UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 18
Kinetic Studies of Heavy Metal Removal from Industrial
Wastewater by Using Natural Zeolite
Ali Mohammed Salih
1,a*
, Craig Williams
2,b
, Polla Khanaqa
3.c
1
Department of Environmental Science, College of Natural Sciences, University of Sulaimani, Sulaymaniyah, Iraq
2
Department of Chemistry and Forensic Science, School of Biology, Faculty of Science and Engineering, University
of Wolverhampton, Wolverhampton, UK
3
Department of Geology, Kurdistan Institution for Strategic Studies and Scientific Research, Sulaymaniyah, Iraq
E-mail:
a
ali.salih@univsul.edu.iq,
b
c.williams@wlv.ac.uk,
c
polla.khanaqa@kissr.edu.krd
1. Introduction
Zeolites are well-known aluminosilicate minerals that are used in wastewater purification processes and
environmental pollution control. The reduction of heavy metal contamination in aquatic systems is a global problem
(Peng et al., 2009). In the meantime, the treatment of industrial wastewater contaminated by heavy metals has become
a major challenge. According to Bish and Ming (2001) the wastewater collected from municipalities, communities and
industry needs to be treated then returned back to aquatic systems or to the land (Kalló et al., 2001). The main sources
of heavy metal pollution occur in wastewater from various sources such as metal plating facilities, battery manufacturing
processes, mining operations, nuclear power generation, over use of pesticides, vehicle emissions, the ceramic and glass
industries, paints, and treated timber and microplastics. Andras et al. (2012) proved that over limit heavy metals are
recognized as toxic elements and their discharge into the water system affects both human health and the natural
ecosystem (Andras et al., 2012) Since they are not biodegradable, and cause numerous diseases and disorders (Pandey
et al., 2009; Tchobanoglous & Burton 1991). In order to achieve the above goal, adsorption, ion exchange can be used
to remove or reduce heavy metals from wastewaters. According to Panayotova and Velikov (2003) the advantage of
using clinoptilolite is being relatively cheap, abundant in supply, sustainable and it is environmentally friendly.
Kinetic studies were carried out in order to investigate the behaviour of adsorbents and understand the removal
mechanisms involved in the adsorption process. Kinetic studies are critical processes that are used to obtain information
about the process dynamics such as the adsorption rate, contact time and mass transfer parameters including external
mass transfer coefficients and intraparticle diffusivity (Connors, 1998). Margeta et al. (2013) have divided the process
Access this article online
Received on: January 20, 2020
Accepted on: October 19, 2020
Published on: June 30, 2021
DOI: 10.25079/ukhjse.v5n1y2021.pp18-25e.v5n1y2021.ppxx-xx
E-ISSN: 2520-7792
Copyright © 2021 Ali et al. This is an open access article with Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (CC BY-
NC-ND 4.0)
Research Article
Abstract
The present work involves the study of the removal of Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
from synthetic metal solutions
using natural zeolite. Laboratory experiments were used to investigate the efficiency of adsorbents in the uptake of
heavy metals from industrial wastewater. The kinetic study was used to identify the effect of parameters that affect
the rate of adsorption and evaluated their impact on the efficiency of the zeolite in the removal of heavy metals from
industrial wastewater. Natural zeolite (clinoptilolite) as adsorbent contacted with multi-component synthetic
solutions containing Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions without any pre-modifications and every hour 15 ml of the
samples were filtered and taken for metal ion concentration analysis using the ICP-OES. The pH values were
monitored and adjusted regularly. The results showed that the capacity of the adsorbents for the removal of heavy
metals increased with a greater mass of absorbent, increased initial solution pH, increased agitation speed and higher
solution concentration.
Keywords: Kinetic studies, Heavy metals, Wastewater, Natural zeolite, Adsorption.
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 19
of diffusion in the zeolite system into four phases: (I) Diffusion in solution, (II) Diffusion through the film, (III)
Diffusion in pores, and (IV) Ion exchange. These parameters are important in the design and to optimise the operation
of any adsorption experiment in wastewater treatment. Therefore, kinetic studies were used in this work to evaluate the
suitability of natural zeolite for removing heavy metal cations from solution. Kinetic studies also supply information
about the nature of the ionic transport mechanisms that control the exchange rate (Harland, 1994). Factors and their
effect on the efficiency of natural zeolite in removing Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
from solution were investigated in detail.
