The presence of heavy metals in aquatic systems can threaten various living
species that depend on such an environment. Metallurgical operations and manufacturing
industries discharge aqueous effluents containing heavy metals which are non
biodegradable. These non biodegradable species tend to accumulate in living
organisms, causing various kinds of metabolic disorders. Innovative methods
for water and wastewater treatment are continuously being developed to remediate
water containing metallic species. However, these methods are unable to achieve
the standards which have been recommended by international water standards bodies
(Garcia-Sanchez and Alvarez-Ayuso, 2002; Bailey
et al., 1999).
In recent years, many natural adsorbents have been investigated for the removal
of heavy metals from water. Calcite, magnetite and dolomite which fall under
the category of carbonate minerals have been investigated for possible use in
the removal of heavy and radio-active metals from aqueous solutions (Zachara
et al., 1991; Papadopoulos and Rowell, 1988).
Carbonate rich minerals are effective at removing heavy metals through a mechanism
of interaction proposed to be a combination between ion-exchange and precipitation
on the carbonate surface (Garcia-Sanchez and Alvarez-Ayuso,
2002). Natural silicate minerals, of which zeolites are a typical form,
have also been investigated as potential low-cost sorbents for removing toxic
heavy metals (Al-Degs et al., 2006; Sheta
et al., 2003). Sorbents can either be organic or inorganic ion-exchangers.
Among the inorganic ion exchangers zeolites are the most abundant. Inorganic
exchangers such as zeolites have useful properties such as resistance to decomposing
in the presence of ionizing radiation or at high temperatures and compatibility
with the environment (El-Kamash, 2008). They also, show
high selectivity towards heavy metal ions which make them suitable for ion exchange
process (El-Kamash, 2008).
Clinoptilolite is the most abundant and cosmopolitan natural zeolite and it
has been widely exploited for its ion-exchange capabilities since, it can easily
exchange its interstitial cations for external cations in solution (Kuronenm
et al., 2006). Natural zeolites, such as clinoptilolite, are able
to lose and gain water in a reversible manner and to exchange their extra framework
cations, both without the change of crystal structure (Rezaei
and Movahedi, 2009). Ion-exchange is made possible by the presence of extra-frame-work
cations which are located in the regular array of channels and cages that constitute
the rigid anionic framework. Cations are bound to the lattice and to water molecules,
which normally fill the zeolite micropores (Dyer and Townsend,
1981). When the zeolite comes into contact with an electrolytic solution,
the exchangeable cations in the zeolite can be removed from their sites and
replaced by ions in the solution. The substitution is stoichiometric and depends
on the parameters provided during experimental procedures.
In addition to high mechanical strength, natural zeolites have good porosity
and high surface area (Inglezakis et al., 2005).
The determination of the exact type of mechanism for metal sorption by natural
adsorbents such as zeolites is a complex procedure (Frimmel
and Huber, 1996). In this study, the chemical and mineralogical characteristics
of South African clinoptilolite were investigated in order to assess the feasibility
of this zeolite as a low cost sorbent for the removal of Cu2+ and
Co2+ from aqueous solutions and mine water solutions. The objectives
of the study were to: assess the removal efficiency and the mechanism of metal
removal, to determine the effect of solution concentration on Co2+
and Cu2+ removal in single and mixed solutions, determine the adsorption
mechanisms, to apply the sorption kinetics and equilibrium studies.
MATERIALS AND METHODS
All chemicals used in this study were of analytical reagent grade and were
obtained from Sigma Aldrich and Merk. The clinoptilolite used was supplied by
Prattely South Africa and was sourced from the Vulture Creek, KwaZulu-Natal
Province of South Africa. The study was conducted between April, 2007 and December
2008. The experimental procedures are as outlined by Nyembe
et al. (2009).
Preparation of clinoptilolite: Original clinoptilolite was crushed with jaw crushers and sieved through screens to a size range of 2.8-5.6 mm. A fraction of this average particle size was rinsed with distilled water and air-dried for 24 h. In addition to original forms, conditioned forms of Southern African clinoptilolite were also used in this study.
