Relyea [8]. Triplicate 1.000.01-g cement samples per formulation were placed in 50-ml centrifuge tubes. All samples were subjected to three overnight washes with 30 ml of deionized water adjusted to a pH of 12.4 to establish equilibrium between the solution pH and surfaces
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freshly exposed from grinding. The samples were agitated overnight on an orbital shaker at 60 rpm. The tubes were then centrifuged at 10,000 g (1 g=980 cm sÿ2) for 20 min. Finally, the wash solution was extracted with a vacuum pipette and fresh wash solution was reintroduced to the cement. The entire 30 ml of cold wash was removed from each tube after the third cycle in preparation for the introduction of the selenite solution. 30 ml of the selenite solution adjusted to a pH of 12.4 was added to each of the tubes containing the cement samples. After the tubes were oscillated on an orbital shaker at 60 rpm for 7 days at room temperature, they were centrifuged at 5700 g for 45 min to separate the solution and cement phases. 15-ml aliquots were removed using a plastic transfer pipette and placed into counting vials for radioanalysis of the 75Se. The remaining 15 ml was not removed to avoid disturbing the centrifugate. The extracted aliquots were counted on high purity germanium gamma-ray spectrometers to quantify radioactive emissions from the 75Se using the 264-keV gamma line. The aliquots were counted for either 300 or 900 s as necessary to achieve counting errors of less than 10%. All samples had a much higher concentration than the reported minimum detectable selenite for the analytical method of 0.2 mg lÿ1 at the 95% con®dence interval. The distribution coecient (in l kgÿ1) of each formulation was calculated using the following equation: Kd
A V m SA
1
where: A
m V SA
decay corrected 75Se activity sorbed on cement (Bq) calculated by taking the dierence between the activity present in 30 ml of 75Se solution per sample prior to contact with cement (determined by counting ``blank'' samples) and the activity present in the sample after contact with the cement (determined by counting the aliquot), mass of the cement (kg) present in each centrifuge tube prior to introducing the 75Se solution, amount of 75Se solution added to each centrifuge tube (liter), and 75 Se activity present in solution after contact with the cement (Bq)
2.3. Sorption/desorption protocol The reversibility of selenite sorption in cement was studied only in formulations containing 50% water and
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0, 3, or 5% clay. All samples were subjected to three overnight washes with 30 ml of 0.01 M-CaCl2 buer solution instead of deionized water to control ionic strength and maintain matrix stability. CaCl2 prevented the dispersion of small particulates and assured clean separation of dissolved and sorbed selenite. 20 ml of the selenite solution with a concentration of 6.5, 26, 53, 126, 784, or 1510 mg lÿ1, was then added to each tube containing triplicate 1.000.01-g samples of each of the formulations. After the tubes were oscillated on an orbital shaker at 60 rpm for 7 days at room temperature, they were centrifuged at 5700 g for 45 min to separate out the liquid phase. 15 ml of solution was removed from each centrifuge tube and analyzed by gamma-ray spectroscopy as described above. 15 ml of 0.01 M-CaCl2 solution was then added to each centrifuge tube, which also contained 5 ml of residual selenite solution, resulting in a total aqueous volume of 20 ml. The test tubes were allowed to equilibrate for 13 days at room temperature under gentle agitation (orbital shaker at 60 rpm). The tubes were then centrifuged at 10,000 g for 20 min. 15 ml of the solution was again extracted and analyzed. This process was repeated a total of four times with equilibrium times of 13, 13, 11 and 11 days, respectively. The aliquots were analyzed for times long enough to achieve an error less than 10% at the 95% con®dence level. The concentration of selenite sorbed by the cement particles following the original sorption wash and each subsequent desorption wash was then calculated. 2.4. Surface area measurements Surface area measurements of all 50% water-fraction cement formulations were measured using the BET nitrogen adsorption method with a Micrometrics1 Gemini 2370 surface area analyzer before and after the 0.01 M-CaCl2 or deionized water washes. This allowed an evaluation of any changes in surface area caused by the experimental procedure. Samples were dried at 200 C under nitrogen prior to measurements.
