An adsorption study of ⁹⁹Tc using nanoscale zero-valent iron supported on D001 resin

Abstract

Nanoscale zero-valent iron (nZVI) supported on D001 resin (D001-nZVI) was synthesized for adsorption of high solubility and mobility radionuclide ⁹⁹Tc. Re(VII), a chemical substitute for ⁹⁹Tc, was utilized in batch experiments to investigate the feasibility and adsorption mechanism toward Tc(VII). Factors (pH, resin dose) affecting Re(VII) adsorption were studied. The high adsorption efficiency of Re(VII) at pH= 3 and the solid-liquid ratio of 20 g/L. X-ray diffraction patterns revealed the reduction of {{\displaystyle \mathrm{Re}\mathrm{O}}}_{\mathrm{4}}^{-} into ReO₂ immobilized in D001-nZVI. Based on the optimum conditions of Re(VII) adsorption, the removal experiments of Tc(VII) were conducted where the adsorption efficiency of Tc(VII) can reach 94%. Column experiments showed that the Thomas model gave a good fit to the adsorption process of Re(VII) and the maximum dynamic adsorption capacity was 0.2910 mg/g.

Keywords

technetium / nanoscale zero-valent iron (nZVI) / D001 resin / adsorption

Introduction

Radionuclide technetium-99 (⁹⁹Tc) is mainly produced by the fission of ²³⁵U in nuclear reactors. ⁹⁹Tc with b-emitter has a long half-life (2.13 × 10⁵ a) and high yield (6.13%) [¹,²]. Large quantities of ⁹⁹Tc have been released into the environment from atomic weapon manufacturing [³], violation handling of raffinates in nuclear power plants [⁴], the storage of glass waste in the ground [⁵], and medical research. Once the organisms such as plants, animals, and human ingest a considerable amount of ⁹⁹Tc, it will cause a serious radiation hazard. Therefore, it is critical to remove ⁹⁹Tc from the environment.

Pertechnetate anion ({\mathrm{TcO}}_4^{-}) is the main existence form for ⁹⁹Tc in aqueous solution. However, {\mathrm{TcO}}_4^{-}^{has a high solubility and mobility in ground water and soil, which increases the difficulty of removing 99Tc. A potentially effective method to solve the above problem is to reduce {\mathrm{TcO}}_4^{-}in aqueous solution into sparingly soluble TcO₂ by a strong reducing agent, nanoscale zero-valent iron (nZVI), to immobilize {}^{\mathrm{99}}\mathrm{T}{\mathrm{cO}}_4^{-} [6].}

Re, as a chemical substitute for ⁹⁹Tc, adsorbed by nZVI has been reported [⁷]. But nZVI tends to aggregate in the process of synthesis, resulting in a poor reaction performance for the reduction of Re [⁸]. Recently, the aggregation of nZVI has been solved by loading nZVI on the supporting materials, such as layered double hydroxide (LDH) [⁹] and graphene oxide (GO) [¹⁰], to improve the adsorption efficiency of Re. Nevertheless, nZVI supported on GO/LDH has not been studied for the adsorption of radionuclide ⁹⁹Tc. In addition, the morphology of the above composites is layered and unsuitable for column adsorption, making it difficult to treat radionuclides ⁹⁹Tc in ground water on a large scale.

In this study, a new carrier, D001 resin, was proposed to support nZVI (D001-nZVI) for the removal of {\mathrm{TcO}}_4^{-} due to its porous structure and cheapness compared with LDH and GO. Furthermore, D001-nZVI with a reasonable dimension, a higher adsorption efficiency, and a lower bed resistance can perform column adsorption. The specific goals of this study were to prepare and characterize D001-nZVI, to study the adsorption mechanism and influencing factors for removal of Re(VII) (a surrogate for Tc) on D001-nZVI, to investigate the adsorption efficiency of Tc(VII) on D001-nZVI, based on the results of Re(VII) adsorption, and to discuss the column adsorption behavior of Re(VII) on D001-nZVI.

Experimental

Materials

NaBH₄, FeSO₄∙7H₂O, KReO₄, NaOH, and HCl (37 wt.%) supplied by Sinopharm Chemical Reagent Co., Ltd., China were used as analytical reagents. NH₄TcO₄ was obtained from Eckert and Ziegler Isotope Products Laboratory. Ultrapure water produced by GLEA was used in all the solutions. The main parameters of D001 resin provided by Zhengguang Industrial Company were as follows: total ion exchange capacity, 4.50 mmol/g; water content, 45%–55%; its bulk density in the wet state, 0.8–0.9 g/mL (20°C); and particle size range, 0.315–1.25 mm. Prior to use, the resin was first soaked with NaCl solution (10 wt.%) for 24 h, and then washed with ultrapure water.

