Chitosan is only soluble in acidic water. When preparing chitosan/UiO-66 composites, UiO-66 nanoparticles have to be dispersed in aqueous chitosan solution under acidic condition. Thus, the use of water-stable MOFs is highly important for the fabrication of chitosan composites. It has been reported that functionalized UiO-66, namely UiO-66-NH
2 and UiO-66-NO
2, prepared from the BDC organic linkers with a nitro- or amino-group attached, have shown improved water stability under harsh pH conditions compared to the parent UiO-66 [
10,
22]. UiO-66-NO
2 showed particularly high chemical stability in basic condition, with the retention of the crystalline structure after being soaked in NaOH solution [
22]. Therefore, chitosan composites with both UiO-66 nanoparticles and functionalized UiO-66 nanoparticles are prepared and evaluated.
Figure S2 shows the FTIR spectra of UiO-66, UiO-66-NH
2 and UiO-66-NO
2 nanoparticles in this work. The peaks observed at 1578 and 1394 cm
−1 are characteristic of the in- and out-of-phase stretching modes from the carboxylate group associated with the organic linker. For UiO-66-NH
2, the amino group can be distinguished at 3504 cm
−1 and 3381 cm
−1, which can be attributed to asymmetric and symmetric N-H stretches respectively. For UiO-66-NO
2, the asymmetric NO
2 stretching vibration can be assigned to the peak at 1536 cm
−1.
3.1. Freeze-Dried and Post-Treated Chitosan/UiO-66 Composites
Porous composites are obtained after freeze-drying the frozen suspension of UiO-66 nanoparticles in aqueous chitosan solution. As characterized by scanning electronic microscopy (SEM), all the monoliths (with UiO-66, UiO-66-NH
2, and UiO-66-NO
2 nanoparticles) show ice-templated structures and the presence of MOF nanoparticles.
Figure 1 shows the SEM images of the composites (CM/UiO-66-NO
2) made from medium MW chitosan and UiO-66-NO
2 nanoparticles after freeze drying. Similar pore morphologies and MOF nanoparticles were observed for CM/UiO-66 composites and CM-UiO-66-NH
2 composites (
Figures S3 and S4).
The sizes of UiO-66-NO
2 nanoparticles are in the approximate range of 50–100 nm (
Figure 1A).
Figure 1B clearly shows the nanoparticles embedded in and on the surface of the layered porous chitosan structure; the ridges can be clearly seen (
Figure 1C), which are characteristic of the ice-templated materials [
20,
21]. In between the ridges the surface of the chitosan appears to be fairly smooth. After heating treatment, the nanoparticles can still be seen embedded in the material and seem to be of similar sizes (
Figure 1D,E). The surface appears to be rougher than the untreated samples, showing a flaked appearance. Similar results can be seen for the NaOH treated sample, as shown in
Figure 1F,G. The surface appears significantly rougher subsequent to the base treatment, potentially removing some of the nanoparticles from the surface layers. Similar observations were noted for the composites with UiO-66 and UiO-66-NH
2 nanoparticles.
Only the chitosan/UiO-66 composites were treated by chemical crosslinking with glutaraldehyde. This is because of the better performance of UiO-66 nanoparticles and composites for the adsorption of MCPP, discussed below. UiO-66 nanoparticles can be observed (
Figure 1H) and the macroporous structure remains after crosslinking (
Figure 1I). The crosslinking of chitosan with glutaraldehyde can be confirmed by FTIR analysis with a new peak from the C=N bond, resulted from the reaction between the amine group on chitosan and the aldehyde group [
24]. The FTIR spectra of the composite monoliths before and after the crosslinking are very similar (
Figure S5). However, on closer inspection of the peaks in region 1550–1700 cm
−1, overlapping peaks are observable, unlike the smooth peaks seen in spectrum of the un-crosslinked monolith. Double-bonded nitrogen groups are often very hard to distinguish as they exhibit adsorptions close to the carbonyl and alkene double bonding region [
25]. They can therefore often be overlapped with these common functional groups, which are apparent within both chitosan and the MOF nanoparticles. Another problem is that only a small degree of crosslinking was achieved. According to Pratt et al. [
26], the intensity of the imine signature increases with amount of the glutaraldehyde content. Therefore the analysis of the spectra is difficult as the peak intensity will be relatively low. In spite of these difficulties, the crosslinking of chitosan can be confirmed by the improved stability in water treatment, as demonstrated in the section below.
