- Email: [email protected]
Photocatalytic degradation of liquid waste containing EDTA
Desalination 232 (2008) 139–144
Photocatalytic degradation of liquid waste containing EDTA H. Seshadria*, S. Chitrab, K. Paramasivanb, P.K. Sinhab a
Safety Research Institute, AERB, Kalpakkam, India Tel. +91 (44) 27480164; Fax +91 (44) 27480165; email: [email protected] b Centralised Waste Management Facility, BARCF, Kalpakkam, India
Received 28 March 2007; accepted revised 24 December 2007
Abstract Ethylenediaminetetracetic acid (EDTA) is widely used as a decontaminating agent in nuclear industry. The presence of complexant interferes in treatment of radioactive effluent. It is therefore essential to degrade EDTA prior to chemical treatment for effective management of radioactive waste. Photocatalytic treatment of EDTA can be used as a pretreatment step. The photocatalytic degradation of EDTA has been investigated in liquid waste using Degussa P-25 titanium dioxide (TiO2) as the semiconductor photocatalyst in presence of UV light. A cylindrical photoreactor has been used for the present purpose. The process of degradation of EDTA was monitored by titrimetric method using magnesium sulphate as the titrant and Erichrome black-T as indicator. The effects of various parameters such as pH, quantity of the catalyst loading, effect of H2O2 were studied. The presence of amides was detected in the degraded waste. The radioactive liquid waste containing degraded EDTA was then subjected a chemical treatment (precipitation) step, which is generally used as a waste treatment step. Results have shown that the degradation products of EDTA does not interfere in the chemical precipitation step and gives a good decontamination factor for the treatment process compared to the radioactive liquid waste where EDTA degradation has not been carried out. Keywords: Photodegradation; TiO2; EDTA; Photoreactor; Liquid waste; Catalyst
1. Introduction Ethylenediamminetetraactetic acid (EDTA) is a domestic and industrial water contaminant. It is also used widely as a decontaminating agent in
nuclear industry. The presence of EDTA in radioactive liquid waste can cause complexation of some of the target cations (Sr2+) and precipitant cations (Ca2+, Cu2+, Fe3+) resulting in interference in their removal by conventional treatment processes such as chemical precipitation, ion ex-
*Corresponding author. Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006 Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006 0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.12.013
140
H. Seshadri et al. / Desalination 232 (2008) 139–144
change, etc. [1]. Further it might also impart elevated leachability and higher mobility of cationic contaminants from the conditioned wastes, i.e. waste immobilized in cement or other matrices and can negatively influence the quality of the final form of the waste [2]. EDTA is not easily biodegradable [3], scarcely degradable by chlorine [4], hardly retained by activated carbon fibres and resistant to ozone treatment [5,6]. Hence it is essential to carry out pretreatment step for the removal of EDTA to render the liquid waste amenable to treatment. Advanced oxidation processes (AOP’s) using oxidants like UV+H2O2 [7], Fenton’s reagent [8], ultrasound (US) [9], solar ferrioxalate [10], MCM41 catalyst [11], wet oxidation [12] photocatalysis [13–18] etc. alone or in combination are considered as methods of clean and ecologically safe treatment for the degradation of organics. TiO2 has proven as a versatile semiconductor photocatalyst, owing to its exceptional optical and electronic properties, chemical stability, non-toxicity and low cost. The German company, Degussa (P25), produces the most popular commercial form of TiO2. This sample contains around 70% anatase and 30% rutile and possesses an excellent activity. EDTA degradation mechanism was reported in the literature [19]. In this paper, photocatalytic degradation of EDTA has been attempted in radioactive liquid waste using titania (P-25). The xydroxyl radicals (•OH) formed on the illuminated TiO2 surface can easily attack the adsorbed EDTA molecules, leading finally to their degradation. 2. Experimental 2.1. Materials The commercially available TiO2 (Degussa P25 having 70% anatase and 30% rutile, surface area 50 m2/g and particle size 30 nm) obtained as a compliment from M/S Degussa India Limited was used as a photocatalyst. EDTA was obtained
