Microporous and Mesoporous Materials 61 (2003) 127–136
www.elsevier.com/locate/micromeso
Sorption characteristics of zinc and iron
by natural zeolite and bentonite
A.S. Sheta *, A.M. Falatah, M.S. Al-Sewailem, E.M. Khaled, A.S.H. Sallam
Soil Science Department, College of Agriculture, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia
Received 22 July 2002; received in revised form 15 October 2002; accepted 16 October 2002
Abstract
Understanding the sorption process in natural zeolites is necessary for effective utilization of these minerals as
nutrient adsorbents and consequently as controlled releases of plant nutrients. This research was undertaken to
characterize the ability of five natural zeolites and bentonite minerals to adsorb and release zinc and iron. The
potential for sorption of these ions were evaluated by applying the Langmuir and Freundlich equations. Zinc
sorption data followed the Langmuir equation. The ability for Zn sorption were in the following order: chabazite > analcime > clinoptilolite1 > bentonite > clinoptilolite2 > phillipsite. Diethylene triamine pentaacetic acid (DTPA)
extractable Zn decreased with the increase in successive extractions. All sorbed zinc was extracted by DTPA in
most zeolite species after four successive extractions while only 50% was readily extractable from chabazite. Iron
sorption data followed the S-type isotherm and the results were described by a Freundlich adsorption model. The
iron activity ratio (Feox /Fed ) of sorbed Fe indicated the dominance of amorphous or poorly crystalline phases with
zeolites and crystalline iron oxide phases with bentonite. The results suggest that natural zeolites, particularly
chabazite and bentonite minerals, have a high potential for Zn and Fe sorption with a high capacity for slow release
fertilizers.
Ó 2003 Elsevier Inc. All rights reserved.
Keywords: Zeolite minerals; Bentonite; Zn and Fe sorption; Amorphous iron
1. Introduction
Zeolites and bentonite are naturally occurring
structured and phyllosilicate minerals respectively,
with high cation exchange and ion adsorption capacity. In particular crystalline zeolites are some of
*
Corresponding author. Fax: +966-1-476-8440.
E-mail address: [email protected] (A.S. Sheta).
the most effective cation exchangers known and
they have two to three times the cation exchange
capacity (CEC) than that of most smectites and
vermiculite [1]. Agriculture applications related to
ion exchange, adsorption and desorption of ions by
zeolites have been reported by many investigators
[2–4]. NHþ
4 -exchanged clinoptilolite added to light
and medium textured soils produced a positive
growth response in radishes when banded on soil
[5]. Natural zeolites indicated a potential use as an
þ
NHþ
4 adsorbent and a controlled release NH4 fertilizer [3]. Coarse texture arid land soils are low
1387-1811/03/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved.
doi:10.1016/S1387-1811(03)00360-3
128
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
to very low in extractable micronutrients and many
are regarded as potentially deficient. However,
some micronutrient compounds have been added
to the soils either directly or as an incidental component of other fertilizers. Considerable research
has been conducted concerning the factors which
influence the adsorption and desorption of Zn and
Fe in soils. Zinc adsorption was found to be pH
dependent [6], related to soil CEC [6,7]. Several
investigators have reported that the adsorption
mechanism is a major contributing factor to low
concentrations of Zn in soil solution [8,9]. Zinc
sorption on soils is affected by the soil pH, clay
minerals, and Fe-, Al-, and Mn-hydrous oxides,
organic material, and carbonates [10]. Adsorption
mechanisms suggested include the ion exchange [6]
or physisorption and chemisorption [11]. Soil pH
affects Zn adsorption, either by changing the number of adsorption sites or by changing the concentration of the Zn species which is preferentially
adsorbed [12]. Desorption of native and added zinc
from a range of New Zealand soils in relation to
soil properties has been studied by Singh et al. [13].
