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Effect of Additives on Photocatalysis of Cr(VI)-Methyl Orange
Available online at www.sciencedirect.com
Procedia Environmental Sciences 18 (2013) 625 – 631
2013 International Symposium on Environmental Science and Technology (2013 ISEST)
Effect of additives on photocatalysis of Cr(VI)-methyl orange Yajun Wang*, Lijuan Jiang, Changgen Feng State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
Abstract Photocatalysis technology is an efficient chemical method for wastewater treatment containing organic and inorganic pollutants. Adding additives may affect the photocatalytic process. The photocatalytic processes of 50 mL mixed solution, 40 mg/L MO-20 mg/L Cr(VI), were proceeded under λı420, pH=1. When adding organics (such as isopropanol, methyl alcohol, alcohol, acetone), photocatalytic efficiency of MO was restrained, and that of Cr(VI) was promoted. Adding materials such as formaldehyde with both oxidizing and reducing effects could promote photocatalytic efficiency of MO and Cr(VI) at the same time. When H2O2 added into the composite system, the influence of photocatalysis is relatively complicated. Through controlled experiment, the results show that H2O2Cr(VI) could form Fenton reagent and speed up the photocatalytic degradation of MO. When inorganic cation (such as Na+, Cu2+, Fe3+) and inorganic anion (such as Cl-, NO2-, S2O32-) added into the composite system, there is no obvious rule. Experimental study show that adding cationics make the photocatalytic reduction of Cr(VI) inhibited, and adding anion which has the reducibility could promote the photocatalytic efficiency of Cr(VI). Since NO2- and S2O32- have simple chemical reactions with Cr(VI), photocatalytic degradation of MO under visible light also could proceed successfully. © 2013 The The Authors. Authors. Published Publishedby byElsevier ElsevierB.V. B.V. © 2013 Selection and and/or peer-review under responsibility of Beijing Institute of Technology. peer-review under responsibility of Beijing Institute of Technology. Keywords: photocatalysis; TiO2; MO; Cr(VI); synergistic effect; additive
1. Introduction Semiconductor photocatalysis is a new method for contaminant treatment which was developed in 1972, and it is a kind of catalytic reaction depending on light [1]. The catalytic activity of TiO2 originates from its electronic structure and photoelectric characteristics. The band theory can be used to explain the photocatalytic reaction principle [2]. At present, photocatalytic reaction kinetics is generally described by a classic model, Langmuir-Hinshelwood adsorption kinetic model [3]. Though TiO2 has many advantages, there are two bottlenecks limiting the application: one is the quick combination of electrons and holes,
* Corresponding author. Tel.: +86-10-68912941. E-mail address: [email protected].
1878-0296 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Beijing Institute of Technology. doi:10.1016/j.proenv.2013.04.086
626
Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
and the other is the minimum sun utilization rate. Nowadays many researchers tried to modify TiO2 in order to restrain combination of electrons and holes and increase the scope absorption of sunlight, such as rare earths modification [4-8], non-metallic modification [9], composite catalyst [10-11]. As for the research of photocatalytic application, it can deal with a lot of pollutants, such as dye, pesticide, heavy metal ion. Owing to that actual wastewater is almost mixture, composite system catalytic research have been paid much attention. Furthermore, according to photocatalytic principle, that photocatalytic oxidation and reduction reactions occur simultaneously can restrain combination of electrons and holes so as to improve catalytic activity. Composite pollutant system photocatalysis has been studied [12-14]. The present research is aimed to discuss the effect of additives on the visible-light simultaneous photocatalysis in methyl orange (MO) and Cr(VI) composite system by Ba-TiO2. The competence of photocatalytic removal for MO and Cr(VI), under visible light irradiation (λ≥420 nm) was employed to investigate the photocatalytic activity. 