Facile preparation and adsorption performance of low-cost MOF@cotton fibre composite for uranium removal

A novel composite MOF@cotton fibre (HCF) was prepared and characterized by FTIR, SEM, XPS and TGA. The effect of various parameters on the adsorption efficiency, such as the solution pH, contact time, initial U(VI) concentration and temperature, was studied. The maximal sorption capacity (Qm) is 241.28 mg g−1 at pH 3.0 for U(VI) according to the Langmuir isotherm adsorption model, and the kinetic and thermodynamic data reveal a relatively fast entropy-driven process (ΔH0 = 13.47 kJ mol−1 and ΔS0 = 75.47 J K−1 mol−1). The removal efficiency of U(VI) by HCF is comparable with that of pure cotton fibre and as-prepared MOF (noted as HST). However, the HST composite with cotton fibre significantly improved the treatment process of U(VI) from aqueous solutions in view of higher removal efficiency, lower cost and faster solid–liquid separation. Recycling experiments showed that HCF can be used up to five times with less than 10% efficiency loss.

Effect of the pH and ion strength on the U removal. The chemistry species of uranium in the solutions significantly varies at different pH, which makes the solution pH greatly affect the adsorption efficiency. The effect of the solution pH of 2-8 and ion strength with different concentrations of NaClO 4 (0.001, 0.01, 0.1 and 1.0 M) on the uranium adsorption is shown in Fig. 3. In Fig. 3 (insert), the removal rate of U(VI) by HCF and CF was maximal at pH = 3. It is well known that the most predominant uranium species is UO 2 2+ at low pH; therefore, the removal rate was low due to the competition between abundant H + and UO 2 2+ ions 55 . With increasing solution pH when U is mainly present as negatively charged species (UO 2 ) 3 (OH) 7 and (UO 2 ) (OH) 3 -, the removal efficiency increased due to the electrostatic attraction between the negatively charged U(VI) species and the positively charged sorbent 56 . When the pH further increased, the hydrolysation of UO 2 2+ and formation of U(VI)-carbonate species (UO 2 ) 2 (CO 3 )(OH) 3− inhibited the U(VI) adsorption, which decreased the adsorption capacity 57 . Therefore, an optimal pH of 3.0 was used in subsequent experiments. In addition, the ion strength clearly had little effect on the adsorption process, which suggests that the inner-sphere surface complexation mechanism plays a main role in the adsorption process 58 .  www.nature.com/scientificreports/ Effect of the contact time and kinetic studies. The adsorption efficiency of the adsorbents can be evaluated by the adsorption equilibrium time and kinetic process. Figure 4 presents the effect of the contact time of 5-120 min on the U(VI) sorption by HCF and CF in regard to the kinetics at an initial U(VI) concentration of 10 mg L −1 at pH 3.0 at room temperature. With the extension of time, the adsorption efficiency significantly increased until it reached an equilibrium within 30 min. The removal rate of HCF for U(VI) was nearly 100% within 30 min, while the removal efficiency of CF was poor (only approximately 70%). Pseudo-first-order and pseudo-second-order models were used to study the adsorption kinetics of U(VI) on HCF. Figure 4 (insert) clearly shows that the pseudo-second-order model had a superior correlation coefficient (R 2 ) compared to the pseudo-first-order model, which indicates that the U(VI) adsorption processes of HCF well fit the pseudosecond-order model.
Adsorption isotherms and thermodynamic studies. Langmuir and Freundlich adsorption isotherm models are expressed in Eqs. (1) and (2), respectively 59 . According to Eqs. (1) and (2), the maximal adsorption capacity (Q m ) was fitted when the initial U(VI) concentration was 5-150 mg L −1 (Fig. 5). The calculated Langmuir and Freundlich isotherm parameters from the fitting processes are listed in Table 1. Table 1 shows that Langmuir isotherm fitted the experimental data well with a higher correlation coefficient (R 2 ), and the maximum adsorption capacity was 241.28 mg g −1 for the adsorption of HCF. Moreover, Fig. 5 shows that HCF is a promising sorbent for the removal of uranium from aqueous solutions in terms of the preparation cost, adsorption efficiency and simple solid-liquid separation for uranium from aqueous solutions.  