Introduction

Nano-sized materials are a new solid that takes a large portion of attention in recent years due to its special properties (such as small size (1–100 nm), numerous specific surface area [1,2,3]. Thus, the nano-materials are able to provide a selective high sorption capacity towards trace mineral ions.

A previous study showed that TiO2 nanoparticles could adsorb heavy metal ions [4] and rare earth elements [5]. The disadvantage in using nanometal oxides as a sorbent is the agglomeration due to van der Waals interactions [6] as a result for the increase in surface area, which makes metal oxide particles unstable. Thus, the metal oxide may lose selectivity, mechanical strength, and high adsorption capacity. To overcome this disadvantage, polymeric supports, with a large surface area, adjustable surface chemistry, perfect mechanical hardness, and pore size distribution, could be used for metal oxides immobilization [6, 7].

Polyethylene glycol (PEG) is a hydrophilic polymer capable of creating a uniform surface, smooth coating and well-defined particles [8], as it reduces cracked surface formation in the immobilizer system [9]. Most reports on nano TiO2 immobilized with PEG have been identified to enhance photosynthesis due to porosity effect and greater specific surface area [10, 11]. Trapalis et al. [12] found that as the weight percent of PEG introduced in the TiO2 film increases, the porosity consequently increases. However, no detailed study was conducted on the sorption behavior of TiO2 immobilized with PEG towards radioactive waste.

A large amount of radioactive wastewater has been produced in conjunction with the advances in nuclear industry. 90Sr, 134Cs, and 60Co are the most important radionuclides in low-level aqueous wastes in terms of a possible dose of humans [13,14,15]. Adsorption of radionuclides from their solutions is one of the various steps used in the treatment of radioactive liquid-waste using suitable adsorbent, (activated and impregnated carbon, clays, alumina silicates, organic, and inorganic gels and others, have been reported) [16,17,18,19,20,21,22].

Therefore, this work aims to prepare TiO2/PEG PNC and explain the physicochemical characteristics using FTIR, TEM, X-ray diffraction, DTA-TG, particle size distribution, and pore size analysis. The sorption of 134Cs and 60Co on TiO2/PEG PNC is studied. Optimization of the reaction conditions such as (pH, shaking time, sorbent concentration, and temperature have been investigated. Moreover, the specifications of the sorption reaction as kinetics mechanism, isotherms models, and thermodynamic parameters are studied.

Experimental

Chemicals

All chemicals used were of (AR) and were used without additional purification. Titanium tetra isopropyl orthotitanate (C12H28O4Ti ≥ 97%) obtained from Merck Germany, Isopropyl alcohol (C3H8OH ≥ 99.7%) obtained from Prolabo, England. Ammonium hydroxide (NH4OH) 33% from Edwic El-Nasr, Egypt. Polyethylene glycol 1500, H (OCH2CH2)n OH Extra Pure Obtained from Kosdaq. China. Dehydrated alcohol, absolute ethanol C2H5OH, 99.9% obtained from International Company for Sup. & MED. Industries, Egypt.

Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were obtained from El-Nasr Co. (ADWIC), Egypt, used to adjust the pH of the solutions. Cesium chloride (CsCl) and cobalt chloride (CoCl2) were purchased from Merck, (Germany). For all experiments, (DDW) double-distilled water was used for the preparation of solutions.

The radionuclides of two elements (134Cs and 60Co) were produced by irradiation of cesium chloride and cobalt chloride powders in the Second Egyptian Research Reactor (ET-RR-2). The initial activity were 42.2 KBq/L, and 14.3 KBq/L for 134Cs and 60Co radionuclides respectively.

A stock solution of (134Cs and 60Co) were prepared by dissolving a certain amount of cesium and cobalt salts in double-distilled water and spiked with the 134Cs and 60Co radionuclides. These solutions were kept as stock radioactive solutions for all experiments.

Preparation of nano-sized TiO2 powder (TiO2 NPs)

Nano-sized TiO2 is prepared by the sol–gel hydrolysis technique using titanium tetra isopropyl orthotitanate as a precursor. The preparation technique includes the hydrolysis of alkoxide (tetra isopropyl orthotitanate) to form sol and then the gel phase is formed after polymerization of sol using a catalyst [23]. The flow chart in Fig. 1a summarizes the steps for TiO2 NPs preparation.

