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Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst Parisa Abdi a, Ali Farzi a, Afzal Karimi b,∗ a b
Faculty of Chemical and Petroleum Engineering, University of Tabriz, Tabriz, Iran Department of Biotechnology, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences, Tehran Iran
a r t i c l e
i n f o
Article history: Received 21 June 2016 Revised 1 November 2016 Accepted 16 November 2016 Available online xxx Keywords: Advanced oxidation Decolorization Glucose oxidase Metal-foam Photocatalyst
a b s t r a c t A recycling open channel reactor was utilized for removal of organic contaminant of malachite green with flow rate of 5 mL/min. TiO2 nanoparticles and FeSO4 powder immobilized on metal-foam (MF) were applied as a photo-Fenton catalyst. Glucose oxidase (GOx) was also coupled with catalyst for in-situ generation of hydrogen peroxide. Furthermore, UV (6 W) lamp was used as a light source to carried out photo-bio-Fenton reaction in the reactor. Several experiments were carried out to study the influence of various combination of affecting processes on decolorization of the pollutant. Removal of 84.37% of malachite green was obtained in GOx/TiO2 /Fe2+ /MF process under UVA-LED during 20 min. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Many kinds of commercial dyes are available worldwide and about 7 × 105 tons of dyestuff are produced annually. Dyes and dye intermediates are destructive organic compounds for the environment and are very harmful for human health. High percentages of dyes are not biodegradable and have disadvantages like carcinogenic and toxic properties. These pollutants are high solubility in water. Entering dyes in water causes blocking of light path in visible region, which results major troubles for aquatic life [1-3]. The most important methods that are used for the removal of dyes are advanced oxidation processes (AOPs) and biological processes. Production of hydroxyl radical intermediates with strong oxidizing ability is the main characteristic of AOPs. These processes are used for both organic and inorganic components including dye intermediates and azo dyes where biological treatment is difficult [1,3,4]. Due to the existence of toxic compounds and their resistance against decomposition and biological degradation in pollutant mixtures and also high cost of AOPs, none of these two methods (biological and AOPs) can be used independently for complete removal of organic contaminants. To overcome this problem, combination of biological processes and AOPs is necessary for treatment of industrial recalcitrant wastewaters. In this method, firstly removal of biodegradable pollutants is carried out and then lowering of non-toxic contaminants implemented [5]. ∗
Corresponding author. Fax: +98 21 86704606. E-mail address: [email protected] (A. Karimi).
Enzyme-based catalysts act at neutral pH and ambient temperature, and also increase the rate of chemical and biological reactions. One of the most important characteristics of enzymes that is necessary for their catalytic action, is their 3-dimensional structure [6,7]. Working with enzymes has some problems like their low stability and high costs. To overcome these difficulties, immobilizing of enzymes could be a good procedure [8]. Glucose oxidase (GOx) is a commercial catalyst that is a dimmer with molecular weight of 186 × 103 g/mol and contains two flavine adenine dinucleotide (FAD) attached to each dimmer [9,10]. GOx has many applications in pharmaceutical, textile, food industry, construction of biosensors [7,9]. This enzyme can be used for degradation of azo-dye pollutants. Other important methods such as photocatalytic techniques, Fenton, ozonation, sonolysis and hybrid methods such as sonoFenton, sono-photocatalysis, photo-Fenton, UV/ozone, UV/H2 O2 etc. can be utilized for AOP aim [11-14]. Each of these methods work in an almost different path for azo-dye decontamination and combination of them may improve decontamination process like combination of sonolysis and Fenton for sono-Fenton or degrade it such as combination of sono-Fenton and UV or be neutral. Fenton process is a type of AOPs which is used for wastewater treatment. Because of generation of strong hydroxyl radicals (• OH), it is an oxidative process and is used to remove stubborn organic pollutants. Reaction of iron catalyst and hydrogen peroxide is carried out to generate 2.8 V oxidation potential of • OH radicals. Fenton reaction is performed in both homogeneous and heterogeneous phases. Chemical reactions take place in homogeneous phase and
http://dx.doi.org/10.1016/j.jtice.2016.11.022 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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nature of reactants is very important. But in heterogeneous phase, some physical reactions take place on active sites of catalyst together with chemical reactions. Active sites of catalyst in Fenton homogeneous process are Fe2+ or Fe3+ ions while in heterogeneous process they are iron ions in hybrid forms of compounds such as (Fe(OH)2 )+ ; (Fe(H2 O)2+ ; (Fe(H2 O)6 )3+ [15-20]. Progress in nanotechnology increased utilization of nanoparticles and transformed this science to one of the most important technologies for removal of waste industry in 1990s. It is well known that degradation of pollutants and dyes using nano scale catalysts is more efficient compared to micro scaled catalyst particles and utilization of nano scale catalyst particles is more favorable [21,22]. TiO2 particles have properties such as nontoxicity, high yield relative to other photo-catalysts and low cost which makes them a suitable photo-catalysts for AOPs [23]. Anatase, rutile, brookite and TiO2 (B) are four famous types of TiO2 in nature and a mixture of 80% anatase and 20% routile is proved to be the best blend to achieve highest activity [24]. Dispersed TiO2 nanoparticles have been used in many wastewater treatment processes, but they would have many problems for environment. To solve their harmful effects, immobilization of TiO2 particles on suitable carriers are suggested [21,23]. Metal foams with porosity of more than 40% could be used for this purpose. Low weight and exclusive characteristics such as high capacity for energy absorption, high thermal conductivity, and many others are the most important properties of such foams. Size, shape, volume percentage and surface area of pores are the main factors for choosing a metal-foam as carrier [25]. In present study, a porous carrier containing nanotopography pores with silver coating on the carrier was considered. TiO2 particles and GOx enzyme, for in situ generation of H2 O2 , were stabilized on the support. UV lamp was used as light source in an open channel photo-reactor. Also in this research, Fenton process was implemented to investigate the effect of Fe2+ ions on decolorization of malachite green.
lated as 0.023 g/g based on dry-weight of metal-foam with 0.238 g of TiO2 immobilized nanoparticles. 2.3. Immobilization of GOx on TiO2 /AgO metal-foam There are different methods such as physical adsorption, covalent bond and entrapment for immobilization of enzymes [5,26]. Simplicity, low cost and minimum denaturation of enzymes are the most important parameters for choosing suitable procedure [27]. In this research, physical adsorption of enzyme on TiO2 /MF carrier was used. Phosphate buffer (PBS, 0.1 M) was used to prepare GOx solution (0.1 mg/mL) [28]. After emerging the carrier in prepared enzyme solution they were incubated at 160 rpm for 1 h, pH value of 6.8, and temperature of 35 °C. Then carrier was separated from the solution and washed twice using fresh phosphate buffer. The amount of immobilized GOx on TiO2 /MF was measured by free enzyme in the supernatant using calibration curve and UV–vis spectrophotometer (S20 0 0 UV–vis spectrophotometer). In this research, 53 U/g of carrier was obtained for immobilized GOx on TiO2 /MF [29,30].
