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Heterogeneous photocatalytic oxidation of manganese(II) over TiO2
231
.I. Photochem. Photobiol. A: Chem., 69 (1992) 237-240
Heterogeneous A. Lozano
photocatalytic
X. Dom&nech+ (Received
of manganese(U)
over TiO,
and J. Garcia
EUETIT, Departament d’En&yetia
Deptament
oxidation
Qvimic4
UPC, 08222, Termssa (Sprrin)
and J. Casado
de Quimicu, Universitat Autdnoma
de Barcelona, 08193, BarceIona (Spain)
May 28, 1992, accepted August 6, 1992)
Abstract The photocatalytic oxidation of manganese(I1) in aqueous suspensions of TiOz on UV illumination was investigated. The experimental results obtained for manganese(I1) solutions as a function of the initial concentration, mass of semiconductor in suspension, pH, light intensity and temperature are reported. The pH of the solution is a key parameter that determines the efficiency of manganese(U) photo-oxidation. Near to the point of zero charge of TiOl the adsorption of manganese(II) onto Ti02 particles increases sharply, favouring charge transfer reactions at the semiconductor-electrolyte interface. The experimental results obtained from manganese(H) solutions at neutral pH agree with the Langmuir-Hinshelwood kinetic model and the corresponding equilibrium adsorption and rate constants have been deduced.
1. Introduction
The oxidation of manganese(I1) to MnO, is a process of enviromnental concern, mainly in aquatic media. Although manganese(H) is the thermodynamically stable form in aerated aqueous solutions, MnO, can be found in these media if the pH of the solutions is neutral-acidic [l]. This is due to the very slow oxidation kinetics of mangauese(II), which exhibits oxidation rate constants lower than 10m6 min-’ for pH values less than 8 [2]. However, in natural waters the presence of solids in suspension can enhance the oxidation rate of manganese(I1) by a catalysed reaction [3]. The presence of MnO, in water supplies is subject to rigorous controls because this oxide is responsible for the formation of unwanted dark precip itates. One of the most ubiquitous metallic oxide particles present in natural waters is TiO, [4]. It has been shown that this semiconductor oxide possesses good photoactivity promoting the oxidation of numerous substances [5]. On illumination with band gap light, the semiconductor particles absorb incident photons and create electron-hole pairs. These charge carriers can migrate to the surface
‘Author to whom correspondence
lOlO-6030/92/$5.00
should be addressed.
particles and react with suitable redox species in solution [6] . In a previous paper, some experimental results were reported which seemed to indicate that TiOz is an efficient photocatalyst for manganese(I1) photo-oxidation [7]. However, these results referred to manganese(U) photo-oxidation in a very limited set of experimental conditions which precluded any conclusion about the kinetics of manganese(II) photo-oxidation. In this paper, the photocatalytic oxidation of manganese(I1) to MnO, over TiO, under different experimental conditions is investigated and the results discussed in terms of the Langmuir-Hinshelwood kinetic model. 2. Experimental
details
All chemicals were of at least reagent grade and were used as received. As manganese(H) source, MnN0,.4H,O Merck pro-analysis was used. The titanium dioxide used in these experiments (Degussa F-25) was predominantly anatase (80% anatase and 20% rutile approximately) with a Brunauer-Emmet-Teller (BET) surface area of 59.1 m2 g-l and a mean particle size of 27 nm. Photochemical experiments were performed in a thermostatically controlled cylindrical Pyrex cell of 40 ml capacity. A mercury vapour lamp (Philips HPK) (125 W) was used as light source. The IR
8 1992 - Elaevier Sequoia. Ali rights reserved
238
A. Lozano et al. I Photocota&ic
fraction of the beam was removed by the water in the double wall of the cell. The intensity of the incident light inside the cell, measured using a uranyl oxalate actinometer, was 2.34~ lo-’ einsteins min- ‘. For the experiments at lower light intensities, neutral filters were used and were placed between the lamp and the photoreactor. Unless otherwise stated a concentration of TiO, of 2 g 1-l was used. The reactive mixtures inside the cell were maintained in suspension by magnetic stirring. The concentration of manganese(I1) in solution was determined polarographically (Metrohm 626 Polarecord) using potassium chloride as background electrolyte (Eljz = 1.5 ~~1s. saturated calomel electrode (SCE)).
