Preparation of Ice-templated MOF-polymer composite monoliths and their application for wastewater treatment with high capacity and easy recycling

Qingshan Fua,b, Lang Wena, Lei Zhanga, Xuedan Chena, Daniel Punb, Adham Ahmedc, Yonghong Yangb, Haifei Zhangb,*

a College of Material Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, China

b Department of chemistry, University of Liverpool, L69 7ZD, United Kingdom

c Thermo Fisher Scientific, Runcorn WA7 1TA, UK

Abstract:

An ice-templating process was used to fabricate polymer/MOF monoliths, specifically

chitosan/UiO-66, as adsorbents for water treatment. The ice-templated macropores

enhanced mass transport while the monoliths could be easily recovered from solution. This

was demonstrated by the adsorption of methylchlorophenoxypropionic acid (MCPP, a

herbicide compound) from dilute aqueous solution. To enhance the stability, the freeze-

dried monoliths were treated with NaOH solution, solvent exchanged, and dried. The

treated chitosan/UiO-66 monolith achieved an adsorption capacity of 34.33 mg g -1 (a

maximum theoretic value of 334 mg g-1 by the Langmuir model), closer to the capacity (36.00

mg g-1) of the freshly prepared UiO-66 nanoparticles, much higher than that of the NaOH-

washed UiO-66 nanoparticles (18.55 mg g-1), by performing the tests in 60 ppm MCPP

solution. The composite monolith could be easily picked up using tweezers and used for

recycling tests. Over 80 % of the adsorption capacity was retained after 3 more cycles. The

powder x-ray diffraction and N2 sorption studies suggested the crystalline structure of UiO-

66 was destroyed during NaOH washing procedure. This, however, provides the potential to

improve the adsorption capacity by developing methods to fabricate true polymer/MOF

composites.

Keywords: Ice-templating, porous monoliths, polymer/MOF composites, UiO-66, chitosan,

water treatment

1

1. Introduction

Clean water resources are depleting rapidly due to contamination with various pollutants

including organic chemicals.1 Phenoxyacids widely used as pesticides in agriculture are

frequently detected in groundwater. It has been reported that phenoxyacids could cause

liver tumours.2 As such, the permitted concentrations of pesticides in drinking water and

groundwater should not exceed 0.1 μg L-1 for a single compound, or 0.5 μg L-1 for the sum of

all pesticides in Europe.3 Methylchlorophenoxypropionic acid (MCPP), one type of

phenoxyacids, has been widely used as herbicide in spite of its high toxicity. MCPP has been

frequently detected in ground water with the concentrations usually exceeding the

limitation in ground water abstraction wells. In order to remove MCPP from water,

researchers have developed many methods, including advanced oxidation processes,

photocatalytic degradation and secondary rapid sand filters.3-5 Nevertheless, the most

attractive method to remove MCPP from water is adsorption, because it is a simple and low

cost process with mild operating conditions. Some adsorbents such as activated carbons5

and iron oxides6 have exhibited good adsorption capacity of MCPP from water.

Metal-organic frameworks (MOFs) are crystalline microporous (and enlarged mesoporous)

materials with metal nodes and organic linkers. By tuning the metal ions and organic linkers,

it is possible to systematically to adjust pore size, pore morphology, porosity, and surface

functionality of MOFs.7 MOFs have been investigated and used in a wide range of application

including as adsorbents for water treatment.7-10 However, the chemical stability of MOFs in

aqueous solutions particularly with low pH and high pH has long been an issue for water

treatment and other water-based applications.11 Some stable MOFs have been developed,

including ZIF-8, DUT-68, NU-1000, UiO-66, PCN-426-Cr(III), and UiO-66-NO2.11-12 With respect

to MCPP, Bansal et al. used Basolite Z1200 (ZIF-8) to adsorb MCPP from ethanol solution of

MCPP. ZIF-8 as he adsorbent showed high MCPP-removal capacity 13. Seo et al. employed

UiO-66 in adsorptive removal of MCPP from water and found that UiO-66 possesses rapid

2

and high MCPP-adsorbed capacity, which was higher than that of activate carbon especially

at low MCPP concentrations 14.

MOF nanoparticles are usually used for adsorption in liquid phase. This is due to the limited

mass transport in microporous MOFs and higher exposed surface area and hence enhanced

mass transport in nanosized MOFs. This, however, creates difficulty in collecting the MOF

nanoparticles after the adsorption test, usually by centrifugation or infiltration with

nanoporous membranes.14 The preparation and use of MOF-polymer monoliths while

retaining the high adsorption capacity but offering facile recycling will be highly beneficially

and a big step forward. MOF-polymer composites (quite often membranes for gas

uptake/separation) can be prepared by blending MOF particles with polymer solution and

allowing the solvent to evaporate,15 electrospinning of MOF/polymer solution,16 synthesis of

MOFs in porous polymer17-18 or hollow fibers,19 polymerisation of MOF-based Pickering

emulsion.20 MOFs may form on pre-fabricated microparticles.21-22

The selection of polymer for MOF-polymer composites is important, particularly for large

scale application. To achieve sustainable applications, a naturally abundant polymer should

be preferred. And this polymer should also exhibit a good adsorption capacity towards the

target pollutants. Otherwise, the polymer may considerably dilute the adsorption effect of

MOFs in the composites. Chitosan, a type of biopolymer, is a good adsorbent to remove

various kinds of anionic and cationic dyes as well as heavy metal ions.23-25 Therefore, chitosan

can be a good candidate for the preparation of MOF-polymer composites.

In this work, we report the fabrication of UiO-66-chitosan monoliths via an ice-templating

approach. UiO-66 is stable in water11-12 and has been used for the adsorption of MCPP.14 UiO-

66 powders or nanoparticles can be readily prepared by solvothermal methods.26-27 Ice

templating has been a simple and effective route to the preparation of various types of

porous material,28-30 including porous chitosan or its composites.31-32 The ice-templating

3

method involves the freezing process to generate ice crystals and the subsequent freeze-

drying process to remove ice crystals and produce ice-templated macroporous materials.28

Although freeze-drying has been used as a drying method to obtain highly porous MOFs,33

the ice-templating method has rarely been used to synthesize MOFs. To the best of our

knowledge, there has been one report on the synthesis of hierarchically porous HKUST-1

monoliths.34 For the preparation of MOF/polymer composites by ice templating, it is also

highly limited, with the only report being the freeze-drying of UiO-66-based Pickering

emulsion.35 Chitosan can only be dissolved in acidic water. This may be the reason that the

general reports of porous chitosan-MOF composites are scarce, as a large number of MOFs

are not stable in acidic water.7, 11 In this work, the ice-templating method produces porous

materials with highly interconnected porosity which enhances mass transport and promotes

the adsorption of pollutants from water. The ice-templated chitosan-UiO-66 composite

monoliths are evaluated (directly or treated with basic solution) for the adsorption of MCPP

from its diluted aqueous solution. High adsorption capacity close to pure UiO-66 is achieved.

