Ruthenium(II)

concentration, binding constant, and capacity). In this experiment, the composition of the test solution was as follows: [Ru(bpy)3. 2+] ) 0-100 μM; [...
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Environ. Sci. Technol. 1998, 32, 3948-3953

Photocatalytic Production of Hydrogen Peroxide by Tris(2,2′-bipyridine) Ruthenium(II) Using Humic Acids as Electron Donor M A S A M I F U K U S H I M A , * ,† KENJI TATSUMI,† SHUNITZ TANAKA,‡ AND HIROSHI NAKAMURA‡ Department of Hydrospheric Environmental Protection, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba 305-8569, Japan, and Division of Material Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan

The photosensitized production of H2O2 by tris(2,2′bipyridine) ruthenium(II) complex [Ru(bpy)32+] was enhanced ∼120-fold by the presence of humic acid (HA). It was found that methyl viologen (MV2+) as a redox mediator in this system was required to produce H2O2. Moreover, in the present system, H2O2 was not produced in the absence of formic acid. The presumed role of formic acid is to reduce the oxidized HA. The reducing ability of HA assisted the photocatalytic reaction of Ru(bpy)32+. In addition, the results of the effect of inorganic salt on H2O2 production suggest that the binding abilities of Ru(bpy)32+ and MV2+ with HA are determinant in the enhancement of H2O2 production.

Introduction Hydrogen peroxide (H2O2) plays an important role in a variety of pollution control systems, and it plays a major role as a primary oxidant in the degradation of pollutants (1). H2O2, a constituent of oxygenated natural waters, is largely produced via the reduction of oxygen by photoexcited organic matter, such as humic substances (HS) (2, 3). Under normal conditions, the concentration of H2O2, as produced by the photoexcitation of natural organic matter, is below the 10 µM level (2). Recent papers (4, 5) dealing with the photoinduced Fenton reaction and UV irradiation have concluded that, for optimum oxidative degradation of pollutants, H2O2 levels of several hundred micromolar are required. Thus, if H2O2 production, mediated by HS, could be enhanced greatly, this could potentially be incorporated into remedial systems very effectively, via Fenton reaction chemistry. It is noteworthy that the photocatalytic production of H2O2 has been investigated using photosensitized dyes, such as the tris(2,2′-bipyridine) ruthenium(II) complex [Ru(bpy)32+] (6-8). The photoinduced electron transfer reaction of Ru(bpy)32+ with redox mediators, such as methyl viologen (MV2+), is shown in Scheme 1. * Author whom correspondence should be addressed. Phone: +81298-58-8328; fax: +81-298-58-8308; e-mail: [email protected]. † National Institute for Resources and Environment. ‡ Hokkaido University. 3948

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SCHEME 1

The redox reactions in Scheme 1 cannot occur without an electron donor, such as EDTA (9). Because the redox potential of Ru(bpy)33+/2+ is known to be +1.29 V (10), EDTA (+0.4 V) (9) could be effectively used as an electron donor for the reduction of Ru(bpy)33+, the oxidative species. HS, which are widely distributed in aqueous environments, function as reducing agents (11-13). It has been reported that the redox potentials of HS are in the range of +0.5-+0.7 V (11, 12), and it is well-known that HS are capable of reducing chromium(VI) (13). This redox potential (+1.33 V) (14) is near that of Ru(bpy)33+/2+. This suggests that Ru(bpy)33+ may be also reduced by HS. The present paper describes attempts to enhance H2O2 production in aqueous systems, which contain humic acid (HA), by taking advantage of the reducing abilities of HA to the Ru(bpy)32+-photosensitized production of H2O2.