2. Experimental 2.1. Materials All chemicals were purchased from Sinopharm Chemical Reagent Co., China and employed without any further purification. Absolute ethyl alcohol (EtOH), barium nitrate (Ba(NO3)2), acetonum (C3H6O), diphenylcarbazide (C13H13N4O), potassium bichromate (K2Cr2O7), hydrochloric acid (HCl), perchloric acid (HClO4), phosphoric acid (H3PO4), sulfuric acid (H2SO4), NaCl, CuCl2, FeCl3, NaCl, NaNO2, Na2S2O3, CH3COOH, C2H2O4, C4H6O6, HCHO, CH3OH, iso-propyl alcohol, C3H6O and methyl orange (MO) are analytical grade reagents. Tetrabutyl titanate (Ti(OBu)4) is chemical grade. The water for experiment is deionized water. 2.2. Photocatalytic procedures Photocatalysis experiments were performed on an open photoreactor. The light source is provided by a PLS-SXE300UV Xe lamp (300 W) with emission of λ ı 420 nm, which is positioned above the photoreactor. The initial concentration (C0) of MO and Cr(VI) was fixed at 40 mg/L and 20 mg/L respectively both in an absolute system and in the composite system which were adjusted by HClO4 to pH ca. 1. Ba-TiO2 of 130 mg was suspended into a fresh MO-Cr(VI) mixed solution (50 mL) with additives. At given interval of illumination, a sample of suspension (ca. 1 mL) was taken out and filtered with microporous membrane (0.45 Pm). The absorbance of residual MO in solution was analyzed by an APL 752 UV-vis spectrometer at 510 nm. The degradation rates of MO were calculated as dMO. The absorbance of residual Cr(VI) in solution was determined by diphenyl carbazide colorimetric method at 540 nm [15]. The expression of reduction of Cr(VI) was dCr(VI). When the concentration was greater than the detection limit, the operation of dilution was necessary. For MO-Cr(VI) composite system, two absorbances were determined first according to MO and Cr(VI) concentration test methods, then calculated their respective concentration. 3. Results and discussion 3.1. Organic additives Adding additives can change the photocatalytic reaction rate in the photocatalytic reaction system, and even change the photocatalytic reaction thermodynamics and kinetics.
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Fig. 1 shows the effect of isopropanol on the photocatalysis in the composite system. The degradation of MO is suppressed while the reduction of Cr(VI) is promoted. Isopropanol could be used as a holetrapping agent, and play MO-competitive role, so the photocatalytic efficiency of MO is reduced; but for Cr(VI), adding isopropanol is equivalent to adding more hole-trapping agent so as to increase the photocatalytic reaction rate. For comparison, isopropanol was added in single system and the experimental results are shown in Fig. 2. The result is the same as that in Fig. 1. Several other organic materials also affect the photocatalytic process shown in Fig. 3. Only formaldehyde can promote photocatalysis of MO and Cr(VI) simultaneously. Formaldehyde has both oxidizing and reducing abilities, and could be either a hole scavenger or electron capture agent. So in the composite system, formaldehyde promoted the photocatalytic efficiency of MO and Cr(VI).
100
80
80
60
dCr(VI)/%
dMO/%
100
C=0 mol/L C=0.02 mol/L C=0.06 mol/L C=0.1 mol/L
40
C=0 mol/L C=0.02 mol/L C=0.06 mol/L C=0.1 mol/L
40
20
0
60
20
0
10
20
30
40
50
0
60
0
10
20
t/min
30
40
50
60
t/min
Fig. 1. Effect of isopropanol on MO degradation and Cr(VI) reduction in the composite system.
100
50
C=0 mol/L C=0.06 mol/L
C=0 mol/L C=0.06 mol/L
80
dCr(VI)/%
dMO/%
40
30
60
20
40
10
20
0
0
20
40
60
80
100
0
0
t/min
20
40
60
80
100
t/min
Fig. 2. Effect of isopropanol on MO degradation and Cr(VI) reduction in single system.
