Environ. Sci. Technol. 2000, 34, 3742-3748
Effects of Humic Acids on the Photoinduced Reduction of U(VI) in the Presence of Semiconducting TiO2 Particles E L E N A S E L L I , † V EÄ R O N I Q U E E L I E T , ‡ MARIA ROSA SPINI,† AND G I O V A N N I B I D O G L I O ‡,* Environment Institute, Joint Research Center, European Commission, I-21020 Ispra (Va), Italy, and Dipartimento di Chimica Fisica ed Elettrochimica, Universita` di Milano, Via Golgi 19, I-20133 Milano, Italy
The effects of humic acids (HA) on the photoinduced reduction of U(VI) in aqueous suspensions containing titanium dioxide particles was investigated by time-resolved laser induced fluorescence in the pH range 5 to 7. The fluorescence response of both aqueous phase and adsorbed U(VI) species was also analyzed. The rate of the aqueous phase U(VI) photoreduction was found to be enhanced by U(VI) complexation with HA. A photosensitizing effect of HA on the interface reaction was evidenced at high irradiation intensity, with the HA prefilter effect prevailing at low irradiation. A kinetic model was developed, which takes into account U(VI) photoreduction occurring both in the aqueous phase and at the semiconductor oxidewater interface as well as adsorption and reoxidation reactions.
Introduction Natural organic substances strongly influence the fate of metal and radionuclide contaminants in aquatic systems (1, 2). This can occur through complexation in the water phase or by alteration of the sorption properties of mineral and particulate surfaces because of coating. In addition, such organic substances absorb light in the visible and near-UV region and can thus activate or take part to sunlight-induced homogeneous and heterogeneous processes occurring in shallow waters (3-5). Although the chemistry of uranium in water solutions has been thoroughly investigated (6, 7), uncertainties still exist on its behavior in conditions comparable to those of aquatic systems. To this purpose, both thermodynamic and kinetic considerations are required. Thermodynamics predicts that in air-equilibrated neutral aqueous solutions the predominant U(VI) species are hydroxo and carbonate complexes, stable against reduction. Thermodynamically unstable U(IV) species might be formed in oxygenated waters through photoreduction from the U(VI) excited state, produced by absorption of solar radiation. In the neutral pH range this process is inhibited by the structural changes consequent to hydrolysis reactions (8). However, U(VI) species adsorbed on mineral oxide particles with semicon* Corresponding author phone: +39-0332.78.93.83; +39-0332.78.56.01; e-mail:
[email protected]. † Universita ` di Milano. ‡ European Commission. 3742
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fax:
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000
ducting properties have been shown to be subject to photoredox equilibria in aqueous suspensions around neutrality (9-11). The presence of humic substances in natural aquatic systems might thus affect both U(VI) photostability in the aqueous phase, through U(VI) complexation, and redox reactions of uranium occurring under solar irradiation at the water-mineral oxide interface. In this paper the attention is focused on the effects of humic acids on the homogeneous and heterogeneous photoinduced processes involving U(VI). Due to its stability (12-14), titanium dioxide was chosen as a model of mineral oxide with semiconducting properties. A laser source (λexc ) 308 nm) was employed both for the production of photoreactive excited states and for inducing the time-resolved fluorescence of U(VI), which was monitored in kinetic studies. These were preceded by a thorough investigation of the adsorption equilibria of U(VI) species in the presence of humic acids and semiconductor oxide particles.