The composite monoliths can be easily picked up from the solution using tweezers and the

recycling adsorption is also demonstrated. The adsorption data are further fitted into

different adsorption isotherm models and the adsorption kinetics are examined.

2. Experimental Section

2.1 Materials

Methylchlorophenoxypropionic acid (MCPP), chitosan (medium molecular weight),

zirconium chloride (ZrCl4), benzoic acid, terephthalic acid (BDC) and N,N-dimethylformamide

(DMF) were purchased from Sigma Aldrich and used without additional purification.

Hydrochloric acid (37 %), deionized water and standard GPR grade solvents were used

routinely as required.

2.1.1 Preparation of UiO-664

ZrCl4 (2 mmol), BDC (2 mmol), benzoic acid (20 mmol), and HCl (37%, 4 mmol) in 36 mL of

DMF were ultrasonically dissolved in a Pyrex vial, covered with aluminium foil. The mixture

was heated in an oven at 120 °C for 48 h. After cooling down to room temperature, a white

powder of UiO-66 nanoparticles (NPs) was harvested by centrifugation and washed with

DMF at room temperature.

2.1.2 Ice-templated chitosan/UiO-66 monoliths

For preparation of chitosan/UiO-66 composite, 0.2 g chitosan powder was dissolved in 20 mL

deionized water added with 120 µL acetic acid with stirring, until a transparent solution

formed. Then 0.2 g or 0.4 g UiO-66 was added into the 1 wt% chitosan solution, respectively.

Following by strong stirring, the two composite solutions (weight ratios of chitosan to UiO-

66 were 1:1 and 1:2) were transferred into several disposable borosilicate glass test tubes

(10x15 mm, purchased from Fisher Scientific), with 1 mL of the solution per test tube. The

tubes containing the solution were slowly immersed into a bath of liquid nitrogen at a rate

of about 0.5 cm min-1 and kept in the bath for 5 min. The frozen samples were freeze-dried

for approximately 48 h in a VirTis Advantage freeze dryer to form dry chitosan/UiO-66

monoliths. The two monoliths with different chitosan-to-UiO-66 ratios were denoted as

Chitosan/UiO-66-1(weight ratio of chitosan to UiO-66 was 1:1) and Chitosan/UiO-66-2

(weight ratio of chitosan to UiO-66 was 1:2). For control, 1 wt% chitosan solution without

UiO-66 was freeze-dried following the same procedure, and the acquired monolith was

denoted as Chitosan.

After freeze-dried, the three kinds of monoliths were soaked in 1 M NaOH solution for 15

min. The NaOH-treated monoliths were washed with deionized water until the pH value

approached to 7. Afterwards all washed monoliths were soaked in acetone for 30 minutes,

replaced with fresh acetone 3 times then soaked in acetone overnight. Finally the monoliths

were soaked in cyclohexane for 1 h before air-drying in fume hood. After these treatments, 5

the three kinds of monoliths: Chitosan, Chitosan/UiO-66-1 and Chitosan/UiO-66-2 were

denoted as Chitosan-W, Chitosan/UiO-66-1-W and Chitosan/UiO-66-2-W, respectively.

2.2 Characterization and measurement

2.2.1 Adsorption procedure

Firstly, a stock solution of MCPP (200 ppm) was prepared by dissolving MCPP powder (100

mg) in deionized water (500 mL). Then a series of MCPP solutions with different

concentrations were achieved by dilution of the stock solution with deionized water. The

concentrations of the MCPP were evaluated by the UV absorbance at 279 nm of the

solutions with a UV spectrophotometer (MQX200, BioTek Instruments, Inc.). A calibration

curve was obtained based on the results of UV absorbance at 279 nm of the MCPP solutions

(2-200 ppm).

The three monolith samples: Chitosan-W, Chitosan/UiO-66-1-W, Chitosan/UiO-66-2-W and

UiO-66 powder were employed for the adsorption assessment. Exact amounts of the

samples (10 mg) were added into the 10 mL MCPP solution with a pre-determined

concentration (20-180 ppm). The MCPP solutions with different adsorbents were stirred

using magnetic stirrers during the adsorption process. At the scheduled time, for the MCPP

solution with UiO-66 powder, 1.5 mL solution was collected and centrifuged at 14000 rpm.

150 μL of the supernatant phase was subject to the UV test. For the MCPP solutions with

monolithic chitosan-UiO-66 composites, 150 μL solution was taken directly for the UV

absorbance test. The MCPP amounts adsorbed by samples at different periods were

calculated according to the following equation:

q t=(C0−Ctm )V (1)

6

Where C 0 and C t (ppm) are the initial concentration and the concentration at time t, m (g) is

the mass of samples, V (L) is the volume of the MCPP solution, q t (mg g-1) is the MCPP

amount adsorbed by the sample at time t.

2.2.2. Material characterization

Sample morphology was determined using scanning electron microscopy (SEM, Hitachi S-

4800). Before evaluation, all samples were sputter-coated with a thin gold layer (~2 nm)

using an automated sputter coater (Emitech K550X). Brunauer-Emmette-Teller (BET) specific

surface areas, pore volumes and Barrette-Joynere-Halenda (BJH) pore size distributions were

obtained by N2 sorption at 77 K using a Micromeritics ASAP 2420 volumetric adsorption

analyser. The samples were degassed for 48 hrs at 120 °C before N2 sorption analysis.

Macropore volumes and macropore size distributions were obtained by Hg intrusion

porosimetry using a Micromeritics Autopore IV 9500 Porosimeter. The samples are dried

under vacuum at 90 oC overnight before Hg porosimetry tests.

