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RESEARCH PAPER
Viscoelastic gels of guar and xanthan gum mixtures provide
long-term stabilization of iron micro- and nanoparticles
Dingqi Xue
•
Rajandrea Sethi
Received: 2 December 2011 / Accepted: 8 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Iron micro- and nanoparticles used for
groundwater remediation and medical applications are
prone to fast aggregation and sedimentation. Diluted
single biopolymer water solutions of guar gum (GG) or
xanthan gum (XG) can stabilize these particles for few
hours providing steric repulsion and by increasing the
viscosity of the suspension. The goal of the study is to
demonstrate that amending GG solutions with small
amounts of XG (XG/GG weightratio 1:19; 3 g/L of total
biopolymer concentration) can significantly improve
the capability of the biopolymer to stabilize highly
concentrated iron micro- and nanoparticle suspensions.
The synergistic effect between GG and XG generates a
viscoelastic gel that can maintain 20 g/L iron particles
suspended for over 24 h. This is attributed to (i) an
increase in the static viscosity, (ii) a combined polymer
structure the yield stress of which contrasts the down-
ward stress exerted by the iron particles, and (iii) the
adsorption of the polymers to the iron surface having an
anchoring effect on the particles. The XG/GG visco-
elastic gel is characterized by a marked shear thinning
behavior. This property, coupled with the low biopoly-
mer concentration, determines small viscosity values at
high shear rates, facilitating the injection in porous
media. Furthermore, the thermosensitivity of the soft
elastic polymeric network promotes higher stability and
longer storage times at low temperatures and rapid
decrease of viscosity at higher temperatures. This
feature can be exploited in order to improve the
flowability and the delivery of the suspensions to the
target as well as to effectively tune and control
the release of the iron particles.
Keywords Nanoscale zero-valent iron Rheology
Yield stress Guar gum Xanthan gum
Groundwater remediation
Abbreviations
ZVI Zero-valent iron
NZVI Nanoscale zero-valent iron
MZVI Microscale zero-valent iron
GG Guar gum
XG Xanthan gum
SBS Single biopolymer solution
BMS Biopolymer mixture solution
WLF Williams-Landel-Ferry theory
List of symbols
G’ Storage modulus (Pa)
G’’ Loss modulus (Pa)
s Downward stress of particle (Pa)
d Average diameter of the particles (m)
D. Xue
Dipartimento Scienza Applicata e Tecnologia—DISAT
and Dipartimento di Ingegneria dell’Ambiente, del
Territorio e delle Infrastrutture—DIATI, Politecnico di
Torino, Turin, Italy
R. Sethi (&)
Dipartimento di Ingegneria dell’Ambiente, del Territorio
e delle Infrastrutture—DIATI, Politecnico di Torino,
Corso Duca degli Abruzzi, 24, 10129 Turin, Italy
e-mail: rajandrea.sethi@polito.it
123
J Nanopart Res (2012) 14:1239
DOI 10.1007/s11051-012-1239-0
q
p
Density of the particles (kg/m
3
)
q
f
Density of the fluid (kg/m
3
)
g Acceleration of gravity (m/s
2
)
v
0
Initial mass magnetic susceptibility (m
3
/kg)
v Mass magnetic susceptibility (m
3
/kg)
Introduction
Nanoscale and microscale zero-valent iron (NZVI and
MZVI, respectively) are object of great interest in the
fields of groundwater remediation and biomedicine
(Zhang 2003; Noubactep et al. 2012; Qiang et al. 2006).
In the former field of application, the effectivity of zero-
valent iron (ZVI) particles has been proven in the
degradation or immobilization of a wide variety of
contaminants (Cantrell et al. 1995;Lietal.2006;Xiu
et al. 2010; Freyria et al. 2011). On the other hand, in
medical applications, ZVI offers potential advantages
over other particles due to its high magnetic moment and
it can maintain superparamagnetism at larger sizes than
their oxides (Huber 2005). This mechanism allows for
an effective use of ZVI for enhancing magnetic
separation, drug delivery, and magnetic resonance
imaging; in addition, in hyperthermia treatments, these
particles have the potential to minimize the amount of
injected material in patients and therefore the potential
to use safer AC magnetic fields (Huber 2005).