Thermogravimetric analysis (TGA): The physical and chemical changes brought about by heat to matter have been scientifically proved to be characteristic to that particular type of matter. There are techniques used in studying these changes and Thermogravimetric analysis (TGA) is one of them. This technique is used to measure the change in mass of a sample in a specified atmosphere as the temperature of the sample is programmed.
For thermal analysis, the TGA (Perkin Elmer Pyris 1) analysis was carried out in this study. The aim was to determine the stability of the zeolite by examining its weight loss with increasing temperatures. The clinoptilolite used was dried at 50°C for 24 h and therefore, it was important to know its thermal stability at very high temperatures. About 10 mg of the original clinoptilolite was used for the analysis. A constant heating rate of 10°C min-1 in air for a range of 50-900°C under a N2 atmosphere was configured.
Infrared spectroscopy: Infra red radiation may be absorbed by covalent
bonds present in a sample resulting in the vibration of the bond at specific
amplitude unique to that type of a covalent bond due to the absorbed frequency.
The absorbed frequency largely depends (among other factors) on the geometry
and the weights of the atoms present in the vibrating covalent bond (Skoog
et al., 1998).
The FTIR technique was used for the HCl-activated, NaCl-activated, KCl-activated and the original forms of clinoptilolite in order to ascertain the effects of chemical conditioning on original clinoptilolite. This was to aid in giving possible explanations about the performances of the clinoptilolite. Dry pellets were prepared by mixing finely milled clinoptilolite (approx. 75 μm) with a bromide binder in a ratio of 1:10 of sample to binder (0.05 g clinoptilolite: 0.5 g binder) and mixing using a pestle and mortar until homogeneity of the mixture was achieved. The pellet was analyzed for its peaks. Results of IR spectra were obtained using a Midac FT-IR 5000 spectrophotometer on CaF2 plates. The IR data are listed with their individual characteristic peaks in wavenumber (cm-1).
X-ray Fluorescence (XRF): For XRF analysis, sample preparation was done by mixing 8 g of dry finely milled (<75 μ) clinoptilolite sample with 20 pellets of HPMA40 binder (approx. 4.1 g) using a milling machine until the mixture was homogeneous. A pressed pellet of the mixed sample was prepared by setting the pressure at 20 tones for 60 sec. The pellets elemental composition was determined using X-ray Fluorescence spectroscopy (XRF, Phillips Magix Pro).
Preparation of synthetic solutions: For AAS analysis solutions of copper and cobalt were prepared by dissolving CoSO4.7H2O and CuSO4.5H2O in deionised water to generate solutions that contained Cu2+ and Co2+ concentrations of 0.0020, 0.0698 and 0.2000 M. These concentrations were arbitrarily chosen on the basis of generating low, middle and high concentrated solutions. These synthetic aqueous solutions were stored at room temperature (approx. 25°C). The samples were used within 48 h after preparation to minimize errors due to precipitation and container plating of the metal ions. The non-mixed Cu2+ and Co2+ aqueous solutions of concentrations 0.0020, 0.0698 and 0.2000 M which corresponded to 0.032, 1.109 and 3.177 g of Cu2+ and 0.030, 1.028 and 2.947 of Co2+ were prepared by dissolving CoSO4.7H2O and CuSO4.5H2O salts in deionised water in 250 mL volumetric flasks. The effects of one cation on the others removal efficiency were studied by varying the metal ion concentration in solution. Studies on the Co/Cu mixed synthetic solutions were done with solutions of copper and cobalt prepared at stoichiometric ratios of Co:Cu -1:1, 1:5, 1:9, 5:1 and 9:1 which corresponded to these concentrations of Co:Cu-0.0020:0.0020 M, 0.0020:0.0698, 0.0020:0.2000, 0.0698:0.0020 and 0.2000:0.0020 M, respectively.
Mine water sampling: In order to assess the applicability of clinoptilolite used in this study for the removal of Cu2+ and Co2+ and other metals from real aqueous solutions, the clinoptilolites capabilities were tested on mine water samples containing other cations and exhibiting a variety of physical parameters. The mine water used was sampled from a gold mining are in Nigel in the Gauteng Province, South East of Johannesburg. The schematic locations of the sampling sites are depicted in Fig. 1. A total of 9 sampling points (Fig. 1) were arbitrarily chosen around the site.
