Simultaneously Remove and Visually Detect Ce 4+ Based on Nanocomposite of UiO-66-NH 2 /CPA-MA

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

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.

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Figure 1

(a) XRD patterns of UiO-66-NH₂ and UA composite. (b) FT-IR spectrum of UiO-66-NH₂ and UA composite. SEM and TEM images of UA (c)–(e). Elements mapping analysis and EDX pattern of UA (f, g).

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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-NH_2. 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 λ = 454 nm 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 ( R 2 ) 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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Figure 2

(a) The UV/Vis diffuse reflection spectroscopy of UA adsorbing Ce⁴⁺ with different concentrations. (b) The linear relationship spectrum .The experiments were parallel performed in five times. The error bars are shown with absolute values of standard deviation (SD) lower than 3%).

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The limits of detection (LOD) are 3.24 mg/L, which can be calculated by follows: (1) LOD = 3 M S , where M refers to the standard deviation obtained in the blank, and S 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⁺.

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Figure 3

(a) Effects of coexisting ions on Ce⁴⁺ adsorption on UA composite (initial Ce⁴⁺ concentration 100 mg/L, adsorbent dose 0.5 g/L). (b) The color of UA when it captures different metal ions.

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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.