2. Material and methods
2.3.1. Decolorization process using Fenton photo-bioreactor A recycling open channel reactor (Fig. 1) was used for decolorization of 20 mg/L MG pollutant with 1 mM of aqueous solution of Fe2+ [31]. Total volume of dye solution was 70 mL and 20 mL of solution was circulating between inlet and outlet of the reservoir. The prepared photo-biocatalyst was placed at bottom and the UV lamp (6 W) was fixed on top of the reactor (3 cm above the solution). Dye solution was poured into the inlet vessel and the control valve was opened in order to fill the reactor by the solution above the photo-biocatalyst. Then, it was flowed to the outlet vessel and was circulated with peristaltic pump with flow rate of 5 mL/min. During the process, samples were taken at specified sampling times and degradation of pollutants was determined. Concentration of dye was obtained from calibration curve and UV– vis spectrophotometer. Decolorization of MG (at wavelength of λmax = 617 nm) was obtained using Eq. (1):
2.1. Material
Decolorization efficiency (% ) =
A silver metal-foam coated with AgO layer was prepared from metal nano-coating company located at Science and Technology Park in Tehran, Iran. TiO2 nanoparticles with normal crystal size of 21 nm (Degussa P25 consisting of about 80% anatase and 20% rutile), Glucose oxidase type II (GOx, EC 1.1.3.4, obtained from Sigma-Aldrich), β -d-glucose, KI (99.99%), NaOH (99.99%), ammonium molybdate tetrahydrate (Mo(Vl).4H2 O), potassium hydrogen phthalate (KHP), FeSO4 , Hydrogen peroxide (30% (v/v), Merck), Malachite green oxalate (MG, C.I. No. 42,0 0 0, C52 H54 N4 O12 , Mw = 927.01 g/mol, λmax = 617 nm) were obtained from Sigma Aldrich.
where C0 and Ct are concentrations of MG solution (mg/L) at times 0 and t, respectively.
2.2. Immobilization of TiO2 nanoparticles on AgO metal-foam A suspension of 0.45 g of TiO2 nanoparticles in 40 mL of methanol was prepared. To prevent pouring of the solution, a thin layer of paraffin was pulled on container of suspended solution and was sonicated for about 15–20 min by Sonoplus Ultrasonic Homogenizer HD 2200 (Germany). To set the pH at about 5, HCl (1 N) was added. After that, prepared suspension was poured on heated MF at about 80 °C. MF and the solution above it, were heated for 5 h. Coated MF was washed with distillated water in order to remove free TiO2 nanoparticles from the carrier. The mass of TiO2 immobilized on MF was calculated from difference between MF weight before and after immobilization. The ratio of TiO2 /MF was calcu-
C − C o t Co
× 100
(1)
3. Results and discussion 3.1. Characterization of photocatalyst and mechanism of Fenton photo-bioreactions Fig. 2(a) and (b) show optical images of metal-foam before and after immobilization of TiO2 nanoparticles. It is clear from color changes of MF that TiO2 powders have coated the metal-foam. Also, Fig. 2(c) and (d) demonstrate surface morphology of MF after immobilization of TiO2 nanoparticles and glucose oxidase enzyme immobilization, respectively. Presence of nano-pores on MF surface and similarity of nanoparticles with carrier in size are the main reasons for adequate immobilization of TiO2 nanoparticles on it. Fig. 2(e) and (f), show nano-pores on MF carrier before and after TiO2 immobilization. Glucose oxidase is one of the well-known enzymes that accelerates electron transfer from substrate (glucose) to oxygen for production of hydrogen peroxide [32]. GOx
C6 H12 O6 + H2 O + O2 → C6 H12 O7 + H2 O2
(2)
Then, ferrous ions react with hydrogen peroxide and produce ferric ions and hydroxyl radicals according to Eq. (3) [16]:
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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Fig. 1. Schematic diagram of continuously circulated open-channel photo fenton bio-reactor.