Fig. 1
figure 1

Flow chart for preparation of a TiO2 NPs and b TiO2/PEG PNC

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Preparation of TiO2/PEG nanocomposite (TiO2/PEG PNC)

The 1:1 PEG/TiO2 nanocomposite was prepared using a wet chemical technique [24] where PEG was adsorbed on the TiO2 surface. 5 g of polyethylene glycol in 20 ml ethanol and added drop wisely to a solution of 5 g of the calcined nano-TiO2 powder. The mixture was lied in ultrasonic for 10 min. The whole mixture was stirred for 3hours. The gelatin was dried at 80 °C for 48 h. The flow chart in Fig. 1b summarizes the steps for TiO2/PEG PNC preparation.

Instruments and apparatus

The scanning electron microscope (SEM) combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction (SEM, FEI Quanta FEG-250, and EDX). The transmission electron micrograph image (TEM), for the particle size was recorded on a TEM, JEM2100, and Jeol.s.b, Japan. The X-ray diffraction (XRD) in a Philips X'PERT multipurpose X-ray diffractometer with copper emission lines. Thermal properties of the samples studied using thermo gravimetric analysis (TGA) performed on a Perkin Elmer TGA6 instrument. The sample was heated from 298 to 973 K with a heating rate of 283 K/min under a nitrogen atmosphere with a flow rate of 20 mL/min. Fourier transform infrared (FT-IR) spectra of the samples were recorded using an IR spectrometer with Fourier transformation (Thermo Nicolet Nexus FT-IR, Waltham, MA, USA. The particle size of the prepared samples determined using Zetasizer Nano-Zs, MALVERN, UK. Pore size distribution and corresponding porosity of the prepared powders investigated applying mercury intrusion porosimetry technique with the aid of Pore-sizer chromatech 9320, USA. The activity of radionuclides have been measured radiometrically by high resolution (7.5%) NaI (Tl) detector model 802-3X3, Canberra, USA.

Sorption batch studies

0.05 g of the TiO2/PEG PNC with (100 mg L−1) of the solution labeled with 134Cs and 60Co radionuclides at various time range (1–120 min), temperatures 25 °C, 40 °C, and 60 °C, and optimum pH. The activity of both radionuclides was determined and the amount of radionuclides, qt (mg g−1), was calculated from the following equation:

$$q_{t} = \frac{{A_{0} - A_{t} }}{{A_{0} }}xC_{0} x\frac{V}{m}$$
(1)

where (Ao and At) are the activity of the solution at initial and any time t, respectively, (Co) is the initial concentration of ions, (V) is the volume of the solution (10 mL), and (m) is the weight of adsorbent (0.05 g).

Sorption isotherm studies

Sorption isotherm studies of 134Cs and 60Co radionuclides [25] were investigated at different concentrations (50–300 mg L−1) onto the prepared materials by adding a certain weight of the sorbent (0.05) to solution of ions labeled by the radionuclides for 30 min. The pH value of the solution was adjusted at (5 ± 0.1). Different isotherm models such as Langmuir and Freundlich isotherm models were studied.

Results and Discussion

Characterization of the prepared nanocompositel

SEM and TEM measurement

Figure 2a represents the dispersion of TiO2 and PEG in TiO2/PEG PNC. There is neither phase separation nor cracking in the prepared composite but a rough and fractured surface is obtained [24, 26]. The white nanoparticles are constituted of TiO2 NPs and the dark surrounding area is constituted of the organic matrix of PEG.

Fig. 2
figure 2

a SEM and b TEM image of TiO2/PEG PNC

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Figure 2b shows a TEM analysis of TiO2/PEG PNC with a significant presence of nanoparticles reveals [27]. The obtained image is mostly composed of aggregates of hexagonal and irregular particles with a particle size below 20 nm. The aggregation attributes to the sol–gel preparation method.