Experimental Section Materials. HA was extracted and purified from Shinshinotsu peat soil using a protocol described by the International Humic Substances Society (IHSS) (15). A stock solution of HA (1 g L-1) was prepared by dissolving it in aqueous 0.01 M NaOH. The total acidity and the carboxyl contents were determined according to a Ba(OH)2 method and a Ca(CH3COO)2 method, respectively (16). The reducing capacity of the HA was determined according to the Walkly-Black method (17). The elemental compositions, the acidic functional group contents, and the reducing capacity of HA are summarized in Table 1. Ru(bpy)32+ was synthesized with Ru(bpy)3Cl2‚6H2O, according to the literature (18). MV2+ was purchased from Kishida Chemicals (Osaka, Japan) as MVCl2‚ 3H2O. A 31% aqueous H2O2 solution (Wako Pure Chemicals, Osaka Japan), standardized by titration with KMnO4, was used as a standard solution (19). Photoirradiation Experiments. The test solutions usually contained 50 µM Ru(bpy)32+, 2.5 mM MV2+, 100 mg L-1 HA, and 0.01 M formate buffer (pH 3.6). For investigating the effect of NaCl concentration, 0.001-0.5 M NaCl was added to this solution. Photoirradiation was carried out by 500 W xenon short arc lamp (Ushio Denki, K.K.) after passing through a colored glass filter (Toshiba Glass Co., Ltd.). An L-42 colored glass filter, which cut off light below 420 nm, was routinely used. The light intensity was set to 1.8 × 105 lx with an illuminometer. Other colored glass filters, which were used to study the effects of wavelength, were UV-25, Y-48, and O-54, and, for these, the transmissible wavelengths were above 290, 480, and 540 nm, respectively. Twenty milliliter aliquots of the test solution were placed in a cylindrical glass vial (28 L × 61 mm). Oxygen gas (99.5% purity) was bubbled into the test solution before the photoirradiation after passing through pure water. The screw cap of the vial was closed, and the solution was then irradiated 10.1021/es970866d CCC: $15.00

 1998 American Chemical Society Published on Web 11/04/1998

TABLE 1. Elemental Composition, Acidic Functional Group Contents, and Reducing Capacity of HA elemental composition %C, 52.2 %H, 5.4 %N, 2.1 %O, 39.9 %S, 0.4

acidic functional groups (mequiv g-1 of HA) total acidity, 11.6 carboxyl groups, 9.7 reducing capacity (mmol of Cr g-1 of HA) 31.7

for 60 min. In the case of experiments for the effect of formic acid, oxygen gas was bubbled into the test solutions during the photoirradiation. The temperature of the test solution was maintained at 25 ( 3 °C. H2O2 was analyzed according to a photomeric method involving the peroxidase-catalyzed oxidation of N,N-diethyl-1,4-phenylenediammonium (19). Prior to the analysis of H2O2, to remove Ru(bpy)32+, MV2+, and HA, the test solution was passed through cation exchanger, sulfopropyl-Sephadex C-25 (C-25), and anion exchanger, diethylaminoethyl-Sephadex A-25 (A-25), columns. It was confirmed that, in these processes, no adsorption of H2O2 on the C-25 and A-25 could be observed. Determination of Binding Species of Ru(bpy)32+ and MV2+ with HA. Under dark conditions, we determined the binding abilities of Ru(bpy)32+ and MV2+ to HA (species concentration, binding constant, and capacity). In this experiment, the composition of the test solution was as follows: [Ru(bpy)32+] ) 0-100 µM; [MV2+] ) 0-200 µM; [HA] ) 100 mg L-1; and 0.01 M formate buffer (pH 3.6). The measurements were carried out according to the A-25 column method (20). The test solution was passed through the A-25 column, and the unbound species of Ru(bpy)32+ or MV2+ in the effluent were determined by absorption spectrophotometry at 452 nm ( ) 15 500 M-1 cm-1) and at 257 nm ( ) 17 300 M-1 cm-1), respectively. The concentration of the bound species was calculated by subtracting the concentrations of unbound species measured from the total concentration initially added. Determination of Formic Acid. A formate solution, at pH 3.6, was prepared by mixing aqueous 0.1 M HCOOH and 0.1 M HCOONa solutions. For the effect of the formic acid concentration, the formate solution was added to the 0.05 M phosphate buffer solution (pH 3.6) containing 50 µM Ru(bpy)32+, 2.5 mM MV2+, and 100 mg L-1 HA. For the other experiments with respect to the effects of formic acid, solution pH values were adjusted by the addition of 0.03 or 0.05 M formate and 0.05 M phosphate buffers. After photoirradiation, the pH of the test solution containing formic acid was acidified to pH 290 nm, and the H2O2 concentrations were very small at wavelengths >420 nm (3.3 µM). As would be expected on the basis of the mechanism described in Scheme 1, ∼200 µM H2O2 had accumulated in the presence of Ru(bpy)32+ and MV2+, using wavelengths >290 and >420 nm. These wavelength ranges included the photoexcitation wavelength of Ru(bpy)32+. Moreover, much smaller amounts of H2O2 were produced VOL. 32, NO. 24, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of HA concentration on the concentration of H2O2. [Ru(bpy)32+] ) 50 µM; [MV2+] ) 2.5 mM, photoirradiation, 60 min; 0.01 M formate buffer (pH 3.6).

