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Surface complexation modeling of uranyl ion sorption on mesoporous silica
Colloids and Surfaces A: Physicochem. Eng. Aspects 221 (2003) 149 /162 www.elsevier.com/locate/colsurfa
Surface complexation modeling of uranyl ion sorption on mesoporous silica K. Sˇtamberg a,*, K.A. Venkatesan b, P.R. Vasudeva Rao b a
Department of Nuclear Chemistry, Faculty of Nuclear Science and Physical Engineering, Czech Technical University, Brˇehova´ 7, 11519 Prague 1, Czech Republic b Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 10 June 2002; accepted 26 March 2003
Abstract Sorption of uranium on MCM-41 was studied as a function of pH, time, [U(VI)] and [CO2 3 ]. In the absence of carbonate the sorption edge occurs at pH 2 and the percentage sorption increased to 95% when the pH of the solution is 6. Further increase in pH results in complete sorption (99%) of uranium. In the presence of carbonate the percentage sorption decreased from 94 to 75% when the pH was varied from 6 to 11. The acid /base property of MCM-41 was investigated by titration of the sorbent with HNO3/NaOH at constant ionic strength. Three surface complexation models, namely, the constant capacitance model (CCM), diffuse layer model and non-electrostatic chemical equilibrium model (CEM) were employed to simulate the amphoteric behaviour of MCM-41 and uranium sorption on it. Various model parameters used for describing the sorption property of the sorbent were obtained from the non-linear regression of the experimental data. The sensitivity analysis and goodness-of-fit indicated that only CEM and CCM could be used to the description of the experimental data. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous silica; Adsorption; Uranium; Surface complexation; Modeling
1. Introduction With the discovery of mesoporous molecular sieve (MCM-41), by Kresge and Beck et al. [1,2], considerable efforts have gone in the way to develop this new family of material and employ them for the removal and separation of various metal ions from aqueous solutions [3 /6]. The * Corresponding author. Tel.: /420-2-243-58205; fax: / 420-2-223-20864. E-mail address: [email protected] (K. Sˇtamberg).
striking features of MCM-41 such as very large BET surface area, large pore volume and fast kinetics of sorption attracted many researchers to utilise them as sorbent and study its sorption behaviour towards chemical and radio toxic metal ions from various waste streams. Mobil corporation [6] patented a sorption separation process for the purification of water using modified MCM-41. Feng et al. [3] have developed highest capacity thiol functionalized monolayers on mesoporous silica, FMMS, for trapping Hg2 from aqueous solution. Grun et al. [7] investigated the properties
0927-7757/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-7757(03)00139-0
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of MCM-41 as packing material in normal-phase HPLC and compared its retention behaviour and selectivity with those of other oxides. Interestingly they found that MCM-41 has the ability to separate all types of analytes (basic, acidic and neutral) with acceptable retention times and good peak shapes. Bruzzoniti et al. [5] studied the partition and retention properties of various environmental pollutants on mesoporous silica and obtained the correlation between chemical structure of the pollutant and sorbent. Recently Shin et al. [8] showed high capacity and fast kinetics of uranyl ion sorption on mesoporous titanosilicate and compared its advantage over the corresponding microporous material. Vidya et al. [9] studied the ion-exchange of template for trapping UO2 in as-synthesised 2 MCM-41 and MCM-48. High sorption capacity achieved in MCM-48 was attributed to strong binding between UO2 and defective sites in the 2 three dimensional pore system of MCM-48. The aim of the present work is to study the behaviour of uranyl ion sorption on calcined MCM-41 as a function of various parameters such as time, concentration of U(VI) and carbonate. The amphoteric nature of MCM-41 was investigated by acid /base titration of the sorbent and the results are also reported. Three types of surface complexation models (SCMs) namely two electrostatic models, i.e. constant capacitance model (CCM) and diffuse double layer model (DLM) and one chemical equilibrium, non-electrostatic model (CEM), were employed to simulate the amphoteric property of MCM-41 and to describe the sorption of uranium on MCM-41.
2. Model approach SCMs have been widely used [10 /14] for the description of the sorption of metal ions from aqueous solution as a function of pH, ionic strength, solution concentration etc. Well-developed SCMs are available for predicting the sorption behaviour of various metal ions from the complex mixtures. SCMs represent a set adsorption reaction occurring between the metal species existing in solution and the surface of sorbent,
resulting in the formation of a surface complexes (SCs). The concentration of the SCs can be estimated using the corresponding set of quasithermodynamic constants that are obtained by non-linear regression analysis of the experimental data. In order to apply SCMs for describing the sorption of uranium on MCM-41, the surface area, total surface site concentration and the protonation constants are needed and that can be obtained by evaluation of acid /base titration data of MCM-41 and surface area measurements. The protolysis reaction of surface groups (XO and XOH0) of MCM-41 are given by XO H XOH0 0
XOH H
K1p
XOH 2
K2p
(1) (2)
where K1p and K2p are the protonation constants for Eqs. (1) and (2), respectively. The total concentration of surface groups, aXOH, can thus be represented by Eq. (3) X [ XOH] [XOH0 ][XOH (3) 2 ][XO ] Depending on the equilibrium pH of the solution, the surface sites of the sorbent can exist in protonated, XOH2, or deprotonated, XO , or neutral form XOH0. The pH at which [XOH2]/ [XO ] is called the point of zero charge (pHpzc). The net charge, Q in mol kg1, on the surface of MCM-41 at any point of acid /base titration curve can be obtained from the titration data. Eq. (4) describes the course of titration relating the surface charge and experimentally measurable parameters and Eq. (5) describes the back titration of completely deprotonated form of the sorbent by acid alone. Q [XOH 2 ][XO ]
(V =m)(Ca Cb [OH ][H ]) (4) X Sa [ XOH] X XOH] Cfb (V =m)([H ][OH ])[ [XOH 2 ][XO ]
(5)
where Ca and Cb is the consumption of acid (HNO3) and base (NaOH), respectively, in mol dm 3; [H] and [OH ] are the bulk concentra-
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tion of hydrogen and hydroxyl ion, respectively, in mol dm 3; V is the volume of liquid phase (dm3); m is the mass of MCM-41 (kg) taken for titration; Sa is the consumption of the acid (HNO3) during the back titration expressed as amol HNO3 per amol of aXOH (Sa /Vca/(m [aXOH]); [aXOH] is the total concentration of active sites on the MCM-41, in mol kg 1; Cfb is the initial concentration of the free base (NaOH) related to the mass of MCM-41, in mol kg1; [XOH0] is the concentration of active sites having the charge equal to zero, in mol kg1. In SCMs, the sorption phenomenon was viewed to proceed with the formation of surface complex, or complexes, as described earlier, between the surface groups of MCM-41 and various species of uranium present in the experimental solution [15,16]. The relative abundance of various uranium species is a strong function of pH and composition of solution, especially carbonates concentration. Table 1 lists aqueous speciation reaction of uranium along with the stability constants, log bi, at zero ionic strength. In the absence of carbonates, complexation of uranium and hydroxyl ion is the likely reaction and in the presence of carbonates, reaction of uranyl ion with CO2 is the most dominant reaction in aqueous 3 medium. These species compete with each other for the formation of a surface complex with the sorbent and are represented in Table 2 as Eqs. (6) / (15). The reaction shown in Eqs. (14) and (15) are of less significance at pH /6 since the pHpzc of MCM-41 is 3.5 /4, as it can be seen later, and hence the sorption probability of HCO3 and CO2 on the sorbent is very small. 3 Table 1 Aqueous speciation of UO2 and stability constants [15,16], bi 2 Aqueous reactions
log bi I/0
UO2 2 /OH /UO2OH 2 2UO2 / 2OH / (UO ) 2 2 2(OH)2 3UO2 2 /5OH /(UO2)3(OH)5 / 7OH / (UO ) (OH) 3UO2 2 2 3 7 2 UO2 2 /CO3 /UO2CO3 2 2 UO2 2 /2CO3 /UO2(CO3)2 2 2 UO2 /3CO3 /UO2(CO3)4 3
8.79 22.36 54.42 69.66 9.62 16.99 21.62
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Table 2 The probable surface reaction of uranyl ion with MCM-41 Surface reactions
Surface complexation constants
UO2 2 XO XOUO2 UO2 OH XO XOUO2 OH0 (UO2 )2 (OH)2 2 XO XO(UO2 )2 (OH) 0 (UO2 )3 (OH) XO XO(UO ) (OH) 2 3 5 5 2 (UO2 )3 (OH) 7 XO XO(UO2 )3 (OH)7
(6) (7) (8) (9) (10)
UO2 CO03 XO XOUO2 CO 3 3 UO2 (CO3 )2 2 XO XOUO2 (CO3 )2 4 UO2 (CO3 )3 XO XOUO2 (CO3 )5 3
(11) K1c (12) K2c (13) K3c
0 HCO 3 XOH2 XOH2 HCO3 CO2 XOH XOH CO 2 3 2 3
(14) K4c (15) K5c
K1 K2 K3 K4 K5
