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Proceedings of the 13th International Conference on Environmental Science and Technology
Athens, Greece, 5-7 September 2013
REMOVAL OF ESCHERICHIA COLI AND HEAVY METALS FROM
WASTEWATER USING SILVER-MODIFIED CLINOPTILOLITE
AKHIGBE* L.O., OUKI* S.K. AND SAROJ* D.P.
*Department of Civil and Environmental Engineering, Centre for Environmental and
Health Engineering, University of Surrey, Guildford, Surrey GU2 7XH, UK.
E-mail: [email protected]
ABSTRACT
The aim of this study was to investigate the removal of Escherichia coli and heavy metals
(Pb2+, Cd 2+ and Zn 2+) from wastewater using silver-modified clinoptilolite under different
experimental conditions through the combined disinfection of pathogenic microorganisms
by the silver ions and sorption of heavy metals on clinoptilolite.
Batch experiments were conducted to look at the effects of initial metal concentration, pH
and contact time on the disinfection, ion exchange and sorption mechanisms involved in
the removal process. Silver-modified zeolites were suspended at 10g/l in single and bicomponent synthetic solutions containing Escherichia coli (108-1010 CFU/100ml) as an
indicator of faecal contamination and metals (Pb2+, Cd 2+ and Zn2+) at concentrations
ranging from 0.1 to 5mg/l.
The silver-modified zeolites exhibited good antibacterial activity with complete elimination
of Escherichia coli in the single and bi-component solutions occurring within the first 30
minutes of contact and high metal removal efficiencies of up to 99% in the single
component metal solutions at pH 5. In the bi-component solutions the presence of metals
at 0.5mg/l did not significantly influence the rate of disinfection. However there was a
slight decrease in the metal removal efficiencies in the presence of Escherichia coli.
The results of this study show that silver-modified zeolites could potentially be applied in
the treatment of complex wastewater effluents while highlighting the need for further
studies to understand the mechanisms occurring in multi-component solutions with a view
to optimizing the system.
KEYWORDS: Silver-modified Clinoptilolite, Ion exchange, Sorption, Disinfection, Heavy
Metals, Escherichia coli
1. INTRODUCTION
Wastewater contains harmful pollutants such as pathogenic microorganisms which cause
waterborne diseases and heavy metals from industrial activities which are persistent nonbiodegradable pollutants with adverse long term effects on human health. Due to rapid
population growth and increased industrial activities, large volumes of wastewater are
being generated requiring more treatment technologies that can handle multiple
pollutants (Wang and Peng, 2010).
The use of natural zeolites for the removal of heavy metals by ion exchange/adsorption
has been established in literature as a cost effective and efficient method of water and
wastewater treatment (Bektas and Kara, 2004; Erdem et al, 2004; Sprynskyy et al, 2006;
Wang and Peng, 2010). Natural zeolites are crystalline porous aluminosilicate minerals
with exchangeable cations making them ideal for use in ion exchange and adsorption.
Clinoptilolite is the most commonly used natural zeolite and several studies have shown
that it is highly selective towards ammonium ions and heavy metals such as lead,
cadmium and zinc (Wang and Peng, 2010).
Silver is known to possess antibacterial properties and a major area of interest in the
application of silver in (waste) water disinfection has been the incorporation of the ions or
nanoparticles into different support materials from which they are slowly released. Natural
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zeolites are good support materials for silver ions due to their non-reactive nature and
good silver exchange properties (Feng et al, 2000; De la Rosa Gomez et al, 2008; Copcia
et al, 2011). The use of silver-modified clinoptilolite in the disinfection of microorganisms
has been investigated and reported in the literature. These studies have shown that
silver-modified clinoptilolite effectively eliminates microorganisms such as Escherichia coli
used as an indicator of faecal contamination (Rivera –Garza et al, 2000; De la Rosa
Gomez et al, 2008; Copcia et al, 2011).
The aim of this study is to investigate the removal of Escherichia coli and heavy metals
(Pb2+, Cd 2+ and Zn2+) from wastewater using silver-modified clinoptilolite under different
experimental conditions through the combined disinfection of pathogenic microorganisms
by the silver ions and sorption of heavy metals on clinoptilolite.
