Adsorption of Sr(II) cations onto phosphated mesoporous titanium dioxide: Mechanism, isotherm and kinetics studies

Journal Pre-proof Adsorption of Sr(II) cations onto phosphated mesoporous titanium dioxide: mechanism, isotherm and kinetics studies Ivan Mironyuk, Tetiana Tatarchuk, Hanna Vasylyeva, Mu. Naushad, Igor Mykytyn

PII:

S2213-3437(19)30553-6

DOI:

https://doi.org/10.1016/j.jece.2019.103430

Reference:

JECE 103430

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

10 July 2019

Revised Date:

14 September 2019

Accepted Date:

20 September 2019

Please cite this article as: Mironyuk I, Tatarchuk T, Vasylyeva H, Naushad M, Mykytyn I, Adsorption of Sr(II) cations onto phosphated mesoporous titanium dioxide: mechanism, isotherm and kinetics studies, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103430

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Adsorption of Sr(II) cations onto phosphated mesoporous titanium dioxide: mechanism, isotherm and kinetics studies Ivan Mironyuka, Tetiana Tatarchukb,c,*, Hanna Vasylyevad, Mu. Naushade, Igor Mykytyna

a

Department of Chemistry, Vasyl Stefanyk Precarpathian National University, 57 Shevchenko Street, 76018 Ivano-Frankivsk, Ukraine, [email protected] b

Educational and Scientific Center of Material Science and Nanotechnology, Vasyl Stefanyk Precarpathian National University, Ivano-Frankivsk, 76018, Ukraine, [email protected] c

d

Uzhhorod National University, [email protected]

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Narodna

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Faculty of Chemical Technology and Engineering, UTP University of Science and Technology, 3, Seminaryjna str., 85-326, Bydgoszcz, Poland Square,

e

88000

Uzhhorod,

Ukraine,

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Department of Chemistry, College of Science, Building #5, King Saud University, Riyadh, 11451, Saudi Arabia, [email protected]

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Graphical abstract

Highlights 

Modification of titania surface by phosphate groups was performed



The phosphate groups led to formation of mesoporous titania with SBET=410 m2·g-1

2 

The maximum adsorption capacity for Sr(II) were calculated 173 mg g−1.



The adsorption mechanisms for Sr(II) removal were elucidated

Abstract

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In this work, phosphate-modified titanium dioxide nanoparticles with mesoporous structure were synthesized and utilised as an efficient nanoadsorbent for Sr(II) removal from aqueous environment. The globular titania particles with a 3-4 nm diameter with chemosorbed on their surfaces phosphate groups =Ti(O2РООH) have been obtained by liquid-phase synthesis. The characterization of titania nanoparticles were performed using XRD, TEM, IR-spectroscopy and Brunauer-Emmett-Teller analyses. The synthesized modified adsorbents exhibited SBET=396– 410 m2·g-1 and mesopore volume of 0.262 – 0.275 сm3·g-1. The isotherm and kinetic models were used for the description of Sr(II) adsorption. The sample 4P-TiO2 showed the best adsorption ability for strontium ions removal. The adsorption capacity of Sr(II) onto modified 2P-TiO2, 4P-TiO2 and 8P-TiO2 samples was found to be 94.1, 172.5, 128.9 mg/g, respectively. An adsorption mechanism for Sr(II) removal was proposed. The adsorption/desorption studies were conducted to find the reusability of phosphated adsorbents. A titanium dioxide-based mesoporous material possesses a significant adsorption activity due to high content of surface acid sites. All the outcomes revealed that the phosphate modified titania showed great potential in Sr(II) removal from aqueous environment and nuclear effluents.

Keywords: Titania; Sr(II); Adsorption; Phosphate; Radionuclide; Aqueous environment *

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Corresponding author: e-mail: [email protected] (Tetiana Tatarchuk) Tel: +38 068 463 24 35

3 1 Introduction Water is an essential and important substance on Earth, because it provides the vital functions of living organisms. However, due to natural and technogenic impacts, the quality of our water resources is constantly deteriorating because of the accumulation of inorganic and organic harmful substances [1,2]. Particularly serious danger to the biosphere causes water pollution by toxic heavy metals such as strontium(II), lead(II), cadmium(II), zinc(II), cupper(II), as well as arsenate, selenate, and fluoride anions [3–6]. Excessive presence of these pollutants in

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the water can causes carcinogenic, mutagenic or teratogenic effects on living organisms. The main origins of environmental pollution are thermal power plants, ferrous metallurgy enterprises, non-ferrous metallurgy enterprises, and chemical industry. The pollutants are

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accumulating in the earth near the enterprises of ferrous and nonferrous metallurgy [7]. More than 95% of them entering into the soils as technogenic dust or with atmospheric precipitations.