3. Results
over TiOl
*
loo-
5600 *
b
20 tJ
::_;’ 3
4
5
6
7
PH
Fig. 1. Percentage of manganese(I1) removed as a function of pH in the dark (A) and on illumination (0) (irradiation time, 30 min; initial manganese@) concentration, 0.2X 10e4 mol 1-l; temperature, 25.0 “C).
and discussion
UV irradiation of 0.2X low4 mol 1-l manganese(l1) solutions in the presence of TiO, at pH 5.2, which is the natural pH of the suspension, produces the gradual elimination of manganese(I1) species in solution and the appearance of a slight dark colouration over the semiconductor particle. This can be explained by the oxidation of manganese(I1) species to insoluble MnOz according to Mn”
oxidation of Mn(Il)
+JO,+H,O-
Mn0,+2H+
in which H+ ions are generated. After 30 min of irradiation the percentage of photo-oxidized manganese(II) is about 43% and the pH is decreased by 0.4 pH units. From dark experiments under otherwise identical experimental conditions, 10% of manganese(I1) is eliminated from the solution, indicating that a certain degree of adsorption of manganese(I1) species onto TiO, particles takes place. The amount of cationic manganese(I1) species adsorbed on the semiconductor particles depends on the point of zero charge (PZC) of the solid. For the TiO, used in these experiments (Degussa P-25), with a PZC of about 6 [S], the percentage of manganese(I1) adsorbed increases sharply from pH 54 to more alkaline media. This is cleariy shown in Fig. 1 where the percentage of manganese(I1) eliminated from 0.2 X low4 mol 1-l manganese(I1) solutions containing TiO* after 30 min of agitation in the dark is given as a function of the initial pH. This behaviour can be explained by considering that, for Ti02 suspensions at a pH higher than PZC, the surface charge of the Ti02 particles becomes negative thus attracting cationic species from the solution. On illumination, more
manganese(I1) is eliminated at all pH values tested (Fig. 1); the yield of manganese(I1) elimination increases with increasing pH. In contrast with the dark experiments, where no change in pH takes place, on illumination a decrease in the pH of the solution is observed, being greater at higher initial pH. So, for initial pH values of 3.0 and 9.1, decreases of 0.1 and 1.5 are found after 30 min of irradiation, which indicates that the oxidation process is taking place [2]. Two factors account for the observed increasing yield of manganese(I1) photo-oxidation with increasing pH: (i) a higher degree of manganese(I1) adsorption onto TiO, particles and (ii) a greater driving force from a thermodynamic standpoint, because the rate of increase in the potential of Mn(II)/MnO, with increasing pH (-0.118 w per pH unit) is greater than the corresponding rate of increase of the oxidizing potential of TiOz (-0.059 V per pH unit) [9]. The yield of manganese(I1) photo-oxidation depends on the mass of semiconductor in suspension and on the intensity of the incident light. In the former case, the percentage of manganese(I1) eliminated increases with an increase in the quantity of suspended TiOz, attaining a limiting value when the concentration of the semiconductor is about 4 g 1-l. In the latter case, the amount of manganese(I1) photo-oxidized increases with an increase in the light flux. In Fig. 2, the number of moles of manganese(I1) photo-oxidized is shown as a function of the number of moles of incident photons for 15 min irradiations of 1.4 X 10m4 and 4.0 x lop4 mol 1-l manganese(I1) solutions containing TiO, in suspension at neutral pH. As can be seen, straight lines are obtained with slopes of 0.0040 (correlation coefficient, 0.956) and 0.0067
A. Lozano et al. / Photocatalytic oxidation of MI(H) overTiOz
239
R= -dc/dr=K,6J=K,K,c/(l+K,c)
P
0
rhotons hd
lrf4
Fig. 2. Moles of manganese@) photcwxidiied as a function of the number of moles of incident photons for 1.4X lo-’ mol I-’ (A) and 4.0~ 10e4 mall-’ (0) solutions at neutral pH (irradiation time, 15 min; temperature, 25.0 “C).
