АDSORPTION OF Co2+ AND RADIOACTIVE 60Со BY MESOPOROUS TiO2

Introduction shows the important of this scientific direction. 60Co with half-life of 5.3 years is one of the few anthropogenic, gamma-emitting radionuclides, that can be detected in aquatic environments affected by liquid effluent discharged from nuclear facilities. The need for control of the content of 60Co in the environment, determines the search for new adsorption materials with high adsorption capacity and chemical, thermal, and radiation resistance. The aim of present work is to investigate the adsorption of Co2+ and 60Co by mesoporous TiO2 from aqueous solutions. Experimental techniques describes the adsorption studies in detail. The mesoporous TiO2 with the initial pore size ratio (Smeso/S = 58 %; Vmeso/V = 64 %) was selected as adsorbent. Synthesis of adsorbents was carried out by the method of liquid phase hydrolysis of aqua complex of TiCl4. The dependence of adsorption value on agitation time, solutions acidity, and equilibrium concentration of Co2+ was investigated in butch mode. The presence of cobalt on the surface of mesoporous TiO2 was confirmed using XRF-analysis. The initial and residual concentration of cobalt was controlled by complexonometric titration with xylenol orange as indicator. Four simplified kinetic models: pseudo-first order and pseudo-second order equations, firstly applied by Lagergren, intraparticle diffusion and Elovich (Roginsky-Zeldovich) kinetic models were applied to experimental data. Langmuir and Dubinin-Radushkevich adsorption theory applied for experimental equilibrium data of adsorption of cobalt cations by mesoporous TiO2. The adsorption energy was measured using Dubinin-Radushkevich equation. The results obtained have shown that the experimental data on the adsorption kinetics of Co2+ by mesoporous TiO2 fit well by Lagergren pseudo-second kinetic model. Applying of Elovich kinetic model gives also high correlation’s coefficients, close to unit (R2 > 0.9). The equilibrium adsorption data are well approximated by Langmuir adsorption theory. Maximal adsorption value obtained experimentally (49±4 mg/g) is in good agreement with calculated by Langmuir adsorption theory (63.81 mg/g). The adsorption energy calculated using Dubinin-Radushkevich equation is 8.104±0.361 kJ/mol, which correspond to physical adsorption mechanism. However, for each values of Polanyi’s potential (ε) (which correspond to certain equilibrium concentration Ce, mg/L) adsorption energy is different. It smooth decreases with increasing concentration of adsorbate in the solution. Although the experimental results are well describing by the Langmuir model, the adsorption energy of Co2+ ions by mesoporous TiO2 depends on the degree of surface filling, which means that the adsorption centers of this sample are not independent. At the low equilibrium concentration of Co2+ (38 mg/L), the adsorption energy is much higher than the corresponding value for adsorption by the physical mechanism. To our opinion, that is why applying of Elovich kinetic model to experimental dada gives high R2. The adsorption of Co2+ by mesoporous TiO2 strongly depends on solutions acidity. To simulate conditions close to real, the adsorption of 60Co by mesoporous TiO2 was investigated. The percentage of 60Co, adsorbed onto TiO2 is more than 90 %. The main conclusion is that mesoporous TiO2 could be useful as an adsorbent for water purification from Co2+ and in decontaminating of radioactive waste containing 60Co.

