Adsorption Behavior of Trace Beryllium (II) onto Metal Oxide Nanoparticles Dispersed in Water

Radioactive trace Be produced in cooling water systems for high-energy accelerators is known to be captured by metal-oxide colloidal nanoparticles generated through corrosion of metal components in water. This study is aimed at investigating the adsorption behavior of trace Be onto various oxide nanoparticles (Al2O3, SiO2, TiO2, Fe2O3, CoO, and CuO) dispersed in water at 25 °C in order to clarify the tendency and features of the interaction of Be with metal oxides. From pH dependence of the distribution ratio of Be between the nanoparticle phase and the aqueous solution phase, the surface complexation constants (βs,n) have been determined for the reaction of Be with the hydroxyl groups on the oxide surface (>S−OH), i.e., Be + n >S−OH ⇄ (>S−O)nBe + n H. The n values are generally 1 and 2 and the sequences of the βs,n values are Fe2O3 > TiO2 ≈ Al2O3 > SiO2 for βs,1 and Fe2O3 > TiO2 > SiO2 > Al2O3 >> CoO ≈ CuO for βs,2. The dependences of the the βs,n values on the kind of oxide are explained based on the electronegativity of the metal (or Si) composing the oxide.

In cooling water systems for high-energy particle accelerators, beryllium-7 ( 7 Be) is produced due to nuclear reactions under an intense radiation field.It is important to know the behavior of radioactive and trace 7 Be in the cooling water.In recent studies, a large part of 7 Be was found to be captured by metal-oxide colloidal nanoparticles generated through corrosion of metal components in water (Itoh et al., 1998(Itoh et al., , 1999;;Matsumura et al., 2009;Bessho et al., 2010;Bessho et al., 2013;Matsumura et al., 2014;Bessho et al., 2015a;Bessho, Matsumura, Takahashi, & Masumoto, 2015b).On the other hand, the adsorption behavior of trace Be 2+ onto CuO nanoparticles in water was evaluated and the complexation constants between Be 2+ and the surface hydroxyl groups of CuO were determined (Bessho, Kanaya, Shimada, Katsuta, & Monjushiro, 2014).In order to clarify the tendency and features of the interaction of Be 2+ with metal oxides, it is further necessary to determine and compare the surface complexation constants for various oxides.

Adsorption Experiments of Be 2+
Aqueous solutions containing 1.0 × 10 −7 mol/dm 3 Be 2+ were prepared.The pH of each solution was adjusted with 5 × 10 −3 mol/dm 3 CH 3 COOH -CH 3 COONa buffer (pH 3.7 to 5.3) or 1 × 10 −2 mol/dm 3 MES -NaOH buffer (pH 5.3 to 6.5).Ionic strength was adjusted to 0.01 with NaNO 3 .The buffer solution of Be 2+ (20 cm 3 ) was mixed with nanoparticles of an oxide (0.001 to 0.02 g) in a plastic vial.Equilibration of the system was done by shaking the vial for 48 h with an Eyela SS-8 shaker in an Eyela SB-24 water bath (25 ± 0.2 °C).The nanoparticles were removed from the system by centrifugation at 15000 rpm with a Kubota 3700 centrifuge.The concentration of Be 2+ in the aqueous phase was determined by graphite furnace atomic absorption spectrometry with a Z-5000 polarized Zeeman atomic absorption spectrometer (Hitachi), and the distribution ratio of Be 2+ between the nanoparticle phase and the aqueous solution phase was calculated.The pH of the aqueous phase was also measured with a glass electrode.