Samples were collected in two instances, before and after rains in 2008. Effluents
from metallurgical operations were collected from all sites. Physical parameters
such as conductivity, pH and turbidity of the water were measured on site per
sampling point and the overall activities noticed around the sampling point
||Site map showing the sampling points around Nigel town
The water samples were drawn within 5 cm below the surface, using glass sampling
bottles. Samples were placed in glass jars and chilled at 6°C before laboratory
analysis which was done within 72 h. The samples were then decanted and assayed
for the presence and content of Co2+ and Cu2+ metal ions.
Assay of samples was done using Atomic Absorption Spectroscopy (AAS Varian 20/20).
Adsorption models of Cu2+ and Co2+ onto clinoptilolite:
Sorption kinetic models can be divided into two main types, namely diffusion-based
models and reaction-based models (Ho et al., 2000).
Diffusion based models External diffusion model: If external diffusion
of metal cations within the diffuse layers outside the sorbent is the rate determining
step then, the Eq. 1 can be fitted into the absorption data
(Lee et al., 1990):
||The initial concentration of adsorbate at time t (mol g-1)
The surface concentration of adsorbate at time t (mol g-1)
The external diffusion coefficient (mol g-1)
||The sorption time
This model showed that the ion-exchange of Cu2+ and Co2+ ions occurred mainly on the external surface of the adsorbent and this can be described by an external diffusion model.
Internal diffusion mechanism: When internal surface and pore diffusion
of metal cations inside the sorbent are the rate determining steps, then the
absorption data can be presented by the Eq. 2 (Ho
et al., 2000):
||The surface concentration of adsorbate at time t (mol g-1)
||The internal diffusion coefficient (mol g-1, min1/2)
||The sorption time (min)
Reaction-based models: A simple kinetic analysis of Co2+ and Cu2+ absorption can be employed using pseudo-first-order equation.
||The surface concentration at equilibrium (mol g-1)
||The surface concentration at time t (mol g-1)
||The pseudo-first-order rate constant (I min-1)
||The time of reaction (min)
Adsorption isotherms and equilibrium studies Langmuir and freundlich: Two of the most commonly used isotherm theories have been adopted in this study, namely, the Langmuir and Freundlich equilibrium isotherm theories. The Langmuir equation can be represented by the Eq. 4:
||The equilibrium concentration of remaining metal in the solution
The amount of a metal adsorbed per mass unit of sorbent at equilibrium
The amount of solution at complete monolayer coverage (mmol g-1)
b (dm3 mmol-1) is a constant that relates to the heat of adsorption.
Freundlich isotherm equation is represented as follows:
X-ray Fluorescence (XRF): The XRF showed a typical mineralogical diffraction pattern of a crystallite with a composition of 70% SiO2, 12% Al2O3, 2% Na2O, 5% K2O, 2% CaO and 2.5% Fe2O3 and traces (0.2%) of TiO2. The X-Ray Fluorescence (XRF) technique revealed the following composition for the alumino-silicate natural clinoptilolite: 0.2% TiO2, 74% SiO2, 1.3% Na2O, 1.1% MgO, 3.8% KO, 1.5% Fe2O3, 1.5% CaO and 12.4% Al2O3..
FTIR analysis: The FTIR spectra in Fig. 2 show the
functionalities that are present in the various forms of clinoptilolite, natural
and activated forms. The wavelength measured ranged between 500 and 4000 cm-1
for the original, 0.04 M and the 0.02 M Hcl-activated forms of clinoptilolite.
||The FTIR spectra for original and HCl-activated clinoptilolite
forms at concentrations of 0.02 and 0.04 M, showing quartz, calcite and
H2O peaks and bands
The distinct stretchings between 300 and 4000 cm-1 are typical of
water adsorption (Madejova, 2003). This shows that water
adsorption and retention by clinoptilolite is increased by HCl activation at
0.02 M concentration At the range of 2000 and 1500 cm-1, the 0.02
M HCl-activated clinoptilolite showed two intensive peaks and yet again the
original and the 0.04 M activated forms showed none. This could be as a result
of 0.02 M activation washes out the non zeolitic impurities present in the original
clinoptilolite as confirmed by XRD, XRF and SEM-EDS. There were peaks observed
for all the clinoptilolite forms at 1558 cm-1 which may be due to
the bending vibrations of adsorbed water. This is expected since given its porous
structure, desiccation of the zeolite at high temperatures 50°C will increase
its hydrophilic (water absorption) properties (Ng and Mintova,
2008). The stretching between 1500 and 1000 cm-1 observed indicates
the presence of a high content of calcite in the sample as confirmed by SEM-EDS.
