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peroxomonosulphate reagent
Applied Catalysis A: General 368 (2009) 35–39
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Kinetics of degradation of acid red 88 in the presence of Co2+-ion/ peroxomonosulphate reagent J. Madhavan a,b, P. Maruthamuthu a,*, S. Murugesan a,1, M. Ashokkumar b a b
Department of Energy, University of Madras, Guindy Campus, Chennai 600 025, India School of Chemistry, University of Melbourne, VIC-3010, Melbourne, Australia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 June 2009 Received in revised form 29 July 2009 Accepted 6 August 2009 Available online 13 August 2009
Visible light-assisted photo-Fenton-like oxidation of a mono-azo textile dye, acid red 88 (AR88), has been carried out in the presence of Co2+ as the catalyst and peroxomonosulphate (PMS) as the oxidant. Decolorization of AR88 in the Co2+/PMS system was observed to follow zero-order kinetics with respect to the dye. It was also found that the decolorization of AR88 increased as the concentration of cobalt ions was increased. The rate of abatement of the acid red 88 was found to be independent of the concentrations of dye and oxidant (PMS). Mineralization studies were also carried out by monitoring the reduction in total organic carbon (TOC) content during the course of the reaction and a suitable mechanism has been proposed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Acid red 88 Oxone Photomineralization Photo-Fenton oxidation Photocatalysis
1. Introduction Contamination of soil and groundwater, due to discharges of industrial wastewaters into the ecosystem, leads to a serious health and environmental problems. Recently, the advanced oxidation processes (AOPs) have attracted much attention in the area of wastewater treatment and other environmental applications [1–3]. Advanced oxidation technologies involve the generation of highly reactive radical species by several processes that include photocatalysis using semiconductors, photo-Fenton reaction (hn/Fe2+/H2O2), ozonation, and UV photolysis of oxidants such as hydrogen peroxide and persulphate [4–10]. Fenton’s reagent is a mixture of hydrogen peroxide and ferrous ions that produces OH radicals. However, it requires acidic pH (3.0) and is reported to be slow and of poor mineralization (less than 60%). In order to overcome the limitations of the Fenton’s reagent and to find suitable oxidation processes, several attempts were made and those modified Fenton reactions are called Fenton-like reactions. Recent studies by Fernandez et al. [11] showed that the use of Co2+ions in combination with peroxomonosulphate in homogeneous medium led to the generation of sulphate radicals which had greater
* Corresponding author. Tel.: +91 44 22301576; fax: +91 44 22352494. E-mail address: [email protected] (P. Maruthamuthu). 1 Present address: School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India. 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.08.008
efficiencies than and several operational advantages over the conventional Fenton’s reagent. It was also supported from the experimental results carried out to find the favorable metal(s) for the decomposition of potassium peroxomonosulphate (KHSO5) by Ag(I), Ce(III), Co(II), Fe(II), Fe(III), Mn(II), Ni(II), Ru(III) and V(III) [12]. The results showed that cobalt(II) was the best catalyst for the activation of peroxomonosulphate due to its oxidation potential (1.82 eV) being slightly higher than that of the couple Co3+/Co2+ (1.80 eV) which drives the reaction to Co3+ from the initial Co2+-ion upon PMS addition in solution [13]. The potential applicability of Co2+/PMS reagent either in homogeneous [13,14] or heterogeneous form [15,16] for the pollutant degradation is also reported recently. Some of the advantages of using Co2+/PMS reagent for the photocatalytic degradation organic pollutants in industrial wastewaters are: (a) Co2+/PMS reagent can be applied over a wide range of pH, which is evidenced from the Co2+/PMS catalyzed degradation of 2,4-dichlorophenol in the pH range 2.0–8.0 [17]. While Fenton’s reagent has failed to exhibit its catalytic activity when the pH exceeded 3.0, Co2+/PMS reagent showed higher efficiency at neutral pH, which is a significant advantage since the pH of most of the contaminated natural waters falls in the range of 6.0–8.0. (b) Co2+/PMS reagent shows higher mineralization of pollutants; >90% mineralization was reported for the mineralization of Orange II [13] and 2,4-dichlorophenol [17].
