Cu-BTC derived CuO and CuO/Cu2O composite: an efficient adsorption material to iodide ions

The copper benzene tricarboxylic acid (Cu-BTC) cannot be used as an adsorbent in water due to hydrophilicity. However, the calcination process can destroy the benzene ring structure to lose hydrophilicity and retains carbon structure skeleton. The CuO composite and CuO/Cu2O composite based on cubical Cu-BTC (C-Cu-BTC) and dodecahedral Cu-BTC (D-Cu-BTC) are successfully manufactured to absorb radioactive iodine ion from water. Before and after calcination, the SEM and XRD were used to characterize the changes of morphology and material structure. The adsorption experiment for iodine ion showed that their saturated adsorption capacities can reach 28.64 mg g−1 (for C–CuO), 49.63 mg g−1 (for D-CuO), 49.84 mg g−1 (for C–CuO/Cu2O) and 91.91 mg g−1 (for D-CuO/Cu2O), respectively. The iodine ion adsorption of adsorbent is an exothermic reaction as shown thermodynamic curves. Through results of adsorption kinetics it is proved that the iodide ion adsorption of CuO composite is physical adsorption and the iodide ion adsorption of CuO/Cu2O composite is chemical adsorption. Compared with CuO composite, CuO/Cu2O composite had better absorption capacity for iodide ions. Furthermore, the interference of common ion on iodide absorption has also been studied. The different types of ion, such as Cl−, SO4 2− and CO3 2−, have effects on the iodine ions absorption capacity for two types of adsorbent. These ions have a slightly effect on iodine ions adsorption of CuO composite. However, these ions have greater influences on iodine ions absorption capacity of the CuO/Cu2O composite. The biggest influence is CO3 2−, and the CO3 2− reduces the adsorption capacity by 44% iodine ion absorption capacity for the CuO/Cu2O composite.


Introduction
The spent fuel disposal and nuclear accident management problems have been widely concerned [1,2]. Radioactive iodine, which has long half-life (for 129 I) and huge emissions (for 131 I), is one of the inevitable radionuclide of nuclear fission [3]. It will cause pollution and seriously endanger the ecological environment. It can enter the human body through food and water and damage to health. Accordingly, it is necessary to remove the radioactive iodine [4]. For iodine element, I − is the dominant species in water media. For instance, leaked radioactive iodine was mainly present as I − in the Fukushima nuclear accident. In addition, radioactive iodide also is released from radiopharmaceutical production facility due to the application of cancer therapy [5]. Therefore, it is of great significance to study the effective removal of radioactive iodine. However it is still a serious challenge to find adsorption materials with high iodine ion removal efficiency [6].
In recent years, some methods actually have been applied to absorb the iodine ion, such as bio-remediation, reverse osmosis, ion exchange and adsorption [7,8]. Due to the high adsorption capacity, simple operation and good economic, the adsorption method has become the most concerned method in the adsorption application [9][10][11]. The material of adsorsorbent is a key component of the adsorption method. However many kinds of Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. adsorsorbent do not have sufficient advantages for iodide ion absorption. Several kinds of materials have been extensively studied as possible adsorbents, such as silver-containing material [12,13], metallic oxide [14] and metal-organic frameworks (MOFs) [15].
Ag 2 O-T3NT was prepared by Yang's group for adsorption of radioactive iodine ion and excellent results have been achieved. However the high cost of silver-containing materials is not suitable for industrial applications [16]. Ping Mao reported the synthesis of Cu/Cu 2 O hydrides by hydrothermal method and used to remove iodine ion, and the results show that the Cu 2 O greatly enhanced the adsorption of iodide. The Cu 2 O greatly improved the adsorption capacity of iodine anions from 0.02 mmol g −1 to 0.18 mmol g −1 [17]. The MOFs are most potential used for adsorption materials for radioactive iodine ions due to high specific area. The MOFs can be made many kinds of metals metal ions and organic reagents. The zeolitic imidazolate framework-8 (ZIF-8) had been prepared to capture radioactive iodine. The Jiuyu Chen synthetized nano Ag 2 O-Ag 2 O 3 /ZIF-8 composite for efficient capture iodine ion and the adsorption isotherm demonstrated the maximum adsorption capacity was 232.12 mg g −1 for iodine ion [18]. A novel Fe 3 O 4 @ZIF-8 core-shell microsphere is synthesized by Tong Zhang [19]. The collection after adsorption is very simple due to great magnetic properties.
