Research on the Graft Copolymerization of EH-lignin with acrylamide

doi:10.4236/ns.2009.11004

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Natural Science, 2009, 1, 17-22 NS

http://dx.doi.org/10.4236/ns.2009.11004

Research on the Graft Copolymerization of EH-lignin

with acrylamide

Run Fang1,2, Xian-Su Cheng*1, Jian Fu1, Zuan-Bin Zheng1

1College of Material Science and Engineering, Fuzhou University, Fuzhou 350002, China; 2Department of Chemistry and Chemical

Engineering, Minjiang University, Fuzhou 350011, China.

*Corresponding author: [email protected]

Received 13 April 2009; revised 14 May 2009; accepted 23 May 2009.

ABSTRACT

Lignin isolated from enzymatic hydrolyzed corn-

stalks (EH-lignin) is a renewable natural polymer

noted for its versatility and applicability in a vari-

ety of uses. Graft copolymerization of EH-lignin

with acrylamide (AM) and the application of this

copolymer as a flocculant in dye wastewater

treatment were studied in this article. The influ-

ences of some factors on yield of copolymer and

the grafting ratio were investigated and the

structure of EH-lignin/AM graft copolymer was

characterized by FT-IR. According to the yield and

the grafting ratio, the optimum conditions for

graft copolymerization were as follows: initiator

K2S2O8-Na2S2O3 with a quantity 3 wt% of EH-lignin,

mass ratio of AM to EH-lignin was 2~3, reaction

time 4h and temperature at 50℃. It was found that

the absorption capacity of graft copolymer to two

azo-dyes was enhanced with the increase of

grafting ratio. Furthermore, the residue concen-

tration of EH-lignin/AM graft copolymer remained

in the supernatant after flocculation was much

lower than that of pure EH-lignin.

Keywords: Lignin; Acrylamide; Graft Copolymeriza-

tion; Dye Wastewater; Decoloration

1. INTRODUCTION

At present, the fossil resources are rapidly running out

and the environmental pollutions are getting even more

serious throughout the world. Great attention has been

paid to the development of sustainable technologies

based on renewable raw materials [1,2,3,4]. As a natural

polymer, lignin is a renewable and biodegradable re-

source and noted for its versatility and applicability in a

variety of uses. Making use of these biomaterials will

not only enhance the economic benefit of bioengineering

but also diminish environmental pollutions [5,6].

EH-lignin is a novel ornanosolv lignin isolated from

the residue of enzymatically hydrolyzed cornstalks as a

by-product of fuel ethanol industry [7]. Compared with

traditional lignosulfonate or alkali lignin, EH-lignin pos-

sesses some valuable characteristics: lower content of

sugar, less impurities and narrow molecular weight dis-

tribution. Furthermore, since the enzymatic hydrolysis

process of the cornstalks is carried out under relatively

mild conditions, many functional groups such as pheno-

lic hydroxyl, alcoholic hydroxyl and methoxyl are well

preserved in EH-lignin [8,9,10].

Due to its abundant functional groups, EH-lignin can

be used in dye wastewater treatment by adsorbing dyes

through hydrogen bonding under acidic conditions.

However, the concentration of residue lignin remains in

supernatant after flocculation is very high, which may

leads to a secondary pollution. In order to minimize the

potential secondary pollution, graft copolymerization of

EH-lignin with acrylamide (AM) was studied in this

paper. The effects of some factors on the copolymeriza-

tion were investigated. The structure of EH-lignin/AM

copolymer was analyzed by FT-IR and its application in

the dye wastewater treatment was evaluated. The residue

concentration of this flocculant remains in the super-

natant after flocculation was measured.

2. EXPERIMENTAL

2.1. Materials

EH-lignin was supplied by Tianguang fuel ethanol com-

pany (He’nan, China) in powder form and purified in

laboratory according to procedures described in our pre-

vious article [4]. Characteristics of purified EH-lignin

are shown in Table 1. The details of two azo-dyes, acid

red 274 (AR 274) and reactive red X-3B (RR X-3B),

were shown in Fig. 1 and Table 2. Acrylamide were

purchased from Guanghua chemical reagent Co., Ltd,

China. All other reagents were of analytical grade.

Table 1. Characteristics of EH-lignin.

Residual

sugar/%

Ash

Phenolic hy-

droxyl/mmol.g-1 Mw Mw/Mn

EHLignin

0.22 0.394.25 20621.22

18 R. Fang et al. / Natural Science 1 (2009) 17-22

SO3Na

NaO3S

NN O

NaO 3SSO3Na

NHC

(a) (b)

Figure 1. Molecular structure of (a) Acid red 274 and (b) Reactive red X-3B.

