Adsorption behavior of ²⁴¹Am(III) and Eu(III) by silica/polymer-based isoHex-BTP adsorbent from nitric acid solution

Adsorption behavior of 241 Am(III) and Eu(III) by silica/polymer-based iso Hex-BTP adsorbent from nitric acid solution 1 first-author 1 1 1 corresp Corresponding author, yzwei@sjtu.edu.cn Corresponding author, yzwei@sjtu.edu.cn yzwei@sjtu.edu.cn 2 2 2 School of Nuclear Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China School of Nuclear Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China China Institute of Atomic Energy, Beijing 102413, China China Institute of Atomic Energy, Beijing 102413, China The adsorption behavior of 241 Am(III) and Eu(III) by silica/polymer-based iso Hex-BTP adsorbent ( iso Hex-BTP/SiO 2 -P) was investigated by a batch experiment method. iso Hex-BTP/SiO 2 -P exhibited high affinity and selectivity for 241 Am(III) over 152 Eu(III) in 2–4 mol/dm 3 nitric acid solutions. Within the experimental contact time range of 0.5–24 h, iso Hex-BTP/SiO 2 -P showed high selectivity for 241 Am(III) compared to 152 Eu(III) in 3 mol/dm 3 nitric acid solution. However, the adsorption kinetics of 241 Am(III) and 152 Eu(III) was slow. Eu(III) adsorption followed the pseudo-second-order kinetic model, indicating chemical adsorption as the rate-limiting step of the adsorption process. And the adsorption agreed well with the Langmuir adsorption model at various temperatures. The adsorption kinetics and isotherm data indicated that the equilibrium adsorption capacity, the adsorption rate, the maximum adsorption capacity and the adsorption affinity, increased with temperature. The thermodynamic parameters, negative change in Gibbs free energy, and positive change in enthalpy and entropy, suggested that the adsorption of Eu(III) was spontaneous and endothermic process with an increase of entropy. iso Hex-BTP/SiO 2 -P Extraction chromatography High level liquid waste (HLLW) Am(III) Eu(III) I. INTRODUCTION 1 The selective partitioning of trivalent minor actinides (MA(III): Am(III) and Cm(III)) from the bulk of trivalent lanthanides (Ln(III)) present in high level liquid waste (HLLW) is important since some of the lanthanides exhibit large neutron-capture cross-sections, hence the decreased transmutation efficiency of MA in the partitioning and transmutation strategy [ 1 1 ]. Currently, several two-step partitioning processes based on liquid-liquid extraction were designed: Firstly, MA(III) and Ln(III) are extracted from HLLW using oxygen-donor ligands such a s CMPO [ 2 2 ], DMDOHEMA [ 3 3 ], TRPO [ 4 4 ] and DIDPA [ 5 5 ]; Next, Am(III) and Cm(III) are selectively separated from Ln(III). However, the second step, MA(III)-Ln(III)-separation, is an exceptionally challenging task owing to their similar oxidation states, chemical properties, ionic radii, and their disadvantageous ratio of presence in HLLW [ 6 6 ]. This separation is not feasible using common oxygen-donor ligands, but soft nitrogen- or sulphur-donor ligands can achieve sufficient selectivity for MA(III) [ 7 7 ]. Among a variety of nitrogen and sulphur donor extracting agents developed, 2,6-bis(5,6-dialkyl-1,2,4-trizain-3-yl)pyridines (known as BTPs) turn out to be able to perform this difficult separation from highly acidic solutions (HNO 3 > 1 mol/dm 3 ) with good selectivity for MA(III) over Ln(III) [ 8 8 ]. Furthermore, BTPs follow the carbon-hydrogen-oxygen-nitrogen (CHON) principle, i.e., BTPs consist entirely of C, H, O, and N atoms, which makes them fully combustible to safe gaseous products, minimizing the generation of secondary waste [ 9 9 ]. A potential process application for actual MA(III) partitioning requires that extracting agent should be chemically stable, highly soluble