Pretreatment of acrylic fiber manufacturing wastewater by the Fenton

Desalination 284 (2012) 62–65
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Desalination
journal homepage: www.elsevier.com/locate/desal
Pretreatment of acrylic fiber manufacturing wastewater by the Fenton process
Jin Li ⁎, Zhaokun Luan, Lian Yu, Zhongguang Ji
State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing100085, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Received 28 May 2011
Received in revised form 18 August 2011
Accepted 20 August 2011
Available online 9 September 2011
Keywords:
Acrylic fiber manufacturing wastewater
Fenton process
Hydroxyl
Biodegradability
Fenton process was employed to pretreat the acrylic fiber manufacturing wastewater. The operation conditions were as follows: H2O2 and ferrous dosages were 100–800 mg/L; pH value was 1–7; reaction time was
0.5–4.0 h. In terms of COD removal and biodegradability improvement, the optimal conditions were as follows: ferrous content was 300 mg/L; hydrogen peroxide was 500 mg/L; pH value was 3.0; reaction time
was 2 h. With these conditions, the overall COD removal and effluent B/C could arrive at 65.5% and 0.529 respectively. The biodegradability of the acrylic fiber manufacturing wastewater was increased by 429%. It was
unnecessary to achieve complete mineralization of the organic compounds into carbon dioxide and water.
Partial oxidation of intermediate compounds could minimize the consumption of chemical reagents and result in substantial reduction of COD and toxicity. The further biological treatment was favored by acquiring
degradable low molecular weight components.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Based on the generation of hydroxyl free radicals which have high
electro-chemical oxidant potential, advanced oxidation processes
(AOPs) have been used to industrial wastewater treatment or pretreatment. Fenton oxidation process, as one of AOPs, stands out for
the treatment of biorefractory organic pollutants in wastewater [1].
The general mechanism of Fenton oxidation for a free radical chain involves the following key steps:
2þ
þ H2 O2 ¼ Fe
3þ
þ H2 O2 ¼ Fe
3þ
þ ·OOH ¼ Fe
Fe
Fe
Fe
2þ
þ
ð2Þ
2þ
þ ·OOH þ H
3þ
¼ Fe
ð1Þ
þ ·OH þ OH
2þ
·OH þ Fe
−
3þ
þ
ð3Þ
−
ð4Þ
þ H þ O2
þ OH
·OH þ ·OH ¼ H2 O2
ð5Þ
·OH þ H2 O2 ¼ ·OOH þ H2 O
ð6Þ
The hydroxyl free radical can attack and initiate the oxidation of
organic pollutant molecule by several degradation mechanisms listed
below:
·OH þ R−H ¼ H2 O þ R· ¼ further oxidation
⁎ Corresponding author.
E-mail address: [email protected] (J. Li).
0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2011.08.037
ð7Þ
Therefore initially biorefractory compounds can be mineralized or
converted to more readily biodegradable intermediates, which can be
removed by further biological treatment. Besides, since both ferrous
and ferric ions are coagulants, the Fenton process can have the dual
functions of oxidation and coagulation in the treatment process. The
relative importance of oxidation and coagulation depends primarily
on the H2O2/Fe 2+ ratio. Chemical coagulation predominates at a
lower H2O2/Fe 2+ ratio, whereas chemical oxidation is dominant at
higher H2O2/Fe 2+ ratios [2]. Generally, Fenton process is composed
of following stages: pH adjustment, oxidation reaction, neutralization,
coagulation and precipitation [3]. The refractory organic substances
are removed at two stages of the oxidation and the coagulation.
A wide variety of Fenton's reagent applications have been
reported, such as treatment of textile wastewater [4,5], reduction of
Polynuclear Aromatic Hydrocarbons (PAHs) in water [6], removal of
Adsorbable Organic Halogens (AOXs) from pharmaceutical wastewater [7], treatment of paper pulp manufacturing effluent [8], treatment
of leachate [9], removal of citrate and hypophosphite binary components [10], or treatment of wastewater containing nitroaromatic
compounds [11].