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Heavy metals are oxidized in the soils; washed out by rainwater and fall into the reservoirs.

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Different treatment techniques like coagulation, precipitation, ultrafiltration, filtration and adsorption have been used for the removal of toxic pollutants [8–10]. Among them, the adsorption technology has some more benefits – simplicity, accessibility and efficiency [11].

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Strontium-90 originates from nuclear power plants and is well known hazardous element for the environment due to its high harmful effect and radioactivity [12]. The accumulation of

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strontium in the body leads to the destruction of the whole body (general toxic effect). The most

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typical form of this disease is the development of dystrophic changes in the osteo-articular system during the period of body growth (there is a lag in growth, exhaustion, baldness, etc.). This disease is accompanied also by a pronounced disturbance of the calcium-phosphorus ratio in the blood, intestinal dysbiosis. In the presence of strontium, iodine becomes inaccessible to the human organism; as a result, the internal iodine deficiency occurs. In this regard, the problem of Sr(II) removal from water is very important for Ukraine and other counties, which had accidents at nuclear power plants [13].

4 The different types of sorbents have been intensively investigated for toxic pollutants removal from aquatic environment such as sand, clays, kaolin, dolomite, etc. However, such adsorbents have low efficiency, since the adsorption process is very slow and their adsorption capacity is small [14–16]. In addition, the ZrO2 microspheres [17], magnetic graphene oxide [18], activated carbon [19], titanate nanotubes [20], TiO2 nanoparticles [21], hydroxyapatite [22], niobate nanofibers [23] have also been used for Sr(II) adsorption. In could be seen that the titanate adsorbents possess much better adsorption capacity towards strontium ions compared to

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the known adsorbents. It was noted that the adsorptive properties of titanante nanomaterials depend on the synthesis methods. The controlled synthesis creates the opportunities for designing the micro- and mesoporous adsorbents with desired surface, morphology and

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structural properties which increase their activity in various environmental processes.

It was shown that titanium(IV)-based compounds had a capability to adsorb heavy metals

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[24–27], and the surface modification increased their adsorption properties [28,29]. For example, according to the [25], the layered titanium phosphate was investigated as promising adsorbent

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due to chemosorbed phosphate groups located at its surface, which can bind the metal cations in the aqueous environment. However, its adsorption capacity was low due to small porosity and

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low surface area. Earlier our scientific group proposed a new way to obtain mesoporous titania [30] with chemisorbed orthophosphate groups on its surface. The high porosity of the material

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and the existence of chemisorbed phosphate groups on the pores surface could substantially increase its ability to bind Sr(II) cations, as well as cations of other heavy metals. The promising

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phosphatized adsorbents for the removal of Sr(II) ions in water treatment were also reported by Ivanets et al. [31], which were prepared by the soft non-acidic method using thermally activated dolomite and NaH2PO4,Na2HPO4 and Na3PO4 as phosphating reagents. Therefore, the aim of this work was to obtain mesoporous titania with chemosorbed phosphate groups and to investigate their impact on the sorption ability to eliminate Sr(II) from

5 aquatic environment. The mesoporous TiO2 was obtained by the liquid-phase route using a titanium aquacomplex and a modifying reagent Na3PO4 as a precursors.

2 EXPERIMENTAL

2.1 Synthesis of unmodified and modified titania adsorbent

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The titanium aquacomplex precursor [Ті(Н2О)6]3+·3Cl– was obtained as described in [32]. The modification of TiO2 was performed by sodium orthophosphate (2, 4 and 8% (wt.) of Na3PO4). The NaOH was added in a drop wise manner to obtain the pH = 1-2 at temperature

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70oC during 30-40 min. After that, the pH of the solutions was increased to around 6-7. The obtained nanoparticles were detached from solutions by the vacuum filtration, washed and dried

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2.3 Characterization methods

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at temperature 120-140°С.

The phase analysis was performed by XRD (STOE STADI P, Cukα anode (λ = 0.154 nm)).

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The calculation of crystallite sizes was carried out by methods of integral width of diffraction reflexes with the help of WinPLOTR software. The structural and adsorption characteristics were carried out

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using BET-surface area analyzer (Quantachrome Autosorb, Nova 2200e, at 77K). The IR-spectra were recorded using SPECORD M80 spectrophotometer in the range of 4000-300 cm-1. Transmission

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electron microscopy (TEM) micrographs were performed using the JEM-100 CX II and JSM 2100F microscopes.