in organic diluent, high selectivity towards MA(III), fast kinetics and high loading capacity [ 9 9 ]. However, so far the available BTPs suffer from one or more drawbacks in liquid-liquid extraction system [ 10 10 ]. This might result in forming a third phase, multi-stage extraction procedures and disposal of large volumes of organic wastes, which requires further optimization to comply with process constrains [ 10 10 ]. Therefore, a series of BTP derivatives where C-H bonds in the α-positions of the triazinyl rings are replaced by C-C bonds have been developed to improve the properties of extracting agents [ 7 7 , 9 9 , 10 10 ]. On the other hand, these problems can be alleviated by the use of alternative methods such as extraction chromatography [ 11 11 - 14 14 11-14 ]. Compared to liquid-liquid extraction method, the advantages of the MA(III) partitioning process based on extraction chromatography are the total insolubility of adsorbent in aqueous phase, China Academy of Engineering Physics, physical degradation, reduced radioactive waste inventory, compact equipment and no disposal of toxic organic solvents as waste [ 15 15 ]. Furthermore, compared to U and Pu, the minor actinides are significantly less abundant in the spent fuel, so the scale of the separation process for minor actinides from HLLW should be considerably smaller than that of the PUREX process [ 16 16 ]. To avoid the drawbacks of the liquid-liquid extraction process using BTPs as extracting agent and take full advantage of extraction chromatography method, we have reported a direct separation process of MA(III) based on extraction chromatography, which uses a single-column packed with silica/polymer-based iso Hex-BTP adsorbent named as iso Hex-BTP/SiO 2 -P ( iso Hex-BTP: 2,6-bis(5,6-di iso hexyl)-1,2,4-triazin-3-yl)pyridine) ( Fig. 1 Fig. 1 ) [ 17 17 , 18 18 ]. The iso Hex-BTP/SiO 2 -P adsorbent, using silica/polymer composite support (SiO 2 -P) as the inert support and iso Hex-BTP as the extractant, was prepared. SiO 2 -P support contains a macroreticular styrene-divinylbenzene copolymer which is immobilized in porous silica particles with pore size of 0.6 μm and mean diameter of 60 μm. The content of extracting agent iso Hex-BTP reached up to 33.3% of the total mass of the adsorbent, so the adsorbent may overcome the limitation of the low solubility of BTPs in traditional aliphatic diluents. The other advantages of the partitioning process include its high adsorption selectivity and adsorption capacity, a minimal organic solvent, compact equipment, and the clean separation of MA(III). In the present work, effects were made to study the initial nitric acid concentration and contact time on the adsorption of 241 Am(III) and 152 Eu(III) by iso Hex-BTP/SiO 2 -P, with a batch adsorption method. The adsorption kinetics, isotherm and thermodynamics of stable nuclide Eu(III) (as a representative element of Ln(III) and a simulated element of MA(III)) by iso Hex-BTP/SiO 2 -P were also determined. II. EXPERIMENTAL 1 A. Materials 2 Trace amount of 241 Am(III) and 152 Eu(III) (1000 Bq/cm 3 each) were prepared from their stocked solutions. Nitrate of Eu(III) (Eu(NO 3 ) 3 ⋅ 6H 2 O) reagent was of commercial reagent of analytical grade. The solutions of Eu(III) were prepared by dissolving Eu(NO 3 ) 3 ⋅ 6H 2 O into nitric acid solutions. The synthesized iso Hex-BTP reagent was of 99% purity. The iso Hex-BTP/SiO 2 -P adsorbent was prepared as described in the previous studies of Wei et al . [ 19 19 , 20 20 ] by impregnating iso Hex-BTP molecules into the pores of the silica/polymer composite support (SiO 2 -P). iso Hex-BTP/SiO 2 -P contained 0.5 g of iso Hex-BTP in 1.0 g of SiO 2 -P, i.e. the content of extracting