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Removal of Lead(II) by Mordenites.

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Dev.Chem. Eng. Mineral Process., 6(3/4),pp. 171-184, 1998.
Removal of Lead(I1) by Mordenites
S. Dai, Y. Kato, D. Guang, and R.Amal*
Centrefor Particle and Catalyst Technologies, School of Chemical
Engineering and Industrial Chemistry, Universiw of New South Wales,
Sydney, New South Wales 2052, AUSTRALIA.
Batch studies were conducted to investigate the removal of PbZ+using mordenite.
Effects of lead concentration, solution pH, particle size and dosage of mordenite on
the removal kinetics and the removal capacity of lead were investigated. It was found
that the main removal mechanism was ion exchange, with Na, Ca, and K as major
exchange cations. The exchange kinetics and the exchange capaciw of lead with the
cations in mordenite increased as the lead concentration increased. The solution pH
affected the exchange rate but not the lead exchange capacit)z The reaction rate at
p H 5 was faster than that at pH 3. As the particle sue of mordenite decreased, the
reaction rate increased The exchange rate of lead also increased as the mordenite
dosage increased A significant removal of lead by mordenite was observed For a
lead concentration of 3 ppm, 99% of the lead was removed, and for a high lead
concentration (2OOppm), 70% lead removal can still be achieved It can be concluded
that mordenite h a a great potential to remove leadfiom wastewaters.
Introduction
A common problem in many today’s industrial processes is that of dealing with the
by products and waste streams produced by their operations. Practices of using toxic
waste dumps, dilution and discharge to the environment are likely to become more
* Author for correspondence.
171
S. Dai,
Y. Kato, D.Guang and R. Amal
and more unacceptable. New information is continually being published on the
environmental damage, as well as harm to health of human, plants, and animals,
which some wastes cause.
The presence of heavy metals in waters imparts threat to humgn life and
environment. The heavy metals, such as lead, cadmium, and chromium, are
poisonous to fish and to intervertebrates at a concentration well below 1 mg/l. These
metals are normally present in the waste waters generated from industries such as
metal finishing, electroplating, mining and metallurgical applications. Methods which
are widely used to remove these metals are precipitation (by hydroxides, sulfides,
etc.), followed by an adsorption by activated carbon process, an ion exchange process
or a reverse osmosis process. Adsorption, ion exchange and reverse osmosis are
normally used only as polishing treatment processes because of their high costs in
treating high concentration heavy metals.
Zeolites are crystalline microporous aluminasilicate which cany negative charges
in their structure resulting fiom the isomorphous replacement of the tetrahedral
framework silicon cation by aluminium. This provides a high exchange capacity
which enables zeolites to absorb, adsorb and freely exchange positively charged ions
and particles. Townsend [ 11 gave a good review of ion exchange in zeolites. The ion
exchange kinetics in zeolites depends on several factors: diffusion of ions through a
near static boundary layer between the external solution and the zeolite surface (i.e.
film controlled diffusion), exchange in the adsorption sites and diffusion through the
channel of the crystal structure (i.e. particle controlled diffusion). The use of natural
zeolites to remove cations, especially ammonia and lead, fiom waters has been
studied extensively for the last decade or so, due to the low cost of the natural zeolites
compared to other resins [2,3,4].
There are different types of natural zeolite minerals found in nature and each type
has its own chemical characteristics. Most publications which discussed the potential
of zeolite minerals always tended to suggest that natural zeolites were like spandex
where one size fits all, meaning that one zeolite mineral, usually clinoptilolite, could
be utilised in virtually all applications.