The fiber industry has continuously developed with the growth of
the economy, accomplishing technological development for synthesizing and processing several highly valuable polymer materials
such as those used for electrical, electronic and biomedical devices.
Acrylic fiber is one of the major synthetic fibers commonly used in
the mass production of clothing. The chemical synthesis of acrylic
fiber is carried out by polymerization of the acrylonitrile (AN) monomers. The quantity of acrylic fiber manufacturing wastewater is inclined to increase with the increment of acrylic fiber used, and the
components of wastewater discharged are complicated and variable.
The biodegradability of the acrylic fiber manufacturing wastewater
J. Li et al. / Desalination 284 (2012) 62–65
2. Materials and methods
2.1. Wastewater characteristics
The acrylic fiber manufacturing wastewater was collected from a
synthetic-fiber factory located at the city of Ningbo, China. The wastewater quality was quite complicated because it consisted of acrylonitrile unit, vinyl acetate unit, oligomers, DMAc, EDTA and sulfate as
Table 1 showed. The molecular formulae of acrylonitrile unit and
vinyl acetate unit were shown in Fig. 1.
2.2. Fenton process
Fenton treatment of acrylic fiber manufacturing wastewater was
carried out at 25 °C and atmospheric pressure according to the following sequential steps. (1) Wastewater sample was put in a beaker
and magnetically stirred; its pH was adjusted by 1 M NaOH. (2) The
scheduled Fe 2+ dosage was achieved by adding the necessary amount
of solid FeSO4·7H2O. (3) A known volume of 30% (w/w) H2O2 solution was added in a single step. (4) After fixed reaction time, calcium
hydroxide (1 M) was added to treated samples to precipitate residual
ferric or ferrous ions and to better coagulate the resulting sludge.
(5) At the end of Fenton treatment, stirring was turned off and the
sludge was allowed to sediment. In each case, all the analyses of treated acrylic fiber manufacturing wastewater were carried out on filtered samples.
Table 1
Characteristics of acrylic fiber manufacturing wastewater.
Acrylonitrile
Vinyl acetate
Oligomers
DMAc
pH
COD
BOD5
BOD5/COD
NH4+-N
SO42−
PO43−
Unit
Range
Average
mg/L
mg/L
mg/L
mg/L
–
mg O2/L
mg O2/L
–
mg N/L
mg S/L
mg P/L
2.99–3.51
0.028–0.038
201.8–218.5
85–115
3.0–3.8
4378–4611
408–467
0.09–0.11
71–88
2061–2262
0.015–0.021
3.33
0.033
208.3
100
3.5
4528
449
0.10
85
2158
0.018
Fig. 1. Molecular formulae of acrylonitrile unit and vinyl acetate unit.
2.3. Analytical procedures
Analytical procedures for the determination of chemical oxygen
demand (COD), biochemical oxygen demand (BOD5) and pH were
conducted according to Standard Methods [16].
3. Results and discussion
3.1. Effect of H2O2 dosage
During the Fenton process, hydrogen peroxide plays a very important role as a source of hydroxyl radical generation. Fig. 2 showed the
effects of hydrogen peroxide dosage on the overall removal, oxidation
removal and coagulation removal of COD. The COD removal efficiency
by oxidation only was 12.9% when the hydrogen peroxide dosage was
100 mg/L. Then it increased sharply and kept at 40% corresponding to
the 500 and 600 mg/L hydrogen peroxide dosages. However, it
dropped when the hydrogen peroxide was added further and stayed
at 32% at last. During the whole process, COD removal by coagulation
was much less than that by oxidation. The maximal COD removal by
coagulation was 20% when the hydrogen peroxide dosage was
500 mg/L. In terms of overall COD removal, the optimal hydrogen
peroxide dosage should be 500 mg/L and the overall removal efficiency could arrive at 60%.