agent iso Hex-BTP was as high as 33.3% of the total mass of the adsorbent. B. Batch adsorption experiment 2 The adsorption of 241 Am(III) and 152 Eu(III) by the iso Hex-BTP/SiO 2 -P adsorbent was studied in a batch adsorption mode. For each batch of experiment, 0.1 g iso Hex-BTP/SiO 2 -P was taken in a glass vial with Teflon stopper. The 5-cm 3 solution containing 241 Am(III) and 152 Eu(III) (1000 Bq/cm 3 each) and nitric acid was added to the adsorbent. The mixture was shaken mechanically at 120 rpm for predetermined contact time at 298 K. The aqueous phase was filtrated through a membrane filter with 0.20 μm pore. The radioactivity of 241 Am or 152 Eu was measured by HPGe-γ-spectrometry (GEM70P-PLUS, Ortec). The distribution coefficient ( K d , cm 3 /g) was calculated by Eq. (1): <math display="block" id="M1"><mrow><msub><mi>K</mi><mtext>d</mtext></msub><mo>=</mo><mo stretchy="false">[</mo><mo stretchy="false">(</mo><msub><mi>A</mi><mn>0</mn></msub><mo>−</mo><msub><mi>A</mi><mtext>f</mtext></msub><mo stretchy="false">)</mo><mo>/</mo><msub><mi>A</mi><mtext>f</mtext></msub><mo stretchy="false">]</mo><mo stretchy="false">(</mo><mi>V</mi><mo>/</mo><msub><mi>W</mi><mtext>R</mtext></msub><mo stretchy="false">)</mo><mn>,</mn></mrow></math> where A 0 and A f (in Bq/cm 3 ) are radioactivity of the radionuclides in initial and final aqueous phase, respectively; V is volume of the aqueous phase (cm 3 ); and W R is weight of the iso Hex-BTP/SiO 2 -P (g). The effects of adsorption parameters including contact time (10 min–72 h), Eu(III) concentration and adsorption temperature (288±1, 298±1 and 308±1 K) on the adsorption of stable nuclide Eu(III) by iso Hex-BTP/SiO 2 -P were studied in a batch adsorption mode as described above. The concentration of Eu(III) in aqueous phase before and after adsorption was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES: Shimadzu ICPS-7510). The equilibrium adsorption capacity, q e (mmol/g), and the adsorption capacity at time t , q t (mmol/g), were obtained using Eqs. (2) and (3). <math display="block" id="M2"><mrow><msub><mi>q</mi><mtext>e</mtext></msub><mo>=</mo><mo stretchy="false">(</mo><msub><mi>C</mi><mn>0</mn></msub><mo>−</mo><msub><mi>C</mi><mi>e</mi></msub><mo stretchy="false">)</mo><mi>V</mi><mo>/</mo><mo stretchy="false">(</mo><mn>1000</mn><msub><mi>W</mi><mtext>R</mtext></msub><mo stretchy="false">)</mo><mn>,</mn></mrow></math> <math display="block" id="M3"><mrow><msub><mi>q</mi><mtext>t</mtext></msub><mo>=</mo><mo stretchy="false">(</mo><msub><mi>C</mi><mn>0</mn></msub><mo>−</mo><msub><mi>C</mi><mi>t</mi></msub><mo stretchy="false">)</mo><mi>V</mi><mo>/</mo><mo stretchy="false">(</mo><mn>1000</mn><msub><mi>W</mi><mtext>R</mtext></msub><mo stretchy="false">)</mo><mn>,</mn></mrow></math> where C 0 (mmol/dm 3 ) and C e (mmol/dm 3 ) are Eu(III) concentration in initial aqueous phase and at equilibrium, respectively; C t (mmol/dm 3 ) is Eu(III) concentration in aqueous phase at a given contact time t ; V is volume of the aqueous phase (cm 3 ); and W R is the weight of iso Hex-BTP/SiO 2 -P (g). Most of the experiments were repeated twice for better accuracy and blank experiments were performed. The experimental error was within ± 5%. III. RESULTS AND DISCUSSION 1 A. Effect of initial nitric acid concentration on adsorption of 241 Am(III) and 152 Eu(III) 2 Figure 2 Figure 2 shows the distribution coefficients of 241 Am(III) and 152 Eu(III) towards iso Hex-BTP/SiO 2 -P at nitric acid concentrations of 0.01–4 mol/dm 3 . From Fig. 2 Fig. 2 , the separation factors between 241 Am(III) and 152 Eu(III) were calculated as SF Am/Eu = 2;7;28;88;61;30 at [HNO 3 ] initial = 0.01, 0.5, 1, 2, 3 and 4 mol/dm 3 , respectively. It can be seen that the