Fe2+ + H2 O2 → Fe3+ +OH• +OH−
(3)
UV-A lamp with λ < 390 nm and energy band of 3.2 eV, was glinted to TiO2 semiconductor photocatalyst to create holes with positive charges and free electrons as follows [33]:
TiO2 + hν → e− cb + h+ ν b
(4)
According to the above reactions, electron–hole pairs are created on catalyst surface and h+ vb reacts with H2 O and OH− as below [34,35]:
h+ ν b + H2 O → OH• + H+
(5)
h+ ν b + OH− → OH•
(6)
Other important reactions for production of hydroxyl radicals that are carried out in this process are shown in the following equations [15,36]:
Fe2+ + H2 O2 + H+ → Fe3+ + OH• + H2 O
(7)
Fe3+ +H2 O +UV → OH• +Fe2+ + H+
(8)
H2 O2 + • O2(ads) − → OH• +OH− +O2
(9)
H2 O2 +UV → OH• + OH−
(10)
In enzymatic reaction, H2 O2 is generated and in Fenton and photolysis reactions it is consumed to produce hydroxyl radicals. Thus it can be claimed that enzymatic and photocatalytic reactions stimulate Fenton process. Another noteworthy point is that no additional hydrogen peroxide was added manually in this process. In AOPs, GOx enzyme plays several roles and flavine adenine dinucleotide (FAD) structure is the most important reason for enzyme activity and it’s characteristics. One of the roles is in-situ production of H2 O2 and another role is inhibiting recombination of charges in the process [37]. 3.2. Photo-catalytic process coupled with bio-Fenton for decolorization of MG In this study, several methods were investigated at different conditions. Firstly, adsorption equilibrium of dye on coated carrier was considered. Concentration changes of fresh solution of dye
were monitored and then the amount of dye adsorption on carrier was determined. In first run, adsorption of dye solution (mg/L) on the carrier was reached to 36% after 2 h. But in second run, it was reduced to 19% and after that no adsorption of dye was detected and the saturated carrier was used to study more decolorization tests. Diagram of adsorption profiles in different runs on carrier is shown in Fig. 3 [30]. Processes that were studied in this work include: (a) Fenton process, (b) enzymatic process, (c) bio-Fenton without UV light, (d) TiO2 /UV, (e) TiO2 /GOx/UV without glucose, (f) TiO2 /GOx/UV + glucose, (g) TiO2 /GOx/glucose/UV/Fe2+ , and (h) TiO2 /UV/Fe2+ . Fig. 4 shows decolorization of MG for different processes. As it is clear, maximum decolorization efficiency was achieved under TiO2 /GOx/UV/Fe2+ process containing glucose as substrate. For comparison, their kinetic constants are shown in Table 1. The most important experimental results from Fig. 4 and Table 1 are as follows: 1. Decolorization of pollutant in TiO2 + Fe2+ and TiO2 + GOx processes reflect the adsorption properties of TiO2 and they are similar to a certain extent. 2. Decolorization amount of pollutant in TiO2 + GOx+Fe2+ and TiO2 + Fe+ +UV processes are partially similar. 3. TiO2 + Fe2+ +UV and TiO2 + GOx+Fe2+ processes have positive synergic with each other. The effect was observed in combined TiO2 + Fe2+ + GOx + UV processes. Fig. 5 shows the differences between efficiency of two processes in presence and absence of glucose. After 20 min, about 15% of difference between efficiency of two processes was observed. The reason for this phenomenon is formation of hydrogen peroxide in presence of substrate, which leads to formation of active hydrogen peroxide radicals. According to obtained results for all processes, the curve of decolorization rate has a steep slope till 20 min and high amount of decontamination was obtained within this short time period. Between 20 to 60 minutes after start of reaction, slope of the curve was decreased and after 1 h it reached to zero. In presence of all of decolorization factors, 84.37% of decolorization was obtained at t = 20 min. 3.2.1. Effect of pH pH of the solution is one of the important factors in AOPs. Because of multi-functional roles of pH on dye decontamination, interpretation of its role on decolorization efficiency is complex. In many researches, optimum pH of Fenton process is reported at
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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Fig. 2. Image of metal-foam (MF): (a) before and (b) after immobilization of TiO2 nanoparticles, (c) SEM image after immobilization of TiO2 , and (d) after immobilization of enzyme on TiO2 /MF, (e) image of nano particles on MF before TiO2 immobilization, and (f) after TiO2 immobilization.
Table 1 Summary of experimental results of MG decolorization under UV light with [MG]0 = 20 mg/L; inlet flow rate = 5 mL/min. MG decolorization process
Decolorization (%) 20 min
60 min
TiO2 +UV TiO2 +Fe TiO2 + Fe+UV TiO2 + GOx TiO2 + GOx+Fe TiO2 + GOx+UV TiO2 + Fe + GOx + UV
42.17 49.27 55.07 50.03 58.55 66.09 84.36
59.15 55.15 64.67 54.64 63.54 71.73 89.85
Decolorization rate constant (k, min−1 )
2.8 × 3.3 × 3.9 × 3.4 × 4.5 × 5.3 × 8.9 ×
10 − 2 10 − 2 10 − 2 10 − 2 10 − 2 10 − 2 10 − 2
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Fig. 3. MG adsorption on Fenton photo-biocatalyst.
Fig. 5. MG decolorization efficiency in TiO2 /UV/Fe/GOx and TiO2 /Fe/UV/GOx (without glucose); [MG] = 20 mg/L, T = 25°C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
acidic pH, positive holes operate as the important oxidation agents while in neutral and alkaline pH, hydroxyl radicals are dominant produced agents which is because of more oxidation of hydroxide ions on TiO2 surface. Thus, Coulombic repulsion force with hydroxyl radicals will occur which is a deterrent factor to decontamination. 2) Ionization of TiO2 surface by H+ ions, leads to adsorption of dye molecules on its surface which are shown by Eqs. (11) and (12). +
TiOH + H+ → TiOH2
(11)
TiOH + OH− → TiO− + H2 O
(12)
3) In acidic medium, TiO2 particles desire to agglomerate. Thus, surface area of photocatalyst for photon absorption and dye adsorption will be decreased [42]. In this study, pH was changed within the range of 0.5–5. Other fixed conditions of current study were dye solution of 20 mg/L, Fe2+ concentration of 1 mM, glucose concentration of 20 mM, and enzyme activity of 120 U/mL. According to Fig. 6, decontamination had the maximum efficiency at pH = 4.5. Since isoelectric point of GOx enzyme is about 4.2, it has negative surface charge at optimum pH [43]. By increasing pH greater than 4.5, decolorization efficiency was reduced. The reason may be the formation of Fe(OH)3 and also H2 O and O2 generation from decomposition of H2 O2 by iron [44]. Thus OH• will be decreased in the process and decolorization efficiency will be reduced.
Fig. 4. MG decolorization efficiency in different processes; [MG] = 20 mg/L, 25°C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
about 3, which means that acidic medium is more suitable for this process. It is related to stability of H2 O2 in acidic solution. Also, since surface charge of photocatalyst in pH < 7 is positive, adjusting pH in acidic region for UV/TiO2 process can be effective on dye decolorization. On the other hand, changing the pH factor can be effective on catalyst surface properties and can change adsorption of dye molecules on catalyst surface which is one of the important factors in photo-oxidation and decontamination [38-42]. Some reasons for positive effect of acidic pH on photocatalytic decontamination process are: 1) Formation of hydroxyl radicals by reaction between positive holes (h+ ) and hydroxide ions (OH − ). Indeed, in vb
3.2.2. Effect of Fe2+ ions Fe2+ ions were introduced to solution by addition of FeSO4 powder. Concentration of Fe2+ ions was changed in the range of 0.5–5 mM. As shown in Fig. 7, concentration of 1 mM Fe2+ had the highest decolorization efficiency. According to Eq. 13, excess amount of Fe2+ ions in the reaction has negative impact on decolorization because of self-consuming of • OH radicals by Fe2+ ions. Since this agent is the most important factor for decontamination, its lack reduces the decolorization [45]. Also excess amount of ferrous ions leads to generation of neutral iron salts in the solution which causes an increase in generation of detrimental and unnecessary solids in solution [39].
