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Improvement in efficiency of lignin degradation by Fenton reaction using synergistic catalytic action
Ecological Engineering 85 (2015) 283–287
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Short communication
Improvement in efficiency of lignin degradation by Fenton reaction using synergistic catalytic action Phisit Seesuriyachan ∗ , Ampin Kuntiya, Arthitaya Kawee-ai, Charin Techapun, Thanongsak Chaiyaso, Noppol Leksawasdi Bioprocess Research Cluster, School of Agro-Industry, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai 50100, Thailand
a r t i c l e
i n f o
Article history: Received 2 December 2014 Received in revised form 2 October 2015 Accepted 7 October 2015 Available online 24 October 2015 Keywords: Central composite design Synergistic catalytic action Fenton reaction Lignin degradation
a b s t r a c t Lignin is a complex aromatic compound and a by-product from the wood pulping process. We used a central composite design to assess the efficiency of synergistic catalysts (Fe0 , Fe2+ and Fe3+ ) and H2 O2 in the Fenton process for removal of lignin, aromatic compounds, and chemical oxygen demand (COD) from lignin solution. Results showed that the Fe0 and H2 O2 concentration were the most significant variables in lignin degradation by Fenton reaction, while Fe2+ and Fe3+ concentration were the next most important variables. The highest removal efficiencies were obtained with concentration levels of 7.61 mg/L Fe0 , 9.89 mg/L Fe2+ , 14.27 mg/L Fe3+ , and 376.88 mg/L H2 O2 at pH 3.0 and 35 ◦ C. Under these optimal conditions, the efficiencies of removal of lignin, aromatic compounds, and COD were 98.45%, 82.63%, and 100%, respectively, while the removal rates of lignin and aromatic compounds were 23.73 mg/L/h and 27.75 mg/L/h, respectively. The optimization results revealed that using synergistic catalysts in the Fenton reaction was more effective in treating wastewater and resulted in decreased cost and time. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lignin is a complex aromatic polymer with a highly branched and heterogeneous three-dimensional structure that depends on the type and growing conditions of plants. Kraft lignin differs from natural lignin as it undergoes a variety of reactions including aryl–alkyl cleavage and strong side chain modification causing the polymer to fragment into smaller alkali-soluble fragments (Chakar and Ragauskas, 2004). To solve this problem, Fenton reaction has been used to treat Kraft lignin (Ninomiya et al., 2013). The Fenton process is an effectively advanced oxidation processes (AOPs) technology for degradation of hazardous and organic pollutants in wastewater due to its relatively good efficiency, low toxicity, simplicity of control and low cost (Oliveira et al., 2014). Several studies have explored the removal efficiency of the Fenton process applied to biologically pretreated products from pulp and paper mills (Catalkaya and Kargi, 2007; Hermosilla et al., 2012; Kazmi and Thul, 2007; Tambosi et al., 2006; Torrades et al., 2011). The focus of most studies has been on Fe2+ /H2 O2 systems with
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Seesuriyachan). http://dx.doi.org/10.1016/j.ecoleng.2015.10.013 0925-8574/© 2015 Elsevier B.V. All rights reserved.
the objective of optimizing Fe2+ and H2 O2 values, which achieved large quantity of ferric (Fe3+ ) residues. However, the use of zero valent iron (Fe0 ) can enhance the system reactivity by reducing Fe3+ residues to ferrous (Fe2+ ) while decreasing the amount of reagent (Fe2+ and H2 O2 ) (Kallel et al., 2009). Fe0 + 2H+ → Fe2+ + H2
(1)
2Fe3+ + Fe0 → 3Fe2+
(2)
Fe0 + H2 O2 + 2H+ → Fe2+ + 2H2 O
(3)
In previous study, the interaction of synergistic catalysts was investigated for decolorization of low-molecular weight compound and it was found that the synergistic catalysts increased the rate and efficiency of the decolorization in comparison with reactions using Fe0 , Fe2+ or Fe3+ alone (Tantiwa et al., 2013). The enhancement of the efficiency of high-molecular weight compounds (alkali lignin, MW 10,000 g/mol) decolorization were thus determined in this study with estimation of the optimal concentration levels of synergistic catalysts (Fe0 , Fe2+ and Fe3+ ) and H2 O2 by the Fenton reaction using response surface methodology via central composite design (CCD).