Experimental Section Materials. Stock solutions of uranyl perchlorate, titanium dioxide (Degussa P25, surface area 50 m2 g-1, average particle size 30 nm, density 3.8 g cm-3), and humic acids (HA, purchased from Aldrich as sodium salts) were prepared and stored as already described (9, 15). The actual concentration of HA was determined spectrophotometrically, after calibration at 250, 400, and 500 nm (extinction coefficients: 25.3, 5.65, and 1.97 cm2 mg-1, respectively). Suspensions containing 100 ppm of TiO2, a 2.0 × 10-5 M initial concentration of U(VI), and different amounts of HA (0-20 ppm) were employed in adsorption and photoreactivity studies, under normal atmospheric conditions (i.e. without CO2 exclusion), after having been shaken for 3 days in the dark. Their acidity was adjusted to the desired pH value by the addition of small volumes of 1 M HClO4 or NaOH solutions. pH measurements were repeated at the end of irradiation (9). Adsorption Measurements. The pH dependence of U(VI) adsorption on TiO2 in the presence of different amounts of HA was determined by measuring the U(VI) content of the surnatant by ICP-MS atomic absorption spectroscopy, after centrifugation for 72 min at 50000 rpm in a Beckman, Model L8-55M, ultracentrifuge. The amount of uranium adsorbed on the oxide was calculated on the basis of blank analyses carried out under exactly the same conditions, except for the presence of TiO2. HA adsorption on TiO2 was measured in aqueous suspensions containing fixed amounts of HA, at different pH values ranging from 2.5 to 7.5. The HA content of the surnatant was determined spectrophotometrically at 290 nm, by comparison with the absorption of a blank solution (without TiO2), containing the same HA amount as the suspension. Time-Resolved Fluorescence Measurements: Apparatus and Procedure. The employed pulsed radiation source, a dye laser pumped at 532 nm by a Nd:YAG laser, and detection system were already described (9). The excitation at 308 nm, with a pulse width of ca. 4 ns and a 20-Hz frequency, was obtained by duplication of the dye laser output at 616 nm. U(VI) fluorescence measurements were made at different irradiation intensity using an attenuator (Newport, Model 935-3) placed on the laser beam. Ferrioxalate actinometry (16) (initial K3Fe(C2O4)3 concentration ) 5.66 × 10-3 M) was used to calibrate the impinging radiation intensity in the investigated intensity range (6 × 10-9-1 × 10-7 Einstein s-1 mL-1). This corresponds to maximal intensities (assuming no filtering effects, see below) on each TiO2 particle equal to 10.1021/es991319q CCC: $19.00
2000 American Chemical Society Published on Web 07/27/2000
1.9 × 103-3.2 × 104 photons s-1, i.e., 97-1600 photons per pulse. The fluorescence emission of irradiated suspensions at 512 nm was measured in the photon-counting mode. Time gating of the fluorescence signal with a delay of 1.3 µs from excitation allowed sufficient time to avoid scattering problems induced by the semiconductor particles and to eliminate the fast emission of humic acids, which totally decays in a few nanoseconds (15, 17). A measurement window of 60 µs and an acquisition time of 2 s were always employed. U(VI) fluorescence lifetimes were measured between 2 and 99 µs with a very low laser intensity, to avoid U(VI) photodegradation. In kinetic studies, suspensions were irradiated in quartz cuvettes under stirring at (19.0 ( 0.5) °C. Due to the high absorption and scattering of the irradiated suspensions, only the U(VI) fluorescence signal originated from a 4-mm thick layer in contact with the irradiated cuvette wall was monitored. The fluorescence of standard solutions, containing U(VI) in 0.75 M H3PO4, was measured before and after each kinetic run, to check the linear response of the detection system. A statistical procedure was used for selecting the best regression equation from the experimental data. This procedure involved their fitting according to different special cases of eq 29 (mono- or biexponential decay, with 3, 4, or 5 parameters, see below) using the nonlinear regression program of the Origin (Microcal) software package, followed by a series of internal comparisons to find the best cutoff point for the number of predictor variables. The sum of squared residuals, the standard deviations of the individual parameters and the partial F-test were used as criteria for the assessment. Uncertainties are expressed as twice standard deviations.
Results and Discussion To identify the uranium species involved in photoredox processes occurring under irradiation at the water-semiconductor oxide interface, homogeneous and heterogeneous equilibria of U(VI) species in the presence of the semiconductor oxide particles and of humic acids were investigated first, under experimental conditions identical to those adopted in photoreactivity studies. Adsorption Equilibria. The effect of humic acids (HA) on the adsorption behavior of 2.0 × 10-5 M U(VI) in aqueous suspensions containing 100 ppm of titanium dioxide at different pH values under dark conditions is illustrated in Figure 1. In the absence of HA, the amount of uranium adsorbed on the semiconductor steadily increases with increasing pH, reaching the maximum value (around 90% of the U(VI) content) at pH 7. In the presence of HA, the percent amount of U(VI) adsorbed at low pH is slightly higher but increases with a lower slope with increasing pH, showing the tendency to reach pH-insensitive plateau values in the pH range 6-7. The percent amount of adsorbed U(VI) in this pH range progressively decreases with increasing HA content. This is likely due to the higher fraction of U(VI) complexed with HA in the liquid phase, even though also U(VI)-HA complexes may be expected to adsorb on the oxide surface, as reported for U(VI) complexes with other organic ligands (10, 11). The higher percent adsorption of U(VI) observed at low pH in the presence of HA may also be interpreted in terms of U(VI) interactions with adsorbed HA. Indeed, under the adopted experimental conditions the percent adsorption of HA on TiO2 particles is around 90% at pH 3 and decreases with increasing pH, being around 10% at pH 7. The observed trend is a consequence of the electrostatic interactions between the ionized, negatively charged, functional groups of HA and the TiO2 surface, which becomes also negatively charged at pH > 6.25, i.e., above its point of zero charge (13).
