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Adsorption of 241Am and 226Ra from natural water by wood charcoal
ARTICLE IN PRESS
Applied Radiation and Isotopes 66 (2008) 95–102 www.elsevier.com/locate/apradiso
Adsorption of
241
Am and
226
Ra from natural water by wood charcoal
C. Miro´a,, A. Baezaa, A. Salasa, J.f. Pastor-Valleb, J. Pastor-Villegasb a
Departamento de Fı´sica Aplicada, Facultad de Veterinaria, Universidad de Extremadura, Avda. de la Universidad s/n, 10071 Ca´ceres, Spain b Departamento de Quı´mica Orga´nica e Inorga´nica Facultad de Formacio´n del Profesorado, Universidad de Extremadura, Avda. de la Universidad s/n, 10071 Ca´ceres, Spain Received 7 March 2006; received in revised form 25 July 2007; accepted 28 July 2007
Abstract The adsorption of 241Am and 226Ra from natural water by a granulated wood charcoal was investigated as a function of the solution pH, in the range 4–10, and of the water flow, in the range 3.5–42 cm3/min. The percentage adsorption of 241Am (fairly constant at 480% for all pHs) was greater than that of 226Ra (which increased with increasing pH from 40% up to 480%). The results are explained by considering the different species of each radionuclide present at the pH values of the solution at the end of the adsorbent column, and the pH of the point of zero charge of the adsorbent. At pH 6, the elimination of 241Am from natural water was independent of the water flow, while the elimination of 226Ra declined linearly as the flow rate was increased. r 2007 Elsevier Ltd. All rights reserved. Keywords: Wood charcoal; Adsorption; Americium; Radium
1. Introduction Water for public consumption can become affected by different radioactive contaminants, amongst the most radiotoxic are 241Am and 226Ra (WHO, 1979). Americium is present in the environment from fallout, reaching 25 Bq/ m2 up to 1990 in the 40–501N belt of latitudes (UNSCEAR, 1993). It also comes from nuclear fuel reprocessing plants (Kershaw and Baxter, 1995) or nuclear accidents such as that of Chernobyl (Broda et al., 1989). The concentration of radium in water depends on the type of rocks through which it has passed and on the amount and distribution of uranium and thorium in the materials constituting the aquifers (Olson and Overstreet, 1964). There are various causes of high radium concentrations: the geology of the catchment area (Sua´rez et al., 2000), the management practices in the routine operation or during restoration of uranium mines (Wang et al., 1993), and the releases from phosphate industries (Paridaens and Vanmarcke, 2001). Corresponding author. Tel.: +34 927 257152; fax: +34 927 257106.
E-mail address: [email protected] (C. Miro´). 0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.07.030
Adsorption is of a great importance from a technological standpoint and for characterization of a diverse range of powders and porous materials; the behaviour of an adsorbent is dependent on its structure and surface chemistry (Rouquerol et al., 1999). The use of activated carbon is considered to be the best currently available technology for removing low-solubility contaminants in water treatment, including trace metals (Derbyshire et al., 2000; Bansal and Goyal, 2005). Different studies of radionuclides removal using carbon adsorbents have been carried out (for example, Rivera-Utrilla et al., 1984; Yamagishi and Kubota, 1989; Abbasi and Streat, 1994; Paajen et al., 1997; Flores et al., 1998; Holm et al., 2000; Turtiainen et al., 2000; Ganzerli et al., 2002; Mikhalovsky and Nikolaev, 2006). Different studies of radionuclides removal using other inorganic adsorbents have also been carried out (for example, Tao et al., 2003; Wang et al., 2004; Chen and Wang, 2007; Han et al., 2007). In accordance with Granados et al. (2006), fundamentals investigations for better understanding of adsorption mechanisms of radionuclides will continue in order to be able to select most suitable materials for a particular requirement.
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The adsorption of metal ions in aqueous solution onto carbon materials is a complex process, which is governed by both electrostatic (Coulombic) and non-electrostatic interactions (which can be of different natures, but are predominantly of van der Waals type due to dispersion forces). The controlling factors of both interactions are: the physical (surface area and pore structure) characteristics and the chemistry of the adsorbent; the nature of the metal ion (given by its speciation diagram), its solubility, and its size; the solution chemistry (solution pH and ionic strength); and the adsorption temperature (Radovic et al., 2001; Lo´pez-Ramo´n et al., 2002, 2003). The more important parameters that influence and determine the adsorption of metals from aqueous solutions are the carbon–oxygen surface functional groups present on the activated carbon and the pH of the solution. Both parameters determine the nature and the concentration of the charge on the activated carbon, and the amount and the concentration of the ionic species in the solution (Bansal and Goyal, 2005). The present work is an initial study of the adsorption of 241 Am and 226Ra dissolved in natural water under a dynamic regime at room temperature, using a wood charcoal manufactured in Extremadura (Spain) in a continuous furnace from eucalyptus wood. This charcoal has been partially characterized by us in recent works (Pastor-Villegas et al., 2006, 2007); obviously, it is a carbon material without activation process, and therefore it is cheaper. The efficiency of the adsorption of each radionuclide was analyzed as a function of the solution pH and of the water flow.
2. Experimental 2.1. Natural water samples and the sampling point The water used in the study was surface water from the River Almonte, which is used to supply the population of the city of Ca´ceres (Spain) of 88,000 inhabitants. The Almonte is a tributary of the River Tagus, and in this zone its waters are mixed with those of the Tagus as part of the 1600 hm3 capacity Alca´ntara reservoir. The Tagus is the principal input of water to that reservoir, and, upstream of this zone, it is used to provide cooling for three nuclear power plants, Zorita, Trillo, and Almaraz. In previous studies, we have quantified the presence of man-made radionuclides in the water of the Alca´ntara reservoir (Baeza et al., 1991, 2001). The sampling point was selected next to the water intake for Ca´ceres’s drinking water plant, which treats 21,000 m3/ day. Water (34 l) was collected on 8 August 1999, and filtered through conventional filter paper. We used 2 l for a physico-chemical analysis, 20 l for the determination of the radioactive content, and the remainder for the different experiments.