172
Removal of lead(II) by mordenites
Many investigations have been carried out on the use of clinoptilolite to remove
ammonia and heavy metals from wastewater [ 5 , 6, 7, 81, since clinoptilolite is widely
distributed in large deposits. The research showed that clinoptilolite was effective in
taking up cations, particularly ammonia and lead [5, 81. Clinoptilolite could also take
up a small amount of other cations, such as copper, zinc, barium, cadmium and cobalt
[ 3 ] . In some cases, clinoptilolite was pretreated with sodium to enhance the uptake of
the heavy metals [ 5 , 6 ] .Loizidou and Townsend [9] studied the exchange of lead into
both sodium and ammonium forms of clinoptilolite, ferrierite and mordenite, and the
preferences of these zeolites for ammonium over sodium. They found that
clinoptilolite had higher exchange capacities for lead than ferrierite and mordenite
did. However, the latter natural zeolites still showed considerable potential for the
removal of lead from effluent.
In this work, batch studies were conducted to investigate the removal kinetics of
lead (Pb2') from synthetic wastewater using mordenite. The effects of lead
concentrations, solution pH, mordenite dosages and particle sizes, on the lead
removal rate and capacity were investigated. The released cations (Na, Ca, K, Mg)
from the natural zeolite due to the uptake of lead was also monitored.
Experimental Technique
Natural zeolite particles obtained from Japanese soil were sieved to obtain the
appropriate particle size distribution. Except for the experiments in which effect of
zeolite particle size on lead removal was studied, for all other experiments the particle
size ranges used are 0.6-0.85 mm. The natural zeolite was thoroughly washed with
ultrapure water and dried in the oven at 8O0C-90"Cto remove impurities. The crystal
phase of the natural zeolite was analysed by X-ray Fluorescence and X-ray
Diffraction. The surface area of the zeolite particles was measured by the B.E.T.
nitrogen adsorption technique. The cation exchange capacity of mordenite was
determined by mixing the natural zeolite with 2 M ammonium acetate for one week
[ 11 and analysing the cations exchanged out from zeolite. The mordenite particles and
50 ml lead solutions were mixed by a temperature controlled shaker machine (Bench
173
S. Dai, Y. Kato, D. Guang and R. Amal
Top Orbital Shaker Incubator Medel 0121 16) for 24 hours. The temperature was
maintained at 25°C. Samples were taken out at different times for analysis. The
concentrations of Pb2+and the cations exchanged out from the zeolite pa’, K’, Ca”,
Mg”) were analysed by ICP-AES (Inductively Coupled Plasma - Atomic Emission
Spectroscopy).
Following is the description of the experimental procedure for different process
conditions.
(i) Effect of lead concentration
In the study of the effect of lead concentration, 0. Ig of mordenite was mixed with 50
ml Pb solutions with concentrations ranging from approximately 3 mg/l to 400 mg/l
(as Pb). The temperature was maintained at 25°C. The experiments were carried out at
two different pH values, 3 and 5 .
(ii) Effect of solution pH
In this investigation, 0.1 g of the natural zeolite was mixed at 25°C with 50 ml of 23.3
mg/l Pb solutions for two different pH values, 3 and 5 .
(iii) Effect of mordenite particle size
In this experiment, mordenite particles were passed through different sieves to obtain
particle size distributions in the range of (a) 0.85-1. I8 mm;(b) 0.6-0.85 mm; (c) 0.30.6 mm; and (d) less than 0.3 mm. 0.lg of mordenite of the appropriate size
distribution was then mixed with 50ml of 23.3 mg/l lead solutions of pH 5 at 25°C.
(iv) Eflect of mordenite dosage
In this experiment, the mordenite dosage was varied from 0.01g to 0.25g and 0.1 g to
1 g, for lead concentrations of 23.3 mg/l and 510 mg/l respectively. The mordenite
particles were mixed with 50 ml of the lead solutions at pH 5 . The temperature was
also maintained at 25°C.
Results And Discussion
X-ray Diffraction shows that the major phase of the natural zeolite is mordenite
((Na2K2Ca),A1,Si,,0,,.28H,O), and X-ray Fluorescence analysis indicates major
exchangeable cations to be Na’, Ca”, K’ (in different ratio), and Mg’+ is only present
I 74
Removal of lead(l1)by mordenites
25
12.5 ppm
-
-a
3.2 ppm
-
0
200
600
400
-
Time Minutes
Figure la. Effect of lead concentrations rangingfrom 3.2ppm to 48.9ppm on
removal of lead, pH 3.