Fig. 3. l/Ri as. l/q for different manganese(I1)
solutions at
PH.
(correlation coefficient, 0.973) for 1.4 x 10m4 and 4.0X 10e4 mol 1-l manganese(II) solutions respectively. These slope values correspond to the ratios between the number of moles of manganese@) photo-oxidized and the number of moles of incident photons. The different slopes of the straight lines in Fig. 2 show that the photo-oxidation yield depends on the initial concentration of manganese(II). From the time course of manganese(I1) concentration in solution, for irradiated solutions of different initial manganese(I1) concentrations (in the range 0.2x 10m4Ax 10m4 mol 1-l) containing Ti02 in suspension at neutral pH, the initial rate (Ri) of manganese(U) elimination has been determined. A plot of l/Ri vs. the inverse of the initial manganese(I1) concentration gives a straight line with a correlation coefficient of 0.9987 (see Fig. 3). This result can be interpreted in terms of the Langmuir-Hinshelwood kinetic model, in which it is assumed that the rate of a unimolecular reaction is proportional to the surface coverage (0) [lo, 111. Assuming that the adsorption equilibrium follows a Langmuir isotherm, then
(1)
where KL and Kz are the reaction rate constant and the equilibrium adsorption constant respectively. From the plot of Fig. 3 and according to eqn. (l), K1 and Kz are deduced to be 6.0~ lo-’ mol 1-l min-l and 3.6 X lo3 1 mol-’ respectively. The high value of K2 compared with that obtained for other solutes on the same semiconductor [5] indicates that manganese(U) is strongly adsorbed on TiOz particles. This favours the oxidation of manganese(U) to MnO, which probably takes place in the adsorption layer by reaction with photogenerated OH radicals according to the scheme shown in Table 1. Although adsorbed OH radicals are the major oxidizing agent in acid media the oxidant is the radical HO* [13, 141 (see second reaction in the scheme in Table 1); this species is probably predominantly responsible for the oxidation of manganese(X) in acidic environments. The yield of manganese@) photo-oxidation also depends on temperature. Table 2 summarizes the percentages of manganese(II) photo-oxidized after 15 min of irradiation of solutions of 4.0~ 10m4 mol 1-l in the presence of TiO, at neutral pH at different temperatures in the range 25-65 “C. As can be seen the yield of manganese(l1) photooxidation increases with increasing temperature. These data obey an Arrhenius-type behaviour with an activation energy of 8.3 k.I mol-‘. This value is significantly lower than that observed for the oxidation of manganese(II) in the dark in both the absence and presence of catalyst [2, 151. TABLE 1. Reactions which generate radicals electrolyte-semiconductor interface [12]
at the illuminated
O2 + esc- ---+ Oz’O;- +H+ d HO; HO; + H+ Hz02 H202+ere- --+ OIT+OHH2t&+Oz’- ---+ OH+OH-+02 OH-+H+ Hz0 f h,+ + OH- fh,+ OR
TABLE 2. Percentages of manganese(U) photo-oxidized at different temperatures (irradiation time, 15 min; pH 7.3; initial mangane.se(ll) concentration, 4.0 X lo-+ mol 1-l) fi(II) 61 69 78 80 92
(k)
T (‘C) 25.0 35.0 45.0 55.0 65.0
240
A. Lozano et al. I Phofocoralylic oxidation of Mn(II) over TiOl
References
4. Conclusions
The results obtained in this work indicate that TiO, behaves as a good photocatalyst for the oxidation of manganese(I1) to MnOz in aqueous solutions. In particular, TiOp can oxidize manganese(H) in neutral to slightly acidic media, where dark homogeneous oxidation is very slow. This fact may be relevant to the behaviour of manganese(H) in the environment. It may be assumed that other semiconductor oxides also present in surface waters, i.e. Fe&J, or ZnO, may promote the oxidation of manganese(I1). Thus photocatalysis plays a role in environmental aquatic chemistry in relation to manganese(I1) photo-oxidation.