UiO-66 nanoparticles were synthesised with established procedures [
11,
22]. The freeze-drying of UiO-66 nanoparticles in aqueous polymer solution was known to retain the crystallinity of UiO-66, as confirmed by powder x-ray diffraction (PXRD) analysis [
14].
Figure S6 confirms the crystallinity of UiO-66-NO
2 was retained in the composites although the heat treatment reduced the crystallinity while the composites treated with NaOH solution showed an amorphous material. Poor crystallinity was observed for the chitosan/UiO-66-NH
2 composites (
Figure S7). Like other MOFs, UiO-66 nanoparticles show high surface areas and the functionalized UiO-66 nanoparticles exhibit lower surface areas due to the presence of functional groups (-NH
2 or -NO
2) in the framework [
11,
22]. Commonly, MOF materials show a range of surface areas, depending on preparation methods, crystallinity, and post-treatment procedure. The UiO-66 nanoparticles synthesized in this study showed a surface area of 1034 m
2 g
−1 and the CM/UiO-66 (1:2) gave a surface area of 339 m
2 g
−1. For water treatment or other liquid phase application, the porosity and macropores can be measured by Hg intrusion porosimetry [
14].
3.2. Adsorption of MCPP from Aqueous Solutions
UiO-66 nanoparticles with different functionalities were first examined with 60 ppm MCPP solution. As shown in
Figure 2, the functionalized UiO-66 nanoparticles (UiO-66-NH
2 and UiO-66-NO
2) yield lower adsorption capacity, particularly with UiO-66-NO
2 nanoparticles being about five times lower. This can be attributed to the lower surface area and hindered access to the micropores due to the presence of the functional groups.
The freeze-dried chitosan/UiO-66 composite monoliths were disintegrated in water, forming cloudy suspensions. However, after heating treatment, the monoliths were largely stable after soaking in 60 ppm MCPP solution for three h, particularly for the chitosan/UiO-66-NO
2 monolith where the MCPP solution was still clear despite the presence of some small pieces (
Figure S8). The adsorption of the heat-treated (HT) composite monoliths with 60 ppm MCPP solution (
Figure 3) shows the improved adsorption capacity for UiO-66-NH
2 and UiO-66-NO
2 compared to their nanoparticle powders. This is because of the relatively high adsorption capacity of chitosan for MCPP and the interconnected macroporosity of the monoliths [
14]. An obvious sigmoidal shape of the adsorption curve is shown for the sample CM/UiO-66-1 (HT). It is less obvious for the other two samples. Generally, this phenomenon may be attributed to the dense skin of the adsorbents after heat treatment. The dense surface or smaller pores impede the uptake of the solution and transport of the MCPP, resulting in low adsorption initially. With time, the adsorbent becomes more hydrated, leading to a more swollen scaffold, facilitating mass transport and higher adsorption. However, the performance of the composite monoliths with functionalized UiO-66 nanoparticles is still lower than that of the standard (un-modified) UiO-66 nanoparticles.
The composite materials prepared with the amino and nitro functionalised MOF nanoparticles were subject to treatment with aqueous NaOH solution, followed by further washes with acetone and cyclohexane. The aim of the base treatment was to remove the residual acid present from the composite fabrication procedure and render the monoliths more stable in aqueous solution. Acetone and cyclohexane were used for a solvent exchange process to reduce shrinkage on drying. It was noticed that the base-treated samples were very stable in water, consistent with our previous findings [
14] (
Figure S9). These functionalised MOFs were shown to be more stable compared to the parent analogue, within acidic and basic conditions [
10,
22]. However, the PXRD analysis on these samples showed that the MOF crystalline structures had been destroyed by the base treatment.