90
80
3
418.0 ppm
--
L
-
193.3 ppm
-
111.1 ppm
I
a
0
200
400
600
-
Time Minutes
Figure 16. Effect of lead concentrations rangingfrom I I I . Ippm to 418ppm on
removal of lead, pH 3.
I75
S.Dai, Y. Kato, D. Guang and R. A m 1
71.5 ppm
20
23.3 ppm
5
-
A
0
v
0
-
3.6 ppm
400
200
600
-
Time Minutes
Figure 2a. Efect of lead concentrations rangingfrom 3.6ppm to 7l.Sppm on
removal of lead, pH 5.
v
90
I
80
-.
0
275.2 ppm
200
400
Time -Minutes
600
Figure 21). Efect of lead concentrations rangingfrom 132.5ppm to 275.2ppm on
removal of lead, pH S.
176
Removal of lead(1I) by mordenites
in a very tiny amount. The specific area of this natural zeolite is approximately 50
m2/g. By mixing the natural zeolite with 2M ammonium acetate for a week, it is
found that the cation exchange capacity of mordenite is 150 meq/l OOg.
As shown in Figures l(a-b) and 2(a-b), the ion exchange kinetics of lead ions
with the cations in mordenite increases as the lead concentration increases for pH
values of 3 and 5. The exchange capacity (in terms of milli-equivalents of Pb”
exchangedl OOg of mordenite) also increases as the lead concentration increases.
Figures 3(a) and (b) show the amount of lead removed (in terms of percentage) when
equilibrium is achieved for pH 3 and 5, respectively. For lead concentrations between
3ppm and 75ppm, more than 99% lead removal is obtained. While for a higher lead
concentration (approximately 200ppm), only 70% of the lead is removed. The ionexchange isotherms of lead with cations in mordenite for pH 3 and 5 are presented in
Figures 4(a) and 4(b). Xc is defined as the fraction of lead exchanged into mordenite
(ratio of meq of Pb taken up by the natural zeolite/lOOg zeolite to maximum
exchange capacity) when equilibrium is reached, and Xs is the corresponding fraction
of lead remaining in the solution (ratio of lead concentration in solution to total
concentration of cations in the solution) [lo]. The maximum cations exchange
capacity is taken as 150 meq/lOOg in the calculations. The slope of the isotherm
indicates the affinity of lead for the mordenite. The steeper slope obtained for pH 5
shows that mordenite has a higher affinity for lead at pH 5 than that at pH 3. Figure 4
also shows that mordenite has high affinity for lead up to 40% of the maximum
exchange capacity. Thereafter the affinity for lead over the other cations becomes
low.
Table 1 summarises the capacity of lead exchanged into mordenite, and the
amount of cations exchanged out from mordenite after equilibrium is achieved at pH3
and 5 for different lead concentrations and mordenite dosage of 0. lg. The amount of
[H’] exchanged into mordenite at pH 3 calculated from the increase of pH after
equilbrium is achieved is also tabulated in Table 1. At pH 5, the amount of Pb”
exchanged into mordenite is balanced by the amount of cations (”a’,
Ca’, K’ , and
Mg”) exchanged out from zeolite. However, at pH 3 for low lead concentrations, the
177
S. Dai,
Y.Kato, D. Guang and R. Amal
Figure 3a. Percentage of lead removal when equilibrium is achieved at p H 3.
100
90
80
--
70 - 60
50
--
~
40
0
100
200
300
Initial lead concentration (pprn)
I
Figure 36. Percentage of lead removal when equilibrium is achieved at pH 5.
178
Removal of lead(1l)by mordenites
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.2
0
0.4
0.6
xs
Figure 4a. Ion exchange isotherm of lead with cations in mordenite at pH 3 .