FIGURE 2. Effect of the concentrations of Ru(bpy)32+ and MV2+ on the accumulation of H2O2. 0.01 M formate buffer (pH 3.6); photoirradiation, 60 min; (a) [MV2+] ) 2.5 mM, [HA] ) 100 mg L-1; (b) [Ru(bpy)32+] ) 50 µM, [HA] ) 100 mg L-1. above 480 and 560 nm, where the wavelength ranges shielded the photoexcitation wavelength of Ru(bpy)32+. These indicate that H2O2 production above 420 nm of irradiation is mainly due to the photosensitization Ru(bpy)32+. Moreover, H2O2 production as a result of the photoexcitation of HA was negligible except for the case when the UV-29 glass filter was used. On the other hand, Figure 2 shows the relationships between the concentrations of H2O2 and Ru(bpy)32+ or MV2+. The H2O2 accumulation for 60 min of irradiation increased with increasing Ru(bpy)32+ or MV2+ concentration. This indicates that both Ru(bpy)32+ and MV2+ are required to produce several-hundred micromolar level of H2O2. The H2O2 concentration accumulated without Ru(bpy)32+ (3.7 µM, Figure 2a) was much smaller than that without MV2+ (21 µM, Figure 2b). These results indicate that the contribution to photoexcitation by HA is minor in the present system. Effect of HA Concentration. In the absence of HA, only 2 µM H2O2 was produced after 60 min of irradiation for the case of the solution containing 50 µM Ru(bpy)32+, 2.5 mM MV2+, and 0.01 M formate buffer (pH 3.6). H2O2 production in this case is due to the reduction of dissolved oxygen by MV+ and/or *Ru(bpy)32+ (6-8). In the presence of 100 mg L-1 HA, 220 µM H2O2 had accumulated after 60 min of irradiation. However, no H2O2 production was observed under dark conditions, even when the solution, containing Ru(bpy)32+, MV2+, and HA, was allowed to stand for 12 h. The effect of HA concentration on the accumulation of H2O2 is shown in Figure 3. The vertical axis in Figure 3 represents the ratio of the concentration in the presence of HA, [H2O2], to that without, [H2O2]0. The ratio increased with increasing HA concentration up to 75 mg L-1. These results suggest that the addition of HA enhances the production of H2O2. However, this ratio decreased for concentrations >100 mg L-1. The reason for this will be discussed below. 3950