The equilibrium constants for the Eqs. (1)/(15) are related to the double layer potential, C, by Boltzmann equation. Thus, the equilibrium constant for the complex formation reaction, for e.g. Eq. (6) can be related to the C by Eq. (16) and similar dependencies also exist for other reaction. [XOUO zi F C 2 ] K1 (16) exp [XO ][UO2 RT 2 ] where K1 is the equilibrium constant for the Eq. (6) at a given ionic strength; C is the electrostatic double layer potential, in volts; [XOUO2] and [XO ] are the concentration of UO2 2 -surface complex and deprotonated form present on MCM-41, respectively, both in mol kg 1; [UO2 2 ] is the bulk uranyl concentration, in mol dm 3; F is the Faraday constant, R is the gas constant, T is the absolute temperature and zi (/ 2) is the charge of i-th component (uranyl species). The advantage of SCMs over other models can be directly seen by the incorporation of the concentration terms of various species existing in solution, for e.g. UO2 in Eq. (16) and especially by 2 the electrochemical potential term that are related to the equilibrium constant Ki. Thus, the wide variation in the activity of different ionic species and pH were taken care during modeling. The relation between surface charge density, sa in C m2, developed on MCM-41 while U(VI) species sorption, in the absence of carbonates, is
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given by Eq. (17). Eq. (18) represents surface charge density, sb, created in the presence of carbonates. In SCMs [10,14] the interdependence of surface charge density (sCCM, sDLM) and surface potential (C) is decided by the type of the assumed model. If CCM is assumed Eq. (19) is valid and if DLM is assumed Eq. (20) holds good. The relation between the surface and bulk concentration of the i-th component is related to surface potential by Eq. (21). For non-electrostatic model, CEM, surface potential is zero (see Eq. (22)). sa ([XOH 2 ][XOUO2 ][XO(UO2 )2 (OH)2 ])
X [ XOH]a [XOH0 ][XOH 2 ][XO ][XOUO2 ]
[XOUO2 OH0 ][XO(UO2 )2 (OH) 2 ] [XO(UO2 )3 (OH)05 ][XO(UO2 )3 (OH)2 7 ] (23)
X [ XOH]b
[XOH0 ][XOH 2 ][XO ][XOUO2 CO3 ] 3 [XOUO2 (CO3 )3 2 ][XOUO2 (CO3 )2 X [ U(VI)]a
(24)
2 V ([UO2 2 ][UO2 OH ]2[(UO2 )2 (OH)2 ]
([XO ]2[XO(UO2 )3 (OH)2 7 ])(F=SP)
3[(UO2 )3 (OH) 5 ]3[(UO2 )3 (OH)7 ])
(17)
0 2[XO(UO2 )2 (OH) 2 ]3[XO(UO2 )3 (OH)5 ]
sb ([XOH 2 ]) ([XO ]3[XOUO2 (CO3 )3 2 ] 5[XOUO2 (CO3 )5 3 ])(F=SP) sCCM GC (in the case of CCM) sDLM 0:1174I
0 m([XOUO 2 ][XOUO2 OH ]
1=2
sin h(zi CF =(2RT))
(in the case of DLM)
(18) (19) (20)
Cis Ci exp(zi F C=(RT))
(21)
C0
(22)
(in the case of CEM)
where SP is the specific surface area of MCM-41, in m2 kg1, [ ] are concentrations of given components, in mol kg 1; G is Helmholtz capacitance, in F m 2, I is the ionic strength; Ci is the bulk concentration of the i-th component, in mol dm 3; Cis is surface concentration of i-th component, in mol dm 3. The total concentration of sorption sites ([aXOH]a and [aXOH]b), uranium ([aU(VI)]a and [aU(VI)]b) and carbonates ([aCO3]b) used in the mass balance calculations are given by the Eqs. (23) /(27) where the subscript ‘a’ and ‘b’ represents the sorption of uranium from the solution in the absence of carbonates and with carbonates, respectively.
3[XO(UO2 )3 (OH)2 7 ]) X [ U(VI)]b
(25)
V ([UO3 CO3 ][UO2 (CO3 )2 2 ] [UO2 (CO3 )4 3 ]) 3 m([XOUO2 CO 3 ][XOUO2 (CO3 )2 ]
[XOUO2 (CO3 )3 2 ]) X [ CO3 ]b V ([UO3 CO3 ]2[UO2 (CO3 )2 2 ]
(26)
3[UO2 (CO3 )4 3 ][H2 CO3 ] 2 [HCO 3 ][CO3 ])
m([XOUO2 CO 3 ] 2[XOUO2 (CO3 )3 2 ] 3[XOUO2 (CO3 )32
(27)
Using the CCM, DLM and CEM models based on the charge and mass balance Eqs. (1) /(27) and Davies equation for calculating the activity coefficients, the sorption and acid /base titration data were fitted by Newton/Raphson’s multidimensional non-linear regression method, using a computer code developed by Sˇtamberg et al. [14]. The optimum values of [aXOH], G , Cfb, equilibrium and surface acidity constants were obtained. All parameters determined in the course of non-
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linear regression procedure are model dependent, because the individual models differ in the form of equation by means of which the surface potential, C*/see Eq. (19) for CCM, Eq. (20) for DLM and Eq. (22) for CEM */and consecutively the surface concentrations, Cis */see Eq. (21) */are calculated. The knowledge of these parameters can enable the simulation and prediction of protonation of sorption process which differs from the present conditions of our study. The goodness-of-fit was evaluated by x2-test [17] which is based on calculating the quantity x2 and the probable quantity Qchi-test. The constant standard deviation of each experimental data point needed to the calculation of x2 was assumed to be equal to 10%. Hence, when Qchi-test /10-3 to 10-4, then there is a good agreement between experimental and calculated data, however, if Qchi-test B/ 10-10, the fitting is regarded as unsatisfactory.
153
3.3. pH titration Nearly 0.25 g of MCM-41 with surface area 711 m2 g1 measured by BET method, was preequilibrated with 100 ml of 0.01 M sodium nitrate solution at 303 K for 3 hours. Nitrogen gas was bubbled through this mixture which was under gentle stirring. The mixture was titrated with dilute nitric acid or sodium hydroxide (0.01 M). The pH of the bulk was measured after 30 min after the addition of /0.5 ml of the titrant. The addition of nitric acid or sodium hydroxide was performed until the pH of the bulk solution was 2.5 while nitric acid addition and pH equals to 9.5 while sodium hydroxide addition added. The results are summarized in Table 3.
Table 3 Acid/base titration data for MCM-41
3. Materials and methods
Primary experimental data Volume of 0.01 M NaOH (ml)
pH
Modified experimental data Volume of 0.01 M HNO3 (ml)
3.1. Materials
18.16 14.94 13.06 8.20 6.44 4.99 3.47 2.42 1.69 0.97 0.47 0.00
9.46 9.43 9.38 9.17 9.12 8.92 8.35 8.23 8.08 7.90 7.69 7.08
0.00 3.22 5.08 9.96 11.72 13.17 14.69 15.74 16.47 17.19 17.69 18.16
Volume of 0.01 M HNO3 (ml) 0.00 0.28 0.53 1.28 2.31 4.24 8.14 13.14 16.95 23.40 26.04
/
/
7.08 4.90 4.64 4.10 3.79 3.44 2.96 2.79 2.66 2.54 2.50
/ 18.44 18.69 19.44 20.47 22.40 26.30 31.30 35.11 41.56 44.20
All the reagents used in the experiment were of analytical grade. Tetraethoxysilane (TEOS) and N -Cetyl-N,N,N -trimethylammonium Bromide (CTAB) were procured, respectively, from Fluka and Loba-Chemie.
3.2. Preparation of MCM-41 MCM-41 was prepared by the procedure reported by Cai et al. [18]. It involves mixing of 205 ml of ammonium hydroxide (25 wt% solution) and 270 ml of distilled water containing 2 g of CTAB. When the solution becomes homogeneous, 10 ml of TEOS was added to it. The whole mixture was stirred for 2 hours. The resulting white product was filtered, washed with distilled water and dried at ambient temperature. Then it was calcined at 823 K for 6 hours to remove CTAB template from the structure.
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3.4. Uranium sorption studies Sorption of uranium as a function of pH in the absence of carbonates was studied by equilibrating, at 301 K, 0.05 g of the sorbent with 10 ml of the solution containing 8.4 /105 M uranium added in the form of uranyl nitrate and 0.01 M sodium nitrate. The pH of the solution was adjusted using diluted nitric acid or sodium hydroxide. The solution was made up with CO2 free deionized water purged with nitrogen and sealed tightly while equilibration. After 6 hours of equilibration the initial and equilibrium concentration of uranium was determined by spectrophotometric method using arzenazo-III as colouring agent. The results are shown in Table 4. Similar experiment was repeated in the presence of 0.1 M sodium nitrate instead of 0.01 M used earlier. Effect of carbonates ion on the uranium was studied at 301 K by equilibrating 0.05 g of the sorbent with 10 ml of the solution containing 8.4 /105 M uranium added in the form of uranyl nitrate and 0.005 M sodium carbonates (I/0.015). The pH of the solution was adjusted with sodium hydroxide from pH 6 to 11. The uranium concentration was determined as described above and the pH was measured after equilibration. The results are shown in Table 4. Langmuir adsorption isotherm was obtained by equilibrating, at 301 K, 0.05 g of the sorbent with
10 ml of the solution containing 0.2 M sodium acetate and varied concentrations of uranium ion, which was added in the form of uranyl nitrate. The concentration of uranium in the solution, having pH approximately 8, was varied from 6.3 /104 to 1.3 /103 M. The uranium concentration in the aqueous solution was estimated using the spectrophotometric method mentioned above. 3.5. Kinetics of sorption To obtain the time required for the sorption of uranium on MCM-41, 0.025 g of the sorbent was equilibrated with 10 ml of the solution containing 4.2 /10 4 M uranium added in the form of uranyl nitrate, at 301 K. The pH of the solution was adjusted to 7 by sodium acetate and acetic acid buffer. The equilibration was stopped at various pre-fixed time intervals and the uranium concentration in the aqueous phase was determined spectrophotometrically as described above.