2.4 Adsorption of Sr(II) The adsorbed amount of Sr(II) was estimated from the difference between the initial concentration of Sr(II) (from 0.001 to 0.1 mol·L-1 SrCl2) and the equilibrium concentration of

6 Sr(II) after its contact with the adsorbent. The adsorbent with certain mass was added to a model solution of metal at 293 K, after 4 hours it was centrifuged and the equilibrium concentration of Sr(II) (Ce) was determined using a complexometry [33] by the following equation: 𝑞𝑒 =

(𝐶𝑖 (𝑆𝑟) − 𝐶𝑒 (𝑆𝑟)) ∗ 𝑉(𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛) 𝑚𝑎𝑑𝑠

(1)

where Ci(Sr) and Ce(Sr) (mg·L-1) are initial and equilibrium concentration of Sr(II) respectively, V(solution) (L) is volume of model solution, and mads (g) is mass of the titania adsorbent. Kinetics of Sr(II) adsorption onto raw and phosphated titania was studied at pH: 7.0 and

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temperature: 298 K in the solution with Co(Sr)= 0.01 mol·L−1; m(ads)=0.05 g and contact time 140 min. At certain intervals, the solution was separated from the adsorbent and analyzed for strontium content. All the data were the average values of three parallel adsorption tests.

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In addition, the point of zero charge (pHPZC) of titania adsorbents was also measured by

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drift method [34] and the adsorption/desorption experiments were performed to estimate the

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reusability of titania adsorbents.

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3 RESULTS AND DISCUSSION

3.1. Modification of titania adsorbent

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The titanium aquacomplex [Ti(H2O)6]3+·3Cl- as a precursor allows to obtain a titania nanoparticles with desired physic-chemical properties by liquid-phase route [30]. The NaOH

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addition leads to the formation of molecules Ti(OH)4·2H2O according to the scheme (2): ОН−

[Ti(OH2)6] ·3Cl → 3+

-

ОН−

→

ОН−

[TiОН(OH2)5] ·3Cl → 3+

-

ОН−

[Ti(OH)3(OH2)3]+·Cl- →

ОН−

[Ti(OH)2(OH2)4] ·2Cl →

Ti(OH)4·2H2O

2+

-

(2)

The introduction of Na3PO4 modifier into the aquacomplex precursor changes the reaction equilibrium and causes the oxidation of Ti3+ cations. The anions OH-, PO43- form intermediate complex [Ti(O2РООН)(OH2)5]3+·3Cl-. After NaOH addition, the pH is increased and

7 intermediate complexes are converted into the molecules Ti(O2РООH)3(OH)3·2Н2О, which become as centers of origin and growth of primary oxide particles (eq.3): ОН− ,Na3PO4

[Ti(OH2)6]3+·3Cl- →

3NaOH

[Ti(O2РООН)(OH2)5]3+·3Cl- →

Ti(O2РООH)3(OH)3·2Н2О (3)

The peculiarity of the titania particles growth process is that the =Ti(O2РООH) groups are focused on the primary particles surface. Thereafter, the condensation process is changed in such a way that the surface with chemosorbed phosphate groups forms the interparticle pores between

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the particles, increase the surface area and pore volume in the titania adsorbents.

3.2. Characterization

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3.2.1. X-ray diffraction analysis

The diffractograms of the TiO2 samples are depicted in Fig. 1 and proved that the samples

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of the unmodified titania (TiO2) and the modified titania (4P-TiO2) were single-phase. Their crystalline structure belongs to the anatase structure (space group I41/amd). Structural parameters

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of synthesized TiO2 samples are specified in Table 1. It could be concluded that there was a tendency to decrease the crystallite sizes with increasing the amount of chemosorbed phosphate

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groups. The crystallite sizes were calculated from the XRD data and it could be seen that they are actually match the dimensions of primary particles formed due to the condensation of the

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Ti(OН)4·2Н2О and Ti(O2РООН(ОН)2·2Н2О molecules.

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Fig. 1 The X-ray diffraction analysis of raw and modified TiO2.

Table 1. The structural characteristics of anatase phase for the raw and modified TiO2 Lattice volume, Å3

Crystallite sizes, nm

a = 3.786, с = 9.430

135.170.05

4.7

a = 3.791, с = 9.430

135.520.05

3.4

4P-TiO2

a = 3.795, с = 9.470

136.380.05

3.2

8P-TiO2

a = 3.809, с = 9.500

137.830.05

3.0

TiO2

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2P-TiO2

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Lattice constants, Å

Sample

The data presented in Table 1 shows that the lattice parameters of the modified TiO2

samples are greater than the lattice parameters of the raw TiO2 sample. There is a tendency to increase the cell volume with the increase in phosphate groups amount. Specifically, in the sample 8P-TiO2, the lattice volume was 137.83 Å3, which were 1.7% higher than the lattice volume of the 2P-TiO2 sample and 1.97% higher than the lattice volume of the unmodified