In general, under the 500 mg/L hydrogen peroxide concentration,
the oxidation removal efficiencies increased with increasing hydrogen peroxide dosages due to the increment of hydroxyl radicals
which were produced through the decomposition of increasing hydrogen peroxide as Eq. (1) showed. However, excess hydrogen peroxide interfered with the measurement of COD. The residual hydrogen
peroxide could consume K2Cr2O7, leading to the increase of COD
shown as follows:
þ
2−
Cr2 O7 þ 3H2 O2 þ 8H ¼ 2Cr
3þ
þ 3O2 þ 7H2 O
ð8Þ
The COD removal by coagulation declined when the hydrogen
peroxide was more than 500 mg/L. On one hand, coagulation removed high molecular weight organics preferentially [17]. Low
80
Removal by oxidation
Removal by coagulation
Overall removal
70
COD removal efficiency (%)
was very low: the ratio of BOD5/COD was about 0.1, and there were
some biorefractory organic pollutants in the wastewater [12]. For
wastewater treatment, most acrylic fiber manufacturing companies
had adopted a conventional biological treatment system followed by
physicochemical methods of neutralization, coagulation and sedimentation [13]. Ultrafiltration (UF) and reverse osmosis (RO) were
also employed to treat acrylic fiber manufacturing wastewater, and
the separation characteristics of wastewater were investigated with
the variations of pressure and temperature [14]. Besides, a combined
three-stage process of thermophilic anaerobic/anoxic denitrification/
aerobic nitrification fluidized bed reactor was used to treat the wastewater and the molecular biotechnology was applied to study the microbial population in the thermophilic anaerobic fluidized bed reactor
[15]. However, there was little report about acrylic fiber manufacturing wastewater treatment by employing Fenton reagent.
In terms of acrylic fiber manufacturing wastewater, due to the
high organic load, toxicity and presence of biorefractory compounds,
biological processes were not efficient. In this work, the Fenton's reagent was used to remove COD from acrylic fiber manufacturing
wastewater prior to biological treatment. The aims of the work
were to analyze the different effects of the operating parameters
and their interactions over the efficiency of Fenton's process in the
treatment of acrylic fiber manufacturing wastewater.
63
60
50
40
30
20
10
0
100
200
300
400
500
600
700
800
H2O2 dosage (mg/L)
Fig. 2. Effect of hydrogen peroxide dosage on removal of COD (Experimental conditions: pH = 4.0; [Fe2+] = 250 mg/L; reaction time = 2 h; temperature = 20 °C).
64
J. Li et al. / Desalination 284 (2012) 62–65
molecular organics produced in the oxidation stage were less prone
to coagulation. On the other hand, the auto decomposition of excessive hydrogen peroxide would produce oxygen bubbles that made
sludge settling difficult [2]. Take both oxidation and coagulation into
consideration, the hydrogen peroxide should be 500 mg/L.
3.2. Effect of ferrous dosage
Ferrous is another main affective factor in Fenton reaction that catalytically decomposes H2O2 to generate ·OH. It is well known that
higher hydrogen peroxide to substrate ratios result in more extensive
substrate degradation, while higher concentrations of iron ions yield
faster rates. Fig. 3 depicted the effect of the amount of Fe 2+ on removal
efficiency for COD with a fixed amount of 500 mg/L hydrogen peroxide. The COD removal by oxidation was 36% when the ferrous dosage
was 100 mg/L. It increased with the increment of ferrous dosage and
arrived at peak of 42.5% corresponding to the ferrous dosage of
300 mg/L. Then it dropped when the ferrous dosage was added further. However, the overall COD removal efficiency scarcely changed
with the ferrous dosage varied between 300 and 600 mg/L. That because, when the ferrous dosage was no more than 600 mg/L, the
COD removal by coagulation increased with the increasing ferrous
content. The decrement of COD removal resulting from oxidation
was offset by the increment through coagulation. When the ferrous
dosage was more than 600 mg/L, the COD removal both by oxidation
and coagulation decreased with the increment of ferrous dosage. So
the optimal ferrous dosage pretreating acrylic fiber manufacturing
wastewater should be of 300 to 600 mg/L.