2.2. Physico-chemical and radiological characterization of the water The collected water was characterized physico-chemically following the analytical procedures set out for drinking water in Spanish legislation (BOE, 1987) and radiologically by gamma spectrometry. A 45% relative efficiency p-type high-purity germanium (HPGe) coaxial detector was used for the gamma spectrometric analysis. The detector has a resolution of 1.9 keV full width at half maximum at 1332.5 keV (60Co gamma emission) and a peak-to-Compton ratio of 64:1, and is coupled to a 4096 channel multichannel analyzer. The gamma spectra were analyzed using an updated version of the program ESPEC (Baeza et al., 1992). The detector counting efficiency was calibrated using calibration sources prepared with a mixed gamma standard solution, Amersham QCY.48, with substrates and geometries identical to those of the sample sources. For the radiological characterization of the natural water, different aliquots were evaporated to dryness on a plastic lamina, at 90 1C, and the residues were collected and placed in Petri dishes (88 mm diameter, 16 mm depth). The 241 Am activity was determined from the 59.54 keV emission (Firestone, 1996), and the 226Ra activity from the 351.9 and 609.3 keV gamma lines of 214Pb and 214Bi, respectively. To this end, the samples were sealed and the measurements were made 1 month later to assure secular equilibrium between 226Ra and its daughters. However, for the cases of the tracer-added aliquots, the 226Ra activity was obtained from the 186.10 keV emissions since it is then neither necessary to ensure secular equilibrium between 226Ra and its closest daughters, nor to consider the process of adsorption of 214Pb and 214Bi on the carbon. The main physico-chemical and radiological characteristics of the natural water were studied exhaustively during the years 1994–1996 (Baeza et al., 2002). Nevertheless, since these are parameters that can vary significantly according to the time at which the sample was collected, we again performed an analysis of these characteristics. The results are given in Table 1. From Table 1, one observes that the water of the river Almonte presents null values of the activities of 241Am and 226 Ra for all practical purposes. 2.3. Addition of tracer and adsorption from the radioactive aqueous solutions The 241Am and 226Ra were supplied by the Centro de Investigaciones Energe´ticas, Medioambientales y Tecnolo´gicas (CIEMAT), Madrid, in a 4 cm3 solution acidified with a mixture of hydrochloric and nitric acids. The chloride and nitrate contents of this solution (o19.5 mg Cl and o19.5 mg NO 3 ) were sufficiently low so as not to alter the physico-chemical characteristics of the water samples it was added to. Sulphate concentration added to the water was 1.5 1011 M with the radium tracer
ARTICLE IN PRESS C. Miro´ et al. / Applied Radiation and Isotopes 66 (2008) 95–102 Table 1 Physico-chemical and radiological characteristics of the natural water Parameter
Value
pH Conductivity (mS/cm) Dry residue 110 1C (mg/l) Carbonate (mg/l) Hydrogencarbonate (mg/l) Chloride (mg/l) Nitrate (mg/l) Sulfate (mg/l) Calcium (mg/l) Sodium (mg/l) Magnesium (mg/l) Potassium (mg/l) 241 Am (Bq/l) 226 Ra (214Pb, 214Bi) (Bq/l)
8.8 544 394 17 37 67 1.2 125 44.81 45.4 15.84 4.69 o0.002 o0.006
solution, which gave an ionic product, 1.95 1014, less than the solubility product, 3.7 1011. Amounts of this tracer solution was added to 12 l of sample, followed by 12 h of gentle stirring in order to guarantee total homogenization. The specific activities of the natural water after the addition and dilution of the tracer were 3072 and 12779 Bq/l for 241Am and 226Ra, respectively. At room temperature, the pH of the samples was then adjusted to values between 4 and 10 by the addition of analytical grade hydrochloric acid or sodium hydroxide. Different aliquots of the radioactive aqueous solutions were then made to flow through a chromatographic column (4.45 cm internal diameter) containing a granulated wood charcoal (1–2 mm size range). After passing through the column, the water was filtered through conventional filter paper in order to eliminate any fine wood charcoal particles present. Then the sample was evaporated to dryness on a plastic lamina, at 90 1C, and the residues were collected and placed in Petri dishes (88 mm diameter, 16 mm depth). It was then analyzed by gamma spectrometry. We had previously verified that filter paper did not adsorb the two radionuclides considered. The adsorption efficiency of the carbon, R(%), was calculated by C1 Rð%Þ ¼ 1 100, (1) C0 where C0 (Bq/l) is the initial concentration of the radionuclides after their addition to the water sample and C1 (Bq/l) is the residual concentration in the water after its passage through the wood charcoal. The parameters that were kept fixed in all the experiments were the mass of the wood charcoal, 30 g, and the volume of the water passing through this carbon adsorbent, 1 l, at a flow rate of 10.5 cm3/min. Different aliquots of the sample were adjusted to the initial pH (pHi) 4, 6, 8, and 10, before being passed through the carbon adsorbent column.
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2.4. Characterization of the wood charcoal The chemical composition and the physical characteristics of the wood charcoal had been determined in a recent study (Pastor-Villegas et al., 2006). The most interesting results from that work are presented in Table 2. In brief, the carbon adsorbent presents: (i) a high carbon content (fixed carbon and elemental carbon), and low contents of ash, hydrogen, nitrogen, and oxygen; (ii) a high total pore volume (Vp41 cm3/g); (iii) a good initial development of its pore structure ranging from narrow micropores to wide macropores, with values of both narrow (WDR(CO2) ¼ 0.225 cm3/g) and wide (WDR(N2) ¼ 0.165 cm3/g) micropore volumes relatively high, a significant amount of macropores (Vma ¼ 0.661 cm3/g), and a high value of the cumulative pore volume (VHg ¼ 0.864 cm3/g); (iv) a BET surface area (SBET(N2) ¼ 387 m2/g) low in comparison with activated carbons, but relatively high taking into account that this carbon adsorbent is a wood charcoal. Also, this wood charcoal contains carbon–carbon double bonds (olefinic and aromatic structures), carbonyl groups, ether structures, and aromatic hydrogens (Pastor-Villegas et al., 2007). Additional characterization of the wood charcoal was carried out in the present study and the results are listed in Table 3. The pH of its point of zero charge (pHPZC) was determined following the mass titration method, a method used for amphoteric solids including inorganic oxides and activated carbons (Noh and Schwarz, 1990). The elemental composition of its ash was determined by inductively coupled plasma emission spectroscopy. Its natural and man-made radioactive content was determined by gamma spectrometry, measuring the 226Ra activity from the gamma lines of 214Pb and 214Bi, and the activities of 228 Ac, 40K, and 137Cs from their 911.1, 1460.7, and 661.7 keV emissions, respectively. From Table 3, one observes that the carbon adsorbent presents a relatively high value of pHPZC. All the radionuclides activities measured, including those that were the
Table 2 Chemical composition and physical characteristics of the wood charcoal (Pastor-Villegas et al., 2006) Proximate composition (wt%, dry basis)
Volatile matter Ash Fixed carbon
8.01 2.06 89.93
Ultimate composition (wt%, dry basis)
C H N S Odiff.