J. Madhavan et al. / Applied Catalysis A: General 368 (2009) 35–39
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Table 1 Chemical structure and absorption maximum of acid red 88. Dye
Chemical structure
(CAR88) vs. time plot suggested that the decolorization of AR88 follows zero-order kinetics with respect to the dye. lmax (nm) 0
Acid red 88 (C.I. 15620)
505
C ¼ k0 t þ C 0
(1)
where C0 and C are the initial dye concentration and the concentration of the dye at any time ‘t’ respectively. In this paper, the term ‘C’ is represented as CAR88. k0 0 is the pseudo zero-order rate constant which can be calculated from the CAR88 vs. time plot. 3. Results and discussion (c) Co2+/PMS reagent is cost-effective as it avoids the costs due to post-neutralization and sludge treatment and it is operative at low metal concentrations. In the present investigation the kinetics of decolorization of acid red 88 in the presence of Co2+/PMS reagent is dealt with in detail. The factors influencing the mineralization of AR88 are also studied. 2. Experimental 2.1. Materials Acid red 88, C20H13N2O4SNa (Table 1), a textile dye that absorbs in the visible region (lmax = 506 nm), was a gift from Atul Ltd, India. Co2+-ion solution was prepared from analytical grade samples of CoSO47H2O (E-Merck). Oxone, the commercial name of potassium peroxomonosulphate, is a triple salt with the composition 2KHSO5KHSO4K2SO4 from Janssen Chimica (Belgium); it was used as received. When preparing the molar solutions, we took the fact that 1 mol of oxone gives 2 mol of peroxomonosulphate as shown by its chemical formula. Sodium nitrite (E-Merck, India) was used as the quenching agent for both sulphate and hydroxyl radicals. Unless otherwise specified, all reagents were of analytical grade and the solutions were prepared using double distilled water. 2.2. Dark decolorization studies
3.1. Dark decolorization studies The following preliminary experiments were performed to check the feasibility of the catalytic degradation of the dye using Co2+/PMS reagent, (i) AR88 + Co2+, (ii) AR88 + PMS, (iii) AR88 + Co2+ + PMS and the results are shown in Fig. 1. It was clearly seen that no appreciable dye decolorization was observed when either Co2+-ion or PMS was added alone whereas a faster and nearly complete decolorization was obtained within 8 min when Co2+-ion in conjunction with PMS was used. That is, the decolorization of the dye was observed only after the conjunction of Co2+-ion with PMS, which clearly proved the efficiency of the Co2+/PMS reagent taken in the present investigation. It was also observed that there is no change in the absorption spectrum of the dye during the addition of Co2+-ion into the dye solution which ruled out the possibility of any complex formation between the Co2+-ion with AR88. 3.1.1. Effect of concentration of the dye Experiments were carried out with various concentrations of the dye, CAR88 = (3–9) 105 M keeping the other experimental parameters, viz., CPMS = 5 mM and C CO2þ ¼ 0:025 mM as constants. Fig. 2 (inset) shows the linear decrease in the concentration of acid red 88 with time for different initial concentrations of acid red 88 used. The obtained dark decolorization rate constant (k0 0) values for the variations in CAR88 are also presented in Fig. 2. As expected
Since the decolorization under light irradiation is very fast, we could not measure it in the present investigation. The decolorization of the dye in the dark was analyzed using the ‘kinetics mode’ available in the Shimadzu UV–vis spectrophotometer (UV 1601). All dark experiments were carried out in a quartz cuvette (5 ml capacity) provided along with the spectrophotometer. The desired concentrations of all the solutions were made up for 3 ml and the order followed in mixing the additives was as follows: dye, cobalt(II)-ion and oxone. 2.3. Photocatalytic reactor and light source for photomineralization studies A 100 ml capacity borosilicate glass with an opening at the top to facilitate periodical removal of the sample was used as the photochemical reactor. Light irradiation was carried out by means of a solar box designed in our laboratory which consists of three 250 W tungsten–halogen lamps (Philips, India) inside a rectangular box fitted with two exhaust fans to considerably lower the temperature within the solar box during irradiation. The short wavelength radiations (l < 310 nm) were removed by the walls of the reaction vessel. 2.4. Kinetic studies The treatment efficiency was measured from the plot of concentration of the dye (which is calculated from the absorbance) vs. time. The straight-line obtained in the concentration of the dye
Fig. 1. Preliminary experiments for the decolorization of acid red 88 in the dark using Co2+/PMS reagent. (*) — AR88 + Co2+-ion; (~)— AR88 + PMS; ()— AR88 + Co2+-ion + PMS; CAR88 = 5 105M; C CO2þ ¼ 0:025 mM; CPMS = 5 mM.
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and [PMS] = 5 mM as constants. The respective k0 0 vs. [Co2+] plot for all variations in [Co2+] is presented in Fig. 3 and the plot of concentration of AR88 with time is also shown in the inset. It should be noted that the rate of the decolorization proportionately increases with increase in concentration of Co2+. This linear dependence of k0 0 with respect to [Co2+] may be due to the increased amount of formation of the active radical species like SO4 and OH with increase in the concentration of Co2+-ion. In the presence of metal-ions, PMS is known to generate SO5 and SO4 in aqueous solution in addition to HO by radical chain reactions [18,19].
Fig. 2. Plot of the pseudo zero-order rate constant for the decolorization of acid red 88 in the dark as a function of CAR88. The insert shows the bleaching of AR88 solutions for different initial dye concentrations. C CO2þ ¼ 0:025 mM; CPMS = 5 mM.