4. Results and discussion 4.1. Evaluation of structure of MCM-41 and its kinetic properties Fig. 1 shows the powder diffraction pattern of MCM-41 and it matches well with the highly
Table 4 Sorption of U(VI) on MCM-41 from aqueous solution Solution without carbonates (I/0.01)
Solution with out carbonates (I/0.10)
Solution with carbonates (I/0.015)
pH
U(VI) [% sorbed]
pH
U(VI) [% sorbed]
pH
U(VI) [% sorbed]
1.85 2.77 3.33 3.88 4.39 5.00 5.68 6.00 6.50 7.52 8.87
1.01 8.47 37.46 67.56 72.34 81.40 90.46 95.50 98.68 98.63 99.71
1.92 2.50 3.54 3.92 4.50 5.26 5.74 6.21 6.89 7.65 8.95
1.0 1.01 35.62 60.54 73.21 82.12 91.23 95.03 98.99 99.51 99.45
6.0 6.50 7.51 9.06 9.82 11.17 / / / / /
94.09 90.96 88.17 85.44 81.89 74.76 / / / / /
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Fig. 1. XRD pattern of MCM-41.
ordered mesoporous silica as reported by Cai et al. [1,18]. An intense peak at 2u /2.38 can be indexed to d(1 0 0) plane of the hexagonal unit cell. The interplanar spacing (d), unit cell dimension (a/ 2d1 0 0/(3)1/2) and the BET surface area of MCM˚ , 44 A ˚ and 711 m2 g1, 41 were found to be 38 A respectively. The time dependence on the sorption of uranium from pH 7 solution by MCM-41 is shown in Fig. 2. It can be seen that the rate of sorption is very rapid in the initial stages. Nearly
70% of uranium is sorbed in 2 min and entire amount of uranium was sorbed with in 30 min. Similar observations were also reported by Shin et al. [8] for the sorption of uranium by mesoporous silica and compared with that of microporous silica. This rapid sorption of uranium on MCM-41 could be attributed the wide porosity and large surface area of mesoporous sorbents facilitates the accessibility of the sorbate towards the sorption sites.
Fig. 2. Variation of percentage sorption of uranium with time (min).
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4.2. Sorption equilibrium isotherm */Langmuir plot Fig. 3 shows the linearized Langmuir plot for the sorption of uranium on MCM-41 from solution having pH 8.3. The classical Langmuir equation in linearized form relating the amount of uranium in solution and solid phase is shown in Eq. (28): Cf =Cs (1=Kb)(Cf =b)
(28)
This equation can be re-written into the original form, namely as Eq. (29): Cs KbCf =(1KCf )
(29)
And it can be easily derived from Eq. (29) that b is the limit of Cs as Cf tends to infinity. Here Cf (in mg l 1) and Cs (in mg g1) are the concentrations of uranium, respectively present in the solution and solid phase at equilibrium. K (in l g1) is the Langmuir constant and b (in mg g1) is the atmost attainable uranium binding capacity under the given conditions. The Langmuir constant and the capacity can be obtained from the linear fitting of the experimental points shown in Fig. 3, which has resulted in 0.8417 l g1 and 58.4 mg g1 (/ 0.245 mmol g1) for K and b. The magnitude of uranium binding capacity, b, obtained in this
study (at pH 8.3) is much higher than the capacity reported by Shin et al. [8] (0.035 mmol g 1 at pH 5.1). But the value of b is comparable with the total concentration of surface sites, [aXOH], of MCM-41. 4.3. Fitting of acid /base titration data The surface acidity constants (K1p and K2p), total site density of MCM-41 and*/in the case of CCM */the value of Helmholtz capacitance (G ) can be obtained by fitting of acid /base titration data using Eq. (4) or Eq. (5). However, it was found that the goodness of the fit was higher when Eq. (5) was used which describes the titration of completely dissociated form of MCM-41, i.e. anionic form, by acid alone. Similar method was followed for modelling of titration curve of hydrous zirconium oxide [19]. Hence, in the present study also, i.e. for modelling of uranium sorption, the acidity constants were taken from the fitting reverse titration data (see modified experimental data in Table 3). The results of non-linear regression analysis of the acid /base titration data using SCMs are given in Table 5. From the statistical quantities, x2 and Qchi-test, it is evident that the titration data can be described by SCMs
Fig. 3. Linearized Langmuir plot (Eq. (28)) for sorption of uranium on MCM-41, where Y/Cf/Cs (g l 1) (Cs is the concentration of uranium in solid phase, in mg g 1 and Cf in solution, in mg l 1).
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Table 5 Results of the evaluation of the titration curve of MCM-41 (I/0.01) Model K1p9/s CEM CCM DLM
K2p9/s
pHpzc
aXOH9/s (mol kg1) G9/s (F m 2)
5.15E89/9.53E7 1.03E-19/1.44E1 3.82 3.829/2.54E-1 1.21E09/2.26E2 4.0 2.129/7.46E-1 1.03E89/4.73E7 3.71E109/4.72E10 8.99E29/1.83E3 6.72(?) 6.189/7.94
in the order CEM :/CCM /DLM. Thus, DLM is not suitable for describing the titration curve, not only owing to the poor statistical quantities, but also to the high magnitude of pHpzc of mesoporous silica resulted from modelling when compared to the pHpzc of silica. Fig. 4 shows the fitting of the acid /base titration data using CEM and Fig. 5 shows the plot of fractional concentration of various forms of the surface sites calculated as a function of pH. It is necessary to point out the very low magnitude obtained for K2p. This sets the protonated form [XOH2] to exist only in strong acidic solution at pH /1, and that could be attributed to the hydrophobic nature [20,21] of MCM-41 resulting in the negligible protonation of the surface hy-
Cfb9/s (mol kg1) x2
/ 3.459/2.57E-1 4.51E19/7.46E-1 5.199/2.71E-1 / 1.27E-49/7.97
Qchi-test
26.1 0.36 15.0 0.65 57.8 8E-6
droxyl groups when compared to silica. It was reported that MCM-41 type of materials exhibit type V adsorption isotherm [20,21] for water indicating the strong hydrophobic nature. Because of this it differs from commercially available silica that the protonation to some extent is not favourable in mild acid conditions and converted to XOH2 form only below pH 1. Thus, the protonated surface [XOH2] does not take part on the sorption process in the pH range of our interest and the involvement of Kp2 in this range is insignificant. Hence, there are some difficulties encountered in modeling of the titration curve in acidic region that has resulted in high standard deviation of the constant K2p and the uncertainty in the magnitude of pHpzc for CCM and CEM.
Fig. 4. Titration curve of MCM-41: pH as a function of consuption of nitric acid related to the total concentration of surface sites of MCM-41 (mol H mol MCM-411). The fitting of experimental data (circules) using Eq. (5) and CEM.
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Fig. 5. The mole fraction (F ) of various sites on MCM-41 (XOH2 , XOH and XO ) as a function of pH calculated using Eq. (5) and CEM.
Similar deviations and differences in the acidity constants (K2p) were also reported by various workers [22 /24]. 4.4. Modeling of sorption of U(VI) in the absence of carbonates Table 6 displays the statistical quantities, x2 and Qchi-test, for the fitting of uranium sorption data obtained at various pH in the absence of carbonates. It is seen that uranium sorption can be described by the both type of models, i.e. by CEM and CCM. Unfortunately, the goodness-of-fit seen in the values of x2 and Qchi-test are poor and it could be due to the simplistic assumption of few U(VI) species in the solution. In this model, in agreement with Allen et al. [16], we have assumed
the involvement of a five different species responsible for the sorption of uranyl ion on to mesoporous silica as shown in Tables 1 and 2. But in geochemical code MINTEQA2 [25] nearly 10 different kind of species are proposed to exist under similar experimental conditions. Thus, it can be expected that for accurate modelling of the current system it is necessary to formulate greater number of sorption reactions and obtain the model parameters. Since, we have formulated only Eqs. (6) /(10) which are responsible for the sorption of uranium, there can be a deviation of predicted and experimental values and thus the possibility of a given model are to some extent limited. However, a reasonable description of the experimental data with minimum of variables parameters (constants) is given in the present paper using non-linear regression analysis. The magnitudes of K1 /K5 with the corresponding standard deviations are given in Table 6. Fig. 6 shows the fitting of experimental data using CEM model and the corresponding fractional concentration (in % of total uranium sorbed) of various complexes of uranium on the sorbent is shown in Fig. 7. It can be observed from this figure that between the pH range 2 and 4, uranium is essentially sorbed by Eq. (6) and between pH 4 and 7, UO2OH forms 1:1 complex with the sorbent. Above pH 7, the uranium primarily exits on the solid phase. The low as XO(UO2)3(OH)2 7 magnitude of constant K3 shown in Table 6 indicates the inability of uranium for the formation of surface complex by the reaction given in Eq. (8) and hence the Eq. (8) can be eliminated while modeling. It follows, from the comparison of speciation of U(VI) as a function of pH in solution [16] with the speciation on the surface of MCM-41 depicted in Fig. 7, that there exists a narrow analogy between the both dependencies. It means
Table 6 Results of the evaluation of U(VI) sorption on MCM-41 from solution (I/0.01) in the absence of carbonates (the values of constants are recalculated for I/0) Model
K19/s
K29/s
K39/s
K49/s
K59/s
x2
Qchi-test
CEM CCM
3.62E69/1.26E6 1.25E69/2.80E5
1.80E69/1.86E6 3.91E59/6.82E5
5.58E29/5.15E6 6.26E-69/2.62E4
1.27E59/7.89E5 9.80E49/2.71E5
2.46E49/1.074 3.53E39/9.54E3
117 102
:/1E-10 :/1E-10
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Fig. 6. Percentage sorption of U(VI) on MCM-41 in the absence of carbonates (I/0.01) as a function of pH: the fitting of experimental data (circles) using CEM.