from M/S REACHEM Limited, India. All the other chemicals, including hydrogen peroxide, MgSO4.7H2O and the other chemicals used were of analytical grade, 99.9% purity. Water used for the present study (Millipore) was double distilled and filtered. 2.2. Photoreactor The reactor (Fig. 1) was a cylindrical type (M/S HEBER Scientific, India), which has a quartz reaction vessel with a capacity of 1200 mL. It has four 16 W low-pressure UV lamps built into the lamp housing with polished anodized aluminum reflectors, which are positioned in such a way that the intensity received by the effluent in the quartz reaction vessel is uniform. The photoreactor has 4 high cooling fans for maintaining the temperature in the range of 25–30°C. The cover of the photoreactor had ports for sampling, gas purging and outlet. Oxygen was passed inside the contents of the reactants through an aerator, which kept the contents of the vessel in agitation. 2.3. Photocatalytic studies The experiments were carried out using as little as 1 mg of the catalyst (P-25). 1000 ml of 1000 ppm EDTA was taken for the photocatalytic studies. 6.25 ml of H2O2 and 1mg of P-25 TiO2 were added into the quartz vessel where the liquid effluent containing EDTA was present. The contents were sonicated using an ultrasonic horn (25 KHz) for 10 min for the purpose of homogenization. The entire solution became slightly turbid owing to the uniform distribution of titania. Samples were drawn periodically from the quartz vessel for monitoring the extent of degradation of EDTA through the analytical method. The temperature was maintained at 30–35°C through high cooling fans. The time at which the UV lamps were turned on was considered time zero or the beginning of the experiment. Dissolved oxygen in the solution played an important role by trapping the conduc-
H. Seshadri et al. / Desalination 232 (2008) 139–144
O2 supply
141
Sampling port
Cooling fan
16 W UV lamp
Quartz reaction vessel
Photoreactor To power supply
Fig. 1. Schematic of cylindrical photoreactor.
tion band electrons forming super oxide ions (O2•–) and thus preventing the electron-hole recombination (O2+ e– → O2•– ) and at the same time H2O2 is formed from O2•–. 2.4. Analytical methods The pH of the solution was measured using a calibrated HACH pH meter at appropriate time intervals and during those time intervals 5 ml of the sample was pipetted out into a conical flask. The samples were analyzed titrimetrically against standard Mg2+ using Erichrome Black T as indicator [20]. At least two runs were carried out for each condition averaging the results. Conductivity and TDS of the samples were determined using HACH conductivity/TDS meter.
3. Results and discussion 3.1. Effect of pH pH of the contents of the effluent as such before the commencement of the reaction was 4.3 (acidic range) and was used as such for acidic pH range studies. The experiments were repeated by altering the pH of the contents to 7.0 and 10.0. It was observed that the kinetics of photocatalytic degradation of EDTA at pH 10.0 was faster than the photocatalytic degradation at pH 4.3 and also at pH 7.0. The kinetics of degradation was found to be in the order alkaline > acidic > neutral. The results are given in Fig. 2. In our study, since the organics was weakly acidic, the faster kinetics for photocatalytic degradation at acidic pH than the
142
H. Seshadri et al. / Desalination 232 (2008) 139–144 110 100
% of EDTA degraded
90 80 70 60
Acidic pH Alkaline pH Neutral pH
50 40 30 20 10 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (h)
Fig. 2. Effect of pH on the photodegradation of EDTA.
