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Nanoparticulate Zeolitic Tuff for Immobilizing Heavy Metals in Soil: Preparation and Characterization

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Nanoparticles derived from natural materials are promising compounds in the field of environmental remediation. The present study produces and characterizes Na-zeolitic tuff in the nanorange, stabilizes the nanotuff in suspension, and investigates the effect of Na-zeolitic nanotuff on sorption of Cd. Breakdown of raw zeolitic tuff with a mean particle size of 109μm to the nanorange was achieved by attrition milling. In the first stage of grinding, a mixture of Al-oxide beads of 1 to 2.6mm diameter was used. The milling process lasted 4h. In the second stage, the dried powder was milled again using a mixture of a fine zirconia beads (0.1mm) and Al-oxide beads (1.0mm). The powder was treated with 1M NaCl solution. Finally, the powder was sonicated in water. After this procedure, the mean and median particle diameters were 47.6 and 41.8nm, respectively. The nanoparticulate zeolitic tuff had a surface area of 82m2 g−1. The estimated zero charge point of the nanoparticle suspension was 3.2. The surface zeta potential was pH dependent. The Na-zeolitic nanotuff increased Cd sorption by a factor of up to 3 compared to the raw zeolitic tuff. Our results indicate that zeolitic nanoparticles can be produced by grinding using a mixture of fine beads in an attrition mill and that this procedure increases their metal immobilizing potential.
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Nanoparticulate Zeolitic Tuff for Immobilizing Heavy
Metals in Soil: Preparation and Characterization
Ayoup M. Ghrair &Joachim Ingwersen &
Thilo Streck
Received: 1 August 2008 / Accepted: 26 January 2009 / Published online: 13 February 2009
#Springer Science + Business Media B.V. 2009
Abstract Nanoparticles derived from natural materials
are promising compounds in the field of environmental
remediation. The present study produces and character-
izes Na-zeolitic tuff in the nanorange, stabilizes the
nanotuff in suspension, and investigates the effect of
Na-zeolitic nanotuff on sorption of Cd. Breakdown of
raw zeolitic tuff with a mean particle size of 109 μmto
the nanorange was achieved by attrition milling. In the
first stage of grinding, a mixture of Al-oxide beads of
1 to 2.6 mm diameter was used. The milling process
lasted 4 h. In the second stage, the dried powder was
milled again using a mixture of a fine zirconia beads
(0.1 mm) and Al-oxide beads (1.0 mm). The powder
was treated with 1 M NaCl solution. Finally, the powder
was sonicated in water. After this procedure, the mean
and median particle diameters were 47.6 and 41.8 nm,
respectively. The nanoparticulate zeolitic tuff had a
surface area of 82 m
2
g
1
. The estimated zero charge
point of the nanoparticle suspension was 3.2. The
surface zeta potential was pH dependent. The Na-
zeolitic nanotuff increased Cd sorption by a factor of
up to 3 compared to the raw zeolitic tuff. Our results
indicate that zeolitic nanoparticles can be produced by
grinding using a mixture of fine beads in an attrition
mill and that this procedure increases their metal
immobilizing potential.
Keywords Nanoparticles .Zeolitic nanotuff .
Surface area .Attrition mill .Heavy metals .Sorption .
Zeta potential
1 Introduction
Heavy metal contamination of soil is a key concern
because of its toxicity, which threatens human life and
the environment (Bhogal et al. 2003; Carmichael
1994; Purves 1985). Cadmium (Cd) is of particular
interest because it is one of the most bioavailable and
mobile heavy metals in soil and the environment.
Cadmium is a highly toxic element that is not known
to be essential for any type of organism (McBirde
1994). Cd is easily absorbed by roots and transported
to shoots. It is then uniformly distributed in plant
organs (Sekara et al. 2005). Uptake of Cd by plants
depends on several plant and soil factors such as
solution-phase concentration, pH, and organic carbon
content. Moreover, the environmental conditions
(temperature and saturation deficit) may play an
important role in controlling Cd uptake (Ingwersen
and Streck 2005). Anthropogenic contamination of
soils by heavy metals (Cd, Pb, and Zn) occurs from
many sources such as mining, atmospheric deposition
Water Air Soil Pollut (2009) 203:155168
DOI 10.1007/s11270-009-9999-6
A. M. Ghrair (*):J. Ingwersen :T. Streck
Biogeophysics Section,
Institute of Soil Science and Land Evaluation,
University of Hohenheim,
Emil-Wolff-Str. 27,
70593 Stuttgart, Germany
e-mail: ghrair@uni-hohenheim.de
from smelting operations, application of sludge, and
mineral fertilizers and pesticides (Alloway 1995).
For on-site remediation of heavy-metal-contaminated
sites, several soil amendments (among others, zeolitic
tuff) have been tested and proposed during the last
decades. Zeolitic tuff is a volcanoclastic deposit that
contains large amounts of zeolites, reflecting the
transformation of magmatic products such as volcanic
glass and primary aluminum silicate minerals (Hay and
Sheppard 2001). Zeolites are secondary minerals that
can be defined as crystalline hydrated aluminosilicates
of alkali and alkaline earth cations having an infinite
three-dimensional framework (Mumpton 1977).
Natural zeolite minerals have been evaluated as
sequestering agents for environmental cleanups and
for immobilizing heavy metals in moderately polluted
soils (Querol et al. 2006; Knox et al. 2003; Tsitsishvili
et al. 1992). Zeolites have been used for a long time
in Japan to improve soil quality (Oste et al. 2002).