3. Results 3.1. Sorption phase Average Kd values determined for each cement formulation are presented in Table 4. The mean Kd value obtained for all formulations studied was 593 l kgÿ1. An Analysis of Variance (ANOVA) test was used to analyze the data from the sorption experiment and allow a comparison between data. The ANOVA test indicated that there was a signi®cant decrease (F=112.4, P=0.001) in selenite sorption as the silica fume content of the cement formulations increased. This eect is illustrated in a box and whisker diagram shown in Fig. 1. The lower and upper edge each box represents the 25th and 75th percentiles of the Kd distribution, respectively. The lower and upper whiskers (error bars) represent the tenth and ninetieth percentiles. The solid line and dotted line within each box represent the median and mean values for the pooled Kd data, respectively. As depicted in Figs. 2 and 3, there were no signi®cant dierences at the 95% con®dence level in selenite sorption as the water or clay contents of the mixes were varied. 3.2. Sorption/desorption phase Sorption data for cement systems is typically expressed as a Freundlich isotherm [9]. The equation describing this model [10] is: log Cs log k n log Cw
2
where CS is the concentration of selenium sorbed on the cement and CW is the concentration of selenium in
2.5. Dissolved silica measurements A LaMotte Smart colorimeter using the silica molybdate complex colorimetric method was used to detect dissolved silica in the wash solutions from the second phase of the study. The wash solutions were also analyzed by Inductively Coupled Plasma±Atomic Emissions Spectroscopy (ICP±AES) for total silica, calcium, and aluminum that comprise the primary constituents of cement and clay. Their presence indicates the degree of dissolution of the cement matrix. From this data, the stability of various cement formulations was examined as a function of the type of wash solution.
Fig. 1. Eect of silica fume on selenite sorption across all formulations.
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Fig. 2. Eect of water content on selenite sorption across all formulations.
Fig. 4. Sorption/desorption isotherms for 50/0/0 formulation.
Fig. 3. Eect of clay content on selenite sorption across all formulations.
Fig. 5. Sorption/desorption isotherms for 50/0/3 formulation.
solution in contact with the cement. Figs. 4±6 present sorption isotherms for the three formulations examined in this phase of the study. In addition to the sorption isotherms, desorption isotherms originating from each of the initial equilibration points are also shown on each plot. The experimental constants (k and n) determined for each formulation by regression analyses are presented in Tables 5 and 6. The value k represents a Kd if the isotherm is linear. The value n represents the deviation from linearity. If n=1, as is the case in this study, a linear relationship exists in which the use of a Kd value would be valid. The Kd values obtained for the three formulations in this phase were similar to the values
determined for these same formulations in the sorption phase of the study. Any discrepancies in the Kd values would mostly likely be a result of using CaCl2 as a ¯occulation promoter in the sorption/desorption phase and not in the sorption phase. Without CaCl2, small particles may remain suspended in solution during washes, which would tend to result in lower Kd values. Desorption results indicate irreversible selenite sorption or very slow desorption kinetics. This is indicated by values of n approaching zero for the desorption data and is shown by the horizontal pattern created by desorption isotherms for each selenium concentration studied. Reversible desorption would have followed the sorption isotherm.
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E.A. Johnson et al. / Waste Management 20 (2000) 509±516 Table 6 Freundlich constants determined for the desorption isotherms Formulation (%water, %silica fume, %clay)
Initial selenium concentration (mm lÿ1)
k
n
R2
50/0/0
1510 784 126 53 26 6.5
24,000 12,000 2300 1000 530 140
0.062 0.080 0.065 0.081 0.080 0.075
0.64 0.81 0.73 0.81 0.74 0.70
50/0/3
1510 784 126 53 26 6.5
24,000 14,000 2400 1000 520 140
0.057 0.049 0.007 0.046 0.035 0.039
0.68 0.78 0.010 0.46 0.32 0.29
50/0/5
1510 784 126 53 26 6.5
24,000 13,000 2300 1000 530 140
0.067 0.046 0.073 0.048 0.048 0.042
0.70 0.66 0.77 0.85 0.37 0.38
Fig. 6. Sorption/desorption isotherms for 50/0/5 formulation.