Particle size statistics and porosity

The particle size statistics is represented in Fig. 3. The sample were ground in the mill and dispersed ultrasonically in non polar solventand water for TiO2/PEG PNC and TiO2 NP, respectively.

Fig. 3
figure 3

Particle size distribution of a TiO2 NPs and b TiO2/PEG PNC

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The nanocharacter appears in TiO2 particles while; the particle size distribution varies between nano-size to fine for TiO2/PEG PNC. The increase in particle size is due to the chemical adsorption of PEG molecules onto TiO2 [28].

The porosity and pore parameters of TiO2/PEG PNC were illustrated in Table 1. The porous structure of TiO2/PEG PNC is clarified as the porosity is 32.78%, TiO2/PEG PNC porosity strongly depends on the amount and molecular weight of PEG [12, 29].

Table 1 Porosity and pore parameters for TiO2/PEG PNC
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FT-IR analysis

Figure 4a, b shows a sequence of FT-IR spectra for TiO2 NPs and TiO2/PEG PNC respectively. Figure 4a represents the spectrum of TiO2 NPs, clearly shows bands at 3862 cm−1, 2979 and 2901 cm−1, assigned to the stretching vibration of the hydroxyl group O–H of the TiO2 NPs. The presence of the O–H group belongs to the remaining water/humidity molecules in the crystals lattice of the TiO2 sample. The band observed around 1633 cm−1, corresponding to bending modes of water Ti–OH; a strong noticeable peak at 1051 cm−1 related to Ti–O modes [30, 31].

Fig. 4
figure 4

FT-IR spectra for: (a) TiO2 NPs, and (b) TiO2/PEG PNC

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The FT-IR spectrum of TiO2/PEG PNC is given in Fig. 4b, a considerable change represents in the spectrum where new bands appear due to the presence of PEG. A strong peak around 1099 cm−1 attributable to C–C and C–O stretching vibrations. A sharp peak at 2872 cm−1 is also observed, corresponding to C–H stretching vibration; while the peak 3862 cm−1 in TiO2 NPs for O–H disappears. This change confirms the chemically adsorption of PEG onto TiO2 [28, 31, 32].

Thermal analysis

Figure 5 shows that PEG/TiO2 PNC had an approximate two endothermic peak at ~ 60 °C and 144 °C with initial weight loss 2.5% attribute to the evaporation of absorbed water and ethanol [28]. Started from 280 °C till 539 °C, three endothermic peaks with total weight loss 51% are attributed to organic matter degradation of PEG in TiO2/PEG PNC [28, 31].

Fig. 5
figure 5

TGA and DTA for the synthesized TiO2/PEG PNC

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X-ray diffraction (XRD)

The result of XRD analysis of TiO2/PEG PNC is represented in Fig. 6 and listed in Table 2. The crystallite structure of the nanocomposite is clarified through the intensity of XRD peaks while the broad diffraction peaks indicate very small size crystallite For TiO2/PEG PNC. The diffraction angle, 2θ = 25.5 [35] confirms TiO2 anatase structure, while 2θ = 23.62, 19.49 confirms crystalline phase of PEG [24].

Fig. 6
figure 6

XRD profiles of the TiO2/PEG PNC

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Table 2 XRD results of TiO2/PEG PNC
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Sorption studies

The effect of pH

Figure 7a shows the effect of pH on the amount adsorbed (qe) of 134Cs and 60Co radionuclides, different experiments were performed in the pH range 1–8. The sorption of both radionuclides is pH-dependent. The amount adsorbed of 134Cs and 60Co increase with increasing pH. At higher pH values, [H+] is low and the negative charge on the surface of the composite will increase enhancement the attraction forces among the surface of the composite and radionuclides increase, so the amount adsorbed of the radionuclides will increase. At lower pH values, the concentration of [H+] is high and competes the sorption of Cs and Co radionuclides on the surface of the composite and so decrease the amount adsorbed of these radionuclides [33].