2. METHODS AND MATERIALS
Materials
Natural clinoptilolite (0.2-0.6mm) was washed with deionised water to remove surface
dust, dried at 60°C and stored in a desiccator for use. The following analytical grade
reagents (purchased from Fisher Scientific, UK) were used to prepare metal stock
solutions in deionised water: Silver nitrate (AgNO3), Lead nitrate (Pb (NO3)2), Cadmium
nitrate tetrahydrate (CdN2O64H2O) and Zinc (II) nitrate hexahydrate (Zn (NO3)2.6H2O).
Nitric acid and Sodium Hydroxide were used for pH adjustment.
Silver Modification of Clinoptilolite
50g of natural clinoptilolite was suspended in 100ml AgNO3 solution (3 %( w/v) Ag+) at
pH5 and shaken for 24h at room temperature in a dark room due to the light sensitivity of
silver. The zeolites were then separated by filtration, washed and dried at 60°C. The
amount of silver taken up by the zeolite was 47.72mg/g. The natural and silver-modified
zeolites are denoted as NZ and AZ respectively in this work.
The chemical composition of the zeolite samples obtained by SEM/EDX analysis is
shown in table 1.
Table 1: Chemical composition of zeolite samples
Oxide (wt%)
Zeolite SiO2
NZ
74.08
AZ
70.47
Al2O3
11.43
10.24
Fe2O3
2.43
1.76
Na2O
1.71
0.35
CaO
1.41
0.64
MgO K2O
0.85 3.63
0.55 3.16
Ag2O Si/Al
5.72
9.44 6.08
Batch Disinfection Experiments
Escherichia coli NCIMB 8545 was grown in nutrient broth at 37°C for 18h and the cells
were centrifuged, washed and suspended in deionised water at 108-1010 CFU/100ml.0.2g
of zeolite was added to 50ml E. coli suspension at pH 5 and solutions containing no
zeolites were used as controls. The samples were shaken for 0-60 minutes at room
temperature and enumeration of viable bacteria was done using the membrane filtration
method. The samples were filtered through 0.45μm membranes which were placed on
absorbent pads saturated with membrane lauryl sulphate broth (MLSB) and incubated at
37°C for 18h, after which bacterial colonies were counted. The amount of silver released
into the treated solutions was analysed by ICP-OES (inductively coupled plasma-optical
emission spectroscopy).
Batch metal sorption experiments
0.2g of zeolite was to 50ml single metal solution at pH 5 and the solutions including
blanks (containing no zeolites) were shaken for 60 minutes at room temperature.
Previous experiments showed that the optimal pH for metals removal was 5 and 60
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minutes of contact was sufficient to reach equilibrium. The treated solutions were filtered
using a 0.45μm syringe filter, acidified to pH<2 and refrigerated prior to ICP-OES
analysis.
The percent metal removal for each metal was calculated using the equation:
% Metal Removal= [(Ci-Cf)/Ci] x 100
Where Ci and Cf are the initial and final metal concentrations respectively.
Simultaneous disinfection and metal sorption experiments
To investigate the performance of the zeolites in mixed solutions, 0.2g of zeolite was
added to 50ml of solution containing E.coli (108-1010 CFU/100ml) and a single metal
(0.5mg/l) at pH 5. The solutions including blanks and controls were shaken for 0-60
minutes at room temperature and enumeration of viable bacteria was done using the
membrane filtration method, while the remaining volume of samples were filtered,
acidified to pH<2 and refrigerated prior to ICP-OES analysis.
All experiments were done in duplicates and mean values were used for calculations.
3. RESULTS AND DISCUSSION
Disinfection efficiency
The disinfection efficiency of the silver-modified clinoptilolite against E.coli is shown in
figure 1. The bacterial count was converted to Log CFU/ml for the purpose of plotting the
graphs. The silver-modified clinoptilolite (EcAZ) exhibited good antibacterial efficiency as
there was a rapid reduction in the number of viable cells as the contact time increased
with complete elimination after 30 minutes of contact.
Figure 1: Disinfection of E.coli in single component solutions (E.coli +silver modified
clinoptilolite (EcAZ); E.coli +natural clinoptilolite (EcNZ))
The amount of silver released into the treated solutions as a function of contact time
during the disinfection process was also measured as shown in figure 2. It can be seen
that as the concentration of silver ions in the sample increased there was reduction in the
bacterial count and at 30 minutes when no viable colonies were detected in the sample,
the amount of silver released into the solution was 1.10mg/l. This suggests that the death
of E.coli is a function of the amount of silver ions released into the solution from the
zeolites during treatment as reported in other studies (Kwakye-Awuah, 2008; De la RosaGomez et al, 2008; De la Rosa-Gomez, 2010).