Acknowledgments
We are grateful for the financial the Fundaci6n Domingo Martinez.
support
from
1 D. Diem and W. Stumm, Geochim. Cosmochim. Acta, 48 (1984) 1571. 2 S. H. R. Davies and J. J. Morgan, J. CoUoid Inmfme Sci., 129 (1989) 63. 3 A. W. Morris and A. J. Bales, Nature, 279 (1979) 318. 4 S. E. Manahan, Environmental Chemishy, Lewis, Boston, 1990, p. 57. 5 M. Schiavello (ed.), Pho~ocarolysiF and Environment, NATO ASI Series, Kluwer, Dordrecht, 1988. 6 G. Hodes and M. GrBtzel, Nouv. L Chim., 8 (1984) $09. 7 K. Tanaka, K. Harada and S. Murata, Sol. Energy, 36 (1986) 159. 8 N. Jaffrezic-Renault, P. Pichat. A. Foissy and R. Mercier, J Phys. Chem., 90 (1986) 2733. 9 S. R. Morrison, ElectrochemishyatSemiconducrorand Oxidized Meral Electrodes, Plenum, New York, 1980, p. 154. 10 H. Al-Ekabi and N. Serpone, J. Phys. Chem., 92 (1988) 5726. 11 C. S. Turchi and D. F. Ollis, 1. Cata!., 122 (1990) 178. 12 K Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Sot. Jpn., 58 (1985) 2015. 13 H. Hidaka, J. Zhao, E. Pelizzetti and N. Serpone, L Phys. Chem., 96 (1992) 2226. 14 D. Lawless, N. Serpone and D. Meisel, 1 Phys. Chem., 95 (1991) 5166. 15 R. W. Coughlin and I. Matsui, .K Cataf., 41 (1976) 108.
.I. Photochem. Photobiol. A: Chem., 69 (1992) 237-240
Heterogeneous A. Lozano
photocatalytic
X. Dom&nech+ (Received
of manganese(U)
over TiO,
and J. Garcia
EUETIT, Departament d’En&yetia
Deptament
oxidation
Qvimic4
UPC, 08222, Termssa (Sprrin)
and J. Casado
de Quimicu, Universitat Autdnoma
de Barcelona, 08193, BarceIona (Spain)
May 28, 1992, accepted August 6, 1992)
Abstract The photocatalytic oxidation of manganese(I1) in aqueous suspensions of TiOz on UV illumination was investigated. The experimental results obtained for manganese(I1) solutions as a function of the initial concentration, mass of semiconductor in suspension, pH, light intensity and temperature are reported. The pH of the solution is a key parameter that determines the efficiency of manganese(U) photo-oxidation. Near to the point of zero charge of TiOl the adsorption of manganese(II) onto Ti02 particles increases sharply, favouring charge transfer reactions at the semiconductor-electrolyte interface. The experimental results obtained from manganese(H) solutions at neutral pH agree with the Langmuir-Hinshelwood kinetic model and the corresponding equilibrium adsorption and rate constants have been deduced.
1. Introduction
The oxidation of manganese(I1) to MnO, is a process of enviromnental concern, mainly in aquatic media. Although manganese(H) is the thermodynamically stable form in aerated aqueous solutions, MnO, can be found in these media if the pH of the solutions is neutral-acidic [l]. This is due to the very slow oxidation kinetics of mangauese(II), which exhibits oxidation rate constants lower than 10m6 min-’ for pH values less than 8 [2]. However, in natural waters the presence of solids in suspension can enhance the oxidation rate of manganese(I1) by a catalysed reaction [3]. The presence of MnO, in water supplies is subject to rigorous controls because this oxide is responsible for the formation of unwanted dark precip itates. One of the most ubiquitous metallic oxide particles present in natural waters is TiO, [4]. It has been shown that this semiconductor oxide possesses good photoactivity promoting the oxidation of numerous substances [5]. On illumination with band gap light, the semiconductor particles absorb incident photons and create electron-hole pairs. These charge carriers can migrate to the surface
‘Author to whom correspondence
lOlO-6030/92/$5.00
should be addressed.
