Abstract
Effective strategies to deal with rare earth pollution are urgently needed due to the overexploitation of rare earths resource. In this study, a novel nanocomposite of UiO-66-NH2/CPA-MA denoted as UA was successfully synthesized, which can simultaneously remove and detect Ce4+ in water. The hybrid consists of UiO-66-NH2 and CPA-MA. Based on the high adsorption performance of UiO-66-NH2, it can remove Ce4+ with high capacity by adsorption. Moreover, it can change its color from olive drab to light cyan depending on the adsorbed Ce4+ concentration, and the chroma is linearly related to the Ce4+ concentration. So, UA can be used to qualitatively and quantitatively detect Ce4+ by its color changing. The kinetics of adsorption course was investigated in details. The anti-inference ability of the nanocomposite in coexisting systems was carefully evaluated. The results indicate that UiO-66-NH2/CPA-MA is highly potential to deal with Ce4+ pollutions due to its bifunctionality.
1. Introduction
Rare earths are important strategic resources, which are eagerly required in many areas of modern industry. However, overexploitation of rare earth has also brought about serious ecological and environmental problems [1, 2], especially in those countries with vast reserves while relatively low exploitation techniques of rare earth. The effective strategies for dealing with rare earth pollution are urgently needed. Detection and removal are two important aspects for the treatments of pollution [3, 4]. Several techniques have been applied to detect rare earth element (REE) in water, including spectrophotometric method, INAA, MAA, ICP-MS, ICP-AES, and EXAFS. However, most of them require expensive instruments and skilled operators and usually are hard to be used on site. As to the removal of REE in water, sorption is one of the most widely applied techniques because of its cheapness and facile operation [5].
In the previous studies, detection and removal were usually the two independent procedures, resulting toilsome steps and enhancing cost. If these two functions can be incorporated into one material and work simultaneously, the treating efficiency would be highly enhanced.
Metal organic frameworks (MOFs) are a kind of novel materials developing fast in the recent years. They have been widely applied in gas storage and separation, contaminant migration and catalysis [6]. UiO-66-NH2 is a Zr-based MOF which has attracted increasing attention because of its high stability, high adsorption ability, and easy modification. In this study, UiO-66-NH2 was incorporated with chlorophosphonazo-MA (CPA-MA) to construct a novel bifunctional nanocomposite denoted as UA. CPA-MA is a typical spectrophotometric reagent for REE based on its distinctive color changing property as capture REE [7]. The chemical structures of UiO-66-NH2 and CPA-MA are shown in Scheme S1. Based on the high adsorption property of UiO-66-NH2 and the distinctive color changing property of CPA-MA, this hybrid UA can not only effectively adsorb REE but also can significantly change its color when captures the certain REE. The certain color and different chrominances appearing at different REE concentrations can be used to qualitatively and quantitatively detect the certain REE.
A common REE of Ce4+ was used as the model contaminant in this study due to its wide distribution and high hazardousness. UA shows high adsorption capacity toward Ce4+. Furthermore, UA exhibits two different color changing phenomenons in different Ce4+ concentration ranges, which could be used to qualitatively and quantitatively detect Ce4+ even via naked eyes in rough level, while the precise measurement can be fulfilled by an inexpensive visible-light spectrophotometer. The present study provides a novel effective method to deal with rare earth pollution.
2. Experimental
2.1. Materials
2-Amino terephthalic acid (NH2-BDC) was bought from Shanghai Macklin Biotechnology Company, China. Cerium sulfate (Ce (SO4)2) was obtained from Shanghai Yuanye Biotechnology Company, China. Chlorophosphonazo-mA (CPA-MA) was bought from Sinopharm Chemical Reagent Co., Ltd, China. Zirconium tetrachloride (ZrCl4) was bought from Dongguan Waxi Chemical Company, China. Other chemicals are all commercial. All materials are directly used without further purification with their analytical grade.
2.2. Experiments
2.2.1. Preparation of UiO-66-NH2
In a typical preparation, 0.2332 g ZrCl4 (1.0 mmol) and 0.1812 g 2-NH2-benzenedicarboxylate (1.0 mmol) were dissolved in 50 mL N,N-dimethyl formamide (DMF) with magnetic stirring for 30 min to produce uniform dispersion. Then, the mixture was placed into a 100 mL Teflon-lined stainless steel autoclave. It was reacted at 393 K for 48 h. After cooled in air to room temperature, the yellow powder was recovered from the mixture by centrifugation and then washed with DMF and anhydrous ethanol for several rounds. In the end, the powder was dried at 353 K for 12 h to get UiO-66-NH2.
