Hydrothermal modification of natural zeolites to improve uptake of ammonium ions

Article (PDF Available)inJournal of Chemical Technology & Biotechnology 80(4):376 - 380 · April 2005with85 Reads
DOI: 10.1002/jctb.1224
Abstract
The modification of natural zeolites was carried out under hydrothermal conditions to improve the effectiveness of the uptake of ammonium ions. Natural zeolites originating in Japan, such as mordenite and clinoptilolite with quartz, feldspar and a trace of layered silicate, were treated with 0.1, 0.3, 1.0 and 3.0 M NaOH solutions at temperatures from 25 to 150 °C under autogenous pressure for 7 days. After the hydrothermal treatment, the transformation of the zeolites to phillipsite, hydroxyl-sodalite and analcime was observed, depending on the temperature and NaOH concentration. The amounts of ammonium ions taken up into the hydrothermally-treated zeolites were compared with those of the starting materials. The treated products, containing mainly phillipsite, took up twice the amount of ammonium ions as the starting materials. The maximum uptake of ammonium ions was 1.92 mmol g−1. The number of ammonium ions taken up into phillipsite was equal to the number of Na+ ions released from phillipsite. These results indicate that the uptake of ammonium ions proceeds by an ion-exchange mechanism with Na+ ions. Copyright © 2005 Society of Chemical Industry
Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 80:376380 (2005)
DOI: 10.1002/jctb.1224
Hydrothermal modification of natural zeolites
to improve uptake of ammonium ions
Yujiro Watanabe,
1,2
Hirohisa Yamada,
1
Junzo Tanaka
1
and Yusuke Moriyoshi
2
1
National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
2
Faculty of Engineering, Hosei University, 3-7-2 Kajino-cho, Koganei, Tokyo 184-8584, Japan
Abstract: The modification of natural zeolites was carried out under hydrothermal conditions to improve
the effectiveness of the uptake of ammonium ions. Natural zeolites originating in Japan, such as mor denite
and clinoptilolite with quartz, feldspar and a trace of layered silicate, were treated with 0.1, 0.3, 1.0 and
3.0
M NaOH solutions at temperatures from 25 to 150
C under autogenous pressure for 7 days. After the
hydrothermal treatment, the transformation of the zeolites to phillipsite, hydroxyl-sodalite and analcime
was observed, depending on the temperature and NaOH concentration. The amounts of ammonium ions
taken up into the hydrothermally-tre ated zeolites were compared with those of the starting materials.
The treated products, containing mainly phillipsite, took up twice the amount of ammonium ions as
the starting materials. The maximum uptake of ammonium ions was 1.92 mmol g
1
. The number of
ammonium ions taken up into phillipsite was equal to the number of Na
+
ions released from phillipsite.
These results indicate that the uptake of ammonium ions proceeds by an ion-exchange mechanism with
Na
+
ions.
2005 Society of Chemical Industry
Keywords: ammonium; clinoptilolite; hydrothermal; mordenite; phillipsite
1 INTRODUCTION
Clinoptilolite and mordenite are the main natural
zeolite minerals found in abundance in many
locations.
1,2
They have high cation exchange capacity
(CEC) and ion adsorption capacity with remarkably
high selectivity for ammonium ions.
3
They are widely
used as low cost ion-exchangers and are especially
used for the removal of ammonium ions in wastewater
treatment.
4–6
Synthesis of high-quality zeolites by hydrothermal
treatment of various materials, such as fly ash,
7–9
kaolinite,
10
smectite,
11,12
bentonite
13
and natural
zeolite,
14,15
has been reported. Kang and Egashira et al
have synthesized sodium phillipsite (Na-P) with a trace
of feldspar from natural clinoptilolite and mordenite
by hydrothermal treatment with 2
M NaOH for 16 h
at 103
C.
14
They have found that when the natural
clinoptilolite is treated hydrothermally it changes to
Na-P, sodium faujasite (Na-X) and hydroxy-sodalite
depending on the reaction conditions.
15
The CEC
of the synthesized Na-P is higher than those of
Na-X, hydroxy-sodalite and the starting materials.
The improvement in the CECs of natural homoionic
zeolites by Na
+
exchange has also been reported by
many researchers.
