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Cite this: DOI: 10.1039/c0xx00000x
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Dimethylphosphatoethyltriethoxysilane (SiP) modified TiO2 mesoporous
particles in the aqueous separation of Ce3+ from hetero-ionic metals and
other homo-ionic REEs.
Mícheál P. Moloney,*a Cedric Loubat, b Agnès Grandjean,a
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Due to their presence in most modern electronic devices and
processes as well as their erratic availability a great deal of
interest has been generated in the acquisition and/or recycling
of REEs. Here we present a simple ethylene glycol based
route for the preparation of SiP functionalised mesoporous
TiO2 particles. Once prepared these particles were then used
to aqueously separate Ce3+ from hetero-valent metals as well
as from other homo-valent REEs.
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1. Introduction
Cerium, along with other rare earth metals, have become more
and more important over the last several decades due to their uses
in the electronics,[1],[2] and biomedical industries,[3, 4] as well
as the use of their compounds in catalysis,[5, 6] and
nanotechnology,[7, 8] luminescence,[9, 10] and chemical
engineering.[2, 11] Rare earth elements (REEs) are to be found
in various products; from audio-visual devices such as mobile
phones, televisions and computer displays to energy storage
devices such as long life batteries. [12, 13] Cerium, for example,
is also used by the automotive industries in catalytic converters,
and by the petroleum industry in refining as a cracking
catalyst.[12, 13] These varied uses make having a constant and
reliable source of REEs quite important. However, despite the
relative abundance of these so called ‘rare earths’ (Ce is the 25 th
must abundant metal in the Earth’s crust; a concentration
comparable to copper) they are still relatively expensive as both
their production level and use has varied over the last decade.[14]
These variations are due, in the most part, to environmental and
geo-political considerations, combined with an increased demand
for high end electronic goods.[15] Consequently, it is with these
factors in mind that new reliable sources of REEs are now being
sought. Recent advances in recycling technology as well as the
ever decreasing ‘shelf life’ of modern electronic devices have
made the idea of device recycling and REEs recuperation a
popular one. Also, it should be noted that as the demand for REEs
begins to outstrip their production the idea of device recycling
begins to be economically viable.[16]
However, the separation of mixtures of REEs can prove
difficult due to a phenomenon known as lanthanide contraction.
This property causes a relatively gradual decrease in ionic size
with increasing atomic number giving the various REEs similar
sizes and therefore making them difficult to separate. Therefore,
This journal is © The Royal Society of Chemistry [year]
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with this in mind we present a clean, simple, aqueous route to
REE separation using a silica-phosphonate functionalised TiO2
mesoparticle system.
Although SiO2 is a more common support for silicaphosphonates, TiO2 will be used as the support of choice in this
work.[11, 17-19] TiO2 was chosen due to its resistance to the
harsh conditions normally associated with REE extraction, i.e.
strong acids and bases. Also unlike silica, TiO2 is not solubilised
by concentrated NaOH. This property proved to be important
later in this work as NaOH was used to activate the functional
groups. TiO2 is amphoteric; this property has been well reported
in the literature, as has its use to control the up-take/release of
ions from solution.[20-26] Moreover, TiO2 is used as an
inorganic support in flow/filtration processes. Therefore, these
TiO2 particles simulate can also be used to simulate a membrane
support system.
The TiO2 particles used in this work were prepared as described
in the literature.[27, 28] The prepared particles were found by
SEM to be around 360nm in size, slightly soluble in water, and
(after a hydrothermal treatment) mesoporous. The Ti precursor
and silano-phosphonate functionalising molecule were mixed
before the TiO2 particles were co-precipitated to ensure
maximum surface coverage. The phosphonate chosen for this
work
was
dimethylphosphatoethyltriethoxysilane
(SiP).
Normally, when grafting a silane group containing molecule such
as SiP the presence of a silica support is preferred. This is done to
increase the likelihood that the silica-phosphonate molecule will
attach at the silane end, leaving the phosphonate head group free.
Therefore, not only does no such guarantee exists here but, it has
been well reported in the literature that phosphonate group
containing molecules co-ordinate onto the lewis acid site
containing Titania surface via the phosphonate oxygen.[29]
However, the difficulty with which the SiP phosphonates’ P-O-R
groups are hydrolysed as well the relative ease which ethoxy
silane groups are hydrolysed are also both well recorded in the
literature. Therefore, it is hoped that the methyl groups present
around the phosphonate head will offer a certain degree of
protection during the TiO2 preparation stage of this work. These –
O-CH3 groups will then later be hydrolysed to –O-H (SiP-TiO2;
AH) and then –O-Na groups (SiP-TiO2; ANa) using a
hydrothermal process and a NaOH(aq) treatment respectively.
BET was used to confirm the porous nature of the SiP-TiO2
[journal], [year], [vol], 00–00 | 1
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(AH), while NMR and FT-IR were used to confirm the presence
of the SiP on the SiP-TiO2 at various points in its preparation.