They reported that CEC and organic carbon were
the dominant soil variables contributing to sorption or desorption of zinc. Seasonal precipitated
iron oxides in vertisol of Southeast Texas were
studied by Golden et al. [14]. They reported that
iron oxides precipitated on exposed surfaces on ped
surfaces and within soil pores were relatively
poorly crystallized while those precipitated on rice
root surfaces were well crystallized. They refer the
presence of poorly crystallized oxides to the presence of soluble Si and P during flooding. The
presence of Mn lowers the crystalinity of geothite
formed from slow oxidation of FeCl2 solution at
pH 7 [15]. Sorption and desorption characteristics of micronutrients (Fe, Zn, Mn, Cu and B)
by natural zeolite minerals is almost scarce in
literature, despite the relative importance of zeolites as natural carrier for nutrients. The objectives of this research included (1) the evaluation
of Zn and Fe sorption characteristics by five natural zeolites and bentonite minerals, and (2) the
quantification of retained Zn and Fe forms through
the extraction using diethylene triamine pentaacetic acid (DTPA) and selective dissolution
methods.
2. Experimental
Five naturally occurring zeolite minerals and
one bentonite were used in this study namely,
clinoptilolite1 and clinoptilolite2 supplied by Aberhills Holdings Inc. 1 Abbotsford, BC, Canada
and Axis Trade Corp., 1 Arizona, USA, respectively. Minerals Research, 1 Clarkson, NY, USA,
supplied phillipsite, chabazite and analcime.
Baroid Saudi Arabia Ltd., 1 Dammam, supplied
the bentonite mineral. The locations, mineralogical properties, CEC (cmol kg1 ), surface area
(m2 g1 ), Si/Al ratio and total Fe2 O3 and ZnO
(g kg1 ) are given in Table 1. Representative zeolites and bentonite samples were used without any
chemical pretreatment. Impurities such as quartz,
mica, feldspars were identified using X-ray diffraction. In preparation for sorption and extraction experiments, the samples were grinded in an
agate mortar and passed through a 1.0 mm sieve.
The zinc sorption experiment were carried out
using the batch equilibrium by weighing triplicate
1.0 g samples in 50 ml centrifuge tubes. The initial
Zn concentrations added to the samples are 0.0,
5.0, 25.0, 50.0, 100.0, 250.0 and 500.0 mg l1 prepared from ZnSO4 . Twenty ml of the initial concentrations were added to each sample, then
suspensions were shaken for 2 h at constant room
temperature (20 °C) followed by centrifugation for
15 min. The zinc concentrations in equilibrium
solutions were measured by atomic absorption
spectrophotometry (AAS) according to [16]. The
amount of Zn sorbed (mmol kg1 ) was calculated
by the difference between Zn added and that remained in the equilibrium solutions.
The Langmuir equation was used to describe the
Zn sorption. The model used is
C=x=m ¼ 1=kb þ C=b
ð1Þ
where C and x=m are the equilibrium Zn concentration (mmol l1 ) and the amount of Zn sorbed
(mmol kg1 ), respectively. The empirical constants
b and k are related to the adsorption maximum
1
Trade names and company names are included for the
benefit of the reader and do not imply endorsement or
preferential treatment of the product.
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
129
Table 1
Selected chemical analyses and textural properties of zeolites and bentonite minerals used in the study
Location
Clinoptilolite1
Clinoptilolite2
Phillipsite
Chabazite
Analcime
Bentonite
Southwestern
USA
Cucurpe,
Sonora Mexico
Pine Valley,
Nevada
Christmas,
Arizona
Barstow,
California
Dammam,
Saudi Arabia
Impuritiesa
Surface areab
(m2 g1 )
Si/Al
ratio
CECc
(cmol kg1 )
Total (g kg1 )
Fe2 O3
ZnO
Quartz, feldspar, mica
Quartz,
mordenite
Mica
407.9
5.50
98.4
13.4
0.08
428.3
5.82
75.6
13.3
0.07
961.7
3.87
223.0
19.3
0.09
Mica, quartz
1100.0
4.54
195.0
18.1
0.05
313.0
3.41
39.0
15.7
0.09
1009.0
2.32
88.0
44.9
0.12
Quartz
Quartz, mica
a
X-ray diffraction.