3. Results and discussion

3.1. Characterization of samples

Monolithic samples were formed after freeze-drying. Their compositions and some of the

properties are given in Table 1. Due to the residual acid in the freeze-dried samples, they

were found to dissolve within a few minutes when they were soaked in water and the MCPP

solution. In order to use them for water treatment, these materials must be made stable in

aqueous solution. This was addressed by washing the freeze-dried monoliths using a base

solution (1M NaOH solution in this study). To avoid significant shrinkage during the drying

process, the washed sample were solvent exchanged with acetone and cyclohexane before

drying in fume hood at room temperature. Figure 1A shows the photos of the three

monoliths after the treatment with NaOH solution and drying. Relatively bigger shrinkage

7

was seen for the chitosan sample whilst the presence of UiO-66 in the composite sample

resulted in smaller shrinkage and maintained the monolith shape. All the three samples

remain intact shapes in aqueous solution with a pH range of 3.6-14, as shown in Figure 1B.

Compared with the UiO-66 powders, the monoliths soaked in MCPP solution (60 ppm) can

be easily taken out at the end of adsorption experiments (Figure 1C). On the other hand,

UiO-66 was suspending in the MCPP solution to form a quite stable but opaque suspension.

A centrifugation process was required to precipitate the UiO-66 nanoparticles and analyse

the supernatant phase. This is time-consuming, inconvenient, and could be the bottle-neck

for potential scale-up applications.

Figure 1. The photos show (A) the three freeze-dried monoliths after washing with NaOH

solution, solvent exchange, and drying; (B) the samples (from left to right: Chitosan-W,

Chitosan/UiO-66-1-W, Chitosan/UiO-66-2-W, UiO-66) soaked in 10 mL 60 ppm MCPP

solution; (C) Chitosan/UiO-66-2-W can be easily taken out after adsorption using tweezers.

Table 1. Porous chitosan and chitosan-MOF composites and their properties

Chitosan Chitosan: MOF BET surface area (m² g-1) Pore Volume from Hg-intrusion

8

solution (mL g-1)

Freeze drying

After washing

Freeze drying Freeze drying

After washing

Chitosan 1% 1:0 1:0 12.4 40.20 4.71

Chitosan/UiO-66-1 1% 1:1 1:0.76 --- --- ---

Chitosan/UiO-66-2 1% 1:2 1:1.44 339 18.01 7.32

The pore structures of the three monoliths before and after alkaline treatment are shown in

Figure 2. Before the alkaline treatment, a highly porous structure can be seen for the

chitosan sample (Figure 2A) while layered porous (or lamellar) structures are observed for

the chitosan-UiO-66 composites (Figure 2E & I). At higher magnifications, the pore wall

surface is smooth for the chitosan sample (Figure 2B) but UiO-66 nanoparticles can be

clearly seen on the surface of Chitosan/UiO-66-1 and Chitosan/UiO-66-2, some embedded

in the chitosan wall and some just on top of the surface (Figure 2F and J). After washing with

NaOH solution, the pore structures are retained for all the samples. However it is obvious

that the porosity and pore size have been reduced considerably. A rough surface structure

can be seen for the sample Chitosan-W (Figure 2D). For the samples Chitosan/UiO-66-1-W

and Chitosan/UiO-66-2-W, the loose UiO-66 nanoparticles seemed to be washed away and

the embedded nanoparticles are available in the washed composites.

In order to find out how much UiO-66 particles were lost during the washing procedure, we

dissolved Chitosan/UiO-66-1-W and Chitosan/UiO-66-2-W in 1M HCl solution, respectively.

Then the solutions were centrifuged at 14000 rpm, and the precipitation was collected. After

dried at 60 oC in a vacuum drier, the precipitation was weighed. The mass loss of UiO-66

after alkaline treatment can be calculated according to the equation:

δ=M 0−M 1

M o×100% (2)

9

where M 0 is the mass of UiO-66 in the freeze-dried Chitosan/UiO-66-1 or Chitosan/UiO-66-

2, M 1 is the mass of the collected precipitation after the monoliths dissolved in HCl solution.

The calculated results show that the mass losses of UiO-66 for Chitosan/UiO-66-1-W and

Chitosan/UiO-66-2-W are 23.79 ± 2.11 % and 28.90 ± 1.95 % (average ± stand error),

respectively.

Figure 2. The pore structures by SEM imaging of (A & B) the freeze-dried chitosan; (C & D)

the chitosan sample after washing treatment; (E & F) the freeze-dried Chitosan/UiO-66-1;(G

& H) the sample Chitosan/UiO-66-1-W (after washing treatment); (I & J) the freeze-dried

Chitosan/UiO-66-2; (K & L) the sample Chitosan/UiO-66--W (after washing treatment).

10

Figure 3. Characterization of freeze-dried chitosan monolith, UiO-66 powder and

chitosan/UiO-66 composite monolith. (A) N2 sorption isotherms, (B) pore size distribution

profiles.

The surface area and porosity of the freeze-dried samples are characterized by N2 sorption.

The freeze-dried chitosan shows a low quantity of adsorbed N2 with a surface area of 12.39

m² g-1 (Figure 3A). This is expected because ice-templating generates macropores which do

not contribute significantly to the surface area. For the UiO-66 nanoparticles, the isotherm

profile (Figure 3A) indicates the presence of micropores, as evidenced by the pore size

distribution calculated by non-linear density functional theory from the N2 sorption data

with some pores maybe from the interstitial space of UiO-66 nanoparticles (Figure 3B). For

the composite Chitosan-UiO-66-2, the adsorbed quantity of N2 is between the UiO-66 and

chitosan but the isothermal curve also shows the presence of micropores. The pore size

11

distribution (calculated from the BJH method) shows the presence of micropores (Figure 3B).

The BET surface areas of Chitosan/UiO-66-2 and UiO-66 are 338.89 m² g-1 and 1034.31 m² g-1,

respectively.

Mercury intrusion porosimetry is a technique to measure macropores and the relevant pore

volume. Due to the shrinkage observed during the washing procedure, Hg porosimetry is

employed to evaluate the change in the size of the ice-templated macropores and the pore

volumes. Based on the curves of cumulative intrusion versus intrusion pressure, the

maximum intrusions are reached for all the samples by the pressure of 1000 psia (Figure 4A).