2. Experimental: Materials and Methods
Natural zeolite (clinoptilolite) was used as “as-received” without any pre-modifications and originally mined in
Anaconda and supplied by the Anaconda mining company, Denver Colorado, USA. It is 97% pure. Synthetic multi-
component solutions of Fe
3+
, Cu
2+
, Pb
2+
and Zn
2+
ions were prepared from analytical grade iron (III) chloride
hexahydrate (H
12
Cl
3
FeO
6
), copper (II) chloride dehydrate (H
4
Cl
2
CuO
2
), Lead(II) acetate trihydrate (C
4
H
12
O
7
Pb) and
zinc acetate dehydrate (C
4
H
10
O
6
Zn). The pH values were monitored and adjusted using a pH meter (Microprocessor
pH Meter – pH 211-HANNA instruments). The pH was adjusted to 2, 4, and 6 ± 0.1 by adding hydrochloric acid (HCl)
or bases sodium hydroxide (NaOH). To observe the effect of agitation speed; agitation in a beaker was obtained by
using a magnetic stirrer (stuart-SB162) at a speed of 100 rpm, 150 rpm and 200 rpm. Zeolite samples with masses 2 g,
4 g and 8 g were contacted with constant volume (100 ml) of multi - component synthetic solutions containing Cu
2+
,
Fe
3+
, Pb
2+
and Zn
2+
ions. They were agitated at agitation speeds of 100, 150 and 200 rpm for agitation times of 60, 120,
180, 240, 300 and 360 minutes in a magnetic stirrer at room temperature. Two different sizes of zeolite particles were
selected <125 µm and <250 µm. The effect of the initial solution concentration on the adsorption process was
determined using multi-component solution concentrations in the range of (50, 100, 200 and 400) mg/l. Every hour 15
ml of the samples were filtered and taken for metal ion concentration analysis using the ICP-OES. The pH values were
monitored and adjusted regularly.
The experiments were duplicated three times in order to examine the reproducibility of the results, while the mean
value was used for all taken data. The deviation between the duplicate samples in analysing the cations was ± 6.4%,
6.3%, 5.6% and 6.4% for Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions respectively.
3. Kinetic Study Results
The results show that the highest adsorption rate of Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions took place in the first hours followed
by a slower adsorption rate later on. The first hour is an initial stage of adsorption when higher rates of adsorption take
place; this may be due to the availability of more adsorption sites and the fact that the metal ions exchange easily on the
surface of the zeolite grains (Inglezakis et al., 2002). The driving force for adsorption is very high in the initial stage of
the adsorption process and this also results in a higher initial adsorption rate. After that, a slower adsorption rate follows
due to slower diffusion of the metal ions into the interior channels. Consequently, these metal ions occupy the
exchangeable positions within the crystal structure of the natural zeolite (Amarasinghe & Williams 2004; Myroslav et al.,
2006).
The data obtained from the kinetic adsorption tests were used to determine the removal capacity,
q
e
(mg/g) of the
different adsorbents using the following equation:
q
e
= (C
o
– C
e
) X V/ m
(1)
The percentage removal of metal ions from solution was also determined using the equation below:
Percentage Adsorbed (% removal)
q
e
= {(C
o
– C
e
) X 100}/ C
o
(2)
When:
q
e
amount of adsorbate adsorbed per unit weight of adsorbent (mg/g)
C
o
and
C
e
are the initial and final metal ion concentrations in solution (mg/l) respectively,
V
is the solution volume (l)
and
m
is the weight of the zeolite used (g).
3.1 Factors that affect the rate of adsorption
A number of parameters that affect the rate of adsorption were studied and described in detail. These include: effect of
adsorbent mass, effect of adsorbent particle size, effect of initial solution pH, effect of initial solution concentration and
effect of agitation speed of adsorbent.
3.1.1. Effect of adsorbent mass
Kinetic experiments were carried out using three different adsorbent masses, 2 g, 4 g and 8 g. The results in Table 1
clearly show that when the adsorbent mass was increased, this resulted in an increase in the adsorption of the heavy
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 20
metal ions. The main reason for this is that as the adsorbent mass increases more adsorption sites are available per mass
of adsorbent surface and thus the total amount of metal that is removed increases.
Table 1. The effect of natural mass on the removal of heavy metals from solution zeolite.
This result indicates that the mineral mass in the solution can affect the adsorption capacity for the removal
of heavy metals as it determines the availability of adsorption sites. The removal of Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions mostly occurs in the early stage, while the percentage of adsorbed Fe
3+
and Pb
2+
reached 90% in the first
hour Figure 1.