Despite the appealing properties of ZVI particles,
their employment is often hindered by poor colloidal
stability. This limitation can be overcome by suspend-
ing the particles in biopolymer solutions, such as guar
or xanthan gum (GG and XG, respectively) (Di
Molfetta and Sethi 2006; Oostrom et al. 2007; Tiraferri
et al. 2008; Phenrat et al. 2008; Zolla et al. 2009). GG
is a galactomannan obtained from the endosperm of
the seeds of Cyamopsis tetragonolobus, while XG is an
extracellular polysaccharide excreted by the bacterium
Xanthomonas campestris (Casas et al. 2000). These
two biopolymers are non-toxic, inexpensive, hydro-
philic, stable, but biodegradable. Previous studies
showed that GG and XG enhance the stability of MZVI
and NZVI by adsorbing to the surface of iron and
providing steric repulsion among the particles and by
increasing the viscosity of the suspension, and there-
fore slowing the aggregation processes (Tiraferri et al.
2008; Comba and Sethi 2009). Furthermore, the shear
thinning rheological behavior of ZVI suspensions in
biopolymer is advantageous in environmental appli-
cations during both the storage and the injection in
porous media. During storage, the high static viscosity
of the suspension can delay particle sedimentation,
while during injection, the viscosity decreases at high
shear rates, requiring lower pumping pressures (Zhong
et al. 2011; Comba et al. 2011; Truex et al. 2011;
Tiraferri and Sethi 2009; Tosco et al. 2012).
However, in order to stabilize the significant iron
load required for field applications (usually greater
than 10 kg/m
3
), high biopolymer concentrations are
necessary, potentially hindering the degradation in the
subsurface. The current study aims at improving the
long-term stabilization of ZVI particles’ dispersion by
exploiting the synergistic effect derived from the
mixing of GG and XG (Amundarain et al. 2009).
When used separately, the water solution of each
polymer is dominated by viscous behavior (Rodd et al.
2000; Wientjes et al. 2000; Choppe et al. 2010; Risica
et al. 2010). Conversely, when mixed together, they
form a viscoelastic gel (Pai and Khan 2002), which
can maintain small particles in suspension even at very
low polymer concentrations. The specific objectives of
this work are (i) to understand the rheological
properties of XG and GG mixtures (at different
concentrations, mixing ratios, and temperatures) and
(ii) to investigate their effectiveness in stabilizing
MZVI and NZVI over long periods.
Materials and methods
Zero-valent iron particles
Commercial reactive NZVI (NANOFER 25S or
N25S) (Fig. 1a) was supplied as liquid slurry by
NANO IRON s.r.o- (Rajhrad, Czech Republic). N25S
has an average particle size of 50 nm and average
surface area of 20–25 m
2
/g. NZVI was separated from
the liquid phase, containing a mixture of organic and
inorganic stabilizers, by a series of washing cycles
with deionized water followed by centrifugation and
sedimentation. After washing, NZVI aggregation and
sedimentation were prevented by continuous ultrason-
ication before mixing with the biopolymer mixtures.
Carbonyl iron powder (B200) was provided by
BASF SE (Ludwigshafen, Germany, Fig. 1b) and
water atomized iron powder (H4) was supplied by
Ho
¨
gana
¨
s AB (Ho
¨
gana
¨
s, Sweden, Fig. 1c). The average
Page 2 of 14 J Nanopart Res (2012) 14:1239
123
particle sizes of B200 and H4 are 4.7 and 41 lm,
respectively.
Preparation of biopolymer solutions and ZVI
suspensions
Deionized water solutions of XG, GG, and their
mixtures were stirred firstly with a magnetic stirrer
and then homogenized using the T25 digital Ultra
Turrax (IKA, Staufen, Germany) high-shear rotor–
stator processor. Finally, the biopolymer solutions
were degassed under vacuum to remove air bubbles
and held for 12 h at room temperature to facilitate
complete dissolution and hydration. ZVI particles
were sonicated for 10 min in deionized water in
order to break up the aggregates formed during
storage before mixing with biopolymer solutions.