Fe2+ + OH• → Fe3+ + OH−
(13)
3.2.3. Effect of glucose concentration In enzymatic processes, glucose has the substrate role and is the activator agent of the reaction. Also it is one of the important
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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Fig. 8. Effect of glucose concentration on MG decolorization; [MG] = 20 mg/L, T = 25 °C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
Eq. (15). While hydroxyl radicals are the main factors of decolorization, their reduction causes reduction of decolorization rate [46-49]. Fig. 6. Effect of pH on decolorization process on GOx/TiO2 /MF carrier; [MG] = 20 mg/L, [Fe2+ ] = 1 mM, T = 25°C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
H2 O2 + OH• → HO•2 + H2 O
(14)
HO•2 + OH• → H2 O + O2
(15)
3.2.4. Effect of MG concentration Effect of initial dye concentration was investigated at different concentrations of MG (10, 20, 30 and 40 mg/L). By increasing dye concentration, efficiency of decolorization was decreased but the rate of decontamination was increased which can be attributed to the fast consumption of OH• radicals at high concentrations of pollutant and also existence of high amounts of dye molecules in the solution. Another reason would be decrease of photon permeation in the solution and subsequently reduction of hydroxyl radicals generation [36,50].
Fig. 7. Effect of Fe2+ ions concentration on MG decolorization; [MG] = 20 mg/L, T = 25 °C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
species for H2 O2 production according to Eq. (2) and its concentration is highly effective on H2 O2 generation. The effect of change of concentration of glucose was studied whose results are demonstrated in Fig. 8. By increasing glucose concentration up to 20 mM, decolorization was increased and increasing concentration of glucose more than this value caused reduction of decolorization efficiency. From Eqs. 14 to 15, it can be concluded that excess amounts of hydroxyl radicals produce agents that could have negative effect on the process and would consume OH• radicals. Also excess amount of H2 O2 leads to generation of hydroperoxyl radicals which is a promoter for hydroxyl radicals consumption according to
3.2.5. Reusability of the photo-biocatalyst Reusability of Fenton photo-biocatalyst is very important for validating experiments. Presence of very small pores on metalfoam leads to protection of immobilized enzyme from UV lamp. Four other cycles were repeated after the first cycle in order to investigate the importance of this parameter on pollutant decontamination. A decreasing trend for decolorization efficiency was observed as shown in Fig. 9. Besides, to prove the accuracy of the results, each cycle of decolorization was repeated 3 times and the average results was presented as error bars on each cycle. In fifth cycle, decolorization was decreased to 51.56% after 20 min. One reason can be gradual washing of enzyme from carrier after each run so amount of enzyme for decolorization of new dye solution will be reduced. The other reason is fouling of the dye on carrier and its immobilized enzyme, so the reaction surface will be polluted and active sites for decolorization will be deactivated. 4. Conclusions The aim of this study was to survey decolorization efficiency of aqueous solutions containing Malachite Green (MG) within a
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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Fig. 9. Reusability efficiency of GOx/TiO2 /MF cycles on MG decolorization process at 20 min (T = 25 °C); [MG] = 20 mg/L, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
photo-biocatalyst reactor coupled with Fenton process. A porous silver metal-foam with a thin-coated AgO layer on it was used as the support for MG decolorization. No erosion of MF and its reusability for many times, was the main reason for choosing this support. The other reason was silver antimicrobial property and strong disinfecting capability of AgO. At first, TiO2 photocatalyst was stabilized on MF and then glucose oxidase enzyme was properly immobilized on TiO2 /MF. Existence of pores on metal-foam with nanotopography creates a good binding with TiO2 nanoparticles and enzyme with nano-size particles. Several tests were conducted inside the open channel-recycling reactor for decolorization of MG as a sample model of azo dye. In presence of TiO2 , enzyme, Fe2+ ions and UV lamp, removal of 84.37% was achieved after 1 h from the start of the process. The fourth reason to choose metalfoam as carrier is that it is a suitable place for protecting enzyme from UV lamp. Also the effect of every reduction agent on pollutant including concentration changes of Fe2+ ions, glucose, MG, and changes on pH were investigated. By setting the values of all of the mentioned factors on their optimum values, the highest decolorization rate was obtained which indicates that lack or excess of each of them leads to generation of inappropriate interactions which ultimately reduces active sites for decolorization. Reusability of photo-biocatalyst was also investigated. After the fifth cycle, in 20 min period, decolorization percentage of MG was reduced to about 51.56%. Acknowledgment The authors thank to University of Tabriz for its collaboration in this research. References [1] Gokulakrishnan S, Parakh P, Prakash H. Degradation of Malachite green by potassium persulphate, its enhancement by 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane nickel(II) perchlorate complex, and removal of antibacterial activity. J Hazard Mater 2012;213-214:19–27. [2] Uma Banerjee S, Sharma YC. Equilibrium and kinetic studies for removal of malachite green from aqueous solution by a low cost activated carbon. J Ind Eng Chem 2013;19:1099–105. [3] Arslan-Alaton I, Tureli G, Olmez-Hanci T. Treatment of azo dye production wastewaters using photo-fenton-like advanced oxidation processes: optimization by response surface methodology. J Photochem Photobiol A: Chem 2009;202:142–53. [4] Huang YH, Tsai ST, Huang YF, Chen CY. Degradation of commercial azo dye reactive Black B in photo/ferrioxalate system. J Hazard Mater 2007;140:382–8.