particles and react with suitable redox species in solution [6] . In a previous paper, some experimental results were reported which seemed to indicate that TiOz is an efficient photocatalyst for manganese(I1) photo-oxidation [7]. However, these results referred to manganese(U) photo-oxidation in a very limited set of experimental conditions which precluded any conclusion about the kinetics of manganese(II) photo-oxidation. In this paper, the photocatalytic oxidation of manganese(I1) to MnO, over TiO, under different experimental conditions is investigated and the results discussed in terms of the Langmuir-Hinshelwood kinetic model. 2. Experimental
details
All chemicals were of at least reagent grade and were used as received. As manganese(H) source, MnN0,.4H,O Merck pro-analysis was used. The titanium dioxide used in these experiments (Degussa F-25) was predominantly anatase (80% anatase and 20% rutile approximately) with a Brunauer-Emmet-Teller (BET) surface area of 59.1 m2 g-l and a mean particle size of 27 nm. Photochemical experiments were performed in a thermostatically controlled cylindrical Pyrex cell of 40 ml capacity. A mercury vapour lamp (Philips HPK) (125 W) was used as light source. The IR
8 1992 - Elaevier Sequoia. Ali rights reserved
238
A. Lozano et al. I Photocota&ic
fraction of the beam was removed by the water in the double wall of the cell. The intensity of the incident light inside the cell, measured using a uranyl oxalate actinometer, was 2.34~ lo-’ einsteins min- ‘. For the experiments at lower light intensities, neutral filters were used and were placed between the lamp and the photoreactor. Unless otherwise stated a concentration of TiO, of 2 g 1-l was used. The reactive mixtures inside the cell were maintained in suspension by magnetic stirring. The concentration of manganese(I1) in solution was determined polarographically (Metrohm 626 Polarecord) using potassium chloride as background electrolyte (Eljz = 1.5 ~~1s. saturated calomel electrode (SCE)).
3. Results
over TiOl
*
loo-
5600 *
b
20 tJ
::_;’ 3
4
5
6
7
PH
Fig. 1. Percentage of manganese(I1) removed as a function of pH in the dark (A) and on illumination (0) (irradiation time, 30 min; initial manganese@) concentration, 0.2X 10e4 mol 1-l; temperature, 25.0 “C).
and discussion
UV irradiation of 0.2X low4 mol 1-l manganese(l1) solutions in the presence of TiO, at pH 5.2, which is the natural pH of the suspension, produces the gradual elimination of manganese(I1) species in solution and the appearance of a slight dark colouration over the semiconductor particle. This can be explained by the oxidation of manganese(I1) species to insoluble MnOz according to Mn”
oxidation of Mn(Il)
+JO,+H,O-
Mn0,+2H+
in which H+ ions are generated. After 30 min of irradiation the percentage of photo-oxidized manganese(II) is about 43% and the pH is decreased by 0.4 pH units. From dark experiments under otherwise identical experimental conditions, 10% of manganese(I1) is eliminated from the solution, indicating that a certain degree of adsorption of manganese(I1) species onto TiO, particles takes place. The amount of cationic manganese(I1) species adsorbed on the semiconductor particles depends on the point of zero charge (PZC) of the solid. For the TiO, used in these experiments (Degussa P-25), with a PZC of about 6 [S], the percentage of manganese(I1) adsorbed increases sharply from pH 54 to more alkaline media. This is cleariy shown in Fig. 1 where the percentage of manganese(I1) eliminated from 0.2 X low4 mol 1-l manganese(I1) solutions containing TiO* after 30 min of agitation in the dark is given as a function of the initial pH. This behaviour can be explained by considering that, for Ti02 suspensions at a pH higher than PZC, the surface charge of the Ti02 particles becomes negative thus attracting cationic species from the solution. On illumination, more