Surface area by ethylene glycol monoethyl ether method.
c
CEC––ammonium acetate method.
b
(mmol kg1 ) and bonding strength (l mmol1 ), respectively. This equation will be used to compare
the adsorption maximum (b) between zeolites and
bentonite since these adsorbents and the experimental conditions are identical as proposed by
Veith and Sposito [17]. The sorbed Zn was
sequentially extracted four times with DTPA, according to the method described by Lindsay and
Norvell [18]. Zinc in all extracts was measured by
AAS.
Sorption of Fe was carried out on the five
mentioned zeolite species and bentonite samples
by the same method as already described for Zn.
The initial concentrations are 0.0, 5.0, 25.0, 50.0,
250.0 and 500.0 mg l1 prepared from FeCl2 Æ
3H2 O salt. Six replicates were prepared in the adsorption experiment in order to characterize the
sorbed Fe. The Freundlich model was used to
describe Fe sorption by zeolites and bentonite
minerals. The linear Freundlich model is
log x=m ¼ log k þ 1=n log C
ð2Þ
where x=m is the amount of Fe sorbed
(mmol kg1 ), k and n are constants, and C is the
equilibrium Fe concentrations (mmol l1 ). The
sorbed Fe was characterized as follows: the available Fe was extracted from the duplicate sample
sequentially (four times) using the DTPA extract
[18]. Other duplicate samples were extracted using
the CBD (sodium citrate–dithionite–bicarbonate)
method [19] for the free Fe oxide form (Fed ) determination. Amorphous and poorly crystalline Fe
oxides form (Feox ) were extracted from another
duplicate sample using NH4 -oxalate in darkness at
pH 3 [20,21]. Soluble Fe in all extracts was measured by AAS [22].
3. Results and discussion
Zinc sorption isotherms of Zn sorbed x=m
(mmol kg1 ) and the equilibrium Zn concentrations C mmol l1 are presented in Fig. 1. The
pattern of the isotherms are quite similar with a
slight difference between zeolites and bentonite. At
lower initial concentrations, the isotherms have a
relatively high slope whereas at higher concentrations the slope was relatively low with a defined
plateau for the adsorption maximum in most of
the samples studied. Sorption isotherms follow the
L-shaped type similar to that described by Sposito
[23]. Such adsorption behavior could be explained
by the high affinity of zeolites and bentonite for Zn
at low concentrations, which decrease as Zn concentration increases. Langmuir constants for Zn
sorption were calculated from the best fitting
straight line between C=x=m and C (Table 2). Data
of maximum adsorption (b mmol kg1 ) have been
130
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
Fig. 1. Sorption isotherms of Zn for zeolite species and bentonite.
l mmol1 ) was in the order: chabazite > analcime > phillipsite > bentonite > clinoptilolite2 >
clinoptilolite1. Results also show that chabazite
in the following decreasing order: chabazite >
analcime > clinoptilolite1 > bentonite > clinoptilolite2 > phillipsite. The binding strength values (k in
Table 2
Langmuir equations and constants (b and k) for Zn sorption by zeolites and bentonite
Sample
Langmuir equations
Clinoptilolite1
Clinoptilolite2
Phillipsite
Chabazite
Analcime
Bentonite
Y
Y
Y
Y
Y
Y
¼ 0:0207x þ 0:0109
¼ 0:0328x þ 0:0136
¼ 0:0385x þ 0:0129
¼ 0:0082x þ 0:0006
¼ 0:0178x þ 0:0031
¼ 0:0212x þ 0:0087
r2
b (mmol kg1 )
k (l mmol1 )
0.9572
0.9925
0.9978
0.9949
0.9831
0.9621
48.31
30.49
25.97
122.00
56.18
47.17
1.90
2.41
2.99
13.67
5.74
2.44
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
mineral has the highest ability for Zn sorption under all the studied concentrations, while bentonite
has an intermediate ability. The amount of Zn
sorbed by chabazite ranges between 92.8 and 75.8%
from the added Zn while it was only 74.4–20.2% for
phillipsite. These data reflect large differences between zeolite minerals for Zn sorption, particularly
in the case of chabazite mineral. Differences in the
mineralogical structure of zeolite species and surface characteristics could play an important role in
the sorption behavior of Zn ions besides the presence of impurities in these natural samples. Data of
the native Zn extracted by successive DTPA extractions (Table 3) decreased drastically after the
first extraction and were almost negligible after the
third extraction in all mineral species. The cumulative amounts of native Zn ranges from 0.91
mg kg1 for bentonite to 0.18 mg kg1 for either
clinoptilolite1 or clinoptilolite2. Therefore, the na-
131
tive available Zn to plants in zeolite species or
bentonite mineral was low and almost negligible.