The preparation process of ZVI-biopolymer suspen-
sions is schematized in Fig. 2. When preparing ZVI
suspensions with biopolymer mixture solution
(BMS), as shown by the red arrows, ZVI particles
were dispersed firstly in GG solution and then XG
solution was added. Suspensions were homogenized
by Ultra Turrax homogenizer for 15 min at
10,000 rpm. The suspensions were ZVI particles (at
a concentration of 20 g/L) dispersed in solutions of
different polymers (at the concentrations of 6, 3, 2,
1.5, and 0.75 g/L).
Rheology measurements of biopolymer solutions
Dynamic rheological measurements were performed
with an Anton Paar MCR-301 rheometer fitted with a
concentric cylinder system. The biopolymer micro-
structure was probed by measuring the shear viscosity,
the storage modulus G’ (or elastic component), which is
a measure of the deformation energy stored by the
sample during the shear process, and the loss modulus
G’’ (or viscous component), a measure of dissipated
energy (Mezger 2006). Dynamic frequency sweep tests
were performed at constant strain amplitude of 1 % set
within the linear viscoelastic region, which was iden-
tified through strain sweep tests. Viscosity was mea-
sured as a function of temperature, from 10 to 40 °C.
Angular frequency sweep tests were conducted between
10 and 70 °C in order to apply the Williams-Landel-
Ferry (WLF) theory known as the ‘‘time–temperature
Fig. 1 Representative STEM images of (a) nanoiron cluster of
N25S, (b) microiron particles of BASF 200, and (c) microiron
particles of H4
J Nanopart Res (2012) 14:1239 Page 3 of 14
123
superposition’’ (Williams et al. 1955) which is used to
explore fluid rheology in frequency ranges that are
otherwise not possible to achieve, neither technically
nor practically (Schramm and Haake 1994).
Solutions of XG, GG, and their mixture, with
concentration values of 6, 3, and 1.5 g/L and XG/GG
weight ratios of 1:4, 2:3, 1:1, 3:2, 4:1, and 1:19, were
used in the rheological tests.
The yield stress of the different solutions was
determined by measuring the strain while increasing
the shear stress in the range from 0.001 to 1 Pa. On a
bi-logarithmic stress–strain plot, the yield stress is the
point where the relationship between strain and stress
deviates from the unitary slope. A linear relation with
a slope approximately equal to unity implies a
Hookean solid-like behavior (Uhlherr et al. 2005). In
terms of biopolymer structure, this represents the
elastic deformation of the structure bonds. When the
stress increases over a certain level, some weaker
bonds start to break. When the stress exceeds the yield
stress, the slope increases significantly and the strain is
no longer a function of stress alone, but also depends
on the rate of stress increase and duration time. The
yield stress represents the transition from elastic solid-
like behavior to viscous liquid-like behavior of the
biopolymer solutions.
Adsorption of biopolymer molecules
Low-shear viscosity is very sensitive to biopolymer
concentration in water, which is decreased by the
adsorption to the particle surface. Differential low-
shear viscosity was calculated to estimate the degree of
adsorption of XG and GG to the iron surface. Viscosity
measurements of XG (at initial concentrations of 3 and
1.5 g/L) or GG (at an initial concentration of 3 g/L)
solutions were carried out, after the removal by
centrifugation of the dispersed ZVI particles (20 g/L),
and compared to the viscosity of the same polymer
solutions before the addition of ZVI particles.
Fig. 2 Preparation process of ZVI-biopolymer suspensions
Fig. 3 Viscosity as a function of shear rate at different concentrations and different temperatures for (a) xanthan gum solutions and
(b) guar gum solutions
Page 4 of 14 J Nanopart Res (2012) 14:1239
123
Sedimentation analysis
Sedimentation tests were used to evaluate the stabil-
ities of N25S, B200, and H4 in XG, GG, and their
mixture solutions. Sedimentation curves were plotted
by exploiting the linear relationship between concen-
tration and magnetic susceptibility, which was logged
by a Bartington MS2G susceptivimeter (Dalla Vecchia
et al. 2009b). Susceptibility is an intrinsic property of
the material; by measuring the mass susceptibility, the
variation of the ZVI’s mass over time can be deduced.
