Adsorption behaviour of Eu(III) on natural bamboo fibres: Effects of pH, humic acid, contact time, and temperature

Adsorption behaviour of Eu(III) on natural bamboo fibres: Effects of pH, humic acid, contact time, and temperature 1 2 corresp first-author Corresponding author. E-mail address: tyu@ecit.edu.cn Corresponding author. E-mail address: tyu@ecit.edu.cn tyu@ecit.edu.cn https://orcid.org/0000-0002-0622-5986 2 2 Engineering Research Center of Nuclear Technology Application (East China University of Technology), Ministry of Education, Nanchang, 330013, China; Engineering Research Center of Nuclear Technology Application (East China University of Technology), Ministry of Education, Nanchang, 330013, China; School of Nuclear Science and Engineering, East China University of Technology, Nanchang, 330013, China School of Nuclear Science and Engineering, East China University of Technology, Nanchang, 330013, China To study the adsorption properties of organic functional groups in plant fibres and identify a highly efficient and affordable adsorbent for radioactive wastewater treatment, natural bamboo fibre (NBF) samples were prepared, and the adsorption properties of Eu(III) on NBFs were studied under given experimental conditions. The effects of the pH, solid-to-liquid ratio, background ions, humic acid, contact time, and temperature on the adsorption behaviour of Eu(III) on NBFs were investigated. Adsorption was greatly influenced by pH, and theadsorption curves presented two inflection pointsat pH≈7 and pH≈11. Moreover, while the ionic strength presented a negative effect at pH<7, the high ionic strength favoured adsorption at pH>7. During adsorption on NBFs, the –OH,C–H, O–H, C–O, and C=O were the main adsorption groups. Hydrolysis occurring on the NBF surface caused the adsorption process to become increasingly difficult at pH>7. The maximum adsorption capacity of NBFs was 147.6 mg/g at 308 K, and the adsorption could be described using the pseudo-second-order kinetic model. The adsorption of Eu(III) on NBFs was a spontaneous and endothermic process according to the thermodynamic parameters of the process, and the adsorption thermodynamics could be well described using the Freundlich adsorption model. Therefore, NBFs were determined to be an efficient, inexpensive, and easily disposable sewage treatment material. Adsorption Eu(III) Natural bamboo fibre Organic functional group 1 Introduction 1 As nuclear science continues to develop, researchers have been focusing on the adsorption and migration of radionuclides on natural adsorbents, in particular the treatment and disposal of radioactive waste [ 1 1 - 3 3 1-3 ]. Eu(III) is a lanthanide rare earth element that is often used to simulate trivalent actinides owing to its physicochemical properties. Because Eu(III) ions are similar to Am(III), it could be used to substitute it, and thus, mitigate the difficulties in producing Am(III) and its high cost. Eu(III) adsorption on soil, clay minerals, and fibres has been intensively studied, and it was determined that Eu(III) usually reacts with adsorbents via cation exchange or surface complexation [ 4 4 ]. Owing to the developments in adsorption research, an increasing number of adsorbents have been prepared and tested. Nevertheless, these adsorbents tended to be expensive and difficult to scale. According to some theories, the adsorption process could be divided into four steps: bulk transport, film diffusion, intraparticle diffusion, and adsorption, where the second and third steps are the control steps [ 5 5 ]. At present, researchers are focusing on four types of adsorbent materials: inorganic materials, organic polymers, carbon family materials, and porous framework materials. Among them, cellulose is considered to be the first most abund ant polymer in nature that can also be used as potential adsorbent owing to its nontoxicity, affordability, and easy functionalisation [ 6 6 ]. Moreover, novel nanomaterials and magnetic materials have been prepared and the adsorption mechanism of heavy metals on them has been analysed using batch techniques, spectral analysis, surface complexation models, and theoretical calculations [ 7 7 , 8 8 ]. China is rich in plant fibre resources, and plant fibres are widely used to treat environmental pollution and agricultural waste, according to previously published papers [ 9 9 , 10 10 ]. Moreover, China is rich in bamboo and wood fibre resources and is the largest producer of bamboo fibre in the world. Bamboo fibre is a green, renewable, and biodegradable fibrous material [ 11 11 , 12 12 ]. In addition to natural bamboo fibre (NBF), other biological materials such as natural or modified chitosan, cellulose, and microorganisms are used for the adsorption or removal of nuclides. These materials possess many organic functional groups, such as –CH, –OH, and –C=O, and exhibit satisfactory adsorption properties for radionuclides [ 13 13 ], thus resulting in good application prospects for treating radioactive waste and preventing nuclear pollution [ 14 14 , 15 15 ]. As lanthanide rare earth element, Eu(III) is often used to simulate trivalent actinides owing to its physicochemical properties. Consequently, Eu(III) adsorption on soil, clay minerals, and fibres has been studied intensively [ 16 16 , 17 17 ]. 2 Experimental 1 2.1 Instruments and reagents 2 An X-650 (Hitachi) scanning electron microscope, an IRPrestige-21 (Shimadzu) Fourier-transform infrared (FT-IR) spectrometer, and an 721E (INESA Analytical Instrument Co., Ltd) visible spectrophotometer were used in this study. The Eu(III) standard solution comprising Eu 2 O 3 was supplied by Ganzhou Deshipu Advanced Material Resource Co., Ltd. Accurately weighed 0.40g Eu 2 O 3 was dissolved in 1.0 mol/L HCl solution, and was then transferred into a volumetric flask and diluted with distilled water to 1000mL.The concentration of obtained Eu(III) solution was 2.273 × 10 −3 mol/L. Arsenazo III(1.0 g/L aqueous solution), pH 2.8 acetic acid–sodium acetate solution, and 0.1% (v/v) ascorbic acid aqueous solution were used as colour developer, buffer, and masking agent, respectively. Distilled water was used for all experiments. 2.2 Preparation of NBF samples 2 The joint and crust were removed from a piece of fresh bamboo stem (which was grown for more than three years), and the stem was cut into 10 cm long and 2 cm wide pieces. Then, the samples were dried at 150 °C for 1 h and ground in a mortar, thus producing rough NBFs. The NBFs were then soaked in 60 °C water for 24 h, then dried and rolled before removing the impurities. Afterwards, the samples were boiled (their temperature was increased from 30 to 120 °C) in a pressure cooker at the heating rate of 20 °C/min for 1 h. Subsequently, the NBF samples were treated with 0.1 mol/L sodium hydroxide (NaOH) solution and were then neutralised with acetic acid. To observe the microfibril structure of the NBF surface, a 10% nitric acid and 10% chromic acid mixed solution (1:1 volume ratio) was used to treat the fibres and remove the hemi-cellulose, lignin, and other non-cellulose elements from the surface of fibres. The procedure was performed at 60 °C for 5 min, and the samples were maintained at 25 °C for 24 h. Then, the samples were washed with distilled water and dried. The obtained NBFs were ground to pass through an 80-mesh sieve (< 0.2 mm) for the standby application. 