Preparation and characterization of D001-nZVI

The D001-nZVI were prepared in the following steps: 2 g of D001 resin was added into 200 mL 0.5 mol/L FeSO₄ solution and the mixture was shaken continuously at 30°C, 120 r/min for 5 h. The resin was filtered by a sand core funnel and added into excess ultrapure water to remove residual Fe²⁺ ion in the resin. The loaded Fe²⁺ ion resin and 200 mL of deoxygenated water (Ultrapure water was deoxygenated by N₂ stream for 1 h.) were placed in a 500 mL three-necked flask. 4 g of sodium borohydride was added into the flask in portions and the reactants were stirred for 2 h. The reaction product was recovered by filtration, ultrapure water washing. Finally, the product was vacuum dried for 12 h. The mechanism of the liquid reduction method by NaBH₄ is shown in Fig. 1.

The Fe distribution within the D001-nZVI particle was viewed with scanning electron microscopy images and an energy dispersive spectrometer (SEM-EDS: Zeiss Ultra Plus (SEM) and INCA X-Act (EDS)). The D001-nZVI was characterized by X-ray diffraction (XRD: Smart Laboratory). The D001-nzvi was rinsed by the H₂SO₄ (10 wt.%) solution and the amount of Fe loaded onto the D001-nZVI was determined by inductively coupled plasma atomic emission (ICP-AES: Shimadzu ICPS-7510).

Batch experiments

Re(VII) and Tc(VII) solutions were obtained by dissolving KReO₄ and NH₄TcO₄ in ultrapure water. The D001-nZVI particles were added into glass conical flask containing Re(VII)/Tc(VII) solutions. Solutions of 0.01 mol/L NaOH or HCl was used for adjusting the solution to the desired pH. To ensure that the nZVI loaded in the D001 resin is not oxidized, the above adsorption solutions were continuously deoxygenated with nitrogen. All the batch experiments were performed in a water-bathing constant temperature vibrator at 25°C with an oscillation frequency of 120 r/min.

The solution was sampled periodically, and placed in a 5 mL centrifuge tube. The sampled solutions were centrifuged at 13500 r/min for 3 min, and then passed through 0.2 mm PVDF filters. The concentrations of Re(VII) and Tc(VII) were determined by an ICP-AES and Liquid Scintillation Counter (LSC7200C: HITACHI ALOKA), respectively.

Column experiment

Column experiments were conducted to discuss the dynamic adsorption behavior of Re(VII) on the D001-nZVI. 0.7126 g of D001-nZVI was filled in a glass column of 5 mm in inner diameter and 50 mm in length. The micropump (SP-T/Y-3201, Nihon Seimitsu Kagaku Co., Ltd.) was used to deliver the Re(VII) solution (2 mg/L, pH 3) to the column at a flow rate of 0.0654 mL/min at 25°C. The effluents were collected by an automatic fraction collector (DC-1500C, Tokyo Rikakikai Co., Ltd.), and the solution concentration in each tube was analyzed by ICP-AES.

Results and discussion

Characterization

The amount of the Fe loaded onto the D001 resin was determined to be 115 mg/g. The SEM images of the D001 resin and D001-nZVI are shown in Fig. 2. Figure 2(b) shows that nano zero-valent iron coats the entire D001 resin, suggesting that nZVI is loaded on the D001 resin. The aggregation phenomenon of nZVI was caused by the magnetic force between iron particles. The porous structure of the D001 resin is displayed in Fig. 2(c). Figure 2(d) reveals that the nZVI particles have been uniformly dispersed on the D001 resin by liquid reduction. The Fe elemental mapping is shown in Fig. 3. It is clearly to see that nZVI is uniformly distributed in the whole D001 resin.

XRD patterns of D001-nZVI and Re(VII)-loaded D001-nZVI were shown in Fig. 4. For the D001-nZVI, the peak at 44.68° represents the Fe⁰ [⁷], which suggests that nZVI has been loaded successfully on the D001 resin. After the D001-nZVI adsorbs Re(VII), the two new peaks which appear at 26.18° and 35.32° are considered to be ReO₂ and Fe₂O₃, respectively [¹¹]. The existence of ReO₂ in the Re(VII)-loaded D001-nZVI further illustrates that the adsorption mechanism of D001-nZVI is the reduction of {\mathrm{ReO}}_4^{-} into ReO₂. The generation of Fe₂O₃ results from the oxidation of Fe⁰ in the reduction process of {\mathrm{ReO}}_4^{-}. Equation (1) gives the specific equation of the reaction [¹²].