Results and discussion
Rheological properties of xanthan gum and guar
gum
XG solutions are characterized by a shear thinning
behavior, as reported in Fig. 3a. The figure shows the
shear rate dependence of viscosity at different polymer
concentrations (i.e., 1.5, 3, and 6 g/L) and at different
temperatures (i.e., 10, 25, and 40 °C). The viscosity
curve is characterized by a Newtonian region at low
Table 1 Low-shear and high-shear viscosities for different biopolymer solutions at 25 °C
Shear
rate
(1/s)
Viscosity (Pas)
XG GG XG GG XG/
GG = 1:19
XG/
GG = 1:1
XG/
GG = 1:19
XG/
GG = 1:1
XG/
GG = 1:4
XG/
GG = 2:3
XG/
GG = 3:2
XG/
GG = 4:1
(3 g/
L)
(3 g/
L)
(6 g/
L)
(6 g/
L)
(1.5 g/L) (1.5 g/L) (3 g/L) (3 g/L) (3 g/L) (3 g/L) (3 g/L) (3 g/L)
0.001 5.3 0.16 94 6.1 1 180 28 380 87 87 65 29
1000 0.0097 0.012 0.017 0.041 0.0068 0.0088 0.019 0.018 0.018 0.013 0.012 0.012
Fig. 4 Master curves of angular frequency sweep measurements determined by the WLF theory at 25 °C for (a) xanthan gum solutions
(1.5, 3 and 6 g/L) and (b) guar gum solutions (3 and 6 g/L)
Table 2 G’ values and its comparison to G’’ for different biopolymer solutions at an angular frequency of 0.1 rad/s, at 25 °C
XG XG GG GG XG/
GG = 1:19
XG/
GG = 1:19
XG/
GG = 1:1
XG/
GG = 1:1
(3 g/L) (6 g/L) (3 g/L) (6 g/L) (1.5 g/L) (3 g/L) (1.5 g/L) (3 g/L)
G’ (Pa) 0.189 1.69 0.00358 0.0489 0.0453 0.315 0.166 0.477
G’’ (Pa) 0.242 1.20 0.00778 0.305 0.0188 0.131 0.103 0.366
Elastic if G’ [ G’’ G’ \ G’’ G’ [ G’’ G’ \ G’’ G’ \ G’’ G’ [ G’’ G’ [ G’’ G’ [ G’’ G’ [ G’’
J Nanopart Res (2012) 14:1239 Page 5 of 14
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shear rates and by a shear thinning behavior at higher
shear rates. In this region, the intermolecular interac-
tions are reduced by the microstructural anisotropy
resulting from the shear deformation. As the shear rate
increases, the orientation of the polymer chains is
forced along the flow direction and produces a drop in
the viscosity (Hyun et al. 2002). Low shear rate
viscosity of XG solution is affected markedly by
temperature. The structure of the XG molecules
changes from a stiff, ordered helical conformation at
lower temperatures to a flexible, disordered structure at
higher temperatures (Norton et al. 1984). This thermo-
rheological behavior is fully reversible between 10 and
70 °C. The tests also demonstrate that low-shear
viscosity increases in a geometric progression when
the concentration is doubled (Table 1).
Also, GG solutions exhibit a shear thinning behav-
ior (Fig. 3b). The low-shear viscosity is much lower
for GG than for XG solutions; conversely, at high
shear rates, the viscosity is higher for GG than for XG
solutions, at equal concentrations. This is due to the
higher molecular weight of XG and its weaker
molecular interactions compared to GG (Flory 1953).
