Selective Separation of Zr(IV) from Simulated High-Level Liquid Wastes by Zeolites

ABSTRACT Effectively separating Zr(IV) from strong acidic and radioactive solutions is crucial for spent fuel reprocessing plants, but it remains a challenging task. This study investigated the adsorption of Zr(IV) in HNO3 solutions using zeolites as adsorbents. The zeolites were characterized by X-ray diffraction (XRD)，scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The effects of adsorption time, HNO3 concentration, the initial concentration of Zr(IV), zeolites dosage, and temperature on the Zr(IV) adsorption behaviors were investigated. The results demonstrated that HY zeolite had a higher adsorption capacity for Zr(IV) than H-SSZ-13 zeolite. The adsorption capacity of HY zeolite was influenced by its Si/Al ratio, which determined the density of adsorption sites. The optimal HY zeolite (HY-25) exhibited a maximum adsorption capacity of 30.438 mg/g in 3 M HNO3 solution. Furthermore, the adsorption isotherms and kinetics of Zr(IV) adsorption were investigated. The adsorption of Zr(IV) on zeolites was endothermic and spontaneous, in accordance with the Freundlich’s isotherm model and pseudo-second-order kinetic model. In a simulated strong acidic solution of Zr(IV) and 10 co-existing cations (Ag+, Ba2+, Cs+, Ce3+, Eu3+, Fe3+, La3+, Nd3+, Sm3+, Sr2+), HY-25 exhibited good selective adsorption of Zr(IV), indicating its potential application in the treatment of high-level radioactive liquid waste.


Introduction
Isotopes of Zr(IV) are nuclides with a high fission yield in spent nuclear fuel (SNF).Previous studies have demonstrated that the most stable valence of zirconium is +IV.The high valence together with a small ion radius endows Zr (IV) with a very high ionic potential.Therefore, Zr (IV) is easily hydrolysed, polymerized and complexed with ligands in aqueous solution.[3] The emulsion is often called the "crud". [4,5]The formation of the crud can not only reduce the recovery rate and the purity of uranium and plutonium but also accelerate the extractant degradation and thus obstruct regular production operations. [6,7]Furthermore, the PUREX process can generate a large amount of high-level liquid waste (HLLW) containing Zr(IV).When the high-level liquid waste (HLLW) is treated by solvent extraction, the existence of Zr(IV) may induce the third phase which refers to the immiscible second organic phase formed between the organic phase and the aqueous phase. [8,9]Therefore, pre-removing Zr(IV) is an effective way to cut off the generation route of the crud and the third phase, and ensure smoother operation for the PUREX process and the treatment of high-level liquid waste. [10]rganic ion exchange resins, [11][12][13][14] sodium alginate gel, [15] hydrous ferric oxide, [16] and silicon-based materials [17][18][19][20][21][22][23][24][25][26][27][28][29][30] have been developed over the last few decades to adsorb or separate Zr(IV).For instance, Takashi et al. investigated the adsorption behaviour of Zr(IV) in alginate polymer gel. [15]The results suggested that the gel had a stronger affinity for Zr(IV) than other fission products and actinides such as Sr, Co, U, Fe, etc., therefore, 95% of the Zr(IV) can be removed from the spent fuel solutions.Zhang et al. synthesized a silica gel with a high specific surface area (998 m 2 /g) to adsorb and separate Zr(IV) in simulated highlevel radioactive liquid waste. [30]The results showed that the maximal adsorption capacity of the silica gel for Zr(IV) is as high as 31.4mg/g.Unfortunately, the silica gel existed with very sluggish kinetics with an adsorption equilibrium time longer than 50 h.And this sluggish kinetics might be an intrinsic feature of silica materials, which can be found in other studies concerning Zr(IV) adsorption on the silica. [23]herefore, these adsorbents still cannot meet the actual operation requirements in extreme conditions of the strong acid and irradiation in the spent fuel reprocessing plant.Organic ion exchange resins are easily decomposed under strong acidic solution and irradiation conditions though they have satisfied adsorption selectivity and adsorption capacity for Zr(IV).Inorganic materials, especially silicon gels suffer from the problem of long adsorption equilibrium time.It is still a great challenge to construct an absorbent to separate Zr(IV) from strong acidic and radioactive solutions in the spent fuel reprocessing plant.
Zeolites are crystalline hydrated aluminosilicates composed of the siliconoxygen tetrahedron and aluminum-oxygen tetrahedron with exceptional features such as selective ion exchange capacity, high specific surface area, excellent radiation stability, and negative charge on the channel structure.The structure and constitution of zeolites can be modified to enhance their adsorption properties, which makes zeolites attract much attention in the field of adsorption.Over the past few decades, natural, synthetic, and modified zeolites have been widely used to adsorb the heavy metal ions and radionuclides such as Pb 2+ , Cd 2+ , Cu 2+ , Mn 2+ , Ba 2+ , UO 2 2+ , Cs + , Sr 2+ from aqueous solutions, [31][32][33][34][35][36][37][38][39][40][41][42][43] nevertheless, the adsorption of Zr(IV) by zeolites is rarely studied. [44,45]Therefore, there is room for further improvement in aspect of the selective adsorption for Zr(IV), and more zeolites need to be tested for effective removal of Zr(IV) from aqueous solutions.
In the present work, we investigated Zr(IV) adsorption behaviours by zeolites with different SiO 2 /Al 2 O 3 ratios and topological structures.The effects of various factors, such as contact time, initial acidity of the solution, initial Zr(IV) concentration, the dosage of zeolites, and temperature on the adsorption behaviours were studied.In addition, the selectivity of zeolites for Zr(IV) adsorption in a simulated high-level liquid waste was investigated.The experimental data were fitted with kinetic, isotherm, and thermodynamic models to interpret the adsorption behaviour of Zr(IV).This work demonstrates that zeolites have potential applications in removing Zr(IV) in high level liquid waste.