for the zero-order kinetics, the decolorization rate remained constant for the entire range of dye concentrations employed in our study inferring that the limiting step is not the reaction of the dye with the radicals in solution but the generation of radicals themselves in the solution. 3.1.2. Effect of concentration of Co2+ The effect of cobalt(II) ion concentration on the decolorization of acid red 88 was studied by varying the concentration of Co2+, [Co2+], from 0.01 to 0.05 mM and maintaining [AR88] = 5 105 M
3.1.3. Effect of concentration of PMS In order to study the effect of concentration of peroxomonosulphate on the decolorization rate of acid red 88, CPMS was varied from 0.5 to 5 mM while maintaining other experimental parameters constant ([AR88] = 5 105 M and C CO2þ ¼ 0:025 mM). The straight-line relationship was found while CAR88 was plotted against time as shown in Fig. 4 (inset) which confirms that this reaction obeys the relation given in Eq. (1). The corresponding pseudo zero-order decolorization rate constant (k0 o) values were presented as k0 o vs. CPMS plot in Fig. 4. It was noted that k0 o remained constant irrespective of the concentration of PMS, proving that the excess CPMS has no influence on the decolorization rate of acid red 88. That is, the decolorization rate is zero-order with respect to CPMS. However, a higher concentration of PMS was used in our study since higher CPMS is beneficial to achieve higher mineralization of the dye [13]. 3.1.4. Kinetics of decolorization of acid red 88 using Co2+/PMS reagent In order to explain the observed kinetic results, we considered the following fundamental reactions that were reported earlier [19–23] for the cobalt-catalyzed decomposition of peroxomonosulphate. Co2þ þ H2 O @ CoOHþ þ Hþ þ
CoOH þ HSO5 ! CoO þ H2 O þ SO4 CoOþ þ 2Hþ @ Co3þ þ H2 O
Fig. 3. Plot of the pseudo zero-order rate constant for the decolorization of acid red 88 in the dark as a function of C CO2þ . The insert shows the bleaching of AR88 solutions for different Co2+-ion concentrations. CAR88 = 5 105 M; CPMS = 5 mM.
K1
þ
K3
(2)
k2
(3) (4)
Fig. 4. Plot of the pseudo zero-order rate constant for the decolorization of acid red 88 in the dark as a function of CPMS. The insert shows the bleaching of AR88 solutions at different PMS concentrations. CAR88 = 5 105 M; C CO2þ ¼ 0:025 mM.
J. Madhavan et al. / Applied Catalysis A: General 368 (2009) 35–39
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SO4 þ Co2þ ! SO4 2 þ Co3þ
k4
(5)
Dye þ SO4 ! Dyeþ þ SO4 2
k5
(6)
Sulphate radical anions are stronger oxidants than hydroxyl radicals, especially at higher pH [24], and are the primary oxidizing species involved in the Co2+-ion mediated decomposition of PMS. Fernandez et al. [13] applied the steady state approximation and derived the rate law from the above basic reactions involved in the degradation of the dye using Co2+/PMS reagent. d½SO4 =dt ¼ k2 :K 1 ½Co2þ ½HSO5 =½Hþ k4 ½Co2þ ½SO4 k5 ½Dye½SO4 ¼ 0
(7)
d½Dye=dt ¼ kCo ½Co2þ
(8)
where kCo = k5 k2.K1[HSO5][Dye]/{[H+](k4[Co2+] + k5[Dye])}. At a defined concentration of Co2+, the zero-order decolorization in Fig. 3 inset can be written as d½Dye=dt ¼ k0 o
(9)
3.1.5. Photomineralization studies Similar to dark decolorization experiments, mineralization studies performed under visible light irradiation also proved that the minerlization process is very significant only when Co2+-ion is coupled with PMS [Fig. 5]. That is, no appreciable photomineralization takes place (< 5%) under visible light irradiation for 1 h with Co2+-ions alone and similar results were also observed in the presence of PMS alone. However, the combination of Co2+-ion with PMS showed a rapid increase in photomineralization efficiency (cal. 66% in 30 min), suggesting that the chain radical reaction operates only when the PMS in conjunction with Co2+-ion was used. Similar results were already reported for the degradation of Orange II and 2,4-dichlorophenol [13,17]. The effects of variation of solution parameters on the TOC removal of AR88 are discussed in the following sections. 3.1.5.1. Effect of CAR88. Fig. 6 shows the percentage of TOC removal for the variations in CAR88 ((3–9) 105 M) while maintaining constant concentrations of Co2+-ion (0.025 mM) and PMS (5 mM). For example, in the case of CAR88 = 5 105 M, about 66.3% TOC removal was obtained in 30 min; however, it reduced to 27.9%
Fig. 5. Plot of the normalized TOC values for the mineralization of acid red 88 under visible light irradiation in 30 min. CAR88 = 5 105 M; C CO2þ ¼ 0:025 mM; CPMS = 5 mM. (W) — AR88 + Co2+-ion; (&)— AR88 + PMS; (D) — AR88 + Co2+-ion + PMS.
Fig. 6. Percentage of TOC removal for different concentrations of acid red 88 in the presence of Co2+-ions (0.025 mM) and PMS (5 mM) under visible light irradiation for 30 min.
when CAR88 is increased to 9 105 M. The reason for the observed decrease in the mineralization extent is that, though the concentration of the dye is increased, the amount of reactive radical species such as HO/SO4 produced by the conjunction of Co2+-ion with PMS remained the same. 3.1.5.2. Effect of C CO2þ . The plot of percentage of TOC removal obtained by varying C CO2þ (0.005–0.075 mM) but with constant CAR88 (5 105 M) and CPMS (5 mM) is presented in Fig. 7. It is evident from Fig. 7 that the percentage of TOC removal is increased when the C CO2þ is increased. About 70% increment in the TOC removal efficiency is seen when the C CO2þ is increased from 0.005 to 0.075 mM. Similar results were also obtained in the dark
Fig. 7. Percentage of TOC removal from acid red 88 (5 105 M) solution as a function of Co2+-ion concentration in the presence of CPMS (5 mM) under visible light irradiation for 30 min.
