Chemical Engineering Journal 447 (2022) 137559
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Study on the sorption properties of (NH4)2TiOF4 particles
Dmitry Sofronov a, b, Tamara Blank a, Sergey Khimchenko a, Alexey Lebedynskiy a,
Pavel Mateychenko b, Victoria Varchenko a, Marharyta Cherniakova a, Miroslaw Rucki c, *,
Wojciech Zurowski c
a
b
c
State Scientific Institution «Institute for Single Crystals», National Academy of Sciences of Ukraine, Prosp. Nauki, 60, Kharkiv 61178, Ukraine
Institute for Single Crystals National Academy of Sciences of Ukraine, Prosp. Nauki, 60, Kharkiv 61178, Ukraine
Kazimierz Pulaski University of Technology and Humanities in Radom, Stasieckiego Str. 51, 26-600 Radom, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Sorption
Uptake
Water treatment
Europium
Lanthanum
Cerium
Yttrium
Scandium
(NH4)2TiOF4
Prevention of water from the pollution with hard metals ions and radionuclides, as well as water treatment
motivate for the research and investigations in the field of the new effective sorbents. This paper presents the
results on (NH4)2TiOF4 sorption properties in the context of the synthesis conditions and related particle for­
mation. It was demonstrated that proper control of the precipitation process could result with either spherical
particles of 50–80 nm diameter or rod-like particles of length up to 10 μm. Moreover, it was found that respective
of synthesis conditions, (NH4)2TiOF4 particles exhibited high sorption efficiency towards lanthanum, yttrium,
scandium, europium, and cerium, above 95% at pH greater than 3. The highest sorption capacity performed
spherical particles synthesized at room temperature. Their respective capacities towards cerium, europium,
lanthanum, yttrium, and scandium at pH 5.5 were 350, 300, 240, 200, and 270 mg/g. The researches indicated
that increased precipitation temperature reduced sorption capacity by 50% and even 70% due to the larger
dimensions of the obtained particles. The tests performed for static and dynamic equilibrium revealed selectivity
of the (NH4)2TiOF4 particles towards the rare earth elements in the presence of divalent and monovalent ions.
1. Introduction
Intense development of the industry and power plants resulted with
increase of the toxic wastes that pose significant threat to the environ­
ment [5,28]. Thus, among main challenges of the today’s society is to
prevent and to clean the environment from technogenic pollution [8].
Sorption is a well-established and effective technology to remove con­
taminants from the aqueous solution [7], especially at the final stages of
water treatment when the concentration of a pollutant is small. There
are many natural and synthetic sorbents available, and they can be
chosen for the particular tasks according to the requirements on cost,
sorption characteristics, chemical stability, regeneration ability, etc.
[53]. The sorption properties depend not only on the sorbent nature, but
also on its structural and morphological features. From this perspective,
development of new sorbents for water treatment and removal of
technogenic pollutants is a very actual task with wide range of
possibilities.
The group of sorbents based on titanium compounds attracts the
attention of the researchers for a long time. Among others, hydrated
titanium dioxide TiO2⋅nH2O [1], as well as titanium phosphates [19],
nanostructured hydrous titanium oxide [55], and TiO2 nanoscale par­
ticles has been reported as sorbents of heavy metal ions [10]. Many
advantages of these substances can be listed, such as high exchange
capacity, chemical and radiation stability, low toxicity, simple synthesis
and relatively low cost [16,21,27]. In particular, hydrated amorphous
titanium dioxide (ha-TiO2) exhibited sorption capacity towards Mn2+,
Cu2+, Fe3+, and Pb2+ of 25, 15, 85, and 620 mg/g, respectively, at pH7
[18]. Youssef and Malhat [52] reported selective removal of lead, cop­
per, zinc, cadmium and iron ions from drinking water at pH7 using ti­
tanium dioxide nanowires, synthesized by hydrothermal method in 10
M NaOH at 493–533 K during two days. The sorption efficiency was 97%
with Pb2+, 75% with Cu2+, 35% with Zn2+, 65% with Cd2+, and 80%
with Fe3+. In turn, sorption capacity of the lead onto TiO2⋅nH2O parti­
cles synthesized through thermohydrolysis of titanium oxysulfate at
373 K, reached 120 mg/g [43]. However, after thermohydrolysis of
hydrated titanium dioxide, the final products performed low sorption
capacity. When calcination was performed at 673 K for 4 h, the
maximum sorption capacity of Pb2+ was estimated ca. 7.4 mg/g [42].