Characterization
X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Bruker D8 advance) applying a CuKα radiation source with a tube voltage of 40 kV and a tube current of 40 mA.The scanning angle range was 2θ = 4°~40°, and the step length was 0.1°/step.Scanning electron microscopy (SEM, Hitachi S4800, 10 kV) and transmission electron microscopy (TEM, JEM-1400Plus, 120 kV) were used to observe the morphology of the samples.To prepare the SEM sample, we apply conductive glue onto the sample stage and sprinkle a small amount of sample powder onto it, followed by blowing off any excess powder.Then, a suitable thickness of gold is sprayed onto the sample for further observing by SEM.For TEM sample preparation, a suspension containing the sample was dropped onto a copper grid covered with a carbon support membrane.Then, the copper grid is allowed to dry naturally in a suitable environment before being placed into the transmission electron microscope.The X-ray photoelectron spectroscopy (XPS) spectra were acquired on a Thermo Scientific Escalab 250Xi electron spectrometer.The binding energy binds are referenced to carbon C1s line at 284.8 eV from adventitious carbon.

Batch adsorption experiments
The Zr(IV) adsorption batch experiments were conducted to investigate the effects of contact time, HNO 3 concentration, adsorbent dosage, and initial Zr(IV) concentration.In a 10-mL glass bottle, a certain amount of zeolites was mixed with the desired amount of Zr(NO 3 ) 4 aqueous solution.The mixture was then shaken in a water bath shaker at the designated temperature.The zeolite was separated by a 0.22 μm filter membrane for further characterization, while the filtrate was analyzed for remaining Zr(IV) using ICP-AES (Thermo Scientific ICAP7400 Duo).
The equilibrium adsorption capacity for Zr(IV) on zeolites, q e (mg/g), together with the removal efficiency of Zr(IV) were calculated according to the following equation: where C 0 and C e are initial and equilibrium concentrations of Zr(IV) in the aqueous solution (mg/L), respectively, m is the mass of the zeolite (g) and V is the volume of the Zr(IV) aqueous solution (L).
The removal efficiency, RE (%) and desorption efficiency E d (%) were determined by the following equations: The distribution coefficient K d (mL/g) and separation factor SF were calculated by using the following equations: where C 0 and C e are the initial and equilibrium concentrations of Zr(IV) in the aqueous solution (mg/L), and C d (mg/L) is the concentration of Zr(IV) in the aqueous solution after desorption (mg/L), respectively.V is the volume of the aqueous solution (mL), and m is the mass of the zeolite (g).
To understand the adsorption mechanism, pseudo-first order and pseudosecond order models were used to study the adsorption kinetics.The pseudofirst order model and pseudo-second order model formulas are as follows [46,47] : where q e is the equilibrium adsorption capacity of the zeolite (mg/g), q t is the adsorption capacity of the zeolite at time t (mg/g), t is the adsorption time (h), k 1 (min −1 ), k 2 (mg•g −1 •min −1 ) are pseudo first-order kinetic constant and pseudo second-order kinetic constant, respectively.