distribution coefficient values of 241 Am(III) increased with initial nitric acid concentration due to the consumption of nitrate in complexation reaction between Am(III) and iso Hex-BTP/SiO 2 -P, and reached the maximum value ( K d = 16246 cm 3 /g) at 3 mol/dm 3 nitric acid, and then decreased slightly at > 3 mol/dm 3 nitric acid due to the competition reaction between Am(III) and HNO 3 with iso Hex-BTP/SiO 2 -P [ 8 8 ]. High distribution coefficients ( > 4869 cm 3 /g) of 241 Am(III) indicated 241 Am(III) quantitative adsorption from 2–4 mol/dm 3 nitric acid medium. Meanwhile, 152 Eu(III) showed weak adsorption within the experimental nitric acid concentration range of 0.01–4 mol/dm 3 . SF Am/Eu was over 60 in 2–3 mol/dm 3 nitric acid solutions. From Fig. 2 Fig. 2 , the adsorbed Am(III) can be stripped into dilute nitric acid ( < 0.1 mol/dm 3 ) or distilled water because Am(III) showed almost no adsorption at ࣘ 1 mol/dm 3 nitric acid and hence adsorption of Am(III) is readily reversible by changing the nitric acid concentration. B. Effect of contac time on adsorption of 241 Am(III) and 152 Eu(III) 2 The distribution coefficient values of 241 Am(III) and 152 Eu(III) from 3 mol/dm 3 nitric acid as a function of contact time are presented in Fig. 3 Fig. 3 . The separation factors between 241 Am(III) and 152 Eu(III) calculated form Fig. 3 Fig. 3 are SF Am/Eu = 55;42;31;26;56;74 at 0.5, 1, 3, 6, 12 and 24 h, respectively. The distribution coefficients of 241 Am(III) by iso Hex-BTP/SiO 2 -P increased rapidly to 6535 cm 3 /g within the first 3 h of contact and the subsequently slow adsorption of 241 Am(III) followed which continued for a relatively long period time until adsorption equilibrium was attained. The distribution coefficient of 241 Am(III) reached up to 19413 cm 3 /g at 24 hours of contact, but the equilibrium state was not still attained. So the kinetics is considerably slow. Contrastively, K d values of 152 Eu(III) were always low and below 504 cm 3 /g within the experimental contact time range. SF Am/Eu was fairly high ( > 50) within 30 min and over 12 hours of contact. From the results above, iso Hex-BTP/SiO 2 -P exhibited a good selectivity and affinity for 241 Am(III) over 152 Eu(III) with relatively high separation factors at nitric acid concentration of 3 mol/dm 3 within the experimental contact time of 0.5–24 h. Prior to the column studies, it was required to study the adsorption kinetics, isotherm and thermodynamics to design MA(III) separation system by extraction chromatography method if suitable for application. Since the adsorption behavior of Eu(III) by BTP/SiO 2 -P adsorbent is similar to that of MA(III) owing to their similar coordination chemistry [ 21 21 ] and Eu(III) is a typical fission lanthanide present in HLLW, as a representative element of Ln(III) and a simulated element of MA(III), the adsorption of stable nuclide Eu(III) by iso Hex-BTP/SiO 2 -P was studied to evaluate the adsorption kinetics, isotherm and thermodynamics in this work. C. Adsorption kinetics of Eu(III) 2 To obtain the information on how the amount of adsorbed Eu(III) changes with contact time and about the process time required to achieve equilibrium between the aqueous and the adsorbent, the adsorption kinetics of Eu(III) by iso Hex-BTP/SiO 2 -P was investigated. In this series of experiments, the initial concentration of Eu(III) was approximately 6 mmol/dm 3 and the initial nitric acid was 3 mol/dm 3 . The adsorption capacity of Eu(III) by iso Hex-BTP/SiO 2 -P against contact time at each temperature is given in Fig. 4 Fig. 4 . It was observed that the adsorption process occurred in two steps. The first step involved rapid Eu(III) uptake within the first 3 h of contact