They are added to the soil to control soil pH and
nitrogen retention. In wastewater treatment, natural
zeolites are used to remove ammonium ions and
heavy metals (Singh and Oste 2001; Zamzow et al.
1990) and in the amendment of sewage sludge
(Sprynskyy et al. 2007; Nissen et al. 2000). The
remediation of soil contaminated by lead and/or
cadmium by applying zeolite significantly reduced
the solubility of these two heavy metals in the soil
(Garau et al. 2007; Fenn et al. 2006).
Kalantari et al. (2006) showed that applying
500 kg ha
1
zeolite increased the growth and yield
of rice growing in contaminated soils and that the
level of plant-available cadmium was reduced by half.
The addition of zeolite reduced the Cd content of the
rice grain by up to 67% compared to the control.
However, the use of zeolites does not always decrease
metal availability (Madrid et al. 2008). Similarly,
Weber et al. (1984) investigated the effect of natural
zeolite amendment on heavy metal uptake by sorghum
from an arable soil. They reported no reduction of
heavy metal uptake even at an application rate of
approximately 6.5% by weight. In addition, other
studies showed that applying zeolite only minimally
affected the plant availability of metals (Chlopecka and
Adriano, 1997,1996;Baydina1996).
A factor that limits the use of zeolitic tuff is the
high application rate needed to effectively immobilize
heavy metals. Zorpas et al. (2003) used an application
rate of 25% by weight to obtain significant effects.
Brannvall (2006) reported that adding 20% zeolite to
polluted sandy soil immobilized 29% Cu, 56% Pb,
and 54% Zn.
One of the limiting properties of natural zeolite is its
relatively small surface area compared to nanomaterial.
Decker et al. (2003) reported that the BET specific
surface area of zeolitic tuff was between 7.8 and
12.3 m
2
g
1
. B.E.T. (or BET) stands for Brunauer,
Emmett, and Teller, the three scientists who optimized
the theory for measuring surface area (Brunauer et al.
1938). In contrast, the BET specific surface area of
nanoparticles is 50100 m
2
g
1
(which is considered
an intermediate surface area; Kockrick et al. 2008).
Pretreating zeolite with NaCl may significantly
increase sorption of pollutants. Athanasiadis and
Helmreich (2005) treated clinoptilolite with 1 M NaCl
solution at room temperature over a 24-h period.
Afterward, the clinoptilolite was washed with ultra-
pure water and ultrasonicated several times, and then
the sample was dried. This pretreatment replaced all
exchangeable cations with Na, which is more easily
exchangeable than bivalent cations such as Ca or Mg
(Cmielewska and Lesny 1995; Bremmer and Schultze
1995).
The starting point of our study was that nanoscale
zeolite particles should be a promising candidate for
heavy metal immobilization because they provide a large
external surface area and can also be easily modified to
effectively bind metals species. Nanoparticles provide
cost-effective solutions to environmental cleanup prob-
lems (Zhang et al. 2004;Zhang2003).
Nanoparticles are defined as particles with an
average characteristic dimension <100 nm (Dutta et al.
2000). Nanoparticles are seen as important auxiliary
materials for soil remediation because of their large
surface area and other beneficial physical properties.
These include magnetic properties, optical properties
(e.g., transparency) along with thermal properties and
chemical properties such as reactivity (Perez et al.
2004). Nanoparticles can be produced following a
bottom-up process, for example synthesis of nano-
particles from solutions, or by a top-down approach in
which nanoparticles are produced, e.g., by mechanical
attrition (De Castro and Mitchell 2003).
Mechanical milling is widely used in the mineral-
processing industry. Here, a powder is subjected to
grinding under high-energy compressive impact
forces (Zhang et al. 2003). Different types of ball
millsattrition mills, planetary mills, tumber mills,
156 Water Air Soil Pollut (2009) 203:155168
shaker mills, and vibrator millshave been manufac-
tured for various purposes. In mechanical attrition, the
size of the bulk material is reduced by milling. This is
a simple technique with low-cost equipment. None-
theless, particles produced by this method usually
have a broad size distribution, and contamination
from the milling machinery is often a problem (Tjong
and Chen 2004; Edelstein and Cammarata 1996,
Ichinose et al. 1992).
An attrition ball mill (attritor) consists of a rotating
vertical drum with a series of impellers which move
inside a grinding tank (Kuhn 1984). Attritors are mills
in which large quantities of powder (0.5 to 40 kg) can
be milled at a time. The grinding tanks are available
in stainless steel or stainless steel coated with
alumina, silicon carbide, silicon nitride, zirconia,
rubber, or polyurethane. Different balls and beads
such as glass, flint stones, stealite ceramic, mullite,
silicon carbide, silicon nitride, silicon, alumina,
zirconium silicate, zirconia, stainless steel, carbon
steel, chrome steel, and tungsten carbide are available.
Operation of an attritor is simple: The powder to be
milled is placed in a container with the balls, and the
mixture is then agitated and rotated at a high speed by
a shaft with arms. This causes the milling media to
exert both shearing and impact forces on the material
(De Castro and Mitchell 2003).
Historically, attrition milling was developed for
producing metal powders for the purpose of alloying.
It was first developed by John Benjamin at the
International Nickel company (Kimura et al. 1999).