In addition to the ZIF-8, Cu-BTC known as HKUST-1 is possible to be a good adsorption material. The Cu-BTC was first successfully synthesized by the Chui's research group in 1999 [20]. It has a 3D structure, open metal sites and high specific surface area. Currently, more and more kinds of Cu-BTC have been studied. The adsorption of Cu-BTC in water was reported by Juan Manuel Castillo in 2008 [21]. They found that the water is preferentially absorbed into the hydrophilic group at low relative pressure. This effect would cause the loss about 50% of surface area. That is similar to that of the micro-porous zeolite. In order to improve adsorption properties in water, Gutiérrez-Sevillano used molecular simulations to prove that Cu-BTC can be selectively enclosed by acetone or dimethyl [22].
Herein, a novel adsorbent was successfully prepared to adsorb iodine ion efficiently from water and the precursor of adsorbent is the Cu-BTC. Because iodide ions cannot be absorbed in water by Cu-BTC, to remove the hydrophilic groups of the Cu-BTC, the CuO composite and CuO/Cu 2 O composite are prepared as absorbent in water. The CuO composite and CuO/Cu 2 O composite are obtained by calcination of Cu-BTC. The Cu-BTC has good thermal stability and there is no sign of damage to the crystal structure in the process. The surface topography, crystallized phases, and chemical bond of adsorbents were characterized. In addition, the adsorbents were applied to study the adsorption kinetics and thermodynamic of iodine ion. The effect of competitive ions on iodine ion capture was also examined.

Chemicals and material
The chemicals purchased commercially includes copper nitrate trihydrate (Cu(NO 3 ) 2 •3H 2 O), 1,3,5benzenetricarboxylic acid (C 9 H 6 O 6 ), N-butyl alcohol (C 4 H 10 O), ethanol (C 2 H 6 O), sodium hydroxide (NaOH) and the lauric acid(C 12 H 24 O 2 ). These chemicals are used to preparation Cu-BTC, CuO composite and CuO/Cu 2 O composite. All reagents are of analytical grade, which are used without further purification. The deionized water and sodium iodide were used to prepare the iodide solution.

Synthesis of CuO composite and CuO/Cu 2 O composite
The Cu-BTC is manufactured based on hydrothermal method [23], as shown figure 1. Firstly, 164 mg Cu(NO 3 ) 2 •3H 2 O is dissolved in 40 ml C 4 H 10 O to prepare mixed solution. And then 80 mg C 9 H 6 O 6 and 14 g C 12 H 24 O 2 are added to the mixed solution and are stir until completely dissolved by ultrasound mixer.
The mixture is placed in a 100 ml of reactor. Then the reactor is moved to the drying incubator in 180°C for 3 h. When the reactor is cooled to room temperature, the mixture is centrifuged, and the blue solid material is achieved.
Finally, after washing with methanol, the upper liquid is removed by centrifugation and the solid material is dried in vacuum for 24 h to obtain cube Cu-BTC, recorded as C-Cu-BTC. The preparation method of dodecahedral Cu-BTC is essentially the same as that of cube Cu-BTC, except for the different amount of C 12 H 24 O 2 added. In manufacturing process of dodecahedral Cu-BTC, the amount of C 12 H 24 O 2 is 7.61g. The dodecahedral Cu-BTC is recorded as D-Cu-BTC.
The 250 mg C-Cu-BTC is tiled on the bottom of crucible and calcined in a tube furnace. The calcination condition is in nitrogen atmosphere at 320°C for 25 min. And then stop nitrogen input, open the tube furnace, oxidize the material in air for 15 min, then cool in air to room temperature. The C-CuO composite are obtained. In preparation process of D-CuO composite, the calcination temperature is 340°C, and other process parameters are the same as the C-CuO composite. The preparation method of CuO/Cu 2 O composite also is simple. the 250 mg Cu-BTC sample is calcined in a tube furnace at 320°C for 40 min. Nitrogen is required throughout the calcination process, and then the sample is cooled in nitrogen to room temperature. The C-CuO/Cu 2 O composite are successfully prepared. However, for the preparation of D-CuO/Cu 2 O composite, the calcination temperature is 340°C, and other steps are can be performed as C-CuO/Cu 2 O composite.