Table 2. Details of the dyes.

Dyes Abbreviation Molecular formula CAS number λmax(nm)

Acid red 274 AR 274 C35H31N3Na2O9S2 72828-83-2 527

Reactive red X-3B RR X-3B C19H10Cl2N6Na2O7S2 12226-03-8 538

2.2. Synthesis and Characteristics of EH-

lignin/ AM Graft Copolymer

Graft copolymerization reactions were carried out in a

jacketed reactor flask equipped with a stirrer and a reflex

condenser under N2 protection. Appropriate amount of

EH-lignin, AM and initiators were dissolved in NaOH

aqueous solution and then reacted at different tempera-

tures for a period of time. When a reaction was finished,

copolymer product was precipitated by acidification and

isolated in a centrifuge. In order to remove monomers,

EH-lignin/AM graft copolymers were washed by dis-

tilled water and then vacuum dried.

Viscosity measurement of lignin/AM copolymer in

water solution was conducted by an Ubbelohde type

viscometer at 30.0±0.1. Extrapolation procedure from ℃

data obtained for 5 concentrations of solutions was used

to calculate [η] from Huggins equation, ηsp/c=[η]+k

[η]2c. The intrinsic viscosity was then used to evaluate

the molecular weight of graft copolymers prepared with

different initiators.

The chemical structure of graft copolymer was ana-

lyzed using FT-IR2000 spectrometer (Perkinelmer, U. S.)

and the spectra were recorded in the range of 500-4000

cm-1.

Yield of EH-lignin/AM copolymer and the grafting

ration were determined by Eq.1 and Eq.2 respectively.

Yield

(%) 100%

YWW



 (1)

Grafting ratio

203

(%) 100%

WWW

GR WW





 (2)

where W0 is the weight of EH-lignin; W1 is the weight of

AM monomer; W2 is the weight of the graft copolymer;

W3 is the weight of lignin remained in the supernatant.

2.3. Adsorption and Decoloration of AR 274

and RR X-3B Dye Wastewater

The adsorption and decoloration of AR 274 and RR

X-3B dye wastewater by EH-lignin and EH-lignin/AM

grafted copolymer was investigated by static adsorption

method. Firstly, a certain amount of flocculant was

weighed and dissolved in 2ml 1%NaOH aqueous solu-

tion. Afterward, 200ml dye wastewater with a concentra-

tion of 500mg/L was added into aforesaid copolymer

solution, stirring 3 min to make sure the mixture well-

mixed and then kept undisturbed for 1h. Finally, the so-

lution was acidified with HCl to pH=4 and then filtered

after another 10 min standing. The concentrations of dye

wastewater before and after treatment were measured at

λmax mentioned in Table 2 by UV-2450 spectropho-

tometer. The total organic carbon (TOC) content of the

supernatant was measured by TOC-V analyzer (Shima-

dzu, Japan) to evaluate the residue amount of the floc-

culant and the dyes remained in the supernatant after

flocculation. The absorption amount, decoloration rate

and TOC removal can be calculated by Eq.3, Eq.4 and

Eq.5 respectively.

Adsorption amount

VCC

Q



)( 0 (3)

Decoloration rate

%100

0





E (4)

TOC removal rate

R. Fang et al. / Natural Science 1 (2009) 17-22 19

%100

0





R (5)

where Q is the adsorption amount, mg/g; C0 and C are

the concentrations of the dye solution before and after

treatment, mg/L; V is the volume of dye solution, L; W

is the amount of graft copolymer, g; E is decoloration

rate; A0 and A are the absorbance of the dye solution

before and after treatment; R is the TOC removal rate;

B0 and B are the TOC values of the dye solution before

and after treatment.

3. RESULTS AND DISCUSSION

The effects of various factors on the yield of copolymer

and the grafting ratio were investigated to determine the

optimum conditions for graft copolymerization. The

basic reaction conditions of these experiments were as

follows: the dosage of EH-lignin was 2.0g, acrylamide

was 4.0g, 100g 1% NaOH aqueous solution was used as

solvent, the amount of K2S2O8-Na2S2O3 was 3% of the

weight of EH-lignin, the reaction temperature was 50 ℃

and the reaction time was 4h.

