Removal of copper and cobalt from aqueous solutions using
natural clinoptilolite
A.F. Mulaba-Bafubiandi,1 B.B. Mamba,2 D.W Nyembe2
1: Minerals Processing and Technology, Department of Extraction Metallurgy, Faculty of
Engineering and the Built Environment, University of Johannesburg, PO Box 526, Wits 2050
2: Department of Chemical Technology, Faculty of Science, University of Johannesburg, PO Box
17011, Doornfontein, South Africa
Email: amulaba@uj.ac.za
Abstract
Clinoptilolite, a natural zeolite, was used in the removal of copper and cobalt from synthetic
solutions and mine water from gold mines in Nigel (Gauteng Province). Removal isotherms
for both copper and cobalt synthetic solutions were constructed. In the single-salt solutions,
uptakes of 79% and 63% for Cu and Co were recorded with 0.02 M and 0.04M HCl
activation, respectively. In the mixed salt solutions, Co and Cu both recorded 79% uptake
with 0.02 M activation. The 0.04 M activation gave percentage removals of 79% and 77% for
Co and Cu respectively. Removal of copper and cobalt from mine water was found to be
100% for certain sites. On the whole, clinoptilolite removal efficiency was not affected by
the presence of more than one heavy metal in dilute solutions which demonstrates its
potential application in the treatment of effluents contaminated with mixed heavy metals.
Introduction
Water pollutants emanating from metallurgical operations have been highlighted as one of
the avenues by which groundwater can be polluted, thus threatening valuable water sources.
The need for clean drinking water is vital for the promotion of good health and with it comes
the challenge to provide safe and high quality drinking water. Groundwater is one of the
major sources of drinking water in South Africa. Unfortunately, these fresh water sources are
faced with an eventuality of vanishing due to the plight of pollution. The need for recovery
and purification of these waters is great if they are to be preserved. A number of purification
methods are currently employed in the water purification industry and these include ion
exchange, ultrafiltration, phytoextraction, electrodialysis and reverse osmosis (Geselbracht,
1996; Schnoor 1997). However, industrial wastewaters often contain a cuisine of metals that
may impose cost implications when it comes to purifying it.
Out of all the water purification technologies, ion exchange is the most commonly used
(Inglezakis et al., 2005). The ion exchange reaction occurs between two or more phases, one
of which is liquid (solution or melt), which exchanges two or more ions (cations or anions),
more or less strongly bound to each phase. The ion exchanger may be a resin, activated
carbon or a zeolite. Zeolites may be synthetic or natural aluminosilicates that have a
framework structure that encloses cavities and tunnels which are occupied by freely moving
hydrated cations that make ion exchange possible(Inglezakis et al., 2005).
Clinoptilolite is the most abundant and cosmopolitan zeolite. It has been widely exploited for
its ion exchange capabilities since it can easily exchange its interstitial sodium for external
cations in solution (Kuronen et al., 2006). Natural zeolites often do not perform the best way
that industrial standards require. When such is the case, chemical conditioning of zeolites to
improve their performance is employed. The chemical removes certain cations from zeolites
that may hinder ion exchange and locate easily exchangeable ones. It has been previously
reported in literature (Zamzow et al., 1992) that sodium type natural clinoptilolite favours the
exchange of Cu2+ over Co2+.
Herein, we report the ability of South African natural and modified clinoptilolite at removing
heavy metals such as copper and cobalt from synthetic solutions and mine water using the
ion exchange capability of clinoptilolite. The effect of concentration of the aqueous solutions
and the degree of HCl-activation of the clinoptilolite in its removal efficiency of the
clinoptilolite was studied. The possibility of establishing the selectivity of clinoptilolite
towards copper and cobalt in the synthetic solutions and mine water was also explored.