The freeze-dried chitosan exhibits the highest intrusion volume (Table 1, Figure 4A) and

lower intrusion pressure of around 100 psia, indicating a highly interconnected macroporous

structure with high pore volume. After washing with NaOH solution, the pore volume of the

chitosan is reduced to nearly one tenth of the original pore volume (4.71 mL g -1). However,

the sample Chitosan-W is still highly porous. For Chitosan-UiO-66-2, the decrease of

intrusion volume after washing with NaOH solution and drying is less than one third (from

18.01 mL g-1 to 7.32 mL g-1), suggesting a stabilizing effect from UiO-66 to the porous

composite monolith. The macropore size distributions of the porous monoliths are given in

Figure 4B. Before the washing procedure, the two kinds of monoliths showed high ratios of

macropores with the size ~100 μm. These macropores disappeared after the washing. In

addition, the chitosan monolith possesses many smaller macropores in the region of 2 - 3

μm and ~5 μm), which disappeared after the washing procedure as well. It is noted that the

pores with the size of 20 – 40 μm increased when the chitosan/UiO-66-2 monolith was

subjected to alkaline treatment.

12

Figure 4. (A) The relationship between pressure and cumulative intrusion for the four

monoliths and (B) macropore pore size distribution for chitosan monolith and chitosan/UiO-

66 composite monolith before and after alkaline treatment.

3.2. Adsorption of MCPP

The quantities of adsorbed MCPP over different samples for various times are shown in

Figure 5. When the initial MCPP concentration was 60 ppm, the adsorption of MCPP was fast

until 120 minutes for all the four samples. After this, the adsorption rates of all the samples

slowed down. Within 360 min of adsorption, UiO-66 and Chitosan/UiO-66-2-W showed

higher adsorption quantities of MCPP (36.00 and 34.33 mg g-1, respectively) than

Chitosan/UiO-66-1-W and Chitosan-W. It is noted that chitosan monolith without MOF can

also exhibit a considerable MCPP-adsorbed capacity (27.33 mg g -1). Remarkably, the

adsorption capacity of Chitosan/UiO-66-2-W is very similar to that of UiO-66 nanoparticles. 13

However, unlike the UiO-66 nanoparticles where the centrifugation or filtration is necessary

to collect the UiO-66 nanoparticles, the chitosan/UiO-66 composite can be easily picked,

washed, and recycled. The quantity of MCPP adsorption remains unchanged after 6 h.

Figure 5. The profiles of the adsorbed quantity of MCPP versus soaking time by immersing

the composites (10 mg) or dispersing UiO-66 nanoparticles in 10 mL of aqueous solution of

MCPP (60 ppm).

In order to obtain the theoretic adsorption capacity and evaluate the adsorption isotherm,

adsorption isotherms of MCPP (from 20 mL MCPP aqueous solution) on all three monoliths

and UiO-66 powder (2 mg of each sample) were investigated. The total adsorption time was

5 h. The amounts of the samples for adsorption tests were 2 mg so that the result could be

directly compared with a previous report.14 For the adsorption of organic materials on solid

surfaces, several mathematical models have been proposed to analyse the experimental

data, of which the Freundlich and Langmuir models are frequently used. In this work, both of

the models are employed to describe the adsorption data. The equation of the Langmuir

isotherm is.36

Ceqe

=Ceqmax

+ 1qmaxk L

(3)

14

Where C e (mg L-1) is the equilibrium concentration of the solution, qe (mg g-1) is the quantity

adsorbed over the samples at equilibrium, qmax (mg g-1) is the maximum adsorption capacity,

k L (L g-1) is the Langmuir constant. The curves of qe - Ce are shown in Figure 6A. A linear

relationship is acquired when C e/qe is plotted versus C e (Figure 6B). The slope and intercept

in Figure 6B are used to calculate k L and qmax. The results are given in Table 2. The

coefficients of determination R2 from the Langmuir equation are closer to 1. This indicates

that the adsorption of MCPP onto UiO-66 and Chitosan/UiO-66-2-W follows the Langmuir’s

model. The maximum adsorption of MCPP on Chitosan/UiO-66-2-W is 337.83 mg g -1, which is

93.24% of that on UiO-66 (362.32 mg g-1). The adsorption quantity on UiO-66 is accordant

with the result of the Seo’s research (370 mg g-1).14

Figure 6. (A) Adsorption isotherms of MCPP on Chitosan/UiO-66-2-W and UiO-66 and (B)

Langmuir plots of the isotherms.15

Table 2. Isotherm parameters for the adsorption of MCPP on UiO-66 and Chitosan/UiO-66-2-W

samples Langmuir Freundlich

qmax (mg g-1) KL (L g-1) R2 1/n kF(L g-1) R2

UiO-66 362.32 0.1323 0.9985 0.2198 125.49 0.9730

Chitosan/MOF-2 337.83 0.1216 0.9991 0.2563 99.31 0.8906

a The concentration of MCPP solution is 20 ppm.

b The weight of sample is 2 mg.

Another parameter RL, a dimensionless equilibrium parameter, can be used to indicate

whether the isotherm is favourable (RL<1), unfavourable (RL>1), linear (RL=1), or

irreversible (RL=0). 37 The RL is defined as follows38:

RL=1

1+k LC0 (4)

Where k L (L g-1) is the Langmuir constant and C0( ppm) is the initial concentration of the

solution. The changes of RL values with C 0 are shown in Figure 7. All the calculated RL

values, under all different initial concentrations (20~160 ppm) of MCPP aqueous solution,

are between 0 and 1, which suggests the two adsorbents are favourable for adsorption of

MCPP.

The Freundlich equation is expressed as follows.39

ln qe=ln k F+1nlnCe (5)

Where k F (L g-1) is a Freundlish constant related to adsorption capacity, 1/n is an empirical

parameter related to adsorption intensity. The linear relationships between ln qe and lnC e

are fitted in Figure 8, whose slope and intercept are employed to calculate the values of k F

16

and1/n. The results are shown in Table 2. UiO-66 possessing the higher value of k F indicates

adsorption is favourable on UiO-66 than on Chitosan/UiO-66-2-W.

Figure 7. RL values of UiO-66 and chitosan/UiO-66-2-W at different initial

concentrations of MCPP aqueous solution.

Figure 8. Freundlich plot of isotherms for MCPP adsorbed on Chitosan/UiO-66-2-W and UiO-

66.