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Figure 2: Death of E.coli as a function of silver released into solution (E.coli (Ec); silver
(Ag))
Several studies suggest the antibacterial action of silver occurs as follows: when bacterial
cells come in contact with silver zeolites, the silver ions released penetrate the cell wall
resulting in detachment of the cytoplasmic membrane and the deposition of dense
granules of silver ions in the cell, DNA molecules and around the cell wall. The silver ions
bind to and damage the cell DNA and there is inactivation of proteins and inhibition of
essential enzymes. These cause irreversible damage resulting in impaired ability to
replicate and cell death (Matsumara et al, 2003; Jung et al 2008; Feng et al, 2000;
Kwakye –Awuah et al, 2008).
There was continued release of silver into the solution after all the bacterial cells were
eliminated possibly due to the exchange of Ag+ ions for H+ ions present in the solution
and the final concentration of silver in the solution after 60 minutes of contact was
2.64mg/l. The natural zeolite (EcNZ) did not exhibit any antibacterial activity compared to
the control (Ec control) as seen in Fig 1 with a slight reduction in the bacterial population
after 60 minutes of contact due to the absence of nutrients necessary for bacterial growth
in the deionised water.
Metal removal efficiency
The performance of the zeolites in single component metal solutions was investigated.
Figure 3 shows the effect of initial concentration on the removal of lead, cadmium and
zinc. It can be seen that the removal efficiency of both silver-modified (AZ) and natural
zeolites (NZ) increased because due to the low initial metal concentrations the available
exchange sites did not become saturated during these experiments (Bektas and Kara,
2004).
Figure 3: Effect on initial concentration (0.1-5mg/l) on heavy metal removal ( silver
modified zeolite (AZ); natural zeolite (NZ))
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Overall the natural and silver-modified zeolites performed well in the single component
solutions and the maximum removal efficiencies (metal concentration=5mg/l) for the
silver-modified zeolites were 98, 99 and 98% for lead, cadmium and zinc respectively as
shown in figure 3. The silver-modified zeolites performed slightly better than the natural
zeolites. This could be due to the replacement of some of the potassium ions present in
the zeolite by silver ions which could easily be exchanged for the metal ions in the
solution; potassium ions are the least exchangeable ions due to their coordination and
location (strong bonding site) within the zeolite framework (Culfaz et al, 2004).
Simultaneous Disinfection and Metal removal efficiency
The performance of the zeolites in mixed solutions containing single metals (0.5mg/l) and
E.coli at fixed concentrations was investigated. Figure 4 shows the removal of E.coli from
single and mixed solutions as a function of time. The results of the experiments show that
the presence of metals at the concentration studied did not significantly affect the
disinfection of E.coli with complete elimination within 30 minutes of contact as observed
in the single solutions (EcAZ).
Figure 4: Disinfection of E.coli in single and mixed solutions
The rate of disinfection in the solution containing zinc was slightly faster due to the
antibacterial properties of zinc ions. Heavy metals are toxic to microorganisms
depending on their concentrations in solution (Hrenovic et al, 2012). However it can be
observed in figure 4 that in the solutions containing metals and E.coli, the metals did not
appear to exhibit any significant antibacterial activity compared to the control solution at
the concentrations studied (0.5mg/l metal + E.coli (108-1010 CFU/100ml)). The natural
zeolites did not exhibit any antibacterial activity (not shown in the figure).
Figure 5: Metal removal in single and mixed solutions
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The metal removal performance of the zeolites in the absence and presence of bacteria
after 60 minutes of contact is shown in figure 5. The initial metal concentration was
0.5mg/l for both the single and mixed solution.
The presence of bacteria in the solutions affected the removal performance of the
zeolites with reduced metal uptake in the mixed solutions which was more significant in
the case of cadmium and zinc. The metal removal efficiencies were (single-91%, with
E.coli-82%), (single-92%, with E.coli -70%) and (single-85%, with E.coli -60%) for lead,
cadmium and zinc respectively. These results suggest competition between the bacteria
and metal ions resulting in some metal ions not being taken up by the zeolites and
possibly sorbed by dead bacterial cells remaining in the solution after treatment.
In all the experiments the pH of the solutions containing zeolites increased from 5 to 5.86.6 after treatment due to the uptake of H+ ions from the solutions and zeolite hydrolysis
(Gunay et al, 2007).