(1){\mathrm{F}\mathrm{e}}^{\mathrm{0}}+{{\displaystyle \mathrm{Re}\mathrm{O}}}_{\mathrm{4}}^{-}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\to {{\displaystyle \mathrm{F}\mathrm{e}}}_{\mathrm{2}}{{\displaystyle \mathrm{O}}}_{\mathrm{3}}+{{\displaystyle \mathrm{Re}\mathrm{O}}}_{\mathrm{2}}+{\mathrm{O}\mathrm{H}}^{-}.

Adsorption of Re(VII)

Effect of initial pH value

The effect of initial pH of the solution on the adsorption of Re(VII) by D001-nZVI was investigated. Experiments of adsorbing Re(VII) were conducted at 298 K with 10 mg/L 50 mL Re(VII) solution and 20 g/L resin at a pH value of 1–9. Figure 5 exhibits the effect of initial pH of the solution on Re(VII) adsorption. When the pH value is increased from 2 to 9, the Re(VII) removal efficiency is decreased from 93.6% to 78%. Li et al. [¹⁰] stated that a lower pH value favored the reduction of Re(VII) to Re(IV) and led to a high level uptake of {\mathrm{ReO}}_4^{-}. Equations (2) and (3) show the reductive immobilization processes of Re(VII). However, at pH= 1, the adsorption efficiency of Re(VII) absorbed by D001-nZVI in 24 h is 20.5%. The reason for the lowest removal efficiency of Re(VII) is considered to be the fact that a large amount of Fe⁰ on the D001-nZVI is exhausted by excessive H⁺ in the solution. In an alkaline condition, the adsorption of Re(VII) was inhibited. The reason for this is that Fe²⁺/Fe³⁺ generates iron hydroxide precipitates coated on the surface of the resin as a result of diminishing the activity of Fe⁰.

(2){\mathrm{3}\mathrm{F}\mathrm{e}}^{\mathrm{0}}+{{\displaystyle \mathrm{2}\mathrm{Re}\mathrm{O}}}_{\mathrm{4}}^{-}+{\mathrm{8}\mathrm{H}}^{+}\to {\mathrm{3}\mathrm{F}\mathrm{e}}^{\mathrm{2}+}+{{\displaystyle \mathrm{2}\mathrm{Re}\mathrm{O}}}_{\mathrm{2}}+4{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O},

(3){\mathrm{3}\mathrm{F}\mathrm{e}}^{\mathrm{2}+}+{{\displaystyle \mathrm{Re}\mathrm{O}}}_{\mathrm{4}}^{-}+{\mathrm{4}\mathrm{H}}^{+}\to {\mathrm{3}\mathrm{F}\mathrm{e}}^{\mathrm{3}+}+\mathrm{2}{{\displaystyle \mathrm{Re}\mathrm{O}}}_{\mathrm{2}}+\mathrm{2}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}.

Effect of resin dose

The effect of resin dose on the adsorption of Re(VII) was studied at 298 K with 50 mL Re(VII) solution with a concentration of 10 mg/L and 5–20 g/L resin at a pH value of 3. The effect of resin dose on the adsorption efficiency of Re(VII) by D001-nZVI was presented in Fig. 6. When the resin is increased from 5 g/L to 20 g/L, the adsorption efficiency of Re(VII) is increased from 72.6% to 93.6%. The result indicates that increasing the resin dose will improve the adsorption efficiency of Re(VII).

Kinetics studies

The adsorption rate of Re(VII) by D001-nZVI was examined. The pseudo-first-order and the pseudo-second-order kinetic models [¹³] were employed to calculate the parameters of kinetic models:

(4)\ln ({{\displaystyle q}}_{\mathrm{e}}-{{\displaystyle q}}_t)=\ln {{\displaystyle q}}_{\mathrm{e}}-{{\displaystyle k}}_{\mathrm{1}}t,