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Table 1 Central composite design (CCD) for lignin degradation by Fenton reaction with the synergistic catalytic action. Std. run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Factors (mg/L)
% Removals
Removal rate (mg/L/h)
X1
X2
X3
X4
Lignin
Aromatic compounds
COD
Lignin
Aromatic compounds
4 12 4 12 4 12 4 12 4 12 4 12 4 12 4 12 0 16 8 8 8 8 8 8 8 8 8
5 5 15 15 5 5 15 15 5 5 15 15 5 5 15 15 10 10 0 20 10 10 10 10 10 10 10
5 5 5 5 15 15 15 15 5 5 5 5 15 15 15 15 10 10 10 10 0 20 10 10 10 10 10
180 180 180 180 180 180 180 180 540 540 540 540 540 540 540 540 360 360 360 360 360 360 0 720 360 360 360
74.88 67.60 75.35 50.74 67.92 40.97 63.75 28.17 69.79 88.22 88.67 72.68 87.97 98.46 80.79 93.76 84.69 61.03 82.70 63.49 83.23 65.19 2.57 80.63 91.81 98.83 96.20
71.30 63.61 71.03 51.01 78.21 60.14 72.91 46.48 79.08 102.03 105.07 75.96 97.40 98.82 92.12 92.28 75.51 70.28 99.63 74.15 79.50 75.52 2.64 85.42 92.85 98.02 98.89
75.00 51.25 75.00 57.14 51.25 37.70 75.00 37.50 25.00 82.10 82.50 72.08 80.39 100.00 87.53 100.00 80.26 57.14 85.71 62.50 71.43 81.94 0.00 100.00 87.67 100.00 100.00
10.52 10.09 10.83 7.98 11.14 7.40 11.01 5.18 11.72 13.11 14.30 12.62 14.04 12.35 14.94 13.01 10.27 10.21 13.99 14.12 9.48 10.78 1.38 14.04 13.28 14.17 13.50
8.32 7.38 8.16 6.08 13.50 10.30 12.48 8.17 14.10 17.49 18.08 12.90 16.61 11.69 15.96 10.81 8.52 12.17 16.67 12.15 13.23 13.07 3.67 14.69 15.35 16.19 16.64
2. Materials and methods
2.4. Experimental design
2.1. Reagents Alkali lignin (Kraft lignin) with 96% (w/w) purity and 4% (w/w) sulfur was purchased from Sigma–Aldrich (St. Louis, MO, USA). Hydrogen peroxide (H2 O2 ) and sulfuric acid (H2 SO4 ) were provided by Merck (Damstadt, Germany). High purity iron power, iron(II) sulfate heptahydrate (FeSO4 ·7H2 O), and iron(III) chloride anhydrous (FeCl3 ) were provided by Fisher Scientific (Leicestershire, UK). All solutions were prepared by dissolving in distilled water containing iron <0.001 mg/L.
We used a central composite design (CCD) in response surface methodology to estimate the coefficients in a mathematical model, predict the response, and check the model applicability of Fe0 (X1 ), Fe2+ (X2 ), Fe3+ (X3 ), and H2 O2 (X4 ) (Table 1), which was described previously by Tantiwa et al. (2013). The steepest ascent method was applied to find the optimum levels (Table 2). The Design Expert 6.0.10 (Stat-Ease, Inc., Minneapolis, MN, USA) was used for the design of experiments and data analysis. The significance of the model equations and the model terms were evaluated in term of p value with a 95% confidence level.
2.2. Experimental procedures
Y = ˇ0 + ˙ˇi Xi + ˙ˇii Xi2 + ˇij Xi Xj
Lignin solution (100 mg/L) was prepared by dissolving alkali lignin with distilled water and adjusted to pH 3.0, and then the solution was added into a 250 mL flask with controlled temperature at 35 ◦ C and was agitated at 250 rpm. The synergistic catalytic action comprised of Fe0 , Fe2+ , and Fe3+ was added into lignin solution with the concentration levels and formula shown in Table 1. After 5 min, H2 O2 was immediately added to initiate reaction. Samples were taken every hour for 10 h. The reactions were stopped instantaneously by the addition of NaOH to elevate pH level to 7.0 (Pesakhov et al., 2007), prior to filtration process using Whatman No. 1 filter paper to remove the chemical precipitate before analysis. 2.3. Analytical method Concentration levels of lignin and aromatic compounds were determined by measuring sample absorbance (Genesys 20 UV–visible spectrophotometer, Thermo scientific, Waltham, MA, USA) at 280 nm and 254 nm, respectively (Janshekar et al., 1981). COD was determined by closed reflux titrimetric method (APHA, 1985). All measurements were performed in triplicate.
(4)