3. CONCLUSIONS
The use of silver-modified clinoptilolite was investigated for its combined metal removal
and antibacterial activity. Overall the experimental results show that the silver-modified
clinoptilolite exhibited good antibacterial properties with complete elimination of bacteria
within 30 minutes of contact and high metal removal efficiencies. The natural zeolites did
not show any antibacterial activity. These findings suggest that silver-modified zeolites
could potentially be used for the simultaneous removal of pathogens and heavy metals
from complex wastewater effluents. In order to optimize the process, further
investigations will focus on understanding the mechanisms occurring in multicomponent
solutions, silver recovery and reuse and application to real wastewater effluents.
4. REFERENCES
1. Bektas, N. & Kara, S. (2004) Removal of lead from aqueous solutions by natural
clinoptilolite:equilibrium and kinetic studies . Separation and Purification Technology
39:189-200
2. Copcia,V.E., Luchian, C., Dunca, S., Bilba, N. & Hristodor, C.M. (2011) Antibacterial
activity of silver-modified natural clinoptilolite. Journal of Material Science 46:71217128
3. Culfaz, M. & Yagiz, M. (2004) Ion exchange properties of natural clinoptilolite: leadsodium and cadmium-sodium equilibria. Separation and Purification Technology
37:93-105
4. De la Rosa-Gomez, I., Olguin, M.T. & Alcantra, d. (2008) Bactericides of coliform
microorganisms from wastewater using silver-clinoptilolite rich tuffs. Applied Clay
Science 40: 45-53
5. De la Rosa-Gomez, I., Olguin, M.T. & Alcantra, d. (2010) Silver-Modified Mexican
Clinoptilolite-Rich Tuffs With various Particle sizes as antimicrobial agents against
Escherichia coli. Journal of the Mexican chemical society 54(3):139-142
6. Erdem, E., Karapinar, N. & Donat, R. (2004) The removal of heavy metal cations by
natural zeolites. Journal of Colloid and Interface Science 280:309-314.
7. Feng, Q.L., Wu, J., Chen, G.Q., Cui, F.Z., Kim, T.N. & Kim, J.O. (2000) A mechanistic
study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus
aureus Journal of Biomedical Materials Research 52(4); 662-8
8. Gunay, A., Arslankaya, E. & Tosun, I. (2007) Lead removal from aqueous solution by
natural and pretreated clinoptilolite: adsorption equilibrium and kinetics. Journal of
Hazardous Materials 146:362-371.
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9. Hrenovic, J., Milenkovic, J., Ivankovic, T. & Rajic, N. (2012) Antibacterial activity of
heavy metal-loaded natural zeolite. Journal of Hazardous Materials. 201-202: 260264
10. Jung, W.K., Koo, H.C., Kim, K.W., Shin, S., Kim, S.H. & Park, Y.H. (2008)
Antibacterial Activity and Mechanim of Action of the Silver Ion in Staphylococcus
aureus and Escherichia coli. Applied Environmental, Microbiology 74(7):2171-2178.
11. Kwakye-Awuah, B., Williams, C., Kenward, M.A. & Radecka, I. (2008) Antimicrobial
action and efficiency of silver loaded zeolite X. Journal of Applied Microbiology 104
(5):1516-1524
12. Matsumara, Y., Yoshikata, K., Kunisaki, S. & Tsuchido, T. (2003) Mode of Bactericidal
action of Silver Zeolite and Its comparison with That of Silver Nitrate. Applied and
Environmental Microbiology: 4278-4281
13. Rivera-Garza, M., Olguın
́ , M.T., Garcıa
́ -Sosa, I., Alcántara, D. & Rodrıg
́ uez-Fuentes,
G. (2000) Silver supported on natural Mexican zeolite as an antibacterial material.
Microporous and Mesoporous Materials 39(3): 431–444
14. Sprynskyy, M., Buszewski, B., Terzyk, A.P. & Namiesnik, J. (2006) Study of the
selection mechanism of heavy metal (Pb2+, Cu2+, Ni2+, and Cd2+ ) adsorption on
clinoptilolite. Journal of Colloidal and Interface Science 304:21-28.
15. Wang, S. & Peng, Y. (2010) Natural zeolites as effective adsorbents in water and
wastewater treatment. Chemical Engineering Journal 156:11-24
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