Effect of Copper Adsorption on Some Charge Characteristics of Nano-Ball Allophane
Mechanisms of change in charge characteristics after copper adsorption on one low Si/Al (KyP) and one high Si/Al (KnP) nano-ball allophane samples were investigated. The CEC values of the two nano-ball allophane samples tended to decrease while AEC increased after copper species adsorption at the initial concentration of 1.60 mM. The change in charge characteristics of the allophane samples was governed by the pH, Cu2+, CuOH+ in the solution and amounts of adsorption. The decline in the CEC values after adsorption was due to the neutralization reactions between the monomeric Cu2+ with Si-O- or Al-O- functional groups. Molecular orbital calculations indicated that Cu2+ and CuOH+ could adsorb not only on the dissociated Si-O- but also with the undissociated Si-OH groups. When copper ion reacted with undissociated Si-OH groups, dissociation reaction of silanol groups accelerated. The Cu2+ ion has a possibility to accelerate the deprotonation of undissociated Si-OH groups near the adsorption sites. The decrease in change CEC values was found to be higher in case of a higher Si/Al ratio (KnP) than that for the lower Si/Al ratio (KyP) counterpart, due to it`s a higher adsorptive capacity for copper species. The slight increase in the AEC values after zinc adsorption was probably due to in part to the initial H+ ions released into the bulk solution.
Protection of soil resources and environment as well as sustainable development of agriculture are important problems lain ahead of humankind with increasing the population and acceleration of industry development (He et al., 1998). Some heavy metals are beneficial whereas some of them are toxicity for the plant growth. Copper is essential to plant, but is toxic when its concentration exceeds a certain critical level (Baker, 1990; Pires and Mattiazo, 2003). Copper enters agricultural ecosystems through applications of Cu-containing fungicides, stable manure and solid wastes from Cu-related mining and manufacturing. There are two types of minerals that involved in the adsorption-desorption of Cu2+ in soils: permanent charge and variable charge. Permanent charge minerals such as montmorillonite carry out a negative charge as a result of ion substitution during the formation of the minerals. Variable charge minerals such as nano-ball allophone carry charge varying from negative to positive depending on pH changes. Copper adsorption and desorption in soil are affected by the proportion of these 2 types of minerals (Atanassova and Okazaki, 1997). Organic matter and pH are the factors that directly or indirectly affect the Cu2+ bioavailability (Rodriguez-Rubio et al., 2003). Organic radicals, as well as iron and aluminum oxides, have OH¯ sites with high potential for specific copper adsorption (Yin et al., 2002). Moreover, the Fe, Al and Mn oxides have a relatively strong affinity (pH dependent) for Cu2+ adsorption (Martins et al., 2003).
The surface charge of variable charge surfaces depends on the solution pH and the pK of the type of surface functional group. For the most agricultural soils, bioavailability of Cu2+ is controlled by adsorption-desorption process (Xie, 1996). The surface charge characteristics are an important factor to limit the behavior of the copper adsorption-desorption (Yin et al., 2002). Remediation of Cu-contaminated soils requires an understanding of Cu2+ adsorption-desorption behavior and the major factors.
Allophane is poorly ordered aluminiumsilicates, with silica to alumina ratio
between 1.0 and 2.0 (Henmi and Wada, 1976). Allophone used to be describing
as amorphous but recent investigations using the sate of art technique has established
the morphology and chemical structure as shown in Fig. 1.
Allophone has CEC values which are strongly dependent on the electrolyte concentration,
type of cation and solution pH. In addition, nano-ball allophane as a pH-dependent
clay mineral has a unique characteristics; it can has both negative and positive
charge simultaneously. The main positive and negative charge is separated each
other in the nano-ball allophane structure as shown in Fig. 1.