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
xs
Figure 46. Ion exchange isotherm of lead with cations in mordenite at pH 5 .
amount of cations exchanged out is significantly higher than the amount of lead taken
up by mordenite. A significant increase of pH (i.e. a decrease of [H'] concentration in
solution) after equilibrium is achieved is also observed at pH 3. The larger amount of
cations exchanged out at pH 3 compared to that at pH 5 is approximately balanced by
179
S. Dai, Y. Kato, D.Guang and R. Amal
the decrease of the amount of [H'] in solution at pH 3. It is also noted that for the lead
concentration of 4 18 ppm, that is no significant pH change.
The effect of solution pH on lead removal for an initial lead concentration of
23.3 ppm is shown Figure 5 . The ion exchange kinetics of lead with the cations in
mordenite at pH 5 is faster than that at pH 3. The amount of lead removed, however,
is approximately the same for both pH values, after equilibrium is achieved. This can
be explained by the fact that at pH 3, the [H'] concentration is high enough that its
exchange with the cations in mordenite significantly affect the exchange kinetics of
lead.
Table 1. Summary of Batch Exchange Results.
~
~
Original lead(I1)
Lead(I1) taken
[H'] taken up by
Na, Ca, K, Mg cations
solution
up by natural
natural zeolite
released from zeolite
I
zeolite
3.15 pprn pH 3.0
1.5 meq/lOOg
24.94 meq/lOOg
25.4 meq/lOOg
12.48 ppm pH 3.0
6.0 meq/lOOg
24.94 meq/lOOg
27.1 meq/lOOg
48.87 ppm pH 3.0
23.2 meq/lOOg
22.5 meq/lOOg
39.7 meq/100g
11 1.07 ppm pH 3.0
48.5 meq/lOOg
16.2 meq/lOOg
64.6 meq/lOOg
193.3 ppm pH 3.0
68.0 meq/lOOg
14.0 meq/lOOg
78 meq/lOOg
418.0 ppm pH 3.0
91.9 meq/lOOg
91.3 meq/lOOg
3.6 ppm pH 5.0
1.7 meq/lOOg
1.O meq/l OOg
1 1.2 meq/l OOg
23.3 ppm pH 5.0
7 1.5 ppm pH 5.0
I 34.2 meqllOOg I
132.5 ppm pH 5.0
I 34.9 meq/l Oog
I
58.3 meq/lOOg
200.4 ppm pH 5.0
61.5 meq/lOOg
68.2 meq/lOOg
I
275.2 ppm pH 5.0
1 1.O meq/l OOg
I
I
68.3 meqll OOg
I
I 84.1 meq/lOOg I
I
I 89.1 meq/lOOg
The effect of mordenite particle size on the lead removal kinetics is shown in
Figure 6. As the particle size increases the rate of ion exchange decreases. From
180
Removal of lead(l1) by mordenites
Figure 6, the relationship between the time required to achieve complete conversion
(T)
and the zeolite particle size was calculated. It was found that T a R ' '.
Figures 7(a) and (b) show the effects of mordenite dosage on the exchange
kinetics of lead (expressed in terms of meq/l) with cations in mordenite. The
exchange rate increases as the dosage of mordenite increases. For a lower lead
concentration (23.3 ppm, 0.225 meq/l), the residual lead concentration after
equilibrium is achieved is almost zero for mordenite dosages ranging from 0.05g to
0.25g. Hence, increasing mordenite dosages will not increase further the removal of
lead. For mordenite dosage of O.OIg, equilibrium is not achieved even after 300
minutes of reaction because of the slow exchange rate. For a higher lead
concentration (5 10 ppm, 4.92 meq/l), the amount of lead removed (in terms of meq/l)
after 300 minutes of reaction increases as the mordenite dosage increases. After
equilibrium is achieved, the same residual lead concentration may be attained for
mordenite dosages of 0.5g and lg, but not for 0.lg of mordenite. The amount of lead
removed (meq/l) is much lower for 0. l g of mordenite. However, when the amount of
lead removed is normalised with the amount of mordenite added (i.e in terms of meq
Pb removedlOOg of zeolite), the lowest amount of zeolite gives the maximum
capacity (see Figure 7c). This can be explained by the fact that once saturation is
achieved, the addition of more mordenite will increase the lead removal, but with a
lower degree of saturation.
