Nano-ball allophane as a variable charge mineral has Cation Exchange Capacity (CEC) strongly dependent on electrolyte concentration, type of cation, solution pH and the method of determination. In addition, nano-ball allophane as a pH-dependent clay mineral has unique characteristics: it can have both negative and positive charges simultaneously. The main positive and negative charges are separated each other in the nano-ball allophane structure as shown in Fig. 1. The positive charges result from aluminol groups (AlOH2+) located at the pore region of the ball of allophane whereas the negative charges come from silanol groups (SiO¯) at inner side of the nano-ball allophane structure. These charge characteristics are different from those of the other pH-dependent clay minerals such as goethite and gibbsite (Parfitt, 1980).
The CEC and AEC values either decrease or increase depending upon materials adsorbed. The increase in CEC and declining AEC values have been reported after sulfate (Padilla et al., 2002), molybdate (Elhadi et al., 2001), citrate (Hanudin et al., 2000) and phosphate (Johan et al., 1999) adsorption on the nano-ball allophane samples. The increase in the CEC values were attributed to charges brought into the nano-ball allophane by anionic forms of the compounds adsorbed. Recently, allophane in soils has been attracting considerable attention because of its significant contribution to the physical and chemical properties of tropical soils.
In our earlier study on Zn adsorption by the nano-ball allophane samples at
various pH values (Ghoneim et al., 2001; Ghoneim, 2002) indicated that
the noticeable amounts of Zn were adsorbed by allophane samples under different
pHs and the equilibrium solution pH decreased after the adsorption. Moreover,
the adsorption was dependent on the difference in the chemical structure between
nano-ball allophane samples and on Zn species. However, the effect of Zn adsorption
on surface chemical characteristics of allophane, such as CEC and surface acidity,
has not been investigated. Because Zn species adsorb strongly to oxygen atoms
of functional groups of allophane (Ghoneim et al., 2001), the Zn-adsorbed
allophane as a newly formed compound has different properties from the original
||Morphology and chemical structure of nano-ball allophane (A:
morphology in section; B: atomic arrangement near the pore; C and D: atomic
arrangement in cross section nears the pore)
The aim of this research was therefore to know the mechanism of change in
charge characteristics of the nano-ball allophane as affected by Zn adsorption.
MATERIALS AND METHODS
Allophane samples: The study was conducted at Environmental Soil Science Laboratory, Ehime University, Japan, during 2001-2005. Two allophane samples were used in this study: one was designated as KyP with low Si/Al ratio (0.67) collected from Tottori prefecture near Mt. Daisen, Japan and KnP with high Si/Al ratio (0.99) collected form Kumamoto prefecture, near Mt. Aso, Japan. Fine clay fraction (< 0.2 μm) was separated from inner part of pumice grains after removing outer part to eliminate any possible contaminations such as volcanic glasses, opaline silica and imogolite. The separation was carried out by ultrasonification at 28 kHz followed by disperation at pH 4 for KyP sample or pH 10 for KnP sample as described by Henmi and Wada (1976). The collected samples were flocculated by NaCl solution, washed with distilled water to remove excess salts and then freeze-dried. The freeze-dried samples were subjected to electron microscopy, IR, DTA and chemical analysis (data not shown) to ascertain the purity of the sample. The results indicate that the sample were free form the impurities described above. Atomic arrangement near the defect (pore) of hollow spherical nano-ball allophane with low Si/Al ratio is shown in Fig. 1B. That of allophane with high Si/Al ratio is shown in Fig. 1C, wherein some accessory silicon is attached and dimeric or polymeric SiO4 tetrahedra are formed.
Zinc adsorption, CEC and AEC measurements: Zinc adsorption was carried out by equilibrating 50.0 mg of the freeze-dried KyP and KnP allophane samples with 100 mL of mixed solutions of ZnCl2 and NaCl in 250 mL pre-weighed centrifuge bottles. The pH levels of the solutions were adjusted between 3 and 10 (initial pH) by adding either HCl or NaOH solutions. The final Zn concentration varied from 0.0 to 0.18 mM and Na concentration as background media was kept at 10 mM. The suspensions were shaken for 24 h, centrifuged at 3500 rpm for 20 min after which equilibrium pH of the supernatant was measured. Further, the level of Zn and Na concentrations in the supernatant were determined by using Polarized Zeeman Atomic Absorption Spectrophotometer (Z-5000). The CEC and AEC values of the samples were determined according to the modified equilibrium method (Schofield, 1949). After the Zn adsorption experiment, the centrifuge bottle plus their content was weighed after decanting the supernatant to calculate the volume of entrained solution. Then, 50 mL of 1M NH4NO3 solution was added to the contents of the centrifuge bottles, shaken for 5 h and then, centrifuged and the NH4NO3 supernatant decanted and kept. The last step was repeated one more time and the levels of Na and Cl concentration in the supernatant were determined. The Na was determined by atomic absorption spectroscopy and Cl by colorimetry according to the method of Huang and Johns (1967). The CEC and AEC values were calculated as the difference in Na and Cl between extracted and entrained. To find out the more possible reactions proposed, MOPAC 2002 program with AM1 basis set as a semi empirical molecular orbital method was employed. The cluster models for allophane were built up with Si tetrahedra and Al octahedra by using bond distances of SiO = 0.1618 nm, AlO = 0.1912 nm and OH = 0.0944 nm.