Fundamental Properties of Nanoparticles Dispersed in Water
Fundamental physical parameters (under dry conditions) of the oxide nanoparticles used in this study were specified by the manufacturer.The zeta potentials, average particle size, and hydroxyl group concentrations were determined for the nanoparticles in an aqueous suspension state.These data, except for the zeta potentials, are summarized in Table 1.All the oxides dispersed in water are 5 to 10 times greater in the average particle size than those in dry state, showing some aggregation of the nanoparticles in the solution.(1.2 ± 0.2) × 10 −4 (7.2 ± 0.4) × 10 −5 (4.9 ± 0.4) × 10 −5 (1.83 ± 0.02) × 10 −4 3.7 × 10 −5 c a. Manufacturer's data for dry nanoparticles.
b. Obtained for nanoparticles dispersed in water (pH 5.5).
c. Bessho et al., 2014.In Figure 2, the zeta potentials of the five kinds of oxide nanoparticles are shown as a function of pH at a constant ionic strength of 0.01.In all cases, the zeta potential shows a continuous decrease with pH.Under the same pH condition as that of the adsorption experiments (pH 3.7 to 6.5), the zeta potential value differs depending on the kind of oxide: Al 2 O 3 (positive) > CoO and CuO (positive ~ neutral) > TiO 2 (positive ~ neutral ~ negative) > SiO 2 and Fe 2 O 3 (negative).Such a difference in the zeta potential may reflect the amount of the hydroxyl groups dissociated or protonated.However, as seen from the pH titration curves (Figure 1), the degree of dissociation of the hydroxyl groups is small for all the oxides in this pH region.(Bessho et al., 2014).As an overall tendency, the D value is larger for the oxide in which the metal (including Si) has a higher valence.This order is not consistent with that of the hydroxyl group concentration (mol/g) of the oxides, i.e., CoO > SiO 2 ≈ Al 2 O 3 > TiO 2 > Fe 2 O 3 > CuO (Table 1).
The adsorption of Be 2+ onto the oxide nanoparticles was evaluated based on the surface complexation model, in which the surface hydroxyl groups on the oxide deprotonate and form complexes with Be 2+ in water.The complexation equilibrium can be written as follows (Bessho et al., 2014).
The following equation is derived from Equation (4).
The left-hand-side of Equation ( 5) was calculated and plotted versus pH as shown in Figure 4.For Al 2 O 3 , SiO 2 , TiO 2 , and Fe 2 O 3 , the slopes of the plots change between 1 and 2, indicating that n = 1 and 2. Only for CoO, the slope is between 2 and 3; this suggests that n = 2 and 3, which is the same result as for CuO (Bessho et al., 2014).As mentioned above, most of the hydroxyl groups are undissociated in the pH region where the adsorption experiments were conducted.Hence, using the hydroxyl group concentrations ([>S−OH] s ) shown in Table 1, the values of β s,n were obtained by nonlinear least squares fitting according to Equation ( 5).The values are summarized in Table 2.The lines in Figure 4 are the regression curves and fit well with the experimental data.
The sequences of the β s,1 , β s,2 , and ), and CuO > CoO (β s,3 ), respectively.It is expected that the oxide having a positive zeta potential is unfavorable to the complexation with the Be 2+ cation.However, Al 2 O 3 (positively charged) have a much larger β s,2 value than CoO and CuO (nearly neutral).In addition, when the n value is 2, the surface charge does not change in the complexation reaction of Equation ( 2).Therefore, it appears that the zeta potential of the oxide nanoparticles is not a dominant factor determining their reactivity for Be 2+ .
From thermodynamic viewpoint, the complexation reaction of Equation ( 2) can be considered to consist of the following two equilibrium processes.