that the knowledge of speciation in solution play an important role in the construction of sorption models. Using the values of K1p, K2p and K1 /K5 for CEM obtained from the modeling of uranium sorption at I /0.01, the uranium sorption dependence was calculated for I/0.1 and the result is compared with experimental data in Fig. 8; the mole fractions of individual SCs are shown in Fig. 9. The good agreement between the experimental points and the calculated dependence indicates the validity of the fitting constants */it can be regarded as the partial validation of the sorption model CEM.
159
Fig. 7. Sorption (in %) of U(VI) species (surface complexes, SC) on MCM-41 in the absence of carbonates (I/0.01) as a function of pH calculated using CEM.SC1, XOUO2 ; SC2, XOUO2OH0; SC3, XO(UO2)2(OH)2 ; SC4, XO(UO2)3(OH)05; SC5, XO(UO2)3(OH)2 7 .
but standard deviations are significantly lower and the goodness-of-fit is much better than in the
4.5. Modeling of sorption of U(VI) in the presence of carbonates Table 7 gives: (i) the values of equilibrium constants, K1C /K3C, and the corresponding standard deviations, (ii) the statistical quantities, x2 and Qchi-test,, acquired from the fittings of data obtained for the sorption of uranium in the presence of carbonates on MCM-41. It is evident that the order of description by SCMs is the same as in the previous experiment, i.e., CEM :/CCM,
Fig. 8. Percentage sorption of U(VI) on MCM-41 in the absence of carbonates (I/0.1) as a function of pH: the calculation of dependence given by experimental data (circles) using CEM (the validation of CEM).
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Fig. 9. Sorption (in %) of U(VI) species (surface complexes, SC) on MCM-41 in the absence of carbonates (I/0.1) as a function of pH calculated using CEM.SC1, XOUO2 ; SC2, XOUO2OH0; SC3, XO(UO2)2(OH)2 ; SC4, XO(UO2)3(OH)05; SC5, XO(UO2)3(OH)2 7 .
previous case. Eventhough, while modelling we have assumed lesser number of U(VI) species in solution i.e. three species (see Table 1) than it was assumed in geochemical modelling, (four species are present in solution according to code MINTEQA2 [25]), the fit obtained by non-linear regression describes very well the sorption curve than it was encountered in modelling of uranium sorption in the absence of carbonate. Thus, Fig. 10 shows the fitting of the experimental data by CEM
and it is evident that sorption of uranium decreases with increase in pH due to the increase in the concentration of CO2 resulting in the 3 conversion of HCO3 to CO2 and to the lower 3 sorption selectivity according to Eqs. (12) and (13), which is in agreement with the earlier report [23]. The calculated sorption dependencies of individual uranyl-carbonate species are shown in Fig. 11. It is seen that the sorption of uranium results in the form of XOUO2CO3 between the pH range 5 and
Table 7 Results of the evaluation of U(VI) sorption on MCM-41 from solution (I/0.015) in the presence of carbonates (the values of constants are recalculated for I/0) Model
K1C9/s
K2C9/s
K3C9/s
x2
Qchi-test
CEM CCM
1.50E69/1.73E5 5.59E59/9.0E4
1.55E49/1.79E3 7.0E39/1.57E3
1.14E39/2.03E1 6.96E39/1.25E2
9.90 18.0
1.9E-2 4.4E-4
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161
5. Conclusions
Fig. 10. Percentage sorption of U(VI) on MCM-41 in the presence of carbonates (I/0.015) as a function of pH: the fitting of experimental data (circles) using CEM.
Fig. 11. Sorption (in %) of U(VI) species (surface complexes, SC) on MCM-41 in the presence of carbonates (I/0.015) as function of pH calculated using CEM.SC1, XOUO2CO3 ; SC2, 5 XOUO2(CO3)3 2 ; SC3, XOUO2(CO3)3 .
8, XOUO2(CO3)3 from pH 6 to approximately 2 11 (with maximum at pH 8) and from pH 8 begins the sorption of uranium as XOUO2(CO3)5 3 .
Sorption of uranium on MCM-41 was studied as a function of various parameters such as pH, time, concentration of U(VI) and carbonates. Sorption edge in the absence of carbonates was found to occur from pH 2 to 6 and the percentage sorption increased with increase in the pH of the solution. Presence of carbonates in the test solution decreased the sorption of uranium by MCM41 due to the formation of anionic carbonate complexes of uranium and to the lower sorption selectivity of these complexes. Very rapid sorption of uranium was observed in the early stages of equilibration and nearly 30 min were required for complete sorption. Uranium sorption on MCM-41 was found to follow Langmuir adsorption model and the most attainable sorption capacity under the given condition was found to be 58 mg g1, i.e., 0.24 mmol g1 U(VI). The acid /base titration data of MCM-41 and the sorption data of uranium on MCM-41 were modelled using three types of SCMs. The statistical analysis of various models indicated that only CEM or CCM could be used to the description of the amphoteric behaviour and uranium sorption on MCM-41. As for the DLM, the poor results of the fitting of titration curve did not enable to be used for the modelling of sorption dependencies. The comparability of CCM with CEM and the non-applicability of DLM pointed to the small role of surface potential, C, in the surface reactions under given conditions. The point of zero charge (pHpzc) of MCM-41 was found to be approximately 3.9 (CCM, CEM). This value is rather high and is not typical for silica (in the database RS3T [26], the values varies between 2 and 3.5). However, thus difference could be attributed to the unique features MCM-41, such as hydrophobicity of MCM-41, playing a significant role in the protolysis of surface hydroxyl groups of MCM-41 and also responsible for the shift pHpzc for MCM-41. There are number of ways in which the adsorption of a metal ion onto the sorbent can be formulated by the assumption of various metal ion species in solution and these species are proposed to be responsible for the sorption thus
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resulting in various equilibrium parameters. The best description of sorption behaviour can be achieved when more number of species is assumed. However, maximisation in the number of equilibrium reactions led to the difficulties in obtaining numerical solutions. Thus, in the present case for simplicity, we have assumed that only few species of uranium existing in solution are responsible for the sorption and fitted the experimental data with reasonable accuracy.
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Hinggins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, J. Liu, K.M. Kemner, Science 276 (1997) 923. [4] J.M. Kim, J.H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem. 99 (1995) 16742. [5] M.C. Bruzzoniti, E. Mentasti, C. Sarzanini, B. Onida, B. Bonelli, E. Garrone, Anal. Chim. Acta 422 (2000) 231. [6] J.S. Beck, D.C. Calabro, S.B. McCullen, B.P. Pelrine, K.D. Schmitt, J.C. Vartuli, US Patent 5 220 101 (1993). [7] M. Grun, A.A. Kurganov, S. Schacht, K.K. Unger, J. Chromatogr. A 740 (1996) 1. [8] Y.S. Shin, M.C. Burleigh, S. Dai, C.E. Barnes, Z.L. Xue, Radiochim. Acta 84 (1999) 37. [9] K. Vidya, S.E. Dapurkar, P. Selvam, S.K. Badamali, N.M. Gupta, Micropor. Mesopor. Mat. 50 (2001) 173. [10] D.A. Dzombak, F.M.N. Morel, Surface Complexation Modeling: Hydrous Ferric Oxide, Wiley, New York, 1990. [11] K.F. Hayes, G. Redden, W. Ela, J.O. Lickie, J. Colloid Interf. Sci. 142 (1991) 448.