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FIGURE 4. Effect of formic acid concentration on the accumulation of H2O2 (a) and the loss of formic acid and the production of H2O2 as the result of irradiation (b). [Ru(bpy)32+] ) 50 µM, [MV2+] ) 2.5 mM; [HA] ) 100 mg L-1; 0.05 M phosphate buffer (pH 3.6); photoirradiation, 60 min; (b) [HCOOH] ) 0.03 M. Effect of Formic Acid. Because the effective pH for the degradation of pollutants by hydroxyl radical is reported to be in the acidic range (pH 3-4) (25), we selected at pH of 3.6 for the present study. To adjust the pH of the test solution, formate and phosphate buffers (0.05 M) were examined. Although 210 µM H2O2 accumulated after 60 min of irradiation in the formate buffer, only 20 µM H2O2 was produced in the phosphate buffer. This suggests that formic acid is effective in the production of H2O2. To confirm the effectiveness of formic acid, we investigated the effect of formic acid concentration on H2O2 accumulation after a period of 60 min of irradiation. As shown in Figure 4a, the H2O2 concentration increased with increasing formate concentration. This suggests that formic acid is required for H2O2 production in the system used here. Effect of Binding Ru(bpy)32+ and MV2+ to HA. It has been reported that the efficiency of the photoinduced electron

FIGURE 5. Scatchard plots for the binding of Ru(bpy)32+ and MV2+ with HA. [HA] ) 20 mg L-1; [Ru(bpy)32+] ) 0-100 µM; [MV2+] ) 0-200 µM, 0.01 M formate buffer (pH 3.6), dark condition. transfer reaction is enhanced by the accumulation of Ru(bpy)32+ and MV2+ into polyions (6, 26-29). Because HAs are also polyelectrolytes with large anionic electrostatic fields, they are capable of binding cations, such as Ru(bpy)32+ and MV2+. Therefore, the binding constants and capacities were evaluated according to the A-25 resin method along with a Scatchard approach (20). Figure 5 shows the Scatchard plot for the case of Ru(bpy)32+ and MV2+, where [bound] and [unbound] represent the concentrations of bound and unbound species, respectively. The linear relationship can be written

[bound]/[unbound] ) KcL - K[bound]

(2)

where K and cL represent the conditional binding constant and the total concentration of binding site, respectively. These could be evaluated by the intercept and slope of the linear lines in Figure 5. The binding capacity was calculated from the cL and HA concentration:

[binding capacity (equiv g-1)] ) cL (mol L-1)/[HA (g L-1)] × 2 (3) The logarithm of the conditional binding constants was also evaluated as Ru(bpy)32+, 5.1 ( 0.2, and MV2+, 3.9 ( 0.1. Moreover, the binding capacities were 540 ( 50 and 360 ( 40 µequiv g-1 of HA for Ru(bpy)32+ and MV2+, respectively. However, the pH used in the present system was 3.6, and the major ionizable groups in HA were undissociated. Therefore, the binding of Ru(bpy)32+ and MV2+ with HA may be due to strong cooperative binding by the interaction of aromatic rings as well as anionic electrostatic forces, as reported for the case of PVS (30). It has been reported that, when the binding species of Ru(bpy)32+ and redox mediator with the polyion are decreased by increasing the concentration of inorganic salts, the fluorescence quenching rate of *Ru(bpy)32+ is also decreased (26). Therefore, the binding Ru(bpy)32+ and MV2+ to HA may be related to H2O2 production. Because of this, NaCl was used as an inorganic salt to investigate this effect on H2O2 production. Figure 6a shows the relationships between the NaCl concentration and the H2O2 concentration that had accumulated for 60 min of irradiation. The H2O2 concentration decreased with increasing NaCl concentration, and the concentration at 0.005 M NaCl was ∼2-fold larger than that at 0.1 M. Figure 6b shows the relationships between the NaCl concentration and the percentages of bound Ru(bpy)32+ or MV2+ with HA under dark conditions. The percentages of Ru(bpy)32+- or MV2+-HA decreased with an increase in NaCl concentration. The H2O2 accumulation for 60 min of irradiation increased with decreasing NaCl concentration,