manganese(I1) is eliminated at all pH values tested (Fig. 1); the yield of manganese(I1) elimination increases with increasing pH. In contrast with the dark experiments, where no change in pH takes place, on illumination a decrease in the pH of the solution is observed, being greater at higher initial pH. So, for initial pH values of 3.0 and 9.1, decreases of 0.1 and 1.5 are found after 30 min of irradiation, which indicates that the oxidation process is taking place [2]. Two factors account for the observed increasing yield of manganese(I1) photo-oxidation with increasing pH: (i) a higher degree of manganese(I1) adsorption onto TiO, particles and (ii) a greater driving force from a thermodynamic standpoint, because the rate of increase in the potential of Mn(II)/MnO, with increasing pH (-0.118 w per pH unit) is greater than the corresponding rate of increase of the oxidizing potential of TiOz (-0.059 V per pH unit) [9]. The yield of manganese(I1) photo-oxidation depends on the mass of semiconductor in suspension and on the intensity of the incident light. In the former case, the percentage of manganese(I1) eliminated increases with an increase in the quantity of suspended TiOz, attaining a limiting value when the concentration of the semiconductor is about 4 g 1-l. In the latter case, the amount of manganese(I1) photo-oxidized increases with an increase in the light flux. In Fig. 2, the number of moles of manganese(I1) photo-oxidized is shown as a function of the number of moles of incident photons for 15 min irradiations of 1.4 X 10m4 and 4.0 x lop4 mol 1-l manganese(I1) solutions containing TiO, in suspension at neutral pH. As can be seen, straight lines are obtained with slopes of 0.0040 (correlation coefficient, 0.956) and 0.0067
A. Lozano et al. / Photocatalytic oxidation of MI(H) overTiOz
239
R= -dc/dr=K,6J=K,K,c/(l+K,c)
P
0
rhotons hd
lrf4
Fig. 2. Moles of manganese@) photcwxidiied as a function of the number of moles of incident photons for 1.4X lo-’ mol I-’ (A) and 4.0~ 10e4 mall-’ (0) solutions at neutral pH (irradiation time, 15 min; temperature, 25.0 “C).
Fig. 3. l/Ri as. l/q for different manganese(I1)
solutions at
PH.
(correlation coefficient, 0.973) for 1.4 x 10m4 and 4.0X 10e4 mol 1-l manganese(II) solutions respectively. These slope values correspond to the ratios between the number of moles of manganese@) photo-oxidized and the number of moles of incident photons. The different slopes of the straight lines in Fig. 2 show that the photo-oxidation yield depends on the initial concentration of manganese(II). From the time course of manganese(I1) concentration in solution, for irradiated solutions of different initial manganese(I1) concentrations (in the range 0.2x 10m4Ax 10m4 mol 1-l) containing Ti02 in suspension at neutral pH, the initial rate (Ri) of manganese(U) elimination has been determined. A plot of l/Ri vs. the inverse of the initial manganese(I1) concentration gives a straight line with a correlation coefficient of 0.9987 (see Fig. 3). This result can be interpreted in terms of the Langmuir-Hinshelwood kinetic model, in which it is assumed that the rate of a unimolecular reaction is proportional to the surface coverage (0) [lo, 111. Assuming that the adsorption equilibrium follows a Langmuir isotherm, then
(1)
