Fabricating MOF/Polymer Composites via Freeze Casting for Water Remediation
Round 1
Reviewer 1 Report
1) More characterizations for the adsorbents are required such as:
A) FTIR on the functionalized samples to check the presence of –NH2 and –NO2 groups
B) PXRD to check the crystallization of MOF
C) Zeta potential and BET to check the surface area, porosity and surface charge of the adsorbents
D) FTIR on the adsorbents before and after adsorption as well as desorption
2) More discussion on:
A) The adsorption mechanisms of MCPP on the different adsorbent
B) Why does the CM/UiO-66-1 composite give better adsorption capacity in comparison with UiO-66?
C) Adsorption kinetics and isotherms studies may be also required
D) Their results should be compared with some other MOF or adsorbents used for MCPP and reported in the literature. For example, the same UiO-66 MOF reported to show adsorption capacity 140 mg/g of MCPP after 3 hrs (https://doi.org/10.1016/j.cej.2015.02.007), while their best composite shows only 45 mg/g after the same time. They consumed a lot of chemicals, energy and time to prepare these composites and at the end they show a stability and adsorptive properties in comparison with those reported in the literature.
3) Other improvements
The definition of UiO-66 should be added to the abstract.
Author Response
1) More characterizations for the adsorbents are required such as:
A) FTIR on the functionalized samples to check the presence of –NH2 and –NO2 groups
Answer: Fig. S2 has been included in the revised manuscript for the FTIR spectra of UiO-66, UiO-66-NH2 and UiO-66-NO2 nanoparticles. We added on page 4:
“Fig. S2 shows the FTIR spectra of UiO-66, UiO-66-NH2 and UiO-66-NO2nanoparticles in this work. The peaks observed at 1578 and 1394 cm-1are characteristic of the in- and out-of-phase stretching modes from the carboxylate group associated with the organic linker.For UiO-66-NH2, the amino group can be distinguished at 3504 cm-1and 3381 cm-1, which can be attributed to asymmetric and symmetric N-H stretches respectively.For UiO-66-NO2, the asymmetric NO2stretching vibration can be assigned to the peak at 1536 cm-1.”
B) PXRD to check the crystallization of MOF
Answer: We cited a reference and also added Fig. S6and Fig. 7 for the crystallinity of the MOF particles in the composites. We added on page 5:
“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]. Fig. S6 confirms the crystallinity of UiO-66-NO2was 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-NH2 composites (Fig. S7).”
C) Zeta potential and BET to check the surface area, porosity and surface charge of the adsorbents
Answer: This study is focusing on monolithic composite materials. Zeta potential cannot be measured for them although zeta potential for UiO-66 nanoparticles may be obtained by measuring their aqueous suspensions. The surface area and porosity of UiO-66 and the composites were measured in a previous study in the group while the surface areas of functionalized UiO-66 nanoparticles were reported in literature. We added on page 5:
“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 (-NH2 or -NO2) 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 m2g-1and the CM/UiO-66 (1:2) gave a surface area of 339 m2g-1. For water treatment or other liquid phase application, the porosity and macropores can be measured by Hg intrusion porosimetry [14].”
D) FTIR on the adsorbents before and after adsorption as well as desorption
Answer: The UiO-66 nanoparticles and the composite monoliths were characterized by FTIR (Fig. S2 and Fig. S3). Due to the low percentage of MCPP adsorbed, useful information was not obtained by FTIR before and after adsorption. Therefore these data are not included in this manuscript.
2) More discussion on:
A) The adsorption mechanisms of MCPP on the different adsorbent.
Answer: The adsorption mechanism of MCPP on chitosan/UiO-66 has been discussed in a previous publication by this group. This manuscript focuses on the stability of the adsorbent materials. We added on page 8:
“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.”
B) Why does the CM/UiO-66-1 composite give better adsorption capacity in comparison with UiO-66?
Answer: There are two reasons for this: (1) The interconnected ice-templated porous scaffold provides enhanced mass transport; (2) Chitosan itself is also a very good adsorbent for MCPP. We added on page 7:
“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].”
C) Adsorption kinetics and isotherms studies may be also required.
Answer; The same response as given for question 2A. We mentioned this in the revised manuscript.
D) Their results should be compared with some other MOF or adsorbents used for MCPP and reported in the literature. For example, the same UiO-66 MOF reported to show adsorption capacity 140 mg/g of MCPP after 3 hrs (https://doi.org/10.1016/j.cej.2015.02.007), while their best composite shows only 45 mg/g after the same time. They consumed a lot of chemicals, energy and time to prepare these composites and at the end they show a stability and adsorptive properties in comparison with those reported in the literature.