In recent years, this technique has been used to
produce a wide range of materials including metastable
structures such as amorphous (Koch et al. 1983)and
quasicrystalline materials (Eckert et al. 1989). The
milling process of material particles to powder size
causes attrition between balls, between the balls and
the container wall, between the balls and the agitator
shaft and the impellers (De Castro and Mitchell 2003).
Studies on the production of nanoparticles by mechan-
ical attrition have been reviewed by several researchers
such as Koch (1993), Siegel (1991), and Gessinger
(1984).
Here, we report for the first time on the preparation
of Na-zeolitic tuff in the nanorange by mechanical
attrition milling. The objectives of our study were (1)
to produce and characterize Na-zeolitic tuff in the
nanorange, (2) to stabilize the zeolitic tuff nanoparticle
in suspension, and (3) to quantify the extent to which
sorption of Cd to zeolitic tuff can be enhanced by
converting the raw material to the nanosize.
2 Materials and Methods
2.1 Raw Zeolitic Tuff
Zeolitic tuff was collected from Tell Rimah volcano,
northeast of Jordan. The sample was crushed and
sieved. The mean particle size was 109 μm (median
119 μm). The total cation exchange capacity (CEC)
was 93.4 cmol
c
kg
1
. The zeolitic tuff was strongly
alkaline (pH 8.8). Exchangeable cations were 6.61%
Ca, 4.55% Mg, 1.67% K, and 0.30% Na (by weight).
2.2 Preexperiments
In preexperiments, different mills (planetary ball and
attrition ball mill), bead materials (Al-oxide beads and
zirconium oxide), bead sizes (from 15 to 0.1 mm),
and grinding times (from 0.5 to 20 h) were tested.
2.2.1 Planetary Ball Milling
A planetary ball mill PM 1000 (Retsch GmbH, Haan,
Germany) was employed. The designation refers to the
rotating support disk, with each jar rotating around its
own axis. The rotation speed was 210 rpm. The
diameter of the tested zirconium dioxide balls was
15 mm. The ball-to-powder weight ratio was 16.
Grinding times ranged between 2 and 20 h without
change in the number or diameter of balls.
2.2.2 Attrition Milling
A laboratory-sized PE/PR attrition mill (Attritor,
Siemens, Germany) was used. The mill had a
chamber and a vertical rotating central shaft with
horizontal arms (impellers). The mill chamber was
filled with 1.62.6 or 1 mm beads of aluminum oxide
(Krahn Chemie GmbH, Hamburg, Germany). The
weight ratio of beads-to-zeolitic tuff powder was 24.
The rotation rate was set to 1,300 rpm. In all cases,
ethanol was used as solvent. The milling process
involved stirring by the agitator and the impellers.
The impellers energize the ball charge. The grinding
vessel was jacketed for cooling (Bilgili et al. 2004;De
Castro and Mitchell 2003).
Water Air Soil Pollut (2009) 203:155168 157
2.3 Preparation of Na-Zeolitic Nanotuff
The Na-zeolitic tuff nanoparticles were prepared
using the following method: 50 g of raw zeolitic tuff
was added to 1,200 g of a mixture of Al-oxide beads
(1 and 1.6-2.6 mm diameter, unit weight ratio,
KRAHN Chemie GmbH, Hamburg, Germany). The
ratio of beads to powder was adjusted to 24 by
weight. In the first stage, the zeolitic tuff was milled
for 4 h. After that, the milling beads were separated
from the suspension with a sieve chain and subse-
quently dried using a vacuum evaporator. The dry
powders were obtained after 15 h of drying at 60°C in
a drying oven. In the second stage, the dried powders
were milled again for 4 h using 1,600 g of a mixture
of 0.1 mm zirconium dioxide beads (YTZ, Tosoh,
Co., Japan) and 1.0 mm aluminum oxide beads
(KRAHN Chemie GmbH, Hamburg, Germany) in a
weight ratio of 2:1. In this stage, the beads to powder
weight ratio was 32:1. The suspension was separated
from the milling media as described above. Ethanol
was separated from the sample in a vacuum evapo-
rator, enabling reuse. The powders were dried in an
oven at 60°C over night. After drying, the powders
were passed through a 20-μm sieve. Subsequently,
each powder was treated with 1 M NaCl solution over
a period of 24 h. Finally, the powders were washed
with ethanol and sonicated at 100% amplitude and
one cycle for 20 min using ultrasonic processors (UP
200S, Germany). To avoid a temperature increase
during the sonication process, an ice coat was used.
2.4 Characterization of Zeolitic Tuff Nanoparticles
Size and shape are important properties of nano-
particles. The size distribution of zeolitic tuff nano-
particles was investigated under transmission electron
microscopy (TEM). TEM studies were performed
using a JEOL100CX II transmission electron micro-
scope (JEOL, Tokyo, Japan) operating at 200 kV and
a magnification of ×200,000. For TEM observation,
nanoparticles were dispersed on a copper grid. The
TEM images were analyzed using the software Image
Tool for Windows, Version 3 (The University of
Health Science Centre at San Antonio, Texas, USA).