The strong band at 1341 cm-1 (due to Si-O stretching) is the main
characteristic band for quartz (Al-Degs et al., 2003).
The peaks observed between 1000 and 600 cm-1 are present in all the
forms of clinoptilolite, one characteristic band appears at 836 cm-1
for all the forms. This is the quartz band which is common with zeolites, especially
those of the Heulandite family (Al-Degs et al., 2003).
The absorbed water in zeolites is driven off by heating at temperatures greater
than 450°C without the structure being decomposed (Mondale,
In order to assess the thermal stability of the clinoptilolite, thermogravimetric
analysis was carried out over a temperature range exceeding 800°C as shown
in Fig. 3. In the temperature range from 25-100°C, the
weight loss due to desorption of physically absorbed water for a 100% weight
is rapid at about 2.5% as it is attributed to the loss of loosely bonded water.
||A typical TGA curve of South African clinoptilolite. Individual
points are not showing due to the proximity of points to one another
In the temperature range from 100 to 200°C, the weight loss is about 3%
whereas, between 200 and 400°C it is 1%. Between, 400 and 600°C the
weight loss was 0.5%. The effective total weight loss when 600°C was reached
is 7%. The dehydration rate in the temperature range from 100 to 400°C is
similar to that of other zeolites characterized as heulandite type-II (Elaiopoulos
et al., 2008). The clinoptilolite was found to be very stable over
a wide range of temperatures which is very important since in this study the
zeolite was dried at 50°C for 24 h from preliminary studies.
Adsorption experiments: The ion exchange kinetics of Co2+ and Cu2+ as a function of their concentrations in their respective solutions were studied at room temperature by varying their concentrations with time and keeping all other parameters constant. From the results shown in Fig. 4 and 5, it can be confirmed that the percentage metal removal decreased with an increase in metal concentration in the aqueous solutions.
Effects of solution concentration on cation removal efficiency: In general, natural clinoptilolite appeared to remove the metals more efficiently from dilute solutions than from concentrated ones. There was a 56% Cu2+ removal from the more dilute (0.0020 M) CuSO4 solution (Fig. 4).
There was a 46 and 28% removal of Cu2+ from aqueous CuSO4 solutions whose concentrations were 0.0698 M and 0.2000, respectively. On the other hand, there was a 36% removal of Co2+ from a CoSO4 synthetic solution whereas, a 14% removal of Co2+ from the 0.2000 M CoSO4 and a 27% removal of Co2+ from a 0.0698 CoSO4 aqueous solution were obtained as shown in Fig. 5.
Percentage removal for Cu2+ and Co2+ was observed to
decrease with increasing metal concentration in the aqueous solutions.
||Cu2+ removal from 0.0020, 0.0698 and 0.2000 M CuSO4
synthetic solutions using original clinoptilolite
||Co2+ removal from 0.0020, 0.0698 and 0.2000 M CoSO4
synthetic solutions using original clinoptilolite
Similar results were obtained with Cu2+, Co2+, Zn2+
and Mn2+ removal from their aqueous synthetic in a study by Erdem
et al. (2004).
Effects of Co/Cu mixing on removal efficiency of Co2+ and Cu2+ removal: The concentration ratios of 1:1 Co:Cu showed the highest removal of Co2+ (40%) and the least removal of in Cu2+ (37%) shown in Fig. 6 and 7, respectively.
The highest Cu2+ removal recorded was 35% and was recorded with a 1:9 Co:Cu ratio. The lowest removal efficiency recorded for Co2+ was 26% and was recorded with the 9:1 Co:Cu synthetic solution where the Co2+ concentration was at its highest while Cu2+ recorded a removal efficiency of 23% with the 1:1 Co:Cu ratio. Another solution where, Co2+ concentration was higher than that of Cu2+ (5:1) also recorded only 31% removal.