3.1. Effects of Some Factors on Graft

Copolymerization

3.1.1. Effects of Different Kinds of Initiators on

Graft Copolymerization

The graft copolymerization of EH-lignin with AM was

carried out in aqueous solution, therefore six water-

soluble radical initiators were chosen and their effect on

grafted copolymerization was studied. The results were

showed in Table 1 and the synthesis conditions were as

follows: weight of lignin was 2.0g, acrylamide 4.0g,

reaction temperature was 50, r℃eaction time was 4h and

the dosage of initiator was 3% of the weight of EH-lig-

nin.

Since the raw materials and the reaction procedures

are identical, the chemical structure of lignin/AM co-

polymers initiated by different initiators is quite similar

to each other. Therefore, higher [η] of a lignin/AM co-

polymer’s aqueous solution may indicates a larger mo-

lecular weight of this copolymer. It can be seen in Table 1

Table 3. Effects of different initiators on yield and intrinsic

viscosity of the graft copolymers.

Initiator Yield/% [η]/mL.g -1

Fe2+-H2O2 34.30 35.76

(NH4)2Ce(NO3)6 30.86 34.51

K2S2O8 37.83 35.37

(NH4)S2O8 34.55 37.15

K2S2O8-NaHSO3 37.40 36.93

K2S2O8-Na2S2O3 39.60 37.86

that the graft copolymer initiated by K2S2O8-Na2S2O3

has the largest yield and highest intrinsic viscosity,

which means this binary-initiating system is more effec-

tive in grafting AM onto EH-lignin. For this reason,

K2S2O8-Na2S2O3 was employed in our further research

on the graft copolymerization of EH-lignin with AM.

3.1.2. Effect of Initiator Dosage on Graft

Copolymerization

The effect of initiator dosage on graft copolymerization

was evaluated considering the yield of copolymer and

the grafting ratio. The results were shown in Fig. 2. It

was found that the yield of the copolymer and the graft-

ing ratio increased with increasing initiator dosage at

first. However, when the initiator dosage was more than

3% of the weight of lignin, Y(%) and GR(%) increased

slowly and then decreased when the dosage of initiator

reach 5%.

In the process of the copolymerization, the binary-

initiating system of K2S2O8-Na2S2O3 generated free

radicals to initiate the polymerization of PAM and the

graft copolymerization of EH-lignin with AM or with

PAM chains. On one hand, high free radical concentra-

tion may enhance the possibility of graft copolymeriza-

tion and increase the yield and molecular weight of co-

polymer; on the other hand, it will lower the polymeriza-

tion degree of PAM that may graft onto EH-lignin and

decrease the molecular size of copolymer. Therefore,

Y(%) and GR(%) reach their maximum when the con-

tradiction reaches a equilibrium and the optimum dosage

of initiator is 3% of the weight of EH-lignin.

3.1.3. Effect of Acrylamide Dosage on Graft

Copolymerization

The mass ratio of acrylamide to EH-lignin is another

important factor that may affect the yield of copolymer

and the grafting ratio. It can be found in Fig. 3 that when

the dosage of AM was no more than 6g, the grafting

ratio of EH-lignin/AM copolymer increased quickly

12345

Grafting ratio (%)

Yield (%)

Mass fraction of initiator/EH-lignin (%)

Y(%)

GR(%)

Figure 2. Effect of initiator dosage on the yield of copoly-

mer and the grafting ratio.

20 R. Fang et al. / Natural Science 1 (2009) 17-22

024681012

Grafting ratio (%)

Yield (%)

Y(%)

GR(%)

Dosage of AM (g)

Figure 3. Effect of AM dosage on the yield of copolymer and

the grafting ratio (the dosage of EH-lignin is 2g).

as the mass ratio of AM to EH-lignin increased. This is

because the higher AM concentration can make it easier

for EH-lignin to come into contact with monomer and

then speed up graft copolymerization and improve

grafting ratio. However, when the concentration of the

AM is higher than a certain level, the probability of the

homopolymerization of AM will increase rapidly. This

reaction, which leads to the formation of PAM, will

compete with graft copolymerization and diminish the

grafting ratio. The yield of copolymer, on the contrary,

decreased gradually as the dosage of AM increased from

1g to 12g. This phenomenon can be ascribed to the rising

water-solubility of the copolymer. It has been mentioned

above that higher monomer concentration will enhance

the probability of homopolymerization of AM and extend

the length of some PAM chains that have been grafted

onto EH-lignin. Thus, the water-solubility of EH-lignin/

AM copolymer increases with the rise of AM dosage and

the quantity of copolymer that can be separated from

aqueous solution declines simultaneously. We can see

from the above analysis that the ideal dosage of acryla-

mide is 4~6g or the mass ratio of AM to EH-lignin is 2~3.