Fig. 4 shows the effect of organic acids with different relative molecular weight on the photocatalytic process in the composite system. The influence of acetic acid on the two substances photocatalytic composite system is small. Oxalic acid has a strong reducing ability and could be oxidized by Cr(VI) to
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CO2 and H2O, while Cr(VI) removed. At the same time, MO has been successfully got rid of by oxalic acid so as to speed up the photocatalytic rate in the composite system. Tartaric acid is an important additive and reducing agent, which could promote Cr(VI) oxidation, while MO oxidation was greatly depressed. Oxalic acid and tartaric acid are reductants, and could react chemically with Cr(VI), but their experimental results are not the same, which is maybe the reason that photocatalytic reaction process is complicated, and the reaction mechanisms are different.
100
100
80
no additive [Isopropanol]=0.06 mol/L [Formaldehyde]=0.06 mol/L [Methanol]=0.06 mol/L [Ethanol]=0.06 mol/L [Acetone]=0.06 mol/L
60
40
dCr(VI)/%
dMO/%
80
40
20
0
no additive [Isopropanol] 0.06 mol/L [Formaldehyde] 0.06 mol/L [Methanol] 0.06 mol/L [Ethanol] 0.06 mol/L [Acetone] 0.06 mol/L
60
20
0
10
20
30
40
50
60
0
0
10
20
30
t/min
40
50
60
t/min
100
100
80
80
dCr(VI)/%
dMO/%
Fig. 3. Effect of organic materials on MO degradation and Cr(VI) reduction in the composite system.
60
no additive Acetic acid Oxalic acid Tartaric acid
40
20
0
0
10
20
30
no additive Acetic acid Oxalic acid Tartaric acid
60
40
20
40
50
60
0
0
10
20
t/min
30
40
50
60
t/min
Fig. 4. Effect of organic acid on MO degradation and Cr(VI) reduction in the composite system.
3.2. Inorganic additives (1) H2O2. As shown in Fig. 5, adding H2O2 affects the photocatalytic reaction results. Liu [16] reported that Cr(VI)-H2O2 could produce Fenton system, resulting in highly oxidative hydroxyl radical (•OH) which accelerate the mineralization of MO. Reaction equations (1)-(8) gives a detailed reaction cycle in the process of photocatalytic degradation of Cr(VI)-H2O2 system. Cr(VI)+H2O2ėCr(III)+HO2噝+H+
(1)
Cr(III)+H2O2ėCr(IV)+噝OH+OH-
(2)
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Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
Cr(IV)+H2O2ėCr(V)+噝OH+OHCr(V)+H2O2ėCr(VI)+噝OH+OH HO2噝+Cr(VI)ėCr(III)+O2+H
(3) -
(4)
+
(5)
噝OH+R1N=NR2ėR3NH2+CO2+H2O+R4
(6)
噝OH+H2O2ėH2O+HO2噝
(7)
H2O2+HO2噝ė噝OH+H2O+O2
(8)
100
dMO/%
80
C=0 mol/L C=0.02 mol/L C=0.06 mol/L C=0.1 mol/L
60
40
20
0
0
10
20
30
40
50
60
t/min
Fig. 5. Effect of H2O2 on MO photocatalytic degradation in the composite system.
(2) Inorganic cations. It can be seen from Fig. 6 that inorganic cations play a more or less inhibited role for photocatalytic treatment of MO and Cr(VI) in the composite system. But there is no regular pattern.
100
100
80
80
dMO/%
dCr(VI)/%
no additive + Na 2+ Cu 3+ Fe
60
60
40
40
20
20
0
0
10
20
30
40
50
60
0
no additive + Na 2+ Cu 3+ Fe
0
10
20
t/min
Fig. 6. Effect of inorganic cations on MO degradation and Cr(VI) reduction in the composite system.