9 titania sample. This dimensional effect was due to the decrease in the dimensions of the crystallite sizes in the investigated TiO2 samples. There is a tendency to reduce the size of the primary TiO2 NPs with the increase of amount of chemosorbed phosphate groups on the titania surface. The necessity of the lattice parameters of the oxide materials from the particle size was due to the Laplace pressure, which could grow to 1.96×106 kPa in the nanoparticles. The convergence of Ti4+ cations and the reduction of Ti-O-Ti valence angle between neighbor TiO6 octahedrons, due to pressure, causes a counteraction that manifests in the increase of the

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interatomic distance in the Ti-O bonds. This is due to the spacial effect caused by Laplace pressure, which was detected also in the fumed silica nanoparticles [35,36].

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3.2.2. Transmission electronic microscopy

The TEM observations displayed that the primary particles were globular in shape with

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diameter 3-6 nm (Fig. 2a). In the acidic reaction medium, the primary particles do not coagulate, since they formed the colloidal micelles with the electrolyte ions. The double electric layer

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determines the micelle’s stability. During the NaOH addition, the aggregate stability of the primary particles was lost and the aggregates with spherical shape and a diameter of 20-60

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nanometers were formed (Fig. 2b). The drying of the water dispersion lets to grow up of spherical aggregates and formed a porous xerogel product. It could be seen that most of particles

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of the xerogel were crystallographically disoriented (Fig. 2c).

10 Fig. 2 TEM: (a) Primary particles of 4P-TiO2; (b) Aggregates of 4P-TiO2 nanoparticles; (c) Nanocrystallites of 4P-TiO2 xerogel.

3.2.3. BET analysis Fig. 3 depicts the N2 adsorption/desorption isotherms by TiO2 samples and the associated parameters given in Table 2. The pore size distributions are depicted in Fig. 4. The textural characteristics of the adsorbents porous structure indicated that the surface-chemical

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modification of samples resulted in a rise in their specific surface area, volume of pores, mostly in the mesopores volume (Vmeso), and the samples possess micro/mesoporous structure.

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Table 2. Morphological characteristics of titania samples S,

Smicro

Smeso

Smeso/S,

V

Vmicro

Vmeso

Vmeso/V,

m2·g-1

m2·g-1

m2·g-1

%

cm3·g-1

сm3·g-1

сm3·g-1

%

TiO2

239

100

139

58.2

0.152

0.054

0.098

64.5

2Р-TiO2

408

54

354

86.8

0.287

0.025

0.262

91.3

4Р-TiO2

410

42

368

89.8

0.290

0.019

0.271

93.4

8Р-TiO2

396

18

378

95.5

0.281

0.006

0.275

97.9

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Adsorbent

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The Table 2 exhibits that the specific surface area of mesopores Smeso for 2Р-TiO2, 4Р-

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TiO2, 8Р-TiO2 was 354, 368, and 378 m2·g-1, respectively, which was 2.55, 2.65 and 2.72 times greater than that of raw titania. The Vmeso for modified samples was 0.262, 0.271 and 0.275 сm3·g-1 for 2Р-TiO2, 4Р-TiO2, 8Р-TiO2, respectively which was in 2.67, 2.76 and 2.80 times greater than that of raw titania. The pore size distribution for unmodified titania showed that most of mesopores had a radius less than 2.2 nm (Fig. 4). Mesopores of this sample had a bimodal structure. The first

11 maximum on the PSD curve corresponded to pores with a radius around 1.3 nm, and second maximum on the PSD curve corresponded to pores with a radius 1.7 nm. The introduction of 2% (wt.) of anions PO43- into TiO2 leds to an increase in the quantity of mesopores with the size 1.7 nm. Moreover, an increase in the amount of chemosorbed phosphate groups in the sample 4P-TiO2 led to the shift of the second maximum on the PSD curve from 1.7 nm to 2.0 nm, and to the increase in the number of those pores. Further increase in the amount of phosphate groups in the 8P-TiO2 sample provided an increase in Vmeso and Smeso, which leds to a

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reduction in mesopore size. The second maximum on the PSD curve was shifted to 1.7 nm. The surface area of phosphated samples increased in 2-2.5 times compared to raw TiO2 due to increase of pore volume. This can be attributed to role of surface phosphate groups, which

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prevent the condensation of small TiO2 particles.

Fig. 3 Nitrogen adsorption/desorption isotherms for TiO2 samples.

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Fig. 4 The PSD with respect to pore radii of raw and modified TiO2.