90.06 1.70 1.30 0.06 4.82
Pore volumes (cm3/g)
Vp WDR(CO2) WDR(N2) Vma VHg
Specific surface area (m2/g)
SBET(N2)
1.044 0.255 0.165 0.661 0.864 387
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Table 3 Value of pH of the point of zero charge and elemental mineral composition of the wood charcoal
pHi
pHf
4.0 6.0 8.0 10.0
6.3 7.3 8.0 10.0
8.5
pH of the point of zero charge
pHPZC
Main natural radioisotope composition (Bq/kg)
226
Main man-made radioisotope composition (Bq/kg)
241
Main ash composition (ppm)
Potassium Sodium Calcium Phosphorus Magnesium Aluminium Lithium Sulfur
Ra ( Ac 40 K
Table 4 Changes in the pH after filtration trough wood charcoal
214
Pb,
228
Am Cs
137
214
Bi)
o3.8 o5.6 110730 o12 o2.2 3848.0 1345.0 854.5 322.1 207.1 133.9 122.2 84.1
Adsorption efficiency (%)
100 Am-241 Ra-226
80 60 40 20 0 0
2
4
6
8
10
pH
object of the present study, were below the detection limit except for the natural radionuclide 40K due to the high potassium content of the wood charcoal. The wood charcoal characteristics (Tables 2 and 3) will condition how it behaves as an adsorbent in removing the radionuclides under study. In particular, it is interesting to note that this charcoal has relatively low oxygen content, which indicates that it must have a basic behaviour in contact with aqueous solutions because carbons with low oxygen content are known to present basic surface properties (Boehm, 1994). The relatively high value of pHPZC is a further indication of this behaviour. 3. Results and discussion 3.1. Adsorption efficiency versus the solution pH After the solution had passed through the column, its pH was again measured to observe whether the adsorbent had caused any change. The results are presented in Table 4. One observes that the pH of the effluent (pHf) increased when the initial pH (pHi) of the radioactive aqueous solution was acidic, but did not change when the initial solution was basic. In other words, the carbon adsorbent behaved as a base for the solutions with pHi values of 4 and 6 by adsorbing protons. Today, there is consensus about the type of surface functionalities that contribute to the acidic character of a carbon material: carboxyl groups, anhydrides, hydroxyls, lactones and lactol groups. In contrast, the type of contributions to the basicity of carbons is still controversial: oxygen-containing functionalities (chromene, pyrone, quinones) and non-heteroatomic Lewis base sites, characterized by regions of p electron density on the carbon basal planes (Bandosz and Ania, 2006). Leo´n y Leo´n et al. (1992), studying the protonation of the basal sites on
Fig. 1. Influence of pH on adsorption efficiency.
carbon concluded that in carbons with low oxygen content there predominates a donor/acceptor type interaction between the graphene layers and the hydronium ions (Cp–H3O+, where Cp are oxygen-free-carbon electron-rich regions within the basal planes of carbon crystallites). Montes-Mora´n et al. (2004) have concluded that pyrones may be the most important basic functional groups at the edges of carbon surfaces. Since the carbon adsorbent used in this study has a relatively low oxygen content, electronrich regions within the graphene layers acting as Lewis base sites may be partially responsible for the carbon’s basicity. It is also possible that soluble mineral matter in the carbon adsorbent contributed to the changes in pHi. The results for the adsorption efficiency, R(%), are shown in Fig. 1. As can be seen, the solution pH plays different roles in the removal of these radionuclides. One observes firstly that the 241Am decontamination efficiency is high, greater than 80% in all cases, the mean adsorption being 85.173.6 (S.D.)%. In contrast, the dependence of the 226Ra adsorption on pH is completely different: the values of the elimination from the water increase with pH from 40% to 85%. As is shown in Fig. 1, the adsorption of 226 Ra as a function of pH fits perfectly the parabola: Rð%Þ ¼ 27:15 þ 20:91pH 0:968pH2 2
(2)
with a correlation coefficient r ¼ 0.999. To interpret these results correctly, one must bear in mind that the most abundant ionic forms of americium in aqueous solutions are: Am3+(III), AmO+ and 2 (V), 3+ AmO2+ (VI). Of these, Am (III) is the commonest state, 2 since powerful oxidation is required to reach higher states (Wolery, 1992). Radium in aqueous solutions is in the divalent form, Ra2+(II) (Clifford, 1991). The ionic radius
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values for Am3+ and Ra3+ are 0.0975 and 0.157 nm, respectively (Cotton and Wilkinson, 1997), which indicates that such cations must to accede to adsorption sites with different difficulty. Moreover, it must be taken into account that various inorganic complexes of Am3+ and Ra2+ will present in aqueous solution. Fig. 2 displays the percentage of the different species in aqueous solution as a function of solution pH. The diagrams were elaborated using the databases EQ 3/6 (Wolery, 1992) and WATEQ4F (Ball and Nordstrom, 1991). One observes in the upper part in Fig. 2 that the predominant species of americium are charged positively, and depend on the solution pH. AmSO+ 4 predominates at the pH values of 4 and 6. At pH 7, AmCO+ 3 predominates,
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+ + but AmSO+ 4 and AmOH2 are significant. AmCO3 also + predominates at pH 8 and Am(OH)2 at pH 10. In the case of radium, one observes in the bottom part in Fig. 2 that Ra2+ predominates at pH 4, but at pH44 the concentration of RaHCO+ 3 increases to pH 7, and the concentrations of Ra2+ and RaSO4(aq) decrease. RaHCO+ 3 predominates at pH values between 6 and 8. At pH46, RaCO3(aq) increases, becoming predominant at pH 10. It is known that carbons have surface charge in aqueous solutions; the carbon surface has a positive charge below point of zero charge and a negative charge above point of zero charge. Since pHPZC of the present carbon adsorbent is 8.5, the net total surface charge (external and internal) will be positive at pHo8.5 and negative at pH48.5. In
Fig. 2. Calculated chemical speciation of americium (top) and radium (bottom) as a function of pH.