30
t/min
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Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
(3) Inorganic anions. Fig. 7 gives the effects of some inorganic anions on the photocatalytic reaction in the composite system. Results show that, NO2- promotes the removal of MO and Cr(VI) at the same time; S2O32- makes Cr(VI) disappeared before irradiation while MO could still be photocatalytic oxidized; Cl- has little effect for MO, while inhibition for Cr(VI).
100
80
80
no additive Cl NO2
60
S2O3
40
dCr(VI)/%
dMO/%
100
2-
no additive Cl NO2
40
20
20
0
60
0 0
10
20
30
40
50
60
0
10
20
30
t/min
40
50
60
t/min
Fig. 7. Effect of inorganic anions on MO degradation and Cr(VI) reduction in the composite system.
4. Conclusions Photocatalysis technology is an efficient method for treating organic and inorganic pollutants. In practice, there are all kinds of materials in the wastewater, so studying the photocatalytic reaction of composite system including additive effect is necessary. The authors adopt Cr(VI)-MO composite system as probe pollutant and discuss the effects of organic additives (isopropanol, methyl alcohol, alcohol, formaldehyde, acetone, acetic acid, oxalic acid, tartaric acid) and inorganic ones (H2O2, Na+, Cu2+, Fe3+, Cl-, NO2-, S2O32-) on the photocatalytic process. The experimental results could provide some useful information for the research of photocatalysis. References [1] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972; 238(5338): 37–38. [2] Didier R, Antoine P, Weber JV. First approach of the selective treatment of water by heterogeneous photocatalysis. Environmental Chemistry Letters 2004; 2(1): 5–8. [3] Konstantinou IK, Albanis TA. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review. Applied Catalysis B: Environmental 2004; 49: 1–14. [4] Ou XQ, Meng JP, Wang QM, Yu JM. Enhanced photoactivity of layered nanocomposite materials containing rare earths, titanium dioxide and clay. Journal of Rare Earths 2006; 24(1): 251–254. [5] Fan CM, Xue P, Sun YP. Preparation of nano-TiO2 doped with cerium and its photocatalytic activity. Journal of Rare Earths 2006; 24(3): 309–313. [6] de la Cruz Romero D, Torres GT, Are´valo JC, Gomez R, Aguilar-Elguezabal A. Synthesis and characterization of TiO2 doping with rare earths by sol-gel method: photocatalytic activity for phenol degradation. Journal of Sol-Gel Science and Technology 2010; 56: 219–226. [7] Xu JJ, Ao YH, Fu DG, Yuan CW. Study on photocatalytic performance and degradation kinetics of X-3B with lanthanidemodified titanium dioxide under solar and UV illumination. Journal of Hazardous Materials 2009; 164: 762–768. [8] Parida KM, Sahu N. Visible light induced photocatalytic activity of rare earth titania nanocomposites. Journal of Molecular Catalysis A: Chemical 2008; 287: 151–158.
