Sustainable Hierarchically Porous Reusable Metal–Organic Framework Sponge as a Heterogeneous Catalyst and Catalytic Filter for Degradation of Organic Dyes

Advanced oxidation processes based on sulfate radical are considered one of the most promising wastewater treatment technologies currently. Among heterogeneous catalysts, cobalt metal–organic framework (MOF) has been widely reported. However, the inherent powder form of MOF hinders its practical application and reusability. Therefore, innovative methods to increase the loading capacity and the accessibility of MOF active sites in monolithic materials are required. Therefore, a simple and scalable method of fabricating a stable, hierarchical porous zeolitic imidazolate framework (ZIF‐67) 3D sponge by growing MOF on a short electrospun fiber network is shown. The sponge can efficiently activate peroxymonosulfate and rapidly degrade an exemplary organic dye (Rhodamine B) with a degradation efficiency of 100%. The resulting multilevel, hierarchical porous structure is beneficial to the mass transfer of reagents making the catalytic process efficient. This also enables the use of the ZIF‐67 as an efficient catalytic filter for continuous removal of dye. The sponge can be recycled and reused for several cycles due to its robustness without loss in efficiency. The proposed research strategy provides a new way to design MOF 3D monolithic materials.

Advanced oxidation processes based on sulfate radical are considered one of the most promising wastewater treatment technologies currently.Among heterogeneous catalysts, cobalt metal-organic framework (MOF) has been widely reported.However, the inherent powder form of MOF hinders its practical application and reusability.Therefore, innovative methods to increase the loading capacity and the accessibility of MOF active sites in monolithic materials are required.Therefore, a simple and scalable method of fabricating a stable, hierarchical porous zeolitic imidazolate framework (ZIF-67) 3D sponge by growing MOF on a short electrospun fiber network is shown.The sponge can efficiently activate peroxymonosulfate and rapidly degrade an exemplary organic dye (Rhodamine B) with a degradation efficiency of 100%.The resulting multilevel, hierarchical porous structure is beneficial to the mass transfer of reagents making the catalytic process efficient.This also enables the use of the ZIF-67 as an efficient catalytic filter for continuous removal of dye.The sponge can be recycled and reused for several cycles due to its robustness without loss in efficiency.The proposed research strategy provides a new way to design MOF 3D monolithic materials.
14a,18] In this work, we first show a procedure of loading ZIF-67 in large amounts (as large as 72%) on a 3D porous fibrous network skeleton of polyimide (PI) electrospun short fibers.In our previous works, we established a method of making PI porous frameworks (sponges) using short electrospun fibers and showed its use as pressure sensors, heat insulation, etc., and skeleton for the growth of covalent organic framework. [19]The present method allows in situ growth of ZIF-67 onto the surface of PI network fibers of the 3D skeleton and thereby bringing ZIF-67 in the shape of a 3D sponge with hierarchical porosity and mechanical stability.The hierarchically porous network facilitates the transport of reagents required for the efficient degradation of organic dyes and at the same time, it exposes a plenty of active sites (ZIF-67) for degradation reaction.After establishing the preparation procedure, the practicability of polymer-MOF sponge as a reusable catalyst to activate PMS for the degradation of organic dye Rhodamine B (Rh B) under different conditions was shown.The unique hierarchical porous structure of the sponge leads to a good water flux and therefore, its utility as a filter for continuous degradation of organic dye is also shown by assembling a simple filter device.This work provides a new avenue for applying MOF-based composites to practical wastewater treatment technologies as efficient reusable heterogeneous materials and filters.