[12] P. Wang, A. Anderko, D.R. Turner, Ind. Eng. Chem. Res. 40 (2001) 4428. [13] N. Marmier, F. Fromage, J. Colloid Interf. Sci. 212 (1999) 252. [14] K. Sˇtamberg, P. Benesˇ, J. Dolansky´, J. Mizera, D. Vopa´lka, Seventh International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, Lake Tahoe, Navada/California, USA, September 26 /October 1, 1999, p. 82. [15] T.E. Payne, T.D. Waite, Radiochim. Acta 52/53 (1991) 487. [16] H.E. Allen, E.M. Perdue, D.S. Brown (Eds.), Metals in Ground Water, Lewis Publishers, London, 1993, p. 370. [17] H. Press, B.P. Flannery, S.A. Teukolsky, V.H. Vetterling, Numerical Recipes in Pascal (The Art of Scientific Computations), Cambridge University Press, Cambridge, 1989, p. 664. [18] Q. Cai, W.Y. Lin, F.S. Xiao, W.Q. Pang, X.H. Chen, B.S. Zou, Micropor. Mesopor. Mat. 32 (1999) 1. [19] K.A. Venkatesan, P.R. Vasudeva Rao, K. .Sˇtamberg, J. Radioanal. Nucl. Chem. 250 (2001) 477. [20] A. Matsumoto, T. Sasaki, N. Nishimiya, K. Tsutsumi, Colloid. Surf. A 203 (2003) 185. [21] S. Iniagaki, Y. Fukushima, K. Kuroda, K. Kuroda, J. Colloid Interf. Sci. 180 (1996) 623. [22] N. Lanonne-Wall, V. Moulin, J.P. Vilarem, Radiochim. Acta 79 (1997) 37. [23] J.D. Prikryl, J. Alka, D.R. Turnar, R.T. Pabalan, J. Contam. Hydrol. 47 (2000) 241. [24] T. Arnold, T. Zorn, H. Zanker, G. Bernhard, H. Nitsche, J. Contam. Hydrol. 47 (2000) 219. [25] J.D. Allison, D.S. Brown, K.J. Novo-Gradac, MinteqA2/ ProdefaA2, Version 3.0A, Geochemical Assessment Model for Environmental System, Environmental Research Laboratory, US Environmental Protection Agency, Athens, 1991. [26] V. Brendler, A Mineral-Specific Thermodynamic Sorption Database (RESXT), Eighth International Conference on Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, MIGRATION’01, Bregenz, Austria, September 16 /21, 2001, p. 108.
Surface complexation modeling of uranyl ion sorption on mesoporous silica K. Sˇtamberg a,*, K.A. Venkatesan b, P.R. Vasudeva Rao b a
Department of Nuclear Chemistry, Faculty of Nuclear Science and Physical Engineering, Czech Technical University, Brˇehova´ 7, 11519 Prague 1, Czech Republic b Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 10 June 2002; accepted 26 March 2003
Abstract Sorption of uranium on MCM-41 was studied as a function of pH, time, [U(VI)] and [CO2 3 ]. In the absence of carbonate the sorption edge occurs at pH 2 and the percentage sorption increased to 95% when the pH of the solution is 6. Further increase in pH results in complete sorption (99%) of uranium. In the presence of carbonate the percentage sorption decreased from 94 to 75% when the pH was varied from 6 to 11. The acid /base property of MCM-41 was investigated by titration of the sorbent with HNO3/NaOH at constant ionic strength. Three surface complexation models, namely, the constant capacitance model (CCM), diffuse layer model and non-electrostatic chemical equilibrium model (CEM) were employed to simulate the amphoteric behaviour of MCM-41 and uranium sorption on it. Various model parameters used for describing the sorption property of the sorbent were obtained from the non-linear regression of the experimental data. The sensitivity analysis and goodness-of-fit indicated that only CEM and CCM could be used to the description of the experimental data. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous silica; Adsorption; Uranium; Surface complexation; Modeling
1. Introduction With the discovery of mesoporous molecular sieve (MCM-41), by Kresge and Beck et al. [1,2], considerable efforts have gone in the way to develop this new family of material and employ them for the removal and separation of various metal ions from aqueous solutions [3 /6]. The * Corresponding author. Tel.: /420-2-243-58205; fax: / 420-2-223-20864. E-mail address: [email protected] (K. Sˇtamberg).
striking features of MCM-41 such as very large BET surface area, large pore volume and fast kinetics of sorption attracted many researchers to utilise them as sorbent and study its sorption behaviour towards chemical and radio toxic metal ions from various waste streams. Mobil corporation [6] patented a sorption separation process for the purification of water using modified MCM-41. Feng et al. [3] have developed highest capacity thiol functionalized monolayers on mesoporous silica, FMMS, for trapping Hg2 from aqueous solution. Grun et al. [7] investigated the properties
0927-7757/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-7757(03)00139-0
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of MCM-41 as packing material in normal-phase HPLC and compared its retention behaviour and selectivity with those of other oxides. Interestingly they found that MCM-41 has the ability to separate all types of analytes (basic, acidic and neutral) with acceptable retention times and good peak shapes. Bruzzoniti et al. [5] studied the partition and retention properties of various environmental pollutants on mesoporous silica and obtained the correlation between chemical structure of the pollutant and sorbent. Recently Shin et al. [8] showed high capacity and fast kinetics of uranyl ion sorption on mesoporous titanosilicate and compared its advantage over the corresponding microporous material. Vidya et al. [9] studied the ion-exchange of template for trapping UO2 in as-synthesised 2 MCM-41 and MCM-48. High sorption capacity achieved in MCM-48 was attributed to strong binding between UO2 and defective sites in the 2 three dimensional pore system of MCM-48. The aim of the present work is to study the behaviour of uranyl ion sorption on calcined MCM-41 as a function of various parameters such as time, concentration of U(VI) and carbonate. The amphoteric nature of MCM-41 was investigated by acid /base titration of the sorbent and the results are also reported. Three types of surface complexation models (SCMs) namely two electrostatic models, i.e. constant capacitance model (CCM) and diffuse double layer model (DLM) and one chemical equilibrium, non-electrostatic model (CEM), were employed to simulate the amphoteric property of MCM-41 and to describe the sorption of uranium on MCM-41.
2. Model approach SCMs have been widely used [10 /14] for the description of the sorption of metal ions from aqueous solution as a function of pH, ionic strength, solution concentration etc. Well-developed SCMs are available for predicting the sorption behaviour of various metal ions from the complex mixtures. SCMs represent a set adsorption reaction occurring between the metal species existing in solution and the surface of sorbent,
resulting in the formation of a surface complexes (SCs). The concentration of the SCs can be estimated using the corresponding set of quasithermodynamic constants that are obtained by non-linear regression analysis of the experimental data. In order to apply SCMs for describing the sorption of uranium on MCM-41, the surface area, total surface site concentration and the protonation constants are needed and that can be obtained by evaluation of acid /base titration data of MCM-41 and surface area measurements. The protolysis reaction of surface groups (XO and XOH0) of MCM-41 are given by XO H XOH0 0
XOH H
K1p
XOH 2
K2p
(1) (2)
where K1p and K2p are the protonation constants for Eqs. (1) and (2), respectively. The total concentration of surface groups, aXOH, can thus be represented by Eq. (3) X [ XOH] [XOH0 ][XOH (3) 2 ][XO ] Depending on the equilibrium pH of the solution, the surface sites of the sorbent can exist in protonated, XOH2, or deprotonated, XO , or neutral form XOH0. The pH at which [XOH2]/ [XO ] is called the point of zero charge (pHpzc). The net charge, Q in mol kg1, on the surface of MCM-41 at any point of acid /base titration curve can be obtained from the titration data. Eq. (4) describes the course of titration relating the surface charge and experimentally measurable parameters and Eq. (5) describes the back titration of completely deprotonated form of the sorbent by acid alone. Q [XOH 2 ][XO ]
(V =m)(Ca Cb [OH ][H ]) (4) X Sa [ XOH] X XOH] Cfb (V =m)([H ][OH ])[ [XOH 2 ][XO ]
(5)
where Ca and Cb is the consumption of acid (HNO3) and base (NaOH), respectively, in mol dm 3; [H] and [OH ] are the bulk concentra-
K. Sˇtamberg et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 221 (2003) 149 /162