3.3 Kinetic studies of MCPP adsorption

For kinetic studies, all four samples (10 mg, respectively) were soaked in 10 mL 60 ppm

MCPP aqueous solution. The pseudo-first-order Lagergren equation and the pseudo-second-

17

order rate equation are used to examine the controlling mechanism of the adsorption

process. The pseudo-first-order Lagergren equation is expressed as40：

log (qe−q t )=log qe−k12.303

t (6)

Where qe and q t are the amounts of MCPP adsorbed at equilibrium and at time t (min),

respectively, k1 (min-1) is the Lagergren constant of adsorption, which can be calculated from

the slope of the fitted linear relationship between log (qe−q t ) and t .

The pseudo-second-order equation is given by41:

tqt

= 12k2qe

2 +tqe

(7)

Where k 2 (min-1) is the pseudo-second-order rate constant, which can be determined from

the intercept of the plot of t /q t versus t .

Figure 9 shows the kinetic profiles based on equations 6 and 7. The relative parameters are

listed in Table 3. From Figure 9, it is clear that the adsorption of MCPP on Chitosan/UiO-66-

2-W and UiO-66 can be better described by the pseudo-second-order model. The

coefficients of determination R2 are almost equal to 1 (Table 3). The two values of qe

calculated according to the pseudo-second-order model were very similar to the

experimental values. The k2 value of UiO-66 is about 1.5 times as much as Chitosan/UiO-66-

2-W.

18

Figure 9. Adsorption kinetics of MCPP adsorbed by Chitosan/UiO-66-2-W or UiO-66: (A)

pseudo-first-order model, (B) pseudo-second-order model.

Table 3 Parameters of pseudo-first-order and pseudo-second-order kinetic models

Samples Pseudo-first-order model Pseudo-second-order modelqe,exp(mg/g) K1(1/min) qe(mg/g) R2 qe,exp(mg/g) K2(1/min) qe(mg/g) R2

UiO-66 36.00 0.01055 4.77 0.9262

36.00 3.07x10-3 35.83 0.9999

Chitosan/UiO-66-2-W

35.22 0.01967 13.66 0.8328

35.22 1.90x10-3 35.32 0.9998

a Estimated from results using 60 ppm of MCPP solution

3.4 Recovery and reusability

Recovery and reusability of an adsorbent are highly important for scale-up or commercial

applications. A good adsorbent needs to be recovered easily from solution and possesses

repetitive high adsorption capacity after a number of cycles. UiO-66 powder has exhibited

19

excellent performance on adsorption of MCPP from MCPP aqueous solution; however, it is

only recovered by centrifugation of the solution or by using microporous membranes, which

will increase processing time and raise the cost of water treatment. In contrast,

chitosan/UiO-66 monoliths can be easily recovered from aqueous solution. As shown in

Figure 1, the chitosan/MOF monolith can be simply picked up from solution by using

tweezers. In addition, after recovered from solution, Chitosan/UiO-66-2-W and UiO-66 could

be washed by deionized water and ethanol (a similar solvent exchange process with acetone

and cyclohexane for Chitosan/UiO-66-2-W), dried under vacuum, and tested for MCPP

adsorption by soaking in 60 ppm MCPP solution for 6 h. Figure 10 shows good recyclability

for all the four tested samples. After 3 cycles, Chitosan/UiO-66-2-W and UiO-66 can adsorb

28.11mg g-1 and 30.67 mg g-1 for 10 mL 60 ppm MCPP aqueous solution, respectively, which

means both of them can retain over 80% adsorbed amount of fresh adsorbents.

Figure 10. Reusability of UiO-66 and Chitosan/UiO-66-2-W for the adsorptive removal of

MCPP from 10 mL 60 ppm MCPP aqueous solution.

3.5 Discussion

Various methods have been employed to prepare MOF-polymer and MOF-inorganic

composites.15-22 The ice-templating method is unique in fabricating higher interconnected

porous materials. It is simple and versatile. Different components can be conveniently mixed 20

together and then processed to form porous composites.28-30 In this work, to the best of our

knowledge, the MOF-polymer composites have been prepared for the first time, in the

context of being used as adsorbents for water treatment. More specifically, Chitosan/UiO-66

composites are prepared, due to their good water stability. Chitosan is soluble in acidic

water but insoluble in neutral water. The chitosan nanofibers prepared by ice templating

were used to adsorb Cu ions from aqueous solutions before.31 UiO-66 has been known as

one of the water-stable MOFs and used as adsorbents for water treatment.8,11-12 UiO-66

nanoparticles can be clearly seen in the chitosan/UiO-66 composites. The N2 sorption data

exhibits the isothermal curve characteristics of micropores and a surface area of 339 m2 g-1

(Figure 3). Furthermore, the power X-ray diffraction (PXRD) analysis shows the presence of

the crystalline UiO-66 nanoparticles in the composite (Figure 11).

Figure 11. XRD pattern of chitosan/UiO-66 composite before and after soaked in 1 M NaOH

solution.

The purpose of preparing the Chitosan/UiO-66 composites was for water treatment with

high efficiency and easy collection after the adsorption tests. This would be highly

advantageous in terms of potential scale-up or commercial applications, compared to the

centrifugation or filtration that has to be used for UiO-66 nanoparticles. We have chosen to

evaluate the adsorption of MCPP, which was investigated recently with UiO-66,14 to

21

demonstrate our concept. However, one of the problems encountered was the

disintegration of the monoliths in aqueous MCPP solutions. This can be attributed to the

residual acid in the freeze-dried composites and the slightly acid nature of aqueous MCPP

solution. This issue was addressed by washing with NaOH solution. The subsequent solvent

exchange with acetone and cyclohexane before drying was to reduce the shrinkage. This

treatment has proven to be effective. The treated Chitosan/UiO-66 monoliths remained

intact in the MCPP solution with the concentration up to 160 ppm.