The negative charge come from the silanol groups (Si-O¯) on the inner side
of the nano-ball allophane structure while the positive charge result from the
aluminol groups (Al-OH2+), which placed at the pore region
of the ball of allophane whereas the. 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 of the nano-ball allophane samples might
be, therefore, decrease or increase along with the type of the adsorbed materials.
The declining in AEC values and increase in CEC values have been reported after
molybdate (Elhadi et al., 2001) and citrate (Hanudin et al., 2000)
adsorption on the nano-ball allophane samples. Recently investigations on Cu2+
adsorption by the nano-ball allophane samples at different pH levels indicated
that the noticeable amounts of copper were adsorbed by the KyP and KnP samples
(Ghoneim et al., 2002).
||Chemical structure of nano-ball allophone (A: Morphology in
section; B: Atomic arrangements near the pore; C and D: Atomic arrangement
in cross section at the pore
In addition, the equilibrium solution pH decreased after adsorption, H+
being replaced by Cu2+ ions (Ghoneim, 2002). Moreover, the adsorption
was greatly controlled by the difference in the chemical structure between the
allophone samples, copper species in the solution and pH (Ghoneim et al.,
2002). When significant amount of copper was adsorbed then, a new compound,
called allophane-copper complex different from the original allophane samples
may be produced. The purpose of this research was to investigate the mechanisms
of change in CEC and AEC of two nano-ball allophone with various Si/Al ratios
upon copper adsorption.
Materials and Methods
Allophane Samples Preparation
Two allophone 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 according to the method of Henmi and Wada (1976). The
KyP and KnP allophone samples have molar ratios of 1.34:2 and 1.98:2, respectively.
The KyP is accordingly considered as a low Si/Al while KnP as higher Si/Al ratio
(Henmi et al., 1981). The chemical structure of nano-ball allophone samples
is shown in Fig. 1.
CEC and AEC Measurements
To 50.0 mg of the freeze dried KyP and KnP samples in pre-weighted 100 mL
centrifuge bottles, an appropriate volume of NaCl and CuCl2 solution
were added and topped with distilled water to attain final concentration of
copper concentration of 1.60 and 10.0 mM for NaCl. Concurrently, the pH of the
solution was adjusted to the levels between 3 and 10 by adding either HCl or
NaOH solutions. After that, the suspensions were shaken for 24 h, centrifuged
at 5000 rpm for 20 min after which the equilibrium pH of the supernatant was
measured. In the next stage, the levels of Cu2+ and Na+
concentrations of the supernatant were determined using the Polarized Zeeman
Atomic Absorption Spectroscopy (Z-5000) and the Cl¯ concentration determined
calorimetrically according to the method of (Huang and Johns, 1967). Here, the
amounts of zinc adsorbed were calculated in each case from the reduction in
the concentrations initially and that remaining after the equilibration. Later,
the CEC and AEC values of the samples were determined according to the modified
equilibrium method of (Schofield, 1949). The centrifuge bottles with their contents
were weighed after decanting the supernatant to calculate the volume of entrained
solutions. Then, 50 mL of 1.0 M 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
process was repeated and the levels of Na+ and Cl¯ concentrations
in the bulked NH4NO3 determined. Finally, the CEC and
AEC values of the samples were estimated as the difference in the concentration
of NH4NO3, which extracted Na+ and Cl¯
ions and that of the entrained solution. The MOPAC 2002 with AM1 basis set as
a semi empirical computational method in Chem 3D program was employed to find
out the more feasible of some of the reactions proposed. Cluster models for
allophane were built up with Si tetrahedra and Al octahedra by using bond distances
of Si-O = 0.1618 nm, Al-O = 0.1912 nm and O-H = 0.0944 nm.