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[9] Jiang JJ, Long MC, Wu DY, Cai WM. Preparation of F-modified nanosized TiO2 and its methyl orange photodegradation mechanism. Chinese Journal of Physical Chemistry 2011; 27(5): 1149–1156. [10] Rawal SB, Sung SD, Lee WI. Novel Ag3PO4/TiO2 composites for efficient decomposition of gaseous 2-propanol under visiblelight irradiation. Catalysis Communications 2012; 17: 131–135. [11] Yang Y, Guo YH, Hu CW, Jiang CJ, Wang EB. Synergistic effect of Keggin-type [Xn+W11O39](12-n)- and TiO2 in macroporous hybrid materials [Xn+W11O39](12-n)--TiO2 for the photocatalytic degradation of textile dyes. Journal of Materials Chemistry 2003; 13(7): 1686–1694. [12] Feng XH, Ding SM, Wu F, Deng NS, Nikolai B. Simultaneous photoreduction of Cr(VI) and photooxidation of methyl orange in water induced by Fe(III)-OH complex. Research of Environmental Science 2005; 18(4): 75–78. [13] Lei XF, Xue XX. Visible light-induced photocatalytic reduction of Cr(VI) in presence of methyl orange and titania-bearing blast furnace slag photocatalysts. Journal of Northeastern University (Natural Science) 2011; 32(8):1133–1136. [14] Papadam T, Xekoukoulotakis NP, Poulios I, Mantzavinos D. Photocatalytic transformation of acid orange 20 and Cr(VI) in aqueous TiO2 suspensions. Journal of Photochemistry and Photobiology A: Chemistry 2007; 186: 308–315. [15] Editorial Committee of Water and Wastewater Monitoring Method, State Environmental Protection Administration. Water and wastewater monitoring method. 4th Edition. Beijing: China Environmental Science Press, 2002, 346–349. [16] Liu ZH, Du J, Liu RL, Mu TM, Tao CY. Degradation of methyl orange solution with microwave-irradiated Cr(VI)-H2O2 system. Chinese Journal of Environmental Engineering 2007; 1(8): 35–39.
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Procedia Environmental Sciences 18 (2013) 625 – 631
2013 International Symposium on Environmental Science and Technology (2013 ISEST)
Effect of additives on photocatalysis of Cr(VI)-methyl orange Yajun Wang*, Lijuan Jiang, Changgen Feng State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
Abstract Photocatalysis technology is an efficient chemical method for wastewater treatment containing organic and inorganic pollutants. Adding additives may affect the photocatalytic process. The photocatalytic processes of 50 mL mixed solution, 40 mg/L MO-20 mg/L Cr(VI), were proceeded under λı420, pH=1. When adding organics (such as isopropanol, methyl alcohol, alcohol, acetone), photocatalytic efficiency of MO was restrained, and that of Cr(VI) was promoted. Adding materials such as formaldehyde with both oxidizing and reducing effects could promote photocatalytic efficiency of MO and Cr(VI) at the same time. When H2O2 added into the composite system, the influence of photocatalysis is relatively complicated. Through controlled experiment, the results show that H2O2Cr(VI) could form Fenton reagent and speed up the photocatalytic degradation of MO. When inorganic cation (such as Na+, Cu2+, Fe3+) and inorganic anion (such as Cl-, NO2-, S2O32-) added into the composite system, there is no obvious rule. Experimental study show that adding cationics make the photocatalytic reduction of Cr(VI) inhibited, and adding anion which has the reducibility could promote the photocatalytic efficiency of Cr(VI). Since NO2- and S2O32- have simple chemical reactions with Cr(VI), photocatalytic degradation of MO under visible light also could proceed successfully. © 2013 The The Authors. Authors. Published Publishedby byElsevier ElsevierB.V. B.V. © 2013 Selection and and/or peer-review under responsibility of Beijing Institute of Technology. peer-review under responsibility of Beijing Institute of Technology. Keywords: photocatalysis; TiO2; MO; Cr(VI); synergistic effect; additive
1. Introduction Semiconductor photocatalysis is a new method for contaminant treatment which was developed in 1972, and it is a kind of catalytic reaction depending on light [1]. The catalytic activity of TiO2 originates from its electronic structure and photoelectric characteristics. The band theory can be used to explain the photocatalytic reaction principle [2]. At present, photocatalytic reaction kinetics is generally described by a classic model, Langmuir-Hinshelwood adsorption kinetic model [3]. Though TiO2 has many advantages, there are two bottlenecks limiting the application: one is the quick combination of electrons and holes,
* Corresponding author. Tel.: +86-10-68912941. E-mail address: [email protected].
