Fabrication of Highly Porous Polymeric Nanocomposite for the Removal of Radioactive U(VI) and Eu(III) Ions from Aqueous Solution

In the present study, a polymeric nanocomposite, CoFe2O4@DHBF, was fabricated using 2,4 dihydroxybenzaldehyde and formaldehyde in basic medium with CoFe2O4 nanoparticles. The fabricated nanocomposite was characterized using FTIR, TGA, XRD, SEM, TEM, and XPS analyses. The analytical results revealed that the magnetic nanocomposite was fabricated successfully with high surface area 370.24 m2/g. The fabricated CoFe2O4@DHBF was used as an efficient adsorbent for the adsorption of U(VI) and Eu(III) ions from contaminated water. pH, initial concentration, adsorption time, and the temperature of the contaminated water solution affecting the adsorption ability of the nanocomposites were studied. The batch adsorption results exposed that the adsorption capacity for the removal of U(VI) and Eu(III) was found to be 237.5 and 225.5 mg/g. The adsorption kinetics support that both the metal ions follow second order adsorption kinetics. The adsorption isotherm well fits with the Langmuir adsorption isotherm and the correlation coefficient (R2) values were found to be 0.9920 and 0.9913 for the adsorption of U(VI) and Eu(III), respectively. It was noticed that the fabricated nanocomposites show excellent regeneration ability and about 220.1 and 211.3 mg/g adsorption capacity remains with U(VI) and Eu(III) under optimum conditions.


Introduction
Radioactive substances can be found in the air, water, and soil, polluting the environment. However, the water source can be contaminated using radionuclides, naturally present in rock and soil or released from human activities, such as medical radiology or nuclear power plants [1][2][3]. The long-term exposure of the radionuclides or drinking of contaminated water can cause cancer and other disorders to the human and animals [4][5][6]. The World Health Organization (WHO) considers that 30 µg L −1 of Uranium is safe, but a clear no-effect concentration has not been definitively derived yet. The US Environmental Protection Agency (EPA) has adapted this value. Therefore, the development of effective approaches or systems to treat the contaminated water represents an urgent demand for researchers. Several methods, including reverse osmosis, membrane filtration, solvent extraction, electrodialysis, chemical precipitation, and adsorption, have been used for the removal of toxic ions form aqueous solution [7][8][9]. Nevertheless, each technology has advantages and disadvantages especially regarding efficiency and costs (Supplementary Table S1). Among these methods, adsorption is the most effective method because of its high efficiency, low cost, and environmentally friendly nature [10][11][12][13]. However, for adsorption, an efficient adsorbent plays the main role and several adsorbents such as The functional group presents in the polymer and in the nanocomposite were determined using FTIR spectra as shown in Figure 2a. In the case of the dihydroxybenzaldehyde-formaldehyde based polymer resin (DHBF) several FTIR peaks were noticed at 3324-3520 cm −1 (O-H), 3044 (C-H aromatic), 2949-2845 (C-H sym and asym), 1663 (C=O), 1564 (C=C), and others [41,42]. Meanwhile, in the case of the magnetic polymer nanocomposite CoFe2O4@ DHBF the C=O band was shifted from 1163 cm −1 to 1649 cm −1 was noticed in the presence of CoFe2O4 nanoparticles and support the interaction between the magnetic nanoparticles and the polymer matrix via hydrogen bonding. Another two FTIR bands were observed at 512 and 627 cm −1 and assigned to the Fe-O and Co-O of the spinal cobalt ferrite [43,44]. The TGA was used to investigate the thermal stability and the interaction between the nanoparticles and the polymer matrix in the nanocomposite (CoFe2O4@ DHBF). The TGA analysis of the