>S−OH
These equations show the deprotonation of the surface hydroxyl group (Equation ( 6)) and the bonding of the negatively charged deprotonated site to Be 2+ (Equation ( 7)), respectively.The reaction of Equation ( 6) should be facilitated with decreasing charge density of the oxygen atom of >S−OH.If the bond between >S−O − and Be 2+ is governed by an electrostatic interaction, the reaction of Equation ( 7) should also depend on the charge density of the oxygen atom of >S−O − ; in this case, an increase in the oxygen charge density facilitates the reaction of Equation ( 7).Therefore, the reactions of Equations ( 6) and ( 7) are oppositely affected by the charge density of the oxygen atom which should decrease with increasing electronegativity of the metal (or Si) composing the oxide.
Tanaka and Ozaki reported the electronegativity (X i ) of the lattice metal ions of oxides (Tanaka & Ozaki, 1967).The X i is given by the following equation.
where Z and X 0 are the valence and Pauling's electronegativity, respectively.The Xi values calculated by Equation ( 8) were used for evaluating activity of catalyst (Tanaka & Ozaki, 1968;Imelik, Naccache, Coudurier, Taarit, & Vedrine, 1985) and Co 2+ adsorption properties of metal oxides (Tamura, Katayama, & Furuichi, 1997).For the oxides examined in this study, the X i values calculated are as follows: in increasing order of X i , CoO (X i = 9.0), CuO (X i = 9.5), Al 2 O 3 (X i = 10.5),Fe 2 O 3 (X i = 12.6), TiO 2 (X i = 13.5), and SiO 2 (X i = 16.2).In Figure 5, the log β s,n (n = 1, 2, and 3) values are plotted against the X i value.The log β s,n values increase with an increase in X i from CoO to Fe 2 O 3 .This result suggests that the complexation of Be 2+ with the oxide nanoparticles (Equation ( 2)) is governed by the deprotonation of >S−OH  −5.17 a a. Bessho et al., 2014.(Equation ( 6)) than the bonding of >S−O − to Be 2+ (Equation ( 7)).A similar result was reported for the complexation of Co 2+ with metal oxides (Tamura et al., 1997) However, in the higher X i region (Fe 2 O 3 , TiO 2 , and SiO 2 ), the β s,n values of Be 2+ decrease with an increase in X i , indicating that the contribution of the coordination of >S−O − to Be 2+ exceeds that of the deprotonation of >S−OH.This may reflect the relatively strong interaction between the small Be 2+ ion and >S−O − .The appearance of maximum in the complexation constant at X i ≈ 13 (Fe 2 O 3 ) is a result of the competition of the two different processes (Equations ( 6) and ( 7)).
Although the complexation abilities of CoO and CuO are smaller as compared to those of the other oxides, only CoO and CuO form (>S−O) 3 Be − complexes on the surface.In order that three >S−O − can bind to a Be 2+ ion, a close positioning of the >S−OH sites is necessary.So we evaluated the surface hydroxyl group density (the amount of hydroxyl groups per unit area of the surface) under some assumptions.Assuming that the nanoparticles in water comprise a hexagonal close-packed array of uniform spherical particles which have a diameter equal to the average particle size of the dry nanoparticles, the packing efficiency of the spheres is 0.74.By considering the packing efficiency, the surface hydroxyl group density can be estimated from the data in Table 1 (Bessho et al., 2014).The densities (unit: mol/m 2 ) are 8 × 10 −6 , 8 × 10 −6 , 1 × 10 −5 , 1 × 10 −5 , 3 × 10 −5 , and 2 × 10 −5 for Al 2 O 3 , SiO 2 , TiO 2 , Fe 2 O 3 , CoO, and CuO, respectively.These values are comparable, in the order of magnitude, to the literature values (Tamura, Mita, Tanaka, & Ito, 2001).The surface hydroxyl group density we calculated suggests that the surface hydroxyl groups are more closely positioned on the CoO and CuO nanoparticles than on the other oxides.The density, however, may also depend on the preparation method of the oxide.