Mine water: From the results in Table 1 it can be
deduced that the degree of removal must be dependent upon the complexity of
the aqueous solutions matrix since, the clinoptilolite was observed to
be capable of effective removal of both Cu2+ and Co2+.
The concentration levels of copper and cobalt ions at the various sites in the
sampling zone vary. In site HVH, site A had copper concentration levels exceeding
20 ppm. Since, the Nigel mine is a gold mine one would generally find copper
in the soil samples. Since, the sampling was also done during the rainy period,
most of the copper had been washed into the nearby small water table where the
sampling was carried out.
||Co2+ removal from Co/Cu mixed synthetic solutions
of different Co:Cu concentrations using original clinoptilolite
||Cu2+ removal from Co/Cu mixed synthetic solutions
of different Co:Cu concentrations using original clinoptilolite
When considering all the sites where copper and cobalt were found, it can be
noted that there was quantitative adsorption of both copper and cobalt, in all
cases exceeding 95% removal. Although, there were other ions, such as silver
ions present in solution, this did not seem to affect the adsorption capability
of the clinoptilolite.
Adsorption models of Cu2+ and Co2+ onto clinoptilolite: It can be noted that for Cu2+ the kF values (external diffusion), are much greater than those observed in kd values (internal diffusion) in Table 2 and 3 under the same conditions.
This indicates the dominance of the external diffusion model for Cu2+ adsorption regardless of the concentrations of Co/Cu or Si:Fe in solution were. On the other hand, the kF values for Co2+ adsorption were only lower where Co:Cu was in a 1:1 concentration ratio and were higher than kd values in all the other solutions, regardless of the amount of Si/Fe added. This means that the Si/Fe added did not influence the type of diffusion model of the two ions (Cu2+ and Co2+). Therefore, the external diffusion model was dominant one.
|| Co2+ and Cu2+ AAS assay before and
after adsorption experiments
|| Parameters for Cu2+ and Co2+ for external
(KF) and internal (Kd) diffusion models in the synthetic
||Parameters for Cu2+ and Co2+ for external
(KF) and internal (Kd) diffusion models in the mine
||Plot of Co2+ internal diffusion model of time against
Considering, the mine water samples and the sorbent used in Table 3, it can be inferred that Cu2+ was still adsorbed externally. However, Co2+ appeared to be mainly adsorbed internally since, the kF values for external diffusion were lower than the kd values.
In general the external diffusion model kF values show good correlation
with the sorption data obtained with the synthetic solutions where, high correlation
coefficients were obtained, thus indicating that the adsorption of Co2+
and Cu2+ was mainly a surface phenomenon that occurs on the external
surface of the clinoptilolite. This was further confirmed by a plot of the Co2+
internal diffusion model of concentration (C) versus time (t). At higher concentrations,
the competition for the surface active sites on the clinoptilolite is high and
consequently lower sorption rates were obtained for both Cu2+ and
Co2+. The linear plot shown in Fig. 8 indicates
the dominance of internal diffusion alongside external diffusion since it does
not pass through the origin.
Adsorption isotherms for Co2+ and Cu2+ Freundlich
isotherms: Even though, high R2 values were obtained for Cu2+
and Co2+ in Fig. 9 and 10, respectively,
the values indicate higher deviations from linearity using the Freundlich isotherm
model for describing Co2+ and Cu2+ compared to Langmuir
models in Fig. 11 and 12.
From the Freundlich isotherm, the KF value obtained for Cu2+
and Co2+ were 2.534 and 2.7188, respectively (Fig.
13, 14). The R2 values obtained were Cu2+
and Co2+ 0.9445 and 0.9006, respectively while, the 1/n (intercept)
values obtained for the two ions were 0.4406 and 0.7185.
||Graph of Cu2+ from the Co/Cu mixed solutions shows
first order kinetics with a correlation value of R2 = 0.8203
||Graph of Co2+ from the Co/Cu mixed solutions shows
first order kinetics with a correlation value of R2 = 0.9902.
solutions shows first order kinetics with a correlation value of R2
From the results of R2 (0.9445 for Cu2+ and 0.9006 for Co2+) obtained the accession that Cu2+ was easier to remove from these solutions can be made.