tion of hydrogen and hydroxyl ion, respectively, in mol dm 3; V is the volume of liquid phase (dm3); m is the mass of MCM-41 (kg) taken for titration; Sa is the consumption of the acid (HNO3) during the back titration expressed as amol HNO3 per amol of aXOH (Sa /Vca/(m [aXOH]); [aXOH] is the total concentration of active sites on the MCM-41, in mol kg 1; Cfb is the initial concentration of the free base (NaOH) related to the mass of MCM-41, in mol kg1; [XOH0] is the concentration of active sites having the charge equal to zero, in mol kg1. In SCMs, the sorption phenomenon was viewed to proceed with the formation of surface complex, or complexes, as described earlier, between the surface groups of MCM-41 and various species of uranium present in the experimental solution [15,16]. The relative abundance of various uranium species is a strong function of pH and composition of solution, especially carbonates concentration. Table 1 lists aqueous speciation reaction of uranium along with the stability constants, log bi, at zero ionic strength. In the absence of carbonates, complexation of uranium and hydroxyl ion is the likely reaction and in the presence of carbonates, reaction of uranyl ion with CO2 is the most dominant reaction in aqueous 3 medium. These species compete with each other for the formation of a surface complex with the sorbent and are represented in Table 2 as Eqs. (6) / (15). The reaction shown in Eqs. (14) and (15) are of less significance at pH /6 since the pHpzc of MCM-41 is 3.5 /4, as it can be seen later, and hence the sorption probability of HCO3 and CO2 on the sorbent is very small. 3 Table 1 Aqueous speciation of UO2 and stability constants [15,16], bi 2 Aqueous reactions
log bi I/0
UO2 2 /OH /UO2OH 2 2UO2 / 2OH / (UO ) 2 2 2(OH)2 3UO2 2 /5OH /(UO2)3(OH)5 / 7OH / (UO ) (OH) 3UO2 2 2 3 7 2 UO2 2 /CO3 /UO2CO3 2 2 UO2 2 /2CO3 /UO2(CO3)2 2 2 UO2 /3CO3 /UO2(CO3)4 3
8.79 22.36 54.42 69.66 9.62 16.99 21.62
151
Table 2 The probable surface reaction of uranyl ion with MCM-41 Surface reactions
Surface complexation constants
UO2 2 XO XOUO2 UO2 OH XO XOUO2 OH0 (UO2 )2 (OH)2 2 XO XO(UO2 )2 (OH) 0 (UO2 )3 (OH) XO XO(UO ) (OH) 2 3 5 5 2 (UO2 )3 (OH) 7 XO XO(UO2 )3 (OH)7
(6) (7) (8) (9) (10)
UO2 CO03 XO XOUO2 CO 3 3 UO2 (CO3 )2 2 XO XOUO2 (CO3 )2 4 UO2 (CO3 )3 XO XOUO2 (CO3 )5 3
(11) K1c (12) K2c (13) K3c
0 HCO 3 XOH2 XOH2 HCO3 CO2 XOH XOH CO 2 3 2 3
(14) K4c (15) K5c
K1 K2 K3 K4 K5
The equilibrium constants for the Eqs. (1)/(15) are related to the double layer potential, C, by Boltzmann equation. Thus, the equilibrium constant for the complex formation reaction, for e.g. Eq. (6) can be related to the C by Eq. (16) and similar dependencies also exist for other reaction. [XOUO zi F C 2 ] K1 (16) exp [XO ][UO2 RT 2 ] where K1 is the equilibrium constant for the Eq. (6) at a given ionic strength; C is the electrostatic double layer potential, in volts; [XOUO2] and [XO ] are the concentration of UO2 2 -surface complex and deprotonated form present on MCM-41, respectively, both in mol kg 1; [UO2 2 ] is the bulk uranyl concentration, in mol dm 3; F is the Faraday constant, R is the gas constant, T is the absolute temperature and zi (/ 2) is the charge of i-th component (uranyl species). The advantage of SCMs over other models can be directly seen by the incorporation of the concentration terms of various species existing in solution, for e.g. UO2 in Eq. (16) and especially by 2 the electrochemical potential term that are related to the equilibrium constant Ki. Thus, the wide variation in the activity of different ionic species and pH were taken care during modeling. The relation between surface charge density, sa in C m2, developed on MCM-41 while U(VI) species sorption, in the absence of carbonates, is
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152
given by Eq. (17). Eq. (18) represents surface charge density, sb, created in the presence of carbonates. In SCMs [10,14] the interdependence of surface charge density (sCCM, sDLM) and surface potential (C) is decided by the type of the assumed model. If CCM is assumed Eq. (19) is valid and if DLM is assumed Eq. (20) holds good. The relation between the surface and bulk concentration of the i-th component is related to surface potential by Eq. (21). For non-electrostatic model, CEM, surface potential is zero (see Eq. (22)). sa ([XOH 2 ][XOUO2 ][XO(UO2 )2 (OH)2 ])
X [ XOH]a [XOH0 ][XOH 2 ][XO ][XOUO2 ]
[XOUO2 OH0 ][XO(UO2 )2 (OH) 2 ] [XO(UO2 )3 (OH)05 ][XO(UO2 )3 (OH)2 7 ] (23)
X [ XOH]b
[XOH0 ][XOH 2 ][XO ][XOUO2 CO3 ] 3 [XOUO2 (CO3 )3 2 ][XOUO2 (CO3 )2 X [ U(VI)]a
(24)
2 V ([UO2 2 ][UO2 OH ]2[(UO2 )2 (OH)2 ]
([XO ]2[XO(UO2 )3 (OH)2 7 ])(F=SP)
3[(UO2 )3 (OH) 5 ]3[(UO2 )3 (OH)7 ])
(17)
0 2[XO(UO2 )2 (OH) 2 ]3[XO(UO2 )3 (OH)5 ]
sb ([XOH 2 ]) ([XO ]3[XOUO2 (CO3 )3 2 ] 5[XOUO2 (CO3 )5 3 ])(F=SP) sCCM GC (in the case of CCM) sDLM 0:1174I
0 m([XOUO 2 ][XOUO2 OH ]
1=2
sin h(zi CF =(2RT))
(in the case of DLM)
(18) (19) (20)
Cis Ci exp(zi F C=(RT))
(21)
C0
(22)
(in the case of CEM)
where SP is the specific surface area of MCM-41, in m2 kg1, [ ] are concentrations of given components, in mol kg 1; G is Helmholtz capacitance, in F m 2, I is the ionic strength; Ci is the bulk concentration of the i-th component, in mol dm 3; Cis is surface concentration of i-th component, in mol dm 3. The total concentration of sorption sites ([aXOH]a and [aXOH]b), uranium ([aU(VI)]a and [aU(VI)]b) and carbonates ([aCO3]b) used in the mass balance calculations are given by the Eqs. (23) /(27) where the subscript ‘a’ and ‘b’ represents the sorption of uranium from the solution in the absence of carbonates and with carbonates, respectively.
3[XO(UO2 )3 (OH)2 7 ]) X [ U(VI)]b
(25)
V ([UO3 CO3 ][UO2 (CO3 )2 2 ] [UO2 (CO3 )4 3 ]) 3 m([XOUO2 CO 3 ][XOUO2 (CO3 )2 ]
[XOUO2 (CO3 )3 2 ]) X [ CO3 ]b V ([UO3 CO3 ]2[UO2 (CO3 )2 2 ]
(26)
3[UO2 (CO3 )4 3 ][H2 CO3 ] 2 [HCO 3 ][CO3 ])
m([XOUO2 CO 3 ] 2[XOUO2 (CO3 )3 2 ] 3[XOUO2 (CO3 )32
(27)
Using the CCM, DLM and CEM models based on the charge and mass balance Eqs. (1) /(27) and Davies equation for calculating the activity coefficients, the sorption and acid /base titration data were fitted by Newton/Raphson’s multidimensional non-linear regression method, using a computer code developed by Sˇtamberg et al. [14]. The optimum values of [aXOH], G , Cfb, equilibrium and surface acidity constants were obtained. All parameters determined in the course of non-
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linear regression procedure are model dependent, because the individual models differ in the form of equation by means of which the surface potential, C*/see Eq. (19) for CCM, Eq. (20) for DLM and Eq. (22) for CEM */and consecutively the surface concentrations, Cis */see Eq. (21) */are calculated. The knowledge of these parameters can enable the simulation and prediction of protonation of sorption process which differs from the present conditions of our study. The goodness-of-fit was evaluated by x2-test [17] which is based on calculating the quantity x2 and the probable quantity Qchi-test. The constant standard deviation of each experimental data point needed to the calculation of x2 was assumed to be equal to 10%. Hence, when Qchi-test /10-3 to 10-4, then there is a good agreement between experimental and calculated data, however, if Qchi-test B/ 10-10, the fitting is regarded as unsatisfactory.
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3.3. pH titration Nearly 0.25 g of MCM-41 with surface area 711 m2 g1 measured by BET method, was preequilibrated with 100 ml of 0.01 M sodium nitrate solution at 303 K for 3 hours. Nitrogen gas was bubbled through this mixture which was under gentle stirring. The mixture was titrated with dilute nitric acid or sodium hydroxide (0.01 M). The pH of the bulk was measured after 30 min after the addition of /0.5 ml of the titrant. The addition of nitric acid or sodium hydroxide was performed until the pH of the bulk solution was 2.5 while nitric acid addition and pH equals to 9.5 while sodium hydroxide addition added. The results are summarized in Table 3.
Table 3 Acid/base titration data for MCM-41
3. Materials and methods
Primary experimental data Volume of 0.01 M NaOH (ml)
pH
Modified experimental data Volume of 0.01 M HNO3 (ml)
3.1. Materials
18.16 14.94 13.06 8.20 6.44 4.99 3.47 2.42 1.69 0.97 0.47 0.00
9.46 9.43 9.38 9.17 9.12 8.92 8.35 8.23 8.08 7.90 7.69 7.08
0.00 3.22 5.08 9.96 11.72 13.17 14.69 15.74 16.47 17.19 17.69 18.16
Volume of 0.01 M HNO3 (ml) 0.00 0.28 0.53 1.28 2.31 4.24 8.14 13.14 16.95 23.40 26.04
/
/
7.08 4.90 4.64 4.10 3.79 3.44 2.96 2.79 2.66 2.54 2.50
/ 18.44 18.69 19.44 20.47 22.40 26.30 31.30 35.11 41.56 44.20
All the reagents used in the experiment were of analytical grade. Tetraethoxysilane (TEOS) and N -Cetyl-N,N,N -trimethylammonium Bromide (CTAB) were procured, respectively, from Fluka and Loba-Chemie.