Adsorption isotherms
A series of Zr(IV) aqueous solution with different concentrations were investigated.Langmuir and Freundlich isotherm models were fitted the experiment data by Origin 9.3's non-linear regression.The Langmuir equation is as follows [48] : where q e is the equilibrium adsorption capacity of Zr(IV) (mg/g), Q m is the maximum adsorption amount at monolayer which indicates that the substance can only be adsorbed on the adsorbent surface with direct contact.C e is the adsorption equilibrium concentration of Zr(IV) in aqueous solution (mg/L), and K L is the Langmuir constant related to the affinity between substance and adsorption sites (L/mg).
The equation for the Freundlich isotherm is as follows [49] : where K F [(mg 1-n •L n )/g] and n are Freundlich constants, which respectively represent the adsorption capacity of the sorbent and the degree of favorability in the adsorption process.Values of n > 1 represent favourable adsorption condition.

Characterization of zeolite
Figure 1 presents the SEM and TEM images of the employed zeolites.It was found that the HY-25 has an irregular shape with a diameter from hundreds of nanometres to two micrometres (Figure 1a,f).The HY-30 (Figure 1b,g), NaY-5 (Figure 1c,h), and H-SSZ-13-20 (Figure 1d,i) zeolites are crystallized with a rhombic shape and a diameter of 1-3 μm.Unlike the above three-dimensional shaped zeolites, K-CHA-4 (Figure 1e,j) has a two-dimensional flake morphology with 3-5 μm in width.Furthermore, the XRD technique was used for the phase identification and the diffraction patterns are shown in Figure 2. The sharp diffraction peaks of HY-25, HY-30 (Figure 2a), and NaY-5 (Figure 2b) can be ascribed to the characteristic peaks of FAU-type zeolites (JCPDS No. 73-2313 for the HY-25 and 30, JCPDS No. 81-2466 for NaY-5), whereas the patterns of H-SSZ-13-20 and K-CHA-4 can be indexed by JCPDS No. 87-2489 and JCPDS No. 85-0976 respectively, indicating they are CHA-type zeolites (Figure 2c,d).

Stability of zeolites
Before Zr(IV) adsorption experiments, the stability of zeolites in 3 mol/L HNO 3 solution was studied.The zeolites were stirred in 3 mol/L HNO 3 solution for 5 hours.The XRD patterns (Figure 3) illustrated that HNO 3 solution damaged the structure of K-CHA-4 and NaY-5 due to their low SiO 2 /Al 2 O 3 ratios.However, HY-25/30 and H-SSZ-13-20 presented excellent stability in the HNO 3 solution due to their high SiO 2 /Al 2 O 3 ratios.Thus, HY-25/30 and H-SSZ-13-20 were used to separate and removal Zr(IV) from the HNO 3 solution.adsorption equilibrium.However, the q e of H-SSZ-13 on Zr(IV) was not satisfied.Based on these results, further experiments were conducted on zeolites with an adsorption time of 180 minutes to ensure a maximum Zr(IV) adsorption.

Effect of interaction time on Zr(IV) adsorption
The kinetic results were fitted by pseudo-first-order and pseudo-secondorder kinetic models, as shown in Table 1.The Zr(IV) adsorption on zeolites can be well fitted by the pseudo-second-order kinetic model (R 2 = 0.99), indicating that Zr(IV) adsorption is dominated by a chemisorption process.

Effect of HNO 3 concentration on Zr(IV) adsorption
The effect of HNO 3 concentration on the q e of Zr(IV) was investigated and the results are presented in Figure 5.With the increase of HNO 3 concentration, q e Table 1.Fitting parameters of pseudo-first-order and pseudo-second-order models for Zr(IV) adsorption on zeolites.

Zeolites Pseudo-first-order model
Pseudo-second-order model q e (mg/g) q e (mg/g) first increased and then decreased.The maximum values were uniformly presented at HNO 3 concentration of 0.5 mol/L.[52] As shown in Figure 5b, the zeta potential of HY-25/30 became gradually negative as the pH values increased, and the zpc value of HY-25/30 was determined to be at pH −0.07 and 0.84.This indicates that when the concentration of HNO 3 is lower than the zpc, the adsorbent surface becomes negatively charged, which facilitates the diffusion of Zr(IV) into the zeolites and the exchange with H + ions.In theory, the adsorption behaviours of Zr(IV) on zeolites are also closely related to the solution acidity and the hydrolysis species of Zr(IV).55][56] The occurrence of non-hydrolyzed Zr 4+ ions are dominant in strongly acidic solutions (pH < 0), [54,55] and the monomeric species become dominant only in very dilute solutions ([Zr] < 10 −4 M). [57,58] Herein, when the concentration of HNO 3 is 0.1 mol/L, Zr(IV) may exist primarily as tetramers, together with a small amount of monomeric and polynuclear species. [54]The tetra-/polynuclear species may be difficult to penetrate the zeolites because of the large size. [59]As a result, the value of q e is relatively low although the competitive adsorption of H + is the lowest.At HNO 3 concentration of 0.5 mol/L, the content of monomers increased, and the competitive adsorption of H + might be relatively low, so the q e achieved the maximal values.When the concentration of H + increased further, the competitive effect of H + was more pronounced, and q e significantly showed a downward trend.This phenomenon is similar to the results obtained by Lin et al. [60]