INTRODUCTION 60 Co with half-life of 5.3 years is one of the few anthropogenic, gamma-emitting radionuclides, that can be detected in aquatic environments affected by liquid effluent discharged from nuclear facilities. 60 Co forms by slow neutron capture in 59 Co, and exist in the materials of the internal structure of the reactor core [1]. In river environment, it can be measured up to 20 km down-stream of NPP. Although cobalt is an essential trace element, it is toxic when concentration levels are too high.
Biosorbents, zeolites, modified zeolites [2], Fe 3 O 4 nanoparticles, and Fe 3 O 4 modified by mercaptobutyric acid (Fe 3 O 4 -MBA), meso-2,3dimercaptosuccimic acid (Fe 3 O 4 -DMSA) or ethylendiaminetetraacetic acid (Fe 3 O 4 -EDTA); metal-organic framework-based adsorbents (MOF), nano-silica or aіluminum silicate are proposed for adsorption removal of Co 2+ [3][4][5][6][7][8]. These adsorbents have certain limitations. Most of adsorbents have low adsorption capacities and selectivity. Their surface needs to be modified by chemical treatment [2]. Modified iron oxide nanoparticles are offered to be used for removal of Tl, Cd, Co, Cu, Ag, Pb [3], but among listed metals, cobalt adsorption is the worst. In addition, iron oxides limitation includes poor regeneration, which increases the cost of nanomaterials [3]. The need in the control of the content of 60 Co in the environment determines the search for new adsorption materials with high adsorption capacity, high recovery, and chemical, thermal, and radiation resistance.
ТіО 2 is known as efficient adsorbent toward bivalent heavy metal cations Ba 2+ , Pb 2+ , Hg 2+ , and radionuclides such as strontium [9][10][11][12]. Among all the TiO 2 synthesis methods, method of liquid phase hydrolysis of aqua complex of TiCl 4 [9][10][11][12][13][14] occupies an important place. Features of TiO 2 synthesis allows to change its surface to obtain the required properties. Mesoporous TiO 2 has a high chemical stability, and saves its adsorption properties even after 10 cycles of regeneration [11]. It is resistant to acidic and alkali medium, it has developed surface area, and is nontoxic. Also, mesoporous TiO 2 has a high thermal stability [11,12] and thus, its radiation stability can be predicted.
In the present investigations, we use a new mesoporous TiO 2 synthesized by reaction of liquid-phase hydrolysis of aqua complex of TiCl 4 for the adsorption of Co 2+ and 60 Co from aqueous solutions.

EXPERIMENTAL TECHNIQUES. SYNTHESIS OF MESOPOROUS ТіО 2
Synthesis of adsorbents was carried out by the method of liquid phase hydrolysis of TiCl 4 aqua complex Conditions of mesoporous TiO 2 synthesis are described in detail in publications [11][12][13][14]. The guidance of mesoporous TiO 2 synthesis, which was used in present investigations, is given in publications [11,12]. This sample relates to mesoporous materials by porous size distribution. However, on its surface there are a considerable number of pores with diameters less than 2 nm [12] classified as micro pores by IUPAC. Modification of the TiO 2 surface by arsenate or carbonate groups increases the volume/surface area of mesoporous TiO 2 , and increases the adsorption capacity of samples toward the heave metal cations with large ionic radius, such as Sr 2+ [11,12]. Mean while, the volume of micro pore remains unchanged or weak decreases after modification [12]. It has been suggested that the initial pore size ratio of the unmodified sample (S meso /S = 58 %; V meso /V = 64 % [12]) may be promising for the adsorption of Co 2+ with relatively small ionic radius. Therefore, TiO 2 synthesized without the addition of modifying reagents was selected as adsorbent.
Mesoporous ТіО 2 was synthesized in anataze modification with cell parameters: a = 3.78 Å; с = 9.5 Å, and crystallite size 4.7 nm. The specific surface area, pore volume, pore size distribution were estimated from N 2 adsorption/desorption isotherms using a BET-surface area analyzer (Quantachrome Autosorb Nova 2200e) at 77 K. The total surface area of this sample is 239.4 m 2 ·g -1 , and surface area of micropores is 100.5 m 2 ·g -1 ; that of mesopores is 138.9 m 2 ·g -1 . Pore radii calculated using DFT method are 1 to 2.5 nm. The point of zero charge of mesoporous TiO 2 pH pzc = 5/35 [11,12].