Results and Discussion
Change in Charge Characteristics
The relationship between equilibrium pH, CEC and AEC values of original
KyP and KnP samples at initial concentration of 1.60 mM in 10 mM NaCl background
solution are shown in (Fig. 2 and 3). With
increasing equilibrium pH, there are increases in the CEC values of the two
allophone samples. Nartey et al. (2001); Elhadi et al. (2001)
and Hanudin et al. (2000) made similar results and demonstrated that
this concave shaped CEC-pH curve of original allophone sample in 10 mM NaCl
background solution is representative of nano-ball allophone morphology.
||Charge characteristics of the KyP sample with and without
copper adsorption at initial Cu concentration of 1.60 mM
||Charge characteristics of the KnP sample with and without
copper adsorption at initial Cu concentration of 1.60 mM
The CEC value for the original KnP allophane sample with a higher Si/Al ratio
(1.98:2) was higher than for the KyP (1.34:2). These results are 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 confirm the existence of more accessory polymeric
SiO4 tetrahedra attached to the main frame of nano-ball allophane
structure; which causes an increase in the Ka value of the Si-OH
group in the nano-ball allophane structure. The fundamental structure of the
nano-ball allophane as proved (Henmi et al., 1997; Matsue and Henmi,
1993) has shown that the Si/Al ratio of 0.5, the imogolite structure and the
additional polymeric SiO4 tetrahedra, which 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 in the amounts of negative charge. The AEC values
decreased as the equilibrium pH increase, indicating that the deprotonation
of the surface OH¯ groups and the consequent, the decline in the positive
charge. Furthermore, AEC was higher in KyP sample than in the higher Si/Al ratio
(KnP). This higher AEC value is attributed to the higher aluminum content per
unit mass of the KyP sample. A lower AEC of KnP is attributed in part due to
the tendency for the attached polymeric SiO4 tetrahedra, which cause
a stearic hindrance effect on the aluminol functional groups at the pore region
of the nano-ball allophane (Johan et al., 1999). Below equilibrium pH
8, it is seen that the two original allophane samples, in 10 mM NaCl have both
negative (CEC) and positive AEC charges (Fig. 2 and 3).
These strikingly unique properties feature of the nano-ball allophane unlike
the other variable charged minerals such as gibbsite and goethite (Parfitt,
1980) is attributed to the difference in the location of the silanol and aluminol
functional groups in the nano-ball allophane structure as shown in (Fig.
1). The silanol groups (Si-O¯), which accounts for a large fraction
of the CEC values in nano-ball allophane are located at the inner surface of
the mineral, while the positively charge aluminol groups (Al-OH2+)
are located at the pore region of the nano-ball allophane. Generally, the CEC
values of the two nano-ball allophane tend to decrease after Cu2+
adsorption at the initial concentration of 1.60 mM as shown in (Fig.
2 and 3) for the KyP and KnP, respectively. The figures
also showed that the decrease in the CEC value from the original sample was
found to be slightly higher in a higher Si/Al ratio (KnP) than for the lower
Si/Al counterpart (KyP) allophane sample. The decline in the CEC values after
copper adsorption was coupled with proton release into the bulk solution (Ghoneim
et al., 2001; Ghoneim, 2002). The release of H+, which was
found to be higher in case of the KnP, with a higher Si/Al ratio than for the
KyP could be an indication of its higher decreasing in the CEC values than for
KyP. The decrease in the CEC value after copper adsorption has been attributed
to the neutralization reactions between the dissociated Si-O¯ functional
groups with the positive species of copper such as Cu2+ and Cu(OH)+.