2.3 Adsorption procedures 2 The adsorption of Eu(III) on NBFs was assessed at different temperatures:25, 35, and 50 °C (298, 308, and 323K, respectively). Certain amounts of NBFs, sodium chloride (NaCl) solution, humic acid (HA) solution, and Eu(III) solution were added to Erlenmeyer flasks. Trace amounts of HCl and NaOH solutions were added to adjust the acidity to the desired pH value. Then, distilled water was added to the flasks to obtain 50mL mixtures. The mixtures were shaken in an oscillator at a certain temperature for a certain time to reach adsorption–desorption equilibrium, when the concentration of Eu(III) in the solution no longer changed. Then, 10 mL solution was pipetted into a centrifuge tube (15 mL) and was centrifuged at the speed of 5000 r/pm for 10 min. Furthermore, 5.0 mL supernatant was pipetted into a colorimetric tube, and 1.0 mL ascorbic acid, 5.0 mL buffer, and 1.0 mL Arsenazo III solutions were added into the tube. The final volume was set with water, and the concentration of Eu(III) was determined using the spectrophotometric method. Lastly, the quantity of adsorbed radionuclide was calculated by comparing the concentration of Eu(III) before and after the adsorption equilibrium. The adsorption percentage ( S , %) and distribution coefficient ( K d ) were calculated using the following formulas: <math display="block" id="M1"><mrow><mi>S</mi><mo> </mo><mrow><mo>(</mo><mi>%</mi><mo>)</mo></mrow><mo>=</mo><mo> </mo><mfrac><mrow><msub><mi>q</mi><mtext>e</mtext></msub></mrow><mrow><msub><mi>c</mi><mn>0</mn></msub></mrow></mfrac><mo>×</mo><mfrac><mi>m</mi><mi>V</mi></mfrac><mo>×</mo><mn>100</mn></mrow></math> and <math display="block" id="M2"><mrow><msub><mi>K</mi><mtext>d</mtext></msub><mo>=</mo><mfrac><mrow><mo stretchy="false">(</mo><msub><mi>c</mi><mn>0</mn></msub><mo>−</mo><msub><mi>c</mi><mtext>e</mtext></msub><mo stretchy="false">)</mo></mrow><mrow><msub><mi>c</mi><mtext>e</mtext></msub></mrow></mfrac><mo>×</mo><mfrac><mi>V</mi><mi>m</mi></mfrac><mo>,</mo></mrow></math> where c 0 (mg/L) is the initial concentration of Eu(III) in the liquid phase, c e (mg/L) is the Eu(III) concentration inthe adsorption equilibrium liquid phase, V (L) is the volume of the mixed solution, and m (g)is the mass of NBFs. 3 Results and Discussion 1 3.1 Characterisation of NBF and Eu-NBF 2 The scanning electron microscopy (SEM) images of NBF and Eu-NBF (after adsorption) are presented in Fig. 1 Fig. 1 . The results indicate that before adsorption, the NBF surface was very rough and rugged, and many irregular pore- and fibre-like structures were observed on it. The presence of these structures greatly increased the surface area of the NBF, thus allowing for more adsorption sites to be exposed. Moreover, the structure provided important adsorption channels and plenty of adsorption space for the adsorption of metal ions from solution. The pore and filament-like structures of the NBF were favourable for Eu(III) adsorption. After adsorption, the number of pore-like structures of the NBF decreased and the valleys became shallower. This indicated that the number of adsorption sites on the surface decreased. The energy dispersive X-ray spectroscopy (EDS) analysis results of the NBF (before adsorption) and Eu-NBF (after adsorption) samples illustrated in Figure 1 Figure 1 are presented in Figure 2 Figure 2 . The composition of NBF mainly included C, O and K, and the corresponding element percentages were 26.8%, 71.8%, and 1.2%, respectively ( Figure 2(a) Figure 2(a) ). Figure 2(b) Figure 2(b) depicts the composition of the sample after adsorption, which mainly included C, O, and Eu, and their element percentages were 22.34%, 62.70% and 14.13%, respectively. Figures 2(c), (d), and (e) Figures 2(c), (d), and (e) illustrate the distribution of C, O, and Eu, respectively, on the surface of the Eu-NBF sample (after adsorption) presented in Figure 1(b) Figure 1(b) . Figures 2(c)-(e) Figures 2(c)-(e) indicate that the elements and adsorption sites were uniformly distributed on the surface of the sample. These findings indicated that the pore structure and filament-like structure of NBFs conferred them strong adsorption capacity for Eu(III), and K was exchanged with Eu(III) during adsorption. To study the function of organic functional groups, the FT-IR spectra of the NBF samples were analysed. Figure 3 Figure 3 illustrates the FT-IR spectra of NBF and Eu-NBF samples at pH4 and 11. The absorption peak at 3313 cm −1 was ascribed to the stretching vibration of the –OH radical and was also the characteristic spectral band for all NBF samples; the absorption peaks at 2920 and 2851 cm −1 were attributed to the telescopic vibration of –CH 2 –; the absorption peak at 1634 cm −1 belonged to the telescopic vibration of the C=O bond; the absorption peak at 1730 cm −1 represented the C=O peak of the –COOH group; the 1634 cm −1 absorption peak was allocated to the =C=O group; the absorption peak at 1544 cm −1 was generated by the –CONH group; the absorption peak at 1237 cm −1 was the characteristic peak of the asymmetric lateral stretching vibration of –C–O–C or wagging vibration of CH 2 (CH 2 OH); the absorption peak at 1021 cm −1 was associated with the –C–O group; and the absorption peak at 601 cm −1 was assigned to –C–X (where X is a halogen) [ 18 18 ]. Figure 3 Figure 3 indicates that the spectra of the Eu-NBF sample after adsorption was very similar to the spectra of the NBF sample (pre-adsorption) at different pH values, and only a couple of peak drifts appeared. These findings indicated that the adsorption process did not destroy the fabric of NBFs. Of all peaks, the skeleton vibration absorption peaks of the hydroxyl –OH, –CH 2 , C–O, C=O, and polycyclic aromatic hydrocarbons were displaced at 513, 41, 4, 1 and 76 cm −1 , respectively, while the positions of the other vibration absorption peaks were essentially unchanged. Therefore, it was concluded that the hydroxyl –OH, C–H, C=O, and C–O were the main adsorption groups of NBFs [ 19 19 ]. At pH4, the peak of –CONH (1544 cm −1 ) was present in the spectra of both the NBF and Eu-NBF samples, however, at pH11the peak disappeared from their spectra. This indicated that the –CONH group hydrolysed under strong alkaline conditions, which led to the significant change