Characterization
The micro-morphology was displayed by scanning electron microscopy (SEM, FEI Quanta 250F, USA). The crystalline phase and crystal structure characterized by x-ray diffraction (XRD) using a Bruker D8 Advance x-ray diffraction with Cu Kα radiation at 40 kV( tube voltage) and 30 mA (tube current). Fourier-transform infra-red spectra were conducted on the spectrometer meter (FT-IR, Bruker Tensor 37) by using potassium bromide disc technique. Thermo gravimetric analysis (TG) was used to thermal stability with a heating rate of 10°C min −1 , temperature measurement range of 50-1000°C, nitrogen flow rate of 30 ml min −1 . The chemical composition and chemical states were tested by x-ray photoelectron spectroscopy ESCALABMK II electron spectrometer.

Adsorption experiment
In order to determine the iodide ion adsorption properties of CuO composite and CuO/Cu 2 O composite, the capture experiment of iodine ion is absorption of solution iodine. Considering the detriment of radioactive iodine, the iodine ion adsorption studies were conducted with non-radioactive 127 I as alternative.
Thermodynamic experiment of absorbent for I − was conducted with different initial I − concentrations (30, 60, 90, 120, 150, 180, 210, 240 mg/l). The mixture was fully mixed for 6 h under the temperature of 298, 308, 318, 328 K respectively. The solid was filtered out by 0.22 μm PES filter. In order to know the concentrations, the iodine ion solution was measured by using ultraviolet spectrophotometer. The influence of interfering ions on iodine ion uptake capacity was also investigated by using Cl − , CO 3 2and SO 4 2− and the interfering anions to iodine ion molar ratio is 100:1. The iodine ion concentration was 210 mg l −1 and the solid-liquid mixture was fully mixed for 6 h. Figure 2 is the TGA curves of two kinds of Cu-BTC morphologies in nitrogen. It is well depicted that the C-Cu-BTC has a similar thermal stability to the D-Cu-BTC. There are two obvious weightlessness processes in the TGA curves. The first weightlessness processes occurred at 50-100°C, mainly the water attached to the surface of the material was evaporated. The second weightlessness processes happened at 310-400°C, the primary reason is that the groups of Cu-BTC begin to gradually decompose and carbon-containing structures are retained. If the temperature continued to increase, the carbon structure would be gradually resolved. The initial decomposition temperature of the material is used as the calcination temperature, in order to ensure the integrity of the material structure.

Characterization
The morphology of the Cu-BTC and the CuO composite and CuO/Cu 2 O composite were displayed using the SEM. As presented in figure 3, The C-Cu-BTC and D-Cu-BTC have some common properties, namely, smooth surface, well-defined shape structure and no obvious agglomeration. Compared to C-Cu-BTC, the particle size of D-Cu-BTC is relatively larger. However the shapes of the two types of Cu-BTC are completely different. The SEM pictures of the CuO composite were shown in figures 3(c) and (d). After calcination, the   surface of CuO composite became extremely rough. The samples still keep clear shape, no overall melting and reunion, and more obvious holes appeared on the surface of the material. The C-CuO composite was contracted slightly, but the overall structure remains intact. The SEM morphology of the CuO/Cu 2 O composite was shown in figures 3(e) and (f), the material obtained by calcination in the N 2 and main body structure retained the completely. Compared with calcination in the air, the surface of C-CuO/Cu 2 O composite did not collapse, but the holes on the surface became larger. The morphology of D-CuO/Cu 2 O composite is different, the edge became raised and the deformation occurs on the surface, but the shape of the composite remains clear, and there is no particle melting each other. In summary, calcination process decomposed the material, but the structure is still relatively complete.