16 18
In the present study, natural zeolites originating in
Japan, such as mordenite and clinoptilolite with traces
of quartz and feldspar, were treated hydrothermally
with 0.1, 0.3, 1.0 and 3.0
M NaOH solutions at
temperatures in the range of 25150
C for 7 days.
The amounts of ammonium ions taken up by the
hydrothermally-treated zeolites were compared with
those of untreated zeolites. The uptake mechanism
of ammonium ions was confirmed by investigation
of the modified material with the highest CEC for
ammonium ions.
2 EXPERIMENTAL PROCEDURE
Two natural zeolitic rocks mined in Shimane
Prefecture, located in the south-west of Japan, were
used as starting materials. The mineral composition
and morphology of this zeolite have been reported
by Watanabe et al
19
The first sample (CLI1) was
clinoptilolite with traces of quartz, feldspar and
layered silicate. The clinoptilolite in CLI1 was a
submicron-sized coffin-shaped crystallite. The Si/Al
molar ratio of CLI1 was 4.94, which is similar to that
published for natural clinoptilolites,
18,20
the surface
area was 91.8m
2
g
1
, and the pore volume was
0.144 mm
3
g
1
. The second one (MOR3) consisted
mainly of mordenite with quartz and a trace of
layered silicate. The mordenite in MOR3 had a
fibrous morphology with the fibres having lengths
of 1 5
µm and diameters of 0.050.1 µm. The Si/Al
molar ratio of MOR3 was 4.85, which is similar to the
Correspondence to: Yujiro Watanabe, National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan
E-mail: watanabe.yujiro@nims.go.jp
(Received 24 October 2003; revised version received 13 October 2004; accepted 21 October 2004)
Published online 17 February 2005
2005 Society of Chemical Industry. J Chem Technol Biotechnol 02682575/2005/$30.00 376
Uptake of ammonium ions by natural zeolites
published value of natural mordenite,
20
the surface
area was 129.6m
2
g
1
, and the pore volume was
0.143 mm
3
g
1
. They were sieved and only particles
with sizes below 50
µm were used as starting materials.
The mixtures consisting of 2.0 g of a starting
material and 30 cm
3
of 0.1, 0.3, 1.0 and 3.0 M
NaOH solutions were placed in Teflon cups fitted into
stainless steal pressure vessels and heated in an oven
at 25, 50, 100 and 150
C under autogenous pressure
for 7 days. The modification of natural zeolites was
performed under alkaline conditions because this
reaction is for recrystallization. The hydrothermal
treatment was carried out for 7 days in order to
remove as much of the impurities as possible. After the
hydrothermal treatment, the resultant products were
filtered through 0.45
µm pore size membrane filters
and washed several times with distilled water. Then
they were freeze-dried for 24 h.
Identification of minerals was carried out by
the powder X-ray diffraction (XRD) method with
monochromatized CuKα radiation at 40 kV and
40 mA using an RIGAKU RINT2200 diffractome-
ter. Morphological changes of the crystallites were
observed by scanning electron microscopy (SEM)
using a Hitachi S-5000 electron microscope. The
chemical composition of the sample obtained was
determined by inductively coupled plasma spec-
troscopy (ICP) (Seiko HVR 1700) with a photomul-
tiplier tube as a detector. The test solutions were
prepared as follows. For the measurement of Si and
Al, 50 mg of a sample were fused with Na
2
CO
3
(0.5g)
and H
3
BO
3
(0.2g) in a platinum crucible at 1000
C
for 10 min. The mixtures were dissolved in HCl solu-
tion after cooling, and distilled water was added to
the solutions to give a final volume of 100 cm
3
.For
themeasurementofNa,K,CaandMg,50mgofa
sample were dissolved in 2 cm
3
of HF (27 mol dm
3
)
and 1 cm
3
of H
2
SO
4
(18 mol dm
3
) and evaporated.
The dried residue was then dissolved in HCl solu-
tion with heating, and distilled water was added to
the solutions to give a final volume of 100 cm
3
.The
specific surface areas were measured by the multi-
point BrunauerEmmetTeller (BET) method using
a Beckman Coulter SA3100 instrument with nitrogen
gas as an adsorbent after degassing at 200
Cfor3h
in vacuum.