where Y represents the response variable, ˇo is the interception coefficient, ˇi is the coefficient for the linear effect, ˇii is the coefficient for the quadratic effect and ˇij is the ijth coefficient of the interaction effect, and Xi Xj are input variables that influence the response variable Y. 3. Results and discussion A total of 27 runs for optimizing the four variables in the current CCD are shown in Table 1. We found that the interaction between synergistic catalysts and H2 O2 improved the removal of lignin, aromatics and COD in the Kraft lignin solution. The lignin removal was in the range of 2.57–98.83%, aromatic compound removal: 2.64–98.89% and COD removal: 0–100%. The rates of removal of lignin and aromatic compounds were 1.38–14.94 mg/L/h and 3.67–18.08 mg/L/h, respectively. At the lowest values, Fenton reaction was not effective due to the absence of H2 O2 (X4 ). These results confirmed that H2 O2 plays an important role in driving catalytic oxidation in the Fenton reaction (Wang et al., 2014). Conversely, the removal efficiency of lignin, aromatic compounds and COD were decreased by about 12–15% in the absence of Fe0 , Fe2+ or Fe3+ , which is compared by the central point of
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285
Table 2 Lignin degradation by Fenton reaction using steepest ascent method. Std run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Factors (mg/L)
% Removals
Removal rate (mg/L/h)
X1
X2
X3
X4
Lignin
Aromatic
COD
Lignin
Aromatic
8.37 9.13 10 65 7.61 7.61 7.61 7.61 7.61 7.61 7.61 7.61 7.61 6.10 4.57 7.61 7.61 7.61 7.61 7.61 7.61 7.61 0 0
9.89 9.89 9.89 10.88 11.87 13.85 9.89 9.89 9.89 9.89 9.89 9.89 9.89 9.89 7.91 5.93 9.89 9.89 9.89 9.89 0 9.89 0
14.27 14.27 14.27 14.27 14.27 14.27 15.70 17.12 19.98 14.27 14.27 14.27 14.27 14.27 14.27 14.27 11.42 8.56 14.27 14.27 0 0 14.27
376.88 376.88 376.88 376.88 376.88 376.88 376.88 376.88 376.88 414.57 497.88 527.63 376.88 376.88 376.88 376.88 376.88 376.88 301.50 226.13 376.88 376.88 376.88
86.70 84.24 90.56 87.18 68.28 70.64 79.77 79.80 84.56 75.56 76.70 81.99 89.89 70.89 88.94 86.56 88.27 88.92 77.73 72.31 65.37 40.43 59.90
92.22 85.47 96.68 89.92 63.63 82.06 81.06 83.98 90.42 70.17 75.83 82.63 97.06 88.76 97.12 88.55 83.13 81.22 73.81 89.01 73.47 58.13 89.35
100 100 100 100 87 100 100 100 100 5 86 88 85 100 100 100 100 100 68 75 100 3 19
20.85 20.10 21.01 20.07 15.61 16.38 18.83 18.68 19.83 17.36 16.65 19.17 20.90 14.86 20.70 20.15 20.36 20.76 17.48 17.00 16.00 9.32 14.25
31.11 22.41 33.07 21.41 15.08 29.45 26.86 25.58 24.11 16.86 16.45 20.21 23.76 20.26 33.85 22.88 20.30 28.27 16.39 31.29 26.98 20.30 31.99
the design. The results were consistent with Chu et al. (2012) who showed that in the absence of Fe0 , phenol degradation was decreased by 12% at a reaction time of 2.5 h. At both low and high concentration levels of Fe0 , Fe2+ , Fe3+ and H2 O2 , the oxidation efficiencies of the Fenton process on removal of lignin, aromatics and COD, and the removal rates of lignin and aromatic compounds were suppressed.
The steepest ascent method was used to estimate the optimal values for lignin degradation using synergistic catalysts in the Fenton reagent as presented in Table 2. The use of only one catalyst in the reaction could mitigate the removal of lignin, aromatic compounds and COD to 65.37%, 73.47% and 100% for Fe0 , 40.43%, 58.13% and 3.00% for Fe2+ and 59.90%, 89.35% and 19.00% for Fe3+ , respectively. The H2 O2 /Fe0 system has better degradation efficiency than
Table 3 Analysis of variance (ANOVA) results and quadratic equation for response parameters. Responses
Lignin
Aromatic compounds
COD
Lignin
Aromatic compounds
Final equation in term of actual factors % Removals 95.61 − 4.83X1 − 3.35X2 − 2.95X3 + 15.72X4 − 4.51X12 − 4.45X22 − 4.17X32 − 12.97X42 − 3.62X1 X2 − 0.60X1 X3 + 7.52X1 X4 0.99X2 X3 + 1.55X2 + 6.84X3 X4 96.59 − 3.64X1 − 3.94X2 + 0.47X3 + 16.84X4 − 4.44X12 − 0.94X22 − 3.28X32 − 12.31X42 − 4.63X1 X2 − 0.57X1 X3 + 4.23X1 X4 − 1.11X2 X3 + 1.24X2 + 1.10X3 X4 95.89 − 2.51X1 + 1.58X2 + 2.92X3 + 15.41X4 − 6.68X12 − 5.33X22 − 4.68X32 − 11.35X42 − 5.78X1 X2 − 1.52X1 X3 + 10.73X1 X4 − 1.40X2 X3 + 1.56X2 + 10.21X3 X4 Removal rate 13.65 − 0.70X1 − 0.009X2 0.021X3 + 2.38X4 − 0.73X12 + 0.22X22 − 0.76X32 − 1.36X42 − 0.49X1 X2 − 0.60X1 X3 + 0.56X1 X4 − 0.068X2 X3 + 0.49X2 + 0.45X3 X4 16.60 − 0.63X1 − 0.66X2 + 0.28X3 + 3.33X4 − 1.24X12 − 0.22X22 − 0.54X32 − 2.44X42 − 0.69X1 X2 − 0.80X1 X3 − 0.085X1 X4 − 0.16X2 X3 + 0.15X2 − 1.38X3 X4
p
LOF
R2
Adj. R2
Pred. R2
AP
CV
0.0001
8.13
0.9224
0.8319
0.5597
12.314
12.89
0.0007
12.51
0.8975
0.7778
0.4118
11.253
13.58
0.0127
5.66
0.9654
0.9308
0.7663
15.363
10.37
0.0001
8.67
0.9237
0.8346
0.5663
12.900
11.12
0.0021
16.50
0.8711
0.7208
0.2632
9.649
20.15
p, probability of error; LOF, lack of fit; Adj. R2 , adjust R2 ; Pred. R2 , predicted R2 ; AP, adequate precision; CV, coefficient of variance.