Table 1 Variation of pH during the photocatalytic degradation of EDTA Time (h)
pH
% of EDTA degraded
0 0.5 1 1.5 2.0 2.5
4.4 7.1 7.7 7.8 8.0 8.0
0 83.33 95.04 96.8 98.58 100
neural pH might be due to the increased adsorption [21,22]. The reason for the increase in kinetics of degradation of EDTA at alkaline pH may be due to the formation of oxide and peroxide ions due to the reaction of hydroxyl radical and H2O2, which contributes to the increase in the rate of degradation [23]. 3.2. Degradation products During the course of EDTA degradation, it was observed that there was an increase in the pH of
the solution from 4.3 to 8.0. The change in pH with time during EDTA degradation study is given in Table 1. It is evident from the above that the acidity of EDTA and hence the chelating ability is lost. This was confirmed though titrimetric determination of EDTA. Since there was a change in pH from acidic to alkaline range, formation of acidic intermediates was ruled out and the formation of either amines or amides was suspected. The qualitative hydroxamic test was positive wherein a deep red coloured solution was obtained on treatment with hydroxylamine and ferric chloride and thus formation of amides was confirmed [7]. This is in good agreement with the mechanism proposed by Babay et al. [24]. 3.3. Effect of hydrogen peroxide 6.25 ml of H2O2 was used in the present EDTA degradation studies. When H2O2 concentration is increased further, the rate of photodegradation does not increase significantly. H2O2 is suitable for trapping electrons by preventing the recombination of electron–hole pairs thus increasing the chances of formation of •OH and O2•– on the sur-
H. Seshadri et al. / Desalination 232 (2008) 139–144
face of TiO2. When the concentration of H2O2 is increased, the amount of •OH formed on the surface of TiO2 increases rapidly and hence the annihilation rates of ·OH and •OH (•OH + •OH → H2O2) are faster than the reaction rates of ·OH and organic contaminants. •OH is annihilated before the reaction of •OH with organic contaminants thereby decreasing the efficiency [22,23]. Experiments were also conducted by reducing the amount of hydrogen peroxide to 3 ml, 2 ml and 1 ml. No significant change in the kinetics of degradation has been found. It is evident from the above that through H2O2 is helpful in trapping electrons, it was confirmed that the contribution of H2O2 in the production of •OH is insignificant. 3.4. Effect of TiO2 loading For the complete degradation of 1000 ml (1000 ppm) of EDTA, 1 mg of P-25 TiO2 was used in the present study. From this it is clear that a very small amount of the catalyst is sufficient for affecting the photodegradation of 1000 ml of 1000 ppm EDTA. This may be attributed to the fact that the nano-sized catalyst has a very high surface area (50 m2/g), which allows more production of hydroxyl radicals and also offers more adsorption sites for EDTA. The experiment was repeated by using 0.5 mg of the catalyst for the similar degradation study. No significant change in the kinetics could be observed. This confirms the requirement of a very small amount of the catalyst. The experiment was also repeated by using 5 mg and 10 mg of P-25 TiO2. The use of higher amounts of the catalyst has been found to decrease the kinetics of EDTA degradation. This is due to the increased light scattering and consequent reduction in light penetration through the solution (masking of light by the suspended excess catalyst particles) [25]. 3.4. DF studies The radioactive effluent containing EDTA af-
143
ter photodegradation studies was subjected to a chemical precipitation step for the treatment of the radioactive effluent. Cesium and strontium were the major nucleides present in the radioactive effluent taken for the present study. Similar radioactive effluent where EDTA was present without degradation was also taken for DF studies for comparison. The studies have shown that the radioactivity of the effluent after chemical treatment was contained in the precipitate that settled at the bottom leaving the supernatant free of any radioactivity. Decontaminant factor (DF = specific activity initial / specific activity final). For the process has also improved significantly when compared to the effluent containing EDTA (without degradation). The results are given in Table 2. From this table it can be seen that the decontamination factors both for strontium and cesium, which are the major sources of radioactivity taken for the present studies, have improved significantly once EDTA in the effluent was degraded when compared to the radioactive effluent where EDTA was present without degradation. This is mainly due to the non-interference of degradation products of EDTA waste. The liquid waste after degradation of EDTA can then be conveniently subjected to chemical treatment.
Table 2 DF obtained for the chemical treatment process in the degraded waste containing EDTA
Sl. Nature of the No. sample
DF obtained for strontium
DF obtained for cesium
1
EDTA containing 7.75 effluent (without EDTA degradation)
8.12
2
EDTA containing 22.12 effluent (after EDTA degradation)
33.6
144
H. Seshadri et al. / Desalination 232 (2008) 139–144
4. Conclusion The result of this investigation clearly reveals that EDTA can be effectively degraded by photocatalytic route using TiO2 in a radioactive effluent containing it. TiO2-assisted photocatalytic degradation offers green chemistry and is highly cost-effective due to the requirement of a very small quantity of the catalyst. It has been found that 1 mg of the catalyst is sufficient for the photodegradation of 1000 ml of 1000 ppm EDTA present in radioactive liquid waste indicating that it is quite economical and safer route for degradation. It was also observed that the usage of a smaller quantity of the catalyst (0.5 mg) does not change the kinetics of the degradation significantly. The results show that the degradation products of EDTA do not interfere in the chemical treatment (precipitation) process and give good decontamination factor for the chemical step. Hence the photocatalytic method can be effectively used for the degradation of EDTA present in liquid waste.