in the adsorption capacity of NBFs. 3.2 Effect of experimental conditions on Eu adsorption 2 Eu(III) adsorption on NBFs as function of the adsorbent concentration (solid-to-liquid ratio, m / V ) was investigated at pH 6 and 298K. As the m / V value of NBFs increased, the adsorption percentage also increased. The experimental error was determined to be less than 5% via repeated experiments at each experimental point. These findings suggested that as the m / V value increased, the number of adsorption groups on the NBF surface increased and the total adsorption capacity of the system was improved [ 20 20 ]. The adsorption percentage of Eu(III) decreased to the lowest value (81.2%) at m / V =0.0428 g/L, while at m / V =6.0 g/L, the adsorption percentage was the highest (95.8%). To better reflect the influence of other factors on the adsorption process, the adsorption percentage of Eu(III) in subsequent experiments should be not too low or too high. Thus, we selected m / V =0.6 g/L as the NBF concentration for the following tests. The effects of the pH and ionic strength on the adsorption process were studied at T =298K and m / V =0.6 g/L. Figure 4 Figure 4 represents the adsorption of Eu(III) on NBFs as function of the pH in the presence of 0, 0.02, 0.04, and 0.1 mol/L NaCl. Eu(III) adsorption was greatly affected by the pH and ionic strength. At pH≈7 and pH≈11, the Eu(III) adsorption curves presented two inflection points at different ionic strengths. At pH < 7 or pH > 11, the adsorption percentage continuously increased as the pH increased, while the adsorption percentage decreased as the pH increased in the pH range of 7-11. While high ionic strength was not conducive for the adsorption process at pH < 7, high ionic strength favoured adsorption at pH > 7. This was because adsorption mainly occurred via cation exchange or outer-sphere complexation at pH < 7 and via inner-sphere complexation at pH > 7. To better reflect the influence of other factors, the following experiments were performed at pH 6. The effects of different cations (0.02 M LiCl, KCl, and NaCl), anions (0.02 M NaNO 3 , NaClO 4 , and Na 2 SO 4 ), and HA on the adsorption of Eu(III) on NBFs were studied under the conditions mentioned above, and the results indicated that Eu(III) adsorption was affected by all these factors. At pH≈7 and pH≈11, the Eu(III) adsorption curve presented turning points in different cations, anions, and HA solutions. The adsorption percentages increased as the pH increased at pH < 7, decreases as the pH increased in the pH range of 7-11, and increased as the pH increased at pH > 11. Moreover, the effects of different ions on the adsorption process could be described as follows: K + > Na + = Li + and SO 4 2− > ClO 4 − = NO 3 − at pH < 7; at pH > 7, the order was reversed. The results also suggested that the presence of HA significantly affected Eu(III) adsorption. At the HA concentrations ( c HA ) of 0.01 and 0.02 g/L, the presence of HA obviously increased the adsorption of Eu(III) on NBFs in the pH range of 2-12. At pH < 7, the increase in adsorption at c HA = 0.02 g/L was higher than that at c HA = 0.01 g/L, and the opposite trend was observed at pH > 7. 3.3 Adsorption mechanism 2 Based on the above characterisation and experimental results, it could be concluded that adsorption occurred via cation exchange or outer-sphere complexation at pH < 7 (Eqs. (3) and (4)) and via inner-sphere complexation or precipitation at pH > 7 (Eqs. (5) and (6)), while Eu(OH) 3 (aq) weakened the adsorption process in the pH range of 7-13 to a certain extent(Eqs.(7)-(10)). Here, 'outer-sphere’ complexes are the weak or 'second coordination sphere’ complexes, while the strong complexes formed between the adsorbate and the adsorbent are usually referred to as 'first coordination sphere’ or 'inner-sphere’ complexes. These results were attributed to the speciation of Eu(III) in the aqueous solution. The speciation of Eu(III) in solution was analysed, and the main species determined were as follows: Eu 3+ at pH < 6, Eu(OH) 2+ and Eu(OH) 2 + at pH 5-9, Eu(OH) 3 at pH 7-12, and Eu(OH) 4 − at pH > 11 [ 21 21 ]. From this speciation distribution, it was concluded that the main species of Eu(III) could participate in cation exchange reactions at pH < 7, and the ionic strength negatively affected adsorption in this pH range. In addition, as the pH increased, the adsorption decreased in the pH range of 7-11, and the background ions positively affected the adsorption process. This could be attributed to the speciation of Eu(III) and surface properties of NBFs. Although the hydrolysis of NBFs and appearance of Eu(OH) 4− and Eu(OH) 3 (aq) significantly decreased adsorption, the presence of background ions (e.g., Na + and Cl − ) changed the distribution of the Eu(III) speciation and promoted a synergistic effect, which enhanced adsorption under ternary system conditions [ 22 22 , 23 23 ]. Additionally, as the NaCl concentration increased, more Na + ions were introduced and became available for possible cation bridging between Eu(III) and the negatively charged functional groups of NBFs. The degree of protonation and point of zero charge decreased as the ionic strength increased, thus facilitating cation adsorption. Therefore, adsorption was promoted to a certain extent at pH > 7 [ 23 23 - 26 26 23-26 ]. The experimental results with respect to the effect of pH indicated that at low pH values, the main adsorption mechanism on NBFs was cation exchange or outer-sphere complexation, while at high pH values, the mechanism changed to inner-sphere complexation. The schematic diagrams of the inner- and outer-sphere complexation mechanisms are presented in Figure 5 Figure 5 . Each complexation mechanism involved three conditions, and the outer-sphere complexation was weaker than inner-sphere complexation. These results indicated that surface complexation was greatly affected by the acidity of the solution, and that cation exchange was affected by background ions. According to the characterisation results of NBFs, the NBF surface contained various organic functional groups, such as –OH and –C=O, and these oxhydryls could turn into adsorption sites of variable charge (≡SO–H) which could capture or release protons. At pH<7, <math display="block" id="M3"><mrow><msup><mrow><mtext>(</mtext><mo>≡</mo><mtext>SO)</mtext></mrow><mtext>-</mtext></msup><msup><mrow><mtext>+ H</mtext></mrow><mtext>+</mtext></msup><mo>⇄</mo><mtext>(</mtext><mo>≡</mo><mtext>SO–H)</mtext></mrow></math> and <math display="block" id="M4"><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>SO</mtext><mo>–</mo><mtext>H</mtext><mo stretchy="false">)</mo><mtext> </mtext><mo>+</mo><msup><mrow><mtext> Eu</mtext></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>⇄</mo><msup><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>SOEu</mtext><mo stretchy="false">)</mo></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup><mo>+</mo><msup><mrow><mtext> H</mtext></mrow><mo>+</mo></msup><mo>.