Figure 1. The effect of the mass of natural zeolite on the adsorption of copper, iron, lead and zinc from solution.
3.1.2. Effect of adsorbent particle size
The effect of adsorbent particle size on adsorption capacities from solutions was investigated by using two different
sizes of <125 µm and <250 µm. It was observed that the smaller particle sized samples adsorbed more of the Cu
2+
,
Fe
3+
, Pb
2+
and Zn
2+
ions. This indicates that any decrease in adsorbent particle size causes an increase in the adsorption
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 21
of the heavy metals ions Figure 2. This is because as the adsorbent particles get smaller more adsorption sites are
available for metal uptake and more contacts are taking place. On the other hand smaller particle sizes result in the
shortening of the diffusion distance that ions have to travel in order to get to an active site; thus adsorption is enhanced
and requires a shorter time to reach equilibrium. This is in agreement with the results obtained by Sprynskyy et al. (2006)
and Inglezakis et al. (2004). They concluded that the larger particle size adsorbent had lower adsorption capacities than
the smaller particle sizes.
Figure 2. The effect of particle size on the adsorption of iron, copper, lead and zinc from solution.
Although particle size can affect the adsorption capacity mostly at the initial stage, as the contact time increases there
is a decrease in the level of the effect of particle size on adsorption and the adsorption process gets slower. The same
results were found by Malliou et al. (1994) and Erdem et al. (2004). The use of very fine particles can also cause some
operational problems such as difficulty in the filtration of the zeolite from solution in batch studies (Inglezakis et al.,
2001).
3.1.3. Effect of initial solution pH
Initial solution pH is a critical parameter for adsorption experiments. This parameter has a significant impact the heavy
metal removal processes since it can influence and impact the adsorbent ability to remove metals and is connected with
the competition of hydrogen (H
+
) ions with heavy metal cations for active sites on the adsorbent surface (Dimirkou,
2007; Inglezakis et al., 2003; Hui et al., 2005). An acidic solution can impact both the character of the exchanging ions
and the character of the adsorbent.
Solutions with different pH values were used as follows: 2, 4 and 6 ± 0.1 for the multi-component solutions. The
results obtained are presented in Figure 3.
The results show that as the solution pH increases, the heavy metal removal efficiency also increases. This is due to
the competition between the hydrogen ions and heavy metal cations for the same exchange sites and electrostatic
repulsion between the heavy metal cations in solution; as more hydrogen ions are adsorbed, the number of protonated
zeolite surfaces increases (Hui et al., 2005). Figure 3 shows how the initial solution pH influences the adsorption capacity
of natural zeolite. Thus an increase in the initial pH resulted in an increase in the adsorption efficiency of natural zeolite
for Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions. Moreno et al. (2001) and Alvarez-Ayuso et al. (2003) observed the same behaviour
using clinoptilolite and stated that the efficiency of metal adsorption depends on the solution pH levels. However, metal
precipitates at high pH values above pH7 and low values below pH2 inhibit the contact of metal ions with the adsorbent.
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 22
Figure 3. The effect of initial solution pH on the adsorption of copper, iron, lead and zinc from solution.
The results show that the ion exchange process increases with an increase in pH up to a maximum value and the best
heavy metal removal efficiency value was obtained between pH values of 4 and 6, while pH values below pH4 or above
pH6 decreased the heavy metal removal efficiency, as shown in Figure 4.
Figure 4. The effect of initial solution concentration and the amount of heavy metals adsorbed qe (mg/g) by natural
zeolite.
A low pH value (< 4) solution can cause dissolution of the zeolite crystal structure and at higher pH values (> 7) the
zeolite structure can be affected and this can result in the reduction of the ion exchange process. This was the same
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 23
result achieved by Oren & Kaya (2006) when they assumed that pH values between pH4 and pH6 are fundamental in
the ion exchange process.
3.1.4. Effect of initial solution concentration
The effect of the initial solution concentration on the adsorption process was determined using multi-component
solution concentrations in the range of (50, 100, 200 and 400) mg/l. The initial solution concentration of the solution
significantly impacts the heavy metal removal process. The results in Table 2 show that generally any increase in the
initial solution concentration results in an increase in the heavy metal removal efficiency and the rate of adsorption, as
shown in Figure 4. This may be a result of the concentration driving force since it is responsible for overcoming the
mass transfer resistance associated with the adsorption of metals from solution by the zeolite (Oren & Kaya 2006).