A laser diffractometer (Malvern Mastersizer 2000,
Malvern Instruments Ltd., UK) was used to measure
the agglomerate size distribution. The surface charge
(Q) of nanoparticles was characterized by a zeta
potential (ζ) measurement. Zeta potential is a measure
of the electrostatic potential generated by accumula-
tion of ions that are organized into an electrical
double layer at the surface of a particle (Sposito
1984). The zeta potential determines the colloidal
stability (Fernandez-Nieves and de las Nieves 1999),
as given in the equation:
Q¼4p""0zr1þkrðÞ ð1Þ
where ɛis the relative dielectric constant of medium,
ɛ
0
is the dielectric constant of vacuum, and κris the
electrokinetic radius. The zeta potential was determined
by an electroacoustic method using an acoustic and
electroacoustic spectrometer (DT-1200, Dispersion
Technology Ltd., USA). This instrument was also used
to measure the particle size distribution. X-ray powder
diffraction patterns were recorded using Cu Kα
radiation source on a Scintag X1 powder diffractometer
(D-500, Siemens AG, Germany). An ultrasonic pro-
cessor (UP 200S, Germany) was used for dispersing
nanoparticles. The chemical composition of the nano-
particles powder and raw zeolitic tuff was determined
by X-ray fluorescence analysis measurements using a
sequential X-ray spectrometer (SRS 200, Siemens AG,
Germany).
2.5 Sorption Experiment in Aqueous System
Sorption experiments were carried out by adding
0.25 g of nanoparticles to 25 ml 0.01 M Ca(NO
3
)
2
solution containing Cd at five different concentrations
(0.2, 0.5, 1.0, 2.0, or 5.0 mg/l). The samples were
shaken for 48 h on a horizontal shaker at 180 rpm and
at room temperature (20±2°C). Afterward, the sam-
ples were centrifuged at 20,000×gfor 30 min (Sorvall
Superspeed Fixed-Angle Rotors, Kendro, USA). A
10-ml aliquot of the supernatant was removed and
stored in a plastic tube at 4°C. Cd solution phase
concentrations were determined by flameless atomic
absorption spectrometry (SpectrAA-800, Varian
Deutschland GmbH, Germany). Sorbed phase con-
centrations were calculated from the mass balance.
All sorption experiments were conducted using 50 ml
polypropylene tubes (Kendro Laboratory Products
GmbH, Germany).
After converting measured solution and sorbed phase
concentrations to logarithms, the log-transformed
158 Water Air Soil Pollut (2009) 203:155168
Freundlich equation was fitted to the measured data
(Streck et al. 1995):
log S¼mlog Cþlog k:ð2Þ
Here, S(mg kg
1
) is the sorbed phase concentra-
tion, C(mg L
1
) is the concentration of dissolved
chemical, kstands for the Freundlich coefficient
(mg
1m
L
m
kg
1
), and mdenotes the Freundlich
exponent (1).
3 Results
In the grinding process, bead size was the key factor
for nanopreparation (Fig. 1). Using 15-mm-diameter
Zr-dioxide beads, the mean particle size of raw
zeolitic tuff after 4 h of grinding was reduced from
109 μm to a powder with a mean particle size of
3.9 μm. Prolonging grinding to 20 h further reduced
the mean particle size to 2.1 μm. Nevertheless, the
nanoscale range (<100 nm) was not achieved. Using
smaller Al-oxide beads (1.62.6 mm diameter)
yielded a mean particle size of 1.3 μm after 4 h of
continuous grinding. The 1-mm-diameter Al-oxide
beads reduced the mean particle size to 1.3 and
1.1 μm after 2 and 6 h, respectively. Zr-dioxide fine
beads with a mean diameter of 0.1 mm were unable to
grind the zeolitic tuff at all. This can be attributed to
the use of a feed particle size whose diameter is larger
than the grinding media.
For optimizing and speeding up the grinding
procedure, a two-stage grinding process was devel-
oped. In the first stage, a mixture of Al-oxide beads
(12.6 mm) tended to achieve faster milling. Two
hours of grinding were sufficient to obtain a particle
mean size of 1.2 μm. At the end of the first stage (4 h),
the mean particle size was 1.1 μm. In the second stage,
a mixture of Zr-dioxide (0.1 mm diameter) and Al-
oxide beads (1 mm diameter) were used. The particles
size in the powder was transferred from the micron
range to the nanorange. After 8 h of grinding, the
nanoparticles tended to form agglomerates (385 nm
diameter; Fig. 1, point a). For particle stabilization, the
powder was washed with 1 M NaCl and sonicated. At
the end of the second stage, the mean particle size of
the nanoparticles was 49 nm (Fig. 1, point b). The size
was the real particle size, as confirmed by TEM
(Fig. 2). The produced nanoparticles had a surface area
of 81.9 m
2
g
1
and a CEC of 154 cmol
c
kg
1
.