Effect of adsorbent dosage on Zr(IV) adsorption
In order to study the effect of adsorbent dosages on Zr(IV) adsorption efficiency, a series of adsorption experiments were studied by increasing the dosage of adsorbents with Zr(IV)-containing solution fixed at 10 mL ([HNO 3 ] = 3 mol/L, [Zr] = 100 mg/L).Figure 6 shows the influence of adsorbent dosage on the adsorption performance of the zeolites for Zr(IV).After a contact of 180 minutes at 25°C, the dosage of 0.2 g and 0.5 g of HY-25 removed 89.9% and 97.7% of Zr(IV), respectively.

Effect of temperature on Zr(IV) adsorption
Batch experiments in the temperature range from 298 K to 323 K were carried out in order to study the effect of temperature on adsorption.
According to the Van't Hoff equation represented as follows [61] : The Gibbs free energy change (ΔG 0 ) of the adsorption can be calculated by the following formula: where ΔH 0 is the enthalpy change (kJ/mol) and ΔS 0 is the entropy change (J•mol −1 •K −1 ) of the adsorption, K 0 is the thermodynamic equilibrium constant, R is the gas constant (8.314J•mol −1 •K −1 ), T is the thermodynamic temperature (K).
Values of K 0 at different temperatures could obtained by fitting a plot of ln(K d ) versus C e at different temperatures (Figure 7), when C e is infinitely close to 0, the intercept can be considered as ln(K 0 ). [62]he plot between ln(K 0 ) versus 1/T was shown in Figure 8, meanwhile, ΔS 0 and ΔH 0 were calculated from the intercepts and the slope.The q e increased as the temperature increased from 298 K to 323 K. ΔH 0 , ΔS 0 , and ΔG 0 of the adsorption calculated according to formulas (11) and (12) were presented in Table 2.It can be observed that, (1) all the values of ΔG 0 are negative, suggesting that the adsorption of Zr(IV) by zeolites is a spontaneous process, and (2) the adsorption of Zr(IV) by zeolites with ΔH 0 >0 indicates that the adsorption process is endothermic.

Isotherm of adsorption of Zr(IV) by zeolites
The effect of the initial concentration of Zr(IV) on the adsorption capacity was studied by changing the initial Zr(IV) concentration from 10 mg/L to 1000 mg/L at different temperatures.As described in Figure 9, the equilibrium adsorption capacity of Zr(IV) increased with increasing initial concentrations.
The maximum equilibrium adsorption capacity is a crucial parameter to measure the performance of an adsorbent.In order to further understand the adsorption process, the adsorption data at 298 K, 303 K, 313 K, and 323 K were fitted by using Langmuir and Freundlich adsorption isotherm models, as shown in Figure 10.The values of the isotherm constants are listed in Table 3.The isotherms of Zr(IV) adsorption by HY-25 and HY-30 agreed well with Freundlich's model.A similar fitting was observed for the adsorption isotherms of various pollutants in literature. [63,64]The parameter n deals with the favorability and intensity of the adsorption, where n > 1 illustrates a favorable adsorption.The maximum adsorption capacity of HY-25 and HY-30 at 298K calculated for Zr(IV) was 30.438 mg/g and 24.427 mg/g, respectively.

Selective adsorption of Zr(IV) by zeolites
The acid-stable zeolites, i.e., HY-25, HY-30, and H-SSZ-13-20 were used to investigate the interference effect of competitive ions in a simulated highlevel radioactive liquid waste with the HNO 3 concentration of 3 M.The cation composition is listed in Table 4. Figure 11 shows the uptake of various metal ions by zeolites in the simulated HLLW.Obviously, HY-25 and HY-30 exhibited exceptional selectivity for Zr(IV), whereas H-SSZ-13-  20 showed poor selectivity under the same condition.To offer more intuitionistic information on the selectivity differences of the various ions in simulated HLLW, the removal efficiency (RE, %), distribution coefficient (K d , mL/g), and separation factor (SF) were calculated and listed in Table S1.The adsorption capacity of the zeolites on competitive ions in the solution is closely related to their ionic potential (charge/radius ratio), the higher ionic potential the ion has, the easier it to be absorbed, which is the reason why the employed zeolites have excellent selectivity to Zr(IV).The outstanding selectivity of HY-25/30 for Zr(IV) in high acidity makes them very promising adsorbents for selective removal of Zr(IV) from actual highlevel radioactive liquid waste.