FIGURE 1. Percent adsorption of U(VI) (total concentration 2.0 × 10-5 M) on TiO2 (100 ppm): (a) in the absence of HA and in the presence of (b) 5 ppm, (c) 10 ppm, and (d) 15 ppm of HA. Similar trends were found for the adsorption of humic acids on γ-alumina and on ZnO (4, 18). U(VI) Speciation. The speciation of a 2.0 × 10-5 M U(VI) aqueous solution, calculated (19, 20) on the basis of the hydrolysis constants reported in the literature (7, 21, 22), predicts the presence of various (UO2)m(OH)n2m-n species, denoted as (m,n). In particular, the (3,5)-hydroxo complex is the predominant species in the pH range 5.5-7.5 (9), while at pH < 5 the uranyl ion (1,0) predominates. The fractions of the (1,1) and (2,2) species are maximal at pH around 5.2 (0.25 and 0.15, respectively). U(VI) aqueous speciation in the presence of different amounts of HA was calculated by means of the Humic IonBinding Model V (23), as combined in the WHAM chemical equilibrium model and computer code (24). This model assumes that only the free metal cation and the first hydroxo complex can bind to humic substances. However, the humic acids employed in the present investigation were shown to bind also other metal species (15), which, carrying different charges, should be subject to different electrostatic effects and consequently have different intrinsic binding constants. In the case of U(VI) the species effectively involved in the binding with humic substances cannot be identified experimentally. Therefore, a pKMHA value of 1.3 for all hydrolyzed U(VI) species was employed in speciation calculations, as previously proposed (23, 25). The results of such analysis evidence that several U(VI) species coexist in 2.0 × 10-5 M U(VI) aqueous solutions containing HA, U(VI) being totally complexed with humic acids only at pH 7 for HA concentrations above 10 ppm, while at pH 5 only 42% of U(VI) is complexed with HA, the predominant species being the uranyl ion. The U(VI) speciation in the TiO2 suspensions was evaluated on the basis of the residual U(VI) concentration in the aqueous phase, i.e., by taking into account the experimentally determined fraction of U(VI) adsorbed at different HA concentrations, Uads, and the residual HA concentration in the aqueous phase obtained from HA adsorption curves on TiO2. As shown in Table 1, also in this case the presence of VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. U(VI) Fractional Distribution in Aqueous Suspensions Containing 100 ppm TiO2, a Total U(VI) Concentration of 2.0 × 10-5 M, and Different Amounts of HAa pH 7 species (1,0) (1,1) (2,2) (3,5) (1,0)-HA (1,1)-HA (2,2)-HA U(VI)ads
no HA
5 ppm
pH 6
10 ppm
15 ppm
no HA
5 ppm
pH 5
10 ppm
15 ppm
0.03 0.10
0.90
0.27 0.06 0.11 0.03 0.80
0.08 0.12 0.80
0.13 0.21 0.01 0.65
0.70
0.14 0.06 0.07 0.73
0.15 0.05 0.80
0.26 0.08 0.01 0.65
no HA
5 ppm
10 ppm
0.21 0.12 0.08 0.09
0.11 0.03 0.04
0.01
0.22 0.02 0.08 0.50
0.31 0.02 0.01 0.65
0.50
15 ppm
0.36 0.03 0.01 0.60
(m,n) indicate (UO2)m(OH)n2m-n species in solution and (m,n)-HA the same species bound with humic acids. U(VI)ads values were determined by adsorption measurements. Fractions below 0.01 were neglected. a
TABLE 2. Fluorescence Lifetimes τf of U(VI) Species in Aqueous Solutions or Aqueous Suspensions (100 ppm TiO2) Containing Different Amounts of HA τf (µs)
FIGURE 2. Emission spectra of 2.0 × 10-5 M U(VI) at pH 7 in aqueous solutions (a) without HA and (b) containing 5 ppm of HA (left-hand ordinate); (c) in aqueous suspensions containing 5 ppm of HA and 100 ppm of TiO2 (right-hand ordinate). HA strongly modifies U(VI) speciation in the aqueous phase. Indeed, U(VI) is totally complexed with HA at pH 6 and 7, while less than 20% of the total U(VI) can be found as (1,0), (1,1), and (2,2) species only at pH 5 in the presence of 5 ppm HA. Moreover, U(VI) prevalently binds to HA as free ion or hydroxo complex, the amount of both free and bound (3,5) species being always negligible. As already mentioned, U(VI)ads is likely to comprise