Figure 4 shows the master curves obtained by
sweep tests over a range of angular frequencies for XG
and GG solutions at 25 °C. By applying the WLF
theory, the frequency sweep curves of 10, 25, 40, 50,
60, and 70 °C between 0.1 and 100 rad/s were
converted into one curve at 25 °C. The short-term
behavior of the samples is represented by high
frequencies and is dominated by an elastic response
when deformation energy is larger than dissipated
energy (G’ [ G’’). On the contrary, the long-term
behavior is represented by low frequencies and is
dominated by a viscous response (G’ \ G’’). The
crossover frequency (where G’ = G’’) identifies the
transition between solid-like and liquid-like behav-
iors. The time for a material to adapt to applied stresses
or deformations is defined as relaxation time and is the
inverse of the crossover frequency. The curves are
influenced by both concentration and temperature.
When the concentration increases, both G’ and G’’
increase, and the crossover point shifts to lower
Fig. 5 Viscosity as a function of shear rate for a BMS with a
XG/GG weight ratio of 1:1
Fig. 6 (a) Low-shear (0.001 s
-1
) and (b) high-shear (1,000 s
-1
) viscosities of BMS (3 g/L) with different XG/GG weight ratios at
different temperatures
Page 6 of 14 J Nanopart Res (2012) 14:1239
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frequencies, which means that the elastic behavior is
prevailing over the viscous behavior (Table 2).
Figure 4 shows that a 3 g/L XG solution exhibits an
elastic behavior above 0.3 rad/s at 25 °C, while for a
GG solution at the same concentration, G’’ is larger
than G’ throughout the frequency range tested. At
6 g/L, XG displays a more pronounced elastic behav-
ior than GG. Therefore, frequency sweep tests dem-
onstrate that the structure of XG solutions is more
suitable to suspend MZVI and NZVI for longer times
than GG solutions at the same concentration.
Rheology of BMS
Figure 5 shows the viscosity curves of BMS with a
XG/GG weight ratio of 1:1 at concentration values of
1.5 and 3 g/L at different temperatures. The BMS
exhibits a pseudoplastic (shear thinning) behavior
affected by temperature, similar to the parent SBS.
Due to the synergistic contribution of the two poly-
mers, the low-shear viscosity of 1.5 g/L BMS with a
XG/GG ratio of 1:1 is higher than that of a 6 g/L XG
solution (Table 1), while the opposite is true at high
shear rates. The extent of the improvement of the
rheological properties depends on the mixing ratio
between XG and GG. The low-shear (0.001 s
-1
) and
high-shear (1,000 s
-1
) viscosities of BMS (3 g/L)
with different mixing weight ratios (i.e., XG/GG ratios
of 1:4, 2:3, 3:2, 4:1, and 1:1) and viscosities of single
XG and GG solutions at different temperatures are
plotted in Fig. 6. At a concentration of 3 g/L, the low-
shear viscosities (at a shear rate of 0.001 s
-1
) of BMS
are one to two orders of magnitudes higher than a pure
XG solution. As temperature increases from 10 to
40 °C, these differences decrease. Comparatively
speaking, the difference in high-shear viscosity (at a
shear rate of 1,000 s
-1
) between BMS and SBS is
significantly smaller, which demonstrates that the
BMS still possesses favorable flowability. It was also
observed that at relatively low temperatures (i.e., 25
and 10 °C), the low-shear viscosity is extremely high
when the weight ratio of XG/GG is close to 1:1.
Fig. 7 G’ and G’’ as a function of angular frequency for BMS with a XG/GG weight ratio of 1:1 at a polymer concentration of (a) 1.5
and (b) 3 g/L
Fig. 8 Viscosity as a function of shear rate for polymer
concentrations of 1.5, 3, and 6 g/L with a XG/GG weight ratio
of 1:19 at 25 °C
J Nanopart Res (2012) 14:1239 Page 7 of 14
123
Figure 7 shows the G’ and G’’ of BMS in frequency
sweep tests. Although the G’ of a 1.5 g/L BMS with a
XG/GG weight ratio of 1:1 is substantially lower than
that of a 6 g/L XG solution throughout the frequency
range, it is higher than G’’ at low frequencies
(Table 2). The marked elastic behavior of the BMS
proves that the interaction between XG and GG
generates a much more stable structure than either
parent SBS.