Conclusion
The adsorption behavior of Be 2+ onto nanoparticles of several metal oxides and SiO 2 in water was quantitatively explained on the basis of the complexation model between Be 2+ and surface hydroxyl groups of the oxides.In general, one Be 2+ ion react with one or two hydroxyl groups on the oxide surface.The complexation constants were found to vary with the electronegativity of the metal (or Si) composing the oxide as follows: CoO ≈ or < CuO < Al 2 O 3 < Fe 2 O 3 > TiO 2 > SiO 2 .The fact that the complexation constants show maximum values for Fe 2 O 3 can be explained based on the opposite effects of the charge density of the oxygen atom on the deprotonation of the surface hydroxyl groups and on the binding of the negatively charged oxygen atoms to Be 2+ .
As can be seen from Equation (4), the distribution ratio of Be 2+ between the nanoparticle phase and the aqueous solution phase depends not only on the surface complexation constants but also on the hydroxyl group concentration (amount of −OH sites per unit mass of the particles).For example, the higher distribution ratio of Be 2+ for CuO than for CoO is explained in terms of the higher hydroxyl group concentration of CuO.Nanoparticles of the oxides, which have extremely large surface area-to-weight ratios, exhibit high adsorption ability for Be 2+ because of their high hydroxyl group concentration.The distribution ratio can be easily estimated from the complexation constants and the hydroxyl group concentration.These findings will provide an effective solution for the removal of 7 Be produced in cooling water systems for high-energy accelerators, and also be applicable to adsorption of other metal ions on metal oxide nanoparticles.

Figure 2 .
Figure 2. Zeta potential versus pH plots for oxide nanoparticles dispersed in water (Al 2 O 3 , 100 mg/dm 3 ; SiO 2 , TiO 2 , Fe 2 O 3 , 200 mg/dm 3 ; CoO, 180 mg/dm 3 ) at a constant ionic strength (0.01 mol/dm 3 ) 3.2 Adsorption Behavior of Be 2+ in Nanoparticle Dispersed Aqueous Solution The partitioning of Be 2+ between the nanoparticle phase and the aqueous solution phase was evaluated in terms of the distribution ratio (D) defined by Equation (1): D={concentration of Be in nanoparticles (mol/g)}/{concentration of Be in aqueous solution (mol/dm 3 )}.(1) In Figure 3, logarithmic values of D are shown as a function of pH.For all the metal oxides, the log D value increases with increasing pH.The D value at pH 6.0 varies in the order, TiO 2 ≈ SiO 2 ≈ Fe 2 O 3 > Al 2 O 3 > CuO >> CoO, where the D value for CuO is cited from the literature(Bessho et al., 2014).As an overall tendency, the D value is larger for the oxide in which the metal (including Si) has a higher valence.This order is not consistent with that of the hydroxyl group concentration (mol/g) of the oxides, i.e., CoO > SiO 2 ≈ Al 2 O 3 > TiO 2 > Fe 2 O 3 > CuO (Table1).

Figure 3 .
Figure 3. Plots of log D as a function of pH for adsorption of Be 2+ onto oxide nanoparticles in waterHere, [X] and [X] s denote the concentration of a material X in the aqueous phase (mol/dm 3 ) and that in the nanoparticle phase (mol/g), respectively.The symbol n represents the number of the surface −O− groups bound to one Be 2+ ion.In the pH range of the present experimental conditions, the main aqueous species of beryllium(II) are Be 2+ , Be(OH) + , and Be(OH) 2 , as expected from the hydrolysis constants, K 1 = [Be(OH) + ][H + ] / [Be 2+ ] = 10 −5.7 and β 2 = [Be(OH) 2 ][H + ] 2 / [Be 2+ ] = 10 −11.68(Schwarzenbach & Wenger, 1969;Chinea et al., 1997).Assuming that Be 2+ is mainly concerned with the surface complexation, D can be expressed as follows.

Figure 5 .
Figure 5. Plots of complexation constants of Be 2+ with hydroxyl groups on oxide surface as a function of electronegativity of the metal (or Si) composing the oxide

Table 1 .
Physical properties of oxide nanoparticles

Table 2 .
Complexation constants of Be 2+ with surface hydroxyl groups of various oxide nanoparticles in water at