The sorbants used in this study were all nitrates with the
exception of GdCl3. This salt was used to determine the effect, if
any, of changing the counter-ion on the absorbance rate of Gd3+
onto the modified SiP-TiO2 particles. In total four REEs nitrates
were examined Ce, Nd, Gd and Yb with the focus been placed on
Ce and the effect, if any, of the present of the other REEs on Ce
sorbance. The effects of the presence of M+ and M2+ on Ce3+
sorbance were also studied.
2. Experimental
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Note: Unfunctionalised TiO2 was prepared exactly as described in
the literature and section 2.2, [27] the only difference being the
lack of SiP.
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2.3. Sorption Experiments
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2.1. Materials
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Dimethylphosphatoethyltriethoxysilane (SiP) was provided by
our industrial partners Specific Polymers. Ethylene Glycol
(HOCH2CH2OH)
and
Titanium
(IV)
Isopropoxide
(Ti(OC(CH3)2)4) were obtained from Sigma-Aldrich. All metal
nitrates with the exception of Yb(NO3)3.xH2O (Fulka) were also
purchased from Sigma-Aldrich. All these materials were used as
received. As AES-ICP was the main workhorse tool in
determining metal concentration all nitrate stock solutions were
prepared to have a metal concentration of ≤ 20 ppm, (mg/L).
Millipore water was prepared in house using a Sartorius water
system (18.2 MΩ.cm).
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2.2. Preparation of SiP functionalised TiO2
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The functionalised TiO2 was prepared using a previously reported
method which has been slightly modified to include the SiP
functionalising molecule.[27]
Briefly, 5g of Ti(OCH(CH3)2)4 and 0.8g of SiP were weighed
out and mixed well together. This mixture was then poured into
50g of ethylene glycol under constant stirring. The solution
turned a deep milk white immediately on addition of the Ti:SiP
mixture. The resulting milk white solution was left stirring overnight after which the white colour disappeared and the solution
was once again clear and colourless. The Ti:SiP ethylene glycol
solution was then poured into 250 ml of acetone under constant
stirring. The white colour reappeared and deepened over time.
After 2 hours of constant stirring at room temperature the SiPTiO2 dispersion was heated to 60 0C and left to stir for 48 hours.
The particles were collected by centrifugation and washed several
times with ethanol. They were then dried overnight in an oven at
60 0C and the resulting powder was washed in a soxhlet, where
hot ethanol was the solvent, for 24 hours and once again dried
overnight in an oven at 60 0C, (After soxhlet, AS). The dried AS
particles were then transferred to a 500 ml RBF and 350 ml of
Millipore water was added. The contents were sonicated till the
particles were well dispersed and then under vigorous stirring
were heated to 120 0C for 6 hours. The particles were then
collected by decanting and centrifugation, washed several times
in millipore water and again oven dried at 60 0C, (After
Hydrothermal, AH). The particles were then dispersed in 150 ml
of NaOH(aq) (15 g/L) and stirred for an hour. They were collected
2 | Journal Name, [year], [vol], 00–00
by centrifugation and washed with millipore water until neutral.
The particles were again dried at 60 0C over night, (After Na
treatment, Ana).
90
All sorption studies were carried out in Millipore water. For the
kinetic experiments 100 ml of ~20 ppm solutions of the target
metals were prepared under constant stirring. Separately, 20 mg
of SiP-TiO2 or TiO2 were sonicated in 20 ml of millipore water
until a homogeneous dispersion was acquired. To start the kinetic
experiment these 20 ml Titania dispersions were added to the 100
ml metal nitrate solutions with the clock starting immediately on
addition. 7ml samples were taken at various intervals and
immediately passed through a Whatman® 200 nm syringe filter
to remove the TiO2 effectively stopping the reaction. The
remaining concentration of metal in the filtered solution was then
determined using AES-ICP.
The Langmuir isotherms were carried out using the same stock
solution concentration as described above; 20 ppm of metal. Each
isotherm contained 10 points/experiments with each point being a
different dilution of the stock solution starting with 2 ppm of
metal and the final point being the stock solution itself. Each
point consisted of 20 ml of solution/dilution with 4 mg of sorbent.
The dispersion were sonicated for 30 minutes to ensure a
homogeneous dispersion and then stirred for 20 hours. Again
Whatman® 200nm syringe filters were used to stop the sorption
process and the remaining metal concentration in solution was
determined using ICP-AES.
2.4. Analytical Techniques
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The morphology of the sample was observed with an ESEM-FIG
(FEI Quanta 200). The N2 sorption-desorption isotherm was
measured at -196 0C using a micromeritics ASAP 2020 Surface
Area and Porosity Analyser. The sample was degassed at 180 0C
overnight. The surface area was calculated by the BrunauerEmmett-Teller (BET) method. The pore size was calculated from
the maximum of the pore size distribution curve calculated by the
Barrett-Joyner-Halenda (BJH) method using the sorption branch
of the isotherm. The total pore volume was calculated by the
single point method. 13C and 31P and 29Si CP/MAS NMR were
measured on a Bruker 400 MHz NMR at 100 Mhz and a sample
spinning frequency of 12 KHz. FT-IR scans were performed
using a Perkin Elmer Spectrum 100 FT-IR spectrometer. Metal
concentration, with the exceptions of sodium and caesium was
determined using a Spectro Arcos ICP-OES spectrometer. Cs and
Na concentration were determined using an ionic chromatograph.