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Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst Parisa Abdi a, Ali Farzi a, Afzal Karimi b,∗ a b
Faculty of Chemical and Petroleum Engineering, University of Tabriz, Tabriz, Iran Department of Biotechnology, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences, Tehran Iran
a r t i c l e
i n f o
Article history: Received 21 June 2016 Revised 1 November 2016 Accepted 16 November 2016 Available online xxx Keywords: Advanced oxidation Decolorization Glucose oxidase Metal-foam Photocatalyst
a b s t r a c t A recycling open channel reactor was utilized for removal of organic contaminant of malachite green with flow rate of 5 mL/min. TiO2 nanoparticles and FeSO4 powder immobilized on metal-foam (MF) were applied as a photo-Fenton catalyst. Glucose oxidase (GOx) was also coupled with catalyst for in-situ generation of hydrogen peroxide. Furthermore, UV (6 W) lamp was used as a light source to carried out photo-bio-Fenton reaction in the reactor. Several experiments were carried out to study the influence of various combination of affecting processes on decolorization of the pollutant. Removal of 84.37% of malachite green was obtained in GOx/TiO2 /Fe2+ /MF process under UVA-LED during 20 min. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Many kinds of commercial dyes are available worldwide and about 7 × 105 tons of dyestuff are produced annually. Dyes and dye intermediates are destructive organic compounds for the environment and are very harmful for human health. High percentages of dyes are not biodegradable and have disadvantages like carcinogenic and toxic properties. These pollutants are high solubility in water. Entering dyes in water causes blocking of light path in visible region, which results major troubles for aquatic life [1-3]. The most important methods that are used for the removal of dyes are advanced oxidation processes (AOPs) and biological processes. Production of hydroxyl radical intermediates with strong oxidizing ability is the main characteristic of AOPs. These processes are used for both organic and inorganic components including dye intermediates and azo dyes where biological treatment is difficult [1,3,4]. Due to the existence of toxic compounds and their resistance against decomposition and biological degradation in pollutant mixtures and also high cost of AOPs, none of these two methods (biological and AOPs) can be used independently for complete removal of organic contaminants. To overcome this problem, combination of biological processes and AOPs is necessary for treatment of industrial recalcitrant wastewaters. In this method, firstly removal of biodegradable pollutants is carried out and then lowering of non-toxic contaminants implemented [5]. ∗
Corresponding author. Fax: +98 21 86704606. E-mail address: [email protected] (A. Karimi).
Enzyme-based catalysts act at neutral pH and ambient temperature, and also increase the rate of chemical and biological reactions. One of the most important characteristics of enzymes that is necessary for their catalytic action, is their 3-dimensional structure [6,7]. Working with enzymes has some problems like their low stability and high costs. To overcome these difficulties, immobilizing of enzymes could be a good procedure [8]. Glucose oxidase (GOx) is a commercial catalyst that is a dimmer with molecular weight of 186 × 103 g/mol and contains two flavine adenine dinucleotide (FAD) attached to each dimmer [9,10]. GOx has many applications in pharmaceutical, textile, food industry, construction of biosensors [7,9]. This enzyme can be used for degradation of azo-dye pollutants. Other important methods such as photocatalytic techniques, Fenton, ozonation, sonolysis and hybrid methods such as sonoFenton, sono-photocatalysis, photo-Fenton, UV/ozone, UV/H2 O2 etc. can be utilized for AOP aim [11-14]. Each of these methods work in an almost different path for azo-dye decontamination and combination of them may improve decontamination process like combination of sonolysis and Fenton for sono-Fenton or degrade it such as combination of sono-Fenton and UV or be neutral. Fenton process is a type of AOPs which is used for wastewater treatment. Because of generation of strong hydroxyl radicals (• OH), it is an oxidative process and is used to remove stubborn organic pollutants. Reaction of iron catalyst and hydrogen peroxide is carried out to generate 2.8 V oxidation potential of • OH radicals. Fenton reaction is performed in both homogeneous and heterogeneous phases. Chemical reactions take place in homogeneous phase and
http://dx.doi.org/10.1016/j.jtice.2016.11.022 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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nature of reactants is very important. But in heterogeneous phase, some physical reactions take place on active sites of catalyst together with chemical reactions. Active sites of catalyst in Fenton homogeneous process are Fe2+ or Fe3+ ions while in heterogeneous process they are iron ions in hybrid forms of compounds such as (Fe(OH)2 )+ ; (Fe(H2 O)2+ ; (Fe(H2 O)6 )3+ [15-20]. Progress in nanotechnology increased utilization of nanoparticles and transformed this science to one of the most important technologies for removal of waste industry in 1990s. It is well known that degradation of pollutants and dyes using nano scale catalysts is more efficient compared to micro scaled catalyst particles and utilization of nano scale catalyst particles is more favorable [21,22]. TiO2 particles have properties such as nontoxicity, high yield relative to other photo-catalysts and low cost which makes them a suitable photo-catalysts for AOPs [23]. Anatase, rutile, brookite and TiO2 (B) are four famous types of TiO2 in nature and a mixture of 80% anatase and 20% routile is proved to be the best blend to achieve highest activity [24]. Dispersed TiO2 nanoparticles have been used in many wastewater treatment processes, but they would have many problems for environment. To solve their harmful effects, immobilization of TiO2 particles on suitable carriers are suggested [21,23]. Metal foams with porosity of more than 40% could be used for this purpose. Low weight and exclusive characteristics such as high capacity for energy absorption, high thermal conductivity, and many others are the most important properties of such foams. Size, shape, volume percentage and surface area of pores are the main factors for choosing a metal-foam as carrier [25]. In present study, a porous carrier containing nanotopography pores with silver coating on the carrier was considered. TiO2 particles and GOx enzyme, for in situ generation of H2 O2 , were stabilized on the support. UV lamp was used as light source in an open channel photo-reactor. Also in this research, Fenton process was implemented to investigate the effect of Fe2+ ions on decolorization of malachite green.
lated as 0.023 g/g based on dry-weight of metal-foam with 0.238 g of TiO2 immobilized nanoparticles. 2.3. Immobilization of GOx on TiO2 /AgO metal-foam There are different methods such as physical adsorption, covalent bond and entrapment for immobilization of enzymes [5,26]. Simplicity, low cost and minimum denaturation of enzymes are the most important parameters for choosing suitable procedure [27]. In this research, physical adsorption of enzyme on TiO2 /MF carrier was used. Phosphate buffer (PBS, 0.1 M) was used to prepare GOx solution (0.1 mg/mL) [28]. After emerging the carrier in prepared enzyme solution they were incubated at 160 rpm for 1 h, pH value of 6.8, and temperature of 35 °C. Then carrier was separated from the solution and washed twice using fresh phosphate buffer. The amount of immobilized GOx on TiO2 /MF was measured by free enzyme in the supernatant using calibration curve and UV–vis spectrophotometer (S20 0 0 UV–vis spectrophotometer). In this research, 53 U/g of carrier was obtained for immobilized GOx on TiO2 /MF [29,30].