J. Madhavan et al. / Applied Catalysis A: General 368 (2009) 35–39
39
and the peroxides formed were decomposed under light irradiation generating two radicals in each case, leading to a new radical chain: ROOH þ hn ! RO þ OH
(18)
Under light irradiation, the additional generation of OH radicals improves the kinetics of dye mineralization, as reported earlier for Fenton systems [28–31]. 4. Conclusions
Fig. 8. Percentage of TOC removal from acid red 88 (5 105 M) solution as a function of PMS concentration in the presence of C CO2þ (0.025 mM) under visible light irradiation in 30 min.
Visible light-assisted photo-Fenton-like oxidation of a monoazo textile dye, acid red 88 (AR88), has been studied in the presence of Co2+ as the catalyst and peroxomonosulphate as the oxidant. A zero-order decolorization kinetics was observed for AR88 in the presence of Co2+/PMS reagent. Enhancement in the decolorization of AR88 was observed on increasing the concentration of cobalt ions due to the increased amount of formation of the active radical species like SO4 and OH. The concentration of PMS was found to have no influence on the decolorization rate of acid red 88 for the abatement of the azo dye. TOC studies showed that the Co2+/PMS reagent is very effective to achieve the mineralization of AR88; hence, the Co2+/PMS system is proved to be a suitable reagent for applications in the field of environmental remediation. Acknowledgements
decolorization experiments. The reason for the observed trend may be the increased generation of HO and SO4 radical species on increasing the concentration of Co2+ -ions. 3.1.5.3. Effect of CPMS. The effect of CPMS on the percentage of TOC removal was obtained by varying the concentration of PMS from 0.5 to 5 mM while keeping the CAR88 (5 105 M) and C CO2þ (0.025 mM) constant [Fig. 8]. The percentage of TOC removal efficiency increased up to CPMS = 3.75 mM and a further increase led to a decrease in TOC values. The reason for the increase in the TOC values up to 3.75 mM was the increased generation of HO and SO4 radicals; however, on further increasing the CPMS, the scavenging reaction between the active radical species, OH/SO4 and SO5 (Eq. (10)) predominates, thereby leading to the reduction of TOC removal efficiency. Similar results were already reported for the degradation of Orange II, when a higher concentration of PMS was employed [13]. HSO5 þ OHorSO4 ! SO5 þ OH orSO4 2 þ Hþ
(10)
3.1.5.4. Mechanism of photomineralization studies. The detailed reactions involving the photocatalyzed mineralization of dyes in the presence of Co2+/PMS reagent are presented below: The presence of organic radicals as intermediates during a reaction is known to form peroxides (ROOH) through radical chain processes [25–27]. Dye þ hn ! Dye
(11)
Dye þ PMS ! Dyeþ þ ðHO þ SO4 2 orHO þ SO4 Þ
(12)
Degraded products þ Co2þ =HO =SO4 ! Radical intermediate ðR Þ (13) R þ PMS=O2 ! CO2 þ H2 O
(14)
R þ O2 ! ROO
(15)
ROO þ RH ! ROOH þ R
(16)
ROO þ ROO ! ROOR þ O2
(17)
The authors thank DST, New Delhi and DEST, Australia for the sanction of the India–Australian strategic research fund (INT/AUS/ P-1/07 dated 19 September 2007) for their collaborative research. References [1] J. Fernandez, J. Bandara, A. Lopez, P. Buffat, J. Kiwi, Langmuir 15 (1999) 185–192. [2] M.R. Dhananjeyan, E. Fine, J. Kiwi, J. Photochem. Photobiol. A: Chem. 136 (2000) 125–131. [3] V. Sarria, S. Parra, N. Adler, P. Peringer, N. Benitez, C. Pulgarin, Catal. Today 76 (2002) 301–315. [4] N. Daneshvar, H.A. Sorkhabi, A. Tizpar, Sep. Purif. Technol. 31 (2003) 153–162. [5] N. Daneshvar, M. Rabbani, N. Modirshahla, M.A. Behnajady, Chemosphere 56 (2004) 895–900. [6] M.D. Sobsey, Water Sci. Technol. 21 (1989) 179–195. [7] J.J. Pignatello, D. Liu, P. Huston, Environ. Sci. Technol. 33 (1999) 1832–1839. [8] Y. Dong, H. Fujii, M.P. Hendrich, R.A. Leising, G. Pan, C.R. Randall, E.C. Wilkinson, Y. Zang, L. Que Jr., B.G. Fox, K. Kauffmann, E. Munck, J. Am. Chem. Soc. 117 (1995) 2778–2792. [9] R.A. Arasasingham, C.R. Cornman, A.L. Balch, J. Am. Chem. Soc. 111 (1989) 7800– 7805. [10] S. Rahhal, H.W. Richter, J. Am. Chem. Soc. 110 (1988) 3126–3133. [11] J. Fernandez, V. Nadtochenko, J. Kiwi, Chem. Commun. (2003) 2382–2383. [12] G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 38 (2004) 3705–3712. [13] J. Fernandez, P. Maruthamuthu, A. Renken, J. Kiwi, Appl. Catal. B: Environ. 49 (2004) 207–215. [14] Yu Zhiyong, L. Kiwi-Minsker, A. Renken, J. Kiwi, J. Mol. Catal. A: Chem. 252 (2006) 113–119. [15] Q. Yang, H. Choi, Y. Chen, D.D. Dionysiou, Appl. Catal. B: Environ. 77 (2008) 300– 307. [16] P. Raja, M. Bensimon, U. Klehm, P. Albers, D. Laub, L. Kiwi-Minsker, A. Renken, J. Kiwi, J. Photochem. Photobiol. A: Chem. 187 (2007) 332–338. [17] G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 37 (2003) 4790–4797. [18] G. Manivannan, P. Maruthamuthu, Eur. Polym. J. 23 (1987) 311–313. [19] J. Kim, J.O. Edwards, Inorg. Chim. Acta 235 (1995) 9–13. [20] D.L. Ball, J.O. Edwards, J. Phys. Chem. 62 (1958) 343–345. [21] R.C. Thompson, Inorg. Chem. 20 (1981) 1005–1010. [22] Z. Zhang, J.O. Edwards, Inorg. Chem. 31 (1992) 3514–3517. [23] C. Marsh, J.O. Edwards, Prog. React. Kinet. 