polymer and the nanocomposites is shown in Figure 2b. The functional group presents in the polymer and in the nanocomposite were determined using FTIR spectra as shown in Figure 2a. In the case of the dihydroxybenzaldehyde-formaldehyde based polymer resin (DHBF) several FTIR peaks were noticed at 3324-3520 cm −1 (O-H), 3044 (C-H aromatic), 2949-2845 (C-H sym and asym), 1663 (C=O), 1564 (C=C), and others [41,42]. Meanwhile, in the case of the magnetic polymer nanocomposite CoFe 2 O 4 @ DHBF the C=O band was shifted from 1163 cm −1 to 1649 cm −1 was noticed in the presence of CoFe 2 O 4 nanoparticles and support the interaction between the magnetic nanoparticles and the polymer matrix via hydrogen bonding. Another two FTIR bands were observed at 512 and 627 cm −1 and assigned to the Fe-O and Co-O of the spinal cobalt ferrite [43,44]. The TGA was used to investigate the thermal stability and the interaction between the nanoparticles and the polymer matrix in the nanocomposite (CoFe 2 O 4 @ DHBF). The TGA analysis of the polymer and the nanocomposites is shown in Figure 2b. Initially a slightly weight loss about to 4.21% and 5.24% was noticed for DHBF and for CoFe2O4@DHBF up to 200 °C temperature due to the evaporation of adsorbed humidity and other solvents. Moreover, between 200 to 375 °C about 62.21% and 47.48% weight loss was found and it is the main degradation stage of the organic moieties for DHBF and for CoFe2O4@ DHBF. The last phase is the cracking of the polymeric materials and at 500 °C about the DHBF decomposed completely while in the case of CoFe2O4@DHBF about to 13.12% weight loss was notices and the residue weight was found about to 31.20 at 800 °C. These outcomes revealed that the CoFe2O4@ DHBF shows excellent thermal stability compared to the polymeric resin, DHBF [45][46][47]. The XRD patterns of CoFe2O4, and CoFe2O4@DHBF are illustrated in Figure 3a. It was noticed that the XRD peaks for pure CoFe2O4 nanoparticles were found at 2θ values 30.14° (220), 35 [48,49]. Moreover, in the case of the CoFe2O4@DHBF, the intensity of the XRD peaks are decreased without changing their position and the amorphous region peaks also appear. These results support that in the nanocomposites the spinal structure of the CoFe2O4 is unchanged and embedded it is pure form without any impurity. The Xray photoelectron spectroscopy (XPS) explained the elemental composition of the CoFe2O4 and CoFe2O4@ DHBF. The XPS survey of the CoFe2O4@DHBF displays the existence of the C, O, Co, and Fe elements as showed in Figure 3b. The deconvoluted spectra of the Co 2p spectra show peaks due to the Co 2p3/2 and Co 2p1/2 at binding energy of 780.76 and 796.65 eV respectively [50,51]. Meanwhile, the satellites peaks, due to the presence of unpaired 3d electron of the high spin Co 2+ and belonging to Co 2p3/2 and Co 2p1/2, appear at a binding energy of 786.19 and 802.95 eV, as shown in Figure 3c. The XPS spectrum of Fe 2p is exposed in Figure 3d and displays two peaks at a binding energy 724.04 and 711.21 eV, assigned to Fe 2p1/2 and Fe 2p3/2 respectively. These results support the presence of Fe 3+ in the invers spinel CoFe2O4. The core-level C1s XPS spectrum is illustrated in Figure 3e and split into four peaks and appeared at binding energy about 283.94, 285.8, 286.80, and 287.72 eV and were assigned to C-C, C=C, C-O, and C=O respectively [52]. The O1s spectrum was split into three peaks and the lattice oxygen appeared at a binding energy of about 529.21, 530.84, and 531.21 eV, Fe-O/Co-O, C-O/C=O, and surface OH, respectively, as illustrated in Figure 3f [53]. Initially a slightly weight loss about to 4.21% and 5.24% was noticed for DHBF and for CoFe 2 O 4 @DHBF up to 200 • C temperature due to the evaporation of adsorbed humidity and other solvents. Moreover, between 200 to 375 • C about 62.21% and 47.48% weight loss