For experiments on the uptake of ammonium
ions, 30.0cm
3
of solutions containing different
concentrations of NH
4
Cl (10
3
–10
2
M, initial pH =
5.1–5.4) were added to 0.1 g of each product in
stoppered polyethylene tubes. The tubes were shaken
at 30 rpm by an end-over-end shaker (Towa Labo
RKVSD 10 101) for 7 days at 25
C, and then
the solids phase was separated by centrifugation at
15 000 rpm for 30 min and filtration using membrane
filters of 0.45
µm pore size.
The concentrations of ammonium ions in the
filtered solutions were determined with an ammonium
specific ion electrode (Toa Dempa Kogyo IM-
20B, Ammonia Electrode Ae-235). The amounts
of ammonium ions taken up were calculated from
differences between the concentrations of ammonium
ions in the filtered solutions and those in the initial
solutions. The values were the averages of three
samples. The concentrations of Na
+
,K
+
,Ca
2+
and
Mg
2+
in the filtered solutions were determined by
ICP analysis (Seiko SPS4000). Before and after the
experiments on uptake, the separated solids were
examined for the confirmation of the changes in the
zeolite structures by the powder XRD method. The
CECs of the samples were determined by extracting
ammonium ions with 1
M KCl solution (pH = 7)
from the ammonium-saturated samples obtained by
repeating centrifugal washing with 1
M CH
3
COONH
4
solution (pH = 7).
14
The amounts of ammonium ions
were determined with an ammonium specific ion
electrode.
3 RESULTS AND DISCUSSION
3.1 Characterization of hydrothermally-
modified products
Changes of the phases of the natural zeolites, CLI1
and MOR3, by hydrothermal treatment are shown in
Table 1. The transformation of the natural zeolites to
phillipsite, hydroxy-sodalite and analcime depended
on the types of the zeolitic rocks, and the reaction
conditions such as hydrothermal temperature and
NaOH concentration.
The structure of CLI1 was not changed after
treatment with 0.1
M NaOH solution at temperatures
below 150
C. The CLI1 samples treated with 0.3 M
NaOH solution at 25, 50 and 100
C also did not show
any change in their XRD patterns. But after treatment
at 150
C, the amount of clinoptilolite, which is the
major zeolite species in CLI1, decreased, and analcime
was formed. CLI1 treated with 1
M NaOH solution
at 25
C did not show any phase change, but at
50
C, the low intensity pattern of phillipsite was
identified. Treatment at 100
C transformed CLI1
mainly to phillipsite with a trace of feldspar, and at
150
C, analcime with a cubo-octahedral shape of
1030
µm in diameter was formed. The CLI1 treated
with 3
M NaOH solution at 25
C did not show any
phase change. At 50
C, the phase consisted mainly
of phillipsite with traces of clinoptilolite and feldspar.
The treatment at 100
C led to phillipsite and hydroxy-
sodalite with a trace of feldspar, and that at 150
Cled
to sodalite with layered silicate.
On the other hand, MOR3 treated with 0.1
M
NaOH solution at temperatures below 150
Cand
with 0.3
M NaOH or 1 M NaOH solution at below
50
C did not show the structural changes. After
treatment with 0.3
M NaOH solution at 100 and
150
C, phillipsite started to appear. After treatment
with 1
M NaOH solution at 100
C, the main phase
became phillipsite with traces of quartz and feldspar,
whereas at 150
C MOR3 was mainly transformed
to analcime with traces of quartz and feldspar. The
MOR3 in 3
M NaOH solution was transformed to
J Chem Technol Biotechnol 80:376380 (2005) 377
Y Watanabe et al
Table 1. The hydrothermal modifications of natural zeolites, CLI1
and MOR3
Starting
material T(
C) C
NaOH
(M)Products
CLI1 25 0.1 Cli, Feld, Qtz, LS
0.3 Cli, Feld, Qtz, L S
1.0 Cli, Feld, Qtz, L S
3.0 Cli, Feld, Qtz, L S
50 0.1 Cli, Feld, Qtz, LS
0.3 Cli, Feld, Qtz, L S
1.0 Cli, Feld, Qtz, LS, Phi
3.0 Phi, Cli, Feld, Qtz, LS
100 0.1 Cli, Feld, Qtz, LS
0.3 Cli, Feld, Qtz, L S
1.0 Phi, Feld
3.0 Phi,Sod,Feld
150 0.1 Cli, Feld, Qtz, LS
0.3 Cli, Ana, Feld, Qtz, LS
1.0 Ana
3.0 Sod, LS
MOR3 25 0.1 Mor, Qtz, LS
0.3 Mor,Qtz,LS
1.0 Mor,Qtz,LS
3.0 Mor,Qtz,LS
50 0.1 Mor, Qtz, LS
0.3 Mor,Qtz,LS
1.0 Mor,Qtz,LS
3.0 Phi,Mor,Qtz,LS
100 0.1 Mor, Qtz, LS
0.3 Mor,Phi,Qtz,LS
1.0 Phi, Q tz, LS
3.0 Phi
150 0.1 Mor, Qtz, LS
0.3 Mor,Phi,Qtz,LS
1.0 Ana, Qtz, Fed
3.0 Phi, Sod
C
NaOH
(M): NaOH concentration (M).