that was followed by a steady stage which finally reached an apparent plateau in 12 h at 298 K. Over 43%, 82% and 93% of the equilibrium adsorption capacity of Eu(III) occurred within the first 3 h at 288, 298 and 308 K, respectively. The sudden increase of q t at the very beginning of the process is attributed to an abundant availability of active sites on internal and external surface area of iso Hex-BTP/SiO 2 -P. With the progressive occupancy of these sites by Eu(III), the process comes into a period of slower adsorption, during which the less accessible sites can be occupied by Eu(III). The equilibrium adsorption capacity ( q e,exp ) of Eu(III) was 0.15 mmol/g at 288 K and 0.16 mmol/g at 308 K ( Fig. 4 Fig. 4 and Table 1 Table 1 ). 1 2 T (K) 3 1 Pseudo-first-order kinetic model 4 1 Pseudo-second-order kinetic model 1 2 Experimental q e,exp (mmol/g) 1 1 q e,calc (mmol/g) 1 1 k 1 (h -1 ) 1 1 R 2 1 1 q e,calc (mmol/g) 1 1 k 2 (g/(mmol h)) 1 1 h (mmol/(g h)) 1 1 R 2 1 1 288 1 1 0.11 1 1 0.08 1 1 0.936 1 1 0.14 1 1 3.56 1 1 0.07 1 1 0.989 1 1 0.15 1 1 298 1 1 0.06 1 1 0.13 1 1 0.574 1 1 0.16 1 1 13.16 1 1 0.33 1 1 1.000 1 1 0.16 1 1 308 1 1 0.04 1 1 0.19 1 1 0.915 1 1 0.16 1 1 25.57 1 1 0.70 1 1 1.000 1 1 0.16 To analyzed the adsorption kinetics of Eu(IIII) by iso Hex-BTP/SiO 2 -P, the pseudo-first-order and the pseudo-second-order kinetic models were applied to the experimental data. The linear form of the pseudo-first-order kinetic model can be written as Eq. (4) [ 22 22 , 23 23 ] <math display="block" id="M4"><mrow><mi>lg</mi><mo stretchy="false">(</mo><msub><mi>q</mi><mtext>e</mtext></msub><mo>−</mo><msub><mi>q</mi><mtext>t</mtext></msub><mo stretchy="false">)</mo><mo>=</mo><mi>lg</mi><msub><mi>q</mi><mtext>e</mtext></msub><mo>−</mo><msub><mi>k</mi><mn>1</mn></msub><mi>t</mi><mn>,</mn></mrow></math> where q e and q t (in mmol/g) are adsorption capacity of the metal ions at equilibrium and time ( t ), respectively; and k 1 (h -1 ) is the adsorption rate constant. The pseudo-first-order kinetic plots for Eq. (4) were made for Eu(III) adsorption by iso Hex-BTP/SiO 2 -P at different temperatures ( Fig. 5 Fig. 5 ). The values of k 1 , the calculated equilibrium adsorption capacity ( q e,calc ) and the correlation coefficients ( R 2 ) at different temperatures were calculated ( Table 1 Table 1 ). R 2 values for this model were fairly low ( < 0.936), and q e,calc decreased with increasing temperature from 288 K to 308 K. The results conflicted with the experimental phenomena, so the experimental kinetic data could not fit to the pseudo-first-order kinetic model. The pseudo-first-order model fits experimental data well for an initial period of the first reaction step, but this model could not provide the best correlation for chemical adsorption process over long periods [ 23 23 ]. The experimental data were also applied to the pseudo-second-order kinetic model. The linear form of the model can be expressed as Eq. (5) [ 23 23 ] <math display="block" id="M5"><mrow><mi>t</mi><mo>/</mo><msub><mi>q</mi><mtext>t</mtext></msub><mo>=</mo><mn>1</mn><mo>/</mo><mrow><mo>(</mo><mrow><msub><mi>k</mi><mn>2</mn></msub><msubsup><mi>q</mi><mtext>e</mtext><mn>2</mn></msubsup></mrow><mo>)</mo></mrow><mo>+</mo><mi>t</mi><mo>/</mo><msub><mi>q</mi><mtext>e</mtext></msub><mn>,</mn></mrow></math> where q t and q e (in mmol/g) are adsorption capacity of the mental ions at time t and equilibrium, respectively; k 2 (g/(mmol h)) is the pseudo-second-order rate constant. The kinetic data of t / q t versus t for Eu(III) adsorption at different temperatures are plotted in Fig. 6 Fig. 6 . The kinetic parameters including k 2 , the calculated equilibrium adsorption capacity ( q e,calc ), h ( <math