When it came to Fenton oxidation, the results indicated that more
Fe 2+ dosage did not mean more oxidation removal because the use of
a much higher concentration of ferrous could lead to the selfscavenging effect of ·OH radical (Eq. (4)) and induce the decrease
in degradation efficiency of pollutants [3]. During the coagulation
process, both the ferrous and ferric ions were coagulant and ferric
ions had a stronger capability of charge neutralization than ferrous
ions according to the Schulz–Hardy rule. When the pH varied between 3.1 and 12.8, zeta potential of acrylic fiber manufacturing
wastewater was negative. So increasing ferrous dosage neutralized
the negative charge and favored coagulation process. However,
when the ferrous dosage was more than 600 mg/L, the pollutants in
the wastewater would be positively charged due to excessive ferrous
and ferric ions. They restabilized again and made the COD removal by
coagulation decreased.
3.3. Effect of initial pH value
The pH value has a decisive effect on the oxidation potential of
·OH because of the reciprocal relationship between the oxidation potential and the pH value (E 0 = 2.8 V and E 14 = 1.95 V). Fig. 4 showed
the effect of initial pH value on COD removal. In general, when the pH
value was 3.0, both the overall COD removal and COD removal by oxidation arrived at peak. They were 66.6% and 46.6% respectively. The
COD removal by coagulation was 4.7% corresponding to the pH of
1.0. Then it increased with the increment of pH value. The coagulation
dominated the Fenton treatment of acrylic fiber manufacturing
wastewater at initial pH over 6.0, and the maximum COD removal
by coagulation was 30.5% corresponding to the pH value of 7.0.
At extremely low pH value (b2.0), oxidation removal decreased sharply due principally to the formation of complex species
[Fe(H2O)6] 2+, which reacted slower with peroxide when compared
to that of [Fe(OH)(H2O)5] 2+ [18]. In addition, the peroxide got solvated in the presence of high concentration of H + ion to form stable peroxone ion [H3O2] +. The peroxone ion led to an electrolytic behavior
on the part of hydrogen peroxide improving its stability and substantially reducing the reactivity with ferrous ion [19]. Moreover, exceptionally low pH could inhibit reaction between Fe 3+ and H2O2 [20].
On the other hand, oxidation removal rapidly decreased with increasing pH above 4.0. The reasons for this inhibition might be explained
not only by the decomposition of hydrogen peroxide, but also by
the deactivation of a ferrous catalyst with the formation of ferric hydroxide complexes leading to a reduction of OH radical. As shown in
Fig. 4, the increase of pH to 7.0 resulted in the extreme decrease of
the oxidation removal efficiency of COD. The oxidation reaction was
no longer predominant in COD removal.
3.4. Effect of reaction time
Reaction time is a key parameter in Fenton process, because not
only treatment performance but also reactor volume is associated
with it. Fig. 5 showed the effect of reaction time on the effluent
BOD5/COD value and COD removal during the Fenton process. In general, both the COD removal by oxidation and by coagulation increased
with the reaction time prolonged. The COD removal efficiency increased rapidly at the beginning of Fenton reaction. The overall COD
removal efficiency in the first 2 h period was 65.5%. Then it increased
slightly and arrived at 78% corresponding to the reaction time of 4 h.