Substantial decrease in the amounts of positive charge (AEC) values has been
reported after molybdate (Elhadi et al., 2001), citrate and oxalate (Hanudin
et al., 2000) and phosphate (Johan et al., 1999) adsorption on
the KyP and KnP nano-ball allophane samples. In these published studies, the
decreases in the AEC values were thought to be due to the neutralization of
the positive charges Al-OH2+ by the anionic form of the
compounds adsorbed. In the current research, a small increase in the AEC values
was found for the KyP and KnP samples after copper adsorption at initial concentrations
of 1.60 mM as shown in Fig. 2 and 3. The
slight increase in the AEC values after adsorption was maybe due to in part
to the initial H+ released into the bulk solution after copper adsorption,
which may have reacted with Al-OH functional group to form the new positive
Figure 4 shows the relationship between the amounts of Cu2+
adsorbed and the net change in the CEC (Δ CEC) of the KyP and KnP samples
at the initial copper concentration of 1.60 mM. From the figure it is seen that
there was a linear decrease in the ΔCEC values with the increasing the
amounts of copper adsorbed by the two allophane samples, indicated that the
very high correlation between ΔCEC value and the amounts of copper adsorbed
(r = 0.99**). Also, at any set point within the pH range, the Δ
CEC values of the nano-ball allophane samples were always smaller than the amounts
of copper adsorbed by the two nano-ball allophane samples. For example, the
amount of copper adsorbed by the KyP allophane sample with a lower Si/Al ratio
at the initial solution pH 4, 5 and 6 were 20.0, 24.5 and 39.5 cmol kg-1,
respectively and the corresponding Δ CEC values were 3.8, 4.0 and 4.33
cmol kg-1, respectively.
||Relationship between amounts of copper adsorbed and changes
in CEC. Initial copper concentration was 1.60 mM
The amounts of copper adsorbed by the KnP allophane sample with a higher Si/Al
ratio were greater than that for the KyP (59.2, 63.3 and 68.3 cmol kg-1)
at the initial pH levels of 4, 5 and 6 and the consequent ΔCEC values also
were higher (7.0, 9.2 and 10.9 cmol kg-1, respectively). The higher
decrease in the CEC value of KnP sample was attributed to its higher adsorptive
capacity for copper species than KyP sample with a lower Si/Al ratio even at
the same solution conditions (Ghoneim, 2002).
Mechanism of Change in Charge Characteristics
The reaction between Cu species and nano-ball allophone resulted in new
compound which has different charge characteristics than the original allophone.
By using the detailed chemical structure of allopahne Fig. 1
and molecular orbital calculation, the mechanisms in development of surface
charge on allophone sample upon copper adsorption could be known. The main adsorption
sites of allophone for the copper adsorption are mainly Al-OH, Al-OH2
and Si-OH functional groups. The reaction of CuOH+ with Al-OH2
or Si-OH groups can describe as:
In these equations, copper adsorbed as CuOH+ (monodentate reaction) with Al-OH2 or Si-OH and one proton released into solution caused decreases in pH. The decrease in CEC values upon copper adsorption at the initial concentration of 1.60 mM was attributed to neutralization reaction as follows:
Reactions 3 and 4 describe the specific adsorption of monomeric Zn+2 either with dissociated Si-O¯ or Al-O¯ as (bidentate reaction) and 2 protons were released for each mole of Cu2+ ion adsorbed. The molecular orbital calculations indicated that when allophone model has one dissociated Si-O¯, the monomeric Cu+2 adsorbed strongly to the Si-O¯ group and weakly adsorbed with undissociated Si-OH groups. The O-H bond lengths of the two silanol groups bonded to copper ion was longer than those of the other silanol groups. These results indicated that, Cu2+ has a possibility to accelerate the deprotonation of undissociated Si-OH groups near the adsorption sites. In addition, Ghoneim (2002) reported that the bond length between Zn2+, Cu2+ ions and the O atoms of the Si-OH groups were shorter for Cu2+ than Zn2+. The little increase in AEC values after copper adsorption could be explained by the next reactions:
In reaction 5, part of 2H+ was released after Cu2+ adsorption into the bulk solution may reacted with, Al-OH group to form Al-OH2+ with the positive charge as shown in reaction 6 and the excess of the proton decreasing the pH. The molecular orbital calculations indicated that when copper ions reacted with undissociated Si-OH, dissociation reaction of silanol groups accelerated due to the bonding formation between copper and the O atoms of silanol groups and coincide with the observed pH decrease (Ghoneim, 2002).
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