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other words, the adsorption of cations is favoured at pH48.5, and the adsorption of anions at pHo8.5. As can be seen in Table 4, the pH values of the solution at the end of the column were 6.3, 7.3, 8.0, and 10.0. These values have to be considered in the adsorption mechanism of the radionuclides. The adsorption of americium and radium takes place at adsorption sites of the wood charcoal used, which are acid surface groups (hard sites) or p-orbitals of the surface polyaromatic units (soft sites), as pointed out Alfarra et al. (2004). It is widely accepted that metal ions are adsorbed on activated carbons by a mechanism of ionic exchange between ionizable protons of surface oxygen groups and metal cations (Radovic et al., 2001; Bansal and Goyal, 2005), having the specific surface area negligible influence. However, this mechanism is harder to explain in the case of basic carbons such as the wood charcoal used in the present study, which is a carbon with low oxygen content having carbonyl groups and ether structures. Recently, RiveraUtrilla and Sa´nchez-Polo (2003) and Sa´nchez-Polo and Rivera-Utrilla (2002), studying the adsorption of Cr(III) and Cd(II), have concluded that ionic exchange of protons between –Cp–H3O+ interactions and cationic species accounts for the adsorption of the latter on basic carbons with positive charge density. Our results indicate that this mechanism is also a plausible explanation at solution pHf below pHPZC. At pHf 8 (a value close to pHPZC), the adsorption of americium falls a little because the anionic 3 species Am(CO3) are repelled by the 2 and Am(CO3)3 negatively charged adsorption sites. At solution pHf higher than pHPZC, the cationic species of the two radionuclides are adsorbed because the net surface charge of the adsorbent is negative. At pHf 10, the adsorption efficiency of radium increases a little, which indicates that radium is removed as RaCO3. This fact is consistent with the speciation diagram (Fig. 2). It is interesting to note that acceptable results are obtained for the efficiency in the decontamination of the two radionuclides at initial pH 6 (pHf 7.3), a value that is within the limits usually required for a water to be considered potable (BOE, 2003). At this point, it is convenient point out that the organic and inorganic chemical composition of natural waters change substantially from source to source, and that variations in pH can change both the surface charge distribution of the carbon adsorbent and the ionization of weak acid or bases, including natural organic material with various functional groups in its structure (Bansal and Goyal, 2005). In particular, humic substances are present in natural waters, varying in concentration depending on their source; these substances may be an important source of complex formation at concentration very low (Silva and Nitsche, 1995). Bautista-Toledo et al. (1994), studying the adsorption of chromium ions from aqueous solutions by activated carbons, pointed out that humic substances may change the surface charge density of the carbon adsorbent, block some adsorption sites, and form organic complexes.
Table 5 Influence of the flow F on the adsorption efficiency R (pHi ¼ 6) F (cm3/min)
R(%) 241
226
9076 7775 7574 9076 9076
8076 6675 6473 6575 3873
Am
3.5 7.0 10.5 21.0 42.0
Ra
In recent study of Wang et al. (2006), it has been concluded that the adsorption of Eu(III) on alumina is strongly influenced by humic acid, it is significantly dependent on pH values and independent of ionic strength. Our results correspond to the removal of radium and americium in natural water, in which other inorganic species are present (Table 1); these species and humic substances may have influence on the adsorption of the radionuclides studied. Further systematic investigation is needed to understand fully the adsorption mechanism because, as mentioned, we have made an initial study. 3.2. Influence of the water flow rate on the adsorption efficiency The influence of the flow rate on the adsorption efficiency was studied by passing 1 l aliquots of sample with the tracer added, adjusted to an initial pH 6, at flow rates (F) in the range 3.5–42 cm3/min. The results are given in Table 5. For 241Am, they indicate that the elimination was independent of the flow rate, with the mean elimination being R(%) ¼ 8478 (S.D.). The elimination of 226Ra, however, declined linearly as the flow rate was increased. The straight line fit to the values of the elimination of this radionuclide over the range of flow rates that was studied is given by Rð%Þ ¼ 7820:91F with r2 ¼ 0.867. These results show that the 241Am species present in the aqueous solutions at pH 6 are more attracted to the adsorption sites available on the carbon adsorbent than the 226Ra species. 4. Conclusions The adsorption of 241Am and 226Ra from natural water by a granulated wood charcoal was investigated as a function of the solution pH and of the water flow. We observed that, relative to 226Ra, the percentage adsorption of 241Am was not only greater but was also less dependent on the solution pH in the range 4–10. In this range, the adsorption efficiency of 226Ra increased from 40% up to 480%, fitting a parabolic function, while in the case of 241Am it was almost constant, 480% in all
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cases. These results were explained by considering the different species of each radionuclide present at the pH values of solution at the end of the adsorbent column, and the pH of the point of zero charge of the wood charcoal. In the adsorption processes at final solution pH values below the pHPZC of the adsorbent, ionic exchange of protons between –Cp–H3O+ interactions and cationic species of the two radionuclides is a plausible explanation. At the highest solution pH, the negative surface charge of the adsorbent attracts the cationic species of americium, and radium is removed as radium carbonate. At pH 6, a value within the pH range of potable natural waters and less than the pHPZC of the adsorbent, the adsorption of 241Am was independent of the water flow, while the adsorption of 226Ra declined linearly as the flow rate was increased. Acknowledgements The present study was made possible by the financial support for the following research projects: project no. 1FD97-0099, financed by the Plan Nacional de I+D, and FEDER and ENRESA-CIEMAT funds; project no. 2PRO1B022, financed by the Consejerı´ a de Educacio´n of the Junta de Extremadura and FEDER. References Abbasi, W., Streat, M., 1994. Adsorption of uranium from aqueous solutions using activitated carbon. Sep. Sci. Technol. 29, 1217–1230. Alfarra, A., Frackowiak, E., Be´guin, F., 2004. The HSAB concept as a means to interpret the adsorption of metal ions onto activated carbons. Appl. Surf. Sci. 228, 84–89. Baeza, A., Del Rı´ o, M., Miro´, C., Paniagua, J., 1991. Radiological impact of the Almaraz Nuclear Power Plant (Spain) during 1986 to 1989 on the surrounding environment. J. Radioanal. Nucl. Chem. 152, 175–188. Baeza, A., Corvo, G., Del Rı´ o, M., Miro´, C., Paniagua, J.M., 1992. The program ESPEC for the analysis of gamma spectra of environmental samples. Appl. Radiat. Isot. 43, 833–839. Baeza, A., Brogueira, A.M., Carreiro, M.C.V., Garcı´ a, E., Gil, J.M., Miro´, C., Sequeira, M.M., Teixeira, M.M.R., 2001. Spatial and temporal evolution of the levels of tritium in the River Tagus in its passage through Ca´ceres (Spain) and the Alentejo (Portugal). Water Res. 35, 705–714. Baeza, A., Dı´ az, M., Garcı´ a, E., Miro´, C., 2002. Influence of interbasin transfers between the Alca´ntara and Guadiloba reservoirs on the radiological quality of the drinking water of the city of Ca´ceres (Spain). J. Radioanal. Nucl. Chem. 252, 441–459. Ball, J.W., Nordstrom, D.K., 1991. User’s manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters. United States Geological Survey Open-File, United States Geological Survey Report, Menlo Park, pp. 91–183. Bandosz, T.J., Ania, C.D., 2006. Surface chemistry of activated carbons and its characterization. In: Bandosz, T.J. (Ed.), Activated Carbon Surfaces in Environmental Remediation. Elsevier, Amsterdam, pp. 159–229. Bansal, R.C., Goyal, M., 2005. Activated Carbon Adsorption. Taylor & Francis, Boca Raton. Bautista-Toledo, I., Rivera-Utrilla, J., Ferro-Garcı´ a, M.A., MorenoCastilla, C., 1994. Influence of the oxygen surface complexes of activated carbons on the adsorption of chromium ions from aqueous