(5){\displaystyle \frac{t}{{{\displaystyle q}}_t}}={\displaystyle \frac{\mathrm{1}}{{{\displaystyle k}}_{\mathrm{2}}{{\displaystyle q}}_{\mathrm{e}}^{\mathrm{2}}}}+{\displaystyle \frac{t}{{{\displaystyle q}}_t}},

where k₁ (min^-1) and k₂ (g/(mg∙min)) are the rate constants of the pseudo-first-order and pseudo-second-order reaction kinetics, respectively; q_e (mg/g) and q_t (mg/g) are the adsorption capacities at equilibrium and time t (min). The fitting results of the two kinetic models were displayed in Figs. 7 and 8. Table 1 tabulates the adsorption rate constants and equilibrium adsorption capacity of the kinetic models. The correlation coefficients (R²) of the pseudo-second-order model is 0.9968, which is higher than the pseudo-second-order model (R² = 0.974), which suggests that the adsorption process of Re(VII) on the D001-nZVI conforms to the pseudo-second-order model and can be further confirmed chemisorption.

Removal of ⁹⁹Tc

The removal of Tc(VII) was studied at different pH values and resin amount. Figures 9 and 10 show the results of Tc(VII) adsorption of 24 h. Figure 9 shows that the adsorption efficiency of Tc(VII) increases continuously with the decrease of pH, and reaches a maximum value of 94.7% at pH= 3. Figure 10 shows that when the resin dose was increased from 2 g/L to 10 g/L, the adsorption efficiency of Tc(VII) increases significantly from 71.9% to 94%. The experimental results demonstrate that the adsorption efficiency of Tc(VII) on the D001-nZVI is related to the pH value and the amount of resin. The effect of the above factors on the adsorption of Tc(VII) is the same as the removal of Re(VII), which further proves that {\mathrm{ReO}}_4^{-} is feasible to be utilized as a surrogate for {\mathrm{TcO}}_4^{-} to immobilize onto D001-nZVI.

Column experiment

Based on the batch experiments of rhenium adsorption, the dynamic adsorption performance of Re(VII) on D001-nZVI was investigated. Figure 11 shows the breakthrough for the adsorption of Re(VII) on the D001-nZVI. It can be seen that the rhenium flowing through the column is completely absorbed before 4 bed volumes, and then the concentration of rhenium in the effluent gradually increases. C_e/C₀ approaches 1 at 277 bed volumes, which means that the nZVI on D001-nZVI is exhausted. The totally absorbed Re(VII) quantity (Q': mg/g) in the column is expressed as

(6)Q^{\text{'}}={{\displaystyle \int }}_{\mathrm{0}}^V{\displaystyle \frac{({{\displaystyle C}}_{\mathrm{0}}-{{\displaystyle C}}_{\mathrm{e}})}{M}}\mathrm{d}V,

where C₀ is the influent Re(VII) concentration (mg/L), C_e is the effluent Re(VII) concentration (mg/L), and M is the mass of the adsorbent (g). The totally absorbed Re(VII) quantity Q' is calculated by graphical integration to be 0.2522 mg/g.

In the dynamic adsorption system, Thomas model is often used to describe and predict the adsorption process. The equation of the model is [¹⁴]

(7){\displaystyle \frac{{{\displaystyle C}}_{\mathrm{e}}}{{{\displaystyle C}}_{\mathrm{0}}}}={\displaystyle \frac{\mathrm{1}}{\mathrm{1}+\exp [{{\displaystyle k}}_T({{\displaystyle q}}_{\mathrm{0}}M-{{\displaystyle C}}_{\mathrm{0}}V)/\theta ]}},

where k_T is the Thomas rate constant (mL/(min∙mg)), q₀ is the maximum mass of the metal adsorbed by the unit mass resin at equilibrium (mg/g), and q is the volume flow rate (L/min). If Thomas model is converted to the linearized form, it is expressed as

(8)\ln \left({\displaystyle \frac{{{\displaystyle C}}_{\mathrm{0}}}{{{\displaystyle C}}_{\mathrm{e}}}}-\mathrm{1}\right)={\displaystyle \frac{{{\displaystyle k}}_T{{\displaystyle q}}_{\mathrm{0}}M}{\theta }}-{\displaystyle \frac{{{\displaystyle k}}_T{{\displaystyle C}}_{\mathrm{0}}V}{\theta }},

where k_T and q₀ can be obtained from the plot of ln[(C₀/C_e) - 1] against q as shown in Fig. 12. The k_T and q₀ obtained from the slope are listed in Table 2. It can be seen from Table 2 that the predicted value of bed capacity q₀ is close to the value of Q', and the correlation coefficient of Thomas model is 0.991. Therefore, it can be concluded that Thomas model is reliable and suitable for predicting column adsorption.

