EFFECT OF SULFATE IONS ON THE OXIDATION OF MTBE BY FENTON S REAGENT Ewa Maria Siedlecka, Anna Januszewska, Piotr Glamowski Department of Environmental Engineering, Faculty of Chemistry, University of Gdańsk, ul. Sobieskiego 18, 80-952 Gdańsk INTRODUCTION Fe 2+ /H 2 O 2, commonly known as Fenton s reagent, and Fe 3+ /H 2 O 2 can be used to oxidize organic pollutants present in industrial wastewater. The mechanisms and kinetics of the decomposition of H 2 O 2 by and in homogenous aqueous solution have been the subject of numerous studies [1,2]. Depending on the nature of the ligands, ph and solvents, different reactive species are supposed to be formed: free and bound hydroxyl radicals, iron complexes. Kinetic model assumes the formation of hydroxyl radicals by the reaction of H 2 O 2 with ferrous ions (reaction 1 in Fig. 1) and the regeneration of ferrous ion from -hydroperoxo complexes (reaction 2 and 3 in Figure 1). H 2 O HO RH R 1 H 2 O 2 H 2 O 2 + O 2 2 [Fe III HO 2 ] 2+ 3 H 2 O 2 Figure 1. Simplified reaction scheme for Fenton s reaction at acidic ph [3]. Recently most of the studies concerning the oxidation of organic pollutants by Fenton s reaction are carried out in the presence of inorganic anions [4,5,6]. They may be present in the solution to be treated or introduced in the solution with a reactants. Inorganic anions may have a significant effect on the overall reaction rates. Sulfate ions was found to decrease the rates of oxidation of chloro- and nitro-phenols, direct dyes, methyl t-butyl ether (MTBE) [5,6,7]. The possible effects of sulfate ions are complexation reactions with ferric and ferrous ions (reaction 4-6) Fe 2+ + SO 4 FeSO 4 (4) Fe 3+ + SO + 4 FeSO 4 (5)
Fe 3+ + 2SO - 4 Fe(SO 4 ) 2 (6) and scavenging of hydroxyl radicals (reactions 7). HSO - 4 + OH SO - 4 + H 2 0 (7) Recently, we undertook a preliminary examination of the effect of selected inorganic anions on the effectiveness of the Fenton advanced oxidative treatment (Fe 2+ /H 2 O 2 ) of waters contaminated with methyl t-butyl ether [4,7]. With respect to chloride, inhibition of oxidation was clearly in evidence, whereas the addition of sulfates rates to a much lesser extent. The present study was undertaken in order to compare the influence of sulfate anions on the degradation rates of MTBE in Fe 2+ /H 2 O 2 and Fe 3+ /H 2 O 2 systems. MATERIALS AND METHODS The initial concentration of MTBE was 5 10-4 M, that of hydrogen peroxide was 7.5x10-3 M. Sulfate concentrations were adjusted to 3,3x10-2 M and 6,6x10-2 M with Na 2 SO 4. The reactions were carried out in a batch reactor (V=0.25L). The oxidation experiments were performed in darkness at 25 ± 1 o C. Reaction mixtures were obtained by taking the appropriate aliquot of MTBE stock solution, adding Fe 2+ (or Fe 3+ ), and adjusting the ph with perchloric acid to ph = 2.8. The reaction was started by the addition of H 2 O 2. During the experiment, samples were collected after various reaction times and immediately quenched with 20µl of 0.01N Na 2 S 2 O 3. The degradation of MTBE was stopped after 90 min. Hydrogen peroxide was determined iodometrically ([H 2 O 2 ]>10-3 M) or spectrofotometricaly using TiCl 4 method ([H2O2]<10-3 M) [8]. MTBE were analyzed directly on a gas chromatograph (Perkin Elmer Clarus 500) coupled to a flame ionization detector (FID) (Perkin Elmer Elite Series PE). RESULTS AND DISCUSSION Fig.2 illustrates the effect of sulfate ions on the degradation of MTBE in the Fe 2+ /H 2 O 2 and Fe 3+ /H 2 O 2 systems in excess hydrogen peroxide ([Fe]:[H 2 O 2 ]=1:7,5). As can be seen from Figure 2 the presence of sulfate ions in system influences this process in different way. In the case of Fe 3+ /H 2 O 2 system used, distinctive inhibition of oxidation can be seen, whereas addition of sulfates to Fe 2+ /H 2 O 2 system influence these rates to much smaller extend. Effectiveness of degradation change significantly too. In presence of 6,6x10-3 M SO 4 after 90 min. of reaction MTBE is degraded down to 3% of the initial concentration