id="M6"><mrow><mi>h</mi><mo>=</mo><msub><mi>k</mi><mn>2</mn></msub><msubsup><mi>q</mi><mtext>e</mtext><mn>2</mn></msubsup></mrow></math> , initial adsorption rate) and R 2 (correlation coefficient) at different temperatures were calculated from the plots, as listed in Table 1 Table 1 . From Table 1 Table 1 , the correlation coefficient of the pseudo-second-order kinetic model ( > 0.989) was much higher than that of the pseudo-first-order kinetic model ( < 0.936) at each temperature. Since chemisorption processes show a good compliance with the pseudo-second-order kinetic model and this model is more likely to predict the kinetic behavior of adsorption with chemical reaction being the rate-controlling step [ 23 23 ], it can be inferred that the adsorption of Eu(III) occurred by chemisorption and the chelation reaction between Eu(III) and iso Hex-BTP /SiO 2 -P is the rate-controlling step of the adsorption process [ 8 8 , 23 23 ]. From 288 K to 308 K, the calculated equilibrium adsorption capacities increased slightly ( q e, calc = 0.14 and 0.16 mmol/g), whereas the rate constant k 2 and initial adsorption rate h increased dramatically from 3.56 g/(mmol h) to 25.57 g/(mmol h) and from 0.07 mmol/(g h) to 0.70 mmol/(g h), respectively. The results were consistent with the experimental phenomena. This may be due to the increase in chemisorption rate as the rate-determining step and acceleration of diffusibility of Eu(III) in solution at higher temperatures. D. Adsorption isotherms of Eu(III) 2 The adsorption capacity of Eu(III) is determined as a function of Eu(III) concentration at a constant temperature that could be explained in adsorption isotherms. The adsorption isotherm of Eu(III) by iso Hex-BTP/SiO 2 -P was obtained at 3 mol/dm 3 nitric acid, contact time of 72 h (to obtain the complete equilibrium state) by changing the initial concentration of Eu(III) within the range of 2–6 mmol/dm 3 , as shown in Fig. 7 Fig. 7 . The adsorption capacity of Eu(III) increased with the equilibrium concentration of Eu(III) in aqueous phase at different temperatures. Since the total existing adsorption sites of iso Hex-BTP/SiO 2 -P were confined, adsorption finally reached an apparent plateau and saturation adsorption at higher concentration of Eu(III). The maximum adsorption capacity ( q m,exp ) of Eu(III) increased slightly by raising the temperature ( Table 2 Table 2 ). 1 2 T (K) 3 1 Freundlich isotherm parameters 4 1 Langmiur isotherm parameters 1 2 Experimental q m,exp (mmol/g) 1 1 n F 1 1 K F (mmol/g) 1 1 R 2 1 1 q m,calc (mmol/g) 1 1 K L (dm 3 /mmol) 1 1 R L 1 1 R 2 1 1 288 1 1 4.14 1 1 0.13 1 1 0.850 1 1 0.16 1 1 4.50 1 1 0.10–0.04 1 1 0.998 1 1 0.15 1 1 298 1 1 4.22 1 1 0.14 1 1 0.638 1 1 0.17 1 1 5.89 1 1 0.08–0.03 1 1 0.992 1 1 0.16 1 1 308 1 1 7.28 1 1 0.15 1 1 0.566 1 1 0.17 1 1 7.73 1 1 0.07–0.02 1 1 0.996 1 1 0.16 The equilibrium relationship between Eu(III) concentration in the liquid phase and the concentration in iso Hex-BTP/SiO 2 -P at individual temperature was analyzed by the Langmuir and Freundlich isotherm models [ 22 22 ]. The Freundlich equation is used to describe the adsorption of an adsorbate on a heterogeneous surface of an adsorbent. The logarithmic linear form of the model is given as Eq. (6) [ 23 23 ] <math display="block" id="M7"><mrow><mi>lg</mi><msub><mi>q</mi><mtext>e</mtext></msub><mo>=</mo><mi>lg</mi><msub><mi>K</mi><mtext>F</mtext></msub><mo>+</mo><mo stretchy="false">(</mo><mi>lg</mi><msub><mi>C</mi><mtext>e</mtext></msub><mo stretchy="false">)</mo><mo>/</mo><mi>n</mi><mn>,</mn></mrow></math> where K F and n are the Freundlich constants which relate to the adsorption capacity and the adsorption intensity, respectively. The plots of lg q e vs. lg C e for