was found and it is the main degradation stage of the organic moieties for DHBF and for CoFe 2 O 4 @ DHBF. The last phase is the cracking of the polymeric materials and at 500 • C about the DHBF decomposed completely while in the case of CoFe 2 O 4 @DHBF about to 13.12% weight loss was notices and the residue weight was found about to 31.20 at 800 • C. These outcomes revealed that the CoFe 2 O 4 @ DHBF shows excellent thermal stability compared to the polymeric resin, DHBF [45][46][47]. The XRD patterns of CoFe 2 O 4, and CoFe 2 O 4 @DHBF are illustrated in Figure 3a. It was noticed that the XRD peaks for pure CoFe 2 O 4 nanoparticles were found at 2θ values 30.14 • (220), 35 [48,49]. Moreover, in the case of the CoFe 2 O 4 @DHBF, the intensity of the XRD peaks are decreased without changing their position and the amorphous region peaks also appear. These results support that in the nanocomposites the spinal structure of the CoFe 2 O 4 is unchanged and embedded it is pure form without any impurity. The X-ray photoelectron spectroscopy (XPS) explained the elemental composition of the CoFe 2 O 4 and CoFe 2 O 4 @ DHBF. The XPS survey of the CoFe 2 O 4 @DHBF displays the existence of the C, O, Co, and Fe elements as showed in Figure 3b. The deconvoluted spectra of the Co 2p spectra show peaks due to the Co 2p 3/2 and Co 2p 1/2 at binding energy of 780.76 and 796.65 eV respectively [50,51]. Meanwhile, the satellites peaks, due to the presence of unpaired 3d electron of the high spin Co 2+ and belonging to Co 2p 3/2 and Co 2p 1/2 , appear at a binding energy of 786.19 and 802.95 eV, as shown in Figure 3c. The XPS spectrum of Fe 2p is exposed in Figure 3d and displays two peaks at a binding energy 724.04 and 711.21 eV, assigned to Fe 2p 1/2 and Fe 2p 3/2 respectively. These results support the presence of Fe 3+ in the invers spinel CoFe 2 O 4 . The core-level C1s XPS spectrum is illustrated in Figure 3e and split into four peaks and appeared at binding energy about 283.94, 285.8, 286.80, and 287.72 eV and were assigned to C-C, C=C, C-O, and C=O respectively [52]. The O1s spectrum was split into three peaks and the lattice oxygen appeared at a binding energy of about 529.21, 530.84, and 531.21 eV, Fe-O/Co-O, C-O/C=O, and surface OH, respectively, as illustrated in Figure 3f [53].  The surface morphology of the nanoparticles and the nanocomposite was determined using the SEM and TEM analysis. As illustrated in Figure 4a, the SEM image of the CoFe2O4 shows the spherical shape with a diameter range of 14-25 nm, the fabricated nanoparticles are aggregated due to their super magnetic nature. While in the case of the CoFe2O4@ DHBF, the CoFe2O4 nanoparticles are well dispersed into the polymer matrix and no aggregation was noticed. The shape and size of the nanoparticles were unchanged in the case of the nanocomposite. The detailed morphology of the nanocomposite was monitored with a TEM image as shown in Figure 4c and showed similar results to the SEM results. The crystalline nature and the interaction with the polymer matrix were further confirmed with HRTEM analysis and illustrated in Figure 4d. The lattice fingers were noticed with d-spacing of 0.262 and 0.291 nm, which were assigned to the (311) and (220) planes of the CoFe2O4 spinal structure [54][55][56]. The existence of pure CoFe2O4 in the polymer nanocomposite was further supported using selected area electron diffraction (SAED) to show the electron diffraction planes as in Figure 4d [49,57]. The surface morphology of the nanoparticles and the nanocomposite was determined using the SEM and TEM analysis. As illustrated in Figure 4a, the SEM image of the CoFe 2 O 4 shows the spherical shape with a diameter range of 14-25 nm, the fabricated nanoparticles are aggregated due to their super magnetic nature. While in the case of the CoFe 2 O 4 @ DHBF, the