1878-0296 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Beijing Institute of Technology. doi:10.1016/j.proenv.2013.04.086
626
Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
and the other is the minimum sun utilization rate. Nowadays many researchers tried to modify TiO2 in order to restrain combination of electrons and holes and increase the scope absorption of sunlight, such as rare earths modification [4-8], non-metallic modification [9], composite catalyst [10-11]. As for the research of photocatalytic application, it can deal with a lot of pollutants, such as dye, pesticide, heavy metal ion. Owing to that actual wastewater is almost mixture, composite system catalytic research have been paid much attention. Furthermore, according to photocatalytic principle, that photocatalytic oxidation and reduction reactions occur simultaneously can restrain combination of electrons and holes so as to improve catalytic activity. Composite pollutant system photocatalysis has been studied [12-14]. The present research is aimed to discuss the effect of additives on the visible-light simultaneous photocatalysis in methyl orange (MO) and Cr(VI) composite system by Ba-TiO2. The competence of photocatalytic removal for MO and Cr(VI), under visible light irradiation (λ≥420 nm) was employed to investigate the photocatalytic activity. 2. Experimental 2.1. Materials All chemicals were purchased from Sinopharm Chemical Reagent Co., China and employed without any further purification. Absolute ethyl alcohol (EtOH), barium nitrate (Ba(NO3)2), acetonum (C3H6O), diphenylcarbazide (C13H13N4O), potassium bichromate (K2Cr2O7), hydrochloric acid (HCl), perchloric acid (HClO4), phosphoric acid (H3PO4), sulfuric acid (H2SO4), NaCl, CuCl2, FeCl3, NaCl, NaNO2, Na2S2O3, CH3COOH, C2H2O4, C4H6O6, HCHO, CH3OH, iso-propyl alcohol, C3H6O and methyl orange (MO) are analytical grade reagents. Tetrabutyl titanate (Ti(OBu)4) is chemical grade. The water for experiment is deionized water. 2.2. Photocatalytic procedures Photocatalysis experiments were performed on an open photoreactor. The light source is provided by a PLS-SXE300UV Xe lamp (300 W) with emission of λ ı 420 nm, which is positioned above the photoreactor. The initial concentration (C0) of MO and Cr(VI) was fixed at 40 mg/L and 20 mg/L respectively both in an absolute system and in the composite system which were adjusted by HClO4 to pH ca. 1. Ba-TiO2 of 130 mg was suspended into a fresh MO-Cr(VI) mixed solution (50 mL) with additives. At given interval of illumination, a sample of suspension (ca. 1 mL) was taken out and filtered with microporous membrane (0.45 Pm). The absorbance of residual MO in solution was analyzed by an APL 752 UV-vis spectrometer at 510 nm. The degradation rates of MO were calculated as dMO. The absorbance of residual Cr(VI) in solution was determined by diphenyl carbazide colorimetric method at 540 nm [15]. The expression of reduction of Cr(VI) was dCr(VI). When the concentration was greater than the detection limit, the operation of dilution was necessary. For MO-Cr(VI) composite system, two absorbances were determined first according to MO and Cr(VI) concentration test methods, then calculated their respective concentration. 3. Results and discussion 3.1. Organic additives Adding additives can change the photocatalytic reaction rate in the photocatalytic reaction system, and even change the photocatalytic reaction thermodynamics and kinetics.
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Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
Fig. 1 shows the effect of isopropanol on the photocatalysis in the composite system. The degradation of MO is suppressed while the reduction of Cr(VI) is promoted. Isopropanol could be used as a holetrapping agent, and play MO-competitive role, so the photocatalytic efficiency of MO is reduced; but for Cr(VI), adding isopropanol is equivalent to adding more hole-trapping agent so as to increase the photocatalytic reaction rate. For comparison, isopropanol was added in single system and the experimental results are shown in Fig. 2. The result is the same as that in Fig. 1. Several other organic materials also affect the photocatalytic process shown in Fig. 3. Only formaldehyde can promote photocatalysis of MO and Cr(VI) simultaneously. Formaldehyde has both oxidizing and reducing abilities, and could be either a hole scavenger or electron capture agent. So in the composite system, formaldehyde promoted the photocatalytic efficiency of MO and Cr(VI).