Materials and methods
Zeolite’s, source, characterization and conditioning
The natural clinoptilolite used in this study was sourced from the Vulture Creek, KwaZulu
Natal province of South Africa. In preparation for mineralogical studies and characterization,
the clinoptilolite was crushed and milled into powder of average particle sizes approximately
75µm. The powder was then examined using an X-ray Powder Diffractometer (XRD)
Phillips X’pert Model 0993 to determine its composition. Its elemental composition was
determined using X-ray Fluorescence spectrometry (XRF, Phillips Magix Pro) while their
surface area was analyzed using BET (Tristar 3000). The measurements were done under
nitrogen atmosphere. Prior to porosity and surface area analysis, 2 g of sample was first
degassed and nitrogen gas was flushed through for 4 hours at 120oC. Clinoptilolite grains of
size in the range of 2.8 mm – 5.6 mm were used for adsorption studies. A fraction of these
was treated in HCl at concentrations of 0.02 M and 0.04 M at room temperature over a period
of 8 hrs. The clinoptilolite was then washed in deionized water to remove the fine fractions
and then dried in the oven at 50 ºC for 24 hr.
Synthetic solutions
For the initial studies, solutions of Copper and Cobalt were prepared by dissolving
CoSO4.7H2O and CuSO4.5H2O in deionized water of pH 6.5, respectively. This procedure
helped to generate single-component solutions with copper and cobalt of concentrations 0.07
M, 0.33 M and 0.66 M. Studies on the copper and cobalt mixed 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 9:5 which corresponded to these concentrations 0.07:0.07, 0.07:0.33, 0.07:0.66,
0.33:0.07 and 0.66:0.07 respectively. These solutions were assayed using atomic absorption
spectroscopy (AAS), model Varian Spectra (20/20).
Batch adsorption studies on synthetic solutions
The ion exchange processes of Copper and Cobalt on the clinoptilolite were conducted at
room temperature. Glass columns of 2 cm diameter and 30 cm of length were pre-loaded
with 25 g of either as-received natural clinoptilolite or HCl-activated clinoptilolite.
A
volume of 25 mℓ of the prepared copper and cobalt bearing solutions of desired
concentrations were passed through each of the two types of zeolites. These were afforded
the same solution-zeolite contact time. The resultant solutions after passing through the
zeolite-paked column were assayed using Atomic Absorption Spectroscopy (AAS) in order
to ascertain the zeolite’s removal efficiency. The flame type used was air-acetylene. The
adsorption wavelengths for the two metals are Cu (324.7 nm) and Co (240.7 nm). Standards
of 1000 ppm, 2000 ppm, 3000 ppm were then prepared and a calibration curve was drawn
using these standards. Dilution was applied stoichiometrically where the concentrations of
the unknown solutions of copper and cobalt exceeded the standards’ concentration range of
the standards.
The metal removal percentage from the solutions and distribution ratio were calculated as
follows:
% Uptake = Co – Cf x 100,
Co
(1)
Co and Cf are the initial and final metal concentrations, respectively.
Kd = Amount of metal in adsorbent x V
Amount of metal in solution
m
(2)
In this study V/m = 25ml/25g = 1
V is the volume of the solution (ml) and m, is the weight of the adsorbent (g). In this study,
the percentage adsorption and Kd can be correlated by the following equation (Khan et al.,
1995).
% adsorption = 100Kd
Kd + 1
(3)
Sampling and storage of mine water
Mine water was sampled from Nigel mining area in the Gauteng province. The samples were
stored in the refrigerator to preserve them prior to sampling.
Ion exchange of mine water samples using clinoptilolite
The ion exchange process of mine water on the clinoptilolite was achieved by allowing 25 g
of clinoptilolite to come into contact with 25 mℓ of previously assayed mine water for 60
minutes. The subsequently treated mine water was assayed using AAS and results were
tabulated.
Results and discussion
Clinoptilolite characterization
Results of the X-Ray Fluorescence (XRF) technique are presented in Table 1. This study
showed that clinoptilolite contained exchangeable ions of sodium, potassium, calcium and
magnesium. This zeolite has a Si/Al ratio of 5.96 (mol/mol). The corresponding ratio of (Na
+ K)/Ca is 3.4. The XRF also showed that this is a high silica clinoptilolite enriched with
Mg, K and sodium. The XRD plot in Figure 1 shows 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.