References [1] A.G.S. Mani, K. Paramasivan, S. Chitra, P.K. Sinha and K.B..Lal, Proc. Indian Environmental Congress, 2004, pp. 276–280. [2] K. Rosikova, J. John, E. Danacikova-Popelova, F. Sobesta and E.W. Hooper, Study of EDTA photodegradation. Proc. Fourth Institute of International Cooperative Environmental Research Florida, Florida State University, Tallahassee, 1998, pp. 379–385. [3] M.L. Hinck, J. Ferguson and J. Puhaakka, Wat. Sci. Technol., 35 (1997) 25–31. [4] H.J. Brauch and S. Schullerer, Vom Wasser, 69 (1987) 155–169. [5] H.J. Brauch and S. Schullerer, Vom Wasser, 72 (1989) 23–29. [6] E. Gilbert and S. Hoffmann-Glewe, Wat. Res., 24 (1990) 39. [7] M. Sorensen and F.H. Frimmel, Z. Naturforsh, 50b
(1995) 1845–1853. [8] S. Chitra, K. Paramasivan, P.K. Sinha and K.B. Lal, J. Adv. Oxid. Technol., 6(1) (2003) 109–114. [9] S. Chitra, K. Paramasivan, P.K. Sinha and K.B. Lal, J. Cleaner Production, 12 (2004) 429–435. [10] C.A. Emilo, W.F. Jardim and M.I. Litter, J. Photochem. Photobiol. A: Chem., 151 (2002) 121–127. [11] K.S. Seshadri, M. Pushpa, P.K. Sinha and K.B. Lal, P. P. Chem., 7(3) (2005) 163–167. [12] M.I. EI-Dessouky, M.M. Abed EI-Aziz, E.H. El Mossalmy and H.F. Aly, J. Radioanal. Nucl. Chem., 249(3) (2001) 643–647. [13] H. Seshadri, S. Chitra, K. Paramasivan and P.K. Sinha, Symposium on Emerging Trends in Separation Science and Technology, 2006, pp. 222–223. [14] A. Mills, R.H. Davies and D. Worsely, Chem. Soc. Rev., 22(66) (1993) 417–425. [15] A. Mills and A. Le Huntle, J. Photochem. Photobiol. A: Chem., 108 (1997) 1–35. [16] M.I. Litter, Appl. Catalysis B: Environ., 23 (1999) 89–114. [17] P.K.J. Robertzon, J. Cleaner Production, 4 (1996) 203–212. [18] N. Gokulakrishnan, A. Pandurangan and P.K. Sinha, Indian Eng. Chem. Res., 82 (2005) 309–326. [19] P.A. Babay, C.A. Emilo, R.E. Ferreyra, E.A. Gautier, R.T. Gettar and M.I. Litter, Int. J. Photoenergy, 3 (2001) 193–199. [20] J. Bassett, R.C. Denney, G.H. Jeffery and J. Mendham, Vogel’s Textbook of Quantitative Inorganic Analysis. Longman, London, UK, 4th ed., 1978, p. 587. [21] D.S. Bhatkhande, V.G. Pangarkar and A.A.C.M. Beenackers, J. Chem. Technol. Biotechnol., 77 (2001) 102–116. [22] S. Sakthivel, B. Neppolian, B. Arabindoo, M. Palanichamy and V. Murugesan, Indian J. Eng. Mater. Sci., 7 (2000) 87–93. [23] M. Zhao, S. Chem and Y. Tao, J. Chem. Technol. Biotechnol., 64 (1995) 339. [24] P.A. Babay, C.A. Emilo, R.E. Ferreyra, E.A. Gautier, R.T. Gettar and M.I. Litter, Wat. Sci. Technol., 44(5) (2001) 179–185. [25] S. Parra, S.E. Stanca, I. Guasaquillo and K. Ravindranathan Thampi, Appl.Catalysis B: Environ., 51 (2004) 107–116.