also HA-complexed forms of U(VI). Fluorescence of U(VI) Species. The various (UO2)m(OH)n2m-n species present in aqueous solution are known to have different lifetimes and different fluorescence efficiencies (21, 26). The fluorescence response of excited U(VI), the only fluorescent oxidation state of uranium under the adopted conditions, was thus investigated in the HA-containing suspensions, to employ fluorescence emission as noninvasive analytical tool for monitoring U(VI) concentration in kinetic studies. Figure 2 shows that the shape of the time-resolved fluorescence emission spectrum of U(VI) at pH 7 under 308 nm irradiation does not change after HA addition, though a not easily quantifiable signal quenching occurs due to U(VI) complexation with HA, an effect which was already observed and attributed to static quenching (27). Much less resolved fluorescence spectra were recorded after the addition of TiO2 (Figure 2(c)), mainly due to light scattering effects of the oxide particles. However, also in this case the spectral shape is not significantly different with respect to the curves (a) and (b) of Figure 2, indicating that the fluorescence spectrum does not undergo major modifications after U(VI) adsorption. The fluorescence lifetimes τf measured in different aqueous solutions and TiO2 suspensions are reported in Table 2. Such values, obtained from the fitting of fluorescence decay data as monoexponential curves, represent the average fluorescence lifetimes of the predominant U(VI) species, no 3744
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[HA] (ppm)
TiO2
pH 5
pH 6
pH 7
0 5 10 0 5
+ +
14.9 ( 0.7 18.9 ( 0.7
33.0 ( 0.6 29.7 ( 0.4 26.0 ( 0.3 25.9 ( 0.4 26.6 ( 0.4
30.5 ( 0.4 28.0 ( 0.5 23.4 ( 0.5 31.2 ( 0.4 31 ( 1
17.1 ( 0.4 21.2 ( 0.4
clear distinction between their single contribution being achievable, due to the small difference in their individual fluorescence lifetimes. Indeed, the τf value in aqueous solution at pH 5 containing HA is lower than those determined at pH 6-7, due to the higher contribution of the (1,0), (1,1) and (2,2) species, having fluorescence lifetimes equal to 2.2, 40.5, and 13 µs, respectively (9, 26). The τf values measured in aqueous HA solutions at pH 6-7, where U(VI)-HA complexes are predominant, are in very good agreement with the values (29-25 µs) measured at pH 6.4 by Laszak (28), who also observed a slight decrease of τf with increasing HA content (see Table 2). In TiO2-HA suspensions at pH 6-7, U(VI) is predominantly in the adsorbed form, U(VI)ads, and aqueous U(VI) is almost totally complexed with HA (Table 1). The τf values measured under these conditions do not show any appreciable variation with respect to the values obtained without HA, perfectly matching the value of 30 µs measured at pH 7 in the absence of HA under conditions of complete U(VI) adsorption (9). In conclusion, in the pH range 6-7 the U(VI) fluorescence spectral shape is not affected by the presence of TiO2 and HA (Figure 2), and almost constant τf values are measured at fixed pH values in TiO2 suspensions, only a slight decrease of τf being observed with increasing amount of HA in the aqueous solutions. Thus the maximum intensity fluorescence signal F emitted at 512 nm appears as most suitable for monitoring U(VI) concentration in kinetic analysis, provided that the different fluorescence efficiencies of the U(VI) species are taken into account, according to the equation
F ) β [U(VI)ads] + γ [U(VI)sol] U(VI)sol indicates in general U(VI) in the aqueous phase, practically coinciding with hydroxo-complexes U(VI)aq in the absence of HA, and with U(VI)-HA complexes in the presence of HA. The proportionality coefficients β and γ, besides being typical of the monitored species, also depend on the experimental setup (9) as well as on shielding factors due to the diffusion of light by the oxide particles and inner filter effects induced by the presence of the absorbing humic acids. However, they are constant within each kinetic run.