15 (1989) 35–75. [24] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, IUPAC, Decker, 1985. [25] C. Morrison, J. Bandara, J. Kiwi, J. Adv. Oxid. Technol. 1 (1996) 160–169. [26] D.F. Ollis, H. Al-Ekabi, Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. [27] M.M. Halmann, Photodegradation of Water Pollutants, CRC Press, Boca Raton, FL, 1996. [28] V. Nadtochenko, J. Kiwi, J. Chem. Soc., Faraday Trans. 93 (1997) 2373–2378. [29] A. Meenakshi, M. Santappa, J. Catal. 19 (1970) 300–3009. [30] B. Ruppert, R. Bauer, G. Heisler, J. Photochem. Photobiol. A: Chem. 73 (1993) 75–78. [31] J. Kiwi, A. Lopez, V. Nadtochenko, Environ. Sci. Technol. 34 (2000) 2162–2168.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Kinetics of degradation of acid red 88 in the presence of Co2+-ion/ peroxomonosulphate reagent J. Madhavan a,b, P. Maruthamuthu a,*, S. Murugesan a,1, M. Ashokkumar b a b
Department of Energy, University of Madras, Guindy Campus, Chennai 600 025, India School of Chemistry, University of Melbourne, VIC-3010, Melbourne, Australia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 June 2009 Received in revised form 29 July 2009 Accepted 6 August 2009 Available online 13 August 2009
Visible light-assisted photo-Fenton-like oxidation of a mono-azo textile dye, acid red 88 (AR88), has been carried out in the presence of Co2+ as the catalyst and peroxomonosulphate (PMS) as the oxidant. Decolorization of AR88 in the Co2+/PMS system was observed to follow zero-order kinetics with respect to the dye. It was also found that the decolorization of AR88 increased as the concentration of cobalt ions was increased. The rate of abatement of the acid red 88 was found to be independent of the concentrations of dye and oxidant (PMS). Mineralization studies were also carried out by monitoring the reduction in total organic carbon (TOC) content during the course of the reaction and a suitable mechanism has been proposed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Acid red 88 Oxone Photomineralization Photo-Fenton oxidation Photocatalysis
1. Introduction Contamination of soil and groundwater, due to discharges of industrial wastewaters into the ecosystem, leads to a serious health and environmental problems. Recently, the advanced oxidation processes (AOPs) have attracted much attention in the area of wastewater treatment and other environmental applications [1–3]. Advanced oxidation technologies involve the generation of highly reactive radical species by several processes that include photocatalysis using semiconductors, photo-Fenton reaction (hn/Fe2+/H2O2), ozonation, and UV photolysis of oxidants such as hydrogen peroxide and persulphate [4–10]. Fenton’s reagent is a mixture of hydrogen peroxide and ferrous ions that produces OH radicals. However, it requires acidic pH (3.0) and is reported to be slow and of poor mineralization (less than 60%). In order to overcome the limitations of the Fenton’s reagent and to find suitable oxidation processes, several attempts were made and those modified Fenton reactions are called Fenton-like reactions. Recent studies by Fernandez et al. [11] showed that the use of Co2+ions in combination with peroxomonosulphate in homogeneous medium led to the generation of sulphate radicals which had greater
* Corresponding author. Tel.: +91 44 22301576; fax: +91 44 22352494. E-mail address: [email protected] (P. Maruthamuthu). 1 Present address: School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India. 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.08.008
efficiencies than and several operational advantages over the conventional Fenton’s reagent. It was also supported from the experimental results carried out to find the favorable metal(s) for the decomposition of potassium peroxomonosulphate (KHSO5) by Ag(I), Ce(III), Co(II), Fe(II), Fe(III), Mn(II), Ni(II), Ru(III) and V(III) [12]. The results showed that cobalt(II) was the best catalyst for the activation of peroxomonosulphate due to its oxidation potential (1.82 eV) being slightly higher than that of the couple Co3+/Co2+ (1.80 eV) which drives the reaction to Co3+ from the initial Co2+-ion upon PMS addition in solution [13]. The potential applicability of Co2+/PMS reagent either in homogeneous [13,14] or heterogeneous form [15,16] for the pollutant degradation is also reported recently. Some of the advantages of using Co2+/PMS reagent for the photocatalytic degradation organic pollutants in industrial wastewaters are: (a) Co2+/PMS reagent can be applied over a wide range of pH, which is evidenced from the Co2+/PMS catalyzed degradation of 2,4-dichlorophenol in the pH range 2.0–8.0 [17]. While Fenton’s reagent has failed to exhibit its catalytic activity when the pH exceeded 3.0, Co2+/PMS reagent showed higher efficiency at neutral pH, which is a significant advantage since the pH of most of the contaminated natural waters falls in the range of 6.0–8.0. (b) Co2+/PMS reagent shows higher mineralization of pollutants; >90% mineralization was reported for the mineralization of Orange II [13] and 2,4-dichlorophenol [17].