Table 5 Freundlich constants determined for the sorption isotherms Formulation (%water, %silica fume, %clay)
k
n
R2
50/0/0 50/0/3 50/0/5
820 1100 1100
0.98 1.00 0.98
1.00 1.00 1.00
3.3. Surface area measurements Results of the surface area measurements indicate that the cement particles undergo little change because of the washes and centrifugation. The dierences between pre-wash and post-wash surface areas ranged from 0 to 8.7%. Slight reductions in pre-wash and postwash surface areas can most likely be attributed to partial dissolution of the amorphous phases. The addition of clay increases the surface area of the unwashed cement from 9.860.01 m2 gÿ1 (50/0/0) to 18.40.01 m2 gÿ1 (50/0/3) and 17.10.01 m2 gÿ1 (50/0/5). The surface area of Attagel 501 is 150 m2 gÿ1 (moisture free value) and likely contributes to the higher surface area measurements obtained for cement formulations containing clay. 3.4. Dissolved silica measurements Total silica determined using colorimetric and ICP± AES methods were compared in order to determine whether the silica present in the wash solutions existed as dissolved silica or in colloidal form. The results
indicated that SiO2 was monomeric (reactive) silica as indicated by the absence of particulates in solution. The presence of colloids in solution would have been detected by ICP±AES, but not by the colorimetric method [11]. The signi®cance of this ®nding is that centrifugation resulted in complete separation of cement particles from solution. In addition, it also implies that the Kd values determined are considered true Kd values. If colloids existed, sorption by these colloids would decrease the calculated Kd values and imply decreased sorption by the cement. 4. Discussion As previously mentioned, selenite (SeO2ÿ 3 ) in an alkaline solution was prepared for use in both the sorption and sorption/desorption phases of the study. No special eort was made to prevent the introduction of oxygen into the cement or solutions during preparation. Under equilibrium conditions, the pH and redox conditions used in this work appear to favor the presence of the selenate species (SeO24 ) [3]. The thermodynamic models, MINEQL+ [12] and MINTEQA2 [13] also predict that selenate is the dominant in these conditions. Although little information on the behavior of selenate and selenite in cement matrices exists in the literature, several studies have investigated the sorptive capacity of these species in soils and clays [14±16]. In general, these studies reported that selenate typically sorbs to soil particles via weakly bonded, outer-sphere surface complexes, whereas selenite sorbs by strongly
E.A. Johnson et al. / Waste Management 20 (2000) 509±516
bonded inner-sphere surface complexes. Compared to selenate, selenite is immobile, and more likely to become irreversibly sorbed. Although selenate is the anticipated solution species according to thermodynamic models, the results of this study suggest that the strong sorption behavior observed can best be described by the presence of selenite. This indicates that little or no transformation of selenite to selenate occurred throughout the duration of the study. The relatively slow oxidation of species such as selenite to selenate has been shown in previous work [17,18]. Kd values ranged from 350 to 930 l kgÿ1 in the sorption phase and 820 to 1100 l kgÿ1 in the sorption/desorption phase of the study. Kd values estimated in the literature typically range from 410ÿ4 to 510ÿ3 l kgÿ1 [5,9,19]. The discrepancy between these Kd ranges is rather large. It should be re-emphasized that the only experimental Kd values for selenium found in the literature were determined for European cements. The results obtained in this study are exceptional only in that they are not consistent with currently accepted Kd values for selenium in cement systems. The results do, however, approximate mean Kd values reported in the literature for selenium sorption in sand, loam, clay, and organic materials of 150, 500, 740, and 1800 l kgÿ1, respectively [20]. Increasing the water content of a cement mix is known to increase the amount of calcium silicate hydrates in the hardened paste early (i.e. 28 days) in the curing process [21]. The increased water content promotes the ¯occulation of the raw cement particles, thereby creating a favorable curing environment. Since calcium silicate hydrates have been shown to be the most important sorbing material in cement mixes [9], it is expected that these mixes would exhibit increased sorption capacity for selenite. However, varying water content did not appear to in¯uence the sorption capacity of the formulations. The range of w/s ratios used in this study (0.45 to 0.55) may not have been large enough to aect a change in selenite sorption. In addition, the use of a water reducer during the preparation of all formulations may have minimized any impact varying the w/s ratios may have had on selenite sorption. Withholding silica fume resulted in cement formulations with greater sorption capacity than those with either 10 or 20% silica fume. Silica fume is known to react with a hydration by-product, calcium hydroxide, to form additional calcium silicate hydrate [22]. However, the reaction is quite slow and may not result in increased levels of calcium silicate hydrate after a standard 28-day cure. Subsequently, the 10 and 20% mixes would most likely contain relatively large amounts of unreacted silica fume at the completion of their cure. This would deleteriously eect the ability of these mixes to sorb selenite because silica fume is known to provide very little sorption capacity for anionic species [23].
