Preparation of a -N-bridged Binuclear Fe(III) Schiff-base Complex

as Catalyst for the Degradation of Emerging Contaminants

Dibutylphthalate (DBP)

Haoyu Shen

, Yufei Wang, Leyi Weng, Zhehao Jiang and Qi Jiang

Ningbo Institute of Technology, Zhejiang University; Ningbo, Zhejiang, 315100, China

Keywords: Fe(III) Schiff base complex; Emerging contaminants (ECs); Dibutyl phthalate (DBP); Degradation;

Fenton-like reaction.

Abstract: Schiff base ligand (ST = N, N'-tetraethylenepentaminebis (salicylideimine)) was prepared by condensation

reaction of salicylaldehyde (SA) and tetraethylenepentamine (TEPA), and further coordinated with iron (III)

via coordination reaction to form a binuclear iron (III) complex, [Fe

(ST)(H

]Cl

(abbreviated as,

(ST)). It was characterized by elemental analysis (EA), Fourier transform infrared spectrometer (FTIR),

and ultraviolet-visible spectroscopy (UV-vis), etc. It was used as catalyst for the degradation of emerging

contaminants, e. g., dibutyl phthalate (DBP) under visible light with hydrogen peroxide solution as oxidant.

The results showed that at pH 3.5-8.0, with the initial concentration of Fe

(ST) complex larger than 5.50

mol·L

-1

, and that of H

larger than 8.16 mmol·L

-1

, the degradation of DBP at initial concentration less than

20.0 mg·L

-1

can be reached to almost 100% within 10 min. The catalytic reaction system has been monitored

by electronic spectrum before or after adding H

at different time intervals. The result showed the catalytic

activity site may be a -N-bridged binuclear Fe(III) centre and a di-Fe(III)-H

transition-state might be

formed, which was favourable to the activation of the H

under visible light. The Fe

(ST) is a potential

effective and green catalyst for the degradation of DBP.

1 INTRODUCTION

Phthalate esters (PAEs) are commonly used synthetic

materials and plasticizers, pesticides, etc. They are

typical environmental hormones and Emerging

contaminants (ECs) (Vrijheid et al., 2016). In recent

years, PAEs residue was detected in the

environmental samples, and even in food. The

pollution of PAEs is becoming more and more

serious (Shen, 2005). How to effectively remove the

PAEs in the environment is a major challenge in the

field of environmental science and technology. At

present, the most effective methods for removing

PAEs are advanced oxidation processes (AOPs)

(Legrini et al., 1993). Among AOPs, Fenton reaction

is an attractive method for effective degradation of

PAEs because of its low cost, the lack of toxicity of

the reagents (Hu et al., 2012). Recently, it has proved

that the Fe

transition metal complex-H

system

has the advantages of high utilization rate of H

than the conventional Fenton oxidation method (Hu

and Xu, 2014). Among them, the metal Schiff-base

complex has attracted much attention due to the

peculiar electronic characteristics, stable structure,

and tailoring the electronic and space effect of

complex accurately and easily by changing the ligand

or metal ion. The iron Schiff-base can act as catalyst

in lots of oxidation reactions, which not only

overcome the shortcomings of high acidity

requirements, but also exhibit some characteristics of

biomimetic reactions. At present, iron Schiff-base

complexes have been reported about the selective

catalytic oxidation of different group mainly in

homogeneous reaction, but few research pays

attention to the heterogeneous photocatalytic

performance in water treatment of toxic organic

pollutants under visible light.

In this work, a novel binuclear iron (III)

complexes with Schiff base [Fe

(ST) (H

]Cl

(abbreviated as: Fe

(ST)) was synthesized and

characterized. It was used for the catalytic

degradation of DBP. The effects of the solution pH

value, the concentration of H

in the system,

(ST) and the initial concentration of DBP were

Shen, H., Wang, Y., Weng, L., Jiang, Z. and Jiang, Q.