3.1.4. Effects of Reaction Time and Reaction

Temperature on Graft Copolymerization

The effects of reaction time and reaction temperature on

the yield of copolymer and the grafting ratio were shown

in Fig. 4 and Fig. 5 respectively. From Fig. 4, we could

see the yield and grafting ratio improved rapidly as the

reaction time increased from 2h to 4h. When the reaction

time was further prolonged, the growth of yield and

grafting ratio became unremarkable. This phenomenon

is similar to the regular pattern of radical polymerization.

The graft copolymerization took place mostly in the pe-

riod of initiation and the speed of the copolymerization

was high at first. However, as the concentration of

acrylamide and initiators declined with elapsed time, the

graft copolymerization slowed down and the yield and

grafting ratio stopped growing.

23456

Yield (%)

Grafting ratio (%)

Y(%)

GR(%)

reaction time (h)

Figure 4. Effect of reaction time on graft copolymerization.

30 40 50 60 70

Grafting ratio (%)

Yield (%)

Y(%)

GR(%)

Reaction temperature (℃)

Figure 5. The influence of reaction temperature on graft co-

polymerization.

The influence of different reaction temperature on

graft copolymerization was shown in Fig. 5. The highest

yield and grafting ratio can both be reached at 50 and ℃

this is the ideal reaction temperature for the bi-

nary-initiating system of K2S2O8-Na2S2O3. It is well

known that low reaction temperature will postpone the

decomposition of initiator and restrain the copolymeri-

zation. On the contrary, when the reaction temperature

was higher than 50, the decomposition of initiator ℃

would be too fast and the possibility of radical transfer

would be greatly enhanced, both of which will lead to

the decrease of yield and grafting ratio.

3.2. FT-IR Spectral Analysis

The FT-IR spectra of EH-lignin and EH-lignin/AM co-

polymer were shown in Fig. 6. It can be seen from the

FT-IR spectrum of EH-lignin/AM copolymer, compared

with pure EH-lignin, the relative intensity of the band at

about 1700 cm-1 increased significantly. This adsorption

peak is assigned to the vibration absorbance of C=O and

the rise of its intensity implies that AM have been

grafted onto EH-lignin. Furthermore, the intensity of the

R. Fang et al. / Natural Science 1 (2009) 17-22 21

4000 3500 3000 2500 2000 15001000

Wavenu m ber / cm -1

Figure 6. FT-IR spectra of EH-lignin and EH-lignin/ AM co-

polymer (a EH-lignin; b EH-lignin/AM copolymer).

band at 1020 cm-1, which is assigned to the absorbance of

N-H, also increased and this is another proof of the suc-

cessful graft copolymerization. All the information pro-

vided by FT-IR analysis had indicated that the product is an

EH-lignin/AM copolymer with numerous functional groups,

such as phenolic hydroxyl (3400cm-1), carbonyl (1700cm-1)

and amide (1550 and 1020cm-1) groups [11,12].

3.3. Adsorption and Decoloration of Dye-

Wastewater by EH-lignin/AM Copolymer

The adsorption and decoloration of AR 274 and RR X-

3B dye wastewater by EH-lignin/AM grafted copolymer

and pure EH-lignin was investigated according to the

procedures described in 2.3. The effect of grafting ratio

on the adsorption capacity of graft copolymer was shown

in Fig. 7. It was found that the absorption capacity of

EH-lignin/AM copolymer to both dye wastewaters in-

creased remarkably with the rising of grafting ratio. When

the grafting ratio was 43.6 %, the maximum adsorption

amount of AR 274 and RR X-3B by graft copolymer may

reach 751mg/g and 512mg/g respectively.

0 10203040

Adsorption amount (102mg/g)

Grafting ratio (%)

AR 274

RR X-3B

Figure 7. Effect of grafting ratio on the adsorption capacity of

EH-lignin/AM copolymer.

The flocculation of dye colloids result from various

mechanisms, including electrostatic attraction, sorption

(related to protonated amine groups and phenolic hy-

droxyl), and bridging (related to the high molecular

weight of the polymer) [13]. There are sulfonic, carbonyl

and amino groups in AR 274 and RR X-3B. The sulfonic

groups can be electrostatic attracted by protonated amide

groups of the EH-lignin/AM graft copolymer and the

amino groups can from hydrogen bonding with phenolic

hydroxyl groups of the copolymer and EH-lignin. Thus,

higher grafting ratio will help to strengthen the su-

pramolecular interaction between dyes and graft co-

polymers, which can bind copolymer molecules closer,

trap and flocculate dyes more effectively.