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Table 4 Studies on degradation of lignin using advanced oxidation processes (AOPs). Sources
Kraft lignin Initial lignin 100 mg/L Kraft lignin Initial lignin 500 mg/L Black liquor Initial lignin OD280 = 1.978 Pulp and paper mill p-Toluenesulponic acid Initial 860 mg/L Alfalfa black liquor Initial lignin 90 mg/L Wheat straw Kraft Initial lignin 100 mg/L
Treatment
Catalyst concentration (mg/L)
Time (h)
Synergistic catalysts of Fenton (H2 O2 /Fe0 + Fe2+ + Fe3+ ) Sonocatalytic (US/TiO2 ) Fenton (H2 O2 /Fe2+ ) (US/TiO2 + H2 O2 /Fe2+ ) Fenton (H2 O2 /Fe2+ )
H2 O2 = 376.45, Fe0 = 7.60, Fe2+ = 9.88, Fe3+ = 14.25 TiO2 = 2000 H2 O2 = 3400 Fe2+ = 280
Fenton (H2 O2 /Fe2+ )
TiO2 UV/TiO2
H2 O2 = 600 Fe2+ = 100 H2 O2 = 34 Fe3+ = 20 TiO2 = 200 TiO2 = 1000
UV/ZnO
ZnO = 1000
Photo-Fenton
H2 O2 = 1500 Fe2+ = 1300
the H2 O2 /Fe2+ or H2 O2 /Fe3+ systems (Tang and Chen, 1996; Tantiwa et al., 2013). Therefore, the combination of the three catalysts (Fe0 , Fe2+ and Fe3+ ) in the Fenton reaction greatly improved the degradation of Kraft lignin. The optimal concentrations for central composite design of Fe0 , Fe2+ , Fe3+ , and H2 O2 for lignin degradation by Fenton reaction were 7.61, 9.89, 14.27, and 376.88 mg/L, respectively. The treatment of 100 mg/L lignin under these optimal conditions resulted in removal of 98.45% of lignin, 82.63% of aromatic compounds and 100% of COD. The removal rates were 23.73 mg/L/h of lignin and 27.75 mg/L/h of aromatic compounds. The optimal values from this study were lower than the optimal values of Torrades et al. (2011) which 1.3 g/L of Fe2+ and 1.5 g/L of H2 O2 to degrade lignin in black liquor was used. The quadratic mathematic equations showed that the two most important variables for the removal of lignin, aromatic compounds and COD, and the removal rate of lignin and aromatic compounds were Fe0 (X1 ) and H2 O2 , (X4 ) while the interaction effect of Fe2+ (X2 ) and Fe3+ (X3 ) were incorporated in all five mathematical models (Table 3). The 3D graphs of the relationships between the significant variables of Fe0 , Fe2+ , Fe3+ , H2 O2 , lignin, aromatic compounds, and COD removals are shown in supplement data (Supp. 1–3). In this study, Fe2+ and Fe3+ concentration levels interacted because the ability of Fe2+ to regenerate by the reaction of Fe3+ with H2 O2 was inhibited at acidic pH (Pignatello, 1992). Table 4 presents investigations of lignin degradation using other AOPs such as titanium dioxide (TiO2 ), sonocatalysis (US/TiO2 ), Fenton reaction, photo-Fenton, sonocatalysis combined with Fenton reagent, ultraviolet (UV) and mesoporous UV/TiO2 . It showed that the combination of catalysts in the synergistic Fenton reaction was more efficient than Fenton reaction or Fenton-like reaction alone or the combination with other processes due to continuous dissolution of iron powder in the absorption of lignin, aromatic compounds and other polyphenol on iron powder surface (Tang and Chen, 1996). The use of Fenton reaction with zero valent iron can be renewable for the reaction system (Kallel et al., 2009) and slightly decreased the amount of Fenton reagents. While, the use of Fenton process by one catalyst consumed large amount of Fenton reagents to get the high efficiency of lignin removal (Kazmi and Thul, 2007; Torrades et al., 2011). The relatively
Removal (%)
References
Lignin
COD
4
98.4
100
2
1.8
–
This study
Ninomiya et al. (2013)
2 2 1.5
49.9 60.0 85.6
94.8
5
98
62
6
47.0
20.0
Kazmi and Thul (2007) Amat et al. (2005)
6 7
27.0 56.0
20.0 72.0
Ksibi et al. (2003)
5
84.0
85.0
Kansal et al. (2008)
Torrades et al. (2011)
low concentration levels of Fenton’s catalysts were enhanced in this study with high efficiency of lignin and COD removal. Therefore, we propose the use of the Fenton process with synergistic catalysts as an alternative wastewater treatment due to its relatively high efficiency, low cost and low catalyst concentration levels.
4. Conclusion This study demonstrated that synergistic catalysts Fe0 , Fe2+ , Fe3+ and H2 O2 could statistically influence lignin, aromatic compounds, and COD removals as evident from response surface methodology via CCD. The optimum concentration levels of the synergistic catalysts were 7.61 mg/L of Fe0 , 9.89 mg/L of Fe2+ , 14.27 mg/L of Fe3+ , and 376.88 mg/L of H2 O2 for removal of lignin, aromatic compounds, and COD by Fenton reaction. The removal efficiencies of lignin, aromatic compounds, and COD under such conditions within 4 h were 98%, 83%, and 100%, respectively. The optimal conditions could enhance the efficiency of the removal of lignin, aromatic compounds, and COD by the synergistic Fenton reaction. After Fenton reaction, iron powder could be successfully recycled and decreased the relative large quantity of Fe2+ , Fe3+ and H2 O2 used previously.
Acknowledgments The authors gratefully acknowledged the financial supports and/or in-kind assistance from Project Funding of National Research University – Chiang Mai University (NRU – CMU) and National Research University – Office of Higher Education Commission, Ministry of Education, Thailand (NRU – OHEC), A New Researcher Scholarship of Chiang Mai University (CMU), and Bioprocess Research Cluster (BRC).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng.2015.10. 013.