Langmuir isotherms: The Langmuir isotherms were observed to fit the ion-exchange data of Co2+ and Cu2+ more than the Freundlich did. The Langmuir gave higher correlation values than did the Freundlich isotherms and from these results it can be deduced that the ion-exchange of Co2+ was more efficient than that of Cu2+.
||Freundlich plot of Cu2+ obtained from the non-mixed
solutions a correlation value of R2 = 0.9445
||Freundlich plot of Co2+ obtained from the non-mixed
solutions a correlation value of R2 = 0.9006
||Langmuir plot of Cu2+ obtained from the mixed Co/Cu
solutions with a correlation value of R2 = 0.9863
||Langmuir plot of Co2+ obtained from the mine water
with a correlation value of R2 = 0.9998
FTIR analysis Effects of solution concentration on removal efficiency:
Athanasiadis et al. (2004) reported that high
metal solution concentrations showed low ion exchange rates compared to solutions
of lower concentration. These researchers concluded that this was due to the
high concentration of the counter ion in the solution which was generally true
for all the analytes. It is possible that more of the smaller size range of
particles was present in this batch. Small particle size increases exchange
efficiency due to surface area to volume ratio. The cation being exchanged moves
a shorter distance in smaller grains than in larger ones thus speeding up the
exchange rate. In Fig. 4 and 5 the percentage
removal for Cu2+ and Co2+ was observed to decrease with
increasing metal concentration in the non-mixed aqueous solutions.
The results in Fig. 4 and 5 indicate that
energetically less favourable sites become involved with increasing metal concentrations
in the aqueous solution (Zachara et al., 1991).
Heavy metal uptake is attributed to different mechanisms of ion-exchange process
as well as to the adsorption process. The phenomena of ion-exchange is also
said to be dependent on the concentration of the aqueous solutions (Zachara
et al., 1991). This is such that the more dilute solutions upload
mainly by means of ion-exchange which proves to be a fast process, while the
more concentrated solutions also use precipitation to upload, which is a slow
process (Papadopoulos and Rowell, 1988). During the
ion-exchange process, metal ions migrate not only through the pores of the zeolite
mass, but also through channels of the zeolite lattice structure and replace
exchangeable cations accessible to them in the clinoptilolite (McBride,
1980). In concentrated solutions, the cation diffusion process is faster
through the pores and is slowed down when the ions move through the smaller
diameter channels of the exchanger such that the ion-exchange process takes
place at a slower pace (McBride, 1980).
The ease of removal observed with the mixed 1:1 Co:Cu solutions must have been
due to that the Cu2+ formed more bulky and stable complexes with
the water molecules in solution which probably then resulted in the availability
of Co2+ ions for sorption. The discrepancies observed in the metal
removal efficiencies could be due to the original zeolites heterogeneous
structure since there was no modification applied to it. It is also possible
that the clinoptilolite surface and pore openings were partially covered by
dust produced during the crushing of the clinoptilolite, resulting in pore clogging
which led to smaller ion-exchange capacity and slower ion-exchange rates (Inglezakis
et al., 1999). Pore clogging by fine particles, which can be reduced
by chemical conditioning of the zeolite, has also been reported as a possible
cause of smaller ion exchange capacity and slower exchange rates (Mondale
et al., 1995). According to literature, pore clogging can affect
the ion-exchange capacity by up to 15%. The distribution of the minerals in
original zeolite is non homo-ionic and is very likely to cause discrepancies
in metal loading. This could also mean that the locations of exchangeable ions
are not ideally distributed within the zeolite, such that there are delays in
the ion exchange process due to exchangeable ions being hindered with other
ions not partaking in the ion-exchange process. The rate at which these metal
ions were being up-loaded into the clinoptilolite was such that the more dilute
solutions were at a high rate than in the more concentrated ones. Such was the
case because more efficient utilization of the adsorption capability of the
ion-exchanger is expected due to a greater driving force by a higher concentration
gradient pressure (Erdem et al., 2004).
