Insights to perfluorooctanoic acid adsorption micro-mechanism over

Fe-based metal organic frameworks: Combining computational

calculation with response surface methodology

Yiqiong Yang a, Zenghui Zheng a, Wenqing Ji a, Jingcheng Xu b, Xiaodong Zhang a *

aSchool of Environment and Architecture, University of Shanghai for Science and Technology,

Shanghai 200093, China

bSchool of Materials Science and Engineering, University of Shanghai for Science and Technolog

y, Shanghai 200093, China

** To whom correspondence should be addressed. Tel. +86 15921267160, Fax. +86 021 55275979 E-mail address: fatzhxd@126.com , zhangxiaodong@usst.edu.cn (X.D. Zhang)

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Text S1. The preparation of MOFs Synthesis of Fe-BTC

Typically, 3.26 g (9.2 mmol) Fe(NO3)3·9H2O and 1.13 g (5.4 mmol) H3BTC

were mixed into 40 mL deionized water. The resultant suspension was maintained

under stirring at room temperature, leading to immediate formation of light orange

solids. After 4 hours, light orange solids were washed with deionized water and

ethanol, and nally dried under air.

Synthesis of MIL-100-Fe

During the preparation, the FeSO4·4H2O were dissolved into water to form a

solution. Next, the H3BTC was added into above solution and the mixture was stirred

for 30 min. The molar ratios of Fe, BTC and H2O is 1:0.67:280. Then the reactant

mixtures were loaded in a Teflon autoclave and kept at 150 °C for 24 h. After that, the

products were purified with water and ethanol at 70 °C for 3 h, respectively. Finally,

the as-obtained orange products were dried overnight under vacuum at 50 °C.

Synthesis of MIL-101-Fe

MIL-101-Fe was prepared following the protocol described earlier. S2 In a

typical synthesis, a mixture of 0.675 g (2.45 mmol) of FeCl3·6H2O, 206 mg of H2BDC

(1.24 mmol), and 15 mL DMF was heated at 110 °C for 20 h in a Teflon autoclave.

The resulting brown solid was filtered off and the raw product was purified by a

double treatment in ethanol at 60 °C for 3 h. MIL-101-Fe was obtained by drying

under vacuum at 60 °C for 7h.

Synthesis of Ce-BTC

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Ce-BTC was synthesized via solvothermal reaction, 3 mmol (0.63 g) H3BTC and

10 mmol (2.17 g) Ce(NO3)3·6H2O were dissolved in 50 mL DMF. Then, the mixture

was sealed in a 100 mL Teflon-lined stainless steel autoclave and heated at 130 °C for

24 h. After cooling to room temperature naturally, the product was filtered, washed by

centrifugal with DMF and ethanol for three times, and vacuum-dried at 80 °C for 24 h

to prepare Ce-BTC materials. 

Synthesis of Mn-BTC

For Mn-BTC, 0.50 g Mn(NO3)2·4H2O and 0.39 g H3BTC were dissolved by

18 ml CH3OH with stirring. The reaction mixture was stirred until completely

dissolved at normal temperature, and then placed in a 150 ml Teflon-lined stainless

steel autoclave at 120 °C for 2 h. After naturally cooling to room temperature in an

autoclave, the precipitate is washed three times with ethanol, and finally dried at

80 °C oven.

Synthesis of Cu-BTC

In a typical preparation, 1.9664 g of H3BTC has dissolved in 20 ml of ethanol.

The mixture was stirred until the complete dissolution of benzene-1,3,5-tricarboxylic

acid. In addition, 4.48 g of Cu(NO3)2·3H2O was added to 10 ml of deionized water in

another flask and mixed thoroughly, until it was completely dissolved. Then the two

solutions were mixed and stirred at room temperature for 30 min. The resulting

viscous mixture was introduced into Teflon-lined stainless steel autoclave. The

autoclave was heated at 140 °C under hydrothermal conditions for 24 h. The reaction

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vessel was then cooled to room temperature. Blue crystals of Cu-BTC were recovered

by filtration, washed thoroughly with deionized water and dried at 100 °C overnight. 