CoFe 2 O 4 nanoparticles are well dispersed into the polymer matrix and no aggregation was noticed. The shape and size of the nanoparticles were unchanged in the case of the nanocomposite. The detailed morphology of the nanocomposite was monitored with a TEM image as shown in Figure 4c and showed similar results to the SEM results. The crystalline nature and the interaction with the polymer matrix were further confirmed with HRTEM analysis and illustrated in Figure 4d. The lattice fingers were noticed with d-spacing of 0.262 and 0.291 nm, which were assigned to the (311) and (220) planes of the CoFe 2 O 4 spinal structure [54][55][56]. The existence of pure CoFe 2 O 4 in the polymer nanocomposite was further supported using selected area electron diffraction (SAED) to show the electron diffraction planes as in Figure 4d [49,57]. The N2 adsorption and desorption isotherm was used to determine the porous properties of the polymer and the nanocomposite. It was noticed that the adsorption of N2 was increased with increasing the relative pressure up to P/PO < 0.8. As shown in Figure 5a, the N2 adsorption-desorption shows type IV hysteresis loop and supports the mesoporous nature of the CoFe2O4@DHBF. The BET results demonstrate that the surface area of the CoFe2O4 nanoparticles and the CoFe2O4@DBF was found to be 96.54 and 370.24 m 2 /g respectively. As shown in inserted figure in Figure 5a the pores size were found to be between 12-18 nm and 24-28 nm an indicate both the polymer and the CoFe2O4@DHBF has mesoporous nature and suitable for the adsorption of the pollutants from aqueous solution. The magnetic behaviors of the pure CoFe2O4 nanoparticles and the CoFe2O4@ DHBF were determined using VSM (vibrating-sample magnetometer) analysis and the results were illustrated in Figure 5b. It was observed that the magnetization curves were performed S-shaped with the applied magnetic field and the saturation magnetization (Ms) were found to be 48.50 emu/g and 34.39 emu/g for CoFe2O4 nanoparticles and the CoFe2O4@DHBF respectively. In the case of the CoFe2O4@DHBF, the magnetization was decreased due the nonmagnetic weight ratio of the polymer, DHBF. Hence, the CoFe2O4@DHBF contains enough magnetization and could be easily and rapidly separated from aqueous solution using a magnet in a very short time. The N 2 adsorption and desorption isotherm was used to determine the porous properties of the polymer and the nanocomposite. It was noticed that the adsorption of N 2 was increased with increasing the relative pressure up to P/P O < 0.8. As shown in Figure 5a, the N 2 adsorption-desorption shows type IV hysteresis loop and supports the mesoporous nature of the CoFe 2 O 4 @DHBF. The BET results demonstrate that the surface area of the CoFe 2 O 4 nanoparticles and the CoFe 2 O 4 @DBF was found to be 96.54 and 370.24 m 2 /g respectively. As shown in inserted figure in Figure 5a the pores size were found to be between 12-18 nm and 24-28 nm an indicate both the polymer and the CoFe 2 O 4 @DHBF has mesoporous nature and suitable for the adsorption of the pollutants from aqueous solution. The magnetic behaviors of the pure CoFe 2 O 4 nanoparticles and the CoFe 2 O 4 @ DHBF were determined using VSM (vibrating-sample magnetometer) analysis and the results were illustrated in Figure 5b. It was observed that the magnetization curves were performed S-shaped with the applied magnetic field and the saturation magnetization (Ms) were found to be 48.50 emu/g and 34.39 emu/g for CoFe 2 O 4 nanoparticles and the CoFe 2 O 4 @DHBF respectively. In the case of the CoFe 2 O 4 @DHBF, the magnetization was decreased due the nonmagnetic weight ratio of the polymer, DHBF. Hence, the CoFe 2 O 4 @DHBF contains enough magnetization and could be easily and rapidly separated from aqueous solution using a magnet in a very short time.