* Corresponding author.
E-mail address: m.rucki@uthrad.pl (M. Rucki).
https://doi.org/10.1016/j.cej.2022.137559
Received 25 April 2022; Received in revised form 11 June 2022; Accepted 13 June 2022
Available online 15 June 2022
1385-8947/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/).
D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Fig. 1. XRD analysis of the (NH4)2TiOF4 powders synthesized from fluoride
solutions: a) without additions at 293 K, b) without additions at 363 K, c) with
OP9 and hydrochloric acid addition at 293 K.
Further papers reported successful application of the hydrated titanium
dioxide for the uptake of radionuclides thorium Th4+ [12], uranium U
(VI) [22], cesium Cs+ [9], americium Am3+ [11], and plutonium Pu4+
[38].
Titanium dioxide nanopowder combined with pectin can serve as an
example of the hybrid material used for sorption of Cu2+, Cd2+, Zn2+,
and Pb2+. It exhibited maximum adsorption capacities 90, 76, 33, and
170 mg/g, respectively [4]. A composite material SiO2–TiO2 was pro­
posed for removal of cesium [13]. The authors discussed the effect of
mass proportion of TiO2, pH, time, cesium concentration, and calcina­
tion temperature on the sorption capacity towards cesium. Moreover,
rather low selectivity was noted. Another TiO2-based composite for
strontium and yttrium removal was TiO2–La [34]. The authors demon­
strated dependence of the sorption process on the interaction time and
solution acidity, and proved that the chemisorption was the main uptake
mechanism for yttrium. The sorption capacity of TiO2–La composite was
79 mg/g with strontium and 134 mg/g with yttrium.
Among promising sorbents, there are materials containing the
functional groups with electron donating properties. These are metal
phosphates, in particular, titanium phosphates (TiPs) [32]. Compared to
the zirconium phosphate, TiP exhibited higher coefficient of cesium
distribution and thus was recommended for liquid radioactive waste
treatment [26]. Amorphous titanium hydroxyphosphate Ti(OH)1.36(H­
PO4)1.32⋅2.3H2O was demonstrated effective in cesium removal in wide
range of pH from 2 to 10 [33] and TiO(OH)H2PO4⋅H2O exhibited ca­
pacity towards Cu2+, Zn2+, Mn2+ of 3.6, 8.5 and 2.8 mg/g [49]. Despite
the simplicity and ease of synthesis, titanium hydroxyphosphates have a
relatively low sorption capacity for heavy metals.
From the above short review it can be stated that, on the one hand,
titanium compounds have high sorption potential for removal and
concentration metal ions out of the water environment, but on the other
hand, there are still many issues to be solved. In particular, new mini­
malistic approaches to the synthesis are welcomed [31], particle
morphology should be investigated, selectivity and other sorption
properties of the variety of titanium compounds require deeper insight.
In the previous research [45], it was demonstrated that after calci­
nation of titanium nitrate at 973 K the TiO2 powder can be obtained,
containing anatase and rutile phases. The calcined powder consisted of
agglomerates of small spherical particles below 100 nm. Thermal
decomposition of titanium oxysulfate and oxyfluoride provided TiO2
powder of anatase structure that consisted mainly of spherical particles
with dimensions 100–200 and 100–300 nm, respectively. However, ti­
tanium dioxide particles obtained from thermal decomposition of pre­
cursors did not exhibit high sorption properties.
Fig. 2. SEM image of the (NH4)2TiOF4 particles synthesized from fluoride so­
lutions: a) without additions at 293 K, b) with OP9 and hydrochloric acid
addition at 293 K, c) without additions at 363 K.
Another widely used method of TiO2 synthesis is precipitation from
aqueous solutions of precursors with subsequent thermolysis [24,36].
When the fluoride solutions of titanium are treated with ammonia
aqueous solutions, titanium diammonium oxytetrafluoride (NH4)2TiOF4
appears in form of insoluble precipitation [44]. Presumably, this sub­
stance itself can exhibit sorption properties, but extensive literature
search did not reveal any result in this area, though some information on
its synthesis could be found [35]. Thus, we decided to investigate the
sorption characteristics of titanium diammonium oxytetrafluoride in the
context of its particle formation under various synthesis conditions.
