JHGT 2017.08, Vol.24, No.6(337-344) Journal of the Hwa Gang Textile 華岡紡織期刊 第二十四卷 第六期 ISSN 1025-9678 http://www.jhgt.org.tw/pdf/jhgt-24.6(337-344)(2017-08).pdf

Electrochemical decolorization of PET depolymerization waste liquid with

Chitosan/Fe3O4 dispersed nano-electrodes

Mengjuan Li1,2, Xiaoqiang Li1,2, Qiang Gao1,2, Yanan Zhu 1,2，Yang Lu 3 1 Key Laboratory of Eco-Textiles (Jiangnan University), Ministry of Education

2College of Textile & Clothing, Jiangnan University 3Key Laboratory of Food Nutrition and Safety, Ministry of Education, School of Food Engineering and

Biotechnology, Tianjin University of Science & Technology

Mengjuan Li : mjjn@jiangnan.edu.cn

Abstract

This work aimed at decolorizing PET depolymerization waste liquid (DWL) by electrochemical

method. PET fabrics were glycolsized by utilizing excess EG and the glycolsis product was purified

through repeated crystallization to get BHET crystal. The waste liquid of the depolymerization process

was electrochemical decolored by utilizing chitosan/Fe3O4 nanoparticles as the dispersed electrodes

under a 15 V DC voltage. The absorption at 338, 531 and 635 nm which were due to the dyes in DWL

decreased with the electrolysis time, while the absorption of EG (322nm) changed slightly. The removal

ratio of dyes was described with A/A0 and achieved to 78.94 %. PET fabrics were depolymerized by

using the decolorized DWL and mixture of decolorized DWL with EG (1:1 v / v). The maximum yield

of BHET was 58% and 72.3%, respectively.

Keywords：PET Depolymerization, Waste Liquid Treatment, Electrochemical Decolorization,

Dispersed Nano-Electrodes, Chitosan/Fe3O4 Nanoparticles

Journal of the Hwa Gang Textile, 2017.08, Vol.24, No.6

Introduction

Poly(ethylene terephthalate) (PET), a common

polymer material with excellent properties, is wildly

used in daily life and industrial process [1, 2]. There are

more than 13 million t PET produced and consumed

every year [3]. In order to reuse the waste PET as the raw

to produce valuable products, such as alkyd resins,

textile dyestuffs, unsaturated polyester, polyurethane

foams, etc. [4-9], PET waste is depolymerized into

oligomer and/or monomer by chemical recycling

techniques [10, 11].

Glycolysis of PET waste is a convenient method to

recycle polymer because of the relative mild reaction

conditions and decent yield of oligomer. PET wastes

could be glycolysized into dimethyl terephthalate

(DMT), bis(2-hydroxyethyl) terephthalate (BHET) or

terephthalic acid (TPA) by utilizing excess ethylene

glycol (EG) [12, 13]. The glycolysis products were

mixed with water and purified through repeated

crystallization to get BHET crystal. Theoretically, the

residual depolymerization waste liquid (DWL) mainly

including ethylene glycol (EG) and water could be

purified and reused to depolymerize waste PET [14]. In

the process of reusing the depolymerization waste liquid,

the dyes, pigments and textile auxiliary agents in the

residual depolymerization waste liquid which were

added into/onto PET products in the productive process

would affect the color and decline the purity of the

glycolysis products. Therefore, the decolorizing process

was the essential and important step of the reusing of

depolymerization waste liquid.

Generally, the decolorization process could be

carried out by biological, physical and chemical methods.

However, biological processes usually cost relative long

time and require large storage areas[15]. Physical

method is to adsorb the dyes and/or pigments by utilizing

high efficiency adsorbents like active carbon, active

alumina (Al2O3), zeolite, mesoporous molecular sieves

and polymeric adsorbents[16-19]. The advantages of

adsorption decolorization are its high decolorization

ratio and decolorization efficiency, while the

disadvantages are the long processing time, large dosage

and difficulty of dealing with the used adsorbents. In

contrast, chemical decolorization processes rely on the

formation of oxidising agent to remove colors.

Electrochemical decolorization is a novel and clean

chemical decolorization technique to use electrons as the

provider of free radical to degrade organics. Furthermore,

electrochemical decolorization technique was impressed

by the advantages of high removal efficiency, short

treating time and without of any secondary pollutant [20,

21].