Fig. 7
figure 7

a Effect of pH on the amount of adsorbed of 134Cs, 60Co on TiO2/PEG PNC at room temperature and b stability of prepared TiO2/PEG PNC powder in different pH

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The speciation diagram of Cs+ and Co+2 ions at different pH values in aqueous solution is shown in Fig. 8 using Hydra/Medusa chemical equilibrium software [34]. As pH increases, hydrolysis precipitation most probably would start due to the formation of various hydro complexes in aqueous solution [35].

Fig. 8
figure 8

Speciation of: a cesium and b cobalt ions at different pH and room temperature

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At initial metal ions concentration, 50 mg L−1, room temperature and different pH values (1–12). From Fig. 8a it is clear that the monovalent cesium (Cs+) is the dominant species at all pH ranges. It is obvious that in Fig. 8b, the divalent species, Co2+, is the dominant cobalt species at pH below 8, at the pH values > 8, the cobalt ion was precipitated as Co(OH)2. So, the removal of cobalt below the pH ˂ 8 is due to the sorption process but at a higher value of pH the hydrolysis of Co2+ [35], leading to the formation of [CoOH]+ and Co(OH)2, therefore, the precipitation process is the predominant mechanism at the higher pH values. So all the sorption experiments were carried out at pH 5 [35]. The result of chemical stability tests is shown in Fig. 7b. The prepared powder material was highly stable at pH 5 with weight loss of about 4%, and it witnessed considerable weight loss of 22, 25 and 27% at pH values 3, 7 and 8, respectively. In light of these findings, further experiments were performed in a solution of pH 5 to avoid any possible hydroxide precipitation and also ensure usage of the material in its most stable state.

The pH- distribution coefficient relation is shown in Table 3. The results obtained showed that Kd is pH-dependent. As the pH increases, kd will increase for both radionuclides confirming the increase in amount sorbed of radionuclides as the pH increase.

Table 3 Distribution coefficients (Kd) of 134Cs and 60Co radionuclides sorbed by TiO2/PEG PNC at different pH
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Effect of shaking time& temperature

The effect of shaking time on the sorption behavior of the studied radionuclides at temperatures 25 °C, 40 °C, and 60 °C is represented in Fig. 9. A certain weight (0.05 g) of the prepared nanocomposite and optimum pH 5. The results indicated that the sorption process was fast and equilibrium was reached within 30 min sorbents. The rapid radionuclides sorption onto TiO2/PEG PNC sorbents can be explained by the availability of active sites on sorbent surfaces. The percent uptake was found to be 45.3, 52.5 and 58.1% for Cs+ ions, and 83.3, 87.3 and 94.5% for 60Co respectively. The amounts sorbed of 134Cs radionuclide at equilibrium is 22.65, 26.25 and 29.05 mg g−1 at 25 °C, 40 °C and 60 °C respectively while the sorbed amounts of 60Co radionuclide at equilibrium is 41.65, 43.75 and 47.25 mg g−1 at 25 °C, 40 °C and 60 °C respectively.

Fig. 9
figure 9

Effect of contact time on the amount sorbed of a134Cs and b60Co onto TiO2/PEG PNC at different temperatures, pH 5 and V/m = 200 mL/g

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Sorption kinetics modeling

Kinetic models are used to determine the rate of sorption reaction and to investigate the mechanism of the sorption process.

The pseudo-first-order model was studied by applying the Non-linear pseudo-first- order Eq. (2) [36]:

$$q_{t} = \, q_{e} \left( {1 - {\text{e}} -^{{k_{1} t}} } \right)$$
(2)

where qe (mg g−1), and qt (mg g−1), are the quantities of metal ion sorbed at equilibrium and at time t, respectively, and k1 (min−1), is the rate constant of the first-order equation.

The pseudo-second-order non –linear equation is expressed as [36]:

$$q_{t } = \frac{{k_{2} q_{e}^{2} t}}{{1 + \, k_{2} q_{e} t}}$$
(3)

where k2 is a rate constant of the second-order model (g/mg. min). Figure 10 illustrates the fitting of the relation and the value of k2 and qe were determined and listed in Table 4. The pseudo-second-order equation is used with the assumption that the interaction between the adsorbate and adsorbent material is due to the chemisorption process.