where KL and Kz are the reaction rate constant and the equilibrium adsorption constant respectively. From the plot of Fig. 3 and according to eqn. (l), K1 and Kz are deduced to be 6.0~ lo-’ mol 1-l min-l and 3.6 X lo3 1 mol-’ respectively. The high value of K2 compared with that obtained for other solutes on the same semiconductor [5] indicates that manganese(U) is strongly adsorbed on TiOz particles. This favours the oxidation of manganese(U) to MnO, which probably takes place in the adsorption layer by reaction with photogenerated OH radicals according to the scheme shown in Table 1. Although adsorbed OH radicals are the major oxidizing agent in acid media the oxidant is the radical HO* [13, 141 (see second reaction in the scheme in Table 1); this species is probably predominantly responsible for the oxidation of manganese(X) in acidic environments. The yield of manganese@) photo-oxidation also depends on temperature. Table 2 summarizes the percentages of manganese(II) photo-oxidized after 15 min of irradiation of solutions of 4.0~ 10m4 mol 1-l in the presence of TiO, at neutral pH at different temperatures in the range 25-65 “C. As can be seen the yield of manganese(l1) photooxidation increases with increasing temperature. These data obey an Arrhenius-type behaviour with an activation energy of 8.3 k.I mol-‘. This value is significantly lower than that observed for the oxidation of manganese(II) in the dark in both the absence and presence of catalyst [2, 151. TABLE 1. Reactions which generate radicals electrolyte-semiconductor interface [12]
at the illuminated
O2 + esc- ---+ Oz’O;- +H+ d HO; HO; + H+ Hz02 H202+ere- --+ OIT+OHH2t&+Oz’- ---+ OH+OH-+02 OH-+H+ Hz0 f h,+ + OH- fh,+ OR
TABLE 2. Percentages of manganese(U) photo-oxidized at different temperatures (irradiation time, 15 min; pH 7.3; initial mangane.se(ll) concentration, 4.0 X lo-+ mol 1-l) fi(II) 61 69 78 80 92
(k)
T (‘C) 25.0 35.0 45.0 55.0 65.0
240
A. Lozano et al. I Phofocoralylic oxidation of Mn(II) over TiOl
References
4. Conclusions
The results obtained in this work indicate that TiO, behaves as a good photocatalyst for the oxidation of manganese(I1) to MnOz in aqueous solutions. In particular, TiOp can oxidize manganese(H) in neutral to slightly acidic media, where dark homogeneous oxidation is very slow. This fact may be relevant to the behaviour of manganese(H) in the environment. It may be assumed that other semiconductor oxides also present in surface waters, i.e. Fe&J, or ZnO, may promote the oxidation of manganese(I1). Thus photocatalysis plays a role in environmental aquatic chemistry in relation to manganese(I1) photo-oxidation.
Acknowledgments
We are grateful for the financial the Fundaci6n Domingo Martinez.
support
from
1 D. Diem and W. Stumm, Geochim. Cosmochim. Acta, 48 (1984) 1571. 2 S. H. R. Davies and J. J. Morgan, J. CoUoid Inmfme Sci., 129 (1989) 63. 3 A. W. Morris and A. J. Bales, Nature, 279 (1979) 318. 4 S. E. Manahan, Environmental Chemishy, Lewis, Boston, 1990, p. 57. 5 M. Schiavello (ed.), Pho~ocarolysiF and Environment, NATO ASI Series, Kluwer, Dordrecht, 1988. 6 G. Hodes and M. GrBtzel, Nouv. L Chim., 8 (1984) $09. 7 K. Tanaka, K. Harada and S. Murata, Sol. Energy, 36 (1986) 159. 8 N. Jaffrezic-Renault, P. Pichat. A. Foissy and R. Mercier, J Phys. Chem., 90 (1986) 2733. 9 S. R. Morrison, ElectrochemishyatSemiconducrorand Oxidized Meral Electrodes, Plenum, New York, 1980, p. 154. 10 H. Al-Ekabi and N. Serpone, J. Phys. Chem., 92 (1988) 5726. 11 C. S. Turchi and D. F. Ollis, 1. Cata!., 122 (1990) 178. 12 K Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Sot. Jpn., 58 (1985) 2015. 13 H. Hidaka, J. Zhao, E. Pelizzetti and N. Serpone, L Phys. Chem., 96 (1992) 2226. 14 D. Lawless, N. Serpone and D. Meisel, 1 Phys. Chem., 95 (1991) 5166. 15 R. W. Coughlin and I. Matsui, .K Cataf., 41 (1976) 108.