Moreover, mixing such minerals with soils low in
available Zn cannot support the plants need for Zn.
On the other hand, data in Table 3 indicated that
more Zn was extracted after the fourth extraction
by DTPA from chabazite treated samples compared with other samples. For example, at higher
initial Zn added, chabazite releases 97 mg kg1 in
the fourth extraction while clinoptilolite1, clinoptilolite2, phillipsite, analcime and bentonite releases
45, 27.8, 80.5, 67, and 64.4 mg kg1 , respectively.
Also, the percentage of cumulative extracted Zn (%
from sorbed) varied considerably with the sorbed
amounts and with zeolite species or bentonite.
Chabazite showed the lowest desorbed percentage
particularly at high levels of sorbed Zn. Only 53% is
readily extractable by DTPA after four successive
extractions leaving 47% Zn retained by the mineral
Table 3
Amounts of sorbed Zn and DTPA extractable Zn (four successive extractions) from Zn treated
P
Zn added
Extractable/
Sorbed Zn
Sorbed Zn
DTPA
1
1
(mg kg )
sorbed (%)
(mg kg1 )
(mg kg )
extractable Zna
(mg kg1 )
and untreated zeolites and bentonite
P
Extractable/
DTPA
sorbed (%)
extractable Zna
(mg kg1 )
0
100
500
1000
2000
5000
10000
Clinoptilolite1
0
88.2
403
600
1020
2610
2860
0.18
43
286
547
994
2272
2743
–
49
71
91
97
87
96
Clinoptilolite2
0
84.2
360.6
550
960
1690
1867
0.18
73
303
445
812
1357
1558
–
87
84
81
85
80
83
0
100
500
1000
2000
5000
10000
Phillipsite
0
74.4
379.8
529.2
1016
1530
1600
0.27
73
325
475
973
1362
1547
–
98
86
90
96
89
97
Chabazite
0
89.6
463.6
819.4
1733.4
4410
7580
0.31
72
302
583
893
2801
3993
–
81
65
71
52
64
53
0
100
500
1000
2000
5000
10000
Analcime
0
89
457.8
800
1520
2650
3660
0.35
65
301
683
1318
1884
2560
–
73
66
85
87
71
70
Bentonite
0
88.8
452.6
604
1072
2610
2860
0.91
46
257
454
899
2024
2368
–
52
57
75
84
78
83
a
Summation of DTPA extractable Zn (four successive extractions).
132
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
even though the mineral showed the highest amount
of Zn extracted in the fourth extraction. This finding may reflect clearly the positive role on the possibility of using chabazite as a slow release for Zn.
Analcime showed a relatively similar trend since it
retained about 30% of the sorbed Zn after four
extraction treatments by DTPA. These results
agrees well with bonding energy data calculated
from the Langmuir model (Table 2). Other minerals
show a relatively low ability to retain Zn against the
extraction by DTPA, for example, phillipsite releases up to 97% and clinoptilolite1 about 96%
at the highest sorbed amounts.
Iron sorption isotherms for zeolite species and
bentonite are presented in Fig. 2. All the isotherms
could be described by the S-type isotherm [23]. The
slope of the isotherm initially increased with the
increase in Fe concentration, but eventually
showed a low slope at relatively high equilibrium
Fe concentrations. This type of isotherm indicates
that at lower concentrations the surface has a low
affinity which increases at higher concentration
[23]. Nevertheless, the S-type isotherm has rarely
been observed for the adsorption of heavy metals
in soil [24]. There were some differences between
the studied samples, particularly at high concen-
Fig. 2. Iron sorption isotherms for zeolite species and bentonite.