Results and Discussion
The preparation process of PI/PAN@ZIF-67 sponge is shown in Figure 1.First, PI short fibers, polyacrylonitrile (PAN), and Co(NO 3 ) 2 •6H 2 O are dispersed in dimethyl sulfoxide (DMSO), and the PI/PAN/Co 2þ sponge with honeycomb frame structure is obtained by freeze-drying.PI short fibers were prepared by mechanical cutting of a PI nonwoven prepared by electrospinning as described in our previous work. [20]PI fibers had an average diameter d = 521 AE 143.0 nm and length L = 77 AE 33 μm, respectively.Fifty short fibers were randomly selected in scanning electron microscope (SEM) images for calculating the average diameter and length using ImageJ software (Figure S1, Supporting Information).PAN as a binder ensures the stability of PI short fibers in the 3D structure and makes Co 2þ uniformly dispersed on the surface of PI short fibers, providing a great deal of sites for the growth of ZIF-67.Then the PI/PAN/ Co 2þ sponge was put into the precursor solution of ZIF-67 (aqueous solution of Co nitrate hexahydrate and 2-methylimidazole), and ZIF-67 grew in situ on the surface of the PI fiber to obtain a sponge in which ZIF-67 was grown on the surface of fibers making network porous PI/PAN template sponge.The final sponge is designated as PI/PAN@ZIF-67.It is worth noting that stirring is necessary during the preparation of PI/PAN@ZIF-67 sponge.If there is no stirring, the nucleation of ZIF-67 is slow, and it is easy to generate larger sized particles, which leads to nonadherence and easy detachment from the sponge surface giving defects and low ZIF-67 loading (Figure S2, Supporting Information).The loading of ZIF-67 was determined according to the mass change of PI/PAN/Co 2þ sponge after the growth of ZIF-67.The loading of ZIF-67 could be as high as 72%.
The morphology of the samples was analyzed by SEM. Figure 2a shows the optical picture of PI/PAN@Co 2þ sponge.A honeycomb structure with uniform macroscopic and microscopic pores was visible (Figure 2b).The wall of the sponge is composed of PI short fibers interwoven with each other, and the PI short fibers have a relatively smooth and clean surface (Figure 2c,d).It can be clearly observed that the wall of the PI/PAN@Co 2þ sponge is a porous network structure with micron-scale pores (4-30 μm) formed by percolation of fibers.This porous network structure provides amount of growth sites and reaction space for ZIF-67 formation, indicating its excellent performance as a carrier.The energy dispersive spectrometer (EDS) spectrum of PI/PAN/Co 2þ sponge proves that Co 2þ was successfully attached to the surface of PI short fibers and dispersed uniformly (Figure 2e-h).This provides lots of nucleation sites for the growth of ZIF-67.The honeycomb porous network structure of PI/PAN@Co 2þ sponge was maintained after in situ growth of ZIF-67 (Figure 2j).The surface of PI short fibers is completely covered by lots of ZIF-67 particles (Figure 2k,l).The in situ growth of ZIF-67 on the PI short fiber enables the internal active sites in the ZIF-67 structure to be well preserved.The nanopores of the ZIF-67 itself, the micropores between the cross-linked PI short fibers, and the macroscopic and regular honeycomb macropores of the sponge form a 3D structure of multilevel and hierarchical macro-micropores.This structure can not only expose the active site of ZIF-67 to promote the activation of PMS, but also facilitate the intermolecular mass transfer in the solution to accelerate the degradation of the dye.
We used X-Ray diffraction (XRD) to analyze the crystal properties of PI/PAN and PI/PAN@ZIF-67 sponges, as shown in Figure 3a.For the PI/PAN sponge, 2θ at 17.5°corresponding to the (0 0 1) has a broad characteristic diffraction peak. [21]13a] In the XRD pattern of PI/PAN@ZIF-67 sponge, the characteristic diffraction peaks of PI/PAN and ZIF-67 could be well confirmed.This indicates the successful preparation of PI/PAN@ZIF-67 sponges.
The Fourier transform infrared (FT-IR) spectrum of the PI/PAN@ZIF-67 sponge is shown in Figure 3b.The characteristic stretching bands at 425, 992, and 1570 cm À1 are attributed to the Co─N, C─N, and C═N of ZIF-67. [22]The peak at 1364 cm À1 is the C─N stretching of the PI imide ring; 1715 and 1779 cm À1 are the symmetric and asymmetric stretching vibration peaks of C═O in the PI imide ring, respectively. [23]2240 cm À1 is attributed to the C≡N stretching vibration peak in PAN. [24]he mechanical properties and shape recovery ability of the sponge are particularly important for any practical use, and they were evaluated by cyclic compression experiments.Figure 4a,b shows the cyclic compressive stress-strain curves of PI/PAN and PI/PAN@ZIF-67 sponges, respectively.It can be found that the compressive strength of PI/PAN is only 5.0 kPa (50% strain), and the sponge height is compressed by about 25.8% after 50 cycles of compression, indicating that the internal structure has undergone serious collapse.The compressive strength of the PI/PAN@ZIF-67 sponge was 28.1 kPa (50% strain), and the height decreased only by 9.9% after 300 cycles of compression.This is due to the fact that the PI/PAN@ZIF-67 sponge consists of two parts: the soft PI short fibers and the hard ZIF-67.The hard ZIF-67 grown on the surface of PI short fibers in situ can significantly improve the mechanical properties of PI/PAN@ZIF-67 sponge and ensure its structural stability.In addition, the 0.1 g PI/PAN@ZIF-67 sponge can withstand  a weight of 200 g without collapsing (Figure 4c), further demonstrating its excellent robustness.