FIGURE 6. Effect of NaCl concentration on the production of H2O2 (a) and the concentration of Ru(bpy)32+- or MV2+-HA (b). 0.01 M formate buffer (pH 3.6); (a) [Ru(bpy)32+] ) 50 µM, [MV2+] ) 2.5 mM, [HA] ) 100 mg L-1, photoirradiation, 60 min; (b) [Ru(bpy)32+] ) 20 µM, [MV2+] ) 10 µM, [HA] ) 40 mg L-1, dark condition. and the percentages of the binding species of Ru(bpy)32+ and MV2+ also increased with decreasing NaCl concentration. These results suggest that the binding effects of Ru(bpy) 32+ and MV2+ with HA contribute to the enhancement of the H2O2 production. Morawetz et al. (31) reported that, when the polyion poly(vinyl sulfate) (PVS) was added into the redox system of Co(III)/Fe(II), PVS accelerated the electron transfer reaction at lower polyion concentrations but decreased it at higher concentrations. This acceleration at lower polyion concentrations is due to effective local reactant accumulation in the polymer domain. The deceleration at higher concentrations can be attributed to the fact that the reactants are spread over each domain, thus resulting in a decrease in the local concentration of reactants. In Figure 3, the ratio [H2O2]/[H2O2]0 corresponds to the acceleration factor for H2O2 production. Hence, the increase in [H2O2]/[H2O2]0 indicates the local accumulation of Ru(bpy)32+ and MV2+ into HA, and the decrease is attributed the reactants being spread over each domain of HA. This also supports the conclusion that the enhancement in H2O2 production is due to the binding of Ru(bpy)32+ and MV2+ with the HA polymer domain. Electron Transfer Pathways. As shown in Scheme 1, Ru(bpy)32+ is excited by irradiation by visible light at 450 nm, resulting in *Ru(bpy)32+ being oxidized to Ru(bpy)33+ by MV2+. To confirm the electron transfer reaction from the *Ru(bpy)32+ to MV2+, we utilized excited state luminescence quenching. The electron transfer rate from *Ru(bpy)32+ to MV2+ is related to the luminescence quenching coefficient of *Ru(bpy)32+ (26-29). Moreover, because *Ru(bpy)32+ also acts as an oxidative agent (32), the reductive species of HA, HAred, would also be expected to be oxidized by *Ru(bpy)32+ as

*Ru(bpy)32+ + HAred f Ru(bpy)3+ + HAox

(4)

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FIGURE 8. Possible electron transfer pathways.

FIGURE 7. Comparison of Stern-Volmer plots of *Ru(bpy)32+/MV2+ and *Ru(bpy)32+/HA systems. The concentration of the quencher, [quencher], for HA was calculated from the reducing capacity: [HA (mol L-1)] ) [reducing capacity (mol g-1)] × [HA (g L-1)]; [Ru(bpy)32+] ) 20 µM; 0.01 M formate buffer (pH 3.6), [NaCl] ) 0.1 M. electron transfer reaction may result in the luminescence quenching of *Ru(bpy)32+ by the dynamic processes. Moreover, *Ru(bpy)32+ may be also quenched by binding with HA with the static processes. It was known that, in the interaction between HA and fluorophore, the luminescence quenching of fluorophore involved both dynamic and static processes (33). It is therefore supposed that part of the quenching of *Ru(bpy)32+ by HA is due to the electron transfer as represented in eq 4. If eq 4 represents the predominant reaction, Ru(bpy)32+photosensitized production of H2O2 would not be observed. The luminescence quenching coefficient of the *Ru(bpy)32+/ MV2+ system was compared with that of the *Ru(bpy)32+/HA system. The Stern-Volmer approach was applied to these systems (Figure 7), and the Stern-Volmer constants, KSV, were evaluated from the slopes of the linear lines [*Ru(bpy)32+/MV2+, 704 M-1; *Ru(bpy)32+/HA, 105 M-1]. Because the excited state lifetime of *Ru(bpy)32+ was reported to be 6.0 × 10-7 s-1 (34), the quenching constants (kq) could be calculated from the slope of the Stern-Volmer plot. The kq value of the *Ru(bpy)32+/MV2+ system (1.2 × 109 M-1 s-1) was much larger than that of the *Ru(bpy)32+/HA system (1.9 × 108 M-1 s-1). Moreover, it was reported that when both Ru(bpy)32+ and quencher ions were adsorbed on the polyion, luminescence quenching took place by dynamic bimolecular processes involving energy transfer or electron transfer (27). This suggests that the quenching of *Ru(bpy)32+ by MV2+ in the presence of HA may also be due to the dynamic bimolecular processes. Therefore, the reduction of MV2+ by *Ru(bpy)32+ would likely be the predominant reaction in the system. On the other hand, the investigation described in Figure 4a shows that formic acid contributes to the Ru(bpy)32+photosensitized H2O2 production. As shown in Figure 4b, formic acid decreased with the accumulation of H2O2. Hoffman et al. (35) showed that formic acid was an effective hole scavenger for the ZnO semiconductor (+1.0-+3.5 V versus NHE), in which formic acid was oxidized to CO2 by the ZnO hole. Hence, this may be due to the oxidation of formic acid to CO2. We cannot clarify which reaction occurs 3952