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Applied Radiation and Isotopes 66 (2008) 95–102 www.elsevier.com/locate/apradiso
Adsorption of
241
Am and
226
Ra from natural water by wood charcoal
C. Miro´a,, A. Baezaa, A. Salasa, J.f. Pastor-Valleb, J. Pastor-Villegasb a
Departamento de Fı´sica Aplicada, Facultad de Veterinaria, Universidad de Extremadura, Avda. de la Universidad s/n, 10071 Ca´ceres, Spain b Departamento de Quı´mica Orga´nica e Inorga´nica Facultad de Formacio´n del Profesorado, Universidad de Extremadura, Avda. de la Universidad s/n, 10071 Ca´ceres, Spain Received 7 March 2006; received in revised form 25 July 2007; accepted 28 July 2007
Abstract The adsorption of 241Am and 226Ra from natural water by a granulated wood charcoal was investigated as a function of the solution pH, in the range 4–10, and of the water flow, in the range 3.5–42 cm3/min. The percentage adsorption of 241Am (fairly constant at 480% for all pHs) was greater than that of 226Ra (which increased with increasing pH from 40% up to 480%). The results are explained by considering the different species of each radionuclide present at the pH values of the solution at the end of the adsorbent column, and the pH of the point of zero charge of the adsorbent. At pH 6, the elimination of 241Am from natural water was independent of the water flow, while the elimination of 226Ra declined linearly as the flow rate was increased. r 2007 Elsevier Ltd. All rights reserved. Keywords: Wood charcoal; Adsorption; Americium; Radium
1. Introduction Water for public consumption can become affected by different radioactive contaminants, amongst the most radiotoxic are 241Am and 226Ra (WHO, 1979). Americium is present in the environment from fallout, reaching 25 Bq/ m2 up to 1990 in the 40–501N belt of latitudes (UNSCEAR, 1993). It also comes from nuclear fuel reprocessing plants (Kershaw and Baxter, 1995) or nuclear accidents such as that of Chernobyl (Broda et al., 1989). The concentration of radium in water depends on the type of rocks through which it has passed and on the amount and distribution of uranium and thorium in the materials constituting the aquifers (Olson and Overstreet, 1964). There are various causes of high radium concentrations: the geology of the catchment area (Sua´rez et al., 2000), the management practices in the routine operation or during restoration of uranium mines (Wang et al., 1993), and the releases from phosphate industries (Paridaens and Vanmarcke, 2001). Corresponding author. Tel.: +34 927 257152; fax: +34 927 257106.
E-mail address: [email protected] (C. Miro´). 0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.07.030
Adsorption is of a great importance from a technological standpoint and for characterization of a diverse range of powders and porous materials; the behaviour of an adsorbent is dependent on its structure and surface chemistry (Rouquerol et al., 1999). The use of activated carbon is considered to be the best currently available technology for removing low-solubility contaminants in water treatment, including trace metals (Derbyshire et al., 2000; Bansal and Goyal, 2005). Different studies of radionuclides removal using carbon adsorbents have been carried out (for example, Rivera-Utrilla et al., 1984; Yamagishi and Kubota, 1989; Abbasi and Streat, 1994; Paajen et al., 1997; Flores et al., 1998; Holm et al., 2000; Turtiainen et al., 2000; Ganzerli et al., 2002; Mikhalovsky and Nikolaev, 2006). Different studies of radionuclides removal using other inorganic adsorbents have also been carried out (for example, Tao et al., 2003; Wang et al., 2004; Chen and Wang, 2007; Han et al., 2007). In accordance with Granados et al. (2006), fundamentals investigations for better understanding of adsorption mechanisms of radionuclides will continue in order to be able to select most suitable materials for a particular requirement.
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The adsorption of metal ions in aqueous solution onto carbon materials is a complex process, which is governed by both electrostatic (Coulombic) and non-electrostatic interactions (which can be of different natures, but are predominantly of van der Waals type due to dispersion forces). The controlling factors of both interactions are: the physical (surface area and pore structure) characteristics and the chemistry of the adsorbent; the nature of the metal ion (given by its speciation diagram), its solubility, and its size; the solution chemistry (solution pH and ionic strength); and the adsorption temperature (Radovic et al., 2001; Lo´pez-Ramo´n et al., 2002, 2003). The more important parameters that influence and determine the adsorption of metals from aqueous solutions are the carbon–oxygen surface functional groups present on the activated carbon and the pH of the solution. Both parameters determine the nature and the concentration of the charge on the activated carbon, and the amount and the concentration of the ionic species in the solution (Bansal and Goyal, 2005). The present work is an initial study of the adsorption of 241 Am and 226Ra dissolved in natural water under a dynamic regime at room temperature, using a wood charcoal manufactured in Extremadura (Spain) in a continuous furnace from eucalyptus wood. This charcoal has been partially characterized by us in recent works (Pastor-Villegas et al., 2006, 2007); obviously, it is a carbon material without activation process, and therefore it is cheaper. The efficiency of the adsorption of each radionuclide was analyzed as a function of the solution pH and of the water flow.
2. Experimental 2.1. Natural water samples and the sampling point The water used in the study was surface water from the River Almonte, which is used to supply the population of the city of Ca´ceres (Spain) of 88,000 inhabitants. The Almonte is a tributary of the River Tagus, and in this zone its waters are mixed with those of the Tagus as part of the 1600 hm3 capacity Alca´ntara reservoir. The Tagus is the principal input of water to that reservoir, and, upstream of this zone, it is used to provide cooling for three nuclear power plants, Zorita, Trillo, and Almaraz. In previous studies, we have quantified the presence of man-made radionuclides in the water of the Alca´ntara reservoir (Baeza et al., 1991, 2001). The sampling point was selected next to the water intake for Ca´ceres’s drinking water plant, which treats 21,000 m3/ day. Water (34 l) was collected on 8 August 1999, and filtered through conventional filter paper. We used 2 l for a physico-chemical analysis, 20 l for the determination of the radioactive content, and the remainder for the different experiments.