The NaOH-treated chitosan monoliths showed good performance in the adsorption of

MCPP. This may be explained by the following reasons. Firstly, the interconnected pore

structure in the ice-templated chitosan monolith and the rough surface after NaOH

treatment increased the exposed surface area, which can supply more adsorption sites for

MCPP. Secondly, chitosan contains high contents of amino and hydroxyl groups,42-43 which

result in strong interaction with the carboxyl group of MCPP. With the addition of UiO-66 in

the composite monoliths, the adsorption of MCPP has been notably improved, close to one

third of increase in adsorption capacity. The maximum adsorption capacity of Chitosan/UiO-

66-2-W (1:1.44 w/w of polymer:MOF) can reach 93.2 % of that of UiO-66 nanoparticles

(Figure 5). This may be understandable because some of the UiO-66 nanoparticles are

embedded in the chitosan wall. As a result, some of micropores and the nanoparticle surface

are blocked. This is reflected by the surface area of the freeze-dried chitosan/UiO-66-2. The

mass ratio of chitosan to UiO-66 is 1:2 in Chitosan/UiO-66-2. If all UiO-66 nanoparticles in

Chitosan/UiO-66-2 can function as UiO-66 powder does for N2 sorption, the BET surface area

may be around 690 m² g-1, two thirds of the BET surface area of UiO-66(1034 m² g-1).

However, the BET surface area of Chitosan/UiO-66-2 is just 339 m² g -1, which suggests that

some UiO-66 particles do not contribute to N2 adsorption.

22

The Langmuir model bases on the assumption that the adsorption occurs on a homogenous

surface and no interaction between adsorbates in the plane of surface, while the Freundlich

model assumes the adsorption happens on a heterogeneous surface.36 By comparing the

coefficients from the two models, we found that the Langmuir model fits better to the

adsorption isotherm than Freundlich model, which suggests that the adsorption of MCPP

over Chitosan/UiO-66-2-W or UiO-66 was more a homogeneous surface adsorption. The

results of kinetic studies suggest that adsorption of MCPP on UiO-66 and Chitosan/UiO-66-2-

W fit the pseudo-second-order kinetics better, which suggests that adsorption rate was

influenced by structural properties of the adsorbent.25

The selection of UiO-66 for the chitosan/MOF composites was based on its high stability in

water.11,12 However, the stability of UiO-66 has been mostly reported in neutral water or

acidic water.11 So it was important to characterize the NaOH-treated monolith

(chitosan/UiO-66-2-W) although it has exhibited good adsorption performance for MCPP.

The PXRD pattern of chitosan/UiO-66-2-W in Figure 11 shows the disappearance of

diffraction peaks while the freeze-dried composite (chitosan/UiO-66-2) exhibits the

diffraction pattern nearly the same as that of UiO-66.26,27 This indicates the collapse of UiO-

66 crystalline structure after the treatment with NaOH washing, although the shape and size

of the original UiO-66 nanoparticles seemed unchanged (Figure 2). The very low surface area

of chitosan/UiO-66-2-W (~ 3 m2 g-1) further confirms the loss of the crystalline microporous

structure. Thus, it is quite remarkable to see that the sample chitosan/UiO-66-2-W still

exhibits adsorption capacity of MCPP close to the UiO-66 nanoparticles (Figure 5).

Considering that the crystalline porous structure of UiO-66 has been destroyed, it may be

more meaningful to compare the adsorption of MCPP for chitosan/UiO-66-2-W and NaOH-

washed UiO-66 nanoparticles. As shown in Figure 12, the NaOH-treated UiO-66

nanoparticles produces a lower adsorption capacity (18.55 mg g-1) in 360 min, a decrease of

23

48.47 % compared to that of UiO-66 nanoparticles in the same time. Although the

microporosity has been lost, the surface of the treated UiO-66 nanoparticle still leads to a

substantial adsorption capacity of the MCPP. This is not surprising considering that various

types of inorganic nanoparticles have been used for water treatment.44-45 The composite

monolith (chitosan/UiO-66-2-W) achieves a higher adsorption capacity (34.33 mg g -1) than

the individual components (27.33 mg g-1 for treated chitosan and 18.55 mg g-1 for treated

UiO-66 nanoparticles). Therefore, not only the composite monolith exhibits the obvious

advantage of easy collection & recycling, but also a synergic effect to enhance the

adsorption capacity.

Figure. 12 The profiles of the adsorbed quantity of MCPP versus soaking time by dispersing

10 mg UiO-66 nanoparticles or NaOH-treated UiO-66 nanoparticles in 10 mL of aqueous

solution of MCPP (60 ppm).

Due to the higher adsorption capacity of UiO-66 than NaOH-treated UiO-66, it would be

more advantageous to actually fabricate the porous chitosan/MOF composites. Instead of

washing the chitosan/MOF composites with NaOH solution, the scaffold may be chemically

crosslinked (e.g., with glutaraldehyde, glyoxal, formaldehyde) in water at neutral pH.23-24 This

can improve the stability of chitosan in water while maintaining the integrity of the MOF

(UiO-66) structure. Another option is the use of MOF particles which are stable under both

24

acidic and basic conditions. There are not many of them available but UiO-66-NO2 can be a

good candidate.11 Furthermore, this method may be applied to other pollutants where UiO-

66 shows high adsorption capacity and/or other MOFs for different types of pollutants. The

key advantage of this method is to maintaining the high adsorption capacity whilst the

porous polymer-MOF composites can be recycled easily.

4. Conclusion

An ice-templating method was employed to fabricate porous polymer/MOF composites. This

was achieved by simply freezing aqueous chitosan solution containing suspended UiO-66

nanoparticles and the subsequent freeze-drying. The ice-templated layered porous structure

was observed in the chitosan/UiO-66 composites with the crystalline UiO-66 nanoparticles.

The composite with a mass ratio of 1:2 of chitosan:UiO-66 exhibited a BET surface area of

339 m2 g-1, lower than the surface area calculated based on the percentage of UiO-66. This

was attributed to the embedded UiO-66 particles and blocked micropores.

Both the chitosan and chitosan/UiO-66 composite monoliths were evaluated for water

treatment, demonstrated by the adsorption of MCPP from dilute aqueous solution. In order

to enhance their stability during adsorption, chitosan and chitosan/UiO-66 composite

monoliths were treated with NaOH solution and dried after the solvent exchange process.

The chitosan monolith showed an adsorption capacity of 27.33 mg g -1 and the chitosan/UiO-

66-2-W exhibited a capacity of 34.33 mg g-1 (close to the capacity achieved by UiO-66

nanoparticles, 36 mg g-1), when tested in 60 ppm aqueous MCPP solution. While a

centrifugation process with high rpm was required to collect the UiO-66 nanoparticles, the

chitosan/UiO-66 monoliths could be easily picked up using tweezers and recycled for further

tests. The adsorption data were analyzed based on adsorption isotherms and kinetic

mechanism. The theoretic maximum adsorption capacity based on Langmuir equation was

25

337.83 mg g-1 for the sample chitosan/UiO-66-2-W (close to the UiO-66 value of 362.32 mg g -

1).