2.2.2. Synthesis of UiO-66-NH2/CPA-MA
The hybrid of UiO-66-NH2/CPA-MA was synthesized by following procedure: 10 mg CPA-MA was dissolved in 40 mL anhydrous ethanol to form a clean solution. 40 mg UiO-66-NH2 was added and distributed. After that, the suspension was vibrated at 180 rpm for 12 h at 303 K. Finally, the powder was recovered, washed, and dried to get the composite of UiO-66-NH2/CPA-MA. The synthesized procedure of UiO-66-NH2/CPA-MA was shown in Scheme S1. The nanocomposite was denoted as UA for the expression convenience.
2.2.3. Characterization
An X-ray powder diffractometer (Cu Ka, Rigaku III/B max) was used to test the samples. Fourier transform infrared (FT-IR) spectra investigation of the samples was conducted on an ALPHA-T FT-IR spectrometer (Bruker, Germany), and the testing range was set as from 4000 to 400 cm-1. The scanning electron microscopy (SEM) imagines of the samples were recorded by a FEI Quanta 200F SEM. The color change of the composite after Ce4+ loading was qualitatively detected by naked eye and quantitatively measured by a solid-state visible spectrophotometer (AvaSpec, China).
2.2.4. Measurement and Removal of Ce4+
Ce4+ can be visually detected and simultaneously removed by the UA nanocomposite. 5 mg UA nanocomposite was placed into 10 mL solutions containing different initial concentrations of Ce4+ (from 10 to 300 mg/L). The suspensions were vibrated at 180 rpm for 12 h at 298 K. After that, the composite was collected by filtrations. To determine the adsorbed Ce4+, the residue Ce4+ concentration was measured by CPA-MA according to the previous report [5, 6], and the adsorbed Ce4+ can be calculated by the difference between the original and the residue concentrations. Naked eyes and a solid-state visible spectrophotometer were used to qualitatively and quantitatively investigate the color change of the hybrid after Ce4+ loading.
2.2.5. Kinetic Analysis
Kinetic analysis was studied at 298 K with the adsorbent dose of 0.5 g/L. The UA nanocomposite was added into 100 mg/L Ce4+ solutions. The mixtures were vibrated at 180 rpm. At different time intervals, the mixtures were sampled to determine the related Ce4+ concentration.
2.2.6. Effects of Coexisting Ion Investigation
The coexisting ions effect (Mg2+, Cu2+ and Zn2+) toward Ce4+ sorption were studied at 298 K. The initial Ce4+ concentration was 100 mg/L, while the coexisting ion concentration was set as 0, 100, 150, 200, 250, and 300 mg/L, respectively. The mixtures were vibrated at 180 rpm at 298 K for 12 h. The adsorbent dose is 0.5 g/L.
3. Results and Discussion
The XRD patterns of pristine UiO-66-NH2 and UA composite are indicated in Figure 1(a). The pristine UiO-66-NH2 exhibits characteristic peaks at the 7.3°, 8.3°, and 25.6° which are in consistent with the early report, showing the successful synthesis of UiO-66-NH2 and its high crystallization [8]. These characteristic peaks also appear in the XRD pattern of UA composite, showing the existence of crystal UiO-66-NH2 in the UA nanocomposite.
(a)
(b)
(c)
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(e)
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FT-IR spectra of UiO-66-NH2 and UA are indicated in Figure 1(b). As to UiO-66-NH2, the peaks at 1420 and 1580 cm-1 were attributed to C-C vibrational modes and C-O bonding in carboxylates, respectively [9]. 1258 cm-1 is ascribed to the C-N stretching of aromatic amines [10]. The adsorption bands at 764 cm-1 are ascribed to N − H rocking vibration [9]. The bands located 3475 and 3352 cm-1 are designated to the asymmetric and symmetric vibrations of N-H bond in NH2, respectively [11–13]. These characteristic bands are in consistence with early literatures, confirming the successful synthesis of UiO-66-NH2. As to CPA-MA, the band located 1676 cm-1 is ascribed to the stretching of C=O [14]. It can be found that the typical bands of UiO-66-NH2 and CPA-MA appear in the spectrum of UA, indicating the well incorporation of UiO-66-NH2 and CPA-MA, furthermore, peaks related to the NH2 of UiO-66-NH2 (3475 and 3352 cm-1) and the peak related to the C=O of CPA-MA (1676 cm-1) are all significantly weakened in the spectrum of UA, showing that the NH2 groups in UiO-66-NH2 have condensed with the C=O groups in CPA-MA to form the composite of UA.