TABLE 4. Relevant Reactions Involved in Uranium Photoredox Processes Occurring in the Presence of Titanium Dioxide and Humic Acids
FIGURE 3. Fluorescence signal decay in U(VI) solutions containing (a) 5 ppm and (b) 10 ppm of HA and in U(VI) suspensions (100 ppm of TiO2) containing (c) 10 ppm of HA and (d) no HA. I° ) 1.8 × 10-8 Einstein s-1 mL-1, pH 7. Solid lines represent model curves.
TABLE 3. First-Order Rate Coefficients kr of Aqueous Phase U(VI) Photoreduction in the Presence of Different Amounts of Humic Acidsa 104 × kr (s-1)
a
[HA] (ppm)
pH 6
pH 7
0 5 10
1.83 ( 0.14 1.90 ( 0.07 1.58 ( 0.15
1.43 ( 0.15 3.02 ( 0.07 6.02 ( 0.12
Irradiation intensity I° ) 3.1 × 10-8 Einstein s-1 mL-1.
Kinetic Studies on Photoinduced Reactions. To ascertain if HA affects U(VI) photostability in the homogeneous phase, kinetic studies were first performed in aqueous solutions in the absence of oxide particles. While a stable fluorescence signal was recorded from irradiated solutions at pH 5, a signal decrease was observed at pH 6-7 and in the presence of HA (Figure 3(a,b)), following a first-order decay. The rate coefficients obtained under different experimental conditions are reported in Table 3. U(VI) photodegradation in the aqueous phase occurs at a relatively higher rate at high pH in the presence of a fixed amount of HA and becomes faster with increasing HA content. This is clearly related to the capability of HA of binding U(VI) and favoring its reduction in excited state U(VI)-HA complexes. Indeed, both U(VI) and HA absorb light at 308 nm: under the adopted experimental conditions U(VI) absorbs ca. 3.3% of the incident radiation, while the fractions of impinging light absorbed by 5, 10, and 15 ppm of HA are around 10%, 18%, and 25%, respectively. U(VI) photoreduction proceeds at a higher rate when HA are negatively charged and can more easily transfer electrons to the metal cations. However, such homogeneous phase photoreaction causes less than 15% decrease of the fluorescence signal (Figure 3(a,b)). The fluorescence signal decreases much faster in TiO2 suspensions (Figure 3(c,d)), indicating that photoredox processes involving uranium species at the semiconductorwater interface occur at a much higher rate than in solution. Indeed, the flat band potential of TiO2 allows the photopromoted conduction band electrons to induce U(VI) reduction to U(IV) (9, 11, 29, 30). The presence of humic acids has several opposite effects: the U(VI) fluorescence signal decays faster with increasing pH also in the presence of HA, but with increasing HA amount at fixed pH the fluorescence signal decays more slowly (Figure 3(c,d)), while the dependence of the decay rate on pH is progressively less pronounced the higher is the HA content. Moreover, as expected, the decay rate becomes faster with increasing the intensity of the laser beam.