3. Results and Discussion
3.1 Characterisation of SiP-TiO2
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A.
B.
C.
5
10
P=O
1244cm-1
15
D.
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50
P=O-Ti
1047cm-1
E.
Fig. 1 Characterisation of SiP-TiO2; above, ESEM images of SiP-TiO2 before (A) and after (B, C) the hydrothermal treatment. Below left; FT-IR of
Dimethylphosphatoethyltriethoxysilane (SiP-black) and SiP-TiO2 (red). Below right; N2 sorption-desorption isotherm with pore size distribution inset.
Figure 1 shows the FT-IR spectrum of both SiP-TiO2 (red) and
the SiP molecule itself (black), the N2 sortion-desorption
isotherm of the SiP-TiO2 after hydrothermal treatment and ESEM
of the material both before and after hydrothermal treatment.
ESEM indicates that particles are smooth surfaced and spherical
with a size distribution of 366 nm ± 57 nm before hydrothermal
treatment, (Figure 1A). Once they are subjected to the
hydrothermal treatment (6 hours at 110 0C) they retain their shape
to a large degree, however, the surface becomes visibly rougher,
(Figure 1B & 1C). The size and size distribution also changes
slightly after the hydrothermal treatment to 399 nm ± 52 nm. The
N2 sorption-desorption isotherm which was carried out on the
hydro-thermally treated SiP-TiO2 resulted in a type IV Isotherm
with a H1/H4 hysteresis loop, indicating a mesoporous solid,
(Figure 1D).[32] While the type IV isotherm is characteristic of
capillary condensation taking place in a therefore mesoporous
solid, the hysteresis loop itself exhibits aspects of both a H1 and
H4 loop. This indicates that while individual particles are
mesoporous they are also aggregated. This aggregation effect was
also confirmed by the ESEM images, (Figure 1B). FT-IR of the
SiP molecule (black spectrum) shows Si-O-Si bending as well as
asymmetric and symmetric stretches at 1061,791 and 457 cm-1. PO-C asymmetric stretches can be seen at 1028 cm-1 and 952cm-1
while the P=O signal is present at 1244 cm-1 CH3 deformations
are at 1480cm-1 and 1394cm-1 while the C-O signals arising from
the silane ethoxy groups are present at 1169 cm-1 and 1076 cm-1,
(Figure 1E). After functionalisation (red spectrum) the P-O-C
stretch has shifted to 1171 cm-1 while the P=O signal can now be
seen at 1276 cm-1, (Figure 1E). The signal at 1428 cm-1 was
assigned to P-C stretching vibrations. The peak at 1047 cm-1
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although in a region overlapped by the C-O stretches of ethylene
glycol and Si-O-Si stretches of intermolecular SiP bonds could
very well be due to the presence of P=O...Ti bonds. However,
when taking this possibility into account the continued presence
of the P=O stretch at 1276 cm-1 as well as the position of the P31
MAS-NMR peaks must also be noted, indicating that there are
still free P=O groups present. Although it should be also noted
that as this molecule is capable of forming intermolecular bonds
via its silane groups it is entirely possible that a “multi-layer”
system of SiP is present on the TiO2 surface; with one
phosphonate group anchoring the bilayer on the TiO2 surface and
the “other” phosphonate group pointing away from the substrate.
Finally, the FT-IR peak at 1630 cm-1 was assigned to bound
surface water.
13
C, 31P and 29Si MAS-NMR were also used to confirm the
presence of the SiP molecule on the TiO2 surface, (Figure 2 and
supporting info). 13C MAS NMR (as well as FT-IR) of the SiPTiO2 after the both the soxhlet (AS) and hydrothermal (AH)
processes confirm that while the soxhlet has very little effect on
the concentration of surface bound ethylene glycol the
hydrothermal treatment removes it completely (Supporting info).
13C MAS NMR of the SiP-TiO (AS) shows the presence of the
2
SiP molecule, with characteristic peaks at 7, 20 and 50 ppm,
(Figure 2A). While 13C NMR scans of the SiP-TiO2 (AH) and
(ANa) still show the presence of the P-O-CH3 peak at 50-51 ppm
its relative intensity is now half of what it was in the SiP-TiO2
(AS), (Supporting Info). Although peak integration in 13C MAS
NMR is not as precise as in 1H NMR, the now equal intensities of
the peaks at 7ppm and 20 ppm which correspond to only one C
apiece and the peak at 50 ppm which corresponds to the two
Journal Name, [year], [vol], 00–00 | 3
series of complex overlapping signals between 35 ppm and 70
O
O
P
C.