Using Rh B as a typical example, we explored the potential of PI/PAN@ZIF-67 sponge as a catalyst in activating PMS for the degradation of organic dyes.Figure 5 shows the degradation of Rh B under different catalytic systems.To rule out the possible adsorption effect of the sponge in the removal of Rh B from the aqueous solution, we tested the change in the dye concentration after putting PI/PAN sponge, PI/PAN@ZIF-67 sponge, and ZIF-67 in the Rh B solution, respectively.The results showed a small decrease (4.9%, 6.2%, and 5.2%, respectively) in Rh B concentration after 30 min as determined by UV-vis spectrophotometer (the concentration of Rh B was evaluated at a wavelength of 555 nm).A Rhodamine solution with only PMS (no sponge) showed 12.9% of Rh B degradation.This indicates that without the activation by a catalyst, PMS cannot effectively degrade dyes.However, when PMS was activated by PI/PAN@ZIF-67 sponge, the Rh B dye was degraded up to 97.4% within 5 min and completely degraded within 30 min.These results indicate that PI/PAN@ZIF-67 sponge can efficiently activate PMS.In addition, we also studied the degradation of Rh B in the presence of ZIF-67 particles and PMS, and the results showed that 93.6% of Rh B degraded within 5 min.This shows that in comparison to ZIF-67 powder, the degradation effect of PI/PAN@ZIF-67 sponge is not compromised.In addition, the degradation kinetics of Rh B conform to the pseudo-first-order rate law-ln C t /C 0 = kt, and the kinetic rate is shown in Figure S3, Supporting Information.
Dye solution of different concentrations (from 25 to 100 mg L À1 ) was used to study the degradation by PI/PAN@ZIF-67 sponge-activated PMS (Figure 6a).When the initial concentration of the dye was reduced from 100 to 25 mg L À1 , the degradation of Rh B within 5 min increased from 68.8% to 100%.It is worth mentioning that when the dye concentration is 25 mg L À1 , the dye can be completely degraded within 20 s as shown in Figure S4, Supporting Information.However, when the dye concentration was 20 mg L À1 or below, the degradation was immediate.The degradation kinetics could not be followed due to the very fast rate of degradation for  concentrations below 20 mg L À1 .Therefore, 50 mg L À1 dye concentration was chosen as a benchmark for further experiments.
pH of the solution can have a significant effect on the degradation process.Figure 6b shows the effect of different pH (3.0-9.0)values on the degradation of dye by PI/PAN@ZIF-67 sponges.When pH = 3.0, only 67.8% of Rh B degraded within 5 min.This may be because, under acidic conditions, H þ makes HSO 5 À more stable, which inhibits the formation of free radicals and reduces the degradation rate. [25]When pH = 5.0, 82.1% of Rh B was degraded within 5 min, was close to that of pH = 7.0.However, when pH = 9.0, only 56.9% of Rh B could be degraded within 5 min.Under alkaline conditions, PMS selfdecomposes, and the content of free radicals generated therefore decreases slowing down the degradation of dyes. [26]But fortunately, the degradation of Rh B can be as high as more than 99% under different pH values within 30 min.This indicated that PI/PAN@ZIF-67 sponge can activate PMS to degrade organic pollutant dyes in a wide pH range.When the pH is 3.0, 5.0, 7.0, and 9.0, the rate constant K values for degradation are 0.24, 0.30, 0.47, and 0.09 min À1 , respectively (Figure S5, Supporting Information).