Attrition milling of raw zeolitic tuff with a mixture of
fine Zr-dioxide and Al-oxide beads, during the second
stage of grinding, produced submicron-size agglomer-
ates composed of nanosize particles. Figure 2ashows
the two types of nanoparticles (single and agglomer-
ated) obtained. TEM imaging showed that the shape of
larger nanoparticles tended to be irregularly with
smooth edges and wavy grain boundary. The smaller
particles were more spherical. The electron diffraction
patterns confirmed that the zeolitic tuff nanoparticles
were still in the nanocrystalline phase (Fig. 2b). The
electron diffraction pattern demonstrated that more
than one crystal was overlapping. Hence, the crystal
structure was conserved. TEM imaging of the nano-
particles revealed that the mean particle diameter was
0.1
1
10
100
1000
a
b
0 5 10 15 20
Grinding time (hours)
Mean particle size (µm)
15 mm zirconium dioxide balls
1.6 - 2.6 mm Al-oxide beads
1 mm Al-oxide beads
0.01 0246 810
0.1
1
10
100
1000
Grindin
g
time (hours)
Mean particle size (µm)
First stage, mixture of Al-oxide
beads (1.0-2.6 mm)
Second stage, mixture of Zr-dioxide
and Al-oxide fine beads (0.1-1.0
mm), after stabilization
Second stage, before stabilization
b
First Stage Second stage
a
Fig. 1 a Mean particle size of zeolitic tuff as a function of
grinding time using different grinding material. bGrinding
progress in the optimized two-stage grinding scheme. Point a
represents the mean agglomerates size before stabilization,
while point brepresents the mean (single) particle size of the
zeolitic nanotuff after stabilization in suspension. Results are
means of three replicates
Water Air Soil Pollut (2009) 203:155168 159
48 nm (median 42 nm). Based on TEM image
analysis, the particle size distribution was log-normal
(Fig. 2c).
The laser diffractometry (LD) results (Fig. 3c)
confirm those obtained from TEM image analysis
(Fig. 2c). Fifty percent of the agglomerate diameters
were <359 nm. After stabilizing, the mean particles
size was 49 nm (median 48 nm). The particle size
distribution was normal before and after stabilization.
The difference between the particle size distribution
curves, means, and medians obtained using TEM
versus LD is attributed to particle shape, morphology,
and overlapping. Under the operating conditions of
the LD, the particle size distributions were different
either because the LD Malvern software is strictly
valid for spheres or because the particles adopt
preferential orientations in the measurement cell (Gabas
et al. 1994). LD considers the measured diameter of
any particle as the diameter of a sphere even if the
particle is a rod, ellipse, or irregularly shape.
The X-ray diffractogram patterns revealed that the
mineral content of zeolitic nanotuff is phillipsite,
chabazite, faujasite, calcite, and smectite (Fig. 4).
Figure 4shows that the peak heights (intensity) and
width of half heights decrease as grinding time
increases and particle size decreases.
The zeta potential measurements show that the
surface of the produced zeolitic tuff nanoparticles was
negatively charged at pH 3.89.3 (Fig. 5). At pH 8.6,
the zeta potential of zeolitic nanotuff was 46.3 mV, and
the zeta potential of Na-zeolitic nanotuff was 64.6 mV.
However, the point at which the graph passes through
Fig. 2 a Zeolitic tuff nano-
particles observed by TEM at
200 kV and ×200,000
magnification. bThe inset
shows the electron diffrac-
tion pattern. cHistogram of
the size distribution of
zeolitic tuff nanoparticles
based on TEM image
analysis. Log-normal curve
was fitted to the particle size
distribution
160 Water Air Soil Pollut (2009) 203:155168
zero potential was not attainable. For the zeolitic
nanotuff, extrapolation of the zeta potential measure-
ments using polynomial third order curves gave an
estimated pH of 3.9. In the case of Na-zeolitic nanotuff,
the estimation yields pH 3.2. The high negative values
of the zeta potential led to a stable nanoparticle
suspension, which enables the zeta potentials to be
measured over a wide range of pH.
The chemical composition of the raw and modified
zeolitic nanotuff is presented in Table 1. The change
in the element contents after the Na treatment of the
zeolitic nanotuff was due to the change in the
exchangeable counter cations. Na partially replaced
cations such as Ca, Mg, and K. For example, Na
concentration increased by up to 4.1 percentage
points from 0.4% to 4.5% by mass. The content of
Ca and K decreased by up to 2.9 and 0.8 percentage
points, respectively. The Zr-dioxide content was
below 0.5% by weight. No increase of the Al-oxide
concentration in the final powder was observed when
the Al-oxide beads were used.
Sorption Isotherms To investigate the ability of Na-
zeolitic nanotuff to immobilize Cd in solution,
sorption batch experiments were carried out at various
Cd concentrations and pH. The Cd sorption isotherms
at pH 4.5, 6.5, and 7.5 are presented in Fig. 6.
Figure 6demonstrates that increasing pH values shift
the isotherms toward a higher sorption of Cd. In
addition, the double logarithm isotherm was fitted to
the linear Freundlich isotherm equation. A positive
relation between adsorption and the Cd concentration
was found. This is in line with results of many previous
studies (Gürel 2006; Voegelin and Kretzschmar 2003;
Gao et al. 1997). The isotherm equations reveal that by
increasing the pH value from 4.5 to 6.5 or 7.5, the
Freundlich coefficient (k)increased34-or123-fold,
respectively.
Figure 7compares Cd sorption isotherms of Na-
zeolitic nanotuff, Na-raw zeolitic tuff, and raw zeolitic
tuff. The slope of the isotherm (m) was similar for Na-
zeolitic nanotuff and raw zeolitic tuff (0.69 and 0.68,
respectively). The Freundlich coefficient of the Na-
zeolitic nanotuff was 5,129 mg
1m
L
m
kg
1
, which is
three times higher than that of the raw zeolitic tuff
(k=1,585 mg
1m
L
m
kg
1
). Treating raw zeolitic tuff
with 1 M NaCl increased the Freundlich coefficient to
2,089 mg
1m
L
m
kg
1
. Yet, transferring the raw
zeolitic tuff to the nanorange had a greater effect on
sorption than NaCl treatment.