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3.2.4. Infrared (IR) spectroscopy

IR-spectra of the raw titania sample and modified samples are depicted in Fig. 5. It can be

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concluded, that the bands near 340, 460, 575 and 730 cm-1 corresponded to the vibrations of Ti-

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O bonds in the TiO6 polyhedrons [32,37]. The bands at 1630 cm-1 corresponded to vibrations of water [38–40]. According to [41–44], the asymmetric asym and symmetric sym stretching

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vibrational modes of the P–O and P–OH bonds in chemisorbed phosphate groups for modified samples are observed at 780, 972, 1048 cm-1 (Fig. 5). These data indicated that covalent

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attachment of РО43- anions to cations Ti4+ from titanium aqua complex precursor was carried out by dentate bonds and leads to the formation of chemisorbed =Ti(O2POOH) groups on the

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nanoparticles surface.

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Fig. 5 IR spectra of raw and modified TiO2.

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3.3. Adsorption of strontium(II)

3.3.1. Surface charge (pHPZC)

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The magnitude of pHPZC is a useful characteristic of the surface, since it indicates the range of pH in which the adsorbent is positive or negative charged. If рН˃рНPZC, the negatively

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charged nucleic centers TiО- are appear on the TiO2 surface: TiОН + ОН-  TiО- + Н2О

(4)

If рН < рНPZC, so the positively charged nucleic centers TiОH2+ appear on the TiO2 surface: TiОН + Н+  TiОН2+

(5)

The hydroxylated surface of TiO2 behaves like an amphoteric electrolyte and it can exhibit both acidic and basic properties.

14 The point of zero charge of each sample was measured by drift method. The measured magnitude of the pHPZC for raw TiO2 was 5.35, while for 2P-TiO2, 4P-TiO2, and 8P-TiO2 samples the magnitudes of the pHPZC were 3.4, 3.1 and 3.1 respectively. The chemisorption of =Ti(O2POOH) groups on the titania particles surface displaces the pHPZC into the acidic area and extend the functionality of the adsorbent.

3.3.2. Adsorption isotherms

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The adsorption of Sr(II) cations were conducted for raw and modified TiO2 samples in order to know the maximum adsorption ability. The adsorption isotherms are displayed in Fig. 6a. The results specified that the adsorption capacity of the raw titania reached to 70.9 mg/g

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compared to phosphated samples 2P-TiO2, 4P-TiO2, and 8P-TiO2, which demonstrated much higher adsorption capacity 94.1 mg/g, 172.5 mg/g and 128.9 mg/g, respectively. The modified

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adsorbent 4P-TiO2 had significantly high adsorption capacity among the modified specimens and thus more effective in comparison with unmodified sample TiO2.

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The Langmuir, Freundlich and Dubinin-Radushkevich isotherm models [45–48] were involved to determine the mechanism of strontium(II) adsorption by raw and phosphate-modified

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titania adsorbents (Fig. 6b-d) and the model parameters are displayed in Table S1.

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Fig. 6. (a) Sr(II) adsorption data on the raw and phosphated titania adsorbents, fitted to (b)

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Langmuir, (c) Freundlich, (d) Dubinin-Radushkevich isotherm models (pH 7.0, V = 5 mL, m(ads) = 50 mg, T = 293 K, contact time 4 h).

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It was established that adsorption data fitted well by Langmuir model due to the high

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R2 values (0.9763–0.9912). The calculated magnitudes of qmax are in good consent with experimental obtained qmax and this conclusion indicates that the adsorption of Sr(II) cations onto

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titania samples is monolayer process. The rooting of titanium-phosphate groups =Ті(О2РООН) onto the titania surface increases the proton-donor ability of surface hydroxyls and they become

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able to bind the Sr(II) cations. The 4P-TiO2 sample has highest value of KL=0.0013 L/mg. The qmax, obtained from Langmuir isotherm, shown the following capacity order: 4P-TiO2 > 8P-

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TiO2 > 2P-TiO2 > TiO2. It is well known that the interactions between individual components can significantly alter sorption characteristics. The results obtained in the current study showed

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that Sr(II) removal by titania-based adsorbents involved monolayer adsorption on the welldistributed active sites of the adsorbent with no interactions between the neighboring adsorbate ions. The Freundlich model does not fitt well adsorption data due to lower R2 (0.9531-0.9799) (Table S1). The parameters obtained from Dubinin-Radushkevich model, which was utilized in order to get conclusion about physical and chemical adsorption, are represented in Table S1. The data indicated the high R2 value (more than 0.9606). The application of Dubinin-Radushkevich