Effect of pH, Initial Concentration, Initial Contact Time
The effect of the pH, initial concentration of U(VI) and Eu(III), the dose of the CoFe2O4@DHBF and the effect of contact time on the removal was thoroughly studied. The pH of the solution is one of the important factor for the removal of the metal ions one CoFe2O4@DHBF [58]. It was noticed, when the pH of the solution was increased from 2 to 7 the percentage adsorption was increased and found to be 95% and 92.2% (with adsorption capacity 237.5 and 225.5 mg/g) with U(VI) and Eu(III) respectively as display in Figure 6a. Therefore, CoFe2O4@DHBF has excellent adsorption performance for both the metals, and was far superior most conventional adsorbents listed in Supplementary Table  S2. To understand the effect of the pH for the removal of the metal ions pHpzc was determined because the adsorbent surface zeta potential significantly influenced the removal of heavy metal. As shown in Supplementary Figure S1, the pHpzc of the CoFe2O4@DHBF was found to be 5.13. Therefore, at pH < pHpzc (point of zero charge), the surface charge of the nanocomposite was positive due to extra protons (H + ), thus the struggle with the metal ions to bind with the adsorption sites, resulting in the active sites of CoFe2O4@DHBF being protonated and the adsorption capacity for the adsorption of metal ions decreasing due to the presence of extra protons. Moreover, the maximum adsorption was noticed at pH 7. However, when the pH of the solution was further increased, the adoration capacity was decreased due to the formation of the insoluble hydroxide of the corresponding metal ions. The contact time of the adsorbent with the adsorbate affects the adsorption of both the metal ions on CoFe2O4@DHBF. As shown in Figure 6b, the adsorption of both the metal ions at different time form 5 min to 200 min were studied. It was noticed that the contact time between both U(VI) and Eu(III) increased and the adsorption of both the metal ions increased initially within 30 min, when about 77.9% and 74.0 % of U(VI) and Eu(III) were adsorbed. When the contact time was increased to 60 min, it reaches equilibrium and about 237.5 and 225.5 mg/g adsorption capacity was noticed against the U(VI) and Eu(III) respectively. However, increasing the time further only slightly changed the adsorption capacity . The effects of the initial concentration during the adsorption of the metal ions on to CoFe2O4@ DHBF was investigated at varying initial concentration from 5-300 mg/L and the results were illustrated in Figure 6c. It was noticed that, when the initial concentration of both the U(VI) and Eu(III) ions was increased, the adsorption percentage of was decreased, while the adsorption capacity of the CoFe2O4@ DHBF was increased with the initial concentration [59]. Resulting, the initial concentration of 100 mg/L show the highest percentage adsorption and were found to be 237.5 and 225.5 mg/g with U(VI) and Eu(III) respectively within 60 min. Additionally, for the industrial application, the adsorption of both the metal ions at their lower concentration in the range (0.05 to 2 mg/L) were also observed and the results are illustrated in Supplementary Figure S2.

Effect of pH, Initial Concentration, Initial Contact Time
The effect of the pH, initial concentration of U(VI) and Eu(III), the dose of the CoFe 2 O 4 @DHBF and the effect of contact time on the removal was thoroughly studied. The pH of the solution is one of the important factor for the removal of the metal ions one CoFe 2 O 4 @DHBF [58]. It was noticed, when the pH of the solution was increased from 2 to 7 the percentage adsorption was increased and found to be 95% and 92.2% (with adsorption capacity 237.5 and 225.5 mg/g) with U(VI) and Eu(III) respectively as display in Figure 6a. Therefore, CoFe 2 O 4 @DHBF has excellent adsorption performance for both the metals, and was far superior most conventional adsorbents listed in Supplementary Table S2.