2. Material and methods
2.3.1. Decolorization process using Fenton photo-bioreactor A recycling open channel reactor (Fig. 1) was used for decolorization of 20 mg/L MG pollutant with 1 mM of aqueous solution of Fe2+ [31]. Total volume of dye solution was 70 mL and 20 mL of solution was circulating between inlet and outlet of the reservoir. The prepared photo-biocatalyst was placed at bottom and the UV lamp (6 W) was fixed on top of the reactor (3 cm above the solution). Dye solution was poured into the inlet vessel and the control valve was opened in order to fill the reactor by the solution above the photo-biocatalyst. Then, it was flowed to the outlet vessel and was circulated with peristaltic pump with flow rate of 5 mL/min. During the process, samples were taken at specified sampling times and degradation of pollutants was determined. Concentration of dye was obtained from calibration curve and UV– vis spectrophotometer. Decolorization of MG (at wavelength of λmax = 617 nm) was obtained using Eq. (1):
2.1. Material
Decolorization efficiency (% ) =
A silver metal-foam coated with AgO layer was prepared from metal nano-coating company located at Science and Technology Park in Tehran, Iran. TiO2 nanoparticles with normal crystal size of 21 nm (Degussa P25 consisting of about 80% anatase and 20% rutile), Glucose oxidase type II (GOx, EC 1.1.3.4, obtained from Sigma-Aldrich), β -d-glucose, KI (99.99%), NaOH (99.99%), ammonium molybdate tetrahydrate (Mo(Vl).4H2 O), potassium hydrogen phthalate (KHP), FeSO4 , Hydrogen peroxide (30% (v/v), Merck), Malachite green oxalate (MG, C.I. No. 42,0 0 0, C52 H54 N4 O12 , Mw = 927.01 g/mol, λmax = 617 nm) were obtained from Sigma Aldrich.
where C0 and Ct are concentrations of MG solution (mg/L) at times 0 and t, respectively.
2.2. Immobilization of TiO2 nanoparticles on AgO metal-foam A suspension of 0.45 g of TiO2 nanoparticles in 40 mL of methanol was prepared. To prevent pouring of the solution, a thin layer of paraffin was pulled on container of suspended solution and was sonicated for about 15–20 min by Sonoplus Ultrasonic Homogenizer HD 2200 (Germany). To set the pH at about 5, HCl (1 N) was added. After that, prepared suspension was poured on heated MF at about 80 °C. MF and the solution above it, were heated for 5 h. Coated MF was washed with distillated water in order to remove free TiO2 nanoparticles from the carrier. The mass of TiO2 immobilized on MF was calculated from difference between MF weight before and after immobilization. The ratio of TiO2 /MF was calcu-
C − C o t Co
× 100
(1)
3. Results and discussion 3.1. Characterization of photocatalyst and mechanism of Fenton photo-bioreactions Fig. 2(a) and (b) show optical images of metal-foam before and after immobilization of TiO2 nanoparticles. It is clear from color changes of MF that TiO2 powders have coated the metal-foam. Also, Fig. 2(c) and (d) demonstrate surface morphology of MF after immobilization of TiO2 nanoparticles and glucose oxidase enzyme immobilization, respectively. Presence of nano-pores on MF surface and similarity of nanoparticles with carrier in size are the main reasons for adequate immobilization of TiO2 nanoparticles on it. Fig. 2(e) and (f), show nano-pores on MF carrier before and after TiO2 immobilization. Glucose oxidase is one of the well-known enzymes that accelerates electron transfer from substrate (glucose) to oxygen for production of hydrogen peroxide [32]. GOx
C6 H12 O6 + H2 O + O2 → C6 H12 O7 + H2 O2
(2)
Then, ferrous ions react with hydrogen peroxide and produce ferric ions and hydroxyl radicals according to Eq. (3) [16]:
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Fig. 1. Schematic diagram of continuously circulated open-channel photo fenton bio-reactor.