2.2. Physico-chemical and radiological characterization of the water The collected water was characterized physico-chemically following the analytical procedures set out for drinking water in Spanish legislation (BOE, 1987) and radiologically by gamma spectrometry. A 45% relative efficiency p-type high-purity germanium (HPGe) coaxial detector was used for the gamma spectrometric analysis. The detector has a resolution of 1.9 keV full width at half maximum at 1332.5 keV (60Co gamma emission) and a peak-to-Compton ratio of 64:1, and is coupled to a 4096 channel multichannel analyzer. The gamma spectra were analyzed using an updated version of the program ESPEC (Baeza et al., 1992). The detector counting efficiency was calibrated using calibration sources prepared with a mixed gamma standard solution, Amersham QCY.48, with substrates and geometries identical to those of the sample sources. For the radiological characterization of the natural water, different aliquots were evaporated to dryness on a plastic lamina, at 90 1C, and the residues were collected and placed in Petri dishes (88 mm diameter, 16 mm depth). The 241 Am activity was determined from the 59.54 keV emission (Firestone, 1996), and the 226Ra activity from the 351.9 and 609.3 keV gamma lines of 214Pb and 214Bi, respectively. To this end, the samples were sealed and the measurements were made 1 month later to assure secular equilibrium between 226Ra and its daughters. However, for the cases of the tracer-added aliquots, the 226Ra activity was obtained from the 186.10 keV emissions since it is then neither necessary to ensure secular equilibrium between 226Ra and its closest daughters, nor to consider the process of adsorption of 214Pb and 214Bi on the carbon. The main physico-chemical and radiological characteristics of the natural water were studied exhaustively during the years 1994–1996 (Baeza et al., 2002). Nevertheless, since these are parameters that can vary significantly according to the time at which the sample was collected, we again performed an analysis of these characteristics. The results are given in Table 1. From Table 1, one observes that the water of the river Almonte presents null values of the activities of 241Am and 226 Ra for all practical purposes. 2.3. Addition of tracer and adsorption from the radioactive aqueous solutions The 241Am and 226Ra were supplied by the Centro de Investigaciones Energe´ticas, Medioambientales y Tecnolo´gicas (CIEMAT), Madrid, in a 4 cm3 solution acidified with a mixture of hydrochloric and nitric acids. The chloride and nitrate contents of this solution (o19.5 mg Cl and o19.5 mg NO 3 ) were sufficiently low so as not to alter the physico-chemical characteristics of the water samples it was added to. Sulphate concentration added to the water was 1.5 1011 M with the radium tracer
ARTICLE IN PRESS C. Miro´ et al. / Applied Radiation and Isotopes 66 (2008) 95–102 Table 1 Physico-chemical and radiological characteristics of the natural water Parameter
Value
pH Conductivity (mS/cm) Dry residue 110 1C (mg/l) Carbonate (mg/l) Hydrogencarbonate (mg/l) Chloride (mg/l) Nitrate (mg/l) Sulfate (mg/l) Calcium (mg/l) Sodium (mg/l) Magnesium (mg/l) Potassium (mg/l) 241 Am (Bq/l) 226 Ra (214Pb, 214Bi) (Bq/l)
8.8 544 394 17 37 67 1.2 125 44.81 45.4 15.84 4.69 o0.002 o0.006
solution, which gave an ionic product, 1.95 1014, less than the solubility product, 3.7 1011. Amounts of this tracer solution was added to 12 l of sample, followed by 12 h of gentle stirring in order to guarantee total homogenization. The specific activities of the natural water after the addition and dilution of the tracer were 3072 and 12779 Bq/l for 241Am and 226Ra, respectively. At room temperature, the pH of the samples was then adjusted to values between 4 and 10 by the addition of analytical grade hydrochloric acid or sodium hydroxide. Different aliquots of the radioactive aqueous solutions were then made to flow through a chromatographic column (4.45 cm internal diameter) containing a granulated wood charcoal (1–2 mm size range). After passing through the column, the water was filtered through conventional filter paper in order to eliminate any fine wood charcoal particles present. Then the sample was evaporated to dryness on a plastic lamina, at 90 1C, and the residues were collected and placed in Petri dishes (88 mm diameter, 16 mm depth). It was then analyzed by gamma spectrometry. We had previously verified that filter paper did not adsorb the two radionuclides considered. The adsorption efficiency of the carbon, R(%), was calculated by C1 Rð%Þ ¼ 1 100, (1) C0 where C0 (Bq/l) is the initial concentration of the radionuclides after their addition to the water sample and C1 (Bq/l) is the residual concentration in the water after its passage through the wood charcoal. The parameters that were kept fixed in all the experiments were the mass of the wood charcoal, 30 g, and the volume of the water passing through this carbon adsorbent, 1 l, at a flow rate of 10.5 cm3/min. Different aliquots of the sample were adjusted to the initial pH (pHi) 4, 6, 8, and 10, before being passed through the carbon adsorbent column.
97
2.4. Characterization of the wood charcoal The chemical composition and the physical characteristics of the wood charcoal had been determined in a recent study (Pastor-Villegas et al., 2006). The most interesting results from that work are presented in Table 2. In brief, the carbon adsorbent presents: (i) a high carbon content (fixed carbon and elemental carbon), and low contents of ash, hydrogen, nitrogen, and oxygen; (ii) a high total pore volume (Vp41 cm3/g); (iii) a good initial development of its pore structure ranging from narrow micropores to wide macropores, with values of both narrow (WDR(CO2) ¼ 0.225 cm3/g) and wide (WDR(N2) ¼ 0.165 cm3/g) micropore volumes relatively high, a significant amount of macropores (Vma ¼ 0.661 cm3/g), and a high value of the cumulative pore volume (VHg ¼ 0.864 cm3/g); (iv) a BET surface area (SBET(N2) ¼ 387 m2/g) low in comparison with activated carbons, but relatively high taking into account that this carbon adsorbent is a wood charcoal. Also, this wood charcoal contains carbon–carbon double bonds (olefinic and aromatic structures), carbonyl groups, ether structures, and aromatic hydrogens (Pastor-Villegas et al., 2007). Additional characterization of the wood charcoal was carried out in the present study and the results are listed in Table 3. The pH of its point of zero charge (pHPZC) was determined following the mass titration method, a method used for amphoteric solids including inorganic oxides and activated carbons (Noh and Schwarz, 1990). The elemental composition of its ash was determined by inductively coupled plasma emission spectroscopy. Its natural and man-made radioactive content was determined by gamma spectrometry, measuring the 226Ra activity from the gamma lines of 214Pb and 214Bi, and the activities of 228 Ac, 40K, and 137Cs from their 911.1, 1460.7, and 661.7 keV emissions, respectively. From Table 3, one observes that the carbon adsorbent presents a relatively high value of pHPZC. All the radionuclides activities measured, including those that were the
Table 2 Chemical composition and physical characteristics of the wood charcoal (Pastor-Villegas et al., 2006) Proximate composition (wt%, dry basis)
Volatile matter Ash Fixed carbon
8.01 2.06 89.93
Ultimate composition (wt%, dry basis)
C H N S Odiff.