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in the present system. Because the redox potential of the oxidation of formic acid to CO2 is at -0.19 V (36), it is possible to reduce Ru(bpy)33+ to Ru(bpy)32+. Because this potential is lower than that for the production of superoxide ion (O2-) (-0.32 V) (8), formic acid may not contribute to the production of O2-. Enhancement of H2O2 production was not observed in the absence of HA. This suggests that HA mediates the electron transfer from formic acid to Ru(bpy)33+, as shown in the following reactions:

Ru(bpy)33+ + HAred f Ru(bpy)32+ + HAox

(5)

HAox + HCOO- f HAred + CO2 + H+

(6)

Considering the results and concepts mentioned above, possible electron transfer pathways in the present system are summarized in Figure 8. The formation of H2O2 via O2is due to disproportionation as described below (6-8):

O2- + H+ h HO2

(pKa ) 4.8)

HO2 + HO2 f H2O2 + O2

(7) (8)

Moreover, it is known that, under deaerated conditions, MV+ accumulates as a result of photoirradiation in a solution containing Ru(bpy)32+, MV2+, and an electron donor such as EDTA (6, 9). Although we attempted to detect MV+, none could be observed after photoirradiation of a solution containing Ru(bpy)32+, MV2+, and HA. However, as shown in Figure 2b, the production of H2O2 at the several-hundred micromolar level could not be observed in the absence of MV2+, and this suggests that MV+ contributes to the formation of O2-. In the present system, the following back electron transfer reaction (dotted arrows in Figure 8) can also be expected:

Ru(bpy)33+ + MV+ f Ru(bpy)32+ + MV2+

(9)

HAox + MV+ f HAred + MV2+

(10)

Although eq 9 was reported in the Ru(bpy)32+/MV2+ system (9), eq 10 was unknown. It has been reported that MV+ could be observed easily when EDTA is used as an electron donor (9). Therefore, we investigated the effect of HA concentration on the accumulation of MV+ with EDTA. Figure 9 shows the absorption spectra of MV+, which had accumulated after 10 min of irradiation. As reported in the literature (37, 38), peaks at 605 and 395 nm, consistent with the presence of MV+, could be observed. The absorbance of MV+ decreased with increasing HA concentration. This indicates that the HA

FIGURE 9. Effect of HA concentration on the accumulation of MV+. [Ru(bpy)32+] ) 20 µM; [MV2+] )1.0 mM; [EDTA] ) 0.5 mM; photoirradiation, 10 min; 0.025 M formate buffer (pH 3.6). interferes with the accumulation of MV+. Therefore, in the absence of O2, eq 10 may be the dominant electron transfer reaction. Collectively, the data show that the addition of HA was effective in Ru(bpy)32+-photosensitized H2O2 production. In the system studied here, the reducing ability of HA assisted the photocatalytic reaction of Ru(bpy)32+. The reducing ability of HA was maintained by the oxidation of formic acid. Moreover, the binding effect of Ru(bpy)32+ and MV2+ with HA also resulted in an enhancement in H2O2 production.

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Received for review September 30, 1997. Revised manuscript received September 30, 1998. Accepted September 30, 1998. ES970866D

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