100
80
80
60
dCr(VI)/%
dMO/%
100
C=0 mol/L C=0.02 mol/L C=0.06 mol/L C=0.1 mol/L
40
C=0 mol/L C=0.02 mol/L C=0.06 mol/L C=0.1 mol/L
40
20
0
60
20
0
10
20
30
40
50
0
60
0
10
20
t/min
30
40
50
60
t/min
Fig. 1. Effect of isopropanol on MO degradation and Cr(VI) reduction in the composite system.
100
50
C=0 mol/L C=0.06 mol/L
C=0 mol/L C=0.06 mol/L
80
dCr(VI)/%
dMO/%
40
30
60
20
40
10
20
0
0
20
40
60
80
100
0
0
t/min
20
40
60
80
100
t/min
Fig. 2. Effect of isopropanol on MO degradation and Cr(VI) reduction in single system.
Fig. 4 shows the effect of organic acids with different relative molecular weight on the photocatalytic process in the composite system. The influence of acetic acid on the two substances photocatalytic composite system is small. Oxalic acid has a strong reducing ability and could be oxidized by Cr(VI) to
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Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
CO2 and H2O, while Cr(VI) removed. At the same time, MO has been successfully got rid of by oxalic acid so as to speed up the photocatalytic rate in the composite system. Tartaric acid is an important additive and reducing agent, which could promote Cr(VI) oxidation, while MO oxidation was greatly depressed. Oxalic acid and tartaric acid are reductants, and could react chemically with Cr(VI), but their experimental results are not the same, which is maybe the reason that photocatalytic reaction process is complicated, and the reaction mechanisms are different.
100
100
80
no additive [Isopropanol]=0.06 mol/L [Formaldehyde]=0.06 mol/L [Methanol]=0.06 mol/L [Ethanol]=0.06 mol/L [Acetone]=0.06 mol/L
60
40
dCr(VI)/%
dMO/%
80
40
20
0
no additive [Isopropanol] 0.06 mol/L [Formaldehyde] 0.06 mol/L [Methanol] 0.06 mol/L [Ethanol] 0.06 mol/L [Acetone] 0.06 mol/L
60
20
0
10
20
30
40
50
60
0
0
10
20
30
t/min
40
50
60
t/min
100
100
80
80
dCr(VI)/%
dMO/%
Fig. 3. Effect of organic materials on MO degradation and Cr(VI) reduction in the composite system.
60
no additive Acetic acid Oxalic acid Tartaric acid
40
20
0
0
10
20
30
no additive Acetic acid Oxalic acid Tartaric acid
60
40
20
40
50
60
0
0
10
20
t/min
30
40
50
60
t/min
Fig. 4. Effect of organic acid on MO degradation and Cr(VI) reduction in the composite system.
3.2. Inorganic additives (1) H2O2. As shown in Fig. 5, adding H2O2 affects the photocatalytic reaction results. Liu [16] reported that Cr(VI)-H2O2 could produce Fenton system, resulting in highly oxidative hydroxyl radical (•OH) which accelerate the mineralization of MO. Reaction equations (1)-(8) gives a detailed reaction cycle in the process of photocatalytic degradation of Cr(VI)-H2O2 system. Cr(VI)+H2O2ėCr(III)+HO2噝+H+
(1)
Cr(III)+H2O2ėCr(IV)+噝OH+OH-
(2)
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Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
Cr(IV)+H2O2ėCr(V)+噝OH+OHCr(V)+H2O2ėCr(VI)+噝OH+OH HO2噝+Cr(VI)ėCr(III)+O2+H
(3) -
(4)
+
(5)
噝OH+R1N=NR2ėR3NH2+CO2+H2O+R4
(6)
噝OH+H2O2ėH2O+HO2噝
(7)
H2O2+HO2噝ė噝OH+H2O+O2
(8)
100
dMO/%
80
C=0 mol/L C=0.02 mol/L C=0.06 mol/L C=0.1 mol/L
60
40
20
0
0
10
20
30
40
50
60
t/min
Fig. 5. Effect of H2O2 on MO photocatalytic degradation in the composite system.