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
trations, that could be explained by the unique
framework structure of zeolite species and the
phyllosilicate structure of bentonite. For example,
at the highest Fe applications clinoptilolite1 seems
to retain the highest amount, followed by bentonite, while analcime was the lowest one. Sorption data followed the Freundlich adsorption
equation (Table 4) and the fit was better for all the
studied mineral species in comparison with the fit
for the Langmuir equation. The results show that
most of the obtained plots were quite similar in
shape and varied by the Freundlich sorption constants, i.e., log k and n (Table 4). The calculated n
value was qualitatively related to the distribution
of site-bonding energies [25]. The n values for all
the studied mineral samples were lower than 1,
which may indicate that the distribution of bonding energies will vary with adsorption density [26].
The small differences encountered in Freundlich
sorption constants could be related to the heterogeneity of sorption sites in zeolites and bentonite.
The close similarity of these constants for bentonite and zeolites may indicate that the unique
structure of different zeolites have no clear role in
the sorption behavior of Fe added under the experimental conditions in comparison with the
bentonite structure. The percentage Fe sorbed/
added data (Table 5) were increased with the increase in initial Fe added up to 250 mg l1 . It then
decreased for all samples. It appears that most of
the added Fe at initial concentrations <250 mg l1
could be retained on the exchange sites or precipitated as insoluble Fe compounds. The extent of
such processes was related to the characteristics of
zeolites and bentonite minerals as well as to the
chemical composition of the equilibrium system.
In that respect Krishnamurtti and Huang [27] reported the formation of lepidocrocite (c-FeOOH)
133
and maghemite (c-Fe2 O3 ) crystalline minerals in
FeCl2 –NH4 OH systems at pHs of 6.0 and 8.0, respectively. Therefore, it was likely that the application of Fe to different zeolites or bentonite
minerals may lead to oxidation and precipitation
of different forms of Fe. However, no attempt was
made to protect the systems from possible oxidation by air or even changing the suspension pH
during the experiment. The nature of precipitates
was attributed to the influence of different surface
properties present in the samples including the
accessory minerals, oxides, hydroxides and the
amorphous aluminosilicates. Data of Fed (Table 6)
indicated that the untreated zeolite species and
bentonite have considerable amounts of native
Fed and the highest amount was extracted from
phillipsite (6760 mg kg1 ) and bentonite (3720
mg kg1 ). Other zeolite minerals have relatively
low contents of native Fed , which range from 1000
mg kg1 for chabazite to 798 mg kg1 for clinoptilolite1. Data also indicated that the amounts of
Fed increased with the increase of sorbed Fe and
this was clear from the differences between Fe in
treated and untreated samples. Bentonite showed
the highest Fed from the sorbed Fe compared with
zeolite minerals which may indicate the possible
role of bentonite surfaces for precipitation of added Fe to form free iron oxides compared with
zeolite surfaces. In that respect Golden et al. [14]
found that iron oxides precipitated on exposed
surfaces of flooded montmorillonitic soils within
soil pores were relatively poorly crystallized while
those precipitated on rice-root surfaces were well
crystallized. On the other hand, data of amorphous Fe extracted by ammonium oxalate (Feox )
from untreated samples indicated that phillipsite
contains relatively high amounts (3249 mg kg1 )
followed by bentonite (1155 mg kg1 ). Also,
Table 4