To understand the effect of the pH for the removal of the metal ions pH pzc was determined because the adsorbent surface zeta potential significantly influenced the removal of heavy metal. As shown in Supplementary Figure S1, the pH pzc of the CoFe 2 O 4 @DHBF was found to be 5.13. Therefore, at pH < pH pzc (point of zero charge), the surface charge of the nanocomposite was positive due to extra protons (H + ), thus the struggle with the metal ions to bind with the adsorption sites, resulting in the active sites of CoFe 2 O 4 @DHBF being protonated and the adsorption capacity for the adsorption of metal ions decreasing due to the presence of extra protons. Moreover, the maximum adsorption was noticed at pH 7. However, when the pH of the solution was further increased, the adoration capacity was decreased due to the formation of the insoluble hydroxide of the corresponding metal ions. The contact time of the adsorbent with the adsorbate affects the adsorption of both the metal ions on CoFe 2 O 4 @DHBF. As shown in Figure 6b, the adsorption of both the metal ions at different time form 5 min to 200 min were studied. It was noticed that the contact time between both U(VI) and Eu(III) increased and the adsorption of both the metal ions increased initially within 30 min, when about 77.9% and 74.0 % of U(VI) and Eu(III) were adsorbed. When the contact time was increased to 60 min, it reaches equilibrium and about 237.5 and 225.5 mg/g adsorption capacity was noticed against the U(VI) and Eu(III) respectively. However, increasing the time further only slightly changed the adsorption capacity. The effects of the initial concentration during the adsorption of the metal ions on to CoFe 2 O 4 @ DHBF was investigated at varying initial concentration from 5-300 mg/L and the results were illustrated in Figure 6c. It was noticed that, when the initial concentration of both the U(VI) and Eu(III) ions was increased, the adsorption percentage of was decreased, while the adsorption capacity of the CoFe 2 O 4 @ DHBF was increased with the initial concentration [59]. Resulting, the initial concentration of 100 mg/L show the highest percentage adsorption and were found to be 237.5 and 225.5 mg/g with U(VI) and Eu(III) respectively within 60 min. Additionally, for the industrial application, the adsorption of both the metal ions at their lower concentration in the range (0.05 to 2 mg/L) were also observed and the results are illustrated in Supplementary Figure S2 revealed that the adsorption percentage of both metal ions was increased with decreasing concentration in both the case distilled water and in synthetic wastewater. Moreover, these results revealed that the adsorption of metal ions slightly decreased in the case of the synthetic wastewater sample due to the presence of co-existence ions.
revealed that the adsorption of metal ions slightly decreased in the case of the synthetic wastewater sample due to the presence of co-existence ions.
Initially, the adsorption capacity was increased with the initial concentration. This is because the contact between metal ions and the adsorptive sites of CoFe2O4@DHBF was increased. However, at high initial concentration, the availability of the adsorptive sites of CoFe2O4@DHBF were regularly decreased and the saturation took place, resulting in a decreased adsorption capacity with both the metal ions. Moreover, the effects of temperature for the adsorption of both the metal ions were also investigated and the results are displayed in Figure 6d. The adsorption results revealed that the adsorption of both the metal ions was decreased with increasing the temperature of the aqueous solution. Therefore, room temperature is suitable for the adsorption of both the metal ions and was used as an optimum temperature.