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Additionally, silica that does not react during hydration is potentially free to compete with selenite for residual surface charges on the cement. Silica has been shown to exhibit a stronger sorption capacity than selenite in some materials [24]. An increase in selenite sorption would be expected at curing times beyond that examined in this study. Clays do not readily react with cement hydration products, and their surfaces have little sorption capacity for anionic species such as selenite or selenate [25]. The amount of clay used in each of the mixes (0±5%) may not have been substantial enough to aect change in the selenite sorption in each of the hardened cement pastes studied. A decrease in selenite sorption would be expected in cements with > 5% clay due to the replacement of calcium silicate hydrates with a relatively nonreactive clay additive. 5. Conclusion No eort was made to determine an optimal formulation for immobilizing selenite in cement. However, the study was designed to identify trends in selenite sorption as w/s ratio, silica fume content, and clay content of a cement mix was varied. Results from this study suggest that Kd values for selenium in cement currently used in performance assessments may substantially underestimate the sorption capacity of the ®nal cement waste form. Selenium sorption in cement systems appears to be greatly in¯uenced by redox conditions which should be considered when predicting its behavior in the environment. Acknowledgements Support with statistical analyses was provided by Susan Franson from the US Environmental Protection Agency, Las Vegas, NV. Guidance with chemical analysis was provided by Steven George of the Department of Chemistry of the University of Nevada Las Vegas. References [1] US Department of Energy, Fernald Oce. Treatability study work plan for Operable Unit 4. Fernald, OH: Fernald Environmental Management Project, 1992. [2] Reynolds Electrical and Engineering Company. Cotter concentrate treatability study plan: revision 1. Las Vegas (NV), 1995. [3] Wedepohl KH, editor. Handbook of Geochemistry: Volume II/3. New York: Springer-Verlag, 1978. [4] Angus MJ, Glasser FP. The chemical environment in cement matrices (Scienti®c Basis for Nuclear Waste Management 50). Pittsburgh (PA): Materials Research Society, 1985. [5] McKinley IG, Scholtis A. A comparison of radionuclide sorption databases used in recent performance assessments. J Contam Hydrol 1993;13:347±63.
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[6] American Society for Testing and Materials (ASTM). Practice for making and curing concrete test specimens in the laboratory (ASTM Standard C192-90a. In: 1994 Annual book of ASTM standards, Vol. 4.02. Philadelphia (PA): ASTM, 1990. [7] US Environmental Protection Agency. Code of Federal Regulations. 40CFR Part 141.62. Washington (DC): US Government Printing Oce, 1998. [8] Relyea JF, Serne RJ, Rai D. Methods for determining radionuclide retardation factors: status report (PNL-3349). Richland (WA): Paci®c Northwest Laboratory, 1980. [9] Bradbury MH, Sarott FA. Sorption databases for the cementitious near-®eld of a L/ILW repository for performance assessment (PSI Bericht. nr. 95-06). Wurenlingen/Villigen: Paul Scherrer Institut, 1995. [10] Stumm W, Morgan JJ. Aquatic chemistry: chemical equilibria and rates in natural waters. 3rd Ed. New York: John Wiley & Sons, Inc, 1996. [11] Iler RK. The chemistry of silica: solubility, polymerization colloid and surface properties, and biochemistry. New York: John Wiley & Sons, Inc, 1979. [12] Schecher WD, McAvoy DC. MINEQL+, a chemical equilibrium modeling system: Version 4.0 for Windows users manual. Hallowell, ME: Environmental Research Software, 1998. [13] Allison JD, Brown DS, Gradec KJ. MINTEQA2/PRODEFA2, a geochemical assessment model for environmental systems: version 3.0 users manual (EPA/600/3-91/021). Washington (DC): US Government Printing Oce, 1991.
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