The XRD pattern of two types of Cu-BTC were shown in figure 4(a), and there are four characteristic peaks at 6.7°, 9.5°, 11.6°, and 13.4°, corresponding to the (200), (220), (222), and (400) crystal surfaces of the material respectively, which is consistent with the reported results. The crystal surface types of the two types of Cu-BTC are the same, but the proportion of the crystal surface is different. Compared with C-Cu-BTC, D-Cu-BTC contains higher proportion of (220) crystal surfaces. The (220) crystal surface has more number of unsaturated Cu atoms per unit area than the (222) crystal surface, therefore the (220) crystal surface has higher adsorption capacity [24]. This means the D-Cu-BTC has greater potential in adsorption properties.
As shown in figure 4(b), the XRD results of CuO composite proved that the crystal surface types of the two types of Cu-BTC are the same. The curves appeared four characteristic peaks at 35.5°, 38.7°, 48.6°and 5.7°, corresponding to (002), (111), (−202) and (202) crystal surfaces of CuO, respectively [25]. Same as figure 4(a), the proportion of the crystal surface for two types of CuO composite is different. Different types of crystal surfaces have different atomic densities, electronic structure, chemical bonds and activity [26]. This indicates that the adsorption capacity of two types of CuO composite for iodine ion is different. The CuO composite contained tiny amounts of CuO 2 , the reason was that the material oxidation was not complete.
The  As shown in figure 5(b), the characteristic peaks appeared about at 1500 cm −1 and 1380 cm −1 , the peak was attributed to the vibration of the aromatic ring and the C-O [28]. Before calcination, these peaks appeared at 1448 cm −1 and 1374 cm −1 [29]. Because calcination accelerates the vibration frequency of C-O bonds and aromatic ring, these characteristic vibration bands shifts towards the long-wave. However, it can be observed that the characteristic peaks of the samples are weakened significantly after calcination. The results mean that the sample structures have been broken down after calcination. The peak of the combine Cu with the O-C=O group appeared at 624 cm −1 and 750 cm −1 , and these peaks are located at 730 cm −1 and 761 cm −1 before the calcination [30]. The above results indicate that the numerous groups were destroyed, and the main structure was retained. The material structures of the two shapes are similar, matching the previous XRD results.
The FTIR spectrums of CuO/Cu 2 O composite were shown in figure 5(c), the characteristic peaks of Cu-BTC were located at 1646 cm −1 , 1448 cm −1 and 761 cm −1 . The characteristic peaks can also be observed in the FTIR spectrums of CuO/Cu 2 O composite. This indicated that the calcined material still retained some organic skeleton. The absorption peaks presented at 3370 cm −1 and 3220 cm −1 , these attributed to O-H bond vibration. That proves that the sample produced hydroxyl group after nitrogen calcination. The absorption peaks incinerated in nitrogen are more than these in air atmosphere for the organic skeleton, as shown between 500 cm −1 and 1600 cm −1 [31]. That means the more skeleton can be retained in the nitrogen atmosphere. The absorption peak at 524 cm −1 is corresponds to Cu-O vibration, that means the calcination products are copper oxide and cuprous oxide.

Iodine absorption study
As shown in figure 6, compared with high concentration, the absorption efficiency of low concentration is higher at the same temperature. The main reason is that the adsorbent can provide adsorption active site is limited. The more adsorption points, the stronger the adsorption driving force. The adsorption driving force affects adsorption capacity. When the solution rises a degree of concentration and the number of adsorption active site is relatively small, there is no the driving force. Therefore the adsorption capacity does not change. Cu-BTC has no adsorption capacity for iodide ions, the iodine absorption experiment confirms that calcination can destroy material hydrophily, and change some skeleton structure and produce the active site for adsorption. The adsorption isotherm data of CuO composite and CuO/Cu 2 O composite were analyzed by Langmuir and Freundlich isotherm models to understand the appropriate I − adsorption process. The expression was represented by: Where, q e (mg/g) represents the uptake capacity of adsorbent at equilibrium. K L is Langmuir constant. q m (mg/ g) is the theoretical maximum adsorption capacity of adsorbent for I − . C e (mg/L) is the equilibrium concentration of adsorbent for I − . K F and n are the Freundlich constants [32].