As the main source of free radicals, the added amount of PMS is very important in the degradation process of dyes.The effect of PMS content on the degradation of Rh B is shown in Figure 6c.When the content of PMS was 0.5 mM, only 37.9% of the Rh B was degraded within 5 min, and only 72.7% of the dye was degraded within 30 min.However, when the content of PMS was increased to 1 mM, the degradation of dye increased to 76.1% within 5 min and Rhodamine degradation reached 98.1% within 30 min, which significantly improved the degradation situation.The results showed that an optimum amount of PMS is required so that sufficient free radicals are generated making the degradation occur effectively.This shows that the content of added PMS is very important to the degradation of the dye.Furthermore, when the content of PMS continued to increase to 2 mM, there was little difference compared to the time when PMS was 1.5 mM.This is because the content of the PI/PAN@ZIF-67 sponge was fixed, and the active sites of the PI/PAN@ZIF-67 sponge for activating PMS had reached saturation.Even if the content of PMS continues to increase, the catalyst cannot continue to activate the excess PMS.Therefore, the PMS content of 1.5 mM is the best choice.This result is also reflected in the kinetics.When the PMS content increases from 0.5 to 2.0 mM, the k values were 0.06, 0.22, 0.44 and 0.44 min À1 , respectively (Figure S6, Supporting Information).
Figure 6d shows the effect of the amount of catalyst (sponge) on the degradation of dyes.The degradation of the dye increased from 87.8% to 97.4% within 5 min on increasing the amount of catalyst from 50 to 100 mg L À1 .The k value increased from 0.31 to 0.44 min À1 (Figure S7, Supporting Information).On further increase in the catalyst content to increase to 125 mg L À1 , 98% of the Rh B was degraded within 5 min (k = 0.44 min À1 ).Compared with the catalyst addition of 100 mg L À1 , there is no obvious increase.This suggests that at high catalyst dosage, the efficiency of PMS to utilize the excess active sites at the PI/PAN@ZIF-67 sponge to generate free radicals is limited.Therefore, 100 mg L À1 catalyst is the optimal input amount.
In addition, we also examined the effect of PI/PAN@ZIF-67 sponge-activated PMS on dye degradation at different temperatures (Figure 6e).There was no significant difference in the % degradation observed on raising the temperature from 25 to 45 °C (Figure S8, Supporting Information).
To understand the catalytic process of PI/PAN@ZIF-67 sponge, we explored the reaction mechanism.In previous reports, it has been proved that Co ions in ZIF-67 can undergo cyclic conversion in divalent and trivalent states, activating PMS to produce sulfate and hydroxide radicals, which have higher oxidation-reduction potential, can eventually oxidize the dye to water and carbon dioxide as in Equation ( S1)-(S4), Supporting Information. [17]We conducted a series of reactive oxygen species (ROS) capture experiments to find out which radical is mainly taking part in the degradation process.It is known that potassium iodide (KI) can effectively capture both sulfate and hydroxide radicals, whereas methanol (MeOH) is selective in capturing sulfate radicals.On the other hand, tert-butanol (TBA) traps hydroxide radicals. [15]As shown in Figure 6f, when KI was added to the catalytic system, only 15.1% Rh B was degraded within 30 min.As KI captures both types of radicals, it could not be said which radical played the major role in the degradation process.When methanol was added to the catalytic system, only 66.5% of the dye was degraded within 30 min.When TBA was added, the Rh B can be degraded up to 99%.The addition of TBA quencher has little effect on the degradation of dye.The results indicated that sulfate radical played major role in the degradation of Rh B. The mechanism of activating PMS by PI/PAN@ZIF-67 sponge to generate free radical degradation of Rh B is shown in Figure S9, Supporting Information.The degradation mechanism of Rh B by the attack of ROS generating several different intermediates through de-ethylation, followed by deamination, dealkylation, decarboxylation, and chromophore cleavage to form other smaller molecule intermediates, is described in the literature.Finally, these small molecules are further mineralized by ROS into H 2 O and CO 2 . [27]or any practical sustainable application, it is important that the catalysts are stable and reusable.Figure 7 shows the degradation efficiency after reusing the regenerated PI/PAN@ZIF-67 sponge.After the first degradation cycle, the PI/PAN@ZIF-67 sponge only needs to be removed from the reaction solution with tweezers.There is no need for elaborate centrifugation or filtration.It is very simple and convenient.Then wash it three times with water and methanol respectively.After vacuum drying at 80 °C, it is directly used for the next cycle.As a result, after five cycles of use, degradation of the dye was negligibly reduced by only 3%.The XRD patterns of the PI/PAN@ZIF-67 sponge before and after use are also shown in Figure S10, Supporting Information.Through comparison, it can be found that the diffraction peaks of the used PI/PAN@ZIF-67sponge and fresh PI/PAN@ZIF-67 sponge are basically consistent, indicating that the PI/PAN@ZIF-67 sponge exhibits good stability during the degradation process of Rh B.