www.nature.com/scientificreports/ where Q e (mg g −1 ) is the equilibrium adsorption capacity; C e (mg L −1 ) is the uranium concentration at equilibrium; Q m (mg g −1 ) is the maximum adsorption capacity; K L (L mg −1 ) and K F (mg 1−n L n g −1 ) are Langmuir constant and Freundlich constant, respectively; n is Freundlich adsorption exponent. Dubinin-Radushkevich (D-R) isotherm is usually employed to explain the adsorption mechanism with respect to Gaussian energy distribution onto a heterogeneous surface and determine the adsorption nature as physical or chemical based on the mean free energy (E) 60 . The D-R isotherm model is expressed by Eq. (3). Model parameters ε, β and E can be determined and calculated by Eqs. (4) and (5).  www.nature.com/scientificreports/ where β (mol 2 (J 2 ) −1 ) is D-R isotherm constant, and ε (J mol −1 ) is Polanyi potential. The calculated D-R isotherm parameters are listed in Table 1. From the obtained E value, the physical or chemical sorption mechanism can be revealed. According to the literature 61 , if E is 8-16 kJ mol −1 , the sorption process chemically occurs, whereas E < 8 kJ mol −1 follows the physical sorption. For HCF, the low E value of 0.56 kJ mol −1 in this study suggests that the U adsorption was a physical adsorption process due to electrostatic or Van der Waal's attractions. The linear form of the Toth equation was as following 62 . A linear relationship can be obtained by plotting (c e /q e ) T against (c e ) T at different T values, and then the values of q T , b T and R 2 can be calculated.
where q T (mg g −1 ) and b T (L mg −1 ) are the parameters of the Toth equation, T is exponent of the Toth equation. Figure 6 shows the regression results for the adsorption of uranium on HCF. The best regression line is identified when T is above 0.8 and the R 2 value of the line is closest to 1. The calculated values of q T , b T and R 2 are also listed in Table 2. Table 2 indicates that the values of R 2 increase with the increase of the values of T. As a result, the Toth equation (T > 0.8) reveals better-fitting results than the Langmuir, Freundlich and D-R equations. According to the reference 63 , T is 0.6-1.0 for the adsorption of uranium, meaning that the adsorption occurs mostly on homogeneous surfaces in this study.
A comparison of the maximum adsorption capacity (Q m ) of various adsorbents is shown in Table 3. According to Table 3, the as-synthesized HCF clearly presents higher Q m than other cellulose-based adsorbents. This increased adsorption capacity of HCF for U(VI) may be attributed to its fibrous structure and the addition of HST (3) ln Q e = ln Q m − βε 2 ,  www.nature.com/scientificreports/ with higher specific surface area and excellent adsorption efficiency 64 in the surface of CF. Q m of the as-prepared HCF shows that HCF is a promising adsorbent for the treatment of uranium-bearing wastewater.
To evaluate the adsorption thermodynamic parameters, the effect of temperature on the uranium removal was investigated using 20 mL solutions containing 0.008 g of HCF and 10 mg L −1 U(VI), which was shaken for 24 h at pH 3.0. After the adsorption equilibrium, adsorption results of U(VI) by HCF were obtained, and the plots of lnK d versus 1/T onto HCF is shown in Fig. 7. Thermodynamic parameters (i.e., enthalpy (ΔH 0 ), entropy (ΔS 0 ) and standard free energy ΔG 0 ) from 303 to 333 K in the adsorption process were calculated from the slope and intercept of the linear line of lnK d versus 1/T according to Vander Hoff Eqs. (7) ~ (9) 65 . The evaluated ΔH 0 value is 13.47 kJ mol −1 , which reflects that the adsorption reaction was endothermic. The obtained positive ΔS 0 and negative ΔG 0 values (Fig. 7 insert) suggest that the adsorption process was spontaneous.  Regeneration and reuse of HCF. The adsorbent reuse is an important index to further evaluate their adsorption performance and reduce the treatment cost. After adsorption and filtration, the adsorbent loaded U(VI) was collected and treated with excess HNO 3 (0.1 mol L −1 ) for 24 h in a shaker. Then, the adsorbent was separated from the liquid by filtration and washed several times by DW to be used for the next cycle adsorption experiment. The reuse experiments of HCF were performed for five cycles. According to Fig. 8, the U(VI) removal percentage with HCF was 99.11% after the first cycling experiment. After five recycling experiments, the U(VI) removal percentage remained at 88.24%, which suggests that HCF can be recovered and reused several times and has favourable recycling capability. Finally, the products were dried at 100 ℃ in vacuum.