Cli: Clinoptilolite, Mor: mordenite, Phi: phillipsite, Ana: analcime, Sod:
sodalite, Qtz: quartz, Feld: feldspar, LS: layered silicate.
phillipsite or hydroxy-sodalite depending on treatment
temperature.
The chemical composition of MOR3 obtained after
treatment with 3
M NaOH solution at 100
Cas
determined by ICP analysis showed that the Si/Al
molar ratio was about 1.64, and the amount of Na
+
ions was 13.0 wt%, which is nearly equal to the Si/Al
molar ratio and CEC of synthetic PHI.
3
K
+
,Ca
2+
and
Mg
2+
were not detected by ICP analysis.
The SEM image of the phillipsite showed aggregates
or intergrown fine rosette crystallites, submicrons in
size (Fig 1). These results indicate that the phase
transformation of natural zeolites as the result of
hydrothermal treatment depends on the compositions
of starting zeolite materials, hydrothermal temperature
and NaOH concentration.
3.2 Ammonium uptake
Experiments on the uptake of ammonium ions were
carried out with the samples modified by hydrothermal
treatment with 1 and 3
M NaOH solutions at 100
C.
5µm
Figure 1. The SEM image of phillipsite obtained after treatment of
natural mordenite (MOR3) with 3
M NaOH solution at 100
C.
The relationship between the amounts of ammonium
ions taken up by the modified samples of CLI1
and MOR3, and the equilibrium concentrations of
ammonium chloride are shown in Figs 2 and 3,
respectively.
Phillipsite obtained from MOR3 treated at 100
C
in 3
M NaOH solution has a greater capacity for
the uptake of ammonium ions than the untreated
sample, as reported by Watanabe et al
19
In the
present study, the amount of ammonium ions ion-
exchanged by phillipsite (1.92 mmol g
1
in 10 mM
NH
4
Cl) was about two-fold greater than that obtained
by MOR3. The surface area of phillipsite was
31.4m
2
g
1
, which is lower than that obtained
from MOR3 (129.6m
2
g
1
), and the pore volume
was 0.119 mm
3
g
1
. These results indicate that
the surface area is not related to uptake of
ammonium ions.
, phillipsite with trace of feldspar (1 M NaOH, 100°C)
, phillipsite with traces of hydroxy-sodalite
and feldspar (3
M NaOH, 100°C)
, starting material (CLI1)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
The uptake of ammonium ions (mmol g
-1
)
0.0 1.0 2.0 3.0 4.0 5.0 6.0
The equilibrium concentration of ammonium chloride (m
M)
Figure 2. The adsorption isotherms for modified clinoptilolite.
19
378 J Chem Technol Biotechnol 80:376380 (2005)
Uptake of ammonium ions by natural zeolites
0.0 1.0 2.0 3.0 4.0 5.0 6.0
The equilibrium concentration of ammonium chloride (mM)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
The uptake of ammonium ions (mmol g
-1
)
, phillipsite with trace of feldspar (1M NaOH, 100°C)
, starting material (MOR3)
, phillipsite (3
M NaOH, 100°C)
Figure 3. The adsorption isotherms for modified mordenite.
19
Similar results were obtained from CLI1 treated
with 1
M NaOH solution at 100
C. When CLI1
was transformed to phillipsite, the uptake of ammo-
nium ions increased. The increase in the uptake of
ammonium ions by phillipsite is related to the larger
number of ion-exchange sites in the phillipsite frame-
work compared with those in mordenite and clinop-
tilolite lattices.