Preparation of a -N-bridged Binuclear Fe(III) Schiff-base Complex as Catalyst for the Degradation of Emerging Contaminants Dibutylphthalate (DBP).

DOI: 10.5220/0008184900290033

In The Second International Conference on Materials Chemistry and Environmental Protection (MEEP 2018), pages 29-33

ISBN: 978-989-758-360-5

investigated. The presumed degradation mechanism

was deeply investigated in present work.

2 EXPERIMENTAL

2.1 Preparation of Fe

(ST)

0.2 mmol (37.9mg) of TEPA and 0.4 mmol (48.8mg)

of SA was dissolved in 10 mL methanol,

respectively. SA methanol solution was added to the

TEPA solution dropwise under vigorous stirring. The

reaction was continued at 60

C for 2 hrs. After the

mixture solution was cooled down, the yellow colour

Schiff base ligand, in which ST is the N,

N'-tetraethylenepentamine-bis (salicylide-imine),

was obtained. The yellow crystals of ST were

collected by filtering under vacuum, washed with a

small amount of water and methanol for three times,

and dried under vacuum at 60

C for 12 hrs, yield:

96.5%. 0.2 mmol (54.6 mg) of FeCl

6H

O and 0.1

mmol (35.2 mg) of ST was dissolved in 10 mL

methanol, respectively. FeCl

6H

O methanol

solution was added to the ST solution dropwise under

vigorous stirring. The 0.1 mol·L

-1

NaAc solution at

was then added dropwise to adjust the mixture

solution pH to 8-9. The reaction was continued at 60

C for 2 hrs. After the mixture solution was cooled

down, the brown microcrystalline product was

obtained, and collected by filtering under vacuum,

washed with a small amount of water and methanol

for three times, and dried under vacuum at 60

C for

12 hrs, yield: 89.6%. The overall preparation

procedure was shown in Scheme 1.

CHO

FeCl

/NaAc

Scheme 1: Preparation of Fe

(ST).

2.2 Catalytic Degradation of DBP by

(ST)

Catalytic degradation experiments were carried out

in 150 mL stoppered flasks, each of which contained

25.00 mL of DBP acetonitrile solution at

concentration of 20 mg·L

-1

, 40 L of

(ST)

acetonitrile solution at concentration of 4.0 mg·L

-1

was added, followed adding 20 uL of 30% H

. The

mixture was shaken at 150 rpm in a thermostatic

shaker, sampling at every 1 min, followed by adding

1 drop of 10% Na

solution to stop the reaction.

The experiments of traditional Fenton system was

carried out similarly by replacing Fe

(ST) with

FeSO

acetonitrile solution of at concentration of 4.0

mg·L

-1

. HPLC method was applied for the

determination of the residue concentrations of DBP.

The degradation rates of DBP under different loading

amount and different pH conditions were calculated

according to Eq. (1):

%100







(1)

Where D

is the degradation rate of DBP; A

and A

are the HPLC peak area of DBP at time t and at time

0, respectively.

The effects of initial pH value, usage amount of

, initial concentration of Fe

(ST) DBP

concentration on the degradation of DBP were

investigated.

3 RESULTS AND DISCUSSION

3.1 Characterization of Fe

(ST)

(ST) was characterized by EA, TG/DTG, UV/Vis

and FTIR, etc. The EA results showed that the

elemental percentage of Fe

(ST) were (experimental

(theoretical), %): C: 36.58 (36.65), H: 5.25 (5.17), O:

13.52 (13.31), N: 9.52 (9.71) and Fe: 15.22 (15.49),

respectively. The molar conductivity of Fe

(ST) was

determined by using acetonitrile as a solvent. It was

found that Fe

(ST) is a 1: 4 type electrolyte (Geary,

1971). The concentration of chlorine ion (Cl

) was

obtained by titration of the Fe

(ST) solution in

acetonitrile with AgNO

solution, and found to be

19.96% (19.67%, theoretically). TG/DTG (Figure 1)

shows that Fe

(ST) has 10.2% weight lose at ~120℃,

which would be relevant to 4 molecular of water

(theoretical data: 9.99%). Combined with the results

of its conductivity and Cl

content, it would be

deducted that the four water molecules might be in

the inner boundary of the Fe

(ST) and coordinated

with Fe (III), while the four Cl

ions are in the outlay

of the complex.