J. Madhavan et al. / Applied Catalysis A: General 368 (2009) 35–39
36
Table 1 Chemical structure and absorption maximum of acid red 88. Dye
Chemical structure
(CAR88) vs. time plot suggested that the decolorization of AR88 follows zero-order kinetics with respect to the dye. lmax (nm) 0
Acid red 88 (C.I. 15620)
505
C ¼ k0 t þ C 0
(1)
where C0 and C are the initial dye concentration and the concentration of the dye at any time ‘t’ respectively. In this paper, the term ‘C’ is represented as CAR88. k0 0 is the pseudo zero-order rate constant which can be calculated from the CAR88 vs. time plot. 3. Results and discussion (c) Co2+/PMS reagent is cost-effective as it avoids the costs due to post-neutralization and sludge treatment and it is operative at low metal concentrations. In the present investigation the kinetics of decolorization of acid red 88 in the presence of Co2+/PMS reagent is dealt with in detail. The factors influencing the mineralization of AR88 are also studied. 2. Experimental 2.1. Materials Acid red 88, C20H13N2O4SNa (Table 1), a textile dye that absorbs in the visible region (lmax = 506 nm), was a gift from Atul Ltd, India. Co2+-ion solution was prepared from analytical grade samples of CoSO47H2O (E-Merck). Oxone, the commercial name of potassium peroxomonosulphate, is a triple salt with the composition 2KHSO5KHSO4K2SO4 from Janssen Chimica (Belgium); it was used as received. When preparing the molar solutions, we took the fact that 1 mol of oxone gives 2 mol of peroxomonosulphate as shown by its chemical formula. Sodium nitrite (E-Merck, India) was used as the quenching agent for both sulphate and hydroxyl radicals. Unless otherwise specified, all reagents were of analytical grade and the solutions were prepared using double distilled water. 2.2. Dark decolorization studies
3.1. Dark decolorization studies The following preliminary experiments were performed to check the feasibility of the catalytic degradation of the dye using Co2+/PMS reagent, (i) AR88 + Co2+, (ii) AR88 + PMS, (iii) AR88 + Co2+ + PMS and the results are shown in Fig. 1. It was clearly seen that no appreciable dye decolorization was observed when either Co2+-ion or PMS was added alone whereas a faster and nearly complete decolorization was obtained within 8 min when Co2+-ion in conjunction with PMS was used. That is, the decolorization of the dye was observed only after the conjunction of Co2+-ion with PMS, which clearly proved the efficiency of the Co2+/PMS reagent taken in the present investigation. It was also observed that there is no change in the absorption spectrum of the dye during the addition of Co2+-ion into the dye solution which ruled out the possibility of any complex formation between the Co2+-ion with AR88. 3.1.1. Effect of concentration of the dye Experiments were carried out with various concentrations of the dye, CAR88 = (3–9) 105 M keeping the other experimental parameters, viz., CPMS = 5 mM and C CO2þ ¼ 0:025 mM as constants. Fig. 2 (inset) shows the linear decrease in the concentration of acid red 88 with time for different initial concentrations of acid red 88 used. The obtained dark decolorization rate constant (k0 0) values for the variations in CAR88 are also presented in Fig. 2. As expected
Since the decolorization under light irradiation is very fast, we could not measure it in the present investigation. The decolorization of the dye in the dark was analyzed using the ‘kinetics mode’ available in the Shimadzu UV–vis spectrophotometer (UV 1601). All dark experiments were carried out in a quartz cuvette (5 ml capacity) provided along with the spectrophotometer. The desired concentrations of all the solutions were made up for 3 ml and the order followed in mixing the additives was as follows: dye, cobalt(II)-ion and oxone. 2.3. Photocatalytic reactor and light source for photomineralization studies A 100 ml capacity borosilicate glass with an opening at the top to facilitate periodical removal of the sample was used as the photochemical reactor. Light irradiation was carried out by means of a solar box designed in our laboratory which consists of three 250 W tungsten–halogen lamps (Philips, India) inside a rectangular box fitted with two exhaust fans to considerably lower the temperature within the solar box during irradiation. The short wavelength radiations (l < 310 nm) were removed by the walls of the reaction vessel. 2.4. Kinetic studies The treatment efficiency was measured from the plot of concentration of the dye (which is calculated from the absorbance) vs. time. The straight-line obtained in the concentration of the dye
Fig. 1. Preliminary experiments for the decolorization of acid red 88 in the dark using Co2+/PMS reagent. (*) — AR88 + Co2+-ion; (~)— AR88 + PMS; ()— AR88 + Co2+-ion + PMS; CAR88 = 5 105M; C CO2þ ¼ 0:025 mM; CPMS = 5 mM.