Possible adsorption mechanisms of Co2+ and Cu2+:Different
physico-chemical reactions such as dissolution, ion-exchange, adsorption and
possibly surface precipitation are known to prevail in a heavy metal solution-clinoptilolite
interaction. Investigation of this interaction is important for their application
in environmental chemistry such as in heavy metal removal from industrial waste
waters and in acid mine drainage remediation. Surface ion-exchange is one of
the processes in a heavy metal solution-clinoptilolite interaction. The forces
involved in this process range from weak vander waals forces and electrostatic
outer-sphere complexes, such as strong chemical interactions. Chemical interactions
can include inner-sphere complex formation that involves a ligand exchange mechanism,
covalent bonding, hydrogen bridges and steric effects (Doula
and Ioanoou, 2003). During the initial stages of surface ion-exchange, outer-sphere
complexes form on the external surface sites of the sorbent. The outer-sphere
complex formation process involves ion-exchange reactions between metal ions
in the aqueous solution and surface counterbalance cations on the ion-exchanger.
As the metal concentration increases on the surface of the ion-exchanger, metal
ions are forced into the internal surface sites, thus, forming inner-sphere
complexes that form during internal ion-exchange.
An increase in the amount of metal cations absorbed on the surface of the clinoptilolite
to a higher surface coverage during adsorption causes the metal ions to exchange
on the clinoptilolite surface. At low surface coverage, complex formation tends
to dominate. As surface coverage increases the formation of distinct aggregates
on the absorbents surface occurs and as surface loading increases surface
ion exchange become the dominant mechanism. Surface ion-exchange is a method
of metal loading common with more concentrated solutions than dilute ones. The
predominant mechanism of sorption of metal ions from dilute solutions is internal
ion-exchange, although both mechanisms occur simultaneously at different rates
in the same solution (Scheidegger and Sparks, 1996).
Dissolution also takes place in a heavy metal solution-clinoptilolite interaction.
The complexes formed on the zeolites surface sites can cause dissolution
of Al3+ and/or Si4+. The tendency of a sorbents
surface to dissolve depends on the type of surface species that make up an inner-sphere
complex. A highly electronegative ligand such as a halide e.g., Cl¯ ions
facilitates the detachment of a central metal ion and enhances dissolution.
This is due to the electron density shift from the ligands towards the central
metal ion at the surface (Doula and Ioanoou, 2003).
This excess of electron density brings the negative charge into the coordination
sphere of the Lewis acid centre and simultaneously enhances the surface protonation
and can labilize the critical Si-O lattice bonds of the clinoptilolite, thus
causing detachment of the central metal ion. This may then result in the total
collapse of the zeolite structure (Scheidegger and Sparks,
Calcite is a principal component of the clinoptilolite as confirmed by FTIR
in this study and therefore the cation exchange capacity may be due to the targeted
metal ions affinity to the surface of calcite (Bolto and Pawlowski,
1987). The ionic radius of M2+ cations and metals of ionic radius
close to that of Ca2+ display stronger displacement than other metals
during ion-exchange. This can provide a reasonable explanation for the higher
adsorption capacity of Co2+ compared to Cu2+. During ion-exchange,
the cations saturate natural zeolites. This means that the metal ions exhaust
all available possible sites of the zeolite and further adsorption can only
be possible at new clinoptilolite surfaces. The cation exchange capacities of
Co2+ and Cu2+ indicate the selectivity to be in favour
of Co2+ over Cu2+ in the Co/Cu synthetic solutions. These
metal cations are present in solution as hexa-aqua (Cu2+) and tetra-aqua
(Co2+) complexes with six and four, respectively, surrounding water
molecules in their non-mixed solutions (Jama and Yucel,
1990). However, in multi-component systems, such as the Co/Cu solutions,
the complexes formed by these ions may not be as simple as the ones in the non-mixed
cation solutions. They may be attached to other cations which may also be attached
to others leading to even bulkier complexes.