Text S2. Experimental methods

In order to study the adsorption performance of PFOA to Fe-based MOFs, batch

experiments including isotherm adsorption, kinetic adsorption and pH factors are

implemented. All the batch experiments are conducted at 298K and pH value is

adjusted to 3.3±0.1. 20 mg Fe-based MOFs are put into 20 mL PFOA solutions at the

concentration range of 50-1000 mg/L in the isotherm adsorption. For the kinetic

adsorption and pH factor experiments, the initial concentration of PFOA is 500 mg/L.

The solutions pH is adjusted by the 1M NaOH and 1M HCl. After adsorption, the

supernatant is filtered by the 0.22 μm anylon syringe filter to be detected. The

concentrations of PFOA is quantified according to the method in Text S2. The data

analysis for adsorption kinetics and isotherms is described in Text S3.

Text S3. Analysis of PFOA

After the sorption experiments, the supernatant was filtered with a 0.22 μm

polyethersulfone membrane, which showed a negligible adsorption for the PFOA. The

PFOA concentrations were determined by a UltiMate 3000 HPLC with a TSQ

Vantage conductivity detector from Thermo (USA); HPLC employed a column

(Hypersil Gold, 150 × 2.1 mm i.d., 5 μm particle size) using methyl cyanide/0.01 M

CH3COONH4 (60/40 for PFOA, v/v) as the mobile phase at 0.3 mL/min flow rate.

The sample volume injected was 5 μL.

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Text S4. Response surface methodology

Response surface methodology experiments and results are designed and analyzed

by the Design Expert software (version, 8.0.6) and central composite design (CCD).

The CCD approach with 3 levels (-1, 0, 1) can offer the precise reports of the

interaction between three key parameters including the initial concentration of PFOA

(A), the dosage of Fe-BTC (B) and pH (C), which are given in the Table S8.

According to the CCD, 20 experiments have been designed to study the interaction of

three parameters. The following quadratic polynomial model expresses the

mathematical relationship of the interaction of three independent parameters and the

predicted responses:

Y=β0 +∑i=1

k

β i Xi +∑i=1

k

∑i=1

k

βij Xi X j+∑i=1

k

βii Xi2 +ε (S1)

,where Y is the value of computational responses, β0 is the constant coefficient, βi, βii

and βij are the coefficients of the linear, quadratic and interactive terms individually

for the parameters Xi, which have great effect on the predicted response. The

correlation coefficient (R2) is used to judge the match degree of the quadratic

polynomial and the value of F is used for statistical significance of the model.

Text S5. Data analysis for adsorption kinetics and isotherms.

he adsorption amount of PFOA onto the Fe-based MOFs at equilibrium is calculated

by the following equation:

qe =(C0- Ce )∙Vm

(S2)

,where qe (mg/L) is the adsorption amount of PFOA onto the Fe-based MOFs at

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equilibrium, C0 (mg/L) is the initial concentration of PFOA, Ce (mg/L) is the

equilibrium concentration of PFOA, V (L) is the volume of the mixed solutions, and

the m (mg) is the volume of Fe-based MOFs in this adsorption system.

The equation of Langmuir [1] model is the following:

qe =qm KLCe

1+ KLCe (S3)

,where qm (mg/L) is the adsorption capacity for PFOA, KL (L/mg) is the constant of

Langmuir model.

The Freundlich [2] model is an empirical equation, which assumes that the reaction

process is multi-layered and occurs on the heterogeneous surface. The equation of

Freundlich model is the following:

qe =KF Ce

1n (S4)

,where KF (mg1-(1/n)·L1/n/g) is a constant of Freundlich model, and 1/n is a constant of

the reaction intensity of Freundlich model.

The intra particle diffusion model, the pseudo second-order-rate (PSO) model and the

pseudo first-order-rate (PFO) [3] model are used to fit the kinetic adsorption data.

And three equations of these model are following:

dqt

qt=k1(qe−q t) (S5)

dqt

qt=k2(qe−q t)

2 (S6)

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q t=k i t0.5+C (S7)

,where qt (mg/mL) is the amount of the PFOA adsorbed at the time, k1 (min-1) is

the constant of the PFO model, k2 (mg󠄕·mL-1·min-1) is the constant of PSO model; ki is

the diffusion rate constant (mg/(g·h0.5)), and C (mg/mL) is the constant related to the

thickness of the boundary layer. If the rate limiting step is intra particle diffusion, the

graph of q t and the square root of time should be a straight line and pass through the

origin (C = 0). The deviation between the graph and the linearity shows that the rate

limiting step should be boundary layer (film) diffusion.