Conclusions

D001-nZVI was synthesized by reducing ferrous ion on the D001 resin, supporting the nZVI on the D001 resin. The iron content in D001-nZVI was determined to be 115 mg/g. The D001-nZVI was a reductive adsorbent for the removal of Re(VII) and the optimal pH value for the adsorption of Re(VII) was 3. The adsorption of Re(VII) was suitable for the pseudo-second-order kinetic model when the rate constant was 0.012 g/(mg∙min). In the column experiment, it was found that Thomas model was suitable to describe the dynamic adsorption performance of Re(VII) on the D001-nZVI. The maximum mass of Re(VII) by the unit mass resin at equilibrium from the Thomas model was 0.2910 mg/g. Based on the optimal adsorption conditions of Re(VII), the adsorption of Tc(VII) on the D001-nZVI was conducted. The optimal removal efficiency for Tc(VII) was 94% at pH= 3. In conclusion, D001-nZVI is an effective adsorbent for the removal of radionuclide ⁹⁹Tc.

References

[1] Chen L S, Wang T H, Hsieh Y K, Jian L W, Chen W H, Tsai T L, Wang C F. Accurate technetium-99 determination using the combination of TEVA resin pretreatment and ICP-MS measurement and its influence on the Tc-99/Cs-137 scaling factor calculation. Journal of Radio Analytical and Nuclear Chemistry, 2014, 299(3): 1883–1889

[2] Zhu L, Sheng D, Xu C, Dai X, Silver M A, Li J, Li P, Wang Y, Wang Y, Chen L, Xiao C, Chen J, Zhou R, Zhang C, Farha O K, Chai Z, Albrecht-Schmitt T E, Wang S. Identifying the recognition site for selective trapping of T cO4− 99. Journal of the American Chemical Society, 2017, 139(42): 14873–14876

[3] Gee G W, Oostrom M, Freshley M D, Rockhold M L, Zachara J M. Hanford site vadose zone studies: an overview. Vadose Zone Journal, 2007, 6(4): 899–905

[4] Liang L, Gu B, Yin X. Removal of technetium-99 from contaminated groundwater with sorbents and reductive materials. Separations Technology, 1996, 6(2): 111–122

[5] Icenhower J P, Qafoku N P, Martin W J, The geochemistry of technetium: a summary of the behavior of an artificial element in the natural environment. Report PNNL-18139. Richland, Washington, USA: Pacific Northwest National Laboratory, 2008

[6] Rard J A, Rand M H, Anderegg G, Chemical Thermodynamics of Technetium. Amsterdam: Elsevier, 1999

[7] Lenell B A, Arai Y. Perrhenate sorption kinetics in zerovalent iron in high pH and nitrate media. Journal of Hazardous Materials, 2017, 321: 335–343

[8] Tosco T, Petrangeli Papini M, Cruz Viggi C, Sethi R. Nanoscale zerovalent iron particles for groundwater remediation: a review. Journal of Cleaner Production, 2014, 77: 10–21

[9] Sheng G, Tang Y, Linghu W, Wang L, Li J, Li H, Wang X, Huang Y. Enhanced immobilization of ReO4− by nanoscale zerovalent iron supported on layered double hydroxide via an advanced XAFS approach: implications for TcO4− sequestration. Applied Catalysis B: Environmental, 2016, 192: 268–276

[10] Li J, Chen C, Zhang R, Wang X. Reductive immobilization of Re (VII) by graphene modified nanoscale zero-valent iron particles using a plasma technique. Science China Chemistry, 2016, 59(1): 150–158

[11] Liu H F, Qian T W, Zhao D Y. Reductive immobilization of perrhenate in soil and groundwater using starch-stabilized ZVI nanoparticles. Chinese Science Bulletin, 2013, 58(2): 275–281

[12] Ding Q, Qian T, Yang F, Liu H, Wang L, Zhao D, Zhang M. Kinetics of reductive immobilization of rhenium in soil and groundwater using zero valent iron nanoparticles. Environmental Engineering Science, 2013, 30(12): 713–718

[13] Yoon I H, Bang S, Chang J S, Gyu Kim M, Kim K W. Effects of pH and dissolved oxygen on Cr(VI) removal in Fe(0)/H₂O systems. Journal of Hazardous Materials, 2011, 186(1): 855–862

[14] Xiong C, Yao C, Wang L, Ke J. Adsorption behavior of Cd(II) from aqueous solutions onto gel-type weak acid resin. Hydrometallurgy, 2009, 98(3–4): 318–324

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