(2) Inorganic cations. It can be seen from Fig. 6 that inorganic cations play a more or less inhibited role for photocatalytic treatment of MO and Cr(VI) in the composite system. But there is no regular pattern.
100
100
80
80
dMO/%
dCr(VI)/%
no additive + Na 2+ Cu 3+ Fe
60
60
40
40
20
20
0
0
10
20
30
40
50
60
0
no additive + Na 2+ Cu 3+ Fe
0
10
20
t/min
Fig. 6. Effect of inorganic cations on MO degradation and Cr(VI) reduction in the composite system.
30
t/min
40
50
60
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Yajun Wang et al. / Procedia Environmental Sciences 18 (2013) 625 – 631
(3) Inorganic anions. Fig. 7 gives the effects of some inorganic anions on the photocatalytic reaction in the composite system. Results show that, NO2- promotes the removal of MO and Cr(VI) at the same time; S2O32- makes Cr(VI) disappeared before irradiation while MO could still be photocatalytic oxidized; Cl- has little effect for MO, while inhibition for Cr(VI).
100
80
80
no additive Cl NO2
60
S2O3
40
dCr(VI)/%
dMO/%
100
2-
no additive Cl NO2
40
20
20
0
60
0 0
10
20
30
40
50
60
0
10
20
30
t/min
40
50
60
t/min
Fig. 7. Effect of inorganic anions on MO degradation and Cr(VI) reduction in the composite system.
4. Conclusions Photocatalysis technology is an efficient method for treating organic and inorganic pollutants. In practice, there are all kinds of materials in the wastewater, so studying the photocatalytic reaction of composite system including additive effect is necessary. The authors adopt Cr(VI)-MO composite system as probe pollutant and discuss the effects of organic additives (isopropanol, methyl alcohol, alcohol, formaldehyde, acetone, acetic acid, oxalic acid, tartaric acid) and inorganic ones (H2O2, Na+, Cu2+, Fe3+, Cl-, NO2-, S2O32-) on the photocatalytic process. The experimental results could provide some useful information for the research of photocatalysis. References [1] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972; 238(5338): 37–38. [2] Didier R, Antoine P, Weber JV. First approach of the selective treatment of water by heterogeneous photocatalysis. Environmental Chemistry Letters 2004; 2(1): 5–8. [3] Konstantinou IK, Albanis TA. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review. Applied Catalysis B: Environmental 2004; 49: 1–14. [4] Ou XQ, Meng JP, Wang QM, Yu JM. Enhanced photoactivity of layered nanocomposite materials containing rare earths, titanium dioxide and clay. Journal of Rare Earths 2006; 24(1): 251–254. [5] Fan CM, Xue P, Sun YP. Preparation of nano-TiO2 doped with cerium and its photocatalytic activity. Journal of Rare Earths 2006; 24(3): 309–313. [6] de la Cruz Romero D, Torres GT, Are´valo JC, Gomez R, Aguilar-Elguezabal A. Synthesis and characterization of TiO2 doping with rare earths by sol-gel method: photocatalytic activity for phenol degradation. Journal of Sol-Gel Science and Technology 2010; 56: 219–226. [7] Xu JJ, Ao YH, Fu DG, Yuan CW. Study on photocatalytic performance and degradation kinetics of X-3B with lanthanidemodified titanium dioxide under solar and UV illumination. Journal of Hazardous Materials 2009; 164: 762–768. [8] Parida KM, Sahu N. Visible light induced photocatalytic activity of rare earth titania nanocomposites. Journal of Molecular Catalysis A: Chemical 2008; 287: 151–158.
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