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Fig. 3. Effect of pH on the uptake efficiency T (%) toward different metals; particles (NH4)2TiOF4 synthesized at: a) without additions at 293 K, b) without additions
at 363 K, c) with OP9 and hydrochloric acid addition at 293 K.
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Fig. 4. Sorption isotherms for cerium at pH 5.5, sorbent particles (NH4)2TiOF4 synthesized at different conditions: 1 – at 293 K without additions; 2 – at 363 K
without additions; 3 – at 293 K with OP9 and hydrochloric acid addition.
Fig. 5. Sorption isotherms for europium at pH 5.5, sorbent particles (NH4)2TiOF4 synthesized at different conditions: 1 – at 293 K without additions; 2 – at 363 K
without additions; 3 – at 293 K with OP9 and hydrochloric acid addition.
2. Experimental
2.1. Precipitation of (NH4)2TiOF4 particles
In the researches, concentrated nitric acid HNO3, hydrofluoric acid
HF, aqueous ammonia NH4OH, emulsifier OP9, and metallic titanium Ti
delivered by Reachem company (Russia) were used. Emulsifier OP9 is a
light colored oil-like paste obtained by treating mono- and dia­
lkylphenols with ethylene oxide. Solid phase extraction (SPE) CHRO­
MABOND columns delivered by Macherey-Nagel company were used.
All the reagents were qualified as chemically pure ones, and distillated
water was used for preparation of the aqueous solutions.
The particles of titanium diammonium oxytetrafluoride (NH4)2TiOF4
were obtained through precipitation of the fluoride solutions with
aqueous ammonia. The titanium fluoride solution was obtained by dis­
solving titanium in the hydrofluoric acid. The procedure was following:
10 g of metallic Ti were put to the teflon vessel of 100 mL volume, and
40 mL of the HF acid were added in portions 10 mL each. After out­
gassing from the reaction mixture was completed, the nitric acid was
added drop by drop until the solution became colorless. Then evapora­
tion was performed in order to remove excessive HF, until a crystalline
precipitate appeared. Next, heating was stopped, and immediately dis­
tillated water was added to the heated solution until the overall titanium
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Fig. 6. Sorption isotherms for lanthanum at pH 5.5, sorbent particles (NH4)2TiOF4 synthesized at different conditions: 1 – at 293 K without additions; 2 – at 363 K
without additions; 3 – at 293 K with OP9 and hydrochloric acid addition.
Fig. 7. Sorption isotherms for scandium at pH 5.5, sorbent particles (NH4)2TiOF4 synthesized at different conditions: 1 – at 293 K without additions; 2 – at 363 K
without additions; 3 – at 293 K with OP9 and hydrochloric acid addition.
fluoride solution volume reached 100 mL.
To the 20 mL of obtained solution, 30 mL of distillated water was
added at the temperature of 293 K. With simultaneous mixing, aqueous
ammonia was added to reach pH 11–12, and the solution was mixed for
another 30 min. Then, the precipitation was filtered out, washed with
distillated water several times, and then dried up at the room
temperature.
times, and then dried up at the room temperature.
2.3. Uptake efficiency and sorption isotherms
The uptake efficiency was determined using the model solutions with
metal ions (Sc, Y, La, Eu. Ce, Pb, Cd, Fe, Zn, Sr, Co). Initially, metal
concentration in the solution was 1 mg/L. To plot the isoterms, onecomponent metal solutions were used at pH 5.5 at different concentra­
tions of metal (Sc, Y, La, Eu. Ce, Co). The concentration range for cerium
was 5–600 mg/L, europium 5–350 mg/L, lanthanum 5–650 mg/L,
scandium 5–500 mg/L, yttrium 5–600 mg/L, and cobalt 5–200 mg/L. To
the chemical glass of 100 mL volume, 50 mL of the model solution was
added, and required pH between 3 and 9 was adjusted by adding some
nitric acid or aqueous ammonia. Then, 0.05 g of sorbent was added and
mixed for 40 min. In 10 min intervals, pH was measured and corrected to
the required value when the difference larger than pH 0.1 was found.