Answer: The paper has already been cited in the manuscript [15]. However, because UiO-66 nanoparticle sizes were not mentioned, surface area was different, and the test conditions (e.g., MCPP concentration) were not exactly the same, comparison with this paper may not be very convincing. Furthermore, because the main objective of this study was to show the advantage of making porous composite scaffolds, i.e., high performance and easy recycling (as demonstrated in Fig. 5), instead of centrifuging or high-pressure filtration to remove MOF nanoparticles, we have chosen not to compare with literature values. Instead we have compared with our own reference samples, i.e., UiO-66 nanoparticles, composites treated differently, which were tested under the same conditions.
3) Other improvements
The definition of UiO-66 should be added to the abstract.
Answer: This has been added in the abstract in the revised manuscript.
Reviewer 2 Report
The paper deals with the preparation, characterization and methylchlorophenoxypropionic acid removal from water solution using the chitosan/UiO-66NP freeze dried monoliths. The use of Zr MOF composites for application in toxic agent removal from water is actually of certain interest and the materials used display interesting features. The paper can be published after some revision as follows:
-characterization of UiO-66 usually requires BET analysis, especially when they are obtained as small NP as the external surface are is usually important. Can the authors provide these analysis? Or at least report the literature data?
-XRPD patterns of the three UiO samples should be reported, at least in SI.
-the use of the three MOF with the Chitosan in two different weight percentage and with three different post treatment is a bit confusing for the reader. Maybe a table with a summary of the samples used could be usefull.
-The SEM imagess of the other MOF and composites should be reported
- what is the origin of the sigmoidal shape of the absorption curve for CM/UiO-66HT shown in figure 3?
Author Response
The paper deals with the preparation, characterization and methylchlorophenoxypropionic acid removal from water solution using the chitosan/UiO-66NP freeze dried monoliths. The use of Zr MOF composites for application in toxic agent removal from water is actually of certain interest and the materials used display interesting features. The paper can be published after some revision as follows:
-characterization of UiO-66 usually requires BET analysis, especially when they are obtained as small NP as the external surface are is usually important. Can the authors provide these analysis? Or at least report the literature data?
Answer: The surface area and porosity of UiO-66 and the composites were measured in a previous study in the group while the surface areas of functionalized UiO-66 nanoparticles were reported in literature. We added on page 5:
“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 (-NH2 or -NO2) 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 m2g-1and the CM/UiO-66 (1:2) gave a surface area of 339 m2g-1. For water treatment or other liquid phase application, the porosity and macropores can be measured by Hg intrusion porosimetry [14].”
-XRPD patterns of the three UiO samples should be reported, at least in SI.
Answer: We cited a reference and also added Fig. S6 and Fig. S7 for the crystallinity of the MOF particles in the composites. We added on page 5:
“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]. Fig. S6 confirms the crystallinity of UiO-66-NO2was 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-NH2 composites (Fig. S7).”
-the use of the three MOF with the Chitosan in two different weight percentage and with three different post treatment is a bit confusing for the reader. Maybe a table with a summary of the samples used could be useful.
Answer: Table S1 has been included in the SI as suggested. This was mentioned on page 3 of the revised manuscript:
“Table S1 lists the chitosan/UiO-66 samples prepared and their descriptions, with post-treated samples also included.”
-The SEM images of the other MOF and composites should be reported
Answer: They are now included in the revised manuscript as Fig. S3 and Fig. S4. We mentioned this on page 4 in the revised manuscript:
“Similar pore morphologies and MOF nanoparticles were observed for CM/UiO-66 composites and CM-UiO-66-NH2composites (Fig. S3 and Fig. S4).”
- what is the origin of the sigmoidal shape of the absorption curve for CM/UiO-66HT shown in figure 3?
Answer: This is likely caused by the dense skin surface 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 more swollen scaffold, facilitating mass transport and higher adsorption. We added on page 6:
“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 more swollen scaffold, facilitating mass transport and higher adsorption.”
Round 2
Reviewer 2 Report
I am satisfied on the changes made to the manuscript. It can be now accepted. However I suggest the authors to report the experimental BET analysis in future works with MOFs rather than only cite literature data. It is a mandatory analysis for a correct characterization of MOF