Photocatalytic degradation of liquid waste containing EDTA H. Seshadria*, S. Chitrab, K. Paramasivanb, P.K. Sinhab a
Safety Research Institute, AERB, Kalpakkam, India Tel. +91 (44) 27480164; Fax +91 (44) 27480165; email: [email protected] b Centralised Waste Management Facility, BARCF, Kalpakkam, India
Received 28 March 2007; accepted revised 24 December 2007
Abstract Ethylenediaminetetracetic acid (EDTA) is widely used as a decontaminating agent in nuclear industry. The presence of complexant interferes in treatment of radioactive effluent. It is therefore essential to degrade EDTA prior to chemical treatment for effective management of radioactive waste. Photocatalytic treatment of EDTA can be used as a pretreatment step. The photocatalytic degradation of EDTA has been investigated in liquid waste using Degussa P-25 titanium dioxide (TiO2) as the semiconductor photocatalyst in presence of UV light. A cylindrical photoreactor has been used for the present purpose. The process of degradation of EDTA was monitored by titrimetric method using magnesium sulphate as the titrant and Erichrome black-T as indicator. The effects of various parameters such as pH, quantity of the catalyst loading, effect of H2O2 were studied. The presence of amides was detected in the degraded waste. The radioactive liquid waste containing degraded EDTA was then subjected a chemical treatment (precipitation) step, which is generally used as a waste treatment step. Results have shown that the degradation products of EDTA does not interfere in the chemical precipitation step and gives a good decontamination factor for the treatment process compared to the radioactive liquid waste where EDTA degradation has not been carried out. Keywords: Photodegradation; TiO2; EDTA; Photoreactor; Liquid waste; Catalyst
1. Introduction Ethylenediamminetetraactetic acid (EDTA) is a domestic and industrial water contaminant. It is also used widely as a decontaminating agent in
nuclear industry. The presence of EDTA in radioactive liquid waste can cause complexation of some of the target cations (Sr2+) and precipitant cations (Ca2+, Cu2+, Fe3+) resulting in interference in their removal by conventional treatment processes such as chemical precipitation, ion ex-
*Corresponding author. Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006 Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006 0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.12.013
140
H. Seshadri et al. / Desalination 232 (2008) 139–144
change, etc. [1]. Further it might also impart elevated leachability and higher mobility of cationic contaminants from the conditioned wastes, i.e. waste immobilized in cement or other matrices and can negatively influence the quality of the final form of the waste [2]. EDTA is not easily biodegradable [3], scarcely degradable by chlorine [4], hardly retained by activated carbon fibres and resistant to ozone treatment [5,6]. Hence it is essential to carry out pretreatment step for the removal of EDTA to render the liquid waste amenable to treatment. Advanced oxidation processes (AOP’s) using oxidants like UV+H2O2 [7], Fenton’s reagent [8], ultrasound (US) [9], solar ferrioxalate [10], MCM41 catalyst [11], wet oxidation [12] photocatalysis [13–18] etc. alone or in combination are considered as methods of clean and ecologically safe treatment for the degradation of organics. TiO2 has proven as a versatile semiconductor photocatalyst, owing to its exceptional optical and electronic properties, chemical stability, non-toxicity and low cost. The German company, Degussa (P25), produces the most popular commercial form of TiO2. This sample contains around 70% anatase and 30% rutile and possesses an excellent activity. EDTA degradation mechanism was reported in the literature [19]. In this paper, photocatalytic degradation of EDTA has been attempted in radioactive liquid waste using titania (P-25). The xydroxyl radicals (•OH) formed on the illuminated TiO2 surface can easily attack the adsorbed EDTA molecules, leading finally to their degradation. 2. Experimental 2.1. Materials The commercially available TiO2 (Degussa P25 having 70% anatase and 30% rutile, surface area 50 m2/g and particle size 30 nm) obtained as a compliment from M/S Degussa India Limited was used as a photocatalyst. EDTA was obtained
from M/S REACHEM Limited, India. All the other chemicals, including hydrogen peroxide, MgSO4.7H2O and the other chemicals used were of analytical grade, 99.9% purity. Water used for the present study (Millipore) was double distilled and filtered. 2.2. Photoreactor The reactor (Fig. 1) was a cylindrical type (M/S HEBER Scientific, India), which has a quartz reaction vessel with a capacity of 1200 mL. It has four 16 W low-pressure UV lamps built into the lamp housing with polished anodized aluminum reflectors, which are positioned in such a way that the intensity received by the effluent in the quartz reaction vessel is uniform. The photoreactor has 4 high cooling fans for maintaining the temperature in the range of 25–30°C. The cover of the photoreactor had ports for sampling, gas purging and outlet. Oxygen was passed inside the contents of the reactants through an aerator, which kept the contents of the vessel in agitation. 2.3. Photocatalytic studies The experiments were carried out using as little as 1 mg of the catalyst (P-25). 1000 ml of 1000 ppm EDTA was taken for the photocatalytic studies. 6.25 ml of H2O2 and 1mg of P-25 TiO2 were added into the quartz vessel where the liquid effluent containing EDTA was present. The contents were sonicated using an ultrasonic horn (25 KHz) for 10 min for the purpose of homogenization. The entire solution became slightly turbid owing to the uniform distribution of titania. Samples were drawn periodically from the quartz vessel for monitoring the extent of degradation of EDTA through the analytical method. The temperature was maintained at 30–35°C through high cooling fans. The time at which the UV lamps were turned on was considered time zero or the beginning of the experiment. Dissolved oxygen in the solution played an important role by trapping the conduc-
H. Seshadri et al. / Desalination 232 (2008) 139–144
O2 supply
141
Sampling port
Cooling fan
16 W UV lamp
Quartz reaction vessel
Photoreactor To power supply
Fig. 1. Schematic of cylindrical photoreactor.