ANALYSIS OF Co 2+ AND 60 Co
The dependence of adsorption value from agitation time, solutions acidity and equilibrium concentration of Co 2+ were investigated in butch mode with liquid : solid phase ratio equal to 100 (m ads = 0.05 g, V sol = 5 ml). The investigations of adsorption value dependence on agitation time and initial concentration of Co 2+ were provided in neutral medium in the concentration ranges of 38-5497 mg/L. The effect of solution acidity on adsorption processes was investigated using certain amount of HNO 3 or NH 4 OH and was controlled by a pH meter "Bilorus' 2003". The initial and residual concentration of Co 2+ was determined using direct complexonometric titration with xylenol orange [15].
The adsorption value and separation factor were measured by equations (1) and (2): (1) where C o (mg/L) and C e (mg/L) initial and residual concentration of cations respectively; V (L) -volume of solution, m (g) -mass of adsorbent; A e -adsorption value (mg/g); 1/ 1 (2) where K L -constant of Langmuir equation [16].
For quality control of the adsorption experiments, replicate assays at least two times were carried out in different days under the same experimental conditions. The kinetic study of each experiment started with the addition of the mesoporous TiO 2 to the solution with dissolved cobalt compound. The solution was filtered after 5, 10, 20 etc. minute, then immediately analyzed for Co 2+ .
The constant β is related to adsorption energy by equation (6) . (6) When the linear approximation was applied, the value of R 2 was calculated using Microsoft Office Excel or Origin Pro 8. In nonlinear approximation, the equation (7) was used to calculate R 2 according to literature [16]: A exp (mg/g) is the amount of adsorbate uptake at equilibrium, A cal (mg/g) is the amount of adsorbate uptake achieved from the model using the 'Solver add-in', and A mean (mg/g) is the mean of the A exp values [16].
To simulate conditions close to real, the radioactive isotopes of 60 Co were obtained using reaction: 59 Co (n,γ) → 60 Co.
For this purpose, 20 g of CoCl 2 was positioned near neutron source. Pu (α) Be compound was used as a source of neutrons (φ = 1.3·10 6 n/cm 2 ·s; E n = 1-10 MeV). The highest cross section for interaction between 59 Co and neutron, according to [19] is for neutrons with energies 100 eV. So, the neutrons from Pu (α) Be sources were slowed down by paraffin to the ranges of energies 100 eV -1 MeV. The thickness of paraffin was calculated using Fermi age equation. The exposure time of the compound was at least 90 days [19]. The sample activity was detected by a scintillate spectrometer with NaI (Tl) crystal [1].
The adsorption conditions of 60 Co by mesoporous TiO 2 were the same as for stable Co 2+ : solution acidity was neutral, duration of interaction was at least 60 min, initial concentration of ( 60 Co)CoCl 2 was 0.005 M; only the mass of the adsorbent was doubled (m = 0.1 g) for the convenience of gamma-spectrometry and XRF analysis.
The decontamination factor (DF) was calculated by the following equation (8): % 100 (8) where (A i ) and (A f ) are the initial and final activities in (Bq/mL) of the radioactive solutions.

XRF ANALYSIS
The elemental chemical composition of the samples were done by XRF-analysis. The analysis was provided in the scan mode using an S2Ranger ©2010 Bruker AXS GmbH under next conditions: voltage 50 kV; tube current 1000 μA; pressure 1000 mBar; filter 250 mm Cu. The peaks of cobalt were observed with the energy 6.93 keV. The quantity of Co 2+ on sorbent surface is 0.146 % which equal to 1.46 mg of Co 2+ per 1 g of adsorbent for initial concentration of Co 2+ 50 mg/L.

RESULTS AND DISCUSSIONS. KINETICS OF ADSORPTION OF Со 2+ BY MESOPOROUS ТіО 2
Contact time between adsorbent and solution of corresponding metal has an important role in understanding of adsorption processes. Kinetics of adsorption of Со 2+ by mesoporous ТіО 2 is shown in Fig. 1.
Adsorption of Co 2+ from aqueous solution strongly depends on the time of interaction. Application of kinetic models to the results of adsorption of Со 2+ by mesoporous ТіО 2 is shown in Fig. 2 (a-d).  The equilibrium of Co 2+ adsorption was achieved after 60 minutes of interaction between TiO 2 and solution of Co 2+ . The Lagergren's pseudosecond order kinetic model provided better correlation than other kinetic models. As well known, a pseudo-second order reaction is a third order reaction in nature in which one of the reagents is in excess (in our case adsorbent). According to [11,12], adsorption of bivalent cations by mesoporous TiO 2 occurs by interaction with surface {≡Ті-OH} groups. High correlation coefficient of the pseudo-second order equation means that bivalent Co 2+ can interact with two {≡Ті-OH} groups. We suppose the physical mechanism of adsorption of Co 2+ by mesoporous TiO 2 as the main mechanism, but the first step of this process may be surface complexation between Co 2+ and 2[≡Ті-OH] groups: That is why applying of Elovich kinetic model to present experimental data gives also high R 2 value (R 2 = 0.94).