RESULTS AND DISCUSSION
Change in charge characteristics: Charge characteristics of the two nano-ball allophane samples, KyP and KnP, with and without Zn adsorption in 10 mM NaCl background solution are shown in Fig. 2 and 3. The CEC values of the original allophane samples tended to increase while the AEC decreased with increasing equilibrium solution pH. Shape of the pH-CEC curves of the original samples was a representative of nano-ball allophane as reported by Padilla et al. (2002), Nartey et al. (2001), Hanudin et al. (2000) and Johan et al. (1999). The CEC value for the original KnP allophane sample was higher than for the KyP. This result is attributed to the difference in the chemical structure between the two nano-ball allophane samples as shown in Fig. 1. The higher amounts of the negative charges for the KnP sample prove the existence of more polymeric SiO4 tetrahedra attached to the main frame of nano-ball allophane structure, which causes an increase in the Ka value of the SiOH group in the nano-ball allophane structure. Fundamental structure of the nano-ball allophane as proved (Henmi et al., 1997; Matsue and Henmi, 1993) has Si/Al ratio of 0.5, the imogolite structure and the additional polymeric SiO4 tetrahedra increase the ratio. Therefore, the KnP allophane sample with a higher Si/Al ratio is considered as the SiO4 adsorption product of the lower Si/Al ratio (KyP) and the adsorbed SiO4 tetrahedra let to increase the amounts of negative charge.
Figure 2 and 3 also shows AEC plots, evaluates
the positive charge sites on the allophane samples as a function of solution
pH. As expected, the AEC values decreased as the equilibrium solution pH increased,
reflecting the deprotonation of the surface hydroxyl groups and the consequently
the reduction in the positive charge.
||Charge characteristics of the KyP sample before and after
Zn adsorption. Initial concentration was: 0.18 mM
||Charge characteristics of the KnP sample before and after
Zn adsorption. Initial concentration was: 0.18 mM
The AEC values were found to be generally higher in KyP allophane sample with
a lower Si/Al ratio than for KnP. This was attributed to the higher Al content
per unit mass of the KyP sample. The aluminol group content per unit mass decreased
with the rising of the Si/Al ratio of the allophane sample (Son et al.,
1998). In addition, abundance of the attached polymeric SiO4 tetrahedra,
which cause a steric hindrance effect on the aluminol groups at the pore region
of the nano-ball allophane, explain the lower AEC values for the KnP than for
the KyP even at the same equilibrium pH values.
It was seen between equilibrium pH of 4 and 8, that the two original allophane samples have both negative and positive charges (Fig. 2 and 3). These strikingly unique characteristics of the nano-ball allophane unlike the other pH-dependent clay minerals such as gibbsite and goethite (Parfitt, 1980) are attributed to the difference in the location of the silanol and aluminol functional groups in the nano-ball allophane structure (Fig. 1).
Effect of zinc adsorption on CEC and AEC: The obtained CEC values of the two nano-ball allophane samples tended to decrease after Zn as shown in Fig. 2 and 3 for the KyP and KnP, respectively. The decrease in the CEC value from the original sample was found to be slightly higher in KnP than in KyP. The decrease in the CEC values after Zn adsorption on nano-ball allophane was accompanied by release of H+ into bulk solution (Ghoneim et al., 2001; Ghoneim, 2002). The release of H+, which was found to be higher in case of KnP than for the KyP, could be an indication of its greater decrease in the CEC values than for KyP. The observed increase in CEC after anions adsorption was attributed to the charge carried by the anions and to the new negative charges created on SiOH groups near the adsorption site due to an inductive effect (Padilla et al., 2002; Nartey et al., 2001). The decrease in the CEC value with Zn adsorption is attributed to neutralization reactions between the dissociated SiO¯ functional groups with the positive species of Zn.
Substantial decrease in AEC values has been reported after adsorption of anions on the nano-ball allophane samples (Padilla et al., 2002; Nartey et al., 2001; Elhadi et al., 2001; Hanudin et al., 2000). In these studies, the decreases in AEC values were thought to be due to neutralization of positive charges AlOH2+ by the anionic form of the compounds adsorbed. In the current work, a slight increase in the AEC values was found for the two nano-ball allophane samples after Zn adsorption (Fig. 2 and 3). This is probably due to in part to the initial H+ released into the bulk solution after Zn adsorption, which may have reacted with AlOH functional group to form the new positive charge, AlOH2+ (Ghoneim, 2002).