Decreasing the XG concentration in the BMS is
essential for environmental applications since this
polymer is more resilient to biodegradation in aquifer
systems and can hinder reactivity of ZVI particles
(Born et al. 2005). Figures 8 and 9 show the
rheological properties of a BMS with a XG/GG ratio
of 1:19. Although the properties of other mixing ratios
(i.e., 1:4, 2:3, 1:1, 3:2, and 4:1) are superior (Table 1),
the characteristics of this BMS are still preferable to
SBS. For the BMS, the temperature/frequency analogy
is not valid and the WLF theory cannot be applied
(Fig. 9). The values of the measured moduli are
summarized in Table 2. At an angular frequency of
0.1 rad/s (the lowest frequency that can be tested), as
the concentration of the SBS decreases, the G’/G’’ ratio
drops from above to below unity as a result of the
weakening of the structure of the fluid. On the contrary,
although decreasing concentration reduces both G’ and
G’’, the structures of BMS are still characterized by a
G’/G’’ ratio above unity at an angular frequency of
0.1 rad/s and thus are sufficiently stable.
Temperature dependence of viscosity
Temperature can affect the viscosity of biopolymer
solutions due to the change in thermal motion and
conformation of the molecules (Dea 1989). The
temperaturedependence of viscosity is reversible (Milas
and Rinaudo 1986) and is shown for XG, GG, and XG/
GG solutions in Fig. 10 for different mixing ratios and
concentrations. When BMS is characterized by a XG/
GG weight ratio of 1:19, or when XG solutions are
heated, viscosity decreases slowly as temperature
increases from 10 to 40 °C. Instead, when the XG/GG
weight ratio ranges from 1:4 to 4:1, and in particular at a
Fig. 9 G’ and G’’ as a function of angular frequency for polymer concentrations of (a) 1.5 and (b) 3 g/L with a XG/GG weight ratio of
1:19 at 25 °C
Fig. 10 Viscosity as a function of temperature at a shear rate of
0.001 1/s for different biopolymer solutions
Page 8 of 14 J Nanopart Res (2012) 14:1239
123
ratio of 1:1, low-shear viscosity drops sharply in a
narrow temperature interval (from 20 to 30 °C, Fig. 10).
This behavior is probably due to the extensive interac-
tion between XG and GG molecules and the order–
disorder transition of XG molecules from helix to
random coil conformation during heating (Dea et al.
1977). So, the mixture of XG and GG forms a
thermoreversible soft elastic network structure, which
explains why the BMS is gel at low temperatures. This
behavior can be exploited to stabilize and store ZVI
suspensions at low temperatures and vice versa to
improve their flowability by heating. A similar strategy
can be used to speed up the sedimentation and the
release of the particles from the polymer network.
Figure 11 shows how 20 g/L suspensions of H4 and
B200 MZVI dispersed in 1.5 and 0.75 g/L of BMS (XG/
GG ratio 1:1), respectively, behave at different temper-
atures. At 20 °C, both the suspensions can be stored
without sedimentation for more than 48 h. Neverthe-
less, when heated to 35 and 40 °C, respectively, the
MZVI settles down within 0.5 h.
Adsorption effect
When ZVI particles are dispersed in XG or GG, they
tend to adsorb part of the polymer to their surface,
determining a decrease in the viscosity of the suspen-
sion, thus causing a reduction in its stability. Fig-
ure 12a displays the low-shear viscosities of XG or
GG solutions before the dispersion of particles and
after its removal. It can be clearly seen that the
decrease in viscosity of the remnant solution is more
significant for smaller particles (N25S and B200) due
to their higher specific surface area. High adsorption
would lead to an increase of the steric stabilization
(especially for NZVI); on the other hand, it would also
Fig. 11 Thermal effect on sedimentation of 20 g/L MZVI in BMS (XG/GG = 1:1): (a) and (b) H4 in 1.5 g/L BMS at 20 °C after 48 h
and at 35 °C after 0.5 h, respectively; (c) and (d) B200 in 0.75 g/L BMS at 20 °C after 48 h and at 40 °C after 0.5 h, respectively
Fig. 12 Viscosity reduction of pure XG and GG solution
(3 g/L) after adsorption to particles of different sizes at 0.001
L/s shear rate
J Nanopart Res (2012) 14:1239 Page 9 of 14
123
decrease the solution viscosity and therefore promote
sedimentation (particularly of MZVI).