Several considerations should be taken into account in the interpretation of kinetic results. First of all, prefilter effects surely have not secondary roles, as both TiO2 and HA, besides U(VI), absorb light at 308 nm. The presence of HA also causes considerable variations in adsorption equilibria, i.e., on the surface concentration of U(VI) (Table 1), thus affecting photoredox reactions occurring both at the oxide-solution interface and in the aqueous phase. Moreover, kinetic results obtained in the presence of semiconductor particles can hardly be compared with those obtained in their absence, due to the major light scattering effects induced by the oxide particles. Finally, photoinduced processes of uranium appear to be partly reversible in the dark, as a stable modest restoration of the fluorescence signal was observed 3 h after the end of irradiation, in both solutions and suspensions. Mechanism of U(VI) Photoreduction. Several equilibria and photoredox reactions have to be taken into account in order to analyze kinetic results under different experimental conditions. The most important steps of the proposed mechanism are listed in Table 4. Starting from the photogeneration of charge carriers on the semiconductor (eqs 1-4) (13, 14), U(VI) reduction may occur through two reaction paths involving both U(VI) adsorbed on the semiconductor, U(VI)ads, and U(VI) species in the aqueous phase, U(VI)sol: (i) U(VI)ads interacts with the electrons photopromoted in the conduction band, giving U(V) species (eq 5) that easily undergo further reduction at VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the interface, leading to U(IV) (9, 11); (ii) U(VI)sol (either in the form of aquo and hydroxo ions, U(VI)aq, in the absence of HA, or mainly in the complexed form U(VI)-HA, in the presence of HA) may undergo photoreduction in the aqueous phase (eqs 10-11). The second reaction path is slower than the first one (Figure 3). The reduction product U(IV) is not stable in the presence of oxygen (6, 7) and may undergo reoxidation to U(VI) (9-11) either by the oxidizing species present on the illuminated semiconductor (mainly OH radicals, but also adsorbed oxygen (31)) (eqs 7-8) or in the aqueous phase (eqs 12-13). As U(IV) is mainly formed directly on the TiO2 surface and due to the competitive formation of insoluble uranium oxides from U(IV)aq (eq 14) (9-11, 32), the rate of U(IV) partial reoxidation in the aqueous phase can be assumed negligible during irradiation. This reaction is effective only after the end of irradiation and is responsible of the modest recovery of the fluorescence signal observed in irradiated suspensions left in the dark for some hours. Finally, besides complexation equilibria (eqs 15-16), involving several U(VI) species, adsorption reactions involving aqueous U(VI) and complexed U(VI) (eqs 17-20) are also relevant in kinetic analysis. In particular, as U(VI) reduction is faster at the semiconductor-water interface than in the aqueous phase, steps 17 and 19 should be taken into account as a source of replenishment of U(VI)ads on the semiconductor surface. The following rate equations can be written for U(VI) adsorbed on the semiconductor, U(VI)ads, and for U(VI) in the aqueous phase, U(VI)sol:
Equation 26 can be written as
d[U(VI)ads] ) -C1[U(VI)ads] + UTC2 + C3[U(VI)sol]0e-C4t dt (27) • where C1, C2, and C3 depend on [ecb] and [ OH], and integrated as follows
[U(VI)ads] )
{
By taking into account the different fluorescence efficiencies of U(VI)ads and U(VI)sol, the following temporal dependence of the fluorescence signal F is thus obtained from eqs 23 and 28:
Depending on the absence or presence of HA, ka represents k16 or k17 and kr represents k10 or k11, respectively. Equation 22 can be easily integrated, giving
}
UT ) [U(VI)ads]0 + [U(VI)sol]0,
F0 ) U T
(24)
• k6[ecb] + k8[ OH]
UT ) [U(VI)ads] + [U(VI)sol] + [U(IV)]
(25)
U(V) concentration being always negligible. After substitution of eqs 23-25 in eq 21 one finally gets
UT
3746
9
k7k8[•OH]2 • k6[ecb] + k8[ OH]
(
6
cb
+ ka -
8
k7k8[•OH]2 • k6[ecb] + k8[ OH]
)
×
[U(VI)sol]0e-(kr+ka)t (26)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000
ka k-a
βka + γk-a ka + k-a
[
]