Table 1: The composition of the alumino–silicate natural clinoptilolite as revealed by XRF.
Ion in clinoptilolite
% Abundance
Ti2+
0.2%
Si2+,
Na+
Mg2+
K+
Fe3+
Ca2+
Al3+
74%
1.3%
1.1%
3.8%
1.5%
1.5%
12.4%
24 000
12000
30 000
10 000
8000
-1
Intensity (s )
10000
6000
4000
2000
0
0
20000
40000
60000
80000
100000
2 (°)
Bragg angle 2 θ (Cu K
α)
Figure 1: Mineralogical pattern of the natural clinoptilolite.
Metal removal from synthetic solutions
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. The results are shown in Figure 2 and 3.
“as-rec
Percentage removal decreased with increase of metal concentration in the aqueous solutions.
This may be due to the fact that particles always move faster when they are less packed, thus
they move faster in dilute solutions and slowly in concentrated solutions.
0.07M
0.33M
0,66M
30
0.07M
0.33M
0.66M
25
40
35
30
25
15
% removal
% removal
20
10
20
15
10
5
5
0
0
0
10
20
30
40
50
-5
60
0
10
20
Time (min)
30
40
50
60
Time (min)
Figure 2. Percentage removal of copper
from its sulphate solutions of 0.07M,
0.33M and 0.66M by clinoptilolite at
0.02M HCl 30 minutes activation.
Figure 3. Percentage removal of cobalt
its sulphate solutions of 0.07M, 0.33M
and 0.66M by clinoptilolite at 0.02M
HCl 30 minutes activation.
Metal removal from the double-component systems
The copper/cobalt mixed solutions exhibited results similar to the single-salt solutions in the
sense that removal efficiency was high with dilute solutions than it was with concentrated
osolutions. The results are shown in Figure 4, 5, 6 and 7. High uptakes ranging between 70 –
79% were recorded with both Cu and Co removal in the concentration ratios of 0.07:0.07 M.
80
70
50
40
30
60
50
40
30
20
20
10
10
0
10
20
30
40
50
60
Time (min)
Figure 4 : Co removal from double
component system using 0.02 M HClactivated clinoptilolite
1:1
1:5
1:9
5:1
9:1
70
% Removal
60
% Removal
80
1:1
1:5
1:9
5:1
9:1
0
10
20
30
40
50
60
Time (min.)
Figure 5 : Cu removal from double
component system using 0.02 M HClactivated clinoptilolite
80
70
70
60
% removal
% Removal
60
50
40
30
1:1
1:5
1:9
5:1
9:1
80
1:1
1:5
1:9
5:1
9:1
50
40
30
20
20
10
10
0
0
10
20
30
40
50
60
Time (min.)
Figure 6 : Co removal from doublecomponent system using 0.04 M HClactivated clinoptilolite.
0
10
20
30
40
50
60
Time (min.)
Figure 7 : Cu removal from doublecomponent system using 0.04 M HClactivated clinoptilolite.
In solutions where there was more cobalt than copper by ratio (Co:Cu = 9:1 and 5:1) there
seems to be difficulty in removing either of the metals. Their removal efficiencies were
reduced. It is possible that there is an intermediate complex that is formed between Co, Cu
and water that is too bulky (Mortimer, 1986) to favour fast removal of these metals from the
aqueous solution. The presence of Cu in high concentrations (Co:Cu = 1:9 and 1:5) in the
double-component solution favours the removal of both Co and Cu. This is in contradiction
with the notion that less concentrated metal solutions favour fast removal of that specific
metal than concentrated solutions.
Removal from mine water samples
In the mine water sample, however metal uptake was highly efficient for both copper and
cobalt. The results are shown in Tables 2 and 3. Only the results for sites that recorded the
presence of these metals are shown. It was observed that the selectivity of the exchanger
towards the cation increases with increasing dilution of the aqueous solution. The mine water
results also confirm this.