90.06 1.70 1.30 0.06 4.82
Pore volumes (cm3/g)
Vp WDR(CO2) WDR(N2) Vma VHg
Specific surface area (m2/g)
SBET(N2)
1.044 0.255 0.165 0.661 0.864 387
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Table 3 Value of pH of the point of zero charge and elemental mineral composition of the wood charcoal
pHi
pHf
4.0 6.0 8.0 10.0
6.3 7.3 8.0 10.0
8.5
pH of the point of zero charge
pHPZC
Main natural radioisotope composition (Bq/kg)
226
Main man-made radioisotope composition (Bq/kg)
241
Main ash composition (ppm)
Potassium Sodium Calcium Phosphorus Magnesium Aluminium Lithium Sulfur
Ra ( Ac 40 K
Table 4 Changes in the pH after filtration trough wood charcoal
214
Pb,
228
Am Cs
137
214
Bi)
o3.8 o5.6 110730 o12 o2.2 3848.0 1345.0 854.5 322.1 207.1 133.9 122.2 84.1
Adsorption efficiency (%)
100 Am-241 Ra-226
80 60 40 20 0 0
2
4
6
8
10
pH
object of the present study, were below the detection limit except for the natural radionuclide 40K due to the high potassium content of the wood charcoal. The wood charcoal characteristics (Tables 2 and 3) will condition how it behaves as an adsorbent in removing the radionuclides under study. In particular, it is interesting to note that this charcoal has relatively low oxygen content, which indicates that it must have a basic behaviour in contact with aqueous solutions because carbons with low oxygen content are known to present basic surface properties (Boehm, 1994). The relatively high value of pHPZC is a further indication of this behaviour. 3. Results and discussion 3.1. Adsorption efficiency versus the solution pH After the solution had passed through the column, its pH was again measured to observe whether the adsorbent had caused any change. The results are presented in Table 4. One observes that the pH of the effluent (pHf) increased when the initial pH (pHi) of the radioactive aqueous solution was acidic, but did not change when the initial solution was basic. In other words, the carbon adsorbent behaved as a base for the solutions with pHi values of 4 and 6 by adsorbing protons. Today, there is consensus about the type of surface functionalities that contribute to the acidic character of a carbon material: carboxyl groups, anhydrides, hydroxyls, lactones and lactol groups. In contrast, the type of contributions to the basicity of carbons is still controversial: oxygen-containing functionalities (chromene, pyrone, quinones) and non-heteroatomic Lewis base sites, characterized by regions of p electron density on the carbon basal planes (Bandosz and Ania, 2006). Leo´n y Leo´n et al. (1992), studying the protonation of the basal sites on
Fig. 1. Influence of pH on adsorption efficiency.
carbon concluded that in carbons with low oxygen content there predominates a donor/acceptor type interaction between the graphene layers and the hydronium ions (Cp–H3O+, where Cp are oxygen-free-carbon electron-rich regions within the basal planes of carbon crystallites). Montes-Mora´n et al. (2004) have concluded that pyrones may be the most important basic functional groups at the edges of carbon surfaces. Since the carbon adsorbent used in this study has a relatively low oxygen content, electronrich regions within the graphene layers acting as Lewis base sites may be partially responsible for the carbon’s basicity. It is also possible that soluble mineral matter in the carbon adsorbent contributed to the changes in pHi. The results for the adsorption efficiency, R(%), are shown in Fig. 1. As can be seen, the solution pH plays different roles in the removal of these radionuclides. One observes firstly that the 241Am decontamination efficiency is high, greater than 80% in all cases, the mean adsorption being 85.173.6 (S.D.)%. In contrast, the dependence of the 226Ra adsorption on pH is completely different: the values of the elimination from the water increase with pH from 40% to 85%. As is shown in Fig. 1, the adsorption of 226 Ra as a function of pH fits perfectly the parabola: Rð%Þ ¼ 27:15 þ 20:91pH 0:968pH2 2
(2)
with a correlation coefficient r ¼ 0.999. To interpret these results correctly, one must bear in mind that the most abundant ionic forms of americium in aqueous solutions are: Am3+(III), AmO+ and 2 (V), 3+ AmO2+ (VI). Of these, Am (III) is the commonest state, 2 since powerful oxidation is required to reach higher states (Wolery, 1992). Radium in aqueous solutions is in the divalent form, Ra2+(II) (Clifford, 1991). The ionic radius
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values for Am3+ and Ra3+ are 0.0975 and 0.157 nm, respectively (Cotton and Wilkinson, 1997), which indicates that such cations must to accede to adsorption sites with different difficulty. Moreover, it must be taken into account that various inorganic complexes of Am3+ and Ra2+ will present in aqueous solution. Fig. 2 displays the percentage of the different species in aqueous solution as a function of solution pH. The diagrams were elaborated using the databases EQ 3/6 (Wolery, 1992) and WATEQ4F (Ball and Nordstrom, 1991). One observes in the upper part in Fig. 2 that the predominant species of americium are charged positively, and depend on the solution pH. AmSO+ 4 predominates at the pH values of 4 and 6. At pH 7, AmCO+ 3 predominates,
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+ + but AmSO+ 4 and AmOH2 are significant. AmCO3 also + predominates at pH 8 and Am(OH)2 at pH 10. In the case of radium, one observes in the bottom part in Fig. 2 that Ra2+ predominates at pH 4, but at pH44 the concentration of RaHCO+ 3 increases to pH 7, and the concentrations of Ra2+ and RaSO4(aq) decrease. RaHCO+ 3 predominates at pH values between 6 and 8. At pH46, RaCO3(aq) increases, becoming predominant at pH 10. It is known that carbons have surface charge in aqueous solutions; the carbon surface has a positive charge below point of zero charge and a negative charge above point of zero charge. Since pHPZC of the present carbon adsorbent is 8.5, the net total surface charge (external and internal) will be positive at pHo8.5 and negative at pH48.5. In
Fig. 2. Calculated chemical speciation of americium (top) and radium (bottom) as a function of pH.