30 ppm
24.15%
A.
50 ppm
5
ppm.
40 ppm
75.84%
O
H
5 ppm
O
O
20 ppm
Si
O
O
O
10
Ti O
O
P
OH
Ti O
Ti
Ti
O
Si O
O
O
Ti
O
Ti
O
15
H
O
H
O
D.
30 ppm
70.75%
B.
O
O
O
P
P
O
O
40 ppm
29.24%
O
P
O
H
O
O
P
O
Si
O
Si
O
Si
Ti
O
O
20
O
O
Ti
O
Si O
O
O
Ti
O
TiO2
25
X
30
35
40
45
Fig 2. (A) & (B) 13C and 29Si MAS NMR of SiP-TiO2 (AH). Structural inset in (A) shows the positions of the assigned carbons while the inset in (B)
shows one of several possible forms of attachment to the TiO2 surface. (C) & (D) 31P MAS NMR of SiP-TiO2 (AS) and (AH), respectively. Structure
insets show theSiP-TiO2 before and after hydrolysation. Once hydrolysation was taken place the P-OH groups can go on to form bonds with the surface
Ti-OH groups.
methyl-carbons would seem to indicate that around half of the PO-CH3 groups have been hydrolysed to P-OH groups, (Figure 2C
& 2D). 29Si MAS NMR of the SiP-TiO2 (AH) and (ANa) shows a
series of complex overlapping signals between 35 ppm and 70
ppm. These include the Ti-O-Si peaks at -41 ppm and -45ppm as
well as several doublets (JP-Si = 36.5 Hz) representing the SiP
surface molecule in various states of cross-linkage, (Table I).
Considering the high degree of intermolecular reactively seen in
silanes and that very similar 29Si MAS NMR data to this has been
previously presented in the literature for the ethyl analogue of SiP
these results were expected. Finally as reported by Pan et al. for
acidified SiP-SBA15 the 31P MAS NMR shows two signals (41
ppm and 31 ppm; peak height ration 1:3.14 respectively) close to
the position of the original SiP phosphonate peak. [Liquid 31P
4 | Journal Name, [year], [vol], 00–00
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NMR on the SiP molecule in CDCl3 showed the phosphonate
peak to be at 36 ppm, (supporting info).] The second peak is due
to the partially hydrolysed phosphonate forming surface bonds
with the TiO2. 31P MAS NMR of the SiP-TiO2 before and after
the hydrothermal treatment showed a stark reversal in the
intensities of the two peaks. Once the particles were treated in the
boiling water the peak at ~ 30 ppm replaced the peak at 40 ppm
as the dominate peak, (1:2). As reported by Pan et al. this is due
to the continual hydrolysation of the P-OH groups, which then go
to form hydrogen bonds with the surface Ti-OH groups. The
NaOH treatment seems to further enhance this effect as it
continues to hydrolysis the SiPs P-O-CH3 to P-OH groups. His
can be seen by the continued increase in the relative height of the
31P MAS NMR peak at 31 ppm, (supporting info).
This journal is © The Royal Society of Chemistry [year]
Table I 39Si MAS NMR peak assignments for monomeric and oligomeric
species obtained from the hydrolysis of SiP. O- represent oxygen’s which
are bonded to the TiO2 surface.
a
Formula
δ ppm
RSi(OEt)3
RSi(O-)3
R(O-)2SiOSi(O-)¬2R
R(O-)2SiOSi(O-)ROSi(O-)2R
-47.4
-42.4
-51.3
-60.4
Cerium Sorption Studies-Kinetics
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Cardenas et al.