20
The real CECs of CLI1, MOR3
and the treated zeolites are shown in Table 2. The
CECs of phillipsite obtained from CLI1 and MOR3
treated with 3
M NaOH at 100
Cwere2.88and
3.13 meq g
1
, respectively, but the values were nearly
two times larger than those of CLI1 (1.40 meq g
1
)
and MOR3 (1.12 meq g
1
). From these results we
conclude that the change of clinoptilolite and mor-
denite to phillipsite improves CEC and uptake of
ammonium ions.
The CECs of the zeolites used in this study are
compared with other published data on zeolites in
Table 3. This comparison indicates that the treated
zeolites composed of mainly phillipsite have similar
but slightly higher exchange capacities than reported
previously for other zeolites.
15,17,21,22
Table 2. Cation exchange capacities of CLI1, MOR3 and
treated zeolite
Starting
material T(
C) C
NaOH
(M)CEC(meqg
1
)
CLI1 1.40
100 1.0 2.76
100 3.0 2.88
MOR3 1.12
100 1.0 2.30
100 3.0 3.13
C
NaOH
(M): NaOH concentration (M).
Table 3. Cation exchange capacities of various zeolites
Zeolite origin
Cation exchange
capacity meq g
1
Reference
CLI1 1.40
MOR3 1.12
CLI1 treated, 3
M, at 100
C2.88
MOR3 treated, 3
M, at 100
C3.13
Natural clinoptilolite 1.75 15
Na-P 4.18 15
Natural clinoptilolite (Na type) 0.44 17
Faujasite (Na type) 3.20 21
Natural clinoptilolite (Na type) 2.05 22
3.3 Ion-exchange mechanism of ammonium
ions
Phillipsite modified from MOR3 at 100
Cin
3
M NaOH solution, with the highest ion-exchange
capacity for ammonium ions, was used to investigate
the uptake mechanism of ammonium ions. Figure 4
shows the relationship between the uptake of
ammonium ions by phillipsite and the amounts of Na
+
released from phillipsite. The numbers of ammonium
ions taken up by phillipsite were equal to those of Na
+
released from phillipsite. The amounts of K
+
,Ca
2+
and Mg
2+
were not detected. These results indicate
that the uptake of ammonium ions proceeds by an ion-
exchange mechanism. X-ray diffraction patterns did
not show any structural change of phillipsite after ion-
exchange, which proves that the framework structure
is stable under this condition.
4 CONCLUSION
Natural zeolitic rocks, such as mordenite and
clinoptilolite with traces of quartz and feldspar,
were treated hydrothermally with 0.1, 0.3, 1.0 and
3.0
M NaOH solutions at temperatures below 150
C.
The phase changes of these zeolites depended on
6
4
2
0
The uptake of ammonium ions (meq dm
-3
)
0246
Na
+
ions concentration in solutions (meq dm
-3
)
Figure 4. The relationship between the amounts of Na
+
ions released
and the amounts of ammonium ions taken up by phillipsite (3
M
NaOH, 100
C).
J Chem Technol Biotechnol 80:376380 (2005) 379
Y Watanabe et al
reaction conditions such as hydrothermal temperature
and NaOH concentration, as well as the chemical
compositions of the starting materials.
The amount of ammonium ions taken up by the
product modified from MOR3 at 100
Cin3M
NaOH solution, which contained mainly phillipsite,
was two-fold greater (1.92 mmol g
1
) than that taken
up by the starting material. This result is explained
by the higher CEC of the phillipsite framework
in comparison to mordenite and clinoptilolite. The
mechanism of ammonium uptake by phillipsite
involved ion-exchange of Na
+
ions with ammonium
ions. The framework structure of phillipsite was
stable in ammonium ion solutions, and therefore
we conclude that phillipsite is ideal for use as an
industrial ion-exchanger to remove ammonium ions
from wastewater.
ACKNOWLEDGMENTS
The authors thank Prof Y Komatsu, Kanazawa Insti-
tute of Technology, Japan, Prof J Michalik, Institute
for Nuclear Chemistry and Technology, Poland and
Dr Takeshi Kasama, Universitat Muenster, Germany
for fruitful discussion and precious comments. We
also wish to thank Mr Y Yajima for his help in ICP
analysis, and Mr H Komori for SEM operation.