</mo></mrow></math> At pH>7, <math display="block" id="M5"><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>SO</mtext><mo>–</mo><mtext>H</mtext><mo stretchy="false">)</mo><mtext> </mtext><mo>+</mo><mtext> </mtext><msup><mrow><mrow><mo>(</mo><mrow><mtext>EuOH</mtext></mrow><mo>)</mo></mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup><mo>⇄</mo><msup><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>S</mtext><mo>–</mo><mtext>OEuOH</mtext><mo stretchy="false">)</mo></mrow><mo>+</mo></msup><mo>+</mo><msup><mrow><mtext> H</mtext></mrow><mo>+</mo></msup></mrow></math> and <math display="block" id="M6"><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>SO</mtext><mo>–</mo><mtext>H</mtext><mo stretchy="false">)</mo><mtext> </mtext><mo>+</mo><mtext> </mtext><msup><mrow><mrow><mo>(</mo><mrow><mtext>Eu</mtext><msub><mrow><mrow><mo>(</mo><mrow><mtext>OH</mtext></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msub></mrow><mo>)</mo></mrow></mrow><mo>+</mo></msup><mo>⇄</mo><mo stretchy="false">(</mo><mo>≡</mo><mtext>S</mtext><mo>–</mo><mtext>OEuOH</mtext><mo stretchy="false">)</mo><mtext> </mtext><mo>+</mo><msup><mrow><mtext> H</mtext></mrow><mo>+</mo></msup><mo>.</mo></mrow></math> Moreover,the hydrolysis process was simulated as follows: <math display="block" id="M7"><mrow><mrow><mo>(</mo><mrow><mo>–</mo><mtext>C</mtext><mo>=</mo><mtext>O</mtext></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mtext>H</mtext><mo>–</mo><mtext>C</mtext><mo>–</mo></mrow><mo>)</mo></mrow><mo>⇄</mo><mrow><mo>(</mo><mrow><mo>–</mo><mtext>C</mtext><mo>–</mo><mtext>C</mtext><mo>–</mo><mtext>OH</mtext></mrow><mo>)</mo></mrow></mrow></math> <math display="block" id="M8"><mrow><mrow><mo>(</mo><mrow><mo>–</mo><mtext>C</mtext><mo>–</mo><mtext>C</mtext><mo>–</mo><mtext>OH</mtext></mrow><mo>)</mo></mrow><mo>⇄</mo><mrow><mo>(</mo><mrow><mo>–</mo><mtext>C</mtext><mo>–</mo><mtext>C</mtext><mo>–</mo><msup><mtext>O</mtext><mo>−</mo></msup></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><msup><mrow><mtext> H</mtext></mrow><mo>+</mo></msup></mrow></math> <math display="block" id="M9"><mrow><mrow><mo>(</mo><mrow><mo>–</mo><mtext>CONH</mtext></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><msub><mrow><mtext> H</mtext></mrow><mn>2</mn></msub><mtext>O</mtext><mo>⇄</mo><msup><mrow><mrow><mo>(</mo><mrow><mo>–</mo><mtext>COO</mtext></mrow><mo>)</mo></mrow></mrow><mo>−</mo></msup><mo>+</mo><msup><mrow><mtext> H</mtext></mrow><mo>+</mo></msup><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mo>–</mo><msub><mrow><mtext>NH</mtext></mrow><mn>2</mn></msub></mrow><mo>)</mo></mrow></mrow></math> <math display="block" id="M10"><mrow><msup><mtext>H</mtext><mo>+</mo></msup><mo>+</mo><msup><mrow><mtext> OH</mtext></mrow><mo>−</mo></msup><mo>⇄</mo><msub><mtext>H</mtext><mn>2</mn></msub><mtext>O</mtext></mrow></math> where (–C–H) is the abbreviated structural formula of –C–H 2 –, which comprised the basic structure of NBF,and (–C–C–OH) is the speculative product of (–C=O) and (–C–H) under alkaline conditions, which could be protonated and deprotonated. This hydrolysis process rendered the cation exchange and complexation reaction increasingly difficult at pH > 7 and was also the important reason for the decrease in adsorption rate. The effect of HA on adsorption was also experimentally studied, and the results suggested that the adsorption mechanism of Eu(III) on NBFs could be postulated using the following equations: <math display="block" id="M11"><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>SO–H</mtext><mo stretchy="false">)</mo><msup><mrow><mtext> + HOOC–HA–OH +Eu</mtext></mrow><mrow><mtext>3+</mtext></mrow></msup><mo>⇄</mo><mo stretchy="false">(</mo><mo>≡</mo><msup><mrow><mtext>S–O)–OC–HA–O–Eu</mtext></mrow><mrow><mtext>3+</mtext></mrow></msup><mo>+</mo><msub><mrow><mtext> H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext></mrow></math> <math display="block" id="M12"><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>S</mtext><mo>–</mo><mtext>O</mtext><mo stretchy="false">)</mo><mo>–</mo><msup><mrow><mtext>OC–HA–O–Eu</mtext></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>+</mo><msub><mrow><mtext> H</mtext></mrow><mn>2</mn></msub><mtext>O</mtext><mo>⇄</mo><mo stretchy="false">(</mo><mo>≡</mo><mtext>SO–H</mtext><mo stretchy="false">)</mo><mtext> </mtext><mo>+</mo><msup><mrow><mtext> HO–HA–COOH + Eu</mtext></mrow><mrow><mtext>3+</mtext></mrow></msup></mrow></math> <math display="block" id="M13"><mrow><mo stretchy="false">(</mo><mo>≡</mo><mtext>S–O</mtext><mo stretchy="false">)</mo><mo>–</mo><msup><mrow><mtext>Eu</mtext></mrow><mrow><mtext>3+</mtext></mrow></msup><mtext>+ HO–HA–COOH</mtext><mo>⇄</mo><mo stretchy="false">(</mo><mo>≡</mo><mtext>SO–H</mtext><mo stretchy="false">)</mo><mtext> </mtext><mo>+</mo><msup><mrow><mtext> HO–HA–COO–Eu</mtext></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup></mrow></math> Here, HA was written as 'HO–HA–COOH’ according to its major functional groups. According to the experimental results and above equations, the adsorption strongly depended on pH. Moreover, it was concluded that the –OH and –COOH groups were the main functional groups that affected adsorption. These results suggested that the speciation of Eu(III) was Eu-HA in solution, and Eu-BNF (binary complexation), Eu-HA-NBF, or HA-Eu-NBF (ternary complexation)on the NBF surface. These conclusions were consistent with the information previously reported in the literature [ 27 27 , 28 28 ]. 3.4 Adsorption kinetics 2 The kinetic study of the adsorption of Eu(III) on NBFs is presented in Figure 6 Figure 6 . Figure 6(a) Figure 6(a) displays the effect of the contact time at 298 and 308 K (25 and 35 °C, respectively). The results indicated that the contact time could greatly affect the adsorption process at different temperatures. As the adsorption time increased, the adsorption percentage increased and reached its maximum after 15 h. At 298 and 308 K, the maximum adsorption percentages were 93.4%, and 94.1%, respectively. Consequently, the contact time was selected to be 15 h for the following experiments. To quantitatively analyse the adsorption kinetics of Eu(III) on NBFs, the pseudo-first- and pseudo-second-order kinetic equations were used (Eqs. (14) and (15), respectively) [ 29 29 , 30 30 ]: <math display="block" id="M14"><mrow><mi>ln</mi><mrow><mo>(</mo><mrow><msub><mi>q</mi><mtext>e</mtext></msub><mo>−</mo><msub><mi>q</mi><mtext>t</mtext></msub></mrow><mo>)</mo></mrow><mo>=</mo><mi>ln</mi><msub><mi>q</mi><mtext>e</mtext></msub><mo>−</mo><mo> </mo><msub><mi>k</mi><mn>1</mn></msub><mi>t</mi><mo>,</mo></mrow></math> and <math