The net change in the CEC (ÄCEC) was calculated as the difference between
CEC values before and after Zn adsorption at a same equilibrium pH value.
||Relationship between change in CEC and amounts of Zn adsorbed.
Initial concentration was: 0.18 mM
Figure 4 shows relationship between amounts of Zn adsorbed
and ΔCEC of the nano-ball allophane samples at initial Zn concentration
of 0.18 mM. The amounts of Zn adsorbed on KyP were 14.9, 15.5 and 16.7 cmol
kg1 at initial solution pHs of 4, 6 and 7, respectively and the
consequent ΔCEC values were -1.9, -6.4 and -9.3 cmolc kg1,
respectively. For KnP sample, the amounts of Zn adsorbed was higher than that
for KyP (19.9, 24.1 and 25.0 cmol kg1) at initial solution pHs of
4, 6 and 7, respectively and the equivalent ΔCEC values were also higher
(-12.1, -14.1 and -14.4 cmolc kg1, in that order). The
calculated ΔCEC values for KnP was higher than for KyP and this was most
likely due to the fact that the KnP, allophane sample with a higher Si/Al ratio,
showed a higher adsorptive capacity for Zn species than for the KyP sample (Ghoneim
et al., 2001; Ghoneim, 2002).
In all cases, the amounts of Zn adsorbed were higher than ΔCEC of the two samples (Fig. 4). It is noticed here that the obtained CEC value in this study is the amount of Na adsorbed on allophane sample when the Zn adsorption is in equilibrium. The higher amount of Zn adsorption compared with decrease in CEC (decrease in adsorbed Na) indicates that Zn exchanged not only exchangeable Na but also H of SiOH group on allophane.
The amount of Zn adsorption was higher in KnP than for KyP sample at a same
initial Zn concentration (Ghoneim et al., 2001).
||Relationship between CEC before and after Zn adsorption. Constant
adsorption of: 15.0 cmol kg-1
To compare the effect of Zn adsorption on CEC between the two allophane samples
at a same Zn adsorption level, constant adsorption experiments were carried
out. Figure 5 shows the pH-CEC plots of KyP and KnP samples
with a constant Zn adsorption of 15.0 cmol kg1, together with those
of the original samples (Zn = 0). The higher the equilibrium pH, the greater
the ΔCEC or Na adsorption, for the two samples. This means that at higher
pH, the added Zn tended to preferentially exchange Na adsorbed on SiO¯
group on allophane as a cation exchange reaction. The results in Fig.
5 also indicate that the decrease in CEC (ΔCEC) was higher in case
of KnP sample than for KyP, at a same equilibrium pH. A possible change in the
form of Zn, from the polyvalent Zn2+ to monovalent ZnOH+
at higher pH, could be another factor controlling the adsorption. These results
suggest that the equilibrium pH, amounts of adsorption and species of Zn are
the main factors controlling adsorption of Zn by nano-ball allophane samples.
The molecular orbital calculation confirmed that the bond length between Zn
atom and O atom of the silanol group (SiO¯) was shorter for ZnOH+
than for Zn2+, indicating that the possibility of ZnOH+
adsorbed stronger than Zn2+ on the nano-ball allophane at higher
Proposed mechanism of the change in charge characteristics: The adsorption sites of the nano-ball allophane for the positive forms of Zn are mainly AlOH2, AlOH and SiOH functional groups. The adsorption mechanisms might therefore be written as follows. AlOH2 and AlOH existed at the pore region of nano-ball allophane and the reactions are schemed as:
Reaction with SiOH is illustrated as:
Equations 1 to 3 portray Zn adsorption
as monodentate reactions, where one H+ was released into the bulk
Zinc would react with two silanol or aluminol groups (reactions 4 and 5), by binuclear reaction in which two H+ are released into the solution.
The adsorption of Zn was represented as specific adsorption with release of two H+ for each mole of Zn2+ ions adsorbed and bring about decrease in solution pH as observed experimentally (Ghoneim, 2002).
Molecular orbital calculation indicated that, when the cluster model of allophane has one dissociated silanol group (SiO¯), the monmeric Zn2+ adsorbed strongly to the dissociated silanol group and also weakly to two undissociated silanol groups. The OH bond length of silanol groups near the Zn adsorption site were longer than those of the other silanol groups. This means that the Zn2+ has a possibility to accelerate the deprotonation of undissociated silanol groups near the adsorption site. This induced increase in Brønsted acidity of the near silanol groups may interpret smaller ΔCEC as compared to Zn adsorbed (Fig. 4).
The following reactions explain the slight increase in the AEC values after Zn adsorption on the allophane samples.
In the reactions 6 and 7, part of the H+, which was initially released into the bulk solution after zinc adsorption (Ghoneim et al., 2001) may have reacted with AlOH group to form the new positive charges, AlOH2+ (Eq. 7) and the excess of the protons remain in the bulk solution and decrease the solution pH (final H+ release). This is in conformity with the results in the previous experiment (Ghoneim et al., 2001) showing decrease in solution pH after Zn adsorption.