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Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Short communication
Improvement in efficiency of lignin degradation by Fenton reaction using synergistic catalytic action Phisit Seesuriyachan ∗ , Ampin Kuntiya, Arthitaya Kawee-ai, Charin Techapun, Thanongsak Chaiyaso, Noppol Leksawasdi Bioprocess Research Cluster, School of Agro-Industry, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai 50100, Thailand
a r t i c l e
i n f o
Article history: Received 2 December 2014 Received in revised form 2 October 2015 Accepted 7 October 2015 Available online 24 October 2015 Keywords: Central composite design Synergistic catalytic action Fenton reaction Lignin degradation
a b s t r a c t Lignin is a complex aromatic compound and a by-product from the wood pulping process. We used a central composite design to assess the efficiency of synergistic catalysts (Fe0 , Fe2+ and Fe3+ ) and H2 O2 in the Fenton process for removal of lignin, aromatic compounds, and chemical oxygen demand (COD) from lignin solution. Results showed that the Fe0 and H2 O2 concentration were the most significant variables in lignin degradation by Fenton reaction, while Fe2+ and Fe3+ concentration were the next most important variables. The highest removal efficiencies were obtained with concentration levels of 7.61 mg/L Fe0 , 9.89 mg/L Fe2+ , 14.27 mg/L Fe3+ , and 376.88 mg/L H2 O2 at pH 3.0 and 35 ◦ C. Under these optimal conditions, the efficiencies of removal of lignin, aromatic compounds, and COD were 98.45%, 82.63%, and 100%, respectively, while the removal rates of lignin and aromatic compounds were 23.73 mg/L/h and 27.75 mg/L/h, respectively. The optimization results revealed that using synergistic catalysts in the Fenton reaction was more effective in treating wastewater and resulted in decreased cost and time. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lignin is a complex aromatic polymer with a highly branched and heterogeneous three-dimensional structure that depends on the type and growing conditions of plants. Kraft lignin differs from natural lignin as it undergoes a variety of reactions including aryl–alkyl cleavage and strong side chain modification causing the polymer to fragment into smaller alkali-soluble fragments (Chakar and Ragauskas, 2004). To solve this problem, Fenton reaction has been used to treat Kraft lignin (Ninomiya et al., 2013). The Fenton process is an effectively advanced oxidation processes (AOPs) technology for degradation of hazardous and organic pollutants in wastewater due to its relatively good efficiency, low toxicity, simplicity of control and low cost (Oliveira et al., 2014). Several studies have explored the removal efficiency of the Fenton process applied to biologically pretreated products from pulp and paper mills (Catalkaya and Kargi, 2007; Hermosilla et al., 2012; Kazmi and Thul, 2007; Tambosi et al., 2006; Torrades et al., 2011). The focus of most studies has been on Fe2+ /H2 O2 systems with
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Seesuriyachan). http://dx.doi.org/10.1016/j.ecoleng.2015.10.013 0925-8574/© 2015 Elsevier B.V. All rights reserved.
the objective of optimizing Fe2+ and H2 O2 values, which achieved large quantity of ferric (Fe3+ ) residues. However, the use of zero valent iron (Fe0 ) can enhance the system reactivity by reducing Fe3+ residues to ferrous (Fe2+ ) while decreasing the amount of reagent (Fe2+ and H2 O2 ) (Kallel et al., 2009). Fe0 + 2H+ → Fe2+ + H2
(1)
2Fe3+ + Fe0 → 3Fe2+
(2)
Fe0 + H2 O2 + 2H+ → Fe2+ + 2H2 O
(3)
In previous study, the interaction of synergistic catalysts was investigated for decolorization of low-molecular weight compound and it was found that the synergistic catalysts increased the rate and efficiency of the decolorization in comparison with reactions using Fe0 , Fe2+ or Fe3+ alone (Tantiwa et al., 2013). The enhancement of the efficiency of high-molecular weight compounds (alkali lignin, MW 10,000 g/mol) decolorization were thus determined in this study with estimation of the optimal concentration levels of synergistic catalysts (Fe0 , Fe2+ and Fe3+ ) and H2 O2 by the Fenton reaction using response surface methodology via central composite design (CCD).