The structure and composition of the sample determine the adsorption type, but the proportion of crystal surface does not change adsorption type. These two kinds of CuO composite belong to the same absorption type. Compared with the Freundlich model, the iodine adsorption data of CuO composite and CuO/Cu 2 O composite are more consistent with Langmuir isotherm model. Although the adsorption caption capacity is very different, the adsorption type is the same. The Langmuir adsorption model was based on the condition that monolayer adsorption occurs at the specific homogeneous adsorption sites. However the Freundlich isotherm model applies to heterogeneous surface adsorption. Therefore, the I − adsorption of CuO composite and CuO/Cu 2 O composite should be monolayer adsorption.
As shown in figure 6, temperature significantly affected the adsorption capacity of iodine ions. In order to find out energetic changes in adsorption process, thermodynamic analysis was tested under 298, 308, 318 and 328 K. Because the environmental temperature from 298 K rises to 328 K, the equilibrium adsorption capacity of CuO composite is gradually reduced. This phenomenon indicates that the exothermal reaction occurs in the adsorption process. When the adsorption temperature increases, the saturated concentration of iodide ions was increased. That means the temperature increase makes the adsorption equilibrium more difficult. Compared with C-CuO composite and C-Cu 2 O composite, the saturation concentration of D-CuO composite and D-Cu 2 O composite were lower. The Langmuir constant (K L ) calculated from Langmuir isotherm model, and it was utilized to calculate the thermodynamic parameters. Adsorption thermodynamic parameters (ΔH°, ΔG°a nd ΔS°) based on fundamental thermodynamic concepts can be determined as follows: where ΔG o (kJ mol) −1 , ΔH o (kJ mol) −1 and ΔS o (J/mol/K) are the standard Gibbs free energy, enthalpy and entropy change, respectively; R (8.314 J mol −1 K −1 ) represents the universal gas constant; T (K) is the absolute temperature [33]. The ΔG o , ΔH o and ΔS o values for iodine ion removal by CuO composite are lised in Table 1.
ΔG o <0, that shows that the adsorption is a spontaneous. The negative value of ΔS o indicates that the degrees of internal freedom of the system were decreased during adsorption. The negative ΔH o value implied that iodine ion adsorption is exothermic, so a decrease of temperature encourages iodine ion adsorption.
In order to evaluate the adsorption performance of samples for iodine ion, the curve of adsorption capacity versus time was recorded. As shown in figure 7, the CuO composite and CuO/Cu 2 O composite quickly adsorbed iodine ions within 100 min, reached slowly to their saturation uptake capacity between 100 min to 300 min. The relatively high increasing trend of uptake capacity within 100 min could be attributed to the abundance of adsorbates and active sites on the adsorbents, and a fairly large amount of iodide from solution transported easily to the adsorbent active sites. These results indicated that not only the adsorption capacity of CuO/Cu 2 O composite increased, but also the adsorption efficiency of sample was relatively high.
The To determine the significant information about the iodine ion adsorption at solid-solution interface, the pseudo-first-order and pseudo-second-order kinetic models were used to analyze the experimental data. The mathematical representations can be described by equations (5) and (6):  where q e (mg g) −1 is the equilibrium adsorption capacity and q t (mg g) −1 is the adsorption amount at time t (min). t represents adsorption time. k 1 (g mg min) −1 and k 2 (g mg min) −1 are the rate constants of pseudo-firstorder and pseudo-second-order, respectively [38]. The plot q t versus t of adsorbents is shown in figure 8. The kinetic curve of CuO composite corresponds to pseudo-first-order kinetic models. The R 2 of CuO composite was at least 0.99. The q e was 49.63, 28.64 mg g −1 for C-CuO composite, D-CuO composite, respectively. The results indicated that the iodine-ion adsorption reactions of two kinds of CuO composite are physical adsorption. However, the kinetic curve of CuO/Cu 2 O composite indicated that experimental result can be described by the pseudo-second-order model. Previous research suggested that the pseudo-second-order model was relatively high adsorption capacity and the uptake process should be chemical sorption. The q e was 91.91, 49.84 mg g −1 for D-CuO/Cu 2 O composite, C-CuO/Cu 2 O composite, respectively. The experimental results of iodine ion adsorption agree with the model research conclusion. For materials with similar structure, the adsorption capacity of chemical adsorption is higher than that of physical adsorption.
