Abstract: Effect of zinc adsorption on the change in surface acidity of two nano-ball allophane samples with varying Si/Al ratio was studied. Zinc adsorption caused the enhancement in the surface acidity of nano-ball allophane samples. The enhancement in the surface acidity after zinc species adsorption is attributed to the inductive effect on the Si–OH groups near the adsorption sites. Molecular orbital calculations indicated that Zn2+ ion has the possibility to accelerate deprotonation of the Si–OH functional groups near the adsorption site. The result showed that the surface acidity of the allophane sample with a higher Si/Al ratio (KnP) was stronger than the KyP sample with lower Si/Al ratio. This tendency was observed under RH between 0 and 76%; then, the acid strength for the two nano-ball samples was the same at RH of 100%. The stronger acidity of the KnP sample is attributed to its a higher number of accessory polymeric silanol groups attached to the main frame structure, which cause the enhancement of the Brønsted acidity. The higher increase in the acid strength of the KnP could be also attributed to its higher adsorptive capacity for zinc species.
INTRODUCTION
Allophane formed in weathered volcanic ash and pumice has morphology of aggregates composed of fine hollow spherules with diameters of 3.5 to 5.0 nm. The chemical composition of the wall of hollow spherules varies in Si/Al atomic ratio form 1:2 to 2:2. There is a regular tendency in physicochemical characteristics such as structural stability against grinding, heating, change in CEC, AEC and surface acidity depending on Si/Al ratio of allophane samples (Henmi et al., 1997). Nano-ball allophane has surface acidity that is weaker than kaolinite and montmorillonite clay minerals, but stronger than for imogolite. The functional groups (Si–OH and Al¯OH) in the nano-ball allophane structure are able to behave as the Brønsted acidity sites. The surface acidity of a lower Si/Al ratio (KyP) sample was found to be increase upon sulfate (Padilla, 2003), phosphate adsorption (Johan, 1999) on the nano-ball allophane samples. In these previous studies, the increase in the acidity was attributed to the enhancement of the Brønsted acidity of the silanol (Si–OH) functional groups on the allophane structure. Elhadi et al. (2001) using molecular orbital calculations indicated that Brønsted acidity of Si–OH groups of allophane near the adsorption sites increased with molybdate adsorption. In our previous study (Ghoneim et al., 2001; Ghoneim, 2002) we found that zinc was strongly adsorbed on nano-ball allophane at different pH.The adsorption brought about a change in CEC and AEC. It is expected that the adsorption would also affect the surface acidity of allophane. Therefore the aim of this study was to know the influence of zinc adsorption on the surface acidity of nano-ball allophane and the mechanism by means of molecular orbital analysis.
MATERIALS AND METHODS
Allophane Samples Preparation
The study was initiated at the Faculty of Agriculture, Environmental Soil
Science Laboratory, Ehime University, Matsuyama, Japan during 2002 to 2005.
Two nano-ball allophane samples used in this study were separated from weathered
pumice grains taken from two different volcanic ash locations in Japan. The
KyP sample was collected from Kurayoshi, Tottori prefecture and KnP sample from
Kakino, Kumamoto prefecture. Fine clay fraction (<0.2 μm) was separated
from the inner part of pumice grains after removing the outer part to eliminate
any possible contamination of allophane samples. The separation was carried
out by ultrasonification at 28 kHz and then, disperation at pH 4 for KyP sample
or pH 10 for KnP sample according to Henmi and Wada (1976). The obtained samples
were flocculated by NaCl solution, washed with water to remove excess salts
and then freeze-dried. The sample were analyzed by electron microscopy, IR,
DTA and chemical analysis (data not shown) to ascertain the purity of the sample.
Atomic arrangement near the defect (pore) of hollow spherical nano-ball allophane
with low Si/Al ratio is shown in Fig. 1C, while the allophane
with high Si/Al ratio is shown in Fig. 1B, in which some accessory
silicon is attached and dimeric or polymeric SiO4 tetrahedra are
formed.
Surface Acidity Measurement
Zinc adsorption experiment was carried out by equilibrating 150.0 mg of
the freeze-dried KyP and KnP allophane samples with 100 mL of ZnCl2
solutions with different concentrations. The initial zinc concentration varied
from 0 to 0.18 mM with the initial pH between 4 and 7 using either HCl or NaOH
solution. The total Na+ concentration was kept at 10.0 mM by adjusting
the amount of NaCl added. The suspensions were then shaken for 24 h, centrifuged
after which the supernatant was decanted. The levels of Zn concentration in
the supernatant was determined by an atomic absorption spectrophotometer (Hitachi
Z-5000) and the amounts of zinc adsorbed by nano-ball allophane samples was
calculated from the difference in concentrations before and after adsorption.