Adsorption Isotherms
To determine the interaction and the adsorption mechanism for the adsorption of U(VI) and Eu(III) onto CoFe2O4@DHBF, absorption isotherm including Langmuir, Freundlich and Temkin models have been used (the details of adsorption isotherms are given in the Supplementary Materials) [60][61][62]. The nonlinear fittings for these models are displayed in Figure 7, and the results are summarized in Table 1. The results revealed that the experimental data are well fitted with the Langmuir isotherm model and the correlation coefficient (R 2 ) values were found to be 0.9920 and 0.9913 for the adsorption of U(VI) and Eu(III), respectively. The calculated adsorption capacity was found to be 330.63 mg/g and 310.70 mg/g with U(VI) and Eu(III) respectively, which were closed to the experimental values at room temperature (298 K). Additionally, the effects of temperature on the adsorption isotherm were also evaluated and the results revealed that, at the increased temperature, the adsorption of both the metal ions was decreased. These outcomes support the fact that adsorption Initially, the adsorption capacity was increased with the initial concentration. This is because the contact between metal ions and the adsorptive sites of CoFe 2 O 4 @DHBF was increased. However, at high initial concentration, the availability of the adsorptive sites of CoFe 2 O 4 @DHBF were regularly decreased and the saturation took place, resulting in a decreased adsorption capacity with both the metal ions. Moreover, the effects of temperature for the adsorption of both the metal ions were also investigated and the results are displayed in Figure 6d. The adsorption results revealed that the adsorption of both the metal ions was decreased with increasing the temperature of the aqueous solution. Therefore, room temperature is suitable for the adsorption of both the metal ions and was used as an optimum temperature.

Adsorption Isotherms
To determine the interaction and the adsorption mechanism for the adsorption of U(VI) and Eu(III) onto CoFe 2 O 4 @DHBF, absorption isotherm including Langmuir, Freundlich and Temkin models have been used (the details of adsorption isotherms are given in the Supplementary Materials) [60][61][62]. The nonlinear fittings for these models are displayed in Figure 7, and the results are summarized in Table 1. The results revealed that the experimental data are well fitted with the Langmuir isotherm model and the correlation coefficient (R 2 ) values were found to be 0.9920 and 0.9913 for the adsorption of U(VI) and Eu(III), respectively. The calculated adsorption capacity was found to be 330.63 mg/g and 310.70 mg/g with U(VI) and Eu(III) respectively, which were closed to the experimental values at room temperature (298 K). Additionally, the effects of temperature on the adsorption isotherm were also evaluated and the results revealed that, at the increased temperature, the adsorption of both the metal Polymers 2020, 12, 2940 9 of 16 ions was decreased. These outcomes support the fact that adsorption followed the Langmuir isotherm and homogenous monolayers adsorption due to the chemisorption between both the metal ions.
Polymers 2020, 12, x FOR PEER REVIEW 9 of 16 followed the Langmuir isotherm and homogenous monolayers adsorption due to the chemisorption between both the metal ions.    To determine the adsorption rate and adsorption rate constant the adsorption kinetics during the adsorption of U(VI) and Eu(III) on to CoFe 2 O 4 @DHBF was determine using pseudo-first order, pseudo-second order, and interparticle diffusion method and the experimental data was fitted with non-liner model [19,63,64]. The resulting adsorption kinetics parameters were summarized into Table 2. As shown in Figure 8a,b, the pseudo-second order model is well fitted and correlates with the experimental results, the correlation coefficient (R 2 ) value was found to be close to 1 (0.9907). Meanwhile, in the case of pseudo-first order and interparticle diffusion, the values of R 2 were found to be 0.9510 and 0.9514, respectively, during the adsorption of U(VI). The maximum adsorption capacity (q e ) was determine using pseudo-second order model and found to be 263.89 mg/g and 253.31 mg/g with U(VI) and Eu(III), these values are close to the experimental values. These outcomes support that the adsorption of both the metal ions were follow the pseudo-second order kinetics model and the adsorption capacity of both the metal ions remain constant with time after equilibrium and the available active sites for adsorption depends on the concentration of the metal ions at equilibrium. The adsorption of both the metal ions was chemisorption and the interaction between both the metal ions and the CoFe 2 O 4 @DHBF via coordination of electrons and the covenant.  The thermodynamics parameters, including change in enthalpy (ΔH), change in entropy (ΔS), and the Gibbs free energy (ΔG), during the adsorption of U(VI) and Eu(III) were determined using the van't Hoff equation and the details are given in supplementary information [65,66]. The results are illustrated in Figure 9a, and they reveal that the adsorption of both the metal ions proceeds via an exothermic reaction and the negative value of the ΔG supports the spontaneous reaction. The values of ΔG increased with the temperature of the solution and the results are summarized in Table  3.   The thermodynamics parameters, including change in enthalpy (∆H), change in entropy (∆S), and the Gibbs free energy (∆G), during the adsorption of U(VI) and Eu(III) were determined using the van't Hoff equation and the details are given in supplementary information [65,66]. The results are illustrated in Figure 9a, and they reveal that the adsorption of both the metal ions proceeds via an exothermic reaction and the negative value of the ∆G supports the spontaneous reaction. The values of ∆G increased with the temperature of the solution and the results are summarized in Table 3. The thermodynamics parameters, including change in enthalpy (ΔH), change in entropy (ΔS), and the Gibbs free energy (ΔG), during the adsorption of U(VI) and Eu(III) were determined using the van't Hoff equation and the details are given in supplementary information [65,66]. The results are illustrated in Figure 9a, and they reveal that the adsorption of both the metal ions proceeds via an exothermic reaction and the negative value of the ΔG supports the spontaneous reaction. The values of ΔG increased with the temperature of the solution and the results are summarized in Table  3.