16 model gave the mean adsorption energies of 11.04, 10.78, 12.31 and 11.79 kJ·mol−1 for TiO2, 2P-TiO2, 4P-TiO2, and 8P-TiO2 respectively. These values indicated that the Sr(II) adsorption on the raw and phosphated titania is ion-exchange process. Fig. 7 shows the removal efficiencies of raw and phosphated titania adsorbents for 0.001M Sr(II) solution at 298 K. The removal efficiency for Sr(II) adsorption onto adsorbents followed the order: 4P-TiO2 (83%) > 8P-TiO2 (78%) > 2P-TiO2 (65%) > TiO2 (58%) suggesting a larger Sr(II) removal efficiency for 4P-TiO2 sample. The EDS analysis was involved to evidence

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the presence of phosphorus and adsorbed strontium on the modified titania samples (Fig. 8).

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Fig. 7 The removal efficiencies of Sr(II) from 0.001 mol/L solution of SrCl2 onto unmodified

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and modified titania.

Fig. 8 EDS of 4P-TiO2 sample after Sr(II) adsorption.

Compared to other known adsorbents (Table 4), the adsorption capacity of raw and phosphated

17 mesoporous titania adsorbents towards strontium ions is much better and demonstrate the effectiveness of synthesized in current study titania adsorbents for the Sr(II) elimination from aquatic environment.

Table 4. Comparing of the adsorption ability for reported adsorbents for the Sr(II) elimination qe (mg·g-1)

References

magnetic chitosan beads

11.58

[16]

phosphate-modified montmorillonite

12.5

[15]

Egyptian soils

12.53

TiO2 NPs

25.00

Titanate nanotubes

66.72

Hydroxyapatite

24.5 (0.28 mmol·g-1)

[49]

70.9

This study

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TiO2 2P-TiO2

[14] [21] [20]

94.1

This study

128.9

This study

172.5

This study

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3.3.3. Kinetics studies

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8P-TiO2 4P-TiO2

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Adsorbent

In order to estimate the rate of adsorption, which is one of the useful parameter in the

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adsorbent activity evaluation [50], the adsorption kinetics studying was carried out and the

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adsorption mechanism was proposed. The adsorption capacities (qt, mg/g) of unmodified and modified titanium adsorbents for the Sr(II) adsorption vs. the contact time (t, min) depicted in Fig. 9 (for 0.01 mol/L initial strontium(II) concentration and for contact time 140 min). The adsorption was fast at the beginning and reached the equilibrium after 60 minutes.

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m(ads)=0.05 g, Co(Sr)= 0.01 mol·L−1, V=5 ml).

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Fig. 9 Kinetics of Sr(II) adsorption onto raw and phosphated titania (pH=7.0, 298 K,

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A pseudo-first-order model, a pseudo-second-order model, an Elovich model, and an intraparticle diffusion model [48] were applied to explain the kinetics of adsorption. All kinetics parameters, estimated from experimental data and the above-mentioned models, are presented in

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Table S2 and depicted in Fig. 10(a-d). Fitting degree of the kinetic models was evaluated by

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correlation coefficient R2.

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Fig. 10. Kinetics studies: (a) PFO; (b) PSO; (c) Elovich; (d) intraparticle diffusion model (pH = 7.0, 298 K, mads = 50 mg, V = 20 mL).

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The results indicated that the obtained data well described by the PSO kinetic model (R2= 0.9980 – 0.9999). The calculated from PSO model equilibrium adsorption capacities qe(calc) are in

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the good agreement with experimentally obtained adsorption capacity qe(exp). The initial sorption

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rate h (mg·g-1·min-1) was estimated by the Eq. 6:

ℎ = 𝑘2 ∙ 𝑞𝑒2

(6)

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where k 2 (g·mg-1·min-1) is the rate constant of the PSO model; qe (mg·g-1) is the equilibrium amount of adsorbate. It could be seen that obtained values of initial sorption rate h are 2.95; 4.58;

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23.53 and 19.65 mg·g-1·min-1 for TiO2, 2P-TiO2, 4P-TiO2, and 8P-TiO2, respectively. These data suggested that the titania modified by 4%(wt.) of phosphate revealed the maximum h. It should

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be indicated that the PSO model fit well the processes means that it involved more than one stage for adsorption from aquatic medium between the adsorbent and adsorbate species [51], what is taking place in the current study (please see the explanation below). The Elovich model was applied to explore the adsorption initial rate 𝛼 and the desorption constant 𝛽. From Fig. 10c it is clear that plot qt vs. ln(t) show good linear correlation. The slope and intercept give us the Elovich constants, which are tabulated in Table S2. The highest