3.2. Preparation of MCM-41 MCM-41 was prepared by the procedure reported by Cai et al. [18]. It involves mixing of 205 ml of ammonium hydroxide (25 wt% solution) and 270 ml of distilled water containing 2 g of CTAB. When the solution becomes homogeneous, 10 ml of TEOS was added to it. The whole mixture was stirred for 2 hours. The resulting white product was filtered, washed with distilled water and dried at ambient temperature. Then it was calcined at 823 K for 6 hours to remove CTAB template from the structure.
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3.4. Uranium sorption studies Sorption of uranium as a function of pH in the absence of carbonates was studied by equilibrating, at 301 K, 0.05 g of the sorbent with 10 ml of the solution containing 8.4 /105 M uranium added in the form of uranyl nitrate and 0.01 M sodium nitrate. The pH of the solution was adjusted using diluted nitric acid or sodium hydroxide. The solution was made up with CO2 free deionized water purged with nitrogen and sealed tightly while equilibration. After 6 hours of equilibration the initial and equilibrium concentration of uranium was determined by spectrophotometric method using arzenazo-III as colouring agent. The results are shown in Table 4. Similar experiment was repeated in the presence of 0.1 M sodium nitrate instead of 0.01 M used earlier. Effect of carbonates ion on the uranium was studied at 301 K by equilibrating 0.05 g of the sorbent with 10 ml of the solution containing 8.4 /105 M uranium added in the form of uranyl nitrate and 0.005 M sodium carbonates (I/0.015). The pH of the solution was adjusted with sodium hydroxide from pH 6 to 11. The uranium concentration was determined as described above and the pH was measured after equilibration. The results are shown in Table 4. Langmuir adsorption isotherm was obtained by equilibrating, at 301 K, 0.05 g of the sorbent with
10 ml of the solution containing 0.2 M sodium acetate and varied concentrations of uranium ion, which was added in the form of uranyl nitrate. The concentration of uranium in the solution, having pH approximately 8, was varied from 6.3 /104 to 1.3 /103 M. The uranium concentration in the aqueous solution was estimated using the spectrophotometric method mentioned above. 3.5. Kinetics of sorption To obtain the time required for the sorption of uranium on MCM-41, 0.025 g of the sorbent was equilibrated with 10 ml of the solution containing 4.2 /10 4 M uranium added in the form of uranyl nitrate, at 301 K. The pH of the solution was adjusted to 7 by sodium acetate and acetic acid buffer. The equilibration was stopped at various pre-fixed time intervals and the uranium concentration in the aqueous phase was determined spectrophotometrically as described above.
4. Results and discussion 4.1. Evaluation of structure of MCM-41 and its kinetic properties Fig. 1 shows the powder diffraction pattern of MCM-41 and it matches well with the highly
Table 4 Sorption of U(VI) on MCM-41 from aqueous solution Solution without carbonates (I/0.01)
Solution with out carbonates (I/0.10)
Solution with carbonates (I/0.015)
pH
U(VI) [% sorbed]
pH
U(VI) [% sorbed]
pH
U(VI) [% sorbed]
1.85 2.77 3.33 3.88 4.39 5.00 5.68 6.00 6.50 7.52 8.87
1.01 8.47 37.46 67.56 72.34 81.40 90.46 95.50 98.68 98.63 99.71
1.92 2.50 3.54 3.92 4.50 5.26 5.74 6.21 6.89 7.65 8.95
1.0 1.01 35.62 60.54 73.21 82.12 91.23 95.03 98.99 99.51 99.45
6.0 6.50 7.51 9.06 9.82 11.17 / / / / /
94.09 90.96 88.17 85.44 81.89 74.76 / / / / /
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Fig. 1. XRD pattern of MCM-41.
ordered mesoporous silica as reported by Cai et al. [1,18]. An intense peak at 2u /2.38 can be indexed to d(1 0 0) plane of the hexagonal unit cell. The interplanar spacing (d), unit cell dimension (a/ 2d1 0 0/(3)1/2) and the BET surface area of MCM˚ , 44 A ˚ and 711 m2 g1, 41 were found to be 38 A respectively. The time dependence on the sorption of uranium from pH 7 solution by MCM-41 is shown in Fig. 2. It can be seen that the rate of sorption is very rapid in the initial stages. Nearly
70% of uranium is sorbed in 2 min and entire amount of uranium was sorbed with in 30 min. Similar observations were also reported by Shin et al. [8] for the sorption of uranium by mesoporous silica and compared with that of microporous silica. This rapid sorption of uranium on MCM-41 could be attributed the wide porosity and large surface area of mesoporous sorbents facilitates the accessibility of the sorbate towards the sorption sites.
Fig. 2. Variation of percentage sorption of uranium with time (min).
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4.2. Sorption equilibrium isotherm */Langmuir plot Fig. 3 shows the linearized Langmuir plot for the sorption of uranium on MCM-41 from solution having pH 8.3. The classical Langmuir equation in linearized form relating the amount of uranium in solution and solid phase is shown in Eq. (28): Cf =Cs (1=Kb)(Cf =b)
(28)
This equation can be re-written into the original form, namely as Eq. (29): Cs KbCf =(1KCf )
(29)
And it can be easily derived from Eq. (29) that b is the limit of Cs as Cf tends to infinity. Here Cf (in mg l 1) and Cs (in mg g1) are the concentrations of uranium, respectively present in the solution and solid phase at equilibrium. K (in l g1) is the Langmuir constant and b (in mg g1) is the atmost attainable uranium binding capacity under the given conditions. The Langmuir constant and the capacity can be obtained from the linear fitting of the experimental points shown in Fig. 3, which has resulted in 0.8417 l g1 and 58.4 mg g1 (/ 0.245 mmol g1) for K and b. The magnitude of uranium binding capacity, b, obtained in this
study (at pH 8.3) is much higher than the capacity reported by Shin et al. [8] (0.035 mmol g 1 at pH 5.1). But the value of b is comparable with the total concentration of surface sites, [aXOH], of MCM-41. 4.3. Fitting of acid /base titration data The surface acidity constants (K1p and K2p), total site density of MCM-41 and*/in the case of CCM */the value of Helmholtz capacitance (G ) can be obtained by fitting of acid /base titration data using Eq. (4) or Eq. (5). However, it was found that the goodness of the fit was higher when Eq. (5) was used which describes the titration of completely dissociated form of MCM-41, i.e. anionic form, by acid alone. Similar method was followed for modelling of titration curve of hydrous zirconium oxide [19]. Hence, in the present study also, i.e. for modelling of uranium sorption, the acidity constants were taken from the fitting reverse titration data (see modified experimental data in Table 3). The results of non-linear regression analysis of the acid /base titration data using SCMs are given in Table 5. From the statistical quantities, x2 and Qchi-test, it is evident that the titration data can be described by SCMs
Fig. 3. Linearized Langmuir plot (Eq. (28)) for sorption of uranium on MCM-41, where Y/Cf/Cs (g l 1) (Cs is the concentration of uranium in solid phase, in mg g 1 and Cf in solution, in mg l 1).
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Table 5 Results of the evaluation of the titration curve of MCM-41 (I/0.01) Model K1p9/s CEM CCM DLM
K2p9/s
pHpzc
aXOH9/s (mol kg1) G9/s (F m 2)
5.15E89/9.53E7 1.03E-19/1.44E1 3.82 3.829/2.54E-1 1.21E09/2.26E2 4.0 2.129/7.46E-1 1.03E89/4.73E7 3.71E109/4.72E10 8.99E29/1.83E3 6.72(?) 6.189/7.94
in the order CEM :/CCM /DLM. Thus, DLM is not suitable for describing the titration curve, not only owing to the poor statistical quantities, but also to the high magnitude of pHpzc of mesoporous silica resulted from modelling when compared to the pHpzc of silica. Fig. 4 shows the fitting of the acid /base titration data using CEM and Fig. 5 shows the plot of fractional concentration of various forms of the surface sites calculated as a function of pH. It is necessary to point out the very low magnitude obtained for K2p. This sets the protonated form [XOH2] to exist only in strong acidic solution at pH /1, and that could be attributed to the hydrophobic nature [20,21] of MCM-41 resulting in the negligible protonation of the surface hy-
Cfb9/s (mol kg1) x2
/ 3.459/2.57E-1 4.51E19/7.46E-1 5.199/2.71E-1 / 1.27E-49/7.97
Qchi-test
26.1 0.36 15.0 0.65 57.8 8E-6
droxyl groups when compared to silica. It was reported that MCM-41 type of materials exhibit type V adsorption isotherm [20,21] for water indicating the strong hydrophobic nature. Because of this it differs from commercially available silica that the protonation to some extent is not favourable in mild acid conditions and converted to XOH2 form only below pH 1. Thus, the protonated surface [XOH2] does not take part on the sorption process in the pH range of our interest and the involvement of Kp2 in this range is insignificant. Hence, there are some difficulties encountered in modeling of the titration curve in acidic region that has resulted in high standard deviation of the constant K2p and the uncertainty in the magnitude of pHpzc for CCM and CEM.
Fig. 4. Titration curve of MCM-41: pH as a function of consuption of nitric acid related to the total concentration of surface sites of MCM-41 (mol H mol MCM-411). The fitting of experimental data (circules) using Eq. (5) and CEM.