Table 2: Co removal from mine water
Name of site
Nigel river
Big Dam
HVH mine site A
Shaft No 2 A
B
C
Piet farm A
Artisanal point
Conc. before
removal
(ppm)
2
1
8
2
2
1
1
8
Conc. after
removal with
‘as-received’
clinoptlolite
(ppm)
1
0
3
1
1
0
0
4
Conc. after
removal with
0.02 M HClactivated
clinoptilolite
(ppm)
0
0
1
0
0
0
0
0
Conc. after
removal with
0.04 M HClactivated
clinoptilolite
(ppm)
1
0
3
0
0
0
0
5
Table 3: Cu removal from mine water using ‘as-received’ and activated clinoptilolite
Name of site
Nigel river
Big Dam
HVH minesite A
Shaft No 2A
B
C
Piet farm A
C
D
E
Artisanal point
Conc. before
Conc. after
removal (ppm) removal with
‘as-received’
clinoptlolite
(ppm)
5
3
20
2
2
4
6
3
2
6
2
4
1
16
1
1
3
2
1
2
2
1
Conc. after
removal with
0.02 M HClactivated
clinoptilolite
(ppm)
1
0
2
0
0
0
1
0
0
0
1
Conc. after
removal with
0.04 M HClactivated
clinoptilolite
(ppm)
2
1
5
1
1
3
1
1
1
1
1
The selectivity of a zeolite for one ion over the other in a matrix is a result of
physicochemical and stereochemical factors. These are hydrated radii, the hydration enthalpy
of the cation and the space requirements in the micropores of clinoptilolite (Inglezakis et al.,
2005). Every metal cation that is dissolved in water possesses a hydrated layer which has a
characteristic thickness and degree of stability. It has been proven that ions with large radii
often show large rejections while smaller ions are often favoured by exchangers (Tansel et
al., 2006). Cu2+ is a smaller ion with a small hydrated radius than Co2+ and therefore Cu2+ is
highly favored by clinoptilolite within the parameters in this study.
It is also known that the hydration enthalpy of Cu2+ is low due to the number of water
molecules solvating it (Carrillo-Tripp et al., 2006). The easy removal of copper compared to
cobalt can also be attributed to the geometry of the hydrated copper ion (Inglezakis et al.,
2005). Cu2+ forms a square-planar complex in water, [Cu(H2O)4]2+, while Co2+ forms a
tetrahedral aqua-complex, [Co(H2O)6]2+ due to
its hexavalent ion. The square planar
complex is less bulky and therefore moves easily in solution compared to the bulky
tetrahedral aqua-complex. However, it cannot be ruled out that ion exchangers generally have
greater affinities for ions with increasing valences. Among ions with the same charge higher
affinities are observed for heavier ions. This is because heavier ions have a smaller hydrated
radius and this property makes them to be easily picked by an ion exchanger (Carrillo-Tripp
et al., 2006, Inglezakis et al., 2005).
The strength of the exchange sites on the zeolite also affects the exchange rate. Natural
clinoptilolite is weakly acidic hence the rate of ion exchange in clinoptilolite is expected to
be high. This is even more so with the acid activated clinoptilolite. However, the
concentration of the acid used to modify the zeolite has to be monitored since it can reduce
the pore volume and consequently lower the exchange ability as observed in BET results
shown in Table 1.
Effect of solution concentration
In general, natural clinoptilolite appeared to remove the metals efficiently from dilute
solutions than concentrated ones. Athanasiadis and Helmreich (2004) documented that high
metal solution concentrations showed low ion exchange rates compared to low
concentrations. They concluded that this was due to the high concentration of the counter ion
in the solution which was generally true for all the analytes. However, an outliar was
observed with Co removal from the cobalt mixed soution using 0.04 M HCl-activated
clinoptilolite (Figure 6). This must have been due to an unevenly mixed size propotion of
the zeolite used. It is possible that more of the smaller size range was present in this batch. A
small particle size increases exchange efficiency due to surface area : volume ratio. The
cation being exchanged travels a shorter distance in smaller grains than in larger ones thus
speeding the exchange rate.