Conclusions
Mordenite has great potential to remove lead from wastewaters. For lead
concentrations less than 75 ppm, 99% lead removal can be achieved. As the lead
concentration increases, both the exchange capacity and kinetics of lead removal
increases. At pH 3, the H' ions compete with lead ions in the exchange reactions with
the cations in mordenite, resulting in a slower lead exchange rate. Both the particle
size and amount of mordenite affect the lead exchange kinetics. The lead exchange
kinetics increases as the particle size decreases and as the mordenite dosage increases.
181
S. Dai, Y. Kato, D.Guang and R. A m 1
This shows that removal of lead by mordenite is affected by the solution conditions
and the properties of mordenite.
14
pH 5.0
12
10
8
6
4
2
0
E
0
Y
100
200
300
-
400
Time Minutes
Figure 5. Efect of solution p H on removal of lead.
I
12
Particle Size Range
+c
0.3 mm
- 0.6 mm
-0.3
+0.6
+,+
-
0.85 mm
0.85 -1.18 mm
0
100
200
Time -Minutes
300
Figure 6. Effect of mordeniteparticle size on lead removal.
182
400
Removal of lead(II) by mordenites
Dosane
-0.01
g
+0.05 g
-0.1
-0.15
9
g
0.25 g
100
0
200
300
400
-
Time Minutes
Figure 7a. Efect of mordenite dosage on lead removal, for 23.4ppm of lead(II).
I
1.og
A
0.w
0.lg
L
0
I00
200
300
400
-
Time Minutes
Figure 76. Efect of mordenite dosage on lead removal (in term of meq/l)for lead
concentration of 510ppm.
183
S. Dai,Y. Kato, D. Guang and R. A m 1
80 --
0.lg
L
Figure 7c. EfJect of mordenite dosage on lead removal (in terms of meq/l OOg) for
lead concentration of 5lOppm.
References
1. Townsend, R.P., 1991. Ion exchange in zeolites. In Introduction To Zeolite Science and Practice, H.
Van Bekkum, E. M. Flannigen and J.C. Jansen. Elsevier, Amsterdam, pp. 359-390.
2. Klieve J.R. and Semmens M.J. 1980. An evaluation of pretreated natural zeolites for ammonium
removal. Wuter Research 14, pp161-168.
3. Semmens, M.J. and Seyfarth, M. 1978. In Natural Zeolites: Properties Occurence and Uses,
Pergamon Press, UK,p. 517.
4. Townsend, R.P. and Loizidou, M.,1984. Ion-exchange properties of natural clinoptilolite, ferrerite
and mordenite. 1. Sodium-ammonium equilibria. Zeolites, 4(2), pp.191-195.
5. Aharon, A., Howe, R.F. and Aldridge, L.P. 1995. Proceedings AWWA 16th Federal Convention,
pp.823-829.
6. Blanchard, G., Maunaye, M. and Martin, G.1984. Water Research, 18(12) pp501-1507.
7. Jorgensen, S.E., Libor, O., Lea Graber, K. and Barkacs, K. 1976. Water Research, vol. 10, pp. 213224.
8. Kamarowski, S., Yu, Q., Jones, P. and Machugall, A. 1995. Proceedings AWWA 16th Federal
Convention, pp.86 1-868.
9. Loizidou, M.and Townsend, R.P. 1987. Zeolites, vol 7, pp153-159.
10. Dyer, A., Enamy, H.,and Townsend, R. P. 1981. The plotting and interpretation of ion exchange
isotherm in zeolite systems. Sep. Sci. Technol.; 16(2), pp.173-183.
Received 2 1 February 1997; Accepted after revision: 14 March 1997.
184
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