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158
Fig. 5. The mole fraction (F ) of various sites on MCM-41 (XOH2 , XOH and XO ) as a function of pH calculated using Eq. (5) and CEM.
Similar deviations and differences in the acidity constants (K2p) were also reported by various workers [22 /24]. 4.4. Modeling of sorption of U(VI) in the absence of carbonates Table 6 displays the statistical quantities, x2 and Qchi-test, for the fitting of uranium sorption data obtained at various pH in the absence of carbonates. It is seen that uranium sorption can be described by the both type of models, i.e. by CEM and CCM. Unfortunately, the goodness-of-fit seen in the values of x2 and Qchi-test are poor and it could be due to the simplistic assumption of few U(VI) species in the solution. In this model, in agreement with Allen et al. [16], we have assumed
the involvement of a five different species responsible for the sorption of uranyl ion on to mesoporous silica as shown in Tables 1 and 2. But in geochemical code MINTEQA2 [25] nearly 10 different kind of species are proposed to exist under similar experimental conditions. Thus, it can be expected that for accurate modelling of the current system it is necessary to formulate greater number of sorption reactions and obtain the model parameters. Since, we have formulated only Eqs. (6) /(10) which are responsible for the sorption of uranium, there can be a deviation of predicted and experimental values and thus the possibility of a given model are to some extent limited. However, a reasonable description of the experimental data with minimum of variables parameters (constants) is given in the present paper using non-linear regression analysis. The magnitudes of K1 /K5 with the corresponding standard deviations are given in Table 6. Fig. 6 shows the fitting of experimental data using CEM model and the corresponding fractional concentration (in % of total uranium sorbed) of various complexes of uranium on the sorbent is shown in Fig. 7. It can be observed from this figure that between the pH range 2 and 4, uranium is essentially sorbed by Eq. (6) and between pH 4 and 7, UO2OH forms 1:1 complex with the sorbent. Above pH 7, the uranium primarily exits on the solid phase. The low as XO(UO2)3(OH)2 7 magnitude of constant K3 shown in Table 6 indicates the inability of uranium for the formation of surface complex by the reaction given in Eq. (8) and hence the Eq. (8) can be eliminated while modeling. It follows, from the comparison of speciation of U(VI) as a function of pH in solution [16] with the speciation on the surface of MCM-41 depicted in Fig. 7, that there exists a narrow analogy between the both dependencies. It means
Table 6 Results of the evaluation of U(VI) sorption on MCM-41 from solution (I/0.01) in the absence of carbonates (the values of constants are recalculated for I/0) Model
K19/s
K29/s
K39/s
K49/s
K59/s
x2
Qchi-test
CEM CCM
3.62E69/1.26E6 1.25E69/2.80E5
1.80E69/1.86E6 3.91E59/6.82E5
5.58E29/5.15E6 6.26E-69/2.62E4
1.27E59/7.89E5 9.80E49/2.71E5
2.46E49/1.074 3.53E39/9.54E3
117 102
:/1E-10 :/1E-10
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Fig. 6. Percentage sorption of U(VI) on MCM-41 in the absence of carbonates (I/0.01) as a function of pH: the fitting of experimental data (circles) using CEM.
that the knowledge of speciation in solution play an important role in the construction of sorption models. Using the values of K1p, K2p and K1 /K5 for CEM obtained from the modeling of uranium sorption at I /0.01, the uranium sorption dependence was calculated for I/0.1 and the result is compared with experimental data in Fig. 8; the mole fractions of individual SCs are shown in Fig. 9. The good agreement between the experimental points and the calculated dependence indicates the validity of the fitting constants */it can be regarded as the partial validation of the sorption model CEM.
159
Fig. 7. Sorption (in %) of U(VI) species (surface complexes, SC) on MCM-41 in the absence of carbonates (I/0.01) as a function of pH calculated using CEM.SC1, XOUO2 ; SC2, XOUO2OH0; SC3, XO(UO2)2(OH)2 ; SC4, XO(UO2)3(OH)05; SC5, XO(UO2)3(OH)2 7 .
but standard deviations are significantly lower and the goodness-of-fit is much better than in the
4.5. Modeling of sorption of U(VI) in the presence of carbonates Table 7 gives: (i) the values of equilibrium constants, K1C /K3C, and the corresponding standard deviations, (ii) the statistical quantities, x2 and Qchi-test,, acquired from the fittings of data obtained for the sorption of uranium in the presence of carbonates on MCM-41. It is evident that the order of description by SCMs is the same as in the previous experiment, i.e., CEM :/CCM,
Fig. 8. Percentage sorption of U(VI) on MCM-41 in the absence of carbonates (I/0.1) as a function of pH: the calculation of dependence given by experimental data (circles) using CEM (the validation of CEM).
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Fig. 9. Sorption (in %) of U(VI) species (surface complexes, SC) on MCM-41 in the absence of carbonates (I/0.1) as a function of pH calculated using CEM.SC1, XOUO2 ; SC2, XOUO2OH0; SC3, XO(UO2)2(OH)2 ; SC4, XO(UO2)3(OH)05; SC5, XO(UO2)3(OH)2 7 .
previous case. Eventhough, while modelling we have assumed lesser number of U(VI) species in solution i.e. three species (see Table 1) than it was assumed in geochemical modelling, (four species are present in solution according to code MINTEQA2 [25]), the fit obtained by non-linear regression describes very well the sorption curve than it was encountered in modelling of uranium sorption in the absence of carbonate. Thus, Fig. 10 shows the fitting of the experimental data by CEM
and it is evident that sorption of uranium decreases with increase in pH due to the increase in the concentration of CO2 resulting in the 3 conversion of HCO3 to CO2 and to the lower 3 sorption selectivity according to Eqs. (12) and (13), which is in agreement with the earlier report [23]. The calculated sorption dependencies of individual uranyl-carbonate species are shown in Fig. 11. It is seen that the sorption of uranium results in the form of XOUO2CO3 between the pH range 5 and
Table 7 Results of the evaluation of U(VI) sorption on MCM-41 from solution (I/0.015) in the presence of carbonates (the values of constants are recalculated for I/0) Model
K1C9/s
K2C9/s
K3C9/s
x2
Qchi-test
CEM CCM
1.50E69/1.73E5 5.59E59/9.0E4
1.55E49/1.79E3 7.0E39/1.57E3
1.14E39/2.03E1 6.96E39/1.25E2
9.90 18.0
1.9E-2 4.4E-4
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5. Conclusions
Fig. 10. Percentage sorption of U(VI) on MCM-41 in the presence of carbonates (I/0.015) as a function of pH: the fitting of experimental data (circles) using CEM.
Fig. 11. Sorption (in %) of U(VI) species (surface complexes, SC) on MCM-41 in the presence of carbonates (I/0.015) as function of pH calculated using CEM.SC1, XOUO2CO3 ; SC2, 5 XOUO2(CO3)3 2 ; SC3, XOUO2(CO3)3 .
8, XOUO2(CO3)3 from pH 6 to approximately 2 11 (with maximum at pH 8) and from pH 8 begins the sorption of uranium as XOUO2(CO3)5 3 .
Sorption of uranium on MCM-41 was studied as a function of various parameters such as pH, time, concentration of U(VI) and carbonates. Sorption edge in the absence of carbonates was found to occur from pH 2 to 6 and the percentage sorption increased with increase in the pH of the solution. Presence of carbonates in the test solution decreased the sorption of uranium by MCM41 due to the formation of anionic carbonate complexes of uranium and to the lower sorption selectivity of these complexes. Very rapid sorption of uranium was observed in the early stages of equilibration and nearly 30 min were required for complete sorption. Uranium sorption on MCM-41 was found to follow Langmuir adsorption model and the most attainable sorption capacity under the given condition was found to be 58 mg g1, i.e., 0.24 mmol g1 U(VI). The acid /base titration data of MCM-41 and the sorption data of uranium on MCM-41 were modelled using three types of SCMs. The statistical analysis of various models indicated that only CEM or CCM could be used to the description of the amphoteric behaviour and uranium sorption on MCM-41. As for the DLM, the poor results of the fitting of titration curve did not enable to be used for the modelling of sorption dependencies. The comparability of CCM with CEM and the non-applicability of DLM pointed to the small role of surface potential, C, in the surface reactions under given conditions. The point of zero charge (pHpzc) of MCM-41 was found to be approximately 3.9 (CCM, CEM). This value is rather high and is not typical for silica (in the database RS3T [26], the values varies between 2 and 3.5). However, thus difference could be attributed to the unique features MCM-41, such as hydrophobicity of MCM-41, playing a significant role in the protolysis of surface hydroxyl groups of MCM-41 and also responsible for the shift pHpzc for MCM-41. There are number of ways in which the adsorption of a metal ion onto the sorbent can be formulated by the assumption of various metal ion species in solution and these species are proposed to be responsible for the sorption thus
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resulting in various equilibrium parameters. The best description of sorption behaviour can be achieved when more number of species is assumed. However, maximisation in the number of equilibrium reactions led to the difficulties in obtaining numerical solutions. Thus, in the present case for simplicity, we have assumed that only few species of uranium existing in solution are responsible for the sorption and fitted the experimental data with reasonable accuracy.
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