Eu(III) adsorption by iso Hex-BTP/SiO 2 -P are depicted in Fig. 8 Fig. 8 . The n and K F constants calculated from the slop and intercept of the lines at different temperatures are listed in Table 2 Table 2 . The Langmuir model which is widely used for modeling adsorption data at equilibrium is valid for monolayer adsorption on a surface. Mathematically, the model may be represented in the linear form given in Eq. (7) [ 23 23 ] <math display="block" id="M8"><mrow><msub><mi>C</mi><mtext>e</mtext></msub><mo>/</mo><msub><mi>q</mi><mtext>e</mtext></msub><mo>=</mo><mn>1</mn><mo>/</mo><mo stretchy="false">(</mo><msub><mi>K</mi><mtext>L</mtext></msub><msub><mi>q</mi><mtext>m</mtext></msub><mo stretchy="false">)</mo><mo>+</mo><msub><mi>C</mi><mtext>e</mtext></msub><mo>/</mo><msub><mi>q</mi><mtext>m</mtext></msub><mn>,</mn></mrow></math> where q e (mmol/g) is the equilibrium amount of an adsorbate adsorbed per specific amount of adsorbent, C e (mmol/dm 3 ) is the equilibrium concentration of an adsorbate in solution, q m (mmol/g) is the maximum adsorption capacity of metal ions required to form a monolayer onto an adsorbent surface, and K L (dm 3 /mmol) is Langmuir adsorption constant related to the adsorption affinity. The fundamental characteristic of a Langmuir isotherm parameter ( R L ) can be expressed in terms of a dimensionless separation factor or an equilibrium parameter, which is defined by the following equation [ 22 22 ]: <math display="block" id="M9"><mrow><msub><mi>R</mi><mtext>L</mtext></msub><mo>=</mo><mn>1</mn><mo>/</mo><mo stretchy="false">(</mo><mn>1</mn><mo>+</mo><msub><mi>K</mi><mtext>L</mtext></msub><msub><mi>C</mi><mn>0</mn></msub><mo stretchy="false">)</mo><mn>,</mn></mrow></math> where C 0 (mmol/dm 3 ) is the initial concentration of an adsorbate. According to the value of R L , the isotherm shape may be interpreted as follows: • R L > 1: unfavorable adsorption, • R L =1: linear adsorption, • 0 < R L < 1: favorable adsorption, • R L =0: irreversible adsorption [ 22 22 ]. A plot of C e / q e versus C e would result in a straight line with a slope (1/ q m ) and interpret of (1/ K L q m ) as given in Fig. 9 Fig. 9 . Langmuir parameters for fitting adsorption data at various temperatures are listed in Table 2 Table 2 . The adsorption data was much better described with the Langmuir isotherm model ( R 2 ≥ 0.992) over the whole concentration range at various temperatures compared to the Freundlich isotherm model ( R 2 ࣘ 0.850) based on the correlation coefficient values. The better fit to the Langmuir model can be explained the monolayer adsorption of Eu(III) by iso Hex-BTP/SiO 2 -P and each adsorptive site can be occupied only once in a one-on-one manner [ 22 22 ]. The maximum adsorption capacity ( q m,calc and q m,exp ) increase slightly with temperature. This may be attributed to the utilization of all available active sites for adsorption at higher temperatures. The adsorption constant K L increased by raising the temperature, indicating the adsorption affinity increasing. The value of R L (0 < R L < 1) reveals that the adsorption of Eu(III) by iso Hex-BTP/SiO 2 -P is favorable. E. Adsorption thermodynamics of Eu(III) 2 In order to explain the increase in adsorption with temperature (as described in the results of adsorption kinetics and isotherms), the thermodynamic parameters, i.e. change in enthalpy (Δ H ), entropy (Δ S ) and Gibbs free energy (Δ G ) associated to the adsorption process were determined using the following equations [ 23 23 , 24 24 ]: <math display="block" id="M10"><mrow><mi>ln</mi><msub><mi>K</mi><mtext>d</mtext></msub><mo>=</mo><mi>Δ</mi><mi>S</mi><mo>/</mo><mi>R</mi><mo>−</mo><mi>Δ</mi><mi>H</mi><mo>/</mo><mo stretchy="false">(</mo><mi>R</mi><mi>T</mi><mo