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3.2 Sorption Studies
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Initial tests were carried out using Ce(NO3)3.6H2O on both SiPTiO2, and unfunctionalised TiO2. By comparing functionalised
and unfunctionalised TiO2 it was possible to see, firstly, if TiO2
itself had any affinity for Ce3+ while, secondly, observing the
ability of the SiP molecule to sorb Ce3+ ions. Surprisingly
however, it was found that the difference between the
unfunctionalised and SiP-TiO2 (AH) was negligible. The
unfunctionalised TiO2 sorbed 22 mg of Ce per gram of TiO2
while the SiP-TiO2 was only slightly above this at 29 mg/g. These
results were found to be almost identical when compared to those
observed for Ce3+ sorbed onto both plain and SiP functionalised
SBA-15 (literature standard) when the results were normalised
for molar mass, (supporting info). ). It should be also noted that a
batch of SiP-TiO2 (AH) particles were also boiled in 12M HCl as
is recommended in the literature, however, this seemed to
diminish, not enhance, its metal sorption properties. However,
unlike SBA-15, TiO2 is not solubilised by strong bases and
therefore it was possible to treat the SiP-TiO2 (AH) particles with
concentrated NaOH(aq). That is to carry out a base (and not acid)
hydrolysis of any remaining P-O-CH3 groups. Briefly, SiP
functionalised mesoporous TiO2 (AH) was treated with
concentrated NaOH(aq), (15g/L) and washed till neutral. These
SiP-TiO2 (ANa) particles were then used as the sorbent
throughout these studies. The supporting information shows four
Ce3+ absorption isotherms representing functionalised and
unfunctionalised TiO2, which were both treated, and not treated,
with NaOH(aq). While the NaOH wash has little effect on the
unfunctionalised TiO2 it increases the sorption ability of the
functionalised SiP-TiO2 by a factor of 3. The enhancement of the
SiP-TiO2 (ANa) particles ability to sorb Ce3+ after the NaOH
wash is due to firstly, a base hydrolysis of any remaining surface
P-O-CH3 groups and secondly the replacement of any Ti-OH and
P-OH groups with Ti-ONa and P-ONa groups. As mentioned
earlier in this section SiP-TiO2 (AH) sorption of cerium is almost
identical to that of acidified SiP-SBA-15, therefore we believe the
increased affinity of the SiP-TiO2 (ANa) for Ce3+ (and other
metals) is more a result of the greater ease with which Ce3+ can
be exchanged for Na+ over H+ than an increase in the number of
available P-OH sites. The relatively small change in the
intensities of the 31P MAS NMR peaks before and after the
NAOH treatment also suggests that the number of hydrolysed
phosphonate groups remains to a large extent unchanged
(Supporting Info). Ionic chromatography was used to confirm
that equimolar amounts of Na+ were released as other metals Cs+,
Sr2+ and Ce3+ were sorbed by the SiP-TiO2 (ANa).
This journal is © The Royal Society of Chemistry [year]
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Initial separation studies with the NaOH washed SiP-TiO2 (ANa)
particles were carried out to determine if Ce3+ could be separated
from a solution of di- and monovalent metals. The industrially
significant elements strontium and caesium were chosen as the
M2+ and M+ metals respectively. In the case of the kinetic
experiments 20 mg of functionalised TiO2 were sonicated in 20ml
of millipore for 30 minutes or until completely dispersed. This
dispersion was then added to a 100ml combined solution of the
three metal nitrates (≥ 20 mg/l each) in millipore water. Samples
were taken at set intervals over a 6 hour period and filtered using
200nm Whatman® hydrophilic filters to remove the SiP-TiO2. It
should be noted that 50% of total cerium sorbtion is done so in
the first five minutes, (Figure 3a). Although Sr2+ is also initially
sorbed within the first five minutes this is quickly reversed and
the metal is released. Therefore, from these results we see that
the ligand is not actually charge specific in that it does not
selectively bind M+3 ions over M+2. However, in this high
concentration, competitive environment previously sorbed Sr+2
ions are quickly released to accommodate any Ce3+ ions present
in solution; this is also true for Cs+1 ions which are sorbed in the
early stages of the process, (Figure 3b). This metal exchange also
demonstrates the SiP molecules’ ability to easily desorb as well
as sorb metal ions.
Cerium Sorption Studies-Langmuir Isotherms
Figure 3b shows an adsorption isotherm plot of the same system
with the Caesium and Sodium data now included. Each point on
the Langmuir curve represents a different experiment. Each
point/experiment involved the stirring overnight of 4 mg of well
sonicated SiP-TiO2 (ANa) in metal nitrate solutions of ever
increasing concentrations. At low concentrations the particles
sorb all three metals indiscriminately. However, as the
concentrations of the metals are increased a point is reached
where only the Ce3+ is sorbed. That is to say it is preferentially
sorbed over the others. This point was also equilibrium
concentration, at which the SiP-TiO2 has sorbed all the metal it
can. Measurement of the equilibrium Na+ concentration in each
point of the adsorption isotherm showed an increase in the
sodium concentration as the initial metal nitrate concentrations
were increased, until it eventually plateaus at 40mg/g, (Figure
3b). Ionic chromatography of the metal nitrate stock solutions
showed the presence of negligible amounts of Na+; therefore the
SiP-TiO2 particles are the only source of Na+ ions in these
isotherms. The Na+ concentration released is dependent on the
concentration of Mn+ sorbed by the SiP-TiO2 (ANa) particles.
This clearly demonstrates that an ion exchange is indeed taking
place. Also, if the amounts of Ce3+, Sr2+ and Cs+ sorbed by the
SiP-TiO2 are converted to millimoles and multiplied by their
corresponding charge, i.e. millimoles of charge, the obtained
number is almost equal to the concentration of Na+ released,
(Table II). When the sodium concentration eventually plateaus
indicating all sodium has been released (20mg/L, Figure 3b) it is
then the previously sorbed Sr2+ and Cs+ that act as the ion
exchangers allowing more Ce3+ to be taken out of solution as they
themselves are released.
Journal Name, [year], [vol], 00–00 | 5
sorption process the experimental data was put through both the
pseudo-first- and pseudo-second-order kinetic models. The
linearized form of the pseudo-first-order rate equation by
Lagergren is given below, (equation 1).