Surprisingly, the adsorption tests with MCPP solution and the monoliths showed unusual results. The concentration of MCPP in the solution was calculated from the UV absorbance at 275 nm, characteristic of MCPP. A decrease in peak intensity at 275 nm over time would be expected for the adsorption of MCPP by the chitosan/MOF monolith. However, for the base-treated chitosan/UiO-66-NO
2, an increase in the intensity of the absorbance peak at 275 nm was observed, along with an increased peak appearing at around 230 nm and a shoulder forming around 300 nm (
Figure S9). This was believed to be due to the composite leaking materials that present with UV absorbance in the same region as MCPP peak. To prove this, the composite material was immersed in pure deionised water. After a period of time, the solution was subject to UV-Vis analysis, which showed the appearance of absorbance peaks corresponding to those appearing within MCPP solution (
Figure S10). This confirms the material leaking from the base treated monolith. A similar observation was noted for the amino functionalised monolith after base treatment but there was no such problem for the chitosan/UiO-66 composites after the base treatment (
Figure S11). Further investigation is required to identify the leaked materials but it indicates that the base treatment is not suitable for chitosan/UiO-66-NO
2 and chitosan/UiO-66-NH
2 monoliths.
Due to the relative low performance of UiO-66-NO
2 and UiO-66-NH
2 nanoparticles & monoliths, the study on chemical crosslinking of the composite monoliths with glutaraldehyde was focused on chitosan/UiO-66. This procedure was found to be highly effective at stabilising the monolith within MCPP solution. The monolith could be easily picked out of solution using tweezers after the six-hour adsorption testing period (
Figure S12). This offers the potential of facile recyclability which is advantageous for industrial purpose.
The adsorption profile of the glutaraldehyde-crosslinked absorbent is compared with that of the nanoparticles and the base-treated samples, as shown in
Figure 4. The crosslinked absorbent shows a superior adsorption profile than both UiO-66 nanoparticles and the base-treated composite monolith, approximately 30% increase in the absorption capacity compared to the sole UiO-66 nanoparticles. The better performance of the crosslinked composites can be attributed to: (1) The interconnected ice-templated porous scaffold provides enhanced mass transport; (2) Chitosan itself is also a very good adsorbent for MCPP [
14]. This crosslinked monolith material therefore presents as a promising material for the absorption of MCPP.
Adsorption kinetics and isotherm studies and adsorption mechanism for the adsorption of MCPP on chitosan/UiO-66 composites were reported before [
14]. This study is focused on how to improve the stability of the chitosan/UiO-66 composites while maintaining their performance. As such, the recovery and reusability of an adsorbent is highly important. The ideal adsorbent needs to have facile recovery and a repetitively high adsorption capacity after reuse cycles. UiO-66 nanoparticles have previously displayed good performance for the adsorption of MCPP from water. The retrieval for reuse was shown to be restricted by the tedious centrifugation process required [
15]. This time-consuming process raises the cost of water remediation, limiting the potential for process upscaling. Therefore, the use of a stable chitosan/MOF monolith was investigated to tackle this problem, where the monolith could be easily picked up instead of using the energy- and time-consuming centrifugation (or filtration) process. The glutaraldehyde-crosslinked monolith has shown to give a superior adsorption capacity to the nanoparticles, while maintaining mechanical stability. The monolith could be simply picked out of the solution using tweezers, offering the potentials for facile recovery.
The performance of the crosslinked adsorbent was further examined to assess its reuse viability. The monolith was immersed within the MCPP solution for 6 h with stirring. An aliquot of the final solution was taken and subject to UV-vis analysis. Recovery of the monolith was facile, simply being picked out of solution using tweezers. Removal of MCPP from the adsorbent was carried out by soaking in deionized water, followed by three replenishes with ethanol. Finally, the adsorbent was dried under vacuum, and the adsorption process repeated to test the recyclability. The adsorption capacity obtained from three reuse cycles are shown in
Figure 5. After three cycles, the monolith still exhibited a high adsorption capacity, 35.67 mg g
−1, within 10 mL of 60 ppm MCPP aqueous solution. Still presenting a higher capacity than that of the sole UiO-66 nanoparticles. The performance of the recycled monolith reached approximately 75% of adsorption capacity compared to the adsorbed amount by the fresh adsorbent. These results demonstrate the glutaraldehyde-crosslinked chitosan/MOF monoliths as a very promising adsorbent for MCPP.