UV-Vis spectrum (Figure 2(a)) of ST and Fe

(ST)

showed that the characteristic peaks of -*

transition and n-* transition absorption of benzene

ring and imine in ST appeared at 252 nm and 315 nm.

After forming Fe

(ST) complex, these two peaks

were red-shifted to 260 nm and 325 nm, respectively,

along with the absorption peak at 325 nm

MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection

significantly broadening, which can be attributed to

the charge transfer of the p

(N) d

(Fe) in Fe

(ST).

Simultaneously, the d-d transition absorption peak of

Fe(III) was observed at 512 nm. The FTIR of ST and

(ST) (Figure 2(b)) showed that typical peaks at

1642 cm

-1

and 1585 cm

-1

assigned to the absorption

of imine and amino groups of ST appeared. After

coordination with Fe(III), these two peaks were

red-shifted to 1635 cm

-1

and 1445 cm

-1

, respectively,

with a large broadening. New peaks at 620 cm

-1

and

525 cm

-1

, attributed to the characteristic absorption

peaks of Fe-N and Fe-O bonds appeared (Mao et al.,

2009), indicating the successful formulation of

(ST) complex.

100 200 300 400 500 600 700 800

100

-10

-8

-6

-4

-2

Temperature(

TG/%

DTG/(%/min)

307.4

109.4

409.6

437.2

Figure 1: TG-DTG curves of Fe

(ST).

200 300 400 500 600 700 800 900

0.0

0.5

1.0

1.5

260 nm

252 nm

315 nm

512 nm

 (nm)

325 nm

(ST)

(a)

4000 3500 3000 2500 2000 1500 1000 500

Wave number (cm

-1

)

(TS)

1445 cm

-1

1635 cm

-1

1185 cm

-1

1045 cm

-1

525 cm

-1

T/%

620 cm

-1

1642 cm

-1

1585 cm

-1

(b)

Figure 2: (a) UV and (b) FTIR spectra of ST and Fe

(ST).

3.2 Degradation of DBP by Fe

(ST)

Figure 3(a) shows the degradation of DBP by Fe

(ST)

in H

/visible light system. With the increasing of

the reaction time, the degradation rate of DBP

increased from 75.6% to 99.8% within 10 min. The

experimental results of traditional Fenton system are

shown in Figure 3(b). With the reaction time

increasing, the degradation rate of DBP decreases

from 30.2% to less than 0.5% in 8 mins. This might

be due to the fact that ·OH free radical is easily

deactivated generated by the traditional Fenton

system. It can be seen that the Fe

(ST)/H

/visible

light system has higher catalytic degradation

performance for DPB than that of the traditional

Fenton system. The high catalytic degradation

efficiency under visible light conditions might be

contributed to the -N-bridged binuclear Fe(III)

centre of the Fe

(ST).

0 20 40 60 80 100

100

Degradation percentage (%)

Reaction time (min)

Figure 3: Degradation extent of DBP under (a)

(ST)/H

/vis, (b) traditional Fenton catalytic systems.

The electronic spectra of DBP solution before and

after the addition of Fe

(ST)/H

are shown in

Figure 4. With the addition of H

(c, 0 min; d, 1

min; e, 5 min; f, 10 min), the intensity of the

characteristic peak of the charge transfer of the p

(N) d

(Fe) of Fe

(ST) at about 325 nm gradually

increased and broadened. This might be due to the

fact that the formation of an -N-bridged binuclear

Fe(III)-H

transition states during the catalytic

process (Mao et al., 2009), shown in Figure 5. When

was added into the Fe

(ST), a H

bridged

transition state might be formed, which might be

favourable to the activation of H

, leading the

produce of ·OH free radicals, under visible light, the

DBP was mineralized to CO

and H

O. As shown in

Figure 4, the characteristic absorption peaks of DBP

at 230 nm and 275 nm disappeared, indicating a

significant degradation of DBP (Figure 4b). The

COD of the post-degradation solution was tested

according to HJ 828-2017 method and the result was

at 15 mg/L, indicating the mineralization of the DBP.