4 Discussion
Grinding raw zeolitic tuff to the nanorange was best
achieved by applying a two-stage grinding process
using a mixture of large and fine bead sizes. Large
beads have more momentum than smaller ones due to
their higher mass. Using small beads increases the
2 5 12 30 76 194 494 1255
Diameter (µm)
0
10
20
30
40
50
60
70
80
90
100
110
Accumulative particles size
105 158 240 363 550 832 1096 1660
Agglomerate diameter (nm)
0
10
20
30
40
50
60
70
80
90
100
110
Accumulative particles size
27 34 43 54 68 86
Diameter (nm)
0
10
20
30
40
50
60
70
80
90
100
110
Accumulative particles size
0
0.5
1
1.5
2
2.5
3
3.5
PSD, Volume (%)
a
0
1
2
3
4
5
6
7
8
9
Frequency %
b
0
2
4
6
8
10
12
14
PSD,weight basis
c
Fig. 3 a Particle size distribution of raw zeolitic tuff (feed
particle size), bparticle size distribution of zeolitic tuff
agglomerates (median 359 nm), and csize distribution of
single tuff particles after stabilizing. Size distributions were
measured using a laser diffractometer
Water Air Soil Pollut (2009) 203:155168 161
number of collisions because of their higher abun-
dance. It also quickened the milling process. This
observation is consistent with previous studies dealing
with the optimal bead size for grinding (Bilgili et al.
2004;Way2004; Jankovic 2003; Zheng et al. 1996).
The mean feed particle size for the first stage of
grinding was 109 μm, for the second stage about
1μm (Fig. 1). The ratio between the ultimately
selected feeding grain size and ball size was consis-
tent with findings of Way (2004), who reported that
for efficient grinding and dispersion, 90% of the feed
particle size should have a smaller diameter than 1/10
of the beadssize.
Compared to other top-down methods for preparing
zeolite nanoparticles (e.g., pulsed laser induced frac-
ture or synthesis from colloidal solution), top-down
methods present some advantages, such as the low
production costs and high yield (Van Heyden et al.
2008; Nichols et al. 2006). Furthermore, the zeolitic
tuff is one of the low-cost raw materials that are
available to reduce the transfer of heavy metals into the
human food chain (Puschenreiter et al. 2005).
The TEM image analysis of the zeolitic nanotuff
(Fig. 2a) for obtaining the grain size distribution was
time consuming. Particle overlap complicated assigning
a contour to each particle. In contrast, in the stabilized
suspension, the size distribution of single particles could
easily be measured using laser scattering analysis. The
mean particle size (48.8 nm) measured by laser
scattering was nearly the same as that derived by TEM
(47.6 nm). The electron diffraction pattern of the zeolitic
nanotuff indicated a disordered arrangement of white
spotlights (Fig. 2b). This might be due to the over-
lapping of the zeolitic tuff nanocrystalline-lattice area.
The electron diffraction patterns could appear as white
spotlights or circle lights. The white spotlights in the
diffraction pattern represent the distribution of atoms
within the crystals. Areas with orderly arranged spot-
lights are termed the lattice area of single crystal
(Kimura et al. 1999), while the white circle-lights
-70
345678910
-60
-50
-40
-30
-20
-10
0
Zeta potential (mV)
pH of solution
zeolitic nano tuff
Na-zeolitic nano tuff
Fig. 5 Zeta potential of zeolitic nanotuff and Na-zeolitic
nanotuff. Polynomial third-order curves were fitted to the data
points
Fig. 4 X-ray diffractometer
patterns of zeolitic tuff.
aRaw zeolitic tuff, braw
zeolitic tuff after 4 h of
grinding, and craw zeolitic
tuff after 8 h of grinding.
Ph phillipsite, Sm smectite,
Ch chabazite, Fa faujasite,
Fo forsterite, Ca calcite,
Hhematite. dSimple and
penetration twinning crystal
growth of raw zeolite minerals.
The photograph was taken
with a scanning electron
microscope
162 Water Air Soil Pollut (2009) 203:155168
represent the amorphous material (Al et al. 2000).
Figure 2illustrates the microstructure of a powder
composed of nanocrystalline particles surrounded by
very fine amorphous grains. The grain sizes of the
zeolitic tuff ranged from 26 to 86 nm (Fig. 3c).