As shown in Figures 1(c)–1(e), the SEM and TEM imagines show that the UA nanocomposites exhibits a morphology of cube with the sizes of around 60-80 nm. The chemical composition of UA was further determined by the mapping and EDX analysis (Figures 1(f) and 1(g)). It can be seen that the elements of Zr, N, C, Cl, O, P, and S exist and uniformly distribute in UA composite. These results confirm that UiO-66-NH2 was successfully incorporated with CPA-MA to form the composite of UA.
To investigate the effects of adsorbent dosage toward Ce4+ capture, dosage of UA was set from 0.5 to 2 g/L, and the related adsorption capacities are indicated in Figure S1. As one can see that the highest unit adsorption capacity appears when the dosage is 0.5 g/L, so 0.5 g/L was determined as the optimal dosage. The kinetic process of UA adsorbing Ce4+ was investigated (Figure S2 (a)). Results showed that the adsorption course was more preferably described by the pseudosecond-order kinetic model (Figure S2 (b), (c) and Table S1). The adsorption capacity of UA was compared with other previously reported Ce4+ adsorbents, and the results are shown in Table S2, indicating it being among the top Ce4+ adsorbents.
In order to test the response of UA toward Ce4+, the adsorptions were conducted with Ce4+ concentration, and the related diffuse reflectance spectra reflecting were recorded with a visible light spectrophotometer. The results indicate that the adsorbent can change its color as capturing Ce4+ ions with different concentration. The color of crude UA is olive drab. In the testing Ce4+concentration range, the Ce4+ load UA shows light cyan, and the chroma of UA decrease with the increase of Ce4+ concentration (insert in Figure 2(a)), which can be testified by the intensity decrease at with the increase of Ce4+ concentration (Figure2(a)). Furthermore, as plot the Ce4+ concentration vs. intensity value at 454 nm, it can be found that the intensity changing have linear relationship with the Ce4+ concentration, as shown in Figure 2(b). The high coefficience () values of 0.9700 exhibit the high linear dependence, indicating that Ce4+ concentration can be measured by chroma changing of UA.
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(b)
The limits of detection (LOD) are 3.24 mg/L, which can be calculated by follows: where refers to the standard deviation obtained in the blank, and refers to the slope of the linear range in the calibration graph.
So as to study the anti-interference ability, the adsorption capacities of UA toward Ce4+ were tested with several coexisting ions (Mg2+, Zn2+, and Cu2+). In the experiments, 100 mg/L Ce4+ was paired with 0, 100, 150, 200, 250, or 300 mg/L the competing ion, respectively. It can be seen in Figure 3(a) that the existence of competing ions can affect the adsorption of Ce4+ on the nanocomposite in different level, depending on the species. Cu2+ shows the least competitiveness, and the nanocomposite removal rate can be retained ~87%. In the presence of Zn2+, which is the most competitive, the nanocomposite removal rate can be retained ~80%. These consequences show that UA has a relatively high anti-interference capacity as adsorbing Ce4+.
(a)
(b)
The crude UA is olive drab. When UA captures 10 mg/L Ce4+, its color quickly changes to be pale green in seconds. When UA was added to the solution containing other metal ions such as Mg2+, Zn2+, and Cu2+, even their concentrations are 30 times higher than that of Ce4+,and they cannot change UA’ color a bit. The results are shown in Figure 3(b), which indicates that Ce4+ can be qualitatively detect even by naked eye according to UA’s color.
4. Conclusion
In summary, a novel nanocomposite of UA was successfully prepared in this study. It can not only effectively remove Ce4+ by adsorption, but can also qualitative and quantitively measure Ce4+. Furthermore, Ce4+ ions can be detected by UA with the LOD of 3.24 mg/L. UA has a high anti-interference ability. Several competing metal ions can coexist with Ce4+ and will not affect its adsorption and detecting ability. The novel nanocomposite of UA can fast remove hazardous Ce4+ ions and can simultaneously visually detect Ce4+. It is a highly potential multifunctional adsorbent to efficiently deal with rare earth containing water.
Data Availability
The data used to support the findings of this study are included within the article.
Disclosure
This manuscript has been post as preprint in https://www.researchsquare.com/article/rs-779280/v1 already [15].
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
We thank the support of National Natural Science Foundation of China (51978323, 42077162), the Key Research and Development Project of Jiangxi Province (20203BBGL73229), and Graduate Student’s Research and Innovation Fund of Nanchang Hangkong University (YC2020-009).
Supplementary Materials
(1) The effects of adsorbent dosage toward adsorption capacity. (2) The adsorption ability of UA towards Ce4+ at different time interval was tested. (3) The pseudo-first-order kinetic and pseudo-second-order kinetic models were used to describe the adsorption course. (4) Comparison of Ce4+ adsorption capacity () between UA and other previously reported adsorbents. (Supplementary Materials)