20 magnitude of 𝛼 is for 8P-TiO2 sample (𝛼= 2008.11), and desorption constant for phosphated titania are in three-four times smaller (𝛽=0.17-0.28) than that of raw sample (𝛽=0.69 for TiO2). In order to forecast the rate-controlling step of adsorption the intraparticle diffusion model (IPD) [48] has been involved to fit the experimental data and the estimated kinetic parameters were shown in Table S2. The analysis has shown that plot qt vs. t0.5 has one linear ranges for TiO2 and two linear ranges for phosphated samples (Fig.10d). It can be concluded that the adsorption onto raw titania includes surface adsorption and micropore diffusion, while the

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adsorption process onto phosphated adsorbents includes the transportation of strontium cations to the external surface and diffusion of Sr(II) into the mesopores of adsorbent. In addition, the plots did not pass through the origin (Fig. 10d), and constants C1, and C2 were not zero, demonstrating

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that diffusion through boundary layer also occurred in the uptake of Sr(II) onto adsorbents [48]. It can be noticed, the constants C2 are higher than constants C1 indicating that the contribution of

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3.3.4. The influence of pH

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boundary layer to adsorption rate increased from stage 1 to stage 2.

The pH is a parameter that affect the adsorption ability of any adsorbent [52–56]. Fig. 11

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displayed the influence of pH on Sr(II) ions adsorption onto raw and phosphated titania samples.

21 Fig. 11 The impact of pH on Sr(II) ions adsorption onto phosphated titania.

As can be seen from Fig.11, the phosphated sorbents exhibited a significantly higher adsorption ability compared to raw titania in acidic and neutral mediums. In the alkaline solution (pH = 8-9) the raw TiO2 and phosphated adsorbents possess better adsorption ability. For example, the adsorption ability of 4P-TiO2 sample was increased from 51.7 mg/g (at neutral pH) to 73.9 mg/g (at pH=9), while the adsorption ability of raw titania was increased from 21.09

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mg/g to 70.4 mg/g at neutral pH and pH=9, respectively. It was shown that the phosphate groups =Ті(О2РООН) and groups ТіОНδ+ (Brønsted acid centers) are the active centers for Sr(II) binding on the surface of modified TiO2 samples,

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because they are transforming into negatively charged centers =Ті(О2РОО–) and ТіО- in a medium with pH > pHPZC. The structural characteristics of the TiO2 samples were taken into

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account to calculate the average number of ТіОН groups on the titania globules surface [28]. The calculations (Table 6) have shown that the average amount of those groups is 128 units per

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10 nm2 of surface area. According to [57] there are 120-140 ≡ТіОН groups per 10 mn2 on the

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TiO2 surface area and those groups can be neutral ТіОН, basic ТіОНδ- or acidic ТіОНδ+.

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Table 6. The adsorption centers on the phosphated titania surface The amount of adsorption centers

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Adsorbent

(per 10 nm2)

The percentage of adsorption centers (%)

=Ті(О2РООН)

ТіОНδ+

ТіОНδ-

ТіО2*

0

27.0

0

21.1

2Р-ТіО2

1.9

29.7

0.7

25.2

4Р-ТіО2

3.7

54.1

1.8

45.2

8Р-ТіО2

8.0

36.8

1.0

35.8

* the average amount of hydroxyl groups is 128 species per 10 nm2 [28]

22 The results show that the rise in the amount of chemosorbed phosphate groups leads to an increase of titania activity regarding to strontium adsorption. From Table 6 it can be seen that there are 27 ТіОНδ+ active centers per 10 mn2 for non-modified titania adsorbent, what is equal to 21.1% of total number of ≡ТіОН groups. The 2Р-ТіО2 adsorbent surface contain 1.9 phosphate groups and 29.7 active centers ТіОНδ+ per 10 mn2, while the chemisorption of 3.7 phosphate groups per 10 mn2 in the 4Р-ТіО2 sample leads to formation of 54.1 acidic centers ТіОНδ+ (45.2 %) on its surface. In the 8P-TiO2 sample the eight phosphate groups cause the

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formation of 36.8 acidic centers, what is less than in the 4Р-ТіО2 sample. The decreasing of the active centers number in this adsorbent is probably due to higher aggregation of the primary particles, which limits the access of Sr(II) cations to the active centers. The data presented in the

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Table 6 allow us to calculate the ability of chemisorbed titanium-phosphate groups to form acidic centers ТіОНδ+ in the vicinity. In particular, in the 4P-TiO2 adsorbent the one

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=Ті(О2РООН) group generates 14.6 Brønsted centers which are able to bind the strontium cations.