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and [PMS] = 5 mM as constants. The respective k0 0 vs. [Co2+] plot for all variations in [Co2+] is presented in Fig. 3 and the plot of concentration of AR88 with time is also shown in the inset. It should be noted that the rate of the decolorization proportionately increases with increase in concentration of Co2+. This linear dependence of k0 0 with respect to [Co2+] may be due to the increased amount of formation of the active radical species like SO4 and OH with increase in the concentration of Co2+-ion. In the presence of metal-ions, PMS is known to generate SO5 and SO4 in aqueous solution in addition to HO by radical chain reactions [18,19].
Fig. 2. Plot of the pseudo zero-order rate constant for the decolorization of acid red 88 in the dark as a function of CAR88. The insert shows the bleaching of AR88 solutions for different initial dye concentrations. C CO2þ ¼ 0:025 mM; CPMS = 5 mM.
for the zero-order kinetics, the decolorization rate remained constant for the entire range of dye concentrations employed in our study inferring that the limiting step is not the reaction of the dye with the radicals in solution but the generation of radicals themselves in the solution. 3.1.2. Effect of concentration of Co2+ The effect of cobalt(II) ion concentration on the decolorization of acid red 88 was studied by varying the concentration of Co2+, [Co2+], from 0.01 to 0.05 mM and maintaining [AR88] = 5 105 M
3.1.3. Effect of concentration of PMS In order to study the effect of concentration of peroxomonosulphate on the decolorization rate of acid red 88, CPMS was varied from 0.5 to 5 mM while maintaining other experimental parameters constant ([AR88] = 5 105 M and C CO2þ ¼ 0:025 mM). The straight-line relationship was found while CAR88 was plotted against time as shown in Fig. 4 (inset) which confirms that this reaction obeys the relation given in Eq. (1). The corresponding pseudo zero-order decolorization rate constant (k0 o) values were presented as k0 o vs. CPMS plot in Fig. 4. It was noted that k0 o remained constant irrespective of the concentration of PMS, proving that the excess CPMS has no influence on the decolorization rate of acid red 88. That is, the decolorization rate is zero-order with respect to CPMS. However, a higher concentration of PMS was used in our study since higher CPMS is beneficial to achieve higher mineralization of the dye [13]. 3.1.4. Kinetics of decolorization of acid red 88 using Co2+/PMS reagent In order to explain the observed kinetic results, we considered the following fundamental reactions that were reported earlier [19–23] for the cobalt-catalyzed decomposition of peroxomonosulphate. Co2þ þ H2 O @ CoOHþ þ Hþ þ
CoOH þ HSO5 ! CoO þ H2 O þ SO4 CoOþ þ 2Hþ @ Co3þ þ H2 O
Fig. 3. Plot of the pseudo zero-order rate constant for the decolorization of acid red 88 in the dark as a function of C CO2þ . The insert shows the bleaching of AR88 solutions for different Co2+-ion concentrations. CAR88 = 5 105 M; CPMS = 5 mM.
K1
þ
K3
(2)
k2
(3) (4)
Fig. 4. Plot of the pseudo zero-order rate constant for the decolorization of acid red 88 in the dark as a function of CPMS. The insert shows the bleaching of AR88 solutions at different PMS concentrations. CAR88 = 5 105 M; C CO2þ ¼ 0:025 mM.
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SO4 þ Co2þ ! SO4 2 þ Co3þ
k4
(5)
Dye þ SO4 ! Dyeþ þ SO4 2
k5
(6)
Sulphate radical anions are stronger oxidants than hydroxyl radicals, especially at higher pH [24], and are the primary oxidizing species involved in the Co2+-ion mediated decomposition of PMS. Fernandez et al. [13] applied the steady state approximation and derived the rate law from the above basic reactions involved in the degradation of the dye using Co2+/PMS reagent. d½SO4 =dt ¼ k2 :K 1 ½Co2þ ½HSO5 =½Hþ k4 ½Co2þ ½SO4 k5 ½Dye½SO4 ¼ 0
(7)
d½Dye=dt ¼ kCo ½Co2þ
(8)
where kCo = k5 k2.K1[HSO5][Dye]/{[H+](k4[Co2+] + k5[Dye])}. At a defined concentration of Co2+, the zero-order decolorization in Fig. 3 inset can be written as d½Dye=dt ¼ k0 o
(9)
3.1.5. Photomineralization studies Similar to dark decolorization experiments, mineralization studies performed under visible light irradiation also proved that the minerlization process is very significant only when Co2+-ion is coupled with PMS [Fig. 5]. That is, no appreciable photomineralization takes place (< 5%) under visible light irradiation for 1 h with Co2+-ions alone and similar results were also observed in the presence of PMS alone. However, the combination of Co2+-ion with PMS showed a rapid increase in photomineralization efficiency (cal. 66% in 30 min), suggesting that the chain radical reaction operates only when the PMS in conjunction with Co2+-ion was used. Similar results were already reported for the degradation of Orange II and 2,4-dichlorophenol [13,17]. The effects of variation of solution parameters on the TOC removal of AR88 are discussed in the following sections. 3.1.5.1. Effect of CAR88. Fig. 6 shows the percentage of TOC removal for the variations in CAR88 ((3–9) 105 M) while maintaining constant concentrations of Co2+-ion (0.025 mM) and PMS (5 mM). For example, in the case of CAR88 = 5 105 M, about 66.3% TOC removal was obtained in 30 min; however, it reduced to 27.9%
Fig. 5. Plot of the normalized TOC values for the mineralization of acid red 88 under visible light irradiation in 30 min. CAR88 = 5 105 M; C CO2þ ¼ 0:025 mM; CPMS = 5 mM. (W) — AR88 + Co2+-ion; (&)— AR88 + PMS; (D) — AR88 + Co2+-ion + PMS.