In addition, in order to further expand the application of PI/PAN@ZIF-67 sponge in real-life application, we constructed the PI/PAN@ZIF-67 sponge as a simple filtration device to evaluate its performance for the continuous treatment of dye wastewater solutions (Figure 8a).Thanks to the multilevel hierarchical porous structure of the PI/PAN@ZIF-67 sponge, the Rh B solution can quickly pass through the PI/PAN@ZIF-67 sponge under self-gravity and its catalytic action provided a clear solution after passing through the sponge.The removal efficiency of Rh B is still as high as 95% after the filter device continuously treats the Rh B solution for 6 h (as shown in Figure 8b, Video 1, Supporting Information).As a result, the PI/PAN@ZIF-67 sponge can also be used as a catalytic filter to activate PMS for the degradation of dyes which makes it promising for scaling up in the future.
Although the present work was carried out using Rh B as an exemplary dye, our system should be valid for the degradation of other dyes, such as methylene blue, acid yellow-17 (AY), and orange II (AO7) that are known to undergo degradation by PMS activation. [28]

Conclusion
In this work, we prepared PI/PAN@ZIF-67 sponges with high ZIF-67 loading (up to 72 wt%), multilayered, and hierarchical macro-microporous honeycomb structure by in situ growth of ZIF-67 on the surface of PI short fibers.The 3D porous framework and high ZIF-67 loading provided the sponge with excellent mechanical properties and compression resistance, making the PI/PAN@ZIF-67 sponge reusable for practical use.The PI/PAN@ZIF-67 sponge exhibited high efficiency in the removal of organic dye by PMS activation.We explored that the optimal conditions for PI/PAN@ZIF-67 sponge to activate PMS to degrade Rh B organic dye were pH = 7, PMS = 1.5 mM, catalyst (PI/PAN@ZIF-67 sponge) = 100 mg L À1 , and the initial concentration of dye was 50 mg L À1 .Also, filtration experiments using PI/PAN@ZIF-67 sponge as a catalytic filter were very promising.This study opens new opportunities to explore MOF-based materials in the form of porous robust 3D sponges for water remediation.Furthermore, this strategy can also be applied to prepare monolithic materials of other types of MOFs and is expected to be suitable for other applications as well.
Preparation of PI Short Fibers: PI short fibers were prepared by cutting PI electrospun nonwoven through a mechanical cutting process.Briefly, 10 g of PI electrospun nonwovens were first cut with scissors into small pieces and put into 1 L of solvent consisting of isopropanol/water (v/v = 3/1).The mixture was cooled using liquid nitrogen and then cut with a mechanical cutter (Robot Coupe Blixer 4, Rudolf Lange GmbH & Co. KG) at 3000 rpm for 25 h.Then, short PI fibers were filtered and freeze-dried for 24-36 h.
Preparation ZIF-67 Particles: ZIF-67 particles were prepared based on the literature that has been reported with a little modification. [29]In brief, 0.145 g cobaltous nitrate hexahydrate was dissolved in 10 mL of Milli-Q water (designated as solution A); 2.37 g 2-methylimidazole was dissolved in 10 mL of Milli-Q water (designated as solution B).Then solution B was quickly poured into solution A and stirred at room temperature for 24 h to obtain purple particles.The purple particles were collected by centrifugation, washed three times with water and methanol, respectively, and finally, dried at 80 °C in the oven to obtain ZIF-67 particles.
Preparation of Template PI/PAN Sponge: 0.5 g PI short fibers and 0.5 g PAN were dissolved in 12.5 mL DMSO in a 25 mL flask.The mixture was cooled to À20 °C in a refrigerator for 2 h.Afterward, it was freeze-dried to get the PI/PAN sponge.
Preparation of PI/PAN@ZIF-67 Sponge: In the first step, a PI/PAN@ Co 2þ sponge was prepared using the same method as described above for PI/PAN sponge.Briefly, 0.1 g PI short fibers, 0.1 g PAN, and 0.1 g Co(NO 3 ) 2 •6H 2 O were dissolved in 25 mL DMSO in a 50 mL flask.Then free-drying after cooling to À20 °C provided PI/PAN@Co 2þ sponge.0.145 g of Co nitrate was dissolved in 10 mL of Mili-Q water, which was designated as solution A. Subsequently, the PI/PAN@Co 2þ sponge was placed in solution A and allowed to stand for 2 h.2.32 g of 2-methylimidazole was dissolved in Mili-Q 10 mL of water (designated as solution B).Afterward, solution B was quickly poured into solution A. The mixed solution with the sponge inside was stirred at room temperature for 24 h to give a purple sponge.Finally, the sponge was washed with water and methanol several times to remove excess of unreacted materials and dried at 80 °C to obtain PI/PAN@ZIF-67 sponge.