The XPS survey spectra of D-CuO composite represent the CuO composite and that of D-CuO/Cu 2 O composite represent the CuO/Cu 2 O composite. High resolution Cu 2p XPS spectra of D-CuO composite were showed in figure 9(b). It could be seen two individual peaks at binding energy 933.3 and 953.2 eV, corresponding with Cu 2p 3/2 and Cu 2p 1/2 , respectively [39]. The satellite peak at 942 eV peaks divided into 940.6 eV, and 943.5 eV, these corresponded with Cu 2+ [40]. After absorption of iodine, there was no significant change except adding to iodine absorption peak in spectra of the CuO composite (figures 9(c) and (d)). Compared with spectra of CuO composite, the 933.2 eV peak divided into 932.1 eV and 933.7 eV in that of CuO/Cu 2 O composite. The 932.1 eV peak corresponded to the Cu + . For the same reason, the 953.2 eV peak divided into 952.1 eV and 954 eV as shown in figure 9(f). After absorption of iodine, the 933.2 eV peak divided into 932.1 eV, 933.6 and 934.2 eV, and that means the new copper compounds was produced [41]. The Cu 2 O was involved in the reaction, and the reaction equation is [42]:  The contaminated water usually contains some anions besides the iodine ions. These other anions might influence the adsorption of iodine ions for adsorbent. Thus, the uptake test with multiple anions(Cl − , SO 4 2− and CO 3 2− ) was carried out, and the result showed in figure 10. It was observed that CO 3 2− , SO 4 2− and Cl − exhibited a slightly effect on iodine ions adsorption of CuO composite. Compared with CuO composite, these ions have greater influences on absorption capacity of the CuO/Cu 2 O composite. In contrast, the CO 3 2− most greatly impacted on iodine absorption capacity. The reason was the high ionic potential of CO 3 2− , which was easier adsorbed by sample compared with iodine ions. The SO 4 2− could slightly improve the adsorption capacity of absorber, and the adsorption capacity was 29.25 mg g −1 (C-CuO composite), 50.21 mg g −1 (D-CuO composite), 54.16 mg g −1 (C-CuO/Cu 2 O composite) and 93.14 mg g −1 (D-CuO/Cu 2 O composite). The reason is speculated that the adsorbed material generated Cu + and produced CuI with acidic conditions. The chemical reaction promoted iodine ion adsorption. Similarly, CO 3 2− belongs to the weak acid ion, which will make the solution become alkaline after hydrolysis and hindering the iodine ion adsorption. The kinetic adsorption experiment has proved that the adsorption process CuO composite is mainly physical adsorption, so the adsorption performance is not obviously affected. However the adsorption process of CuO/Cu 2 O composite is mainly chemical adsorption. It was observed that CO 3 2− exhibited a strong inhibitory effect on iodine ion adsorption.

Conclusions
In summary, the two different forms of Cu-BTC were successfully synthesized and the prepared materials as the precursor were calcined to generate porous CuO composite and CuO/Cu 2 O composite. As shown the SEM pictures of material, the calcination process decomposed the material, but the samples structure was still relatively complete. FTIR spectral and XRD spectral proved that dodecahedron composite is different from  cubical composite for crystal face type and chemical bond type. The dodecahedron composite is more potential for absorption of iodine ions. The thermodynamic experiment shows that the process of iodine absorption is exothermic and the monolayer adsorption occurs at the surface of the adsorbent. The adsorption properties of the prepared absorber were also compared, the adsorption capacity of dodecahedron composite is higher than that of cubical composite and the adsorption capacity of CuO/Cu 2 O composite is higher than that of CuO composite. The kinetics experiments indicate that the iodine absorption of CuO/Cu 2 O composite accords with pseudo second-order model, therefore the iodine ions absorption mechanism of CuO/Cu 2 O composite is chemical uptake. Additionally, the experiment exhibits that the CuO composite and CuO/Cu 2 O composite have excellent resistance to competitive ions. The absorber might be a cost-effective candidate for the emergent treatment of iodine ions in wastewater.