Stability of ZVI particles in polymer solutions
A series of sedimentation experiments proved that
ZVI dispersions with diluted SBS (XG or GG) are not
stable over long periods of time (Fig. 13). Despite the
high static viscosity, XG solutions are not able to
stabilize MZVI, which aggregates during sedimenta-
tion. The aggregation of MZVI is shown in Fig. 13a–b
as an increase of susceptibility normalized to the
initial value. During sedimentation, compaction of the
iron particles can occur leading to an instantaneous
increase of concentration (and susceptibility) when
the iron passes by the sensor. Thus, although SBS
possesses high low-shear viscosity, the interaction
between free biopolymer molecules in solution and
those adsorbed to the particle surface is not suffi-
ciently strong to prevent long-term sedimentation as a
decrease of susceptibility ratio.
Similarly, Fig. 13c shows that diluted SBS (except
XG at 3 g/L) is not able to suspend NZVI for long
periods of time. Usually, finer particles are character-
ized by smaller sedimentation velocity; however, in
the 3 g/L GG solution, the NZVI (N25S) settles more
rapidly than microparticles (B200). This is due to the
larger specific surface area of NZVI that leads to a
higher adsorption of biopolymer with a consequent
decrease of the viscosity (Fig. 12a) and to the stronger
magnetic interactions occurring among NZVI (Dalla
Vecchia et al. 2009a).
Fig. 13 Sedimentation tests of (a) H4, (b) B200, and (c) N25S dispersed in different biopolymer solutions
Page 10 of 14 J Nanopart Res (2012) 14:1239
123
Fig. 14 Schematic
representation of the
structure formed by the
interaction between XG
and GG molecules
Fig. 15 Schematic
representation of interaction
among XG, GG, and
a ZVI particle
J Nanopart Res (2012) 14:1239 Page 11 of 14
123
The tests performed on SBS proved that the
structure of single biopolymers in solution is unable
to prevent the sedimentation of ZVI particles, due to
the unfavorable alignment, and weak interaction
among molecules (Dea et al. 1977). On the contrary,
BMS performs much better than SBS at an equal
polymer concentration (e.g., 3 g/L), even with a very
low XG content (i.e., XG/GG = 1:19, Fig. 13a, b). By
comparing the viscosities (Table 1) and the corre-
sponding dispersion stabilities (Fig. 13) of BMS and
SBS, the stability of BMS-based ZVI dispersions
cannot be attributed solely to the increase of the low-
shear viscosity. Despite the smaller low-shear viscos-
ities (0.001 s
-1
) of BMS with a XG/GG weight ratio of
1:19 at concentrations of 3, 1.5, and 1 g/L, compared
to single XG solutions at 6, 3, and 1.5 g/L, respec-
tively, their ZVI suspensions are more stable. The
structure arising from the mixture of XG and GG can
be held responsible for this phenomenon. When XG is
dissolved at low temperatures (\40 °C), its molecules
are present as single, double, or triple helices arranged
in an ordered conformation that promotes their inter-
action with the GG molecules (Dea I. C and Morris E
1977; Casas et al. 2000). GG consists of a backbone
chain of mannose units linked to a monomolecular unit
of galactose. Galactose residues are not uniformly
distributed: There are regions without galactose
(smooth regions) and others with multiple galactose
residues (hairy regions). Smooth regions are the ones
that favor the interaction with the XG (Garcı
´
a-Ochoa
et al. 2000; Kim et al. 2009). The interaction between
molecules of these gums forms a continuous network
structure in BMS, as simply depicted in Fig. 14.
Moreover, previous studies showed that GG molecules
are able to adsorb to the ZVI surface (Tiraferri et al.
2008) and that the molecules of both XG and GG have
a nanoscale size with a height of 1.12 nm and a
calibrated width of 1.22 nm (Iijima et al. 2007); thus,
as shown in the schematic representation in Fig. 15,
the GG molecules can be adsorbed to the surface of
ZVI particle to form ‘‘anchors’’ while the xanthan
present in the solution provides stability to the
biopolymer structure. The presence of a continuous
biopolymer gel structure and the adsorption of GG
confer a great stability to ZVI dispersed in BMS.