display="block" id="M15"><mrow><mfrac><mi>t</mi><mrow><msub><mi>q</mi><mtext>t</mtext></msub></mrow></mfrac><mo>=</mo><mfrac><mn>1</mn><mrow><msub><mi>k</mi><mn>2</mn></msub><msubsup><mi>q</mi><mtext>e</mtext><mn>2</mn></msubsup></mrow></mfrac><mo>+</mo><mfrac><mn>1</mn><mrow><msub><mi>q</mi><mtext>e</mtext></msub></mrow></mfrac><mi>t</mi><mo>,</mo></mrow></math> where t (h) is the contact time, k 1 (h −1 ) and k 2 (g/(mg h)) are the pseudo-first- and pseudo-second-order reaction rate constants, respectively, q t (mg/g) is the adsorption capacity of Eu(III) at time t (h), and q e (mg/g) is the equilibrium adsorption capacity. These two kinetic models are presented in Figs. 6(b) and (c) Figs. 6(b) and (c) . From Figs. 6(b) and (c) Figs. 6(b) and (c) , it was determined that the linearity of the experimental data was good at 25 °C. The adsorption parameters were calculated using the slopes and intercepts of the plots generated by graphing the above equations, and the obtained values are given below. For the pseudo-first-order kinetics equation: T =25 °C, R 2 =0.990, k 1 =0.689 h −1 , and q e =0.335 mg/g. For the pseudo-second-order kinetics equation: T =25 °C, R 2  =0.999, k 2 =7.505 g/(mg h), and q e =5.382 mg/g. These results indicated that the pseudo-second-order kinetics model could fit the experimental data better than the pseudo-first-order kinetics one and was more suitable for describing the adsorption of Eu(III). The linear correlation coefficient ( R 2 ) indicated that the pseudo-second-order kinetics model could accurately describe the kinetics of the adsorption reaction because it was very close to 1 [ 29 29 , 31 31 ]. According to the experimental results and data reported in the literature, the adsorption capacity of NBFs for Eu(III) was relatively low, and the time required for the adsorption process to reach equilibrium was relatively long compared with the those of many synthetic or modified materials for other radionuclides(the adsorption capacity is usually higher than 50 mg/g,and the time required for the adsorption process to reach equilibrium is less than 10 h) [ 6 6 , 32 32 , 33 33 ]. The adsorption kinetics of Eu(III) on NBFs could also be described using the diffusion model and Weber–Morris intraparticle diffusion model [ 34 34 ]. The experimental data could be described using the former model as follows: <math display="block" id="M16"><mrow><mi>B</mi><mi>t</mi><mo>=</mo><mtext> </mtext><mo>−</mo><mtext>ln</mtext><mrow><mo>(</mo><mrow><mn>1</mn><mtext> </mtext><mo>–</mo><mi>F</mi></mrow><mo>)</mo></mrow><mtext> </mtext><mo>–</mo><mtext> </mtext><mn>0.4977</mn><mo>,</mo></mrow></math> where B =π 2 Di / d 2 , F = q t / q e , d is the particle radius, and Di is the diffusion coefficient. The latter model could further simulate the diffusion process, and the model equation can be expressed as follows: <math display="block" id="M17"><mrow><msub><mi>q</mi><mtext>t</mtext></msub><mo>=</mo><msub><mi>K</mi><mtext>e</mtext></msub><msup><mi>t</mi><mrow><mn>0.5</mn></mrow></msup><mo>+</mo><mi>C</mi><mo>,</mo></mrow></math> where C is the intercept and K e (kg/(mg h 0.5 )) is the intraparticle diffusion rate constant. The value of K e could be calculated from the slope of the linear plot of q t vs. t 0.5 . The Weber–Morris kinetic model data was divided into two sections, and the Weber–Morris kinetic model related constants are summarised in Table 1 Table 1 . 1 2 T (K) 3 1 Initial linear area 3 1 Second linear area 1 1 K e1 (kg/(mg h 0.5 ) 1 1 C 1 1 1 R 1 2 1 1 K e2 (kg/(mg h 0.5 ) 1 1 C 2 1 1 R 2 2 1 1 298 1 1 0.019 1 1 5.31 1 1 0.981 1 1 0.004 1 1 5.36 1 1 0.929 1 1 308 1 1 0.020 1 1 5.31 1 1 0.932 1 1 0.003 1 1 5.37 1 1 0.967 1 1 323 1 1 0.021 1 1 5.30 1 1 0.992 1 1 0.010 1 1 5.33 1 1 0.973 The linear plots of (−ln(1 − F ) − 0.4977) vs. t at different temperatures were obtained and the related parameters were calculated. The results indicated that the linear plots did not pass through the origin. This suggested that diffusion was an important control step, and was involved in the adsorption process. Moreover, intraparticle diffusion could be divided into two stages: the initial stage of the curve and the buffering or equilibrium stage. The two stages fitted the experimental data well, which suggested that the intraparticle diffusion step was not the only control step, but it was one of the stages of the adsorption process. 3.5 Adsorption isotherms 2 The fitting curves of the adsorption of Eu(III) on NBFs at298, 308, and 323 K were obtained and analysed. The Langmuir adsorption model could be described as follows [ 35 35 ]: <math display="block" id="M18"><mrow><msub><mi>c</mi><mtext>s</mtext></msub><mo>=</mo><mfrac><mrow><mi>b</mi><msub><mi>c</mi><mrow><mtext>smax</mtext></mrow></msub><msub><mi>c</mi><mtext>e</mtext></msub></mrow><mrow><mn>1</mn><mo>+</mo><mi>b</mi><msub><mi>c</mi><mtext>e</mtext></msub></mrow></mfrac><mo>,</mo></mrow></math> <math display="block" id="M19"><mrow><mfrac><mrow><msub><mi>c</mi><mtext>e</mtext></msub></mrow><mrow><msub><mi>c</mi><mtext>s</mtext></msub></mrow></mfrac><mo>=</mo><mfrac><mn>1</mn><mrow><mi>b</mi><msub><mi>c</mi><mrow><mtext>smax</mtext></mrow></msub></mrow></mfrac><mo>+</mo><mfrac><mrow><msub><mi>c</mi><mtext>e</mtext></msub></mrow><mrow><msub><mi>c</mi><mrow><mtext>smax</mtext></mrow></msub></mrow></mfrac><mo>,</mo></mrow></math> where c smax is the maximum equilibrium adsorption capacity (mol/g), c s is the Eu(III) concentration in the solid phase (mol/g), c e is the Eu(III) concentration in the liquid phase (mol/l), and b (L/mol) is an adsorption temperature-related constant. According to equation (19), b and c smax could be determined from intercept and slope, respectively, of the c s vs. c e graph. The linear correlation coefficients of the Langmuir adsorption model were 0.963, 0.925, and 0.969, and the c smax values were 1.09×10 −4 , 9.71×10 −4 , and 1.12×10 −4 mol/g (16.57, 147.6, and 17.02 mg/g), at 298, 308, and 323 K, respectively. The adsorption capacity of NBFs for radionuclides is similar to those of other natural or modified biological adsorbents; however, the cost of NBFs is considerably lower than those of the other adsorbents [ 36 36 ]. Generally, the cost of NBFs is a fraction of that of other biological adsorbents and approximately 1% of the cost of synthetic functional materials. As the temperature increased, the adsorption isotherm positions of Langmuir isotherm decreased, and the c e / c s values also decreased. This dynamic indicated that Eu(III) was distributed more in the solid phase than in the liquid phase at high temperatures and that high temperature could improve the adsorption process. In addition, as c s increased, the Langmuir adsorption isotherms also presented an increasing trend, and the c e / c s values increased as well, which indicated that the increase in c e could reduce the increase rate of c s . Because the number of adsorption sites on the adsorbent surface was finite, the extra radionuclides would transfer from the solid phase to the liquid phase. For a certain concentration range, the fitting curve at high temperature (323 K) intersected the fitting curve at intermediate temperature (308K), which indicated that radionuclides that were combined with organic matter of NBFs could desorb from the adsorbent surface at higher temperature and migrate from the solid phase to the liquid phase. Moreover, the highest and lowest fitting curve temperatures of the Freundlich adsorption isotherms were323 and 298 K, respectively. These results suggested that in the range of 298-323K, high temperature was conducive to the adsorption of Eu(III) on NBFs. According to the Freundlich isotherm model: <math display="block" id="M20"><mrow><msub><mi>c</mi><mtext>s</mtext></msub><mo>=</mo><mi>a</mi><msubsup><mi>c</mi><mtext>e</mtext><mtext>n</mtext></msubsup><mo>,</mo></mrow></math> and its linear form could be written as follows: <math display="block" id="M21"><mrow><mtext>lg</mtext><msub><mi>c</mi><mtext>s</mtext></msub><mo>=</mo><mtext>lg</mtext><mi>a</mi><mo>+</mo><mtext>lg</mtext><msub><mi>c</mi><mtext>e</mtext></msub><mo>,</mo></mrow></math> where a (mol 1−n g −1 L n ) is the equilibrium concentration of adsorbent ions when n =1, and n is the dependence of the adsorption on the equilibrium concentrations. The R 2 values of the Freundlich adsorption model: 0.979, 0.960, and 0.986at 298, 308, and 323 K, respectively, were close to 1. This suggested that the Freundlich adsorption model was more suitable for describing the Eu(III) adsorption behaviour on NBFs than the Langmuir adsorption model. According to the above-mentioned experimental data, the instantaneous adsorption capacity of NBFs for Eu(III) was 5.382 mg/g, and the maximum adsorption capacity was 16.63 mg/g at 298K. This adsorption capacity was relatively mediocre compared with that of other adsorbents, and was significantly lower than those of bentonite and metal–organic framework materials [ 37 37 , 38 38 ]. Using experimental data, the thermodynamic functions of the Eu(III) adsorption on NBFscould be calculated using the following equations: <math display="block" id="M22"><mrow><mtext>ln</mtext><mi>K</mi><mo>=</mo><mfrac><mrow><mo>−</mo><mi>Δ</mi><msup><mi>H</mi><mn>0</mn></msup></mrow><mi>R</mi></mfrac><mo>×</mo><mfrac><mn>1</mn><mi>T</mi></mfrac><mo>+</mo><mfrac><mrow><mi>Δ</mi><msup><mi>S</mi><mn>0</mn></msup></mrow><mi>R</mi></mfrac><mo>,</mo></mrow></math> and <math display="block" id="M23"><mrow><mi>Δ</mi><msup><mi>G</mi><mn>0</mn></msup><mo>=</mo><mi>Δ</mi><msup><mi>H</mi><mn>0</mn></msup><mo>−</mo><mi>T</mi><mi>Δ</mi><msup><mi>S</mi><mn>0</mn></msup><mo>,</mo></mrow></math> where K is the adsorption equilibrium constant at different temperatures, and Δ H 0 , Δ S 0 , and Δ G 0 are the standard enthalpy, entropy, and Gibbs free energy changes, respectively. The values of ln K were obtained by plotting ln K d vs. c e and extrapolating c e to 0. The values of Δ H 0 and Δ S 0 were calculated from the slopes and intercepts of the ln K vs. 1/ T plots. The results indicated that Δ H 0 at 298, 308, and 323 K were positive (4.57kJ/mol), which suggested that the adsorption reaction was an endothermic process. This also suggested that in the temperature range of 25-50 °C (298-323 K), Eu(III) was soluble in water and the hydration of the outer NBF sheath was destroyed before Eu(III)was adsorbed on it, which explained why this process required energy. The amount of required energy exceeded the quantity of heat released from the cation adsorption on the adsorbent surface. This theory demonstrated that the amount of heat absorbed during the desolvation process was higher than the adsorption heat within a certain range. At high temperatures, cationic desolvation occurred easier and therefore Eu(III) was more easily adsorbed [ 39 39 ]. Under the experimental conditions in this study, the adsorption reaction was a spontaneous process because Δ G 0 was negative (−25.21, −26.21, and −27.71 kJ/mol, at 298, 308, and 323 K, respectively). As the temperature increased, the absolute value of Δ G 0 increased gradually, which indicated that temperature could accelerate the adsorption reactions. The Δ S 0 values of the adsorption process were positive, and thus, demonstrated that NBF adsorption was a spontaneous process. 4 Conclusion 1 In this study, NBF samples were prepared and characterised, and the application of NBFs for the removal of Eu(III) from aqueous solutions was studied using batch techniques. The main adsorption sites of NBFs were organic functional groups, and temperature could affect the combination of functional groups and NBF surface properties. Moreover, the pH value determined the hydrolysis of the organic functional groups, which affected the adsorption behaviour and adsorption performance of the NBFs. Furthermore, HA could improve Eu(III) adsorption on NBFs at low pH values and could inhibit its adsorption at high pH values. Cation exchange, outer-, and inner-sphere complexation were the main mechanism of the adsorption process in different pH ranges. The time required to achieve adsorption–desorption equilibrium was approximately 15 h, and the reaction could be described using the pseudo-second-order kinetic model. The maximum adsorption capacity was calculated to be 147.6 mg/g at 308 K. The Freundlich adsorption model fitted well the adsorption isotherms. Therefore, NBFs were determined to be an efficient, inexpensive, and easily available material for Eu(III)remediation from aqueous solutions. References L. Chen , X.J. Yu , Z.D. Zhao , Effect of humic acid, pH and ionic strength on the sorption of Eu(III) on red earth and its solid component . J. Radioanal. Nucl. Chem . 274 , 187 - 193 ( 2007 ). https://doi.org/10.1007/s10967-006-6814-0 https://doi.org/10.1007/s10967-006-6814-0 Effect of humic acid, pH and ionic strength on the sorption of Eu(III) on red earth and its solid component B. Ivanova , M. Spiteller , Adorption of uranium composites onto saltrock oxides-experimental and theoretical study . J. Environ. Radioactiv . 135 , 75 - 83 ( 2014 ). https://doi.org/10.1016/j.jenvrad.2014.03.019 https://doi.org/10.1016/j.jenvrad.2014.03.019 Adorption of uranium composites onto saltrock oxides-experimental and theoretical study X.X. Wang , Y.B. Sun , A. Alsaedi , et al ., Interaction mechanism of Eu(III) with MX-80 bentonite studied by batch, TRLFS and kinetic desorption techniques . Chem. Eng. J . 264 , 570 - 576 ( 2015 ). https://doi.org/10.1016/j.cej.2014.11.136 https://doi.org/10.1016/j.cej.2014.11.136 Interaction mechanism of Eu(III) with MX-80 bentonite studied by batch, TRLFS and kinetic desorption techniques K. Cheng , Y.F. Zhu , W.J. Weng , et al ., Biocompatible Eu-doped TiO2 nanodot film with in situ protein adsorption characterization property . Thin Solid Films . 