The occupation of active sites of clinoptilolite during ion-exchange is a competitive
process whereby one cation is more favoured over the other. This could be attributed
to cation-cation interactions as well as to the water molecules that surround
the cations in solution. Inglezakis et al. (2005)
documented that selectivity of a zeolite for one ion over the other in a matrix
is a result of physico-chemical and stereo-chemical factors which are hydrated
radii, hydration enthalpy of the cation and the space requirements in the micropores
of clinoptilolite in connection with the incoming ions. The rate of Cu2+
and Co2+ ions-exchange on clinoptilolite was determined as a function
of the initial metal concentrations. The plots show that the kinetics of adsorption
of Cu2+ and Co2+ consisted of two phases; an initial rapid
phase where the adsorption process was fast and contributed significantly to
equilibrium uptake and a slower second phase with a relatively small contribution
to the total metal adsorption. The first phase is the instantaneous adsorption
stage or external surface ion-exchange. The second phase is the gradual adsorption
stage is the intra-particle diffusion which controls the adsorption rate and
finally the metal uptake reaches equilibrium.
Kinetic models: The kinetic models outlined in Eq. 1-3
were applied to the kinetic data of Co2+ and Cu2+ adsorption
by the clinoptilolite forms. The internal diffusion theorem states that external
diffusion is the dominant adsorption mechanism if the straight line obtained
from the equation does not pass through the origin. This was true with the plot
obtained for the Co2+ plot. The plot indicates the dominance of external
diffusion at the earlier stages of interaction. Due to the heterogeneous nature
of the adsorbent and the presence of active materials, i.e., calcite and zeolite
minerals, the molecular movement of Co2+ and Cu2+ deep
inside the sorbent particles is likely to be restricted though not totally excluded.
The external and internal diffusion models confirmed that external ion-exchange
occurring mainly on the surface of the clinoptilolite. The KF values
were observed to increase with a decrease in the initial cation concentration,
which can be attributed to the lower competition among the cationic species
for the sorption surface sites at lower concentration as observed with the mine
water samples in Table 3 (Lee et al.,
1999; Ho et al., 2000). At the early stages
of adsorption, the process appears to be largely controlled by external diffusion
mechanism. This serves to confirm that external ion-exchange was the controlling
mechanism during Co2+ and Cu2+ adsorption. The correlation
values of Co2+ and Cu2+ in the first order kinetics plot
(Fig. 9, 10) show that the high correlation
of this model to the sorption data is an indication that Eq. 3
gives an accurate estimation for the equilibrium capacity for the two cations.
The adsorption mechanism was confirmed to follow first order kinetics.
Adsorption isotherms: KF and n are the Freundlich parameters
that show the favorability of the type of adsorption of a particular ion under
study (Frimmel and Huber, 1996). If the slope is high
and the value of n is high, it is a clear indication that adsorption intensity
is favorable over a wide range of the concentrations studied, while a steep
slope where the value of n is low is an indication that the adsorption intensity
is favorable at high concentrations but much less at dilute concentrations (Mohan
et al., 2006). A high value of the intercept KF, indicates
high adsorption potential (Mohan and Singh, 2000; Hasany
et al., 2002). In this study, the KF values obtained were
quite low and this indicates a low adsorption potential and the adsorption potential
of the clinoptilolite over the recorded time was indeed low. The 1/n values
for the systems in this study fall in the range of 1/n<1 ( 0.4406 for Cu2+
and 0.7185), which indicates minimal metal ion adsorption as confirmed by the
percentages removed by the clinoptilolite. A lower description for Cu2+
adsorption data was evident (R2 = 0.9006) according to the b mol
g-1 parameter. Using the Langmuir model, the maximum adsorption capacity
for the metals can be estimated as Co2+ >Cu2+ from
the Co/Cu mixed solutions.
The natural clinoptilolite investigated in this study is an effective sorbent for removing Cu2+ and Co2+ ions from aqueous solution. The natural sorbent is especially suited to retaining Co2+, as Co2+ was found to be more mobile than Cu2+. It was also found that mixing the cations in solution greatly influences metal removal efficiency and this largely depended on the dissolved ratios of these metals. The adsorption data proved that sorption is faster in dilute solutions than in concentrated solutions. It was also found that the main mechanism of adsorption is ion-exchange. Kinetic data showed good correlation to a pseudo-first order and external diffusion models which indicated that sorption of Co2+ and Cu2+ occurred on the external surface of the sorbent with internal diffusion being less significant in the experimental systems investigated. The Langmuir equation fitted the kinetic data better than the Freundlich equation and it also indicated that Co2+ was adsorbed much faster than Cu2+. This implies that the ion-exchange process occurred more on the surface of the clinoptilolite than it did in its inner sites.