Characteristics of the adsorbents

The XRD patterns of the MIL-101s shown in Fig. 2a are agreeable with simulated one48,51, confirming the MIL-101s were successfully prepared and that the crystal structure of pristine MIL-101 does not change with functionalization. However, the XRD intensities of the MIL-101s decreased slightly on modification, particularly those of MIL-101-(OH) 2 and MIL-101-NH 2 , probably because of harsh conditions required for these modifications. The nitrogen adsorption isotherms (Fig. 2b) of the MIL-101s and the BET surface areas (Table 1) obtained from these isotherms show that the MIL-101s have considerable porosities, although functionalization (to introduce –OH, –NO 2 , and –NH 2 groups) reduced the porosities. This reduction could be due to the volumes of the functional groups and/or the decreased crystallinity with modifications (as shown by the XRD patterns). FTIR spectra of the modified MOFs shown in Fig. 2c confirm the grafting was successful based on the presence of the band at 1216 cm−1, which originate from the C-N stretching of the grafting agents52. The band at 1540 cm−1 of the MIL-101-NO 2 is because of the stretching vibration of -NO 2 group53.

Figure 2 (a) XRD patterns, (b) nitrogen adsorption isotherms and (c) FTIR spectra of MIL-101s. Full size image

Table 1 BET surface areas and maximum adsorption capacities (based on weight and surface area of adsorbent) of MIL-101s for naproxen. Full size table

Comparison of adsorbents for naproxen adsorption

Figure 3 shows the quantity of naproxen adsorbed by MIL-101s (based on weight and surface area of adsorbents) and activated carbon at different adsorption times. The figure indicates that naproxen adsorption by MIL-101s and activated carbon was almost complete after 4 h, suggesting relatively rapid adsorption of this material. As illustrated in Fig. 3a, the amount adsorbed (based on weight of adsorbents) decreased in the order MIL-101–OH > MIL-101-NH 2 > MIL-101-(OH) 2 > MIL-101 > activated carbon > MIL-101-NO 2 , which agrees with reported results for virgin MIL-101, MIL-101-NH 2 , and activated carbon46,47. Although the surface area of MIL-101-OH was around 70% that of pristine MIL-101, it adsorbed much more (about 1.53 times after 12 h) naproxen. Moreover, the results show the very high competitiveness (about 1.81 times after 12 h) of the MOFs against conventional adsorbents such as activated carbon. Figure 3b shows the amount of naproxen adsorbed per unit surface area by the MIL-101s and activated carbon, which decreased in the order MIL-101-(OH) 2 > activated carbon > MIL-101-OH > MIL-101-NH 2 > MIL-101 ~ MIL-101-NO 2 . The MIL-101-(OH) 2 showed very high adsorption capacity for naproxen per unit surface area, even though the amount of naproxen adsorbed per unit weight was not very high. Curiously, however, MIL-101-NO 2 was very poor at naproxen adsorption based on both weight and surface area despite the presence of the polar nitro group in the MOF.

Figure 3: Effect of adsorption times on the adsorbed amounts of naproxen over MIL-101s and activated carbon. (a,b) Show the adsorbed amounts of naproxen based on the unit weight and surface area, respectively, of adsorbents. The initial concentration of naproxen was 50 ppm. The legends in (b) are the very same as those in (a). Full size image

Adsorption isotherms and effect of functional groups on adsorption

Isotherms for naproxen adsorption by MIL-101s were obtained at 25 °C after 12 h of adsorption, which is sufficient for equilibrium, and the results are shown in Fig. 4. The adsorbed amounts (based on weight of MIL-101s) at equilibrium decreased in the order MIL-101–OH > MIL-101-NH 2 > MIL-101-(OH) 2 > MIL-101 > MIL-101-NO 2 , which was the same order as observed for quantity adsorbed after various times (Fig. 3a). The adsorbed amounts per unit area decreased in the order MIL-101–(OH) 2 > MIL-101-OH ~ MIL-101-NH 2 > MIL-101 ~ MIL-101-NO 2 , in agreement with Fig. 3b. The maximum adsorbed quantities (Q 0 ) obtained from Langmuir plots (Supplementary Figure 1) are summarized in Table 1, and the results again show that MIL-101s functionalized with –OH groups were highly effective at adsorbing naproxen from water. The amino group was also effective at naproxen adsorption, in agreement with a previous study47 despite the use of a different functionalization method. However, as shown in Figs 3 and 4, the introduction of a nitro group on the surface of MIL-101 was not effective for the adsorption of naproxen, even with the presence of charge separations in the –NO 2 group (positive N and negative O). Very curiously, the MIL-101-NO 2 and pristine MIL-101 showed very similar performances (based on surface area) for naproxen adsorption as shown in Figs 3b and 4b. Considering the functional groups on naproxen, including a carboxylic acid and an ether, the presence of polar groups on MIL-101s was expected to yield effective adsorption of naproxen via, for example, electrostatic interactions54; however, only –OH and –NH 2 groups were efficient for absorption of naproxen from water.