tion band electrons forming super oxide ions (O2•–) and thus preventing the electron-hole recombination (O2+ e– → O2•– ) and at the same time H2O2 is formed from O2•–. 2.4. Analytical methods The pH of the solution was measured using a calibrated HACH pH meter at appropriate time intervals and during those time intervals 5 ml of the sample was pipetted out into a conical flask. The samples were analyzed titrimetrically against standard Mg2+ using Erichrome Black T as indicator [20]. At least two runs were carried out for each condition averaging the results. Conductivity and TDS of the samples were determined using HACH conductivity/TDS meter.
3. Results and discussion 3.1. Effect of pH pH of the contents of the effluent as such before the commencement of the reaction was 4.3 (acidic range) and was used as such for acidic pH range studies. The experiments were repeated by altering the pH of the contents to 7.0 and 10.0. It was observed that the kinetics of photocatalytic degradation of EDTA at pH 10.0 was faster than the photocatalytic degradation at pH 4.3 and also at pH 7.0. The kinetics of degradation was found to be in the order alkaline > acidic > neutral. The results are given in Fig. 2. In our study, since the organics was weakly acidic, the faster kinetics for photocatalytic degradation at acidic pH than the
142
H. Seshadri et al. / Desalination 232 (2008) 139–144 110 100
% of EDTA degraded
90 80 70 60
Acidic pH Alkaline pH Neutral pH
50 40 30 20 10 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (h)
Fig. 2. Effect of pH on the photodegradation of EDTA.
Table 1 Variation of pH during the photocatalytic degradation of EDTA Time (h)
pH
% of EDTA degraded
0 0.5 1 1.5 2.0 2.5
4.4 7.1 7.7 7.8 8.0 8.0
0 83.33 95.04 96.8 98.58 100
neural pH might be due to the increased adsorption [21,22]. The reason for the increase in kinetics of degradation of EDTA at alkaline pH may be due to the formation of oxide and peroxide ions due to the reaction of hydroxyl radical and H2O2, which contributes to the increase in the rate of degradation [23]. 3.2. Degradation products During the course of EDTA degradation, it was observed that there was an increase in the pH of
the solution from 4.3 to 8.0. The change in pH with time during EDTA degradation study is given in Table 1. It is evident from the above that the acidity of EDTA and hence the chelating ability is lost. This was confirmed though titrimetric determination of EDTA. Since there was a change in pH from acidic to alkaline range, formation of acidic intermediates was ruled out and the formation of either amines or amides was suspected. The qualitative hydroxamic test was positive wherein a deep red coloured solution was obtained on treatment with hydroxylamine and ferric chloride and thus formation of amides was confirmed [7]. This is in good agreement with the mechanism proposed by Babay et al. [24]. 3.3. Effect of hydrogen peroxide 6.25 ml of H2O2 was used in the present EDTA degradation studies. When H2O2 concentration is increased further, the rate of photodegradation does not increase significantly. H2O2 is suitable for trapping electrons by preventing the recombination of electron–hole pairs thus increasing the chances of formation of •OH and O2•– on the sur-
H. Seshadri et al. / Desalination 232 (2008) 139–144
face of TiO2. When the concentration of H2O2 is increased, the amount of •OH formed on the surface of TiO2 increases rapidly and hence the annihilation rates of ·OH and •OH (•OH + •OH → H2O2) are faster than the reaction rates of ·OH and organic contaminants. •OH is annihilated before the reaction of •OH with organic contaminants thereby decreasing the efficiency [22,23]. Experiments were also conducted by reducing the amount of hydrogen peroxide to 3 ml, 2 ml and 1 ml. No significant change in the kinetics of degradation has been found. It is evident from the above that through H2O2 is helpful in trapping electrons, it was confirmed that the contribution of H2O2 in the production of •OH is insignificant. 