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The rooting of titanium-phosphate groups =Ті(О2РООН) into the TiO2 surface structure increases the proton-donor ability of surface hydroxyls and they become able to bind the Sr(II)

na

cations. The higher electronegativity of phosphorus atoms compared to titanium atoms causes the displacement of the electron density to the phosphorus atoms in the =Ті(О2РООН) groups.

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The inductive effect of the electron density redistribution is also taking place in the Ti-O-Ti

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bridges attached to the titanium-phosphate groups, therefore, the neighboring hydroxyl groups get proton-donor properties and become as Brønsted acid centers (Scheme (7)):

(7)

23 Thus, in phosphated TiO2 samples, the developed structure of mesopores, their large specific surface, and also the ionogenic nature of chemosorbed groups =Ti(O2POO-) provided the high adsorption activity for Sr(II) cations. The adsorption of cations by phosphated sorbents in acidic

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and neutral media (pH = 2-7) was carried out according to the following scheme (8):

(8)

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In a low alkaline medium (pH~8.0), the formation of cations SrOH+ led to an increase in

na

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their adsorption (Scheme 9):

(9)

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Based on these schemes, Sr adsorption mechanism was mostly electrostatic interaction and

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Sr(II) cations could be adsorbed on the phosphate modified TiO2 because of negatively charge surface caused by deprotonation of TiOH and phosphate groups. However, in a strongly alkaline medium (pH  9.0), chemisorbed phosphate groups were split off from the adsorbent surface. The breaking of Ti-O-P bridge bonds and the loss of chemosorbed groups aligned the adsorption activity of the unmodified and modified sorbents. The adsorption of iodide anions from potassium iodide solution were performed to calculate the amount of basic centers ТіОНδ- onto adsorbents surface. The experiments have shown that

24 non-modified TiO2 does not adsorb anions while the adsorption of iodide anions by the 2P-TiO2, 4P-TiO2, 8P-TiO2 adsorbents is 0.052, 0.126 and 0.066 mmol·g-1 respectively.

3.3.5. Regeneration studies The adsorption/desorption experiments were conducted to explore the reusability of phosphated sorbents. The 0.001M HNO3 solution was used to investigate the desorption of Sr(II) cations from the surface of sorbents. The obtained results are shown in Fig. 12 that indicated that

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adsorbents have good reusability. The adsorbents (TiO2, 2P-TiO2 and 4P-TiO2) did not lose their adsorption capacity during the four cycles of adsorption/desorption, while 8P-TiO2 lose its adsorption capacity from 10.45 mg/g to 2.4 mg/g. Typically, all adsorbed amount of Sr ions was

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desorbed by eluent. However, the 8P-TiO2 sample lose its adsorption capacity. The probable reasons are pore blocking and nanoparticle aggregation. The decrease of adsorption capacity in

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the 8P-TiO2 sample can also be attributed to loss of some phosphate groups from

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the titania surface resulting in decrease of active adsorption centers.

Fig. 12 Reusability of phosphated titania adsorbents (experimental conditions: 0.0025 mol·L-1 Sr(II) solution , temperature 25°C, pH=7.0).

25 4. Conclusions The globular titania particles with a 3-4 nm diameter with chemosorbed on their surfaces phosphate groups =Ti(O2РООH) have been obtained by liquid-phase route. During the gelation and dispersion drying stage, the sites with the chemosorbed phosphate groups blocked the accretion of the primary particles and led to the formation of a xerogel material with a homogeneous mesoporous structure. Such liquid-phase synthesis with the use of 4% (wt.) of modifying reagent allowed obtaining the adsorbent with a mesopore volume of 0.275 cm3∙g-1 and

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specific surface of 368 m2∙g-1, which in 2.8 and 2.6 times exceed the unmodified TiO2. The developed mesopore structure, a large specific surface of the sorbent, as well as the ionogenic nature of chemosorbed groups provided its high adsorption ability for Sr(II) removal.

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Since, chemisorbed phosphate groups shifted the surface charge pHPZC from 5.35 to 3.1-3.4, which expanded the functionality of the adsorbent, and provided an effective adsorption of metal

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cations in the acidic medium. The maximum equilibrium adsorption capacity obtained from the Langmuir model for modified sorbent 4P-TiO2 was noted 172.5 mg.g-1 and the adsorption of

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Sr(II) by phosphated adsorbent was exceeded 1.3-2.4 times compared to unmodified TiO2. Sr adsorption mechanism was mostly electrostatic interaction and Sr(II) cations adsorbed on the

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phosphate modified TiO2 because of negatively charge surface caused by deprotonation of TiOH

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and phosphate groups.

Acknowledgements

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This work was supported by the Ministry of Education and Science of Ukraine (Project

number MESU 0117U002408).

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