Ameyama et al. (1998) and Gleiter (1989)reportedthat
many of the materials milled in mechanical attrition
devices were crystalline and had a wavy grain
boundary and that the crystallite (grain) size diameter
after milling was often between 1 and 10 nm. They
Table 1 Chemical composition of the natural and modified zeolitic tuff nanoparticles
Oxide Oxide content
Raw zeolitic tuff (% by weight) Zeolitic nanotuff (% by weight) Na-zeolitic nanotuff (% by weight) ±SD
a
(% by weight)
SiO
2
44.82 44.09 44.79 2.13
Al
2
O
3
15.64 13.94 14.44 0.89
Fe
2
O
3
11.14 13.64 12.28 0.13
Na
2
O 0.41 0.40 4.47 0.15
CaO 10.05 8.73 7.18 0.07
MgO 10.20 10.81 9.73 0.20
K
2
O 2.18 1.56 1.30 0.15
MnO 2.65 2.40 2.38 0.00
TiO
2
2.32 3.14 2.85 0.04
P
2
O
5
0.40 0.34 0.35 0.01
ZrO
2
0.02 0.60 0.49 0.00
a
Standard deviation of the measurements
k = 141
m = 0.94
R
2
= 0.99
k = 5129
m = 0.69
R
2
= 0.99
k = 17378
m = 0.74
R
2
= 0.99
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
-4.6 -3.6 -2.6 -1.6 -0.6 0. 4
log C (mg/l)
log S (mg/kg)
pH 4.5
pH 6.5
pH 7.5
Fig. 6 Freundlich adsorption isotherms of cadmium for Na-
zeolitic nanotuff at pH 4.5, 6.5, and 7.5. Each data point was
measured in triplicate. The symbols kand mare the parameters
of the Freundlich isotherm log S=mlog C+k
k = 1585
m = 0.68
R2 = 1.00
k = 2089
m = 0.69
R2 = 0.99
k = 5129
m = 0.69
R2 = 0.99
0.80
1.00
1.20
1.40
1.60
1.80
2.00
2.20
2.40
2.60
2.80
3.00
-4 -3.5 -3 -2.5 -2 -1.5 -1
log C (mg/l)
-0.5
log S (mg/kg)
Raw zeolitic tuff
Na- raw ze olit ic tu ff
Na- zeo litic n ano tuff
Fig. 7 Freundlich adsorption isotherms of Cd for raw zeolitic
tuff, Na-raw zeolitic tuff, and Na-zeolitic nanotuff at pH 6.5.
Each data point was measured in triplicate. The symbols kand m
are the parameters of the Freundlich isotherm log S=mlog C+k
Water Air Soil Pollut (2009) 203:155168 163
termed such material as nanocrystalline. Other authors
(Eckert et al. 1989;Schultz1988;Kochetal.1983)
have used attrition milling to produce amorphous
phase formation.
After milling, traces of ZrO
2
were found in the
zeolitic nanopowder (Table 1) as impurities and
contamination, which could not be totally avoided in
mechanical grinding. The ZrO
2
content in the final
powder was not a serious problem. ZrO
2
shows an
adsorption potential comparable to some commercially
available materials such as iron-based oxides and
hydroxides (Hristovski et al. 2008). The ZrO
2
portion
of the final nanomaterial is small (<0.5% by weight).
Therefore, we may assume that it is negligible in Cd
sorption. Nanomaterials in form of milled powders
usually contain contamination elements generated
during milling (Tellkamp et al. 2001;Eckertetal.
1992) because, during mechanical attrition, collision
occurs between the grinding medium and the vessel and
between the grinding balls themselves. Consequently,
both the grinding medium and vessel deteriorate and
abrade. Contamination therefore increases with increas-
ing milling energy and milling time. One method to
avoid contamination is to use a grinding medium and
vessel of the same composition as the powder being
milled (Han et al. 2005; De Castro and Mitchell 2003).
Zeolites are hydrous aluminum silicates. Therefore, Al-
oxide beads were used at the first stage of grinding. At
the second stage of grinding, however, zirconium
dioxide beads (0.1 mm) were used because the fine
bead size (0.1 mm) was only available as zirconium
dioxide. Grinding the raw zeolitic tuff to the nanorange
increased the BET surface area by a factor of 10.
Among nanoparticles, Na-zeolitic nanotuff has an
intermediate surface area (Kockrick et al. 2008).
Zeolitic tuff is a negatively charged volcanic
mineral that attracts positively charged compounds
and traps them in its cage-like structure. According to
the classification scheme of Riddick (1968), the
stability of the Na-zeolitic nanoparticles was very
good to extreme at pH > 8. The suspension stability
was acceptable at pH values between 5.0 and 8.0,
where the zeta potential ranged from 40 to 60 mV.
At pH 3, the particles stuck together in suspension
and eventually formed agglomerates. According to
Rumpf (1962), the two forces influencing agglomer-
ation of particles in solution are Van der Waals force
and the repulsive force. The latter one is strongly
depending on ionic strength and zeta potential. The
Van der Waals force increases with decreasing grain
size (Brar 2000). The point of zero charge (PZC) was
not attained because the suspension was not stable at
pH<3.8. This observation is in agreement with the
estimated point of zero charge of pH 3.2. PZC is the
pH value at which the net total particle charge
becomes zero. At this pH value, particles do not
move in an applied electrical field (electrophoretic
mobility measurement; Sposito 1989). At PZC, the
suspension is unstable because the Van der Waals force
is higher than the repulsive force. The zeta potential
strongly influences the ability of water or liquid to carry
nanoparticles in suspension. At low pH, zeolitic tuff
nanoparticles form large agglomerates and precipitate.
In the present work, the agglomeration problem was
overcome by sonication after replacing the counter
cations with Na. In the presence of monovalent cations,
the diffuse layer tends to enlarge, making the suspension
more stabile.
The X-ray diffraction patterns indicate that the
zeolitic tuff sample contains three types of zeolite
minerals: phillipsite, chabazite, and faujasite. The
average channel diameter was 0.43 nm for phillipsite,
0.37 nm for chabazite, and 0.74 nm for faujasite (Barrer
1978). This means that the nanocrystalline zeolite (26
86 nm) may still consist of a conservative channel
system because crystal size exceeds channel diameter.