The synergetic effect of mixing XG and GG that
arises from the formation of a polymeric network
structure was verified by yield stress tests. In Table 3,
the yield stress of solutions of GG (3 g/L), XG (3 g/L),
and BMS (XG/GG = 1:19, 3 g/L) is reported. XG and
GG solutions are characterized by a yield stress of 0.12
and 6.58 9 10
-3
Pa, respectively; however, when a
GG solution is amended with a small amount of XG,
the yield stress increases by two orders of magnitude.
When the yield stress of the polymer solution exceeds
the downward stress exerted by the particle, the
suspension is stable over a long period. Thus,
comparing these two stresses can provide a direct
evaluation of the effectiveness of polymer solutions in
stabilizing suspended particles. The downward stress
exerted by an iron particle (Table 3) can be calculated
according to the expression
s ¼
d
6
q
p
q
f
g
where d is the particle diameter, q
p
is the density of the
particles (7,900 kg/m
3
), q
f
is the density of the fluid
(1,000 kg/m
3
), and g is the acceleration of gravity. The
Table 3 Comparison of biopolymer yield stress and particle downward stress and estimation of suspension stability
Downward stress of iron particle (Pa)
H4 B200 N25S
0.46 5.27 9 10
-2
5.61 9 10
-4
Gel (3 g/L) Yield stress of polymer solutions (Pa) Yield stress of polymer solutions after partial adsorption on iron (Pa)
GG 6.58 9 10
-3
2.37 9 10
-3
5.11 9 10
-4
1.51 9 10
-4
Very unstable Very unstable Very unstable
XG 0.123 3.2 9 10
-2
1.69 9 10
-2
5.78 9 10
-3
Very unstable Unstable Stable
XG/GG = 1:19 0.478 0.470 0.443 0.389
Stable Very stable Very stable
Page 12 of 14 J Nanopart Res (2012) 14:1239
123
yield and downward stress values are reported in
Table 3 and prove quantitatively the efficacy of low-
concentration BMS in stabilizing both MZVI and
NZVI.
Conclusions
Previous studies have demonstrated that both GG and
XG can improve the stability of concentrated micro-
and nanoscale iron particles (e.g., 20 g/L). These
polymers can adsorb to the surface of the ZVI particle,
preventing particle aggregation and sedimentation,
thanks to an increase in the viscosity of the suspension.
Here, we proposed a strategy to increase the static
viscosity of ZVI suspensions, while lowering their
dynamic viscosity, thus facilitating the injection and
flow in porous media without increasing the polymer
concentration. Using low-concentration XG and GG
mixtures (XG/GG = 1:19, at 3 g/L), we found that
due to the synergistic interaction of these two gums, a
viscoelastic gel is formed, which results in long-term
(more than 24 h) stabilization of both the micro- and
the nanoscale iron particles at concentrations as high
as 20 g/L. The stabilization effectiveness of this gel
was attributed to (i) the greater static viscosity of the
mixture, (ii) the presence of a polymeric structure the
yield stress of which contrasts the downward stress
exerted by the iron particles, and (iii) the adsorption of
GG molecules to the surface of the ZVI particle, which
has an anchoring effect on the particles, coupled with
the stability provided by the XG to the biopolymeric
structure. The marked shear thinning behavior of the
BMS and its low biopolymer concentration guarantee
low viscosity at high shear rates, facilitating the
injection in the subsurface. In addition, the biopolymer
mixtures are characterized by a thermoreversible soft
elastic network structure, such that the viscosity of the
solutions increases at low temperatures and drops
suddenly at higher temperatures. This interesting
property can be exploited for particles storage at low
temperatures and to improve injection and flowability
in porous media at higher temperatures.
Acknowledgments The study was partially funded by the EU
AQUAREHAB research project (FP7, Grant Agreement n.
226565) and by MIUR in the framework of PRIN 2008. The
authors acknowledge Dr. M.Coı
¨
sson at INRiM for STEM
micrographs.
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