Materials
In the above preparation process of HCF, by the hydrothermal synthesis method firstly H 3 BTC react with BA in a Teflon autoclave to obtain the MOF product (noted as HST), and then HST composites with the raw material CF to produce HCF.
Characterization. The structure characteristics of HCF, CF and HST were analysed by Fourier transform infrared (FTIR) spectroscopy (Bruker VERTEX 70, Germany). The morphology characteristics of the samples were obtained on a scanning electron microscope (SEM) (Helios 600i, Japan). X-ray photoelectron spectroscopy (XPS) of HCF and CF were studied using an ESCALAB 250 X-ray photoelectron spectroscope (Thermo fisher, USA). Thermal stability of the products was studied by thermogravimetric analysis (TGA) spectroscopy (Netzsch STA449F5, Germany) from 30 to 900 ℃ at a heating rate of 10 K/min under an argon flow.
Adsorption studies. The U(VI) stock solutions were prepared by dissolving UO 2 (NO 3 ) 2 ·6H 2 O in DW; then, small amounts of concentrated HNO 3 was added to avoid the hydrolysis of UO 2

2+
. The working U(VI) solutions were prepared by appropriately diluting the stock solutions immediately before their use. The adsorption capacities of U(VI) onto HCF and CF were investigated as a function of the solution pH, contact time, initial U concentration and temperature by batch adsorption experiments. The solution pH was adjusted by adding NaOH and HCl and measured using a glass electrode (Leici PHS-3C, China). HCF or CF was added to 20 mL U(VI) solution and shaken in a shaker (Kangshi, China). After filtration, the U(VI) concentration in the solution was determined by a micro-quantity uranium analyser (MUA model, China). All experiments were performed in www.nature.com/scientificreports/ triplicate, and the data are presented as the mean values. The removal efficiency (R (%)) and adsorption amount of U(VI) on HCF or CF (q (mg g −1 )) of U(VI) in solution were calculated using Eqs. (10) and (11) 59 , respectively.
where c 0 (mg L −1 ) is the initial U(VI) concentration, c t (mg L −1 ) is the U(VI) concentration at time t in the solution, V (L) is the solution volume, and m (g) is the adsorbent mass.
In the desorption experiments, the obtained U(VI)-loaded HCF was washed with DW and rinsed in 3 M HNO 3 for 24 h; then, it was thoroughly washed in DW until U(VI) ions were not detected in the rinsing solution. The dried and regenerated adsorbent was reused for further adsorption experiments, and this recycling procedure was repeated five times.

Conclusion
The composite HCF with lower cost was fabricated via a facile solvothermal approach to adsorb U(VI) from aqueous solutions. The preparation cost of the adsorbent was significantly reduced by using BA with lower cost as the ligand to replace part of the traditional ligand H 3 BTC. HCF shows favourable adsorption capacity for U(VI) with maximum adsorption capacities of 241.28 mg g −1 at pH 3.0 compared to CF. HCF shows favourable regenerability, and the U(VI) removal percentage was 88.24% after five cycles. This work offers a new and cost-effective adsorbent HCF, which can be effectively used as a promising sorbent to remove U(VI) from the real multi-component U(VI)-containing nuclear waste influents. Furthermore, due to the advantageous fibrous form, HCF can be easily separated from aqueous solutions, which enhances post-treatment efficiency for further practical applications. www.nature.com/scientificreports/