After the sorption test, the sorbent was filtered out and metal
2.2. Synthesis of (NH4)2TiOF4 nanorods
To the 20 mL of obtained solution, 30 mL of distillated water was
added at the room temperature with continuous mixing. Similarly, 10
mL of hydrochloric acid was added, as well as titanium with organic
additive OP9 in proportion by mass C(Ti):C(OP9) = 1:1. Then the entire
solution was mixed for 15 min, and next aqueous ammonia was added to
reach pH 11–12. As-obtained the solution was mixed for another 30 min.
The precipitation was filtered out, washed with distillated water several
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Chemical Engineering Journal 447 (2022) 137559
Fig. 8. Sorption isotherms for yttrium at pH 5.5, sorbent particles (NH4)2TiOF4 synthesized at different conditions: 1 – at 293 K without additions; 2 – at 363 K
without additions; 3 – at 293 K with OP9 and hydrochloric acid addition.
Fig. 9. Sorption isotherms for cobalt at pH 5.5, sorbent particles (NH4)2TiOF4 synthesized at different conditions: 1 – at 293 K without additions; 2 – at 363 K without
additions; 3 – at 293 K with OP9 and hydrochloric acid addition.
concentration was measured with the inductive coupled plasma atomic
emission spectrometry (ICP-AES) method. To increase reliability of the
results, the series of experiments were repeated three times.
Table 1
Results of the Langmiur isotherms calculations for cerium, europium,
lanthanum, scandium, yttrium and cobalt at pH 5.5.
Metal
Ce
Eu
La
Sc
Y
Co
293 K without
additions
363 K without
additions
293 K with OP9 and
hydrochloric acid
addition
Amax, mg/g
R2
Amax, mg/g
R2
Amax, mg/g
R2
350
300
240
270
200
12
91.2
98.2
97.6
93.3
99.2
98.5
120
140
120
170
120
8
93.6
95.2
91.1
96.7
96.6
94.4
150
250
150
210
140
10
97.5
98.7
98.1
98.1
94.5
92.6
2.4. Dynamic equilibrium tests
These tests were performed using model solution containing Zn, Cd,
Co, Eu, K, Mg, and Sr adding 0.3 g sorbent to empty 2 mL polypropylene
CHROMABOND SPE columns. During the first 5 min, distillated water of
pH5.5 passed through the columns for conditioning, and then the model
solution was applied (pH5.5). Initial concentrations of zinc, cadmium,
cobalt, europium, potassium, magnesium and strontium were 35, 35, 30,
70, 60, 50, and 40 mg/L, respectively. The solution flow ratio was 1 mL/
min. Fractions of the solution after passing through the column were
collected every 5 min. The concentration of metals in the initial solution
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Table 2
Comparison of Eu, Ce, La, Y, and Sc adsorption capacity (q) of different materials.
max
Sorbents
pH
(NH4)2TiOF4
Kaolinite
5.5
6.0
6.0
5.0
4.2
4.5
4.0
5.5
5.0
3.8
5.0
–
3.8
4.0
3.0
2–6
6.0
6.0
5.0
3,5
2–6
4.0
6.0
4.0
6.0
1.6–1.8
Titanates nanotubes
TiO2
Al2O3/expanded graphite
MnO(OH)
Fe3O4/Humic acid
Cellulose acetate (CA) membrane
clinoptilolite-containing tuff
O
Fe
3
4
Amorphous cerium phosphate
Clay/ humic acid
Tangerine (Citrus reticulate) peel
Carbon/3,4-dihydroxybenzaldehyde
goethite
Sargassum fluitans
activated carbon
Tangerine (Citrus reticulate) peel
Na alginate
Ca alginate
titanium phosphate
qmax ∞, mg/g
Refs.
Eu
Ce
La
Y
Sc
300
1.2
–
28
1.5
7.4
50.0
10.6
9.35
20.3
350
240
200
270
-
–
–
-
42
-
-
this work
[17 25]
–
-
2.07
3.2
-
-
[50 56]
[40]
[47]
[46]
[37]
[51 2]
–
–
–
-
–
–
–
-
–
–
–
-
28
155.6
–
–
-
–
–
-
-
-
-
–
–
-
–
–
–
-
116
–
–
-
–
–
2.56
–
–
-
–
21.7
–
–
–
–
162.79
–
–
-
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
144.80
2.56
72.4
175.4
154.86
–
–
–
–
-
–
-
–
-
–
-
–
-
128.2
181.8
99.0
97.08
–
-
42.14
–
and in each collected fraction was measured with ICP-AES atomic
emission spectrometry method.