Preparation of a -N-bridged Binuclear Fe(III) Schiff-base Complex as Catalyst for the Degradation of Emerging Contaminants

Dibutylphthalate (DBP)

200 300 400 500 600 700 800 900

0.0

0.4

0.8

1.2

1.6

a DBP 10 mg L

-1

b DBP after degradation

c Fe

(TS)/H

0 min

d Fe

(TS)/H

1 min

e Fe

(TS)/H

5 min

f Fe

(TS)/H

10 min

g Fe

(TS)/H

20 min

 (nm)

Figure 4: UV spectra of DBP before (a) and after (b) adding

(ST)/H

at different reaction time (c-g).

III

O O

III

+ H

COOCH

Vis

Figure 5: Presumed mechanism of -N bridged binuclear

Fe(III)-H

transition-state and production of OH.

3.2.1 Effect of Solution pH Values

The effect of solution pH was investigated with the

pH values ranging from 3.0 to 9.0. The results (Figure

6) showed that at the pH range of 3.5-8.0, DBP can

degrade almost 100% within 10 min under the

(ST)/H

/visible light catalytic system. The pH

value of the DBP solution is about 6.0. Thus the

catalytic degradation can be carried out without pH

adjustment of the DBP solution. The reason for the

wider pH range of the present catalytic system than

that of the conventional Fenton reaction system may

be due to the fact that the Fe(III) coordinated to the

ST ligand might stable the Fe(III) active centre,

leading its resistance to the effect of the solution pH

varying on the degradation efficiency of DBP (Wang

et al., 2007).

0 20 40 60 80

100

0 5 10 15 20

100

pH=3.0

pH=9.0

pH=3.5

pH=4.0

pH=5.0

pH=7.0

pH=8.0

Reaction time (min)

Degradation percentage (%)

Reaction time (min)

Figure 6: Effect of the solution pH on the

(ST)/H

DEP catalytic degradation system (insert:

pH=3.5-8.0 enlarged).

3.2.2 Effect of Usage Amount of H

The effect of usage amount of H

was investigated

with the concentration of H

in the catalytic system

was in the range of 2.04 to 40.8 mmol·L

-1

, with the

initial concentration of DBP at 20 mg·L

-1

. The results

(Figure 7) showed that when the concentration of

was at 2.04 mmol·L

-1

, nearly 45 mins were

needed for total degradation of DBP. When the

concentration of H

at 4.08 mmol·L

-1

, degradation

of DBP can be realized within 30 mins. With the

concentration of H

increasing, the degradation

rate increased gradually, and when the concentration

of H

at 8.16 mmol·L

-1

, the degradation of DBP

can be realized within 10 mins (Lee and Yoon, 2004).

0 20 40 60 80

100

2.04

4.08

8.16

16.3

24.5

32.6

40.8

mmol L

-1

Degradation percentage (%)

Reaction time (min)

mmol L

-1

mmol L

-1

mmol L

-1

mmol L

-1

mmol L

-1

mmol L

-1

Figure 7: Effect of the usage amount of H

on the

(ST)/H

/DEP catalytic degradation system.

3.2.3 Effect of Usage Amount of Fe

(ST)

The effect of usage amount of Fe

(ST) was

investigated with the concentration of Fe

(ST) in the

catalytic system was at 1.10-11.0 mol·L

-1

. The

results (Figure 8) showed that Fe

(ST) was found to

be the key factor for the generation of the ·OH free

radicals. When the usage amount of Fe

(ST) was low,

the generation rate degradation rate of DBP was very

low. With the usage amount of Fe

(ST) increasing,

the generation rate of the ·OH free radicals increased,

leading a increasing of degradation rate of DBP.