Text S6. Computational calculation

Spin-polarized density functional calculations for the geometry optimization and

energy calculations were performed using generalized gradient approximation (GGA)

with the Perdew-Wang 1991 (PW91) exchange-correlation functional. DFT semi-core

pseudopots core treatment were used with the TS scheme 4 for the dispersion

correction for DFT. [4]. The double numeric polarization (DNP) basis set was used to

describe atomic orbitals [5]. The convergence tolerance for optimization were10-4 Ha

(energy), 0.02 Ha/Å (Max. force), and 0.05 Å (Max. displacement). The conductor-

like screening model (COSMO) using water solvent with dielectric constant of 75.84

was used to treat the solvation effects.

Geometry-optimization of Fe-based MOFs and PFOA was conducted before

calculation. The binding energies (Ebd) between different species of Fe-based MOFs

and PFOA were calculated according to the following equation:

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Ebd = EAB – EA – EB (S8)

where EAB is the total energy for the adsorption of PFOA to the MOFs, EA and EB are

the energies of adsorbent and adsorbate, respectively. A more negative value of Ebd

represents a stronger binding of adsorbent and adsorbate.

Forcite module (a molecular mechanics based module) with the “Universal”

force field in Materials Studio is used to optimize structural simulation of Fe-based

MOFs. The crystalline structure was sequentially optimized using a cascade of

steepest decent, conjugate gradient and quasi-Newton methods. The convergence

criteria for energy, force, stress and displacement are 2×10−5 kcal mol−1,

1×10−5 kcal mol−1 Å−1, 1×10−3 GPa and 1×10−5 Å, respectively. The low energy

adsorption configuration of PFOA molecules adsorbed to the accessible surface of the

Fe-based MOFs were simulated using the Adsorption Locator modules with the

“Universal” force field in Materials Studio.

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5 10 15 20 25 30 35 40

(a)

MIL-101-Fe

MIL-100-Fe

Inte

nsity

(a.u

.)

2Theta (degree)

Fe-BTC

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

500

600

700

Ads

orbe

d vo

lum

e (c

m3 /g

)

Relative pressure (p/p0)

Fe-BTCMIL-100-FeMIL-101-Fe

(b)

Fig. S1. Characterization of Fe-MOFs, (a) for XRD patterns and (b) for N2

adsorption-desorption isotherm

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Fig. S2. pH drift method to obtain pHpzc for Fe-based MOFs

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Design-Expert?SoftwareRemoval rate

Color points by value ofRemoval rate:

78.9

1.2

Actual

Pre

dict

ed

-20

0

20

40

60

80

100

0 20 40 60 80

Fig. S3. The actual and predicted plots for PFOA uptake capacity of Fe-BTC.

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540 538 536 534 532 530 528 526

C-OC=O

Fe-O

Fe-O

C=O

Inte

nsity

(a.u

.)

Binding energy (eV)

C-O

(c) O1s

MIL-101-Fe

MIL-101-Fe-PFOA

540 538 536 534 532 530 528 526

C-OC=O

Fe-O

Fe-O

C=O

Inte

nsity

(a.u

.)

Binding energy (eV)

C-O

(a) O1s

Fe-BTC

Fe-BTC-PFOA

540 538 536 534 532 530 528 526

C-O C=O

Fe-O

Fe-O

C=O

Inte

nsity

(a.u

.)

Binding energy (eV)

C-O

(b) O1s

MIL-100-Fe

MIL-100-Fe-PFOA

Fig. S4 XPS spectra for O1s of Fe- based MOFs before and after exposure to PFOA.

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Fig. S5. The structure of Fe3O cluster.