0.487
[54]
[23],)
[2]
[15]
[14]
[48]
[30]
[50]
[39]
[3]
[48]
[20]
[20]
[41]
precipitated out of fluoride solutions without additions, according to the
section 2.1, at room temperature and at 363К, respectively. Here, the
reflections can be seen related only to the phase of titanium dia­
mmonium oxytetrafluoride (NH4)2TiOF4. In the diffractogram Fig. 1c,
there are reflections of the (NH4)2TiOF4, as well as the reflections related
to the phase NH4Cl.
In Fig. 2, there are SEM images of the synthesized particles. The
powder precipitated without additions at room temperature consists of
small spherical particles of dimensions below 50 nm, seen in Fig. 2a. At
elevated precipitation temperature up to 363К, the main particles
became larger, but also appeared small shapeless formations below 80
nm and large blocks with side dimensions several μm, as it can be seen in
Fig. 2c. In turn, the particles obtained after addition of OP9, formed rodlike shapes up to 10 μm long, shown in Fig. 2b.
2.5. Static equilibrium tests
Test in the static mode were performed using potable water with
additions of lanthanum and europium nitrates. To the chemical glass of
100 mL volume, 50 mL of the water was added and mixed with 0.05 g of
sorbent during 1 h at pH6.0. Then the precipitation was filtered out, and
metal concentration in the water after sorption was measured with ICPAES mass spectrometry method.
2.6. Equipment
Phase composition of the as-obtained powders were examined using
the X-ray diffractometry with graphite monochromator on the initial
beam. The wavelength of Kα radiation of Cu was λ = 1.54187 Å. Phase
identification was performed using Profex tools [6]. To analyze surface
morphology of the synthesized nanoparticles, scanning electron micro­
scope (SEM) type JSM-6390LV made by Jeol Ltd. (Japan) was used. It
provided high resolution of 4.0 nm. The FTIR spectra were obtained with
Spectrum One FTIR Spectrometer made by PerkinElmer (USA), using
potassium bromide tablets, of wavelength range 7800–350 cm− 1, and
resolution 0.5–64 cm− 1. Concentration of metal ions before and after
sorption was measured using the atomic emission spectroscopy method
with the inductive coupled plasma atomic emission spectrometry (ICPAES) method, Trace Scan Advantage device delivered by Thermo Jarrell
Ash (USA).
3.2. Sorption tests
The sorption capacity (qe) and uptake efficiency (T) belong to the
main characteristics of the sorbent that help to asses its practical
applicability [29]. Thus, in the present work, the main attention was
paid to these parameters.
3.2.1. Effect of pH on the uptake efficiency
From the initial researches it was found that the equilibrium could be
reached after 30–40 min, and further prolongation of the process did not
increase its efficiency. Thus, the tests were performed during 40 min. It
should be noted that the synthesis conditions of the (NH4)2TiOF4 par­
ticles had no effect on the uptake efficiency.
Fig. 3 shows the results obtained for different metal ions. It can be
stated that the highest uptake efficiency above 95% at pH3 was reached
for lanthanum, yttrium, scandium, europium, and cerium. Moreover, Pb
uptake efficiency of 95% was reached at pH4. As for iron, copper, zinc,
strontium, and cobalt, increase of pH provided higher values of uptake
efficiency that reached maximal values 90% at pH 7, 95% at pH 5.5,
95% at pH9, 70% at pH9, and 90% at pH9, respectively.
From the obtained results it can be derived that in the range of pH
3–6, the (NH4)2TiOF4 particles exhibited some sort of selectivity toward
3. Results and discussion
3.1. Particles characterization
Before the synthesized (NH4)2TiOF4 powders underwent sorption
tests, their morphology was analyzed. Fig. 1 presents the results of X-ray
phase analysis of the powder obtained by the above methodology. Dif­
fractogramms shown in Fig. 1a and 1b, represent the powder
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
dimension and decrease of their specific surface area.