It was then found that the crystalline structure of UiO-66 was destroyed during NaOH

washing but the shape of the nanoparticles was retained. While the NaOH-washed UiO-66

particles showed a lower adsorption capacity of 18.55 mg g-1, the NaOH-washed

chitosan/UiO-66 composites achieved a higher adsorption capacity than both of the treated

UiO-66 and chitosan. It is anticipated that developing a procedure to fabricate the

chitosan/MOF composite (i.e., no collapse of MOF structures) with good mechanical stability

can offer better adsorption capacity while retaining the advantage of easy recycling.

Author Information

Corresponding author: zhanghf@liverpool.ac.uk

Notes: The authors declare no competing financial interest.

Acknowledgement

We are grateful for the access to the facilities in the Centre for Materials Discovery and in

the Department of Chemistry at the University of Liverpool. Rob Clowes is acknowledged for

some of the N2 sorption tests. QF thanks the Sichuan University of Science and Engineering

for funding his academic visit to the University of Liverpool.

Reference

26

(1) Hasan, Z.; Jhung, S. H. Removal of Hazardous Organics from Water Using Metal-Organic

Frameworks (MOFs): Plausible Mechanisms for Selective Adsorptions. J. Hazard. Mater.

2015, 283, 329-339.

(2) Reddy, J. K.; Azarnoff, D. L.; Hignite, C. E. Hypolipidaemic Hepatic Peroxisome

Proliferators Form a Novel Class of Chemical Carcinogens. Nature 1980, 283, 397-398.

(3) Hedegaard, M. J.; Arvin, E.; Corfitzen, C. B.; Albrechtsen, H.-J. Mecoprop (MCPP) Removal

in Full-Scale Rapid Sand Filters at a Groundwater-Based Waterworks. Sci. Total Environ.

2014, 499, 257-264.

(4) Meunier, L.; Boule, P. Direct and Induced Phototransformation of Mecoprop [2-(4-Chloro-

2-Methylphenoxy)Propionic Acid] in Aqueous Solution. Pest Manage. Sci. 2000, 56, 1077-

1085.

(5) Ignatowicz, K. Selection of Sorbent for Removing Pesticides during Water Treatment. J.

Hazard. Mater. 2009, 169, 953-957.

(6) Clausen, L.; Fabricius, I. Atrazine, Isoproturon, Mecoprop, 2,4-D, and Bentazone

Adsorption onto Iron Oxides. J. Environ. Qual. 2001, 30, 858-869.

(7) Zhou, H. C.; Long, J. R.; Yaghi, O. M. Introduction to Metal–Organic Frameworks. Chem.

Rev. 2012, 112, 673-674.

(8) Dias, E. M.; Petit, C. Towards the Use of Metal-Organic Frameworks for Water Reuse: a

Review of the Recent Advances in the Field of Organic Pollutants Removal and Degradation

and the Next Steps in the Field. J. Mater. Chem. A 2015, 3, 22484-22506.

(9) Van de Voorde, B.; Bueken, B.; Denayer, J.; De Vos, D. Adsorptive Separation on Metal-

Organic Frameworks in the Liquid Phase. Chem. Soc. Rev. 2014, 43, 5766-5788.

(10) Khan, N. A.; Jung, B. K.; Hasan, Z.; Jhung, S. H. Adsorption and Removal of Phthalic Acid

and Diethyl Phthalate from Water with Zeolitic Imidazolate and Metal–Organic Frameworks.

J. Hazard. Mater. 2015, 282, 194-200.

27

(11) Howarth, A. J.; Liu, Y.; Li, P.; Li, Z.; Wang, T. C.; Hupp, J. T.; Farha, O. K. Chemical,

Thermal and Mechanical Stabilities of Metal–Organic Frameworks. Nat. Rev. Mater. 2016, 1,

15018.

(12) Liu, X.; Demir, N. K.; Wu, Z.; Li, K. Highly Water-Stable Zirconium Metal–Organic

Framework UiO-66 Membranes Supported on Alumina Hollow Fibers for Desalination. J. Am.

Chem. Soc. 2015, 137, 6999-7002.

(13) Bansal P, B. L. M., Deep A., Kaushik P. Zn Based Metal Organic Framework as Adsorbent

Material for Mecoprop. Res. J. Recent Sci. 2013, 2, 84-86.

(14) Seo, Y. S.; Khan, N. A.; Jhung, S. H. Adsorptive Removal of ethylchlorophenoxypropionic

Acid from Water with a Metal-Organic Framework. Chem. Eng. J. 2015, 270, 22-27.

(15) Li, S.; Huo, F. Metal-Organic Framework Composites: from Fundamentals to

Applications. Nanoscale 2015, 7, 7482-7501.

(16) Armstrong, M. R.; Arredondo, K. Y. Y.; Liu, C.-Y.; Stevens, J. E.; Mayhob, A.; Shan, B.;

Senthilnathan, S.; Balzer, C. J.; Mu, B. UiO-66 MOF and Poly(vinyl cinnamate) Nanofiber

Composite Membranes Synthesized by a Facile Three-Stage Process. Ind. Eng. Chem. Res.

2015, 54, 12386-12392.

(17) O'Neill, L. D.; Zhang, H.; Bradshaw, D. Macro-/Microporous MOF Composite Beads. J.

Mater. Chem. 2010, 20, 5720-5726.

(18) Pinto, M. L.; Dias, S.; Pires, J. Composite MOF Foams: The Example of

UiO-66/Polyurethane. ACS Appl. Mater. Interfaces 2013, 5, 2360-2363.

(19) Wisser, D.; Wisser, F. M.; Raschke, S.; Klein, N.; Leistner, M.; Grothe, J.; Brunner, E.;

Kaskel, S. Biological Chitin–MOF Composites with Hierarchical Pore Systems for Air-Filtration

Applications. Angew. Chem. Int. Ed. 2015, 54, 12588-12591.