Freundlich sorption equations and constants (log k and n) for Fe sorption on zeolites and bentonite
Mineral
Equations
Clinoptilolite1
Clinoptilolite2
Phillipsite
Chabazite
Analcime
Bentonite
Y
Y
Y
Y
Y
Y
¼ 1:562x þ 0:7961
¼ 1:1507x þ 1:4333
¼ 1:5332x þ 0:607
¼ 1:1909x þ 1:210
¼ 1:0864x þ 1:2766
¼ 1:3166x þ 1:2055
r2
log k (mmol kg1 )
n (kg l1 )
0.8576
0.7581
0.7449
0.8248
0.8991
0.8313
0.796
1.433
0.607
1.210
1.277
1.210
0.64
0.87
0.65
0.84
0.92
0.76
134
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
Table 5
Amounts of Fe sorbed, and DTPA extractable Fe (four successive extractions) for zeolites and bentonite minerals
P
P
Initial
Sorbed/added
DTPA
Sorbed/added
DTPA
x=m (mg kg1 )
x=m (mg kg1 )
a
concentration
(%)
extractable Fe
(%)
extractable Fea
(mg l1 )
(mg kg1 )
(mg kg1 )
0.00
5.0
25.0
50.0
250.0
500.0
Clinoptilolite1
–
23.13
335.2
797.4
4261.2
7600
–
23.1
67.0
79.7
85.2
76.0
35.6
51.2
128.0
406.4
1361.0
1663.6
Clinoptilolite2
–
36
336.4
796
4510
5894
–
36.0
67.3
79.6
90.2
58.9
20.6
36.4
166.4
389.4
1390.0
1542.2
0
5.0
25.0
50.0
250.0
500.0
Phillipsite
–
10.8
346.7
600
4391.3
6352
–
10.8
69.3
60.0
87.8
63.5
10.0
17.20
235.2
539.0
1700.7
1610.2
Chabazite
–
37.4
295.6
595
4393.8
5520
–
37.4
59.1
59.5
87.9
55.2
19.4
35.8
181.0
532.2
1605.2
2813.6
0
5.0
25.0
50.0
250.0
500.0
Analcime
–
48.4
296.6
407.8
3930
5300
–
48.4
59.3
40.8
78.6
53.0
26.8
34.2
141.8
376.8
1018.6
807.0
Bentonite
–
29.4
363.8
836
4139.3
–
–
29.4
72.8
83.6
82.8
–
13.8
31.4
259.0
405.0
1629.0
2320.0
a
Summation of DTPA extractable Fe (four successive extractions).
amounts Feox increased with the increase in
amounts of sorbed Fe and the increase was more
pronounced in all zeolite minerals compared with
bentonite, particularly at low concentrations of
applied Fe. Data of the iron activity ratio (Feox /
Fed ) were <1 in all the untreated samples reflecting
the dominance of well crystalline Fe oxides.
Treated zeolite samples have iron activity ratios >1
in most cases which may reflect the dominance of
amorphous or poorly crystalline Fe oxides phases
as a result of the Fe application to zeolite species.
Bentonite shows iron activity ratios <1, indicating
high amounts of crystalline Fe oxides (Fed ) and
less amounts of poorly crystalline and amorphous
oxides (Feox ). Therefore, these data clearly suggest
the existence of a variable role of zeolite and
bentonite mineral surfaces on the formation of
different Fe forms as a result of the Fe application. Data of the native Fe extracted by successive
DTPA extractions (Table 3) decreased with successive extractions in most of the cases. Analcime
showed the highest extractable native Fe in the
first extraction followed by clinoptilolite1 and
bentonite. In the fourth extraction clinoptilolite1
showed the highest extractable Fe followed by
analcime and chabazite. The cumulative concentrations of native Fe extracted from zeolite minerals ranged from 35.6 mg kg1 for clinoptilolite1
to 10.0 mg kg1 for phillipsite and for bentonite
(13.8 mg kg1 ). Amounts of extractable Fe from
treated samples increased in the first extraction in
all samples with the increase in initial Fe application and with the increase in the sorbed Fe. There
was a general decrease in the amount of extractable Fe with successive extraction, but all samples
indicated relatively high amounts even after the
fourth extraction. For example, at the highest application treatment (500 mg l1 ) about 160 mg kg1
was extracted from treated chabazite followed
by bentonite (153 mg kg1 ) while other zeolites
showed low extractable Fe that range from 54
mg kg1 for clinoptilolite2 to 68 mg kg1 for analcime. The data also indicated that the cumulative
amounts of extractable Fe largely increased with
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
135
Table 6
Amounts of CBD and NH4 -oxalate extractable Fe (mg kg1 ) from Fe treated and untreated zeolites and bentonite minerals