Unlike lanthanides, sorption capacity toward cobalt was ca. 10 mg/g
and it revealed very little dependence on the morphological character­
istics of the (NH4)2TiOF4 sorbent. For the comparison, Table 2 presents
sorption capacity (qmax) values of several materials used for the removal
of europium, cerium, lanthanum, yttrium, and scandium. From this
comparison it can be supposed that the (NH4)2TiOF4 particles have
competitive potential as a sorbent. Its simple and economically advan­
tageous synthesis method provides additional merits.
Fig. 10 shows the FTIR spectra obtained for (NH4)2TiOF4 particles
synthesized at 363 K, before and after sorption tests with lanthanum and
europium removal. In the spectrum seen in Fig. 10a, there is wide
sorption band in the area of 3000–3600 cm− 1 with maxima at 3190 and
3082 cm− 1. Moreover, sorption bands at 1417, 770, and 536 cm− 1 can
be distinguished. Vibrations in the area of 536 cm− 1 can be attributed to
the bonds Ti-O-Ti, while the one at 770 cm− 1 to the bond Ti-F (Hou
et al., 2017). After the sorption of lanthanum (Fig. 10b) and europium
(Fig. 10c), substantial differences in the area of 600–1000 cm− 1 can be
seen. Sorption band of 770 cm− 1 displaced to 894 cm− 1 after removal of
the lanthanum and to 862 cm− 1 after removal of europium. Perhaps this
can be explained by interaction of the removed metals with fluoride
ions. This assumption seems to be confirmed by the XRD analysis shown
in Fig. 11. After sorption process, the sorbent contains fluorides of
lanthanum or europium. It is likely that the process consisted a metal
fluoride formation according to the following scheme:
(NH4)2TiOF4 → TiOF2 + NH4F
3+
3NH4F + La
→ LaF3 + 3NH+
4
(1)
(2)
3.2.3. Static and dynamic equilibrium tests
For the tests in static and dynamic equilibrium conditions, the par­
ticles (NH4)2TiOF4 were chosen that had been synthesized at 293 K.
These particles exhibited the highest sorption capacity toward rare earth
metals. Fig. 12 presents the plots of normalized concentrations C/C0
versus reaction time t, where C0 is initial concentration at t = 0, C is the
concentration at time t.
The experiments revealed that during the entire testing time of 130
min, removal ratio of europium was above 98% (C/C0 < 0.02), while the
divalent and monovalent elements were removed in very small degree.
Fig. 12 shows that the holding time is maximal for cadmium. It remained
in the column after t = 130 min in concentration of 90% compared with
the initial one, while exit concentrations of Mg and K after 30 min is
equal to the initial value (C/C0 = 1). These results provide evidence for
selectivity of the tested sorbent toward europium in presence of singly
and doubly charged ions, so that europium can be separated from them.
It should be noted that ratio C/Co above 1 can be observed in the range
of 25–75 min for potassium and magnesium, while for cobalt in the
range between 45 and 75 min. This effect, apparently, is a consequence
of the rather rapid saturation of the sorbent with these cations in the
initial stage, followed by their partial desorption before equilibrium is
reached.
Static equilibrium tests were performed in potable water with ad­
ditions of solutions of nitrates of cerium, lanthanum, and europium.
After nitrates were added, pH of the mixture reached 6.0 value. Results
of the sorption tests are collected in Table 3.
The experimental results demonstrated that the (NH4)2TiOF4 parti­
cles irrespective of synthesis conditions are capable to remove rare earth
metals from the potable water. The composition of the water itself plays
no significant role. In particular, Fe concentration remained unchanged
after the sorption test, while Ca and Mg concentrations slightly
decreased.
Overall results of the experiments in static and dynamic equilibrium
conditions proved potential feasibility of (NH4)2TiOF4 particles for se­
lective removal of the rare earth elements from water solutions.
Fig. 10. FTIR spectra of the (NH4)2TiOF4 samples synthesized at 363 K without
additions: a) before sorption test, b) after lanthanum sorption at pH5.5, c) after
europium sorption at pH5.5.
rare earth elements lanthanum, yttrium, scandium, europium, and
cerium.
3.2.2. Sorption isotherms
To assess the practical feasibility of the sorbent based on its sorption
capacity (qe), sorption isotherms were plotted. The isotherms were
described with Langmuir equations. For the tests, the metals of high
uptake efficiency were chosen, such as yttrium, lanthanum, scandium,
europium, and cerium, as well as cobalt characterized with low uptake
efficiency. Figs. 4-9 present the sorption isotherms related to the
(NH4)2TiOF4 particles synthesized at different conditions. Results of the
Langmiur model calculations are shown in Table 1.