584 , 9 - 12 ( 2015 ). https://doi.org/10.1016/j.tsf.2015.01.003 https://doi.org/10.1016/j.tsf.2015.01.003 Biocompatible Eu-doped TiO2 nanodot film with in situ protein adsorption characterization property H.N. Tran , S.J. You , A. Hosseini-Bandegharaei , et al ., Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review . Water Res . 120 , 88 - 116 ( 2017 ). https://doi.org/10.1016/j.watres.2017.04.014 https://doi.org/10.1016/j.watres.2017.04.014 Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review Y. Xie , C.L. Chen , X.M. Ren , et al ., Emerging natural and tailored materials for uranium-contaminated water treatment and environmental remediation . Prog. Mater. Sci . 103 , 180 - 234 ( 2019 ). https://doi.org/10.1016/j.pmatsci.2019.01.005 https://doi.org/10.1016/j.pmatsci.2019.01.005 Emerging natural and tailored materials for uranium-contaminated water treatment and environmental remediation Y.H. Wu , H.W. Pang , Y. Liu , et al ., Environmental remediation of heavy metal ions by novel-nanomaterials: A review . Environ. Pollut . 246 , 608 - 620 ( 2019 ). https://doi.org/10.1016/j.envpol.2018.12.076 https://doi.org/10.1016/j.envpol.2018.12.076 Environmental remediation of heavy metal ions by novel-nanomaterials: A review Y. Xie ,  D.D. Shao ,  X.R. Lu , et al ., Spectroscopic Investigation of Enhanced Adsorption of U(VI) and Eu(III) on Magnetic Attapulgite in Binary System . Ind. Eng. Chem. Res.   57 , 7533 - 7543 ( 2018 ). https://doi.org/10.1021/acs.iecr.8b01803 https://doi.org/10.1021/acs.iecr.8b01803 Spectroscopic Investigation of Enhanced Adsorption of U(VI) and Eu(III) on Magnetic Attapulgite in Binary System W.B. Yang , J. Li ,  Y.X. Liu , Study on plant fiber/plastic composites as the substrate of floorboard . J. Forestry Res . 16 , 245 - 246 ( 2005 ). https://doi.Org/10.1007/BF02856825 https://doi.Org/10.1007/BF02856825 Study on plant fiber/plastic composites as the substrate of floorboard J.T. Wang , Y. Zheng , A.Q. Wang , Preparation and properties of kapok fiber enhanced oil sorption resins by suspended emulsion polymerization . J. Appl. Polym. Sci . 127 , 2184 - 2191 ( 2013 ). https://doi.org/10.1002/app.37783 https://doi.org/10.1002/app.37783 Preparation and properties of kapok fiber enhanced oil sorption resins by suspended emulsion polymerization Z.H. Li , M. Kobayashi , Plantation future of bamboo in China . J. Forestry Res . 15 , 233 - 242 ( 2004 ). https://doi.org/10.1007/BF02911032 https://doi.org/10.1007/BF02911032 Plantation future of bamboo in China L. Nayak ,  S.P. Mishra , Prospect of bamboo as a renewable textile fiber, historical overview, labeling, controversies and regulation . Fash. Text . 3 , 2 - 24 ( 2016 ). https://doi.org/10.1186/s40691-015-0054-5 https://doi.org/10.1186/s40691-015-0054-5 Prospect of bamboo as a renewable textile fiber, historical overview, labeling, controversies and regulation M.F. Hamza , J.C. Roux , E. Guibal , Uranium and europium sorption on amidoxime- functionalized magnetic chitosan micro-particles . Chem. Eng. J . 344 , 124 - 137 ( 2018 ). https://doi.org/10.1016/j.cej.2018.03.029 https://doi.org/10.1016/j.cej.2018.03.029 Uranium and europium sorption on amidoxime- functionalized magnetic chitosan micro-particles K.R. Zhu ,  C.L. Chen , J. Li , et al ., Alkali-treated cellulose fibers for U(VI) separation and enrichment . J. Radioanal. Nucl. Chem . 308 , 981 - 990 ( 2016 ). https://doi.org/10.1007/s10967-015-4527-1 https://doi.org/10.1007/s10967-015-4527-1 Alkali-treated cellulose fibers for U(VI) separation and enrichment A.N. Veleshko , S.A. Kulyukhin , I.E. Veleshko , et al ., Sorption of radionuclides from solutions with composite materials based on Mikoton natural biopolymer . Radio. Chemistry 50 , 508 - 514 ( 2008 ). https://doi.org/10.1134/S1066362208050135 https://doi.org/10.1134/S1066362208050135 Sorption of radionuclides from solutions with composite materials based on Mikoton natural biopolymer W.M. Dong , X.K. Wang , J.Z. Du , et al ., Sorption and desorption characteristics of Eu(III) on red earth . J. Radioanal. Nucl. Chem . 242 , 793 - 797 ( 1999 ). https://doi.org/10.1007/BF02347397 https://doi.org/10.1007/BF02347397 Sorption and desorption characteristics of Eu(III) on red earth K. Cheng , Y.F. Zhu , W.J. Weng , et al ., Biocompatible Eu-doped TiO2 nanodot film with in situ protein adsorption characterization property . Thin Solid Films 584 , 9 - 12 ( 2015 ). https://doi.org/10.1016/j.tsf.2015.01.003 https://doi.org/10.1016/j.tsf.2015.01.003 Biocompatible Eu-doped TiO2 nanodot film with in situ protein adsorption characterization property R.X. Cheng , Q.S. Zhang , S.J. Sui , Improvement of softening treatment technology of bamboo . Wood Sci. Technol . 40 , 327 - 335 ( 2006 ). https://doi.org/10.1007/s00226-005-0034-9 https://doi.org/10.1007/s00226-005-0034-9 Improvement of softening treatment technology of bamboo Y.Z. Xu , L.M. Yang , Z. Xu , et al ., Distinguishing malignant from normal stomach tissues and its in vivo, in situ measurement in operating process using FTIR Fiber-Optic techniques . Sci. China Ser. B: Chem . 48 , 167 - 175 ( 2005 ). https://doi.org/10.1360/04yb0135 https://doi.org/10.1360/04yb0135 Distinguishing malignant from normal stomach tissues and its in vivo, in situ measurement in operating process using FTIR Fiber-Optic techniques K. Štamberg , P. BeneŠ , J. Mizera , et al ., Radiotracer study of the kinetics of complexation and decomplexation of Eu(III) with humic acid using ion exchange . J. Radioanal. Nucl. Chem . 258 , 347 - 360 ( 2003 ). https://doi.org/10.1023/A:1026246006953 https://doi.org/10.1023/A:1026246006953 Radiotracer study of the kinetics of complexation and decomplexation of Eu(III) with humic acid using ion exchange T. Yu , S.M. Liang , T. Pan , et al ., Sorption kinetic studies of U(VI) on inorganic and organic modified red earth: comparison of linear and non-linear methods . J. Radioanal. Nucl. Chem . 314 , 297 - 305 ( 2017 ). https://doi.org/10.1007/s10967-017-5343-6 https://doi.org/10.1007/s10967-017-5343-6 Sorption kinetic studies of U(VI) on inorganic and organic modified red earth: comparison of linear and non-linear methods Z.Y. Zha , D.N. Wang , W. Hong , et al ., Influence of Europium speciation on its accumulation in Brassica napus and over-expressing BnTR1 lines . J. Radioanal. Nucl. Chem . 301 , 257 - 262 ( 2014 ). https://doi.org/10.1007/s10967-014-3120-3 https://doi.org/10.1007/s10967-014-3120-3 Influence of Europium speciation on its accumulation in Brassica napus and over-expressing BnTR1 lines Z.Y. Chen , Q. Jin , Z.J. Guo , et al ., Surface complexation modeling of Eu(III) and phosphate on Na-bentonite: Binary and ternary adsorption systems . Chem. Eng. J . 