Treatment
Untreated (0)
Initial Fe concentration (mg Fe l1 )
5.00
25.0
50.0
250.0
500.0
Clinoptilolite1
Fed (mg kg1 )
Fed treated–Fed untreated
Feox (mg kg1 )
Feox treated–Feox untreated
Feox /Fed
798
–
236
–
0.296
820
22
293
57
2.59
1000
202
697
461
2.28
1348
550
1065
829
1.51
3780
2982
4410
4174
1.40
5800
5002
6825
6589
1.32
Clinoptilolite2
Fed (mg kg1 )
Fed treated–Fed untreated
Feox (mg kg1 )
Feox treated–Feox untreated
Feox /Fed
1180
–
570
–
0.483
1236
56
630
60
1.07
1524
344
975
405
1.18
1760
580
1425
855
1.47
4020
2840
4860
4290
1.51
5160
3980
6450
5880
1.48
Phillipsite
Fed (mg kg1 )
Fed treated–Fed untreated
Feox (mg kg1 )
Feox treated–Feox untreated
Feox /Fed
6760
–
3249
–
0.48
6840
80
3345
96
1.2
7160
400
3750
501
1.25
7500
840
4225
976
1.34
10100
3340
7125
3876
1.16
10760
4000
7590
4341
1.09
Chabazite
Fed (mg kg1 )
Fed treated–Fed untreated
Feox (mg kg1 )
Feox treated–Feox untreated
Feox /Fed
1000
–
143
–
0.14
1076
76
195
52
0.68
1260
260
483
340
1.31
1560
560
882
739
1.32
4100
3100
3923
3780
1.22
7480
6480
4725
4582
0.71
Analcime
Fed (mg kg1 )
Fed treated–Fed untreated
Feox (mg kg1 )
Feox treated–Feox untreated
Feox /Fed
840
–
518
–
0.62
900
60
576
58
0.97
1170
330
1002
484
1.47
1472
632
1500
982
1.55
3390
2550
4215
3697
1.45
4120
3280
5295
4777
1.46
Bentonite
Fed (mg kg1 )
Fed treated–Fed untreated
Feox (mg kg1 )
Feox treated–Feox untreated
Feox /Fed
3720
–
1155
–
0.31
3829
109
1252
97
0.89
4200
480
1402
247
0.52
4800
1080
1920
765
0.71
7520
3800
3765
2610
0.69
11000
7280
8400
7245
0.99
Fed ––Free iron oxides (Fe extracted by the CBD method).
Feox ––Amorphous or poorly crystalline iron (Fe extracted by NH4 -oxalate in darkness).
the increase of the sorbed Fe in all zeolite minerals,
and the extent of extractable Fe depends on the
type of zeolite. For bentonite, relatively high cumulative extractable Fe was obtained from the
retained Fe, particularly at high levels. These data
reflect the importance of the studied minerals as a
carrier for Fe, even though it was precipitated or
oxidized in the system. Correlation coefficients
were calculated between the amounts of Fed or
Feox and the cumulative DTPA extractable Fe
from the sorbed Fe. The results indicated that
cumulative DTPA extractable Fe was highly correlated with Fed (r2 ¼ 0:961) and Feox (r2 ¼ 0:88).
Therefore, the results suggest that sorbed or
freshly precipitated Fe on zeolites or bentonite
could be a source for increasing available iron.
136
A.S. Sheta et al. / Microporous and Mesoporous Materials 61 (2003) 127–136
4. Conclusions
The studied zeolite species (clinoptilolite1 and 2,
chabazite, phillipisite and analcime) and bentonite
showed considerable variations in Zn sorption
properties and DTPA extractability. Chabazite
has the highest ability for Zn sorption, while
bentonite has an intermediate ability compared
with other zeolites. Chabazite retains a relatively
high percentage of the sorbed Zn against the extraction by DTPA, even it released more Zn after
the fourth extraction by DTPA. Iron sorption data
were described by a Freundlich adsorption model.
Most of the sorbed Fe by zeolites was present in
the poorly crystalline or amorphous form while
that of bentonite was present in the free iron oxide
form. The results suggest that natural zeolite and
bentonite minerals have a high potential for Zn
and Fe retention. The availability of the retained
Zn and Fe was higher for Zn compared with Fe,
and chabazite seems to have the highest ability for
Zn sorption and extractability by DTPA. Bentonite has intermediate characteristics for Zn and
Fe sorption among the studied zeolite mineral
species.
Acknowledgement
This research was supported in part by the
Saudi Basic Industries Corporation (SABIC).
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