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other words, the adsorption of cations is favoured at pH48.5, and the adsorption of anions at pHo8.5. As can be seen in Table 4, the pH values of the solution at the end of the column were 6.3, 7.3, 8.0, and 10.0. These values have to be considered in the adsorption mechanism of the radionuclides. The adsorption of americium and radium takes place at adsorption sites of the wood charcoal used, which are acid surface groups (hard sites) or p-orbitals of the surface polyaromatic units (soft sites), as pointed out Alfarra et al. (2004). It is widely accepted that metal ions are adsorbed on activated carbons by a mechanism of ionic exchange between ionizable protons of surface oxygen groups and metal cations (Radovic et al., 2001; Bansal and Goyal, 2005), having the specific surface area negligible influence. However, this mechanism is harder to explain in the case of basic carbons such as the wood charcoal used in the present study, which is a carbon with low oxygen content having carbonyl groups and ether structures. Recently, RiveraUtrilla and Sa´nchez-Polo (2003) and Sa´nchez-Polo and Rivera-Utrilla (2002), studying the adsorption of Cr(III) and Cd(II), have concluded that ionic exchange of protons between –Cp–H3O+ interactions and cationic species accounts for the adsorption of the latter on basic carbons with positive charge density. Our results indicate that this mechanism is also a plausible explanation at solution pHf below pHPZC. At pHf 8 (a value close to pHPZC), the adsorption of americium falls a little because the anionic 3 species Am(CO3) are repelled by the 2 and Am(CO3)3 negatively charged adsorption sites. At solution pHf higher than pHPZC, the cationic species of the two radionuclides are adsorbed because the net surface charge of the adsorbent is negative. At pHf 10, the adsorption efficiency of radium increases a little, which indicates that radium is removed as RaCO3. This fact is consistent with the speciation diagram (Fig. 2). It is interesting to note that acceptable results are obtained for the efficiency in the decontamination of the two radionuclides at initial pH 6 (pHf 7.3), a value that is within the limits usually required for a water to be considered potable (BOE, 2003). At this point, it is convenient point out that the organic and inorganic chemical composition of natural waters change substantially from source to source, and that variations in pH can change both the surface charge distribution of the carbon adsorbent and the ionization of weak acid or bases, including natural organic material with various functional groups in its structure (Bansal and Goyal, 2005). In particular, humic substances are present in natural waters, varying in concentration depending on their source; these substances may be an important source of complex formation at concentration very low (Silva and Nitsche, 1995). Bautista-Toledo et al. (1994), studying the adsorption of chromium ions from aqueous solutions by activated carbons, pointed out that humic substances may change the surface charge density of the carbon adsorbent, block some adsorption sites, and form organic complexes.
Table 5 Influence of the flow F on the adsorption efficiency R (pHi ¼ 6) F (cm3/min)
R(%) 241
226
9076 7775 7574 9076 9076
8076 6675 6473 6575 3873
Am
3.5 7.0 10.5 21.0 42.0
Ra
In recent study of Wang et al. (2006), it has been concluded that the adsorption of Eu(III) on alumina is strongly influenced by humic acid, it is significantly dependent on pH values and independent of ionic strength. Our results correspond to the removal of radium and americium in natural water, in which other inorganic species are present (Table 1); these species and humic substances may have influence on the adsorption of the radionuclides studied. Further systematic investigation is needed to understand fully the adsorption mechanism because, as mentioned, we have made an initial study. 3.2. Influence of the water flow rate on the adsorption efficiency The influence of the flow rate on the adsorption efficiency was studied by passing 1 l aliquots of sample with the tracer added, adjusted to an initial pH 6, at flow rates (F) in the range 3.5–42 cm3/min. The results are given in Table 5. For 241Am, they indicate that the elimination was independent of the flow rate, with the mean elimination being R(%) ¼ 8478 (S.D.). The elimination of 226Ra, however, declined linearly as the flow rate was increased. The straight line fit to the values of the elimination of this radionuclide over the range of flow rates that was studied is given by Rð%Þ ¼ 7820:91F with r2 ¼ 0.867. These results show that the 241Am species present in the aqueous solutions at pH 6 are more attracted to the adsorption sites available on the carbon adsorbent than the 226Ra species. 4. Conclusions The adsorption of 241Am and 226Ra from natural water by a granulated wood charcoal was investigated as a function of the solution pH and of the water flow. We observed that, relative to 226Ra, the percentage adsorption of 241Am was not only greater but was also less dependent on the solution pH in the range 4–10. In this range, the adsorption efficiency of 226Ra increased from 40% up to 480%, fitting a parabolic function, while in the case of 241Am it was almost constant, 480% in all
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cases. These results were explained by considering the different species of each radionuclide present at the pH values of solution at the end of the adsorbent column, and the pH of the point of zero charge of the wood charcoal. In the adsorption processes at final solution pH values below the pHPZC of the adsorbent, ionic exchange of protons between –Cp–H3O+ interactions and cationic species of the two radionuclides is a plausible explanation. At the highest solution pH, the negative surface charge of the adsorbent attracts the cationic species of americium, and radium is removed as radium carbonate. At pH 6, a value within the pH range of potable natural waters and less than the pHPZC of the adsorbent, the adsorption of 241Am was independent of the water flow, while the adsorption of 226Ra declined linearly as the flow rate was increased. Acknowledgements The present study was made possible by the financial support for the following research projects: project no. 1FD97-0099, financed by the Plan Nacional de I+D, and FEDER and ENRESA-CIEMAT funds; project no. 2PRO1B022, financed by the Consejerı´ a de Educacio´n of the Junta de Extremadura and FEDER. References Abbasi, W., Streat, M., 1994. Adsorption of uranium from aqueous solutions using activitated carbon. Sep. Sci. Technol. 29, 1217–1230. Alfarra, A., Frackowiak, E., Be´guin, F., 2004. The HSAB concept as a means to interpret the adsorption of metal ions onto activated carbons. Appl. Surf. Sci. 228, 84–89. Baeza, A., Del Rı´ o, M., Miro´, C., Paniagua, J., 1991. Radiological impact of the Almaraz Nuclear Power Plant (Spain) during 1986 to 1989 on the surrounding environment. J. Radioanal. Nucl. Chem. 152, 175–188. Baeza, A., Corvo, G., Del Rı´ o, M., Miro´, C., Paniagua, J.M., 1992. The program ESPEC for the analysis of gamma spectra of environmental samples. Appl. Radiat. Isot. 43, 833–839. Baeza, A., Brogueira, A.M., Carreiro, M.C.V., Garcı´ a, E., Gil, J.M., Miro´, C., Sequeira, M.M., Teixeira, M.M.R., 2001. Spatial and temporal evolution of the levels of tritium in the River Tagus in its passage through Ca´ceres (Spain) and the Alentejo (Portugal). Water Res. 35, 705–714. Baeza, A., Dı´ az, M., Garcı´ a, E., Miro´, C., 2002. Influence of interbasin transfers between the Alca´ntara and Guadiloba reservoirs on the radiological quality of the drinking water of the city of Ca´ceres (Spain). J. Radioanal. Nucl. Chem. 252, 441–459. Ball, J.W., Nordstrom, D.K., 1991. User’s manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters. United States Geological Survey Open-File, United States Geological Survey Report, Menlo Park, pp. 91–183. Bandosz, T.J., Ania, C.D., 2006. Surface chemistry of activated carbons and its characterization. In: Bandosz, T.J. (Ed.), Activated Carbon Surfaces in Environmental Remediation. Elsevier, Amsterdam, pp. 159–229. Bansal, R.C., Goyal, M., 2005. Activated Carbon Adsorption. Taylor & Francis, Boca Raton. Bautista-Toledo, I., Rivera-Utrilla, J., Ferro-Garcı´ a, M.A., MorenoCastilla, C., 1994. Influence of the oxygen surface complexes of activated carbons on the adsorption of chromium ions from aqueous
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