The relationship between the dosage of coagulants

and the decoloration rate of dye wastewater was shown

in Fig. 8. The concentration of AR 274 dye wastewater

was 500mg/L and the grafting ratio of EH-lignin/AM

copolymer was 30.8%. It can be seen in Fig. 8 that the

decoloration rate of dye wastewater increased rapidly as

the dosage of both flocculants increased from 50mg/L to

200mg/L. When the dosage of flocculant reached

800mg/L, the AR 274 dye had almost been removed

completely.

Results also showed that both pure EH-lignin and EH-

lignin/AM graft copolymer performed well in the re-

moval of AR 274 from dye wastewater when the dosage

of flocculant is higher than 200mg/L. As has been men-

tioned in 3.2, EH-lignin and lignin/AM copolymer pos-

sess lots of functional groups, such as phenolic hydroxyl,

carbonyl and amide groups. Meanwhile, AR 274 also

contains alcoholic hydroxyl and carbonyl groups, which

leads to the adsorption of dye molecules on EH-lignin

and EH-lignin/AM copolymer through hydrogen bond-

ing. Subsequently, the flocculant molecules bridge to

each other under acidic environment and form large

flocs, which will trap the dyes dissolved in wastewater

and then precipitate simultaneously.

0100 200 300400 500 600 700 800 900

100

Decoloration rate (%)

Dosage of coagulant (mg/L)

EH-lignin

Graft copolymer

Figure 8. Effect of coagulant dosage on the decoloration rate

of AR 274 wastewater.

22 R. Fang et al. / Natural Science 1 (2009) 17-22

0100200 300 400 500 600700 800 900

100

Dosage of coagulant (mg/L)

EH-lignin

Graft copolymer

TOC removal rate of dye wastewater(%)

Figure 9. TOC removal rate of dye wastewater after flocculation.

The residue of EH-lignin and EH-lignin/AM copoly-

mer in the supernatant after flocculation was measured

by TOC test to evaluate the potential secondary pollution

of this flocculant. These experiments were carried out

with various dosage of flocculant at pH=4. It can be

found in Fig. 9 that, compared with the decoloration rate

of AR 274 wastewater flocculated by both flocculants,

the TOC removal rate of these samples are much lower.

This phenomenon implies that part of EH-lignin and

EH-lignin/AM copolymer remains in the supernatant

after flocculation. Furthermore, TOC removal rate of the

dye wastewater treated by EH-lignin/AM graft copoly-

mer is much higher than that of pure EH-lignin, which

means the graft copolymerization of EH-lignin with AM

will help to minimize the residue amount of this co-

polymer in wastewater and diminish the potential sec-

ondary pollution.

4. CONCLUSIONS

The preparation and the application of EH-lignin/AM

graft copolymer were presented in this article. The op-

timum synthesis conditions were discussed and the func-

tional groups of the copolymer were characterized by

FT-IR The adsorption and decoloration of two azo-dye

wastewater by EH-lignin/AM grafted copolymer and

pure EH-lignin was investigated.

1) The optimum conditions for the graft copolymeri-

zation of EH-lignin with acrylamide were as follows:

initiator K2S2O8-Na2S2O3 with a quantity 3 wt% of EH-

lignin, mass ratio of AM to EH-lignin was 2~3, reaction

time 4h and temperature at 50.℃

2) FT-IR spectrum of EH-lignin/AM copolymer indi-

cated that acrylamide had been grafted onto EH-lignin

successfully and the copolymer had numerous functional

groups, such as phenolic hydroxyl, carbonyl and amide

groups.

3) The absorption capacity of EH-lignin/AM copoly-

mer to dye wastewater was enhanced with the increase

of grafting ratio. Both pure EH-lignin and EH-lignin/AM

graft copolymer performed well in the removal of AR

274 from dye wastewater when the dosage of flocculant

is higher than 200mg/L. However, the residue concen-

tration of EH-lignin/AM graft copolymer remained in

the supernatant after flocculation was much lower than

that of pure EH-lignin.

ACKNOWLEDGEMENTS

The author thanks Key Laboratory of Cellulose and Lignocellulosics

Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of

Sciences for financial support (LCLC-2004-158).

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