Three dimensional electrode reactors are novel

electrochemical devices with many small particles

between the cathodic and anodic electrodes. The

particles can be polarized under the applied electric field

and formed charged dispersed electrodes to increase the

reaction surface area and shorten the transfer distance

between the reactant and electrodes. Therefore, the three

dimensional electrode reactors can promote the

electrolytic efficiency and has been successfully applied

to remove pollutants including organic compounds,

metal ions and dyes from wastewater. Chitosan modified

ferroferric oxide (Chitosan/Fe3O4) magnetic

nanoparticles are good candidates for dispersed

electrodes of three dimensional electrode reactor

because of their role of electron supplier in

electrochemical reactions, good adsorbability, specific

surface areas and convenience of collection and reusing

[22, 23].

In this article, we provided a method to decolorize

the PET depolymerization waste liquid by

electrochemical method. Pt sheet electrodes worked as

the cathodic and anodic electrodes. Chitosan/Fe3O4

magnetic nanoparticles were used as the dispersed

electrodes to facilitate the electrolytic efficiency. The

absorbance of dyes and EG were determined by means

of UV-vis spectra. The removal rates of dyes were

expressed by the variation of concentration (C/C0) which

could be evaluated by the variation of absorbance (A/A0).

Journal of the Hwa Gang Textile, 2017.08, Vol.24, No.6

Experimental

Materials

Poly(ethylene terephthalate) fabrics was washed

with distilled water and dried at 80 °C for 8 hours until

its weight was invariable. Ethylene glycol (EG), acetone

(AC), ethanol, paraffin span-80, acetic acid,

glutaraldehyde, zinc acetate dehydrate, sodium

tetrafluoroborate (NaBF4), hydrogen peroxide (H2O2, 30

wt%), ferroferric oxide (Fe3O4) magnetic nanoparticles

(500nm in diameter), chitosan, all of CP grade, were

purchased from Sinopharm Chemical Reagent Co., Ltd.

(Shanghai, China).

Synthesis of chitosan/ Fe3O4 nanoparticles

Chitosan/ Fe3O4 magnetic nanoparticles were

prepared by the covalent binding of chitosan onto the

surface of Fe3O4 nanoparticles [24]. Fe3O4 magnetic

nanoparticles were washed by ethanol and dispersed in

the mixture of paraffin and span-80 (60:1 v/v), then

chitosan in acetic acid solution (2 wt %) was added. The

suspension was mixed by ultrasonic irradiation and then

mixed with glutaraldehyde solution (25 wt %) with a

mechanical stirrer. After reaction, the chitosan/ Fe3O4

magnetic nanoparticles were achieved and were washed

by water and ethanol, and finally dried in a vacuum oven.

Glycolysis of PET

Glycolysis of dyed PET fabrics was achieved on the

basis of a previously published report [13]. Dyed PET

fabrics, EG and zinc acetate dehydrate (1:3:0.002 wt %)

were added into a four-necked round-bottom glass flask,

the glycolysis reactions were processed at 198 °C under

nitrogen atmosphere for 4 hours. After reaction, the

glycolysis products were mixed with hot water (1:1 v/v)

and purified through repeated crystallization to get

BHET crystal.

Electrochemical decolorization

An illustration of the experimental setup was

showed in Figure 1. Electrolysis experiments were

performed with an undivided cylindrical glass cell of 4.8

cm diameter and 6 cm height. Depolymerization waste

liquid solution with H2O2 and NaBF4 were injected into

the cell. Constant-current electrolyses were performed

with direct-current (DC) power supply (MCH-305D-II,

MCH Instruments Co., Ltd., Shenzhen, China). Two Pt

sheets (10 × 10 × 0.1 mm) were used as the cathode and

anode. Chitosan/ Fe3O4 magnetic nanoparticles were

dispersed in the DWL solution and worked as dispersed

electrodes. The cylindrical reactor cell was placed on a

magnetic stirrer to keep the chitosan/ Fe3O4 magnetic

nanoparticles dispersed in the solution uniformly.