3.4. Effect of TiO2 loading For the complete degradation of 1000 ml (1000 ppm) of EDTA, 1 mg of P-25 TiO2 was used in the present study. From this it is clear that a very small amount of the catalyst is sufficient for affecting the photodegradation of 1000 ml of 1000 ppm EDTA. This may be attributed to the fact that the nano-sized catalyst has a very high surface area (50 m2/g), which allows more production of hydroxyl radicals and also offers more adsorption sites for EDTA. The experiment was repeated by using 0.5 mg of the catalyst for the similar degradation study. No significant change in the kinetics could be observed. This confirms the requirement of a very small amount of the catalyst. The experiment was also repeated by using 5 mg and 10 mg of P-25 TiO2. The use of higher amounts of the catalyst has been found to decrease the kinetics of EDTA degradation. This is due to the increased light scattering and consequent reduction in light penetration through the solution (masking of light by the suspended excess catalyst particles) [25]. 3.4. DF studies The radioactive effluent containing EDTA af-
143
ter photodegradation studies was subjected to a chemical precipitation step for the treatment of the radioactive effluent. Cesium and strontium were the major nucleides present in the radioactive effluent taken for the present study. Similar radioactive effluent where EDTA was present without degradation was also taken for DF studies for comparison. The studies have shown that the radioactivity of the effluent after chemical treatment was contained in the precipitate that settled at the bottom leaving the supernatant free of any radioactivity. Decontaminant factor (DF = specific activity initial / specific activity final). For the process has also improved significantly when compared to the effluent containing EDTA (without degradation). The results are given in Table 2. From this table it can be seen that the decontamination factors both for strontium and cesium, which are the major sources of radioactivity taken for the present studies, have improved significantly once EDTA in the effluent was degraded when compared to the radioactive effluent where EDTA was present without degradation. This is mainly due to the non-interference of degradation products of EDTA waste. The liquid waste after degradation of EDTA can then be conveniently subjected to chemical treatment.
Table 2 DF obtained for the chemical treatment process in the degraded waste containing EDTA
Sl. Nature of the No. sample
DF obtained for strontium
DF obtained for cesium
1
EDTA containing 7.75 effluent (without EDTA degradation)
8.12
2
EDTA containing 22.12 effluent (after EDTA degradation)
33.6
144
H. Seshadri et al. / Desalination 232 (2008) 139–144
4. Conclusion The result of this investigation clearly reveals that EDTA can be effectively degraded by photocatalytic route using TiO2 in a radioactive effluent containing it. TiO2-assisted photocatalytic degradation offers green chemistry and is highly cost-effective due to the requirement of a very small quantity of the catalyst. It has been found that 1 mg of the catalyst is sufficient for the photodegradation of 1000 ml of 1000 ppm EDTA present in radioactive liquid waste indicating that it is quite economical and safer route for degradation. It was also observed that the usage of a smaller quantity of the catalyst (0.5 mg) does not change the kinetics of the degradation significantly. The results show that the degradation products of EDTA do not interfere in the chemical treatment (precipitation) process and give good decontamination factor for the chemical step. Hence the photocatalytic method can be effectively used for the degradation of EDTA present in liquid waste.
References [1] A.G.S. Mani, K. Paramasivan, S. Chitra, P.K. Sinha and K.B..Lal, Proc. Indian Environmental Congress, 2004, pp. 276–280. [2] K. Rosikova, J. John, E. Danacikova-Popelova, F. Sobesta and E.W. Hooper, Study of EDTA photodegradation. Proc. Fourth Institute of International Cooperative Environmental Research Florida, Florida State University, Tallahassee, 1998, pp. 379–385. [3] M.L. Hinck, J. Ferguson and J. Puhaakka, Wat. Sci. Technol., 35 (1997) 25–31. [4] H.J. Brauch and S. Schullerer, Vom Wasser, 69 (1987) 155–169. [5] H.J. Brauch and S. Schullerer, Vom Wasser, 72 (1989) 23–29. [6] E. Gilbert and S. Hoffmann-Glewe, Wat. Res., 24 (1990) 39. [7] M. Sorensen and F.H. Frimmel, Z. Naturforsh, 50b
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