Catalytic Batch Experiments: All degradation experiments were carried out in 150 mL glass containers with magnetic stirring at 300 rpm.The pH value of the solution was adjusted by adding 0.1 M HCl and 0.1 M NaOH, respectively.Typically, a known amount of PI/PAN@ZIF-67 sponge was added to 100 mL of 50 mg L À1 Rh B solution.Subsequently, a certain amount of PMS was added to the solution and the time was noted.Every 5 min 1 mL sample was pipetted out, quenched with 0.5 mL methanol.Then, the degradation of the Rh B was measured using a UV-vis spectrophotometer at a wavelength of 555 nm.The degradation efficiency of Rh B was calculated according to Equation (1) where R is the degradation efficiency of Rh B (%), and C 0 and C t are the concentrations of Rh B at time 0 and time t (mg L À1 ), respectively.And the kinetics of Rh B degradation process was investigated according to the pseudo first-order kinetics Equation ( 2) in which k is a rate constant and t is the degradation time (min).Besides, all batch experiments were repeated, and the average value with the standard deviation was presented for the results.
Reusability of Sponges: After the initial degradation experiment, the PI/PAN@ZIF-67 sponge was taken out from the reaction solution using tweezers, followed by simple cleaning with water and methanol.It was directly used for the next cycle after drying.
Continuous Flow Catalytic Test Using Sponge as Catalytic Filter: The sponge was also tested as a catalytic filter.For this, the PI/PAN@ZIF-67 sponge with a diameter of 2.5 cm and a height of 1.5 cm was loaded in a 20 mL plastic syringe, and the Rh B solution (25 mg L À1 ) containing PMS (1.5 mM) was filtered under the action of gravity, during which the Rh B solution was continuously added to ensure that it can be continuously filtered, and the filtered solution was collected and analyzed to determine the concentration as described above.

Figure 5 .
Figure 5. Plots of C t /C 0 versus time for studying the degradation of Rh B under different conditions.Reaction conditions: Rh B concentration = 50 mg L À1 , PMS concentration = 1.5 mM, catalyst amount = 100 mg L À1 , pH = 7, temperature = 25 °C.

Figure 6 .
Figure 6.Plots of C t /C 0 versus time for studying the effect of a) the dye (Rh B) concentration, b) pH, c) the concentration of PMS, d) the dosage of catalyst (PI/PAN@ZIF-67 sponge), e) temperature, and f ) radical scavengers on the Rh B degradation.The data for (a) were collected using PMS concentration = 1.5 mM, catalyst amount = 100 mg L À1 , pH = 7, temperature = 25 °C.The data for (b) were collected using Rh B solution concentration = 50 mg L À1 , PMS concentration = 1.5 mM, catalyst amount = 100 mg L À1 , and temperature = 25 °C.The data for (c) were collected using Rh B solution concentration = 50 mg L À1 , catalyst amount = 100 mg L À1 , pH = 7, temperature = 25 °C.The data for (d) were collected using Rh B solution concentration = 50 mg L À1 , PMS concentration = 1.5 mM, PH = 7, and temperature = 25 °C.The data for (e) were collected using Rh B solution = 50 mg L À1 , PMS concentration = 1.5 mM, catalyst mount = 100 mg L À1 , pH = 7.The data for (f ) were collected using Rh B solution concentration = 50 mg L À1 , PMS concentration = 1.5 mM, catalyst amount = 100 mg L À1 , pH = 7 and temperature = 25 °C.

Figure 8 .
Figure 8. a) Photograph of the assembly showing degradation of Rh B by filtration process.PI/PAN@ZIF-67 sponge was fitted in a syringe for use as a catalytic filter to degrade Rh B. b) Continuous degradation experiment of Rh B with PI/PAN@ZIF-67 sponge as a filter.Reaction condition: pH = 7.0, 25 °C, dye (Rh B) concentration = 25 mg L À1 , PMS concentration = 1.5 mM.