Figure 1 Illustration of the electrochemical

decolorization setup with chitosan/ Fe3O4

magnetic nanoparticles

UV-vis spectra detection

Absorbances of the original PET-DWL and PET-

DWL after electrochemical decolorization were

determined by an UV-mini1240 spectrophotometer

(Shimadzu, Japan). The absorbance of each sample was

defined as the average value of three parallel samples.

Results and discussion

UV-vis spectroscopy

UV-Vis absorption spectra of EG which was diluted

with acetone were showed in figure 2. The absorption at

322 nm was proportionally decreased with the

decreasing of volume fraction of EG. It indicated that the

absorption at 322 nm was due to EG. DWL was diluted

with acetone or EG proportionally. UV-Vis absorption

spectra of acetone-diluted DWL and EG-diluted DWL

were showed in figure 3 and figure 4 respectively. The

absorption at 322 nm appeared in all curves and

Journal of the Hwa Gang Textile, 2017.08, Vol.24, No.6

proportionally decreased with the dilution of acetone

(figure 3) while proportionally increased with the

dilution of EG (figure 4). It indicated that the absorption

at 322 nm was due to the DWL-including EG. The

absorptions at 338, 531 and 635 nm which decreased

with the dilution of acetone as well as the dilution of EG

were due to DWL-including dyes, pigments and textile

auxiliary agents.

Figure 2 UV-Vis absorption spectra of EG which was

diluted with acetone, the volume fraction of EG were (a)1/8, (b)1/16, (c)1/32, (d)1/64, (e)1/128. The inset demonstrated that the absolute value of absorption at 322nm was proportional to the volume fraction of EG

Figure 3 UV-Vis absorption spectra of DWL which was

diluted with acetone, the volume fraction of DWL were (a)1/8, (b)1/16, (c)1/32, (d)1/64, (e)1/128. The inset demonstrateed that the absolute value of absorption at 338nm (black circles) and 322nm (red circles) were proportional to the volume fraction of DWL

Figure 4 UV-Vis absorption spectra of DWL which was

diluted with EG, the volume fraction of DWL were (a)1/8, (b)1/16, (c)1/32, (d)1/64, (e)1/128. The inset demonstrates the proportionality of absorption at 338nm (black circles) and 322nm (red circles) to the volume fraction of DWL

Figure 5 UV-Vis absorption spectra of DWL at different

electrochemical decolorization time, from 0 to 6 h (from top to bottom) with the interval of 0.5 h

Electrochemical decolorization of DWL

The DWL solutions prepared as the following 3

conditions: (a) 60mL DWL + 5mM NaBF4 + 0.04 g

chitosan/Fe3O4, (b) 60mL DWL + 5mM NaBF4 + 10 mL

H2O2 (30 wt%) and (c) 60mL DWL + 5mM NaBF4 +

0.04 g chitosan/Fe3O4 nanoparticles + 10 ml H2O2 (30

wt%) were electrochemical reacted in a electrolyte cell

with a 15 V direct-current supplied between the anodic

and cathodic electrodes. A sample of UV-Vis absorption

spectra of DWL varies with electrochemical

decolorization time was showed in figure 5. The

Journal of the Hwa Gang Textile, 2017.08, Vol.24, No.6

absorptions at 338, 531 and 635 nm were decreased with

the reacting time while the absolute value of absorption

at 322 nm changed slightly. It indicated that the dyes in

DWL were degraded by means of electrochemical

oxidation while EG in DWL almost has no effect while

the electrochemical process.

According to the Beer-Lambert Law:

A = ε·b·C (1)

Where A is absorbance, ε is the molar absorbtivity

of the sample, b is the path length of the sample and C is

the concentration of the sample in solution, ε and b are

constant in this experiment. Therefore, the variation of

concentration (C/C0) could be expressed by the variation

of absorbance (A/A0) as showed in equation 2.

C/C0 = A/A0 (2)

where C0 is the concentration of dyes (or EG) in the

original PET-DWL, C is the concentration of dyes (or

EG) in the decolorized PET-DWL, A0 is the absorbance

of dyes (or EG) in the original PET-DWL, A is the

absorbance of dyes (or EG) in the decolorized PET-DWL.