stretchy="false">)</mo><mn>,</mn></mrow></math> <math display="block" id="M11"><mrow><mi>Δ</mi><mi>G</mi><mo>=</mo><mi>Δ</mi><mi>H</mi><mo>−</mo><mi>T</mi><mi>Δ</mi><mi>S</mi><mn>,</mn></mrow></math> where K d is the distribution coefficient, T (K) is temperature and R (8.13 J/(mol K)) is the universal gas constant. Figure 10 Figure 10 shows the relationship between 1/ T and ln K d . The calculated values are Δ H =5621 J/mol; Δ S = 54 J/(mol K); and Δ G = -9931, -10471 and -11011 J/mol at 288, 298 and 308 K, respectively. The negative Δ G value indicates that the process is spontaneous and feasible with high preference for Eu(III) by iso Hex-BTP/SiO 2 -P. The increase in negative value of Δ G from -9931 J/mol at 288 K to -11011 J/mol at 308 K implies that adsorption tendency of Eu(III) by iso Hex-BTP/SiO 2 -P increased as expected at higher temperatures. Δ H was positive, showing that the adsorption is endothermic, and heat was consumed to transfer Eu(III) from aqueous phase onto iso Hex-BTP/SiO 2 -P and coordinate between Eu(III) and iso Hex-BTP/SiO 2 -P. The positive value of Δ S suggests the increase in the degree of randomness at the solid-solution interface mostly encountered in Eu(III) binding due to the release of water molecules of the hydration sphere during the fixation of Eu(III) on iso Hex-BTP/SiO 2 -P surface [ 22 22 , 25 25 ]. Furthermore, positive Δ S value indicates that the adsorption process is probably irreversible and favored complexation adsorption. Although Δ H value was positive, Δ G value at each temperature was negative, indicating that the contribution of entropic term makes the free adsorption energy negative. These results were in agreement with the results obtained from the pseudo-second-order kinetic model and Langmuir isotherm model. IV. CONCLUSION 1 In this study, batch adsorption experiments showed that iso Hex-BTP/SiO 2 -P exhibited remarkable selectivity towards 241 Am(III) compared to 152 Eu(III) at nitric acid concentration of 2–4 mol/dm 3 . Within the experimental contact time range of 0.5–24 h, iso Hex-BTP/SiO 2 -P showed the relatively high selectivity for 241 Am(III) over 152 Eu(III) in 3 mol/dm 3 nitric acid. Various factors such as contact time, Eu(III) concentration and temperature had impacts on adsorption of stable nuclide Eu(III) (as a representative element of Ln(III) and a simulated element of MA(III)) by iso Hex-BTP/SiO 2 -P. The obtained adsorption data depending on contact time provided much better fitting for the pseudo-second-order kinetic equation with high correlation coefficient ( R 2 ≥ 0.989) than the pseudo-first-order kinetic model, indicating that Eu(III) adsorption by iso Hex-BTP/SiO 2 -P occurred by chemisorption mechanism as a rate-controlling step in the adsorption process. However, the adsorption kinetics of Am(III) and Eu(III) by iso Hex-BTP/SiO 2 -P is slow. Furthermore, the equilibrium adsorption capacity initial adsorption rate and adsorption rate constant increased with temperature from 288 K to 308 K. The experimental adsorption isotherm fitted better to the Langmuir isotherm model than the Freundlich isotherm model at various temperatures. 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DOI: 10.1007/s13762-012-0127-6 http://doi.org/10.1007/s13762-012-0127-6 Removal of heavy metal ions from aqueous solutions with multi-walled carbon nanotubes: Kinetic and thermodynamic studies 10.13538/j.1001-8042/nst.26.S10301 Supported by National Natural Science Foundation of China (Nos. 11305102, 91126006, 21261140335 and 91226111) and Doctoral Fund of Ministry of Education of China (No. 20130073110046) 2014-05-06 2014-07-19 2014-12-20 2015-02-20 2021-04-11 2015 Nuclear Science and Techniques 1 26