in Fe 2+ /H 2 O 2 system whereas in Fe 3+ /H 2 O 2 to 46% of the initial concentration. As previously observed Pignatello [2], the presence of sulfate ions drastically decreased the rate of decomposition of H 2 O 2 by. As shown in the data in Figure 3, under our experimental conditions ([Fe 3+ ]=1x10-3 M, ph=2,8, I= ) concentration of sulfate 6,6x10-3 M can lead to a 10 times reduction of kinetic constant (k obs ). Absorption spectra of solutions have been measured in order to show the effect of sulfate concentration on the speciation of (Fig 4). The addition of sulfate to solution of in concentration 1x10-3 M leads to an increase of absorbance band close to 304 nm (Fig.3) This is attributed to the formation of iron(iii)
sulfatocomplexes: FeSO 4 + and Fe(SO 4 ) 2 -. The sulfatocomplexes of Fe 2+ (FeSO 4 ) have the higher reactivity as the free ferrous ions for the overall reaction with H 2 O 2 [9,10]. C/C 0 x10-3, M a) b) 0M SO 4 3,3x10-2 M SO 4 6,6x10-2 M SO 4 0,3 0,1 C/C 0 x10-3, M 1,0 0,8 0,6 0 M SO 4 3,3 x 10-2 M SO 4 6,6 x 10-2 M SO 4 time, min. time min Figure 2. Influence of the concentration of sulfate on MTBE degradation by Fe 2+ / H 2 O 2 - A) or Fe 3+ / H 2 O 2 B) Experimental conditions: [MTBE]=1x10-3 mol/l, [Fe 2+ ] 0 =1x10-3 mol/l, [Fe 3+ ] 0 =1x10-3 mol/l, [H 2 O 2 ]=7,5x10 3 mol/l, ph=2,8, T = 25±1 o C). 1,0 0,8 SO 4 free 3,3 x 10-2 M SO 4 6,6 x 10-2 M SO 4 12 10 C/Co 0,6 time, [min] k obs x10-3, min -1 8 6 4 2 0 0 2 4 6 [SO 4 ], x10-2 M Figure 3. Decomposition of H 2 O 2 by and first-order rate constant in the presence of sulfate. (Experimental condition: [Fe 3+ ] 0 =1x10-3 mol/l, ph=2,8). Therefore, under the conditions used in the present study, the extend and rate of mineralization of MTBE was dependent on formation of -sulfatcomlexes. Based on the discussion above -sulfocomplexes in contrast to -sulfocomplexes
inhibit the generation of hydroxyl radicals because they are non or less reactive than free ferric ions to decomposition of H 2 O 2. The Fenton reaction under conditions of excess peroxide, may be predominantly the Fe 3+ /H 2 O 2 process, since Fe 2+ is rapidly oxidized. However, as shown in Fig. 3a, in Fe 2+ /H 2 O 2 system the concentration of hydroxyl radicals were enough to mineralization of MTBE in 97% independent on concentration of sulfate in solution. Substituting Fe 2+ for Fe 3+ (Fig.3b) caused significant decreased of rate and effectiveness of MTBE decomposition. This indicates that sulfates retards the reaction by complexation of Fe 3+ at initial stage, when the ferrous ions is generated from ferric ions. Sulfates ions create competition between hydrogen peroxide and ferric ions leading to the inhibition of hydroxyl radicals generation. Absorbance, l=0,5 1 20x10-2 M 10x10-2 M 5x10-2 M 0 M 0 240 260 280 300 320 340 360 walvelenght, nm Figure4. Absorption spectra of solution in the absence and in the presence of different concentration of sulfate (Experimental conditions: 0 =1x10-3, ph=2,8,, T = 25±1 o C). Acknowledgements Financial support was provided by the Polish Ministry of Education and Research under grants: DS 8270-4-0093-6 and BW 8000-5-0194. The authors want to extend thanks to Joanna Piotrowska to take part in experimental works. REFERENCES [1] M.I. Kremer, J.Phis.Chem.A.(2003),107,1734 [2] J. Pignatello, L. Di, P. Huston, Environ.Sci.Technol (1999) 33, 1832 [3] De Laat J. Le G.T. Legube B, Chemosphere (2004) 55, 715 [4] E.Siedlecka, P. Stepnowski, Separation and Purification Technology (2006) 52,317 [5] E. Lipczyńska-Kochany, G. Sprah, S.Harms, Chemosphere (1995) 30, 9 [6] J. Kiwi, A. Lopez, V. Nadtochenko, Environ.Sci.Technol. (2000) 34, 2162 [7] E.M. Siedlecka, P. Stepnowski, A. Więckowska, in: J.Hupka, R. Aranowski (Eds.), Proceedings of 4th International Conference Oils and Environment, Gdansk University of Technology, Gdansk, 2005, 384 [8] G.M. Eisenberg, Ind. and Eng. Chem. 15 (1943) 327 [9] G.L. Truong, J. De Laat, B. Legube, Water Res. 38 (2004) 2383 [10] De Laat J., Le G.T., Environ.Sci.Technol, 39 (2005) 1811