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Table 1 Central composite design (CCD) for lignin degradation by Fenton reaction with the synergistic catalytic action. Std. run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Factors (mg/L)
% Removals
Removal rate (mg/L/h)
X1
X2
X3
X4
Lignin
Aromatic compounds
COD
Lignin
Aromatic compounds
4 12 4 12 4 12 4 12 4 12 4 12 4 12 4 12 0 16 8 8 8 8 8 8 8 8 8
5 5 15 15 5 5 15 15 5 5 15 15 5 5 15 15 10 10 0 20 10 10 10 10 10 10 10
5 5 5 5 15 15 15 15 5 5 5 5 15 15 15 15 10 10 10 10 0 20 10 10 10 10 10
180 180 180 180 180 180 180 180 540 540 540 540 540 540 540 540 360 360 360 360 360 360 0 720 360 360 360
74.88 67.60 75.35 50.74 67.92 40.97 63.75 28.17 69.79 88.22 88.67 72.68 87.97 98.46 80.79 93.76 84.69 61.03 82.70 63.49 83.23 65.19 2.57 80.63 91.81 98.83 96.20
71.30 63.61 71.03 51.01 78.21 60.14 72.91 46.48 79.08 102.03 105.07 75.96 97.40 98.82 92.12 92.28 75.51 70.28 99.63 74.15 79.50 75.52 2.64 85.42 92.85 98.02 98.89
75.00 51.25 75.00 57.14 51.25 37.70 75.00 37.50 25.00 82.10 82.50 72.08 80.39 100.00 87.53 100.00 80.26 57.14 85.71 62.50 71.43 81.94 0.00 100.00 87.67 100.00 100.00
10.52 10.09 10.83 7.98 11.14 7.40 11.01 5.18 11.72 13.11 14.30 12.62 14.04 12.35 14.94 13.01 10.27 10.21 13.99 14.12 9.48 10.78 1.38 14.04 13.28 14.17 13.50
8.32 7.38 8.16 6.08 13.50 10.30 12.48 8.17 14.10 17.49 18.08 12.90 16.61 11.69 15.96 10.81 8.52 12.17 16.67 12.15 13.23 13.07 3.67 14.69 15.35 16.19 16.64
2. Materials and methods
2.4. Experimental design
2.1. Reagents Alkali lignin (Kraft lignin) with 96% (w/w) purity and 4% (w/w) sulfur was purchased from Sigma–Aldrich (St. Louis, MO, USA). Hydrogen peroxide (H2 O2 ) and sulfuric acid (H2 SO4 ) were provided by Merck (Damstadt, Germany). High purity iron power, iron(II) sulfate heptahydrate (FeSO4 ·7H2 O), and iron(III) chloride anhydrous (FeCl3 ) were provided by Fisher Scientific (Leicestershire, UK). All solutions were prepared by dissolving in distilled water containing iron <0.001 mg/L.
We used a central composite design (CCD) in response surface methodology to estimate the coefficients in a mathematical model, predict the response, and check the model applicability of Fe0 (X1 ), Fe2+ (X2 ), Fe3+ (X3 ), and H2 O2 (X4 ) (Table 1), which was described previously by Tantiwa et al. (2013). The steepest ascent method was applied to find the optimum levels (Table 2). The Design Expert 6.0.10 (Stat-Ease, Inc., Minneapolis, MN, USA) was used for the design of experiments and data analysis. The significance of the model equations and the model terms were evaluated in term of p value with a 95% confidence level.
2.2. Experimental procedures
Y = ˇ0 + ˙ˇi Xi + ˙ˇii Xi2 + ˇij Xi Xj
Lignin solution (100 mg/L) was prepared by dissolving alkali lignin with distilled water and adjusted to pH 3.0, and then the solution was added into a 250 mL flask with controlled temperature at 35 ◦ C and was agitated at 250 rpm. The synergistic catalytic action comprised of Fe0 , Fe2+ , and Fe3+ was added into lignin solution with the concentration levels and formula shown in Table 1. After 5 min, H2 O2 was immediately added to initiate reaction. Samples were taken every hour for 10 h. The reactions were stopped instantaneously by the addition of NaOH to elevate pH level to 7.0 (Pesakhov et al., 2007), prior to filtration process using Whatman No. 1 filter paper to remove the chemical precipitate before analysis. 2.3. Analytical method Concentration levels of lignin and aromatic compounds were determined by measuring sample absorbance (Genesys 20 UV–visible spectrophotometer, Thermo scientific, Waltham, MA, USA) at 280 nm and 254 nm, respectively (Janshekar et al., 1981). COD was determined by closed reflux titrimetric method (APHA, 1985). All measurements were performed in triplicate.
(4)