Figure 4: Adsorption isotherms of naproxen over MIL-101s at 25 °C. (a,b) Show the isotherms of naproxen based on the unit weight and surface area, respectively, of adsorbents. Full size image

Effect of solution pH

The pH of a solution is very important54 in the adsorption of organics from water considering the protonation/deprotonation of adsorbates and/or changes in the surface charges of adsorbents with different pH values. In this work, MOFs such as MIL-101-OH and pristine MIL-101 were studied at various pH values. As shown in Fig. 5, the amounts of naproxen adsorbed by MIL-101 and MIL-101-OH decreased as solution pH increased, which is similar to previous results for pristine MIL-10146,47. This tendency is understandable considering the ready deprotonation of naproxen at high pH (pKa of naproxen ~4.2) and the decreased surface charge (i.e., a change from positive to negative) of MIL-10155 with increasing pH. In other words, repulsive interactions between MIL-101s and naproxen are expected at high pH. Very curiously, the amount adsorbed by MIL-101-OH per unit surface area at a pH 10 was very similar to that of pristine MIL-101 (highlighted with a blue circle in Fig. 5b). This could be due to deprotonation of the –OH group (to form –O−) in MIL-101-OH at pH 10 (considering the pKa of ethanolamine, 9.5), leading to the contribution of H-bonding between naproxen and deprotonated MIL-101-OH becoming negligible, meaning only surface area was important in adsorption (see below).

Figure 5: Effect of pH of solution on the adsorbed amounts of naproxen over MIL-101 and MIL-101-OH. (a,b) Show adsorbed amounts based on the unit weight and surface area, respectively, of adsorbents. Full size image

Competitiveness and reusability of the adsorbent

So far, several adsorbents including carbonaceous materials have been used to adsorb naproxen from water. Table 2 compares the maximum adsorption capacities (Q 0 ) and amounts adsorbed at equilibrium (q 24 h , after 24 h) of studied adsorbents. Table 2 shows that the MIL-101-OH was very competitive when compared to studied adsorbents such as activated carbon11,56,57,58, bone char15, and mesoporous materials with and without modifications (SBA-1511 and MCM-4112), showing the potential applications of MIL-101-OH for adsorptive removal of naproxen from water.

Table 2 Maximum adsorption capacities (Q 0 ) or adsorbed amount after 24 h at equilibration (q 24 h ) of various adsorbents for naproxen. Full size table

Before evaluation of reusability of the MIL-101-OH, the stability of the MOF, after naproxen adsorption, was checked using XRD and SEM. As shown in Supplementary Figure 2, there is little change of XRD patterns of MIL-101 after modification to introduce –OH and after naproxen adsorption. Moreover, SEM images of Supplementary Figure 3 showed similar results. The energy dispersive X-ray spectroscopy (EDX) results (Supplementary Table 2) show that the chemical composition of MIL-101 did not change much with modification to introduce –OH and after naproxen adsorption. However, the contents of C and Cr of the MOF increased and decreased slightly, respectively, upon modification and adsorption. The EDX analysis results are understandable because of introduction of carbonaceous materials on the MOF by modification and adsorption of naproxen. All of these results confirm the stability of the MIL-101-OH in adsorption of naproxen, suggesting the possible reusability in adsorptions.

Reusability of adsorbents is very important for their possible application in commercial plants. As it showed the highest adsorption capacity per unit weight, MIL-101-OH was washed with ethanol, a common solvent, and used again for the adsorptive removal of naproxen from water. As shown in Fig. 6, the performances of MIL-101-OH decreased slightly with increasing the number of recycles. However, the performance after the third cycle was still much higher than that of activated carbon or virgin MIL-101 (highlighted with colored horizontal lines in the figure), showing the competitiveness of MIL-101-OH for naproxen adsorption. FTIR spectra showed not only adsorbed naproxen and successful removal with solvent washing, based on the bands corresponding to naproxen (Fig. 7).

Figure 6: Effect of recycle numbers on the adsorbed amount of naproxen over MIL-101-OH. The upper and lower horizontal lines show the adsorbed amount of naproxen over pristine MIL-101 and activated carbon, respectively (the adsorption time: 12 h and initial concentration of naproxen: 50 ppm). Full size image

Figure 7 FTIR spectra of naproxen, MIL-101-OH, MIL-101-OH (with adsorbed naproxen), and purified MIL-101 (by ethanol washing of the adsorbed MIL-101). Full size image

Even though MIL-101 does not contain toxic Cr(VI) ions, one may suspect the practical applications of MIL-101 containing Cr(III), with or without modifications, in water purifications. The results of this study, however, can suggest the possible applications of suitable MOFs in water purification via adsorption, considering synthesis of highly porous MOFs composed of non-toxic metal ions such as iron, aluminum, titanium, alkali metals and alkaline earth metals.