Fig. 10
figure 10

Non-linear form fitting of pseudo-first order and pseudo-second order plots for the sorption of a134Cs and b60Co radionuclides onto TiO2/PEG PNC at different temperatures

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Table 4 Calculated parameters of the studied kinetic models for adsorption of 60Co and 134Cs onto TiO2/PEG PNC
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Figure 10 expressing the relation fitting. Table 4 illustrated the values of qe, k1, k2 and the correlation coefficient (R2) of both radionuclides at three different temperatures. The values of the calculated capacity of the sorbed radionuclides at equilibrium qe (cal.) must be in identity with the values of the experimental data qe (exp.).

The values of qe (cal.) are commensurate with the results of qe (exp.) for both pseudo-first and pseudo- second- order. But, the high values of R2 for pseudo-second-order elucidate that the sorption of 60Co and 134Cs radionuclides on TiO2/PEG obeys pseudo-second-order kinetics. This fact shows that the pseudo-second-order model is the predominant model and the sorption process can be controlled by the chemical sorption process.

Equilibrium isotherm studies

The adsorption isotherm describes the relationship between the amount of sorbate on the sorbent at equilibrium, qe, and the concentration of dissolved sorbate in the liquid, Ce Fig. 11 shows the plots between the amount of 134Cs and 60Co radionuclides sorbed at equilibrium qe, onto the papered TiO2/PEG PNC and the initial metal ion concentrations Co. The concentration of metal ions ranges 50–300 mg/L. From the Figure, it is observed that the amount sorbed of each radionuclide was increased with the increase of the initial concentration of the metal ions and temperature.

Fig. 11
figure 11

Effect of metal ion concentrations on the amount sorbed of a134Cs and b60Co onto TiO2/PEG PNC at pH 5, different temperatures, and V/m = 200 mL/g

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Langmuir and Freundlichisotherm models

The non-linear form of the Langmuir model can be expressed as the following [37]:

$$q_{e} = \frac{{ q_{m} b \, C_{e} }}{{1 + bC_{e} }}$$
(4)

where qm is the monolayer sorption capacity (mg g−1), b is the constant related to the free energy of sorption (b α e−ΔG/RT) and Ce is the equilibrium metal ion concentration.

Also, the non-linear form of the Freundlich model can be expressed in Eq. (5) as the following [37]

$$q_{e} = \, k_{f} C_{e}^{1/n}$$
(5)

where, kf is the Freundlich adsorption capacity coefficient (mg g−1), 1/n is the adsorption intensity

Figure 12 represents non -linear form fitting of Langmuir isotherm and Freundlich models of 134Cs and 60Co radionuclides sorption onto TiO2/PEG PNC. On the basis of the parameters obtained from the Langmuir model (Table 5), the maximum sorption capacities of TiO2/PEG PNC towards 134Cs and 60Co were calculated to be 163.28 ± 10.98, and 330.16 ± 41.60 mg g−1, respectively at 25 °C. The sorption capacity of 134Cs and 60Co onto TiO2/PEG PNC and the energy of sorption increased with temperature. The available active sites of TiO2/PEG PNC increase as the temperature is raised, consequently the sorption capacity will increase.

Fig. 12
figure 12

Non-linear form fitting of Langmuir isotherm and Freundlich models of 134Cs and 60Co radionuclides sorption onto TiO2/PEG PNC at different temperatures

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Table 5 The values of parameters of Langmuir and Freundlich isotherm models for adsorption ions onto TiO2/PEG PNC:
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Also, the numerical of 1/n < 1 suggested that TiO2/PEG PNC sorption reactions towards 134Cs and 60Co are favorable.

From the tabulated parameters, comparing the correlation coefficients of Langmuir isotherm model (R2 ≥ 0.981 for 134Cs and R2 ≥ 0.957 for 60Co radionuclide) with the correlation coefficients of Freundlich isotherm model (R2 ≥ 0.98 for 134Cs and R2 ≥ 0.98 for 60Co radionuclide), indicating that the Freundlich isotherm model is more applicable.