where Y represents the response variable, ˇo is the interception coefficient, ˇi is the coefficient for the linear effect, ˇii is the coefficient for the quadratic effect and ˇij is the ijth coefficient of the interaction effect, and Xi Xj are input variables that influence the response variable Y. 3. Results and discussion A total of 27 runs for optimizing the four variables in the current CCD are shown in Table 1. We found that the interaction between synergistic catalysts and H2 O2 improved the removal of lignin, aromatics and COD in the Kraft lignin solution. The lignin removal was in the range of 2.57–98.83%, aromatic compound removal: 2.64–98.89% and COD removal: 0–100%. The rates of removal of lignin and aromatic compounds were 1.38–14.94 mg/L/h and 3.67–18.08 mg/L/h, respectively. At the lowest values, Fenton reaction was not effective due to the absence of H2 O2 (X4 ). These results confirmed that H2 O2 plays an important role in driving catalytic oxidation in the Fenton reaction (Wang et al., 2014). Conversely, the removal efficiency of lignin, aromatic compounds and COD were decreased by about 12–15% in the absence of Fe0 , Fe2+ or Fe3+ , which is compared by the central point of
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Table 2 Lignin degradation by Fenton reaction using steepest ascent method. Std run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Factors (mg/L)
% Removals
Removal rate (mg/L/h)
X1
X2
X3
X4
Lignin
Aromatic
COD
Lignin
Aromatic
8.37 9.13 10 65 7.61 7.61 7.61 7.61 7.61 7.61 7.61 7.61 7.61 6.10 4.57 7.61 7.61 7.61 7.61 7.61 7.61 7.61 0 0
9.89 9.89 9.89 10.88 11.87 13.85 9.89 9.89 9.89 9.89 9.89 9.89 9.89 9.89 7.91 5.93 9.89 9.89 9.89 9.89 0 9.89 0
14.27 14.27 14.27 14.27 14.27 14.27 15.70 17.12 19.98 14.27 14.27 14.27 14.27 14.27 14.27 14.27 11.42 8.56 14.27 14.27 0 0 14.27
376.88 376.88 376.88 376.88 376.88 376.88 376.88 376.88 376.88 414.57 497.88 527.63 376.88 376.88 376.88 376.88 376.88 376.88 301.50 226.13 376.88 376.88 376.88
86.70 84.24 90.56 87.18 68.28 70.64 79.77 79.80 84.56 75.56 76.70 81.99 89.89 70.89 88.94 86.56 88.27 88.92 77.73 72.31 65.37 40.43 59.90
92.22 85.47 96.68 89.92 63.63 82.06 81.06 83.98 90.42 70.17 75.83 82.63 97.06 88.76 97.12 88.55 83.13 81.22 73.81 89.01 73.47 58.13 89.35
100 100 100 100 87 100 100 100 100 5 86 88 85 100 100 100 100 100 68 75 100 3 19
20.85 20.10 21.01 20.07 15.61 16.38 18.83 18.68 19.83 17.36 16.65 19.17 20.90 14.86 20.70 20.15 20.36 20.76 17.48 17.00 16.00 9.32 14.25
31.11 22.41 33.07 21.41 15.08 29.45 26.86 25.58 24.11 16.86 16.45 20.21 23.76 20.26 33.85 22.88 20.30 28.27 16.39 31.29 26.98 20.30 31.99
the design. The results were consistent with Chu et al. (2012) who showed that in the absence of Fe0 , phenol degradation was decreased by 12% at a reaction time of 2.5 h. At both low and high concentration levels of Fe0 , Fe2+ , Fe3+ and H2 O2 , the oxidation efficiencies of the Fenton process on removal of lignin, aromatics and COD, and the removal rates of lignin and aromatic compounds were suppressed.
The steepest ascent method was used to estimate the optimal values for lignin degradation using synergistic catalysts in the Fenton reagent as presented in Table 2. The use of only one catalyst in the reaction could mitigate the removal of lignin, aromatic compounds and COD to 65.37%, 73.47% and 100% for Fe0 , 40.43%, 58.13% and 3.00% for Fe2+ and 59.90%, 89.35% and 19.00% for Fe3+ , respectively. The H2 O2 /Fe0 system has better degradation efficiency than
Table 3 Analysis of variance (ANOVA) results and quadratic equation for response parameters. Responses
Lignin
Aromatic compounds
COD
Lignin
Aromatic compounds
Final equation in term of actual factors % Removals 95.61 − 4.83X1 − 3.35X2 − 2.95X3 + 15.72X4 − 4.51X12 − 4.45X22 − 4.17X32 − 12.97X42 − 3.62X1 X2 − 0.60X1 X3 + 7.52X1 X4 0.99X2 X3 + 1.55X2 + 6.84X3 X4 96.59 − 3.64X1 − 3.94X2 + 0.47X3 + 16.84X4 − 4.44X12 − 0.94X22 − 3.28X32 − 12.31X42 − 4.63X1 X2 − 0.57X1 X3 + 4.23X1 X4 − 1.11X2 X3 + 1.24X2 + 1.10X3 X4 95.89 − 2.51X1 + 1.58X2 + 2.92X3 + 15.41X4 − 6.68X12 − 5.33X22 − 4.68X32 − 11.35X42 − 5.78X1 X2 − 1.52X1 X3 + 10.73X1 X4 − 1.40X2 X3 + 1.56X2 + 10.21X3 X4 Removal rate 13.65 − 0.70X1 − 0.009X2 0.021X3 + 2.38X4 − 0.73X12 + 0.22X22 − 0.76X32 − 1.36X42 − 0.49X1 X2 − 0.60X1 X3 + 0.56X1 X4 − 0.068X2 X3 + 0.49X2 + 0.45X3 X4 16.60 − 0.63X1 − 0.66X2 + 0.28X3 + 3.33X4 − 1.24X12 − 0.22X22 − 0.54X32 − 2.44X42 − 0.69X1 X2 − 0.80X1 X3 − 0.085X1 X4 − 0.16X2 X3 + 0.15X2 − 1.38X3 X4
p
LOF
R2
Adj. R2
Pred. R2
AP
CV
0.0001
8.13
0.9224
0.8319
0.5597
12.314
12.89
0.0007
12.51
0.8975
0.7778
0.4118
11.253
13.58
0.0127
5.66
0.9654
0.9308
0.7663
15.363
10.37
0.0001
8.67
0.9237
0.8346
0.5663
12.900
11.12
0.0021
16.50
0.8711
0.7208
0.2632
9.649
20.15
p, probability of error; LOF, lack of fit; Adj. R2 , adjust R2 ; Pred. R2 , predicted R2 ; AP, adequate precision; CV, coefficient of variance.