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J Chem Technol Biotechnol 80:376380 (2005)
    • "Zeolites can be readily synthesised in hydrothermal conditions and a lot of information is readily available concerning synthetic zeolites and chemical resolution for zeolite synthesis (Hawkins, 1981). The chemical preparation of synthetic zeolites from silica and alumina is expensive; zeolite researchers are thus seeking cheaper raw materials for zeolite synthesis to reduce costs, including clay minerals such as kaolinite (Breck, 1974; Barrer et al., 1974; Boukadir et al., 2002), halloysite (Klimkiewicz and Drąg, 2004), illite, smectite, interstratified illite-smectite (Baccouche et al., 1998), montmorillonite (Cañizares et al., 2000) and bentonite (Boukadir et al., 2002; Ruiz et al., 1997), natural zeolites (Kang and Kazuhiko, 1997; Watanabe et al., 2005; Covarrubias et al., 2006), volcanic glasses (Breck, 1974; Barrer, 1982; Vitarelli et al., 1983; Colella et al., 1985; Moirou et al., 2000), diatomite (Anderson et al., 2005; Holmes et al., 2001), high silica bauxite (Puerto and Benito, 1996), oil shale (Shawabkeh, 2004) and natural clinker (Ríos and Williams, 2008; Ríos et al., 2008; Sandoval et al., 2009). "
    [Show abstract] [Hide abstract] ABSTRACT: The present study concerns the synthesis of zeolites from Neogene sedimentary and pyroclastic rocks exposed around the Sochagota Lake, Paipa (Boyacá). Two synthesis methods were used: conventional hydrothermal treatment and alkaline fusion followed by hydrothermal reaction. Both raw materials and synthesised zeolytic products were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Several zeolytic phases were synthesised, including faujasite (FAU), phillip-site (PHI) and sodalite (SOD). The results showed that the alkaline fusion approach was more efficient regarding hydrothermal conversion of the raw materials than conventional hydrothermal treatment, taking into account that zeolytic products having a higher degree of crystallinity and few impurities were obtained in this way. This study was aimed at applying experimental mineralogy to a laboratory simulation of the geological conditions in which zeolites can occur as the basis for defining criteria for exploring natural zeolites in Colombia, with prospects for the profitable exploitation of these mineral resources in different parts of Colombia.
    Article · Dec 2011
    • "The ammonia removal appeared to proceed through rapid kinetics at neutral pH value, with removal capacities up to 0.68 meq NH 4 + /g. Watanabe et al. [61] carried out a modification of Japanese natural zeolites under hydrothermal conditions to improve the effectiveness of the uptake of ammonium ions. The samples were treated with 0.1, 0.3, 1.0 and 3.0 M NaOH solutions at temperatures from 25 to 150 @BULLET C under autogenous pressure for 7 days. "
    [Show abstract] [Hide abstract] ABSTRACT: Natural zeolites are abundant and low cost resources, which are crystalline hydrated aluminosilicates with a framework structure containing pores occupied by water, alkali and alkaline earth cations. Due to their high cation-exchange ability as well as to the molecular sieve properties, natural zeolites have been widely used as adsorbents in separation and purification processes in the past decades. In this paper, we review the recent development of natural zeolites as adsorbents in water and wastewater treatment. The properties and modification of natural zeolite are discussed. Various natural zeolites around the world have shown varying ion-exchange capacity for cations such as ammonium and heavy metal ions. Some zeolites also show adsorption of anions and organics from aqueous solution. Modification of natural zeolites can be done in several methods such as acid treatment, ion exchange, and surfactant functionalisation, making the modified zeolites achieving higher adsorption capacity for organics and anions.
    Full-text · Article · Jan 2010
  • [Show abstract] [Hide abstract] ABSTRACT: The overlay of a wireless local loop system on the CDMA2000 cellular system is proposed. The CDMA2000 system uses 5, 10, 15 and 20 MHz of dispersion bandwidth and the WLL system uses 1.25 MHz of dispersion bandwidth. In this way, it is possible to have 3, 4, 8, 9, 12 and 16 frequency allocations for the WLL system depending on the different bandwidths of the CDMA2000 cellular system. As an overlay system the WLL can use only a fraction of the frequency allocations according to co-frequency allocation reuse distance, similar to FDMA and TDMA cellular systems. With this idea less interference is introduced in the WLL system (in the limit, the capacity will be similar to the isolated cell) keeping at the same value the interference introduced in the CDMA cellular system
    Conference Paper · Feb 2001
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