Fe2+ + H2 O2 → Fe3+ +OH• +OH−
(3)
UV-A lamp with λ < 390 nm and energy band of 3.2 eV, was glinted to TiO2 semiconductor photocatalyst to create holes with positive charges and free electrons as follows [33]:
TiO2 + hν → e− cb + h+ ν b
(4)
According to the above reactions, electron–hole pairs are created on catalyst surface and h+ vb reacts with H2 O and OH− as below [34,35]:
h+ ν b + H2 O → OH• + H+
(5)
h+ ν b + OH− → OH•
(6)
Other important reactions for production of hydroxyl radicals that are carried out in this process are shown in the following equations [15,36]:
Fe2+ + H2 O2 + H+ → Fe3+ + OH• + H2 O
(7)
Fe3+ +H2 O +UV → OH• +Fe2+ + H+
(8)
H2 O2 + • O2(ads) − → OH• +OH− +O2
(9)
H2 O2 +UV → OH• + OH−
(10)
In enzymatic reaction, H2 O2 is generated and in Fenton and photolysis reactions it is consumed to produce hydroxyl radicals. Thus it can be claimed that enzymatic and photocatalytic reactions stimulate Fenton process. Another noteworthy point is that no additional hydrogen peroxide was added manually in this process. In AOPs, GOx enzyme plays several roles and flavine adenine dinucleotide (FAD) structure is the most important reason for enzyme activity and it’s characteristics. One of the roles is in-situ production of H2 O2 and another role is inhibiting recombination of charges in the process [37]. 3.2. Photo-catalytic process coupled with bio-Fenton for decolorization of MG In this study, several methods were investigated at different conditions. Firstly, adsorption equilibrium of dye on coated carrier was considered. Concentration changes of fresh solution of dye
were monitored and then the amount of dye adsorption on carrier was determined. In first run, adsorption of dye solution (mg/L) on the carrier was reached to 36% after 2 h. But in second run, it was reduced to 19% and after that no adsorption of dye was detected and the saturated carrier was used to study more decolorization tests. Diagram of adsorption profiles in different runs on carrier is shown in Fig. 3 [30]. Processes that were studied in this work include: (a) Fenton process, (b) enzymatic process, (c) bio-Fenton without UV light, (d) TiO2 /UV, (e) TiO2 /GOx/UV without glucose, (f) TiO2 /GOx/UV + glucose, (g) TiO2 /GOx/glucose/UV/Fe2+ , and (h) TiO2 /UV/Fe2+ . Fig. 4 shows decolorization of MG for different processes. As it is clear, maximum decolorization efficiency was achieved under TiO2 /GOx/UV/Fe2+ process containing glucose as substrate. For comparison, their kinetic constants are shown in Table 1. The most important experimental results from Fig. 4 and Table 1 are as follows: 1. Decolorization of pollutant in TiO2 + Fe2+ and TiO2 + GOx processes reflect the adsorption properties of TiO2 and they are similar to a certain extent. 2. Decolorization amount of pollutant in TiO2 + GOx+Fe2+ and TiO2 + Fe+ +UV processes are partially similar. 3. TiO2 + Fe2+ +UV and TiO2 + GOx+Fe2+ processes have positive synergic with each other. The effect was observed in combined TiO2 + Fe2+ + GOx + UV processes. Fig. 5 shows the differences between efficiency of two processes in presence and absence of glucose. After 20 min, about 15% of difference between efficiency of two processes was observed. The reason for this phenomenon is formation of hydrogen peroxide in presence of substrate, which leads to formation of active hydrogen peroxide radicals. According to obtained results for all processes, the curve of decolorization rate has a steep slope till 20 min and high amount of decontamination was obtained within this short time period. Between 20 to 60 minutes after start of reaction, slope of the curve was decreased and after 1 h it reached to zero. In presence of all of decolorization factors, 84.37% of decolorization was obtained at t = 20 min. 3.2.1. Effect of pH pH of the solution is one of the important factors in AOPs. Because of multi-functional roles of pH on dye decontamination, interpretation of its role on decolorization efficiency is complex. In many researches, optimum pH of Fenton process is reported at
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Fig. 2. Image of metal-foam (MF): (a) before and (b) after immobilization of TiO2 nanoparticles, (c) SEM image after immobilization of TiO2 , and (d) after immobilization of enzyme on TiO2 /MF, (e) image of nano particles on MF before TiO2 immobilization, and (f) after TiO2 immobilization.
Table 1 Summary of experimental results of MG decolorization under UV light with [MG]0 = 20 mg/L; inlet flow rate = 5 mL/min. MG decolorization process
Decolorization (%) 20 min
60 min
TiO2 +UV TiO2 +Fe TiO2 + Fe+UV TiO2 + GOx TiO2 + GOx+Fe TiO2 + GOx+UV TiO2 + Fe + GOx + UV
42.17 49.27 55.07 50.03 58.55 66.09 84.36
59.15 55.15 64.67 54.64 63.54 71.73 89.85
Decolorization rate constant (k, min−1 )
2.8 × 3.3 × 3.9 × 3.4 × 4.5 × 5.3 × 8.9 ×
10 − 2 10 − 2 10 − 2 10 − 2 10 − 2 10 − 2 10 − 2
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Fig. 3. MG adsorption on Fenton photo-biocatalyst.
Fig. 5. MG decolorization efficiency in TiO2 /UV/Fe/GOx and TiO2 /Fe/UV/GOx (without glucose); [MG] = 20 mg/L, T = 25°C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
acidic pH, positive holes operate as the important oxidation agents while in neutral and alkaline pH, hydroxyl radicals are dominant produced agents which is because of more oxidation of hydroxide ions on TiO2 surface. Thus, Coulombic repulsion force with hydroxyl radicals will occur which is a deterrent factor to decontamination. 2) Ionization of TiO2 surface by H+ ions, leads to adsorption of dye molecules on its surface which are shown by Eqs. (11) and (12). +
TiOH + H+ → TiOH2
(11)
TiOH + OH− → TiO− + H2 O
(12)
3) In acidic medium, TiO2 particles desire to agglomerate. Thus, surface area of photocatalyst for photon absorption and dye adsorption will be decreased [42]. In this study, pH was changed within the range of 0.5–5. Other fixed conditions of current study were dye solution of 20 mg/L, Fe2+ concentration of 1 mM, glucose concentration of 20 mM, and enzyme activity of 120 U/mL. According to Fig. 6, decontamination had the maximum efficiency at pH = 4.5. Since isoelectric point of GOx enzyme is about 4.2, it has negative surface charge at optimum pH [43]. By increasing pH greater than 4.5, decolorization efficiency was reduced. The reason may be the formation of Fe(OH)3 and also H2 O and O2 generation from decomposition of H2 O2 by iron [44]. Thus OH• will be decreased in the process and decolorization efficiency will be reduced.
Fig. 4. MG decolorization efficiency in different processes; [MG] = 20 mg/L, 25°C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
about 3, which means that acidic medium is more suitable for this process. It is related to stability of H2 O2 in acidic solution. Also, since surface charge of photocatalyst in pH < 7 is positive, adjusting pH in acidic region for UV/TiO2 process can be effective on dye decolorization. On the other hand, changing the pH factor can be effective on catalyst surface properties and can change adsorption of dye molecules on catalyst surface which is one of the important factors in photo-oxidation and decontamination [38-42]. Some reasons for positive effect of acidic pH on photocatalytic decontamination process are: 1) Formation of hydroxyl radicals by reaction between positive holes (h+ ) and hydroxide ions (OH − ). Indeed, in vb
3.2.2. Effect of Fe2+ ions Fe2+ ions were introduced to solution by addition of FeSO4 powder. Concentration of Fe2+ ions was changed in the range of 0.5–5 mM. As shown in Fig. 7, concentration of 1 mM Fe2+ had the highest decolorization efficiency. According to Eq. 13, excess amount of Fe2+ ions in the reaction has negative impact on decolorization because of self-consuming of • OH radicals by Fe2+ ions. Since this agent is the most important factor for decontamination, its lack reduces the decolorization [45]. Also excess amount of ferrous ions leads to generation of neutral iron salts in the solution which causes an increase in generation of detrimental and unnecessary solids in solution [39].