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Table 4 Studies on degradation of lignin using advanced oxidation processes (AOPs). Sources
Kraft lignin Initial lignin 100 mg/L Kraft lignin Initial lignin 500 mg/L Black liquor Initial lignin OD280 = 1.978 Pulp and paper mill p-Toluenesulponic acid Initial 860 mg/L Alfalfa black liquor Initial lignin 90 mg/L Wheat straw Kraft Initial lignin 100 mg/L
Treatment
Catalyst concentration (mg/L)
Time (h)
Synergistic catalysts of Fenton (H2 O2 /Fe0 + Fe2+ + Fe3+ ) Sonocatalytic (US/TiO2 ) Fenton (H2 O2 /Fe2+ ) (US/TiO2 + H2 O2 /Fe2+ ) Fenton (H2 O2 /Fe2+ )
H2 O2 = 376.45, Fe0 = 7.60, Fe2+ = 9.88, Fe3+ = 14.25 TiO2 = 2000 H2 O2 = 3400 Fe2+ = 280
Fenton (H2 O2 /Fe2+ )
TiO2 UV/TiO2
H2 O2 = 600 Fe2+ = 100 H2 O2 = 34 Fe3+ = 20 TiO2 = 200 TiO2 = 1000
UV/ZnO
ZnO = 1000
Photo-Fenton
H2 O2 = 1500 Fe2+ = 1300
the H2 O2 /Fe2+ or H2 O2 /Fe3+ systems (Tang and Chen, 1996; Tantiwa et al., 2013). Therefore, the combination of the three catalysts (Fe0 , Fe2+ and Fe3+ ) in the Fenton reaction greatly improved the degradation of Kraft lignin. The optimal concentrations for central composite design of Fe0 , Fe2+ , Fe3+ , and H2 O2 for lignin degradation by Fenton reaction were 7.61, 9.89, 14.27, and 376.88 mg/L, respectively. The treatment of 100 mg/L lignin under these optimal conditions resulted in removal of 98.45% of lignin, 82.63% of aromatic compounds and 100% of COD. The removal rates were 23.73 mg/L/h of lignin and 27.75 mg/L/h of aromatic compounds. The optimal values from this study were lower than the optimal values of Torrades et al. (2011) which 1.3 g/L of Fe2+ and 1.5 g/L of H2 O2 to degrade lignin in black liquor was used. The quadratic mathematic equations showed that the two most important variables for the removal of lignin, aromatic compounds and COD, and the removal rate of lignin and aromatic compounds were Fe0 (X1 ) and H2 O2 , (X4 ) while the interaction effect of Fe2+ (X2 ) and Fe3+ (X3 ) were incorporated in all five mathematical models (Table 3). The 3D graphs of the relationships between the significant variables of Fe0 , Fe2+ , Fe3+ , H2 O2 , lignin, aromatic compounds, and COD removals are shown in supplement data (Supp. 1–3). In this study, Fe2+ and Fe3+ concentration levels interacted because the ability of Fe2+ to regenerate by the reaction of Fe3+ with H2 O2 was inhibited at acidic pH (Pignatello, 1992). Table 4 presents investigations of lignin degradation using other AOPs such as titanium dioxide (TiO2 ), sonocatalysis (US/TiO2 ), Fenton reaction, photo-Fenton, sonocatalysis combined with Fenton reagent, ultraviolet (UV) and mesoporous UV/TiO2 . It showed that the combination of catalysts in the synergistic Fenton reaction was more efficient than Fenton reaction or Fenton-like reaction alone or the combination with other processes due to continuous dissolution of iron powder in the absorption of lignin, aromatic compounds and other polyphenol on iron powder surface (Tang and Chen, 1996). The use of Fenton reaction with zero valent iron can be renewable for the reaction system (Kallel et al., 2009) and slightly decreased the amount of Fenton reagents. While, the use of Fenton process by one catalyst consumed large amount of Fenton reagents to get the high efficiency of lignin removal (Kazmi and Thul, 2007; Torrades et al., 2011). The relatively
Removal (%)
References
Lignin
COD
4
98.4
100
2
1.8
–
This study
Ninomiya et al. (2013)
2 2 1.5
49.9 60.0 85.6
94.8
5
98
62
6
47.0
20.0
Kazmi and Thul (2007) Amat et al. (2005)
6 7
27.0 56.0
20.0 72.0
Ksibi et al. (2003)
5
84.0
85.0
Kansal et al. (2008)
Torrades et al. (2011)
low concentration levels of Fenton’s catalysts were enhanced in this study with high efficiency of lignin and COD removal. Therefore, we propose the use of the Fenton process with synergistic catalysts as an alternative wastewater treatment due to its relatively high efficiency, low cost and low catalyst concentration levels.
4. Conclusion This study demonstrated that synergistic catalysts Fe0 , Fe2+ , Fe3+ and H2 O2 could statistically influence lignin, aromatic compounds, and COD removals as evident from response surface methodology via CCD. The optimum concentration levels of the synergistic catalysts were 7.61 mg/L of Fe0 , 9.89 mg/L of Fe2+ , 14.27 mg/L of Fe3+ , and 376.88 mg/L of H2 O2 for removal of lignin, aromatic compounds, and COD by Fenton reaction. The removal efficiencies of lignin, aromatic compounds, and COD under such conditions within 4 h were 98%, 83%, and 100%, respectively. The optimal conditions could enhance the efficiency of the removal of lignin, aromatic compounds, and COD by the synergistic Fenton reaction. After Fenton reaction, iron powder could be successfully recycled and decreased the relative large quantity of Fe2+ , Fe3+ and H2 O2 used previously.
Acknowledgments The authors gratefully acknowledged the financial supports and/or in-kind assistance from Project Funding of National Research University – Chiang Mai University (NRU – CMU) and National Research University – Office of Higher Education Commission, Ministry of Education, Thailand (NRU – OHEC), A New Researcher Scholarship of Chiang Mai University (CMU), and Bioprocess Research Cluster (BRC).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng.2015.10. 013.
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