Figure 6 Variation of color removal ratio of DWL

with time: (a) 60mL DWL + 5mM NaBF4 + 0.04 g chitosan/Fe3O4, (b) 60mL DWL + 5mM NaBF4 + 10 mL H2O2 (30 wt%), (c) 60mL DWL + 5mM NaBF4 + 0.04 g chitosan/Fe3O4 nanoparticles + 10 mL H2O2 (30 wt%)

Figure 7 Absorbance variation of EG in DWL with time:

(a) 60mL DWL + 5mM NaBF4 + 0.04 g chitosan/Fe3O4, (b) 60mL DWL + 5mM NaBF4 + 10 mL H2O2 (30 wt%), (c) 60mL DWL + 5mM NaBF4 + 0.04 g chitosan/Fe3O4 nanoparticles + 10 mL H2O2 (30 wt%).

The color removal ratio of DWL in the above 3

conditions were expressed by A/A0 at 338 nm and

showed in figure 6. In all curves, the absorbance of dyes

decreased linearly in the first 3 h. The max color removal

ratio was 78.94 % when DWL contained both chitosan/

Fe3O4 magnetic nanoparticles and H2O2. In all conditions,

the absorbance of EG changed slightly (figure 7) which

indicated the EG was not degraded in the

electrochemical decolorization process.

Mechanism of electrochemical decolorization in DWL

The original DWL solution was acidic solution with

pH of 3.75. During the electrochemical decolorization

process, water was electrolyzed on the Pt anode and

cathode in the presence of O2 and produced ozone (O3)

and H2O2, respectively.

H2O (acidic solution) → O3 + H+ + e- (3)

O2 + H+ + e- → H2O2 (4)

The Fenton reactions of FeII/FeIII occurred on the

surface of chitosan/ Fe3O4 particles in the presence of

H2O2. FeII was oxidized by H2O2 to generate FeIII and

hydroxyl radical (equation 5). Meanwhile FeIII could be

oxidized by H2O2 to generate FeII and hydroperoxyl

radical (equation 6). In addition, the Fenton reactions

also took place in the bulk solution by the dissolved iron

Journal of the Hwa Gang Textile, 2017.08, Vol.24, No.6

and H2O2. Fe2+ catalyzed the decomposition of H2O2 and

generated hydroxyl radical (equation 7). While Fe3+

reacted with H2O2 to generate Fe2+ and hydroperoxyl

radical (equation 8) [23].The dyes in DWL could be

decolorized by the generated radical immediately. The

proposed mechanism was showed in figure 8.

FeII + H2O2 → FeII·H2O2

→ FeIII + ·OH + O H- (5)

FeIII + H2O2 → FeIII·H2O2

→ FeII + HOO· + H+ (6)

Fe2+ + H2O2 → Fe3+ + ·OH + OH- (7)

Fe3+ + H2O2 → Fe2+ + HOO· + H+ (8)

Figure 8 Proposed mechanisms of electrochemical

decolorization of PET depolymerization waste liquid

with chitosan/Fe3O4 dispersed nano-electrodes

Reusing of decolorized DWL

The decolorized DWL was purified through

vacuum distillation to remove water and reused to

degrade PET fabrics with the method that mentioned in

section 2.3. The yield of BHET by only using

decolorized DWL was above 58% and by using mixture

of decolorized DWL and EG (1:1 v/v) was 72.3%.

Conclusion

In this study, the residual DWL which was

produced in the PET glycolysis process was decolorized

with an electrochemical method. The DWL which

included EG, water, dyes, pigments and textile auxiliary

agents was electrolyzed in a three dimensional electrode

reactor under 15 V DC voltage. Chitosan/ Fe3O4

magnetic nanoparticles worked as the dispersed

electrodes to facilitated the electrolytic efficiency. The

absorption of the dyes in DWL at 338, 531 and 635 nm

decreased with the electrolysis time, while the

absorption of EG (322nm) changed slightly. The

maximum removal ratio achieved to 78.94 % when the

electrolyte solution contain both chitosan/Fe3O4

nanoparticles and H2O2. PET fabrics were

depolymerized by using the decolorized DWL and

mixture of decolorized DWL with EG (1:1 v/v). The

maximum yield of BHET was 58% and 72.3%,

respectively.

Acknowledgment

This work was financially supported by the

National High-tech R&D Program of China (No.

2016YFB0302901), the project program of Chinese

National Training Programs of Innovation and

Entrepreneurship for Undergraduates (No.

201610295029).

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