Therefore, as the initial concentration increases, the driving force also increases, resulting in improved efficiency of
the heavy metal removal process. Then after the system reaches saturation point, the initial solution concentration
does not show any significant change in the amount adsorbed due to a decrease in the number of active sites
(Panayotova & Velikov 2003).
Table 2. The effect of initial solution concentration on the adsorption capacity of natural zeolite.
Heavy Metals
Initial Concentration (mg/l)
Amount Adsorbed, qe (mg/g)
Copper
50
0.37
100
0.76
200
1.30
400
2.30
Iron
50
0.0
100
0.46
200
1.13
400
2.78
Lead
50
0.62
100
1.29
200
2.89
400
6.68
Zinc
50
0.19
100
0.56
200
0.80
400
1.37
3.1.5. Effect of agitation speed
The effect of agitation speed on the removal of the cations from the solution was determined using a magnetic stirrer
at speeds of 100, 150 and 200 rpm. The results of the effect of agitation are shown in Figure 5.
The results show that the metal removal efficiency increased as the speed of agitation increased. This is in agreement
with the results obtained by Ören & Kaya, (2006). They concluded that an increase in the speed of agitation resulted
in higher adsorption capacities. The agitation helps in overcoming the external mass transfer resistance, which
controls the rate of adsorption. Hence, an increase in the speed of agitation generally results in an increase in ion
mobility in the solution and reduces the mass transfer resistance. At high agitation speed, the external diffusion
coefficient increases and the boundary layer becomes thinner, which usually improves the rate of solute diffusion
through the boundary layer (Barrer 1982). Agitation of the mixture also results in abrasion and the production of more
broken natural zeolite particles. This means that fresh smaller size zeolite particles are produced and more activate
sites are available on the surface. So this mechanical procedure leads to an increase in the surface area, which
significantly improves the efficiency of the heavy metal removal from solution (Trgo & Peric 2003).
4. Conclusions
The performed work leads to the following conclusions:
• The study indicated the suitability of the zeolite used for the removal of Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions from
synthetic wastewater, while considering the economic aspects of wastewater treatment.
• The adsorbent mass, adsorbent particle size, initial solution pH, initial solution concentration and agitation
speed as well as pre-treatment or modification of the adsorbent in the case of batch experiments are usually
the most influential parameters.
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 24
Figure 5. The effect of agitation speed on the adsorption of heavy metals by natural zeolite.
• The efficiency of heavy metal removal was enhanced and faster with increased initial solution pH, increased
agitation speed, increased solution concentration, decreased particle size and greater mass of absorbent as well
as the application of pre-treatments.
• Initial solution pH is the most critical parameter which has a significant impact the heavy metal removal
processes compared to other parameters
• The results suggests that the adsorption of Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions on the selected adsorbents involves
a complex mechanism and a number of possible rate controlling steps can determine the process efficiency
such as boundary layer diffusion due to external mass transfer effects (external solution phase surrounding the
particle), intraparticle diffusion within the exchanger itself, and chemical reaction kinetic control.
• In general the results show that adsorption is a heterogeneous process as the removal rate of Cu
2+
, Fe
3+
, Pb
2+
and Zn
2+
ions mostly occurred early on, but as the contact time increased , there was a decrease in the level of
the effect of the parameters on adsorption and the adsorption process became slower.
References
Alvarez-Ayuso, E., Garcia-Sanchez, A., & Querol, X. (2003) Purification of metal electroplating waste waters using
zeolites. Water Research, 37, 4855-4862.
Amarasinghe, B. & Williams, R. (2004) Tea waste as a low cost adsorbent for the removal of Cu and Pb from wastewater.
Chemical Engineering Journal, 132, 299-309.
Andras, P., Turisova, I. Marino A. & Buccheri G. (2012) Environmental hazards associated with heavy metals at
Ľubietová Cu-deposit (Slovakia). Environmental Science, 28, 259-264.
Barrer, R. (1982) Hydrothermal chemistry of zeolite. London: Academic Press INC.
Cabrera, C., Gabaldon, C. & Marzal, P. (2005) Sorption characteristics of heavy metal ions by a natural zeolite. Journal
of Chemical Technology and Biotechnology, 80, 477-481.
Connors, K. (1998) Chemical Kinetics: The study of reaction rates in solution. John Wiley & Sons:USA.
Dimirkou, A. (2007) Uptake of Zn
2+
ions by a fully iron exchanged clinoptilolite. Water Reserch., 41, 2763–2773.