The separated clay samples (precipitates) was then washed with distilled water
and dried in oven at 105°C for 6 h. The collected samples were then brought
to different Relative Humidity (RH) sets of 0.0% (after heating up to 105°C),
31.0% (saturated CaCl2.2H2O solution), 76.0% (saturated
CH3COONa.3H2O solution) and 100% (water). The most wanted
RH levels were obtained by equilibrating the samples in desiccators with the
suitable medium given in parenthesis for 24 h.
The acid strength was determined according to the method of Benesi (1956) and Walling (1950). The principle of this method is to observe and determine the change in the color of the samples upon the donation of H+ to the neutral base with known pKa values (Hammett, 1940), where suitable indicators are used as bases for measurement of the change in strength of the surface acidity. The indicators used are shown in Table 1. The concentration of each indicator was 1.0% dissolved in benzene. The acidity (H0) was expressed with ordinary numbers rank form I to VI as shown in Table 1. After equilibrating the allophane sample under different RH levels, about two drops of indicators were added to the samples and the change in color was observed by the naked eye. The acid strength was then estimated using Table 1. For example, a sample that gave a red coloration with methyl yellow but was yellow with aminoazatoluene was estimated to have acid strength of H0 = 3.3 to 2.0 or rank III.
| Fig. 1: | Scheme of chemical structure of a unit particle of nano-ball allophane (A: Complete structure; B and C: atomic arrangement in the cross section at the pore region) |
| Table 1: | List of Hammet indicators used for surface acidity measurements and ranking to express H0 values |
RESULTS AND DISCUSSION
Change in Acid Strength
The results generally showed that the acidity of the allophane sample with
a higher Si/Al ratio (KnP) was stronger than for the KyP sample with lower Si/Al
ratio (Table 2). This trend was observed under the RH between
0 and 76% and then the acid strength for the two nano-ball samples was the same
at the RH of 100%. The stronger acidity of the KnP sample is attributed to its
a higher number of accessory polymeric silanol groups attached to the main frame
as shown in Fig. 1 which cause the enhancement of the Brønsted
acidity (Henmi et al., 1997). For the lower Si/Al ratio KyP sample, the
result indicated that with increasing RH from 31 to 100%, there was no apparent
change in the acid strength (H0 = II). This results is attributed
to the water molecules occupying the same number of the acid sites, however
heating the allophane samples up to 105°C, led acid strength increase up
to H0 values of (+ 3.3 ~ + 2.0) or rank III. It is well known that,
when the mineral surfaces become drier the Brønsted acidity and protons
are concentrated in a smaller volume of water resulting in more extreme surface
acidity (Huang, 2000). Even very weak bases, i.e., poor proton acceptors can
be protonated on the surfaces of such mineral.
| Table 2: | Measured acid strength (H0) of the allophane samples with and without zinc adsorption |
(*) above data indicated amounts of Zn adsorbed μmol g-1 |
|
On the other hand, at RH of 100%, the both original allophane samples in 10.0 mM NaCl background solution have the same rank II (H0 = 4.0 ~ 3.3). These results are attributed in part to the 100% moisture level, the water molecules engaged almost all the available sites for the acidity, resulting in the very weak Brønsted acidity. In addition, when the nano-ball allophane samples become wetter, the H+ ions are concentrated in large volumes of the water and subsequently resulting in weak Brønsted acidity.