Figure 6shows that by increasing pH, the Cd
isotherms shift toward higher Cd sorption. The
Freundlich coefficient kis a measure for the sorption
strength of a solute (Ingwersen 2001; Springob and
Böttcher 1998). The increase of the Freundlich
coefficient with increasing pH is attributed to several
factors.
First, increasing the pH increases the electrostatic
interaction with Cd
2+
, which increases sorption.
Electrostatic interactions are important for the sorption
process (Morais et al. 2007). This is confirmed by the
zeta potential drop from 17 mV at pH 3.8 to 64 mV
at pH 9.3. The effect of pH on Cd adsorption partly
reflects changes in the net proton charge on the soil
particles. Second, Naidu et al. (1994) found that the
type of interaction (inner-sphere or outer-sphere sur-
face complexation) between Cd and solid phase
depends on pH. The hydrolysis properties of Cd
2+
play an important role in the relation between sorption
and pH. Surface complexation is influenced by pH,
164 Water Air Soil Pollut (2009) 203:155168
whereas ion exchange is influenced by ionic strength
(Wu et al. 2007;Wangetal.2005). Choi (2006)
reported that, based upon two geochemical models
(surface complex modeling and the Langmuir model)
of Cd adsorption to the reference materials smectite and
vertisol, the basal plane siloxane cavities are apparently
the most important sites for Cd complexation at pH<6.5.
For the pH-dependent sites, the edge-site aluminol
appears to be the dominant functional surface group
responsible for Cd adsorption at pH > 6.5. The combina-
tion of the zeta potential, surface complexation, and
surface functional group of the Na-zeolitic nanotuff and
the hydrolysis properties of the Cd
2+
are responsible for
the high impact of pH on sorption.
The sorption isotherms presented in the results
(Fig. 7) demonstrate that the efficiency of Na-
zeolitic nanotuff for immobilizing Cd from solution
was up to 3.2 times higher than that of the raw zeolitic
tuff and up to 2.5 higher than the Na-raw zeolitic tuff.
The increase of Na-zeolitic nanotuff sorption is
related to decreasing the grain size of the raw zeolitic
tuff to the nanorange and hence to the higher surface
area, higher absolute zeta potential, and total CEC.
Grinding the raw tuff to the nanorange can expose
most of the atoms of a nanoparticle on the surface.
Consequently, the surface atoms can bind with other
atoms and enhance the adsorption (Liang et al. 2000).
Grinding of the raw zeolitic tuff was probably
associated with increasing broken edges, corners,
and chemical bonds. Thus, the number of free
sorption sites may have been increased and the
immobilization potential enhanced.
The increase of Cd sorption on Na-zeolitic nanotuff
by up to three times compared with the raw tuff is less
than the expected. Perhaps the zeolitic channel system
was partially clogged or covered with very fine powder
during the grinding process (Inglezakis et al. 1999;
Carland and Aplan 1995). Another possible explanation
could be addition of HNO
3
to the zeolitic nanotuff
during the sorption experiment (the pH of zeolitic
nanotuff was adjusted by HNO
3
); this enabled H
+
to
occupy part of the free sorption sites on the nano-
particle surfaces and changed the net proton charge of
nanoparticles. Nanotuff needed much more HNO
3
than
raw tuff to adjust the pH to 6.5. Zeolite minerals have a
high affinity for H protons (He et al. 2008; Laborde-
Boutet et al. 2006). Furthermore, nanoparticles might
partly have been agglomerated during the sorption
experiment. Consequently, the increase of surface area
and Cd sorption was less than expected.
5 Conclusions and Outlook
The present study establishes and describes a novel
method for producing and stabilizing zeolitic tuff in
the nanorange. Attrition milling with a mixture of
different beads size was suitable for transforming the
raw zeolitic tuff to the nanorange. The results clearly
demonstrate that a mixture of fine beads effectively
both grinds and disperses the powder particles. The
obtained suspension was stable over a wide range of
pH. Grinding the natural zeolitic tuff to the nanorange
increased its surface area, absolute zeta potential, and
cation exchange capacity, thereby enhancing its
potential for immobilizing heavy metals. Zeolitic
nanotuff can therefore be seen as an important
auxiliary material in the field of soil remediation.
The sorption batch experiments clearly demonstrate
that grinding the raw zeolitic tuff increased Cd
sorption much more than treating the raw tuff with
1 M NaCl solution.
In conclusion, zeolitic nanotuff is a potential
alternative to traditional soil amendments provided
that it can be produced at a reasonable price. Future
research is needed to study the metal immobilizing
effect of the zeolitic nanotuff in soils differing in pH
and texture. Investigations are also needed on the
applicability and efficiency of this material for soil
remediation purposes under field conditions. Nano-
particles have been criticized to cause health problem
(Gatti and Montanari 2008;Seaton2007). The
characteristics of nanoparticles that influence toxicity
is included the number, size, surface area, shape,
solubility, chemical composition, and chemical reac-
tivity (Marconi 2006). We encourage focusing scien-
tific interest on monitoring nanoparticle transport in
soil and groundwater as well as the ecotoxicity of
nanoparticles.
Acknowledgments The authors would like to express thanks
for the support of Prof. Dr. F. Aldinger and Dr. J. Bill from the
Max-Planck Institute for Metal Research (Stuttgart, Germany)
during many fruitful discussions and for providing the attrition
ball mill and testing the nanoparticle powder. This research was
supported by the German Academic Exchange Service
(DAAD).
Water Air Soil Pollut (2009) 203:155168 165
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