Reusability and Regeneration Ability
The reusability and regeneration of the CoFe 2 O 4 @DHBF was carried out with six cycles and the results are illustrated in Figure 9b. It was noticed that the fabricated nanocomposites show excellent regeneration ability and about to 220.1 and 211.3 mg/g adsorption capacity remains with U(VI) and Eu(III) under optimum conditions (pH = 7, room temperature, initial concentration 100 mg/L, dose of adsorbent 0.01 g, volume 25 mL) [67,68]. The slow decrease in the adsorption capacity with each cycle of regeneration may be due to the loss of the adsorbent during the regeneration because no change in the adsorption site was noticed and supported by the XPS spectra after 3 cycles as shown in Supplementary Figure S3. These outcomes revealed that the CoFe 2 O 4 @DHBF exhibits promising regeneration ability for the adsorption of both the metal ions and in future can be used as a potential adsorbent for the adsorption of toxic pollutants form aqueous solution on an industrial scale.

Conclusions
Herein, we have fabricated novel nanocomposite and characterized successfully. As-prepared nanocomposite was utilized as capable adsorbent for the removal of U(VI) and Eu(III) form contaminated water. The batch adsorption results exposed that the adsorption capacity for the removal of U(VI) and Eu(III) was found to be 237.5 and 225.5 mg/g, respectively, at room temperature. The optimum condition of the adsorption of both metal ions were pH = 7, initial concentration 100 mg/L, contact time 60 min, and room temperature. The interaction between the metal ions and the CoFe 2 O 4 @DHBF was determine using the adsorption isotherm and adsorption kinetics. The adsorption of both the metals followed the pseudo-second order reaction model and Langmuir adsorption isotherm. The correlation coefficient (R 2 ) values of the Langmuir isotherm were found to be 0.9920 and 0.9913 for the adsorption of U(VI) and Eu(III), respectively. Additionally, the reusability results exhibit promising regeneration ability for the adsorption of both the metal ions and in future can be used as a potential adsorbent for the adsorption of toxic pollutants form aqueous solution on industrial scale.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4360/12/12/2940/s1, Table S1: A comparison of removal technologies for the removal of radioactive metal ions, Figure S1: Zeta potential of synthesized CoFe2O4@DHBF, Figure S2: (a) Effect of initial concentration for the adsorption of U(VI) and Eu(III) onto CoFe2O4@DHBF (in distilled water) (b) Effect of initial concentration for the adsorption of U(VI) and Eu(III) onto CoFe2O4@DHBF (in synthetic wastewater) (0.01 g adsorbent, 25 mL, optimum pH = 7, optimum time 60 min at room temperature), Table S2: Comparison of adsorption capacities of Eu(III) and U(VI) by various adsorbents, Figure S3: XPS spectra after adsorption of U(VI) and Eu(III) and after desorption 3 cycles.