Reusability of zeolites
The reusability of HY-25 zeolite was examined by batch experiments and the 0.5 M H 2 C 2 O 4 was used as eluent (Figure 12).After a certain number of reuses cycles, HY-25 kept excellent desorption capacity, but the adsorption capacity appeared to be slightly weakened.The adsorption and desorption efficiencies of the HY-25 for Zr(IV) were higher than 50% and 95% after seven adsorption and desorption cycles.

Adsorption mechanism
Typically, zirconium ions are recognized to undergo hydrolysis and polymerize into clusters in acidic solution. [53,55]Due to the limitation of channel diameter, adsorption sites in H-Y [59] (cage: 12 Å, channel: 7.4 Å) and H-SSZ-13 [65] (cage: 7.3 Å × 12 Å, channel: 3.8 Å) exhibited different accessibility to these large zirconium species, leading to their significant different adsorption capacities, as shown in Figure 13.The small difference in   adsorption capacity on Zr(IV) between HY-25 and HY-30 can be explained by the amount of Al in the framework, which dominates the density of adsorption sites.
To further understand the adsorption mechanism, XPS was used to characterize HY-25 before and after the adsorption of Zr(IV), as shown in Figure 14.The survey spectra of HY-25 before and after adsorption, and signals of the Zr 3d, Si 2p, Al 2p, and O 1s were recorded.After adsorption of Zr(IV), a new peak with a binding energy of 184.08 eV(the red curve of Figure 14a) appears, corresponding to Zr(3d). [66]The Zr(3d) core level characteristic peak after adsorption could be deconvoluted into two Gaussian peaks: 185.74 eV for Zr(3d 3/2 ) and 183.44 eV for Zr(3d 5/2 ) (inset of Figure 14a).9]

Conclusions
The ability of zeolites to adsorb Zr(IV) from aqueous solutions was investigated.All the zeolites were able to attain the adsorption equilibrium within 180 minutes and fit well with the pseudo-second-order kinetic model.According to the study, the adsorption capacity of zeolites was affected by the zeolite dosage and the initial ion concentration.The isotherms of Zr(IV) adsorption by HY-25 and HY-30 were well fitted with the Freundlich's model.At the concentration of HNO 3 was 3 mol/ L and the temperature of 298 K, the maximum adsorption capacity of HY-25 and  HY-30 calculated for Zr(IV) was 30.438 mg/g and 24.427 mg/g respectively.Thermodynamics of Zr(IV) adsorption on zeolites was a spontaneous endothermic process.In addition, HY-25/30 showed highly selective adsorption of Zr(IV) a simulated high-level liquid waste containing 10 co-existing cations (Ag + , Ba 2+ , Cs + , Ce 3+ , Eu 3+ , Fe 3+ , La 3+ , Nd 3+ , Sm 3+ , Sr 2+ ) in 3 mol/L HNO 3 solution, indicating that the zeolites are highly potential to be used for the Zr(IV) removal in actual high level liquid waste of high acidity during the spent fuel reprocessing.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 4 Figure 1 .
Figure 4 shows the effect of contact time on Zr(IV)adsorption by zeolites.The results indicate that H-SSZ-13-20 took about 60 minutes, while HY-25 and HY-30 took approximately 180 minutes to achieve

Figure 2 .
Figure 2. The XRD patterns of the employed zeolites.

Figure 3 .
Figure 3.The XRD patterns of five different zeolites before and after stirred in 3 mol/L HNO 3 solution.

Funding
The work was supported by the National Natural Science Foundation of China [U1867206]; Lingchuang Foundation of China National Nuclear Corporation [LC202309000703]; Continuous-Support Basic Scientific Research Project [BJ22002903]; Presidential Foundation of China Institute of Atomic Energy [YZ202212001003].

Figure 14 .
Figure 14.(a) The XPS spectra of HY-25 before and after Zr(IV) adsorption, (b) the scan of O 1s, (c) the scan of Si 2p, and (d) the scan of Al 2p.

Table 2 .
Thermodynamic parameters of Zr(IV) onto the zeolites.

Table 3 .
Isotherm parameters for adsorption of Zr(IV) onto zeolites at different temperatures.T (K)