INVESTIGATION OF EQUILIBRIUM ADSORPTION OF Co 2+ BY MESOPOROUS TiO 2
Adsorption under equilibrium conditions provides fundamental data about the adsorption process. The parameters of equilibrium equations often give some insight into the sorption mechanism, the surface properties, and the capacity of the sorbent [16]. Adsorption equilibrium studies were provided in batch experiments and neutral conditions. Results are shown in Fig. 3.
Curve of isotherm adsorption of Co 2+ by mesoporous TiO 2 has a form close to Langmuir isotherm. According to Langmuir, adsorption is limited to one molecular layer, and locates on adsorption centers, which are independent on each other. Adsorption centers is bonding with one molecule of adsorptive [16].
In coordinates of С е /А е f (С е ) Langmuir isotherm has a linear form, slope and intercept of which correspond to parameters of Langmuir equation, and the square of linear approximation coefficient (R 2 ) indicates the degree of reliability of the calculated results. However, the Tran et al. [16] recommend applying a nonlinear approximation, which gives the values of the parameters of the Langmuir equation as close as possible to the true ones. Other authors [20][21][22][23] confirmed that the use of nonlinear approximation of experimental data of equilibrium adsorption gives values that are reliable relevant for modelling the isotherms of adsorption.
In present investigations linear and nonlinear approximation of Langmuir isotherm were applied. Nonlinear approximation is carried out using "Solver add in" option in Microsoft Office Excel [16].
Results are shown in Fig. 4, and in Table 2.
The experimental data of Co 2+ adsorption by mesoporous TiO 2 are adequately described by Langmuir model. Value of A max (mg/g) calculated using nonlinear approximation is closer to the experimental value of Co 2+ adsorption by mesoporous TiO 2 (Table 2) than the same parameter calculated using linear approximation. R L was measured using parameter of Langmuir equation K L calculated by nonlinear approximation (0.00086 L/mg). Obtained value of separation factor is less than unit (R L < 1, see Table 2), so adsorption of Co 2+ onto mesoporous TiO 2 is favorable [16].
The constant β, which can be determine from dependence А е f(ε 2 ), allows us to calculate the adsorption energy value by equation (8). Arithmetic mean of adsorption energy of Со 2+ by mesoporous ТіО 2 in concentration ranges of CoCl 2 0.001-0.1 mol/L (or 38-5497 mg/L) is 8.104±0.361 kJ/mol. However, for each value of Polanyi's potential (ε) (which corresponds to certain equilibrium concentration C e , mg/L) adsorption energy is different. It smooth decreases with increasing of adsorbate concentration in the solution. Although the experimental results are well described by the Langmuir model, the adsorption energy of Co 2+ ions by mesoporous TiO 2 depends on the degree of surface filling, what means that, according to Adamson [24], the adsorption centers of this sample are not independent.   In the case of physical adsorption of heavy metal by inorganic adsorbents, the value of adsorption energy is equal to Е ≤ 8 kJ·mol -1 [16]. At the low equilibrium concentration C e of Co 2+ (38 mg/L), the adsorption energy is much higher than the corresponding value for adsorption by the physical mechanism (see Table 3).
The mesoporous TiO 2 shows a high affinity to cobalt cations at low concentrations of cobalt in solution, which determines a high decontamination factor for 60 Co and could be useful for purification of aqueous solutions from cobalt waste with high accuracy. The point of zero charge of mesoporous TiO 2 pH pzc = 5.35. Near this value of pH the adsorption of Co 2+ begins and increases with increasing pH from 2.82 (pH = 4) to 20.5 mg/g (pH = 7) (see in Table 5, and Fig. 6).
These patterns of dependence of adsorption Co 2+ on pH are in good agreement with those shown in literature [27,29].
Decontamination factor measured using equation (8) is equal to 98 %. Investigated mesoporous TiO 2 could be useful as an adsorbent for water purification from Co 2+ and in decontaminating of radioactive waste containing 60 Co. CONCLUSIONS Adsorption of Co 2+ by mesoporous TiO 2 was investigated in the batch mode. The influence of agitation time, equilibrium concentration and solutions acidity were determined. The elemental chemical composition of the samples were done by XRF analysis.
The experimental data of adsorption kinetic of Co 2+ by mesoporous TiO 2 fitting well by Lagergren pseudo-second kinetic model. The equilibrium adsorption data well approximation by Langmuir adsorption theory. Maximal adsorption value, obtained experimentally (49±4 mg/g), is in good agreement with that calculated by Langmuir adsorption theory (63.81 mg/g).
Adsorption energy calculated using Dubinin-Radushkevich equation decreases with increasing of adsorbate concentration in the solution. Although the experimental results are well described by the Langmuir model, the adsorption energy of Co 2+ ions by mesoporous TiO 2 depends on the degree of surface filling.
Investigated mesoporous TiO 2 could be useful as adsorbent for water purification from Co 2+ and in decontaminating of radioactive waste containing 60 Co.