(20) Huo, J.; Marcello, M.; Garai, A.; Bradshaw, D. MOF-Polymer Composite Microcapsules

Derived from Pickering Emulsions. Adv. Mater. 2013, 25, 2717-2722.

28

(21) Zhu, Q.-L.; Xu, Q. Metal-Organic Framework Composites. Chem. Soc. Rev. 2014, 43,

5468-5512.

(22) Ahmed, A.; Forster, M.; Jin, J.; Myers, P.; Zhang, H. Tuning Morphology of

Nanostructured ZIF-8 on Silica Microspheres and Applications in Liquid Chromatography and

Dye Degradation. ACS Appl. Mater. Interfaces 2015, 7, 18054-18063.

(23) Wan Ngah, W. S.; Teong, L. C.; Hanafiah, M. A. K. M. Adsorption of Dyes and Heavy

Metal Ions by Chitosan Composites: A Review. Carbohydr. Polym. 2011, 83, 1446-1456.

(24) Chiou, M.-S.; Ho, P.-Y.; Li, H.-Y. Adsorption of Anionic Dyes in Acid Solutions Using

Chemically Cross-Linked Chitosan Beads. Dyes Pigm. 2004, 60, 69-84.

(25) Kamari, A.; Ngah, W. S. W. Isotherm, Kinetic and Thermodynamic Studies of Lead and

Copper Uptake by H2SO4 Modified Chitosan. Colloid Surf., B 2009, 73, 257-266.

(26) Katz, M. J.; Brown, Z. J.; Colon, Y. J.; Siu, P. W.; Scheidt, K. A.; Snurr, R. Q.; Hupp, J. T.;

Farha, O. K. A facile Synthesis of UiO-66, UiO-67 and their Derivatives. Chem. Commun. 2013,

49, 9449-9451.

(27) Zhu, X.; Gu, J.; Wang, Y.; Li, B.; Li, Y.; Zhao, W.; Shi, J. Inherent Anchorages in UiO-66

Nanoparticles for Efficient Capture of Alendronate and its Mediated Release. Chem.

Commun. 2014, 50, 8779-8782.

(28) Qian, L.; Zhang, H. Controlled Freezing and Freeze Drying: A Versatile Route for Porous

and Micro-/Nano-Structured Materials. J. Chem. Technol. Biotechnol. 2011, 86, 172-184.

(29) Deville, S. Ice-Templating, Freeze Casting: Beyond Materials Processing. J. Mater. Res.

2013, 28, 2202-2219.

(30) Gutiérrez, M. C.; Ferrer, M. L.; del Monte, F. Ice-Templated Materials: Sophisticated

Structures Exhibiting Enhanced Functionalities Obtained after Unidirectional Freezing and

Ice-Segregation-Induced Self-Assembly. Chem. Mater. 2008, 20, 634-648.

(31) Qian, L.; Zhang, H. Green Synthesis of Chitosan-Based Nanofibers and their Applications.

Green Chem. 2010, 12, 1207-1214.

29

(32) Ahmed, A.; Hearn, J.; Abdelmagid, W.; Zhang, H. Dual-Tuned Drug Release by

Nanofibrous Scaffolds of Chitosan and Mesoporous Silica Microspheres. J. Mater. Chem.

2012, 22, 25027-25035.

(33) Ma, L.; Jin, A.; Xie, Z.; Lin, W. Freeze Drying Significantly Increases Permanent Porosity

and Hydrogen Uptake in 4,4-Connected Metal–Organic Frameworks. Angew. Chem. Int. Ed.

2009, 48, 9905-9908.

(34) Ahmed, A.; Hasell, T.; Clowes, R.; Myers, P.; Cooper, A.I.; Zhang, H. Aligned Macroporous

Monoliths with Intrinsic Microporosity via a Frozen-Solvent-Templating Approach. Chem.

Commun. 2015, 51, 1717-1720.

(35) Zhu, H.; Zhang, Q.; Zhu, S. Assembly of a Metal–Organic Framework into 3 D Hierarchical

Porous Monoliths Using a Pickering High Internal Phase Emulsion Template. Chem. Eur. J.

2016, 22, 8751-8755.

(36) Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J.

Am. Chem. Soc. 1918, 40, 1361-1403.

(37) Chen, M.; Chen, Y.; Diao, G. Adsorption Kinetics and Thermodynamics of Methylene

Blue onto p-tert-Butyl-calix[4,6,8]arene-Bonded Silica Gel. J. Chem. Eng. Data 2010, 55,

5109-5116.

(38) Weber, T. W.; Chakravorti, R. K. Pore and Solid Diffusion Models for Fixed-Bed

Adsorbers. AIChE J. 1974, 20, 228-238.

(39) Dehghani, M. H.; Sanaei, D.; Ali, I.; Bhatnagar, A. Removal of Chromium(VI) from

Aqueous Solution Using Treated Waste Newspaper as a Low-Cost Adsorbent: Kinetic

Modeling and Isotherm Studies. J. Mol. Liq. 2016, 215, 671-679.

(40) Doğan, M.; Alkan, M.; Demirbaş, Ö.; Özdemir, Y.; Özmetin, C. Adsorption Kinetics of

Maxilon Blue GRL onto Sepiolite from Aqueous Solutions. Chem. Eng. J. 2006, 124, 89-101.

(41) Ho, Y. S.; Chiang, C. C. Sorption Studies of Acid Dye by Mixed Sorbents. Adsorption 2001,

7, 139-147.

30

(42) Ravi Kumar, M. N. V. A Review of Chitin and Chitosan Applications. React. Funct. Polym.

2000, 46, 1-27.

(43) Chu, K. H. Removal of Copper from Aqueous Solution by Chitosan in Prawn Shell:

Adsorption Equilibrium and Kinetics. J. Hazard. Mater. 2002, 90, 77-95.

(44) Hua, M.; Zhang, S.; Pan, B.; Zhang, W.; Lv, L.; Zhang, Q. Heavy Metal Removal from

Water/Wastewater by Nanosized Metal Oxides: A Review. J. Hazard. Mater. 2012, 211, 317-

331.

(45) Simeonidis, K.; Mourdikoudis, S.; Kaprara, E.; Mitrakas, M.; Polavarapu, L. Inorganic

Engineered Nanoparticles in Drinking Water Treatment: A Critical Review. Environ. Sci.:

Water Res. Technol., 2016, 2, 43-70.

31