Fe2+ + OH• → Fe3+ + OH−
(13)
3.2.3. Effect of glucose concentration In enzymatic processes, glucose has the substrate role and is the activator agent of the reaction. Also it is one of the important
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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Fig. 8. Effect of glucose concentration on MG decolorization; [MG] = 20 mg/L, T = 25 °C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
Eq. (15). While hydroxyl radicals are the main factors of decolorization, their reduction causes reduction of decolorization rate [46-49]. Fig. 6. Effect of pH on decolorization process on GOx/TiO2 /MF carrier; [MG] = 20 mg/L, [Fe2+ ] = 1 mM, T = 25°C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
H2 O2 + OH• → HO•2 + H2 O
(14)
HO•2 + OH• → H2 O + O2
(15)
3.2.4. Effect of MG concentration Effect of initial dye concentration was investigated at different concentrations of MG (10, 20, 30 and 40 mg/L). By increasing dye concentration, efficiency of decolorization was decreased but the rate of decontamination was increased which can be attributed to the fast consumption of OH• radicals at high concentrations of pollutant and also existence of high amounts of dye molecules in the solution. Another reason would be decrease of photon permeation in the solution and subsequently reduction of hydroxyl radicals generation [36,50].
Fig. 7. Effect of Fe2+ ions concentration on MG decolorization; [MG] = 20 mg/L, T = 25 °C, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
species for H2 O2 production according to Eq. (2) and its concentration is highly effective on H2 O2 generation. The effect of change of concentration of glucose was studied whose results are demonstrated in Fig. 8. By increasing glucose concentration up to 20 mM, decolorization was increased and increasing concentration of glucose more than this value caused reduction of decolorization efficiency. From Eqs. 14 to 15, it can be concluded that excess amounts of hydroxyl radicals produce agents that could have negative effect on the process and would consume OH• radicals. Also excess amount of H2 O2 leads to generation of hydroperoxyl radicals which is a promoter for hydroxyl radicals consumption according to
3.2.5. Reusability of the photo-biocatalyst Reusability of Fenton photo-biocatalyst is very important for validating experiments. Presence of very small pores on metalfoam leads to protection of immobilized enzyme from UV lamp. Four other cycles were repeated after the first cycle in order to investigate the importance of this parameter on pollutant decontamination. A decreasing trend for decolorization efficiency was observed as shown in Fig. 9. Besides, to prove the accuracy of the results, each cycle of decolorization was repeated 3 times and the average results was presented as error bars on each cycle. In fifth cycle, decolorization was decreased to 51.56% after 20 min. One reason can be gradual washing of enzyme from carrier after each run so amount of enzyme for decolorization of new dye solution will be reduced. The other reason is fouling of the dye on carrier and its immobilized enzyme, so the reaction surface will be polluted and active sites for decolorization will be deactivated. 4. Conclusions The aim of this study was to survey decolorization efficiency of aqueous solutions containing Malachite Green (MG) within a
Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022
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Fig. 9. Reusability efficiency of GOx/TiO2 /MF cycles on MG decolorization process at 20 min (T = 25 °C); [MG] = 20 mg/L, TiO2 /MF = 0.022 g/g, and recycling rate = 5 mL/min.
photo-biocatalyst reactor coupled with Fenton process. A porous silver metal-foam with a thin-coated AgO layer on it was used as the support for MG decolorization. No erosion of MF and its reusability for many times, was the main reason for choosing this support. The other reason was silver antimicrobial property and strong disinfecting capability of AgO. At first, TiO2 photocatalyst was stabilized on MF and then glucose oxidase enzyme was properly immobilized on TiO2 /MF. Existence of pores on metal-foam with nanotopography creates a good binding with TiO2 nanoparticles and enzyme with nano-size particles. Several tests were conducted inside the open channel-recycling reactor for decolorization of MG as a sample model of azo dye. In presence of TiO2 , enzyme, Fe2+ ions and UV lamp, removal of 84.37% was achieved after 1 h from the start of the process. The fourth reason to choose metalfoam as carrier is that it is a suitable place for protecting enzyme from UV lamp. Also the effect of every reduction agent on pollutant including concentration changes of Fe2+ ions, glucose, MG, and changes on pH were investigated. By setting the values of all of the mentioned factors on their optimum values, the highest decolorization rate was obtained which indicates that lack or excess of each of them leads to generation of inappropriate interactions which ultimately reduces active sites for decolorization. Reusability of photo-biocatalyst was also investigated. After the fifth cycle, in 20 min period, decolorization percentage of MG was reduced to about 51.56%. Acknowledgment The authors thank to University of Tabriz for its collaboration in this research. References [1] Gokulakrishnan S, Parakh P, Prakash H. Degradation of Malachite green by potassium persulphate, its enhancement by 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane nickel(II) perchlorate complex, and removal of antibacterial activity. J Hazard Mater 2012;213-214:19–27. [2] Uma Banerjee S, Sharma YC. Equilibrium and kinetic studies for removal of malachite green from aqueous solution by a low cost activated carbon. J Ind Eng Chem 2013;19:1099–105. [3] Arslan-Alaton I, Tureli G, Olmez-Hanci T. Treatment of azo dye production wastewaters using photo-fenton-like advanced oxidation processes: optimization by response surface methodology. J Photochem Photobiol A: Chem 2009;202:142–53. [4] Huang YH, Tsai ST, Huang YF, Chen CY. Degradation of commercial azo dye reactive Black B in photo/ferrioxalate system. J Hazard Mater 2007;140:382–8.
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Please cite this article as: P. Abdi et al., Application of a hybrid enzymatic and photo-fenton process for investigation of azo dye decolorization on TiO2 /metal-foam catalyst, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.022