256 , 61 - 68 ( 2014 ). https://dx.doi.org/10.1016/j.cej.2014.06.096 https://dx.doi.org/10.1016/j.cej.2014.06.096 Surface complexation modeling of Eu(III) and phosphate on Na-bentonite: Binary and ternary adsorption systems N. Janot , P.E. Reiller , M.F. Benedetti , Modelling Eu(III) speciation in a Eu(III)/PAHA/α-Al2O3 ternary system . Colloid. Surface. A: Physicochem. Eng. Aspects . 435 , 9 - 15 ( 2013 ). https://dx.doi.org/10.1016/j.colsurfa.2013.02.052 https://dx.doi.org/10.1016/j.colsurfa.2013.02.052 Modelling Eu(III) speciation in a Eu(III)/PAHA/α-Al2O3 ternary system S. Barnie , J. Zhang , H. Wang , et al ., The influence of pH, co-existing ions, ionic strength, and temperature on the adsorption and reduction of hexavalent chromium by undissolved humic acid . Chemosphere . 212 , 209 - 218 ( 2018 ). https://doi.org/10.1016/j.chemosphere.2018.08.067 https://doi.org/10.1016/j.chemosphere.2018.08.067 The influence of pH, co-existing ions, ionic strength, and temperature on the adsorption and reduction of hexavalent chromium by undissolved humic acid Z.J. Guo , J. Xu , K.L. Shi , et al ., Eu(III) adsorption/ desorption on Na-bentonite: Experimental and modeling studies. Colloids and Surfaces A: Physicochem . Eng. Aspects . 339 , 126 - 133 ( 2009 ). https://doi.org/10.1016/j.colsurfa.2009.02.007 https://doi.org/10.1016/j.colsurfa.2009.02.007 Eu(III) adsorption/ desorption on Na-bentonite: Experimental and modeling studies. Colloids and Surfaces A: Physicochem S.Y. Huang , H.W. Pang , L. Li , et al ., Unexpected ultrafast and high adsorption of U(VI) and Eu(III) from solution using porous Al2O3 microspheres derived from MIL-53 . Chem. Eng. J . 353 , 157 - 166 ( 2018 ). https://doi.org/10.1016/j.cej.2018.07.129 https://doi.org/10.1016/j.cej.2018.07.129 Unexpected ultrafast and high adsorption of U(VI) and Eu(III) from solution using porous Al2O3 microspheres derived from MIL-53 Z.L. Niu , T. Ohnuki , E. Simoni , et al ., Effects of dissolved and fixed humic acid on Eu(III)/ Yb(III) adsorption on aluminum hydroxide: A batch and spectroscopic study . Chem. Eng. J . 351 , 203 - 209 ( 2018 ). https://doi.org/10.1016/j.cej.2018.06.034 https://doi.org/10.1016/j.cej.2018.06.034 Effects of dissolved and fixed humic acid on Eu(III)/ Yb(III) adsorption on aluminum hydroxide: A batch and spectroscopic study K. Vadivelan , Kumar Vasanth , Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk . J. Colloid Interf. Sci . 286 , 90 - 100 ( 2005 ). https://doi.org/10.1016/j.jcis.2005.01.007 https://doi.org/10.1016/j.jcis.2005.01.007 Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk A. Baraka , Investigation of temperature effect on surface-interaction and diffusion of aqueous-solution/porous-solid adsorption systems using diffusion-binding model . J. Environ. Chem. Eng . 3 , 129 - 139 ( 2015 ). https://doi.org/10.1016/j.jece.2014.11.001 https://doi.org/10.1016/j.jece.2014.11.001 Investigation of temperature effect on surface-interaction and diffusion of aqueous-solution/porous-solid adsorption systems using diffusion-binding model C.C. Yao , T.J. Chen , A new simplified method for estimating film mass transfer and surface diffusion coefficients from batch adsorption kinetic data . Chem. Eng. J . 265 , 93 - 99 ( 2015 ). https://doi.org/10.1016/j.cej.2014.12.005 https://doi.org/10.1016/j.cej.2014.12.005 A new simplified method for estimating film mass transfer and surface diffusion coefficients from batch adsorption kinetic data S. Song , S. Zhang , S.Y. Huang , et al ., A novel multi-shelled Fe3O4@MnOx hollow microspheres for immobilizing U(VI) and Eu(III) . Chem. Eng. J . 355 , 697 - 709 ( 2019 ). https://doi.org/10.1016/j.cej.2018.08.205 https://doi.org/10.1016/j.cej.2018.08.205 A novel multi-shelled Fe3O4@MnOx hollow microspheres for immobilizing U(VI) and Eu(III) Y.Z. Hu , C.F. Zhao , L. Yin , et al ., Combining batch technique with theoretical calculation studies to analyze the highly efficient enrichment of U(VI) and Eu(III) on magnetic MnFe2O4 nanocubes . Chem. Eng. J . 349 , 347 - 357 ( 2018 ). https://doi.org/10.1016/j.cej.2018.05.070 https://doi.org/10.1016/j.cej.2018.05.070 Combining batch technique with theoretical calculation studies to analyze the highly efficient enrichment of U(VI) and Eu(III) on magnetic MnFe2O4 nanocubes R. Jain , D. Dominic , N. Jordan , et al ., Higher Cd adsorption on biogenic elemental selenium nanoparticles . Environ. Chem. Lett . 14 , 381 - 386 ( 2016 ). https://doi.org/10.1007/s10311-016-0560-8 https://doi.org/10.1007/s10311-016-0560-8 Higher Cd adsorption on biogenic elemental selenium nanoparticles P. Li , Q.H. Fan , D.Q. Pan , et al ., Effects of pH, ionic strength, temperature and humic acid on Eu(III) sorption onto iron oxides . J. Radioanal. Nucl. Chem . 289 , 757 - 764 ( 2011 ). https://doi.org/10.1007/s10967-011-1153-4 https://doi.org/10.1007/s10967-011-1153-4 Effects of pH, ionic strength, temperature and humic acid on Eu(III) sorption onto iron oxides A.A. Tolba , S. I. Mohamady , S.S. Hussin , et al ., Synthesis and characterization of poly(carboxymethyl)-cellulose forenhanced La(III) sorption . Carbohyd. Polym . 157 , 1809 - 1820 ( 2017 ). https://dx.doi.org/10.1016/j.carbpol.2016.11.064 https://dx.doi.org/10.1016/j.carbpol.2016.11.064 Synthesis and characterization of poly(carboxymethyl)-cellulose forenhanced La(III) sorption X.X. Wang , S.S. Lu , L. Chen , et al ., Efficient removal of Eu(III) from aqueous solutions using super-adsorbent of bentonite-polyacrylamide composites . J. Radioanal. Nucl. Chem . 306 , 497 - 505 ( 2015 ). https://doi.org/10.1007/s10967-015-4115-4 https://doi.org/10.1007/s10967-015-4115-4 Efficient removal of Eu(III) from aqueous solutions using super-adsorbent of bentonite-polyacrylamide composites J. Li , X.X. Wang , G.X. Zhao , et al ., Metal-organic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions . Chem. Soc. Rev . 47 , 2322 - 2356 ( 2018 ). https://doi.org/10.1039/c7cs00543a https://doi.org/10.1039/c7cs00543a Metal-organic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions Q.H. Fan , D.D. Shao , Y. Yu , et al ., Effect of pH, ionic strength, temperature and humic substances on the sorption of Ni(II) to Na-attapulgite . Chem. Eng. J . 150 , 188 - 195 ( 2009 ). https://doi.org/10.1016/j.cej.2008.12.024 https://doi.org/10.1016/j.cej.2008.12.024 Effect of pH, ionic strength, temperature and humic substances on the sorption of Ni(II) to Na-attapulgite 10.1007/s41365-019-0710-3 This study was supported by the National Natural Science Foundation of China (No. 21561001) and the Natural Science Foundation of Jiangxi Province, China (No. 20161BAB203100). 2019-03-06 2019-05-21 2019-05-26 2019-12-19 2020-01-01 2021-04-10 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2020 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-019-0710-3 http://dx.doi.org/10.1007/s41365-019-0710-3 . 2020 Nuclear Science and Techniques 1 31