The obtained experimental data demonstrated that the particles of
(NH4)2TiOF4 exhibited high sorption capacity toward lanthanides. The
highest value of the sorption capacity was noted in the case of the par­
ticles synthesized at room temperature without any additions. Namely,
as-obtained powder exhibited sorption capacity toward cerium and
europium at pH 5.5 was 350 and 300 mg/g, respectively. Elevated
synthesis temperature resulted with decreased sorption capacity by 50%
and even 70%. Perhaps, it can be explained considering larger di­
mensions of the particles obtained at higher temperature, as it is seen in
Fig. 2, and subsequent decrease of their specific surface area. Sorption
capacity of the nanorods formed at 293 K in the presence of OP9 and
hydrochloric acid additions toward lanthanides was also 25–50% lower
compared to the spherical particles synthesized at 293 K without addi­
tions. Similarly, it seems to be caused by the increase of the particles
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Fig. 11. XRD results for the particles (NH4)2TiOF4 synthesized at 363 K without additions: a) after lanthanum sorption at pH5.5, b) of europium after europium
sorption at pH5.5.
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D. Sofronov et al.
Chemical Engineering Journal 447 (2022) 137559
Fig. 12. Plots of normalized concentrations C/C0 for Zn, Cd, Co, Eu, K, Mg, and Sr obtained in the column at pH5.5 with (NH4)2TiOF4 particles synthesized at 293 K
without additions.
can be concluded that the (NH4)2TiOF4 particles exhibited selectivity
toward rare earth elements in the presence of divalent and monovalent
ions.
Comparison with available published data provided ground for the
conclusion that (NH4)2TiOF4 particles, usually treated as a side product
from TiO2 synthesis from the fluoride solutions of titanium treated with
ammonia aqueous solutions, can be themselves used as an effective
sorbent. High uptake potential of titanium diammonium oxy­
tetrafluoride, as well as its relatively simple, cheap and environmentally
friendly synthesis, motivates for further detailed research.
Table 3
Uptake of Ce, La, Eu from the potable water using the particles synthesized at
various conditions.
Metal
Fe
Ca
Mg
Ce
La
Eu
K
Na
Mn
SO2–
4
Cl–
C0 ,
mg/L
C after sorption process, mg/L
Synthesized at
293 K with no
additions
Synthesized at
363 K with no
additions
Synthesized at 293 K
with OP9 and
hydrochloric acid
0.04
24.2
7.92
10.0
10.0
10.0
7.8
185
0.005
84
174
0.04
23.95
7.768
< 0.01
0.0264
0.0248
7.8
180
0.005
80
170
0.04
23.25
7.390
< 0.01
0.0179
0.0080
7.8
182
0.004
75
168
0.04
15.85
6.876
< 0.01
0.0278
0.0276
7.8
184
0.004
70
172
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
4. Conclusions
Data will be made available on request.
The experimental results confirmed the presumptions on the sorption
properties of (NH4)2TiOF4. It was demonstrated that during precipita­
tion from fluoride solutions with ammonia, (NH4)2TiOF4 particles are
formed, and their morphology is dependent on the synthesis conditions.
Namely, it was found that when precipitation took place at room tem­
perature, dimensions of the particles were below 50 nm, but increase of
the temperature resulted with synthesis of larger particles. Application
of OP9 additive and hydrochloric acid provided the formation of rodlike particles of length up to 10 μm.
As-obtained (NH4)2TiOF4 particles exhibited high removal efficiency
above 95% toward lanthanum, yttrium, scandium, europium, and
cerium at pH larger than 3. Removal efficiency toward Pb reached 95%
at pH above 4, and further increase of pH increased uptake of Fe, Cu, Zn,
Sr, and Co. It was found that between pH 3 and 6, (NH4)2TiOF4 particles
exhibited selectivity toward rare earth elements. In particular, sorption
capacity toward cerium and europium at pH5.5 was 350 and 300 mg/g,
respectively. However, elevated temperature during synthesis resulted
in decrease of the sorption capacity by 50–70%, which could be attrib­
uted to the increased dimensions of the synthesized particles.
Moreover, from the results of static and dynamic equilibrium test it
Acknowledgements
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
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