Fig. 6. Percentage of TOC removal for different concentrations of acid red 88 in the presence of Co2+-ions (0.025 mM) and PMS (5 mM) under visible light irradiation for 30 min.
when CAR88 is increased to 9 105 M. The reason for the observed decrease in the mineralization extent is that, though the concentration of the dye is increased, the amount of reactive radical species such as HO/SO4 produced by the conjunction of Co2+-ion with PMS remained the same. 3.1.5.2. Effect of C CO2þ . The plot of percentage of TOC removal obtained by varying C CO2þ (0.005–0.075 mM) but with constant CAR88 (5 105 M) and CPMS (5 mM) is presented in Fig. 7. It is evident from Fig. 7 that the percentage of TOC removal is increased when the C CO2þ is increased. About 70% increment in the TOC removal efficiency is seen when the C CO2þ is increased from 0.005 to 0.075 mM. Similar results were also obtained in the dark
Fig. 7. Percentage of TOC removal from acid red 88 (5 105 M) solution as a function of Co2+-ion concentration in the presence of CPMS (5 mM) under visible light irradiation for 30 min.
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and the peroxides formed were decomposed under light irradiation generating two radicals in each case, leading to a new radical chain: ROOH þ hn ! RO þ OH
(18)
Under light irradiation, the additional generation of OH radicals improves the kinetics of dye mineralization, as reported earlier for Fenton systems [28–31]. 4. Conclusions
Fig. 8. Percentage of TOC removal from acid red 88 (5 105 M) solution as a function of PMS concentration in the presence of C CO2þ (0.025 mM) under visible light irradiation in 30 min.
Visible light-assisted photo-Fenton-like oxidation of a monoazo textile dye, acid red 88 (AR88), has been studied in the presence of Co2+ as the catalyst and peroxomonosulphate as the oxidant. A zero-order decolorization kinetics was observed for AR88 in the presence of Co2+/PMS reagent. Enhancement in the decolorization of AR88 was observed on increasing the concentration of cobalt ions due to the increased amount of formation of the active radical species like SO4 and OH. The concentration of PMS was found to have no influence on the decolorization rate of acid red 88 for the abatement of the azo dye. TOC studies showed that the Co2+/PMS reagent is very effective to achieve the mineralization of AR88; hence, the Co2+/PMS system is proved to be a suitable reagent for applications in the field of environmental remediation. Acknowledgements
decolorization experiments. The reason for the observed trend may be the increased generation of HO and SO4 radical species on increasing the concentration of Co2+ -ions. 3.1.5.3. Effect of CPMS. The effect of CPMS on the percentage of TOC removal was obtained by varying the concentration of PMS from 0.5 to 5 mM while keeping the CAR88 (5 105 M) and C CO2þ (0.025 mM) constant [Fig. 8]. The percentage of TOC removal efficiency increased up to CPMS = 3.75 mM and a further increase led to a decrease in TOC values. The reason for the increase in the TOC values up to 3.75 mM was the increased generation of HO and SO4 radicals; however, on further increasing the CPMS, the scavenging reaction between the active radical species, OH/SO4 and SO5 (Eq. (10)) predominates, thereby leading to the reduction of TOC removal efficiency. Similar results were already reported for the degradation of Orange II, when a higher concentration of PMS was employed [13]. HSO5 þ OHorSO4 ! SO5 þ OH orSO4 2 þ Hþ
(10)
3.1.5.4. Mechanism of photomineralization studies. The detailed reactions involving the photocatalyzed mineralization of dyes in the presence of Co2+/PMS reagent are presented below: The presence of organic radicals as intermediates during a reaction is known to form peroxides (ROOH) through radical chain processes [25–27]. Dye þ hn ! Dye
(11)
Dye þ PMS ! Dyeþ þ ðHO þ SO4 2 orHO þ SO4 Þ
(12)
Degraded products þ Co2þ =HO =SO4 ! Radical intermediate ðR Þ (13) R þ PMS=O2 ! CO2 þ H2 O
(14)
R þ O2 ! ROO
(15)
ROO þ RH ! ROOH þ R
(16)
ROO þ ROO ! ROOR þ O2
(17)
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