100
A.
Sorbed Metal Concentration (mg/g)
90
5
10
80
50
70
60
(qe - qt) = qeexp( – k1t)
50
(1)
40
55
30
20
10
0
-20
30
80
130
180
230
60
Time (minutes)
Where k1 is the sorption rate constant (min-1), while qe and qt are
the amounts of metal sorbed onto the SiP-TiO2 (ANa) in mg of
metal per gram of SiP-TiO2 (ANa) at equilibrium and at time (t)
respectively. The plot of (qe - qt) versus t gives a straight line and
k1 can be calculated by finding the slope. The pseudo-secondorder kinetic model is given with the equation.
100
90
B.
80
Metal Sorbed (mg/g)
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70
60
30
70
10
0
-10
10
30
50
70
90
110
Metal Concentration (mg/L)
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Fig 3. A; Effect of stirring time on the sorption of Ce3+ (red) and Sr2+
(blue) onto SiP-TiO2 (ANa). Initial Sr sorbance is reversed within the
first 15 minutes. B; Sorption Isotherm of the Ce, (red diamonds), Sr, (blue
squares), and Cs, (green/yellow triangles) solution on SiP-TiO2 (ANa).
The red x’s represent the Na+ released by the SiP-TiO2 (ANa) as it
absorbs the other metals.
Table II Millimoles of charge (milliequivalent) of all three studied metals
sorbed, or in the case of sodium released, at a specific concentration point
in the Isotherm in Fig. 3B.
10ppm
20ppm
70ppm
a
1
k2qe2
-
t
qe
(2)
40
20
30
=
50
20
25
t
qt
Ce3+
Sr2+
Cs+
Total
Na+
0.44
0.855
1.92
0.46
0.9
0
0.2
0.1
0
1.1
1.86
1.92
1.22
1.73
1.73
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Footnote text.
Both qe and qt have the same meaning as above and k2 is the rate
constant, (g/mg/min). A plot of t/qt vs. t is linear, and qe and k2
can be calculated from the slope and intercept respectively. The
model parameters and the correlation coefficient obtained for
both equations are seen in table III. While both sets of results fit
well with high R2 values, the pseudo-second-order kinetic
equation has a higher R2 value and its qe value is closer to that
which was observed experimentally. As was pointed out in the
previous section this system behaves as a chelating exchanger,
releasing the previously sorbed Na+ for the metals in solution.
Therefore, it is no surprise that second-order-kinetics can be
applied here.
Of course the support itself must also be considered. The
sorption process on porous solids can be broken down into four
different stages i). bulk diffusion, ii). film diffusion, iii). intraparticle diffusion and iv). sorption (or ionic exchange) of the
adsorbate on the surface. As one or more of these processes are
diffusion limited steps, which diffusion steps are limited they
may determine the rate and amount of material sorbed they must
also be investigated. As the SiP-TiO2 has a porous structure,
further investigation was carried out to determine which diffusion
steps are limited to the entire sorption process. Then the diffusion
model is expressed using the Weber and Morris equation below. 32
35
95
qt = kidt1/2
(3)
Kinetic Model
40
45
The sorption rate of Ce3+ on SiP-TiO2 (ANa) was studied with an
initial (T=0) Ce3+ concentration of 225mg/g over a 3 hour period.
As was seen in figure 3a the kinetics are very fast especially in
the first five minutes, where 50% of the total Ce3+ metal sorbed is
taken-up. However, it takes over 3 hours for equilibrium to be
reached. This equilibrium time is slower than reported for ethyl
analogue on SBA-15 and U(IV).17 To create a clearer picture of the
6 | Journal Name, [year], [vol], 00–00
100
As before qt is the amount of metal sorbed at time t in mg/g. The
constant kid is the diffusion constant (mg/g/h). A plot of qt as a
function of t1/2 gives a straight line from which kid can be found.
The experimental kinetic data was applied to equation (3), as
shown in Figure 10. The points are not linearly distributed but
give four straight lines with four different slopes. Similar kinetics
This journal is © The Royal Society of Chemistry [year]
55
Table III. Kinetic model constants and correlation coefficients highlighting the sorption of Ce3+ onto SiP-TiO2 (ANa). Exp = experiment result.
Pseudo-1st -Order
Pseudo-2nd -Order
qe (mg/g)
k1 (min-1)
R2
qe (mg/g)
86.16
3.36 x 10-2
0.997
92.59
Exp.