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Fig. S6. Typical structures of adsorbed PFOA for representative mechanisms: (a)

Fe3O cluster with H2O; (b) Fe-cluster with ·OH; (c) HB1, PFOA parallel contact with

coordinate H2O of Fe3O cluster; (d) HB2, PFOA contact with coordinate H2O of Fe3O

cluster crosswise; (e) LAB1, Lewis acid/base complex between PFOA and Fe3O

cluster; (f) π-CF1, C-F chain tail reacts vertically with benzene ring; (g) π-CF2, C-F

chain tail reacts parallel with benzene ring; (h) anion-π1, anion-π interaction between

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the dissociated carboxyl group of PFOA and benzene ring of MOFs; (i) HB3, PFOA

parallel contact with coordinate H2O of protonated Fe3O cluster; (j) HB4, PFOA

contact with coordinate H2O of protonated Fe3O cluster crosswise; (k) HB5, PFOA

contact with H+ of protonated Fe3O cluster; (l) LAB2, Lewis acid/base complex

between PFOA and protonated Fe3O cluster; (m) π-CF3, C-F chain tail reacts

vertically with benzene ring of protonated Fe3O cluster; (n) π-CF4, C-F chain tail

reacts parallel with benzene ring of protonated Fe3O cluster; (o) anion-π2, anion-π

interaction between the dissociated carboxyl group of PFOA and benzene ring of

protonated Fe3O cluster.

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Fig. S7. Charge density of each adsorption configuration.

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Fig. S8. Two types of cages of MIL-100-Fe (a) and MIL-101-Fe (b) and the location of PFOA to cages.

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Table S1 N2physisorption results of selected MOFs.

MOFs SBET (m2/g) ReferenceFe-BTC 1051

This workMIL-100-Fe 1237MIL-101-Fe 1811

Ce-BTC 43 [6]Mn-BTC 1542 [7]Cu-BTC 1429 [8]

Table S2 Isotherm adsorption parameters for PFOA adsorption to Fe-based MOFs.

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AdsorbentLangmuir Freundlich

KLa qm

b R2 KFc n R2

Fe-BTC 0.00628 548.2 0.9946 19.0 2.00 0.9509MIL-100-Fe 0.00561 426.6 0.9918 14.8 2.05 0.9729

MIL-101-Fe 0.00558 490.1 0.9793 16.5 2.02 0.9549

a L/mgb mg/gc mg/g/(L/mg)1/n.

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Table S3 Kinetic parameters for PFOA adsorption on Fe-based MOFs.

AdsorbentPseudo-first-order kinetic Pseudo-second-order kinetic

k1a qe

b R2 k2×103c qea R2

Fe-BTC 0.03587 337.1 0.9742 0.0979 404.4 0.9781MIL-100-Fe 0.08831 175.9 0.9443 0.6094 196.7 0.9686

MIL-101-Fe 0.04148 261.7 0.9268 0.1687 303.9 0.9679

a 1/hb mg/gc g/(mg·h).

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Table S4 The fitting parameters of intra-particle model.

AdsorbentStage 1 Stage 2 Stage 3

Kia Cb R2 Kii

a Cb R2 Kiiia Cb R2

Fe-BTC 38 1.0 0.9184 13.3 196 0.9983 1.5 320 0.9123MIL-100-Fe 31 5.7 0.9309 3.5 135 0.9122 1.5 180 0.9595

MIL-101-Fe 23 45.1 0.9987 20.7 57 0.9741 1.8 252 0.7768

a mg/(g·h0.5)b mg/g.

Table S5 Experimental design conditions and response of each experimental run.

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Run

A:Initial concentration

（mg/L）B:Dosage

(mg)C:pH

Actual removal rate

(%)

Predicted value rate

(%)

1 500 50 3 33.4 33.92 1000 50 3 38.6 38.1

3 500 150 3 69.3 71.1

4 1000 150 3 78.9 80.8

5 500 50 11 5.2 2.7

6 1000 50 11 1.2 -1.1

7 500 150 11 13.3 13.2

8 1000 150 11 16 14.9

9 500 100 7 38.8 39.0

10 1000 100 7 39.9 42.0

11 750 50 7 22.5 27.3

12 750 150 7 56.4 53.9

13 750 100 3 60.5 56.8

14 750 100 11 2.3 8.3

15 750 100 7 41 41.0

16 750 100 7 42.1 41.0

17 750 100 7 42.3 41.0

18 750 100 7 40.2 41.0

19 750 100 7 41.6 41.0

20 750 100 7 43.2 41.0

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Table S6 ANOVA test for response function Y (removal rate of PFOA).