Erdem, E., Karapinar, N. & Donat, R. (2004) The removal of heavy metal cations by natural zeolites. Journal of colloidal
and Interface Science, 280(2), 309 – 314.
UKH Journal of Science and Engineering | Volume 5 • Number 1 • 2021 25
Harland, C. (1994) Ion Exchange: Theory and Practice, 2
nd
ed. The Royal Society of Chemistry: London.
Hui, K., Chao, C. & Kot, S. (2005) Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products
from recycled coal fly ash. Journal of Hazardous Materials, 127, 89 -101.
Inglezakis, V., Loizidou, M. & Grigoropoulou, H. (2001) Applicability of simplified models for the estimation of ion
exchange diffusion coefficients in zeolites. Journal of Colloid and Interface Science, 234, 434-441.
Inglezakis, V., Loizidou, M. & Grigoropoulou, H. (2002) Equilibrium and kinetic ion exchange studies of Pb
2+
, Cr
3+
,
Fe
3+
and Cu
2+
on natural clinoptilolite. Water Research, 36, 2784-2792.
Inglezakis, V., Loizidou, M. & Grigoropoulou, H. (2004) Ion exchange studies on natural and modified zeolites and the
concept of exchange site accessibility. Journal of Colloid and Interface Science, 275(2), 570 – 576.
Inglezakis, V., Zorpas, A., Loizidou, M., Grigoropoulou, H. (2003) Simultaneous removal of metals Cu2+, Fe3+ and
Cr3+ with anions SO42- and HPO42- using clinoptilolite. Microporous and Mesoporous Materials, 61, 167-171.
Kalló, D. (2001) Applications of natural zeolites in water and wastewater treatment,. Reviews in Mineralogy and
Geochemistry, 45(1), 519-550.
Malliou, E., Loizidou, M. & Spyrellis, N. (1994) Uptake of lead and cadmium by clinoptilolite. Science of the Total
Environment, 149, 139 – 144.
Margeta, K., Logar, N., Šiljeg, M. & Farkaš, A.(2013) Natural Zeolites in Water Treatment – How Effective is Their
Use, Chapter 5 [online][accessed on 08 November 2016] available at: <http://www.intechopen.com/books/water-
treatment/natural-zeolites-in-water-treatment-how-effective-is-their-use>.
Moreno, N., Querol, X. & Ayora, C. (2001) Utilization of zeolites synthesised from coal fly ash for the purification of
acid mine waters. Environmental Science and Technology, 35, 3526-3534.
Myroslav, S., Boguslaw, B., Artur, T. & Jacek, N. (2006) Study of the Selection Mechanism of Heavy Metal (Pb
2+
, Cu
2+
,
Ni
2+
and Cd
2+
) adsorption on clinoptilolite. Journal of Colloid and Interface Science. 304, 21-28.
Ören, A. & Kaya, A. (2006) Factors affecting adsorption characteristics of Zn
2+
on two natural zeolites. Journal of
Hazardous Materials,13, 59–65.
Panayotova, M. & Velikov, B. (2003) Influence of zeolite transformation in the homoionic form on the removal of some
metal ions from wastewater. Journal Science and Health, A38(3), 545 – 554.
Pandey, P., Sambi, S., Sharma, S. & Singh, S. (2009) Batch Adsorption Studies for the Removal of Cu (II) Ions by
ZeoliteNaX from Aqueous Stream. Proceedings of the World Congress on Engineering and Computer Science, I, San Francisco,
USA.
Peng, J., Song, Y., Yuan, P., Cui, X. & Qiu, G. (2009) The remediation of heavy metals contaminated sediment. Journal
of Hazardous Materials, 161(2 - 3), 633 – 640.
Sprynskyy, M., Boguslaw B., Terzyk, A. & Namiesnik, J. (2006) Study of the selection mechanism of heavy metal (Pb
2+
,
Cu
2+
, Ni
2+
and Cd
2+
) adsorption on clinoptilolite. Journal of Colloid and Interface Science, 304, 21-28.
Tchobanoglous G. & Burton F. (1991) Wastewater Engineering: Treatment, Disposal, Reuse.3
rd
ed. Singapore: McGraw-Hill
International Editions.
Trgo, M. & Peric, J. (2003) Interaction of the zeolitic tuff with Zn-containing simulated pollutant solutions. Journal of
Colloid and Interface Science, 260, 166–175.