R2
qe (mg/g)
0.998
92
k2 (g/mg/min)
5
a
10
15
20
Footnote text.
were observed on the sorption of humic acid by cross-linked
chitosan beads and on the sorption of U(VI) by chitosantripolyphosphate beads, and on the sorption of U(VI) on DPTS
functionalised SBA-15.17 Specifically, the initial steep-sloped
portion represents the bulk diffusion or exterior sorption rate
which is very high. The second and third portions can be
attributed to intra-particle diffusion in first, the outer “shell”
which was created by the hydrothermal process, and second, in
the “core” TiO2. Lastly, the fourth portion corresponds to the
chemical equilibration of Ce3+ in SiP-TiO2. Such kinetics
suggests that intra-particle diffusion may play an important role
in the rate determination in the sorption process but not the sole
rate determining factor because of the deviation of the curves
from the origin and non-linear distribution of the plots.
can be expressed according to the equation below, (Equation 4).34
65
100
75
80
y = 2.484x + 55.508
R2 = 0.9998
3rd
90
80
Ce
qe
70
25
30
9.84 x 10-4
60
y = 0.0963x + 87.608
R2 = 1
4th
+
1
(4)
Qb
Ce is the equilibrium concentration of adsorbate (mg/L), qe
represents the amount of adsorbed adsorbate at equilibrium
(mg/g), and Q and b are the Langmuir constants related to
sorption capacity (mg/g) and the affinity of the binding sites on
sorbent (ml/mg), respectively, (Equation 4). Q and b can be
obtained by plotting Ce/qe versus Ce. Table IV lists the
parameters of the Langmuir model for the Ce3+ sorption on SiPTiO2 (ANa).
Table IV. Cerium Isotherm model constants and coefficients.
70
Q (mg/g) B (ml/mg)
60
y = 5.2755x + 33.216
R2 = 0.9918
2nd
t 50
q
Ce
Q
=
92.59
40
a
30
12
R2
Exp. (mg/g)
0.9998
92.98
ΔGL (J/mol)
-17309
Footnote text.
85
y = 20.707x - 6E-15
R2 = 1
1st
20
35
10
0
-2
3
8
13
18
t0.5
90
Fig 4. Particle diffusion model kinetics of Ce3+ sorbed onto SiP-TiO2
(ANa) particles.
40
95
From the good correlation coefficient of 0.9998 and the fact that
the equilibrium sorption capacity (Q) obtained from Langmuir
model (92.59 mg/g) is practically identical to the experimentally
observed saturation capacity (92.98 mg/g), it can be concluded
that the sorption of Ce3+ onto SiP-TiO2 follows the Langmuir
sorption model. According to the Langmuir model, the
favourability of SiP-TiO2 (ANa) as a Ce3+ sorbent, related to the
separation factor RL, can be obtained from the Langmuir sorption
constant, equation 5, where C0 is the initial metal ion
concentration.
Sorption Isotherm Model
45
50
Adsorption isotherms are fundamental to the understanding of the
sorption mode of an adsorbate on sorbent surface when
equilibrium is attained. Therefore, as shown earlier (Figure 3b), a
sorption isotherm was determined by studying the amount of
Cerium adsorbed onto the SiP-TiO2 (ANa) as a function of metal
concentration in the supernatant at equilibrium (Ce) at near
neutral conditions. The Langmuir isotherm model usually used to
fit the experimental data is based on the assumption that the metal
ions are sorbed as a monolayer, uniform and finite. Also it is
assumed that the sorption energy decreases as the distance from
the surface increases. The linear form of the Langmuir isotherm
This journal is © The Royal Society of Chemistry [year]
RL =
1
1 + bC0
(5)
100
105
0 < RL < 1 designates that the sorbent is a favourable medium for
the sorption of the given metal ion. The calculated RL values were
found to be between 8.6 x 10-3 to 9.3 x 10-4. Since these values
are between 0 and 1, the SiP-TiO2 (ANa) can be considered a
favourable sorbent for Ce3+. The ΔGL, the affinity of the SiP
ligand for Ce3+, was also calculated from this data and was found
to be -17309 J/mol.
Journal Name, [year], [vol], 00–00 | 7
Lanthanide Separation
5
10
15
20
55
The ability of SiP-TiO2 (ANa) to separate Ce3+ from other L3+
ions was also examined. Nitrates of Nd (III), Gd (III), and Yb
(III) were chosen for these studies due to a). their M3+ charge and
b). their every increasing atomic mass. Briefly, millipore water
solutions of Ce3+ and the other lanthanide to be examined were
prepared. To these stock solutions (Ce/Nd, Ce/Gd and Ce/Yb)
was then added the SiP-TiO2 (ANa) using the same procedure
which was described for the earlier kinetic experiments. The
results of these experiments are presented in table V. Again, the
most striking thing about these results is the amount of metal
sorbed within the first 5 minutes. The Ce3+ and Nd3+ mixture
shows almost identical equilibrium concentrations for both
metals. This is not surprising as Ce3+ and Nd3+ have very similar
atomic radii and molar masses. However, the Ce3+/Gd3+ and
Ce3+/Yb3+ mixtures do show some degree of separation. As the
molecular weight of the competitor lanthanide increases so too
does its uptake. These results are highlighted in Table V below,
which shows the equilibrium points of these experiments in
millimoles of metal per gram of SiP-TiO2 (ANa), as well as the
ratio of M3+ to Ce3+.