Temkin model:

Temkin isotherm linear form is given by Eq. (6) [38].

$$q_{e} = \, RT \, \ln \, K_{T} + \, RT/b \, C_{e}$$
(6)

where, KT is the equilibrium binding constant (L mol−1), the adsorption heat is (b), Fig. 13 represents Temkin isotherm plot. Table 6 shows the values of parameters and R2 ≥ 0.96 for 134Cs and ≥ 0.92 for 60Co.

Fig. 13
figure 13

Temkin isotherm plots of 134Cs and 60Co radionuclides sorption onto TiO2/PEG PNC at different temperatures, pH 5 and V/m = 200 mL/g

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Table 6 The values of parameters of the Temkin isotherm model for 134 Cs and 60Co radionuclides sorption onto TiO2/PEG PNC:
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Effect of metal-ion Interference

The sorption interference of specific ions on TiO2/PEG PNC was studied. The sorption mixture of Na (I), Sr (II) and Fe (III) were studied.; Contact time was 2 h with 0.05 g of sorbent dosage. It was shown that the presence of the monovalent, divalent and trivalent ions greatly affect the sorption of both 134Cs and 60Co radionuclides. The sorption capacities of TiO2/PEG PNC towards 134Cs and 60Co increase in the order the 60 Co > 134Cs. In presence of 50 mg L−1 of Sr (II), the amount sorbed from 83.3 to 9.5 mg g−1 and from 45.3 to 1.9 mg g−1 for 60Co and 134Cs, respectively. This might be due to the influence of ionic potential and ionic radius [39]. Figure 14 represents the metal-ion interfering effect on 134Cs and 60Co radionuclides onto TiO2/PEG PNC.

Fig. 14
figure 14

Metal-ion interfering effect on a134Cs and b60Co radionuclides sorption onto TiO2/PEG PNC

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Thermodynamic studies

The sign of the free energy, (ΔG° kJ/mol, determines the favorability of the sorption process. ΔG° is calculated with Eq. (7) aid.

$$\Delta G^{{^\circ }} = - RT\ln K_{c}$$
(7)

where, the universal gas constant is R (8.314 J K−1 mol−1), Temperature is T (K), and the equilibrium constant of the sorption process is Kc. The values of other thermodynamic parameters such as ΔH° (kJ/mol (and ΔS° (J/mol K) have been calculated using the following Eq. (8).

$$\ln \, K_{C} = \, \Delta S^{{^\circ }} /R - \, \Delta H^{{^\circ }} /RT$$
(8)

Figure 15 shows the plot between ln Kc and 1/T. The values of ΔH° are positive emphasize an endothermic sorption reaction of 134Cs and 60Co radionuclides onto TiO2/PEG PNC as shown in Table 7. Also, the positive values of ΔS° indicate an increase in the randomness at the sorbent/radionuclides interface and a good affinity of sorbents towards the radionuclide [40].

Fig. 15
figure 15

Thermodynamic plot for the sorption of a134Cs and b60Co radionuclides onto TiO2/PEG PNC

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Table 7 Thermodynamic parameters for the sorption of 134Cs and 60Co radionuclides onto TiO2/PEG PNC:
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Conclusion

TiO2/PEG PNC was successfully prepared by wet-chemical method and physic-chemically characterized by some analytical techniques such as (TEM), (FTIR), (XRD), (DTA-TGA), particle size distribution, and pore size analysis. TiO2/PEG PNC used for sorption of 134Cs and 60Co radionuclides from aqueous solutions at pH 5. The equilibrium was reached after about 30 min. Pseudo second order model is applicable with the sorption reaction mechanism. The monolayer capacities (at pH 5 and 25 °C) of 134Cs and 60Co radionuclides on TiO2/PEG PNC were found to be 163.28 ± 10.98, and 330.16 ± 41.60 mg g−1, respectively. The thermodynamic studies explain favorability and endothermic nature of the sorption process. Therefore, TiO2/PEG PNC could be used as an efficient sorbent for 134Cs and 60Co radionuclides. The study demonstrates that interference with co-metals is an important factor for consideration in TiO2/PEG PNC sorption application.