The mine water samples were the most dilute of all the analytes (see Tables 1 and 2). There
was total (100%) removal with some water samples depending on the site from which they
were obtained. However, the fact that mine waters are multicomponent systems cannot be
ruled out since the species present in such systems behave in unpredictable ways (ref). A full
assay of the water and study of the behaviour of such a system is required to draw concrete
conclusions.
Effect of HCl conditioning
BET and SEM analyses revealed changes in the zeolite structure. A decreasing pore volume
with increase in HCl concentration was revealed by BET. Analyses of 0.00 M (“asreceived”), 0.02 M, 0.04 M and 0.10 M HCl-activated clinoptilolite revealed that there is a
decrease pore volume and surface area as the HCl concentration increases. The results of the
surface are and pore volume measurements are shown in Table 3.
The decrease in pore volume is expected to hinder ion exchange. The BET results agree with
the results obtained for the performance of the clinoptilolite where 0.02 M HCl-activated
zeolite was the best performer i.e. high percentage extractions were obtained with this form
of zeolite than those obtained with 0.00 M, 0.0 0.04 M concentrations. The 0.02 M acidactivation concentration also did not alter the pore volume of the clinoptilolite. There were
however, some morphological changes brought about by acid activation on clinoptilolite
structure when the two SEM images are compared. The SEM image in Figure 9 shows a
more open stucture of the HCl-activated clinoptilolite’s surface compared to the unactivated
zeolite in Figure 8. This could explain why the acid-activated clinoptilolite performed better
than the “as-received” zeolite. However, in the copper/cobalt mixed solutions were less
affected by these changes in the zeolite structure. The highest uptake of the 1:1 concentration
ratio with 0.04 M HCl-activation was >70%. This can be attributed to the fact that there is
free movement in dilute solutions (Ouki and Kavannagh, 1999). The 0.04 M HCl-activation
concentration in the 1:1 did not affect the metal uptake.
Figure 8. ‘As-received’
clinoptilolite at X2000
magnification
Figure 9. HCl activated
clinoptilolite at X2000
magnification
In the non-mixed salt solutions activated clinoptilolite generally performed better as a metal
up-taker than the “as-received” zeolite. The “as-received” clinoptilolite showed capacity to
remove 37% Cu and
39% Co. Uptakes of 79% and 63% for copper and cobalt were
recorded with 0.02 M and 0.04M HCl activation, respectively. In the mixed solutions, both
cobalt and copper recorded 79% uptake with 0.02 M activation. The 0.04 M activation gave
percentage removals of 79% and 77% for Co and Cu respectively.
It has been documented that acid treatment of natural clinoptilolite improves its adsorbent
properties (Korkuna et al., 2006, Inglezakiz et al., 2001, Athanasiadis et al., 2005). This is
due to decatination, dealumination and dissolving of amorphous silica fragments blocking the
channels. The study by Korkuna et al (2006) also revealed that there is a change on
clinoptilolite structure after acid treatment. Dilute acid activations improved the removal
capacity of the clinoptilolite’s. Studies by Vasylechko et al (1999), and Hernandez (2000) are
also confirm this finding.
Conclusions

Kinetic isotherms for Cu and Co exchange on ‘as-received’ and natural clinoptilolite
were presented for both single- and double- component systems.

Cu was easier to remove than Co from the single-component systems (79%).

Co and Cu were both easy to remove from the double-component systems (79%).

0.02 M HCl-activated clinoptilolite was the best performing form for both Co and Cu
removal. It was followed by 0.04M HCl activation.

The “as-received” clinoptilolite was the least performing of the three forms.

In the solutions where there was more cobalt than copper by ratio (Co:Cu = 9:1)
metal removal was difficult.

The removal efficiency was noted to increase with dilute solutions. It was the highest
with the mine water which recorded 100% removal of Cu and Co.
Acknowledgements
The authors would like to acknowledge financial support from the University of
Johannesburg, Eskom’ TESP and the National Research Foundation (NRF). The authors also
wish to thank Prattley (South Africa) for providing the natural clinoptilolite samples that
were used in this study.
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