Oxidation was dominant throughout the whole Fenton process. At the
80
80
Removal by oxidation
Removal by oxidation
COD removal efficiency (%)
COD removal efficiency (%)
70
Overall removal
60
50
40
30
20
Overall removal
60
50
40
30
20
10
10
0
Removal by coagulation
70
Removal by coagulation
100
200
300
400
500
600
700
800
Ferrous dosage (mg/L)
Fig. 3. Effect of ferrous dosage on removal of COD (Experimental conditions: pH = 4.0;
[H2O2] = 500 mg/L; reaction time = 2 h; temperature = 20 °C).
0
1
2
3
4
5
6
7
pH
Fig. 4. Effect of initial pH value on removal of COD (Experimental conditions: [Fe2+] =
300 mg/L; [H2O2] = 500 mg/L; reaction time = 2 h; temperature = 20 °C).
J. Li et al. / Desalination 284 (2012) 62–65
90
1
B/C
Removal by oxidation
Removal by coagulation
Overall removal
70
0.8
60
0.6
50
B/C
COD removal efficiency (%)
80
40
0.4
30
20
0.2
10
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
65
B/C could arrive at 65.5% and 0.529 respectively. The biodegradability
of the acrylic fiber manufacturing wastewater was increased by 429%.
It was unnecessary to achieve complete mineralization of the organic
compounds into carbon dioxide and water. Partial oxidation of intermediate compounds could minimize the consumption of chemical reagents and result in substantial reduction of COD and toxicity. The
further biological treatment was favored by acquiring degradable
low molecular weight components.
Acknowledgments
The work was supported by Natural S & T Major Project (No.
2009ZX07529-004-2). The authors would like to thank the editor
and the anonymous reviewers for their editing and review.
0
Reaction time (h)
Fig. 5. Effect of reaction time on effluent B/C and removal of COD (Experimental conditions: [Fe2+] = 300 mg/L; [H2O2] = 500 mg/L; pH = 3.0 h; temperature = 20 °C).
beginning, the oxidation removal of COD was low. Then it increased
linearly within first 1.5 h of reaction time and was kept at 45.1% corresponding to the reaction time of 2 h. After 2 h, the oxidation removal of COD changed insignificantly. In contrast, the COD removal by
coagulation was insignificant within first 1 h and increased gradually
with extending the reaction time. When the reaction time was 4 h, it
could arrive at 30%.
The biodegradability of wastewater is indexed by BOD5/COD (B/C)
ordinarily, and an exploitable biodegradability improvement should
be B/C N 0.5 [21]. The B/C value of raw wastewater was about 0.1. It
could increase to 0.226 after 0.5 hour's reaction. Although the COD removal was not efficient during this period, many refractory matters
were degraded into biodegradable ones. Then the B/C value was enhanced with the increment of reaction time and peaked at 0.529 corresponding to the reaction time of 2 h. When the reaction time was
extended further, however, the B/C value dropped. On one hand,
after 2 hours' reaction, hydrogen peroxide was almost exhausted
and there were not enough hydroxyl radicals produced to decompose
the refractory matters. On the other hand, due to the increasing COD
removal efficiency by coagulation, many degradable matters were removed by it. Generally, the B/C value could be enhanced by Fenton
process and the refractory matters were mostly degraded into various
intermediate organic compounds without mineralization during the
process. In many cases, it was unnecessary to achieve complete mineralization of the organic compounds into carbon dioxide and water.
Partial oxidation of intermediate compounds could minimize the consumption of chemical reagents and result in substantial reduction of
COD and toxicity. Additionally, higher biological activity of degradable low molecular weight components was in favor of the further biological treatment [9,22]. So the optimal reaction time should be 2 h
when both the COD removal and B/C value were taken into
consideration.
4. Conclusions
Based on former studies and discussion on the COD removal from
acrylic fiber manufacturing wastewater by the Fenton process, it
could be concluded that:
In terms of COD removal and biodegradability improvement, the
optimal conditions were as follows: ferrous content was 300 mg/L;
hydrogen peroxide was 500 mg/L; pH value was 3.0 h; reaction time
was 2 h. With these conditions, the overall COD removal and effluent
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