SourceSum of squares

dfMean square

F-value

p-value Prob > F

Model 8483.77 9 942.64 84.22 < 0.0001A-Initial

concentration21.32 1 21.32 1.90 0.1976

B-Dosage 1768.90 1 1768.90 158.05 < 0.0001

C-pH 5890.33 1 5890.33 526.30 < 0.0001

AB 15.40 1 15.40 1.38 0.2680

AC 32.40 1 32.40 2.90 0.1197

BC 355.11 1 355.11 31.73 0.0002

A2 0.61 1 0.61 0.06 0.8195

B2 0.38 1 0.38 0.034 0.8571

C2 195. 1 195.10 17.43 0.0019

Residual 111.910

11.19

Lack of Fit 106.4 5 21.28 19.30 0.0028

Pure Error 5.51 5 1.10

Cor Total 8595.6919

R2 = 0.9870

Adjusted R2 = 0.9753

Predicted R2 = 0.9069

adequate precision = 34.638

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Table S7 Binding energies (kJ/mol) of PFOA adsorbed by Fe-based MOFs .

Adsorbents Interactions Binding energy (kJ/mol)

Fe3O cluster

Coordinate H2O -45.99-OH -212.92

HB1 -47.98

HB2 -28.77

LAB1 -65.21

π-CF1 -17.46

π-CF2 -15.49

anion-π1 -7.78

Protonated Fe3O cluster

HB3 -62.59

HB4 -36.51HB5 -61.54

LAB2 -104.27

π-CF3 -29.25

π-CF4 -23.88

anion-π2 -7.11

Table S8 Level of various independent variables at coded values of response surface methodology experimental design.

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Symbol Independent variablesCoded levels

-1 0 1

A Initial concentration (mg/L) 500 750 1000B MOFs dosage (mg) 50 100 150

C pH 3 7 11

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Reference

[1] L. Fang, B. Wu, J.K. Chan, I.M. Lo, Lanthanum oxide nanorods for enhanced

phosphate removal from sewage: a response surface methodology study,

Chemosphere 192 (2018) 209-216.

[2] J. Luo, M. Sun, C.L. Ritt, X. Liu, Y. Pei, J.C. Crittenden, M. Elimelech, Tuning

Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron Oxychloride (FeOCl)

Nanosheets, Environ. Sci. Technol. 53 (2019) 2075-2085.

[3] G.Z. Kyzas, G. Bomis, R.I. Kosheleva, E.K. Efthimiadou, E.P. Favvas, M.

Kostoglou, A.C. Mitropoulos, Nanobubbles effect on heavy metal ions adsorption by

activated carbon, Chem. Eng. J. 356 (2019) 91-97.

[4] K. Liu, S.Y. Zhang, X.Y. Hu, K.Y. Zhang, A. Roy, G. Yu, Understanding the

adsorption of PFOA on MIL-101 (Cr)-based anionic-exchange metal–organic

frameworks: comparing DFT calculations with aqueous sorption experiments

Environ. Sci. Technol. 49 (2015) 8657-8665.

[5] W.L. Sun, H.B. Li, H.M. Li, S. Li, X.Q. Cao, Adsorption mechanisms of

ibuprofen and naproxen to UiO-66 and UiO-66-NH2: Batch experiment and DFT

calculation, Chem. Eng. J. 360 (2019) 645-653.

[6] X.D. Zhang, F.L. Hou, Y. Yang, Y. Wang, N. Liu, D. Chen, Y.Q. Yang, A facile

synthesis for cauliflower like CeO2 catalysts from Ce-BTC precursor and their

catalytic performance for CO oxidation, Appl. Surf. Sci. 423 (2017) 771-779.

[27] X.D. Zhang, X.T. Lv, F.K. Bi, G. Lu, Y.X. Wang, Highly efficient Mn2O3

catalysts derived from Mn-MOFs for toluene oxidation: The influence of MOFs

26

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

5354

precursors, Mol. Catal. (2019) https://doi.org/10.1016/j.mcat.2019.110701.

[8] Y.Q. Yang, H. Dong, Y. Wang, C. He, Y.X. Wang, X.D. Zhang, Synthesis of

octahedral like Cu-BTC derivatives derived from MOF calcined under different

atmosphere for application in CO oxidation, J. Solid State Chem. 258 (2018) 582-587.

27

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242

243

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247

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