Effect of Zinc Adsorption
The effect of zinc adsorption on the acid strength of nano-ball allophane
samples at different RH levels is presented in Table 2. In
general, after zinc adsorption there are increases in the surface acidity of
the two nano-ball allophane samples except for the KnP sample with a higher
Si/Al at RH of 0%. The increase in the acid strength after adsorption is the
same magnitude regardless the amounts of zinc adsorbed (Table
2). For example, the KyP allophane sample with a lower Si/Al ratio, the
acid strength increased from rank II to rank IV at the RH between 31 and 100%,
however after heating the samples up to 105°C, the acid increase to the
rank V. In case of allophane sample with a higher Si/Al ratio (KnP) at the RH
of 31 and 76% levels, the acid strength increased from rank III to V (+ 1.5
~ -3.0). At the 100% moisture level, the two samples have the same acidity of
rank II. Also the results showed that no change in the acid strength was observed
for the KnP sample at 0%, this may be due to the sample after heating up to
105°C losses almost all the water adsorbed on its surfaces, in addition
the rank V is regard as wider range (e.g., H0 = +1.5 ~ -3.0). The
increase in the acid strength after zinc adsorption on the two samples at the
RH between 31 and 76% is attributed to the H2O coordinated the exchangeable
sodium on the allophane surface. The stronger acid strength observed after zinc
adsorption for the KnP at RH between 31 and 76% is probably due to the higher
inherent Na+ content for the KnP allophane sample. The higher increase
in the acid strength of the higher Si/Al ratio (KnP) could be also attributed
to its higher adsorptive capacity for zinc (Ghoneim et al., 2001, 2006;
Ghoneim, 2002). The surface acidity of the clay minerals arises from two main
sources: the Brønsted acidity and the Lewis acidity. The Brønsted
acidity comes from the deprotonation of the H+ on the constant charge,
water molecules, which coordinated to surface bound cations and also from the
dissociation of functional groups. On the other hand, the Lewis acidity originates
from un-saturated structural silicon and aluminum, from constituent ions such
as aluminum and iron that exposed at the edges of the minerals (Sposito, 1984;
Wu et al., 1992; McBride, 1994). Table 3 shows the
possible origins of the surface acidity of nano-ball allophane before Zn adsorption.
Molecular Orbital Analysis
The increase in the Brønsted acidity of the nano-ball allophane samples
after phosphate adsorption was explained by the dissociation of H+
from the silanol (Si–OH) groups near the adsorption site (Johan, 1999),
but Elhadi et al. (2001) reported that the increase in the surface acidity
of nano-ball allophane samples was attributed to the dissociation of the proton
from the adsorbed molybdate (Mo–OH).
| Table 3: | The possible origins of the surface acidity of nano-ball allophane samples before zinc adsorption |
For that reason the molecular orbital calculations were carried out to know and determine the sources of increase in the surface acidity of the allophane samples upon zinc adsorption.
The gas acidity of a molecule is determined from the difference in energy between that of the anion and the neutral molecule by the following equation (Siggel and Thomas, 1986).
A = Ea – Em |
Where A is the gas acidity of the molecule, Ea and Em is the total energy of the anion and neutral molecule, respectively. The Brønsted acidity can be estimated either from the equilibrium constant pKa value or from the change in the free energy value (ΔG°) of the reaction of (Siggel et al., 1988). The ΔG value can be calculated from the difference in the heat of formation (ΔH) between the products and reactants (ΔH = Hproducts¯Hreactants).
The dissociation reaction is expressed as:
RH → R¯ + H+ |
Where RH is the neutral molecule and R- is the anion. The principle of the calculations is when the dissociation of two compounds, RIH and RIIH, with a similar molecular weight occurs, therefore only the heat of formation (H) of the products and the reactants are required.
RIH → RI¯ + H+ |
RIIH → RII¯ + H+ |
| Fig. 2: | Model clusters used for molecular orbital calculations. Models A and D shows allophane with undissociated Si–OH; models B, C, E and F simulates zinc species adsorption with one or two Si–O¯ groups |
However, the heats of formation of proton is canceled out and therefore only the heats of the formation of the dissociated and undissociated forms are needed. Lower value for ΔH indicates the stronger Brønsted acidity.
Figure 2 shows the zinc adsorption on the silanol groups. Model A depict allophane model with no dissociated Si–OH; protons with the asterisk marks in the three models shows those can dissociate and cause the acidity. Models B and C simulates the zinc adsorption with one or two dissociated Si–O- groups, respectively. Even when the allophane model had no dissociated silanol group, the Zn2+ ions was adsorbed and positioned between the three O atoms of the three silanol groups. The O–H bond lengths of the three Si–OH bonded to the Zn2+ (0.097 nm) was longer than those of the other silanol groups (0.095 nm). This means that the protons of the silanol groups bonded to Zn2+ ions were easily dissociated, resultant in a release of the protons into the bulk solution as observed experimentally (Ghoneim et al., 2001; Ghoneim, 2002). For the allophane model with one Si–O- dissociated, the monomeric Zn2+ adsorbed strongly by the dissociated Si–O- and also weakly adsorbed by the two near Si–OH groups (model B). The O–H bond length of the two Si–OH groups, which bonded to Zn2+ were longer than for the other silanol groups. These results indicate that, the Zn2+ ions have the possibility to accelerate the deprotonation of the Si–OH near the adsorption site. For adsorption of ZnOH+, the molecular orbital calculation also carried out (models D, E and F). The monohydroxylated species ZnOH+, also could be adsorbed between three silanol groups but the Zn¯O bond length was longer than those for Zn2+ adsorption.