60
counter-ion a solution of Gd and Sr chloride was prepared. This
was then stirred with SiP-TiO2 (ANa) (as in previous
experiments) and the results were examined. Again, no Sr2+ was
still sorbed by the time the equilibrium point was reached. The
use of either GdCl3 or Gd(NO3)3 as the Gd source made very little
difference to the overall shape of the sorbance curve. The
equilibrium concentration of Gd was slightly higher when GdCl3
was used as the Gd source; (Gd(NO3)3 0.744 mm/g vs. GdCl3
0.795mm/g, however, this difference is well within the bounds of
experiment error. Therefore, the type of counter-ion used seems
to have little effect on the sorption process in these experimental
conditions.
65
Table VI. Gadolinium Isotherm model constants and coefficients.
Q (mg/g)
B (ml/mg) R2
Exp. (mg/g)
ΔGL (J/mol)
120.48
a
41.49
0.9996
121.17
-21022
Footnote text.
70
Table VII. Ytterbium Isotherm model constants and coefficients.
Q (mg/g)
B (ml/mg)
138.8
360
R2
Exp. (mg/g)
ΔGL (J/mol)
25
Table V. Equilibrium concentrations of Ce/Nd, Ce/Gd and Ce/Yb
solutions sorbed onto SiP-TiO2 (ANa) in millimole metal sorbed per gram
of SiP-TiO2. Ratio of Ce3+ to M3+ sorbed is also shown.
Ce (mmol/g) Nd (mmol/g) Gd (mmol/g)
0.428
0.495
0.335
a
Yb (mmol/g)
Ratio
0.479
1: 1.02
1: 1.325
1: 1.429
0.44
0.656
a
75
30
80
35
40
45
50
138.01
-26705
Footnote text.
Conclusions
Footnote text.
As mentioned above the ratio of Ce/Nd sorbed is almost 1:1
while 32.5% more Gd3+ and 42.9% more Yb3+ are sorbed than
Ce3+ in their respective experiments. This seems to suggest that as
the difference in weight between the metal ions increases so too
does the ability of the SiP-TiO2 (ANa) to separate them, with the
sorbent preferring heavier lanthanides. Langmuir adsorption
isotherms were also individually carried out on Gd(NO3)3 and
Yb(NO3)3. These were performed by repeating the experiment
described earlier, (Figure 3b), but replacing the Ce with Gd or
Yb. The results are tabled below. By comparing tables IV, VI
and VII we see that there is an increase in both the Langmuir
constants (Q and b) and the absolute ΔGL values for SiP-TiO2
(ANa) as the MW of the target lanthanide is increased. This
indicates that SiP-TiO2 (ANa) shows a higher affinity for Gd,
than Yb than Ce. We therefore submit that this material could be
used to separate Lanthanides depending on whether they are light
Ln as opposed to heavy Ln.
0.9997
85
90
95
A novel sorbent composed of phosphonate functionalised
mesoporous titania was synthesised by the co-condensation
method and used in Ln3+ sorption, in batch processes. The
sorption of Ln3+ on SiP-TiO2 (ANa) was studied as a function of
various parameters such as time, pH, and Ln 3+ concentration. The
initial kinetics of Ln3+ sorption on SiP-TiO2 (ANa) is ultra-fast
with 50% of total metal sorbed being done so with in the first
minute or two. However, equilibrium time is over 3 hours. The
sorption of Ce3+ was found to follow pseudo-second-order type
sorption kinetics. Intraparticle diffusion plays an important role in
the sorption processes but it could not be accepted as the sole
rate-determining step. The measured maximum sorption capacity
depends on the metal examined starting with Ce at 92 mg/g and
increasing with the molecular weight of the lanthanide. The
sorption isotherms for Ce3+, Gd3+ and Yb3+ have been
successfully modelled by the Langmuir isotherm which revealed
a monolayer chemical sorption. The sodium washed SiP
functionalised TiO2 demonstrated some ability to separate Ln3+
metals according to molecular mass. Finally, it was noted that
changing of the counter-ion from nitrate to chloride had little
effect on the shape or equilibrium concentration of the sorption of
Gd3+.
Counter-ion tests
To investigate the effect on SiP-TiO2 (ANa) of changing the
8 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]
Notes and references
a
5
10
Address, Address, Town, Country. Fax: XX XXXX XXXX; Tel: XX XXXX
XXXX; E-mail: xxxx@aaa.bbb.ccc
b
Address, Address, Town, Country. Fax: XX XXXX XXXX; Tel: XX XXXX
XXXX; E-mail: xxxx@aaa.bbb.ccc
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
1
A. Name, B. Name and C. Name, Journal Title, 2000, 35,
3523; A. Name, B. Name and C. Name, Journal Title, 2000,
35, 3523.
75
80
85
15
1.
20
2.
3.
25
4.
5.
30
6.
7.
35
8.
40
9.
45
10.
11.
50
12.
13.
55
14.
15.
60
16.
17.
65
18.
19.
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