When the usage amount of Fe

(ST) reached 5.50

mol·L

-1

, the degradation rate tends to be stable.

0 20 40 60 80

100

1.10

2.20

3.30

4.40

5.50

6.60

11.0

Degradation percentage (%)

Reaction time (min)

umol L

-1

umol L

-1

umol L

-1

umol L

-1

umol L

-1

umol L

-1

umol L

-1

Figure 8: Effect of the usage amount of Fe

(ST) on the

(ST)/H

/DEP catalytic degradation system.

MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection

3.2.4 Effect of the Initial Concentration of

DBP

The effect of initial concentration DBP was

investigated with the concentration of DBP in the

catalytic system at 14.38-287.77 mol·L

-1

. The

results (Figure 9) showed that when the concentration

of DBP<71.6 mol·L

-1

(50mg·L

-1

), the DBP can be

totally degenerated in 10 mins. The residue

concentration of DBP was at g·L

-1

to mg·L

-1

levels

in the aqueous environment, thus the present

(ST)/H

/vis system can be used for the

treatment of the DBP in the environment.

0 20 40 60 80

100

14.4

28.8

54.0

71.6

143.9

215.8

287.8

Degradation percentage (%)

Reaction time (min)

umol L

-1

umol L

-1

umol L

-1

umol L

-1

umol L

-1

umol L

-1

umol L

-1

Figure 9: Effect of the initial concentration of DBP on the

(ST)/H

/DEP catalytic degradation system.

3.3 Reusability Investigation

The reusable of the Fe

(ST)

was evaluated. Results

were shown in Figure 10, which indicated that

(ST) could be used for at least 10 cycles with

degradation rate higher than 95% upon recovery on

average. No obvious decrease in the degradation

efficiency and iron leaching were found. Compared

with literature reports, the Fe

(ST) is a potential

effective and reusable catalyst for the degradation of

DBP with high degradation effect.

-1 0 1 2 3 4 5 6

100

(a) (b)

Degradation percentage (%)

Cycle number

Figure 10: Recycle of the Fe

(ST) catalyst (a) Method 1:

re-adding DBP after rotating evaporation of the

post-degeneration solution, (b) Method 2: re-adding DBP

directly in the post-degeneration solution.

4 CONCLUSIONS

In this work, a binuclear iron (III) complex (Fe

(ST))

was prepared and characterized by EA, TG/DTG,

UV/Vis and FTIR, etc. It was used as a catalyst for

the Fenton–like reaction of the catalytic degradation

of the emerging contaminants di-butyl-phthalate

(DBP). It is found that under the condition of visible

light, the catalytic degradation of DBP could be

achieved by Fe

(ST)/H

system in aqueous

solution with the pH range from 3.5 to 8.0. Compared

with ordinary Fenton reaction system, a wide range of

pH value for the degradation of DBP achieved. The

degradation of DBP was more than 99% when the

concentration of Fe

(ST) was larger than 5.50

mol·L

-1

, the concentration of H

was larger than

8.16 mmol·L

-1

, and the concentration of DBP was

less than 20.0 mg·L

-1

. With the aid of the electronic

spectrum monitoring, it is found that the transition

state of -N bridged bi-nucleus Fe(III)-H

centre

may form in the catalytic process.

ACKNOWLEDGEMENTS

We would like to thank the National Natural Science

Foundation of China (51608479, 81502421), the

National Natural Science Foundation of Zhejiang

Province (LY14B04003), the National Natural

Science Foundation of Ningbo (2018A610206), the

National College Students’ innovation and

entrepreneurship training program (201813022009),

the Xinmiao Students’ innovation training program

of Zhejiang Province (2018R401181) for the

financial support.

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Preparation of a -N-bridged Binuclear Fe(III) Schiff-base Complex as Catalyst for the Degradation of Emerging Contaminants

Dibutylphthalate (DBP)