C3k-a C2 β k - (k + k-a) exp(-C1t) + βka + γk-a a C1 - C4 C1 a k-a C2 ka + k-a βC3 +γ ‚ exp(-C4t) + β ‚ C1 - C4 βka + γk-a C1 βka + γk-a (30)
]
which is of the type
F/F0 ) A1 exp(-C1t) + A2 exp(-C4t) + A3
[U(IV)] can be estimated by taking into account that at any irradiation time the initial U(VI) concentration, UT, is equal to
2 • 2 d[U(VI)ads] -k5k6[ecb] -k7k8[ OH] + ) [U(VI)ads] • dt k [e ] + k [ OH]
[U(VI)sol]0
)
Consequently
[
• k5[U(VI)ads][ecb] + k7[U(IV)][ OH]
[U(VI)ads]0
and
where [U(VI)sol]0 indicates the concentration of U(VI)sol at the beginning of irradiation. U(V) is an intermediate reactive species, whose concentration may be calculated by applying the steady-state assumption to eqs 5-8:
[U(V)] )
}
At the beginning of irradiation
(23)
[U(VI)sol] ) [U(VI)sol]0e
{
C2 C1
βC3 C3[U(VI)sol]0 exp(-C1t) + + γ [U(VI)sol]0 C1 - C4 C1 - C4 C2 exp(-C4t) + UT β (29) C1
F ) F0
-(kr+ka)t
{
F ) β [U(VI)ads] + γ [U(VI)sol] ) β [U(VI)ads]0 - UT
d[U(VI)ads] • ) -k5[U(VI)ads][ecb] + k8[U(V)][ OH] + dt ka[U(VI)sol] (21) d[U(VI)sol] ) -kr[U(VI)sol] -ka[U(VI)sol] ) dt -(kr + ka)[U(VI)sol] (22)
}
C2 C3[U(VI)sol]0 exp(-C1t) + C1 C1 - C4 C2 C3[U(VI)sol]0 exp(-C4t) + UT (28) C1 - C4 C1
[U(VI)ads]0 - UT
(31)
Equation 31 has general validity. However, two particular conditions can be encountered: (a) when U(VI) is totally adsorbed on the semiconductor (k-a ) 0), the term A2 exp(-C4t) of eq 31 disappears. The expression already obtained under such conditions for the temporal dependence of the fluorescence signal (9) still holds, the term (C2/C1) ) A3 representing the asymptotic steady-state fraction of UT remaining as U(VI) in the presence of competition between U(VI) photoreduction and U(IV) oxidation. (b) When the rate of U(IV) oxidation under irradiation is low compared to the rate of U(VI) reduction, i.e., under high irradiation intensity (33) ([ecb] being proportional to the square root of the irradiation intensity Ia absorbed by TiO2, while adsorbed OH radical concentration is proportional to Ia1/4 (9), leading to C2 , C1), the term A3 of eq 31 is negligible, and a biexponential
TABLE 5. Parameters Obtained from the Fitting of the Fluorescence Signal Decay in the Presence of Different Amounts of Humic Acids in the pH Range 6-7, According to the Equation F/F0 ) A exp(-Ct) + A3a I1 [HA] (ppm) 103 × C (s-1)
FIGURE 4. C4 rate coefficients (eq 31) as a function of pH. U(VI) suspensions without HA, I° ) 3.1 × 10-8 Einstein s-1 mL-1.
4.3 ( 0.6 3.2 ( 0.6 3.6 ( 0.7
5 10 15
I2 A3
103 × C (s-1)
A3
0.051 ( 0.007 0.14 ( 0.02 0.20 ( 0.01
8.9 ( 0.9 7.6 ( 0.9 5.6 ( 0.7
0.035 ( 0.004 0.078 ( 0.009 0.18 ( 0.03
a Irradiation intensity (in Einstein s-1 mL-1): I ) 1.8 × 10-8; I ) 3.1 1 2 × 10-8.
TABLE 6. Parameters Obtained from the Fitting of the Fluorescence Signal Decay in the Presence of Different Amounts of Humic Acids, According to Eq 31a
FIGURE 5. C1 (full symbols) and C4 (open symbols) rate coefficients (eq 31) as a function of irradiation intensity, measured in U(VI) suspensions without HA at pH 5.9 (circles), pH 6.5 (squares), and pH 6.7 (triangles). decay of the fluorescence signal should consequently be observed, with C1 ) k5 [ecb-] (eq 5) and C4 ) kr + ka. When both (a) and (b) apply, a simple monoexponential decay should obviously be observed. The values of the preexponential factors A1 and A2 are not very useful in the analysis of results, being the combination of several parameters. Model Verification. In the present work a radiation intensity higher than in previous studies on the U(VI)-TiO2 system (9) was employed. Consequently, the fluorescence signal monitored in TiO2 suspensions without HA never reached a stationary state but totally disappeared during irradiation (A3 ≈ 0). According to the statistical procedure described above, biexponential decay curves (Figure 3(d)) fit experimental data much better than monoexponential decays, i.e., case (b) apply, with A2 > A1 (eq 31). C1 values, coinciding in this case with k5 [ecb], do not show any marked dependence on pH, as expected from the model, being C1 ) (1.8 ( 0.6) × 10-2 s-1 and C1 ) (2.6 ( 0.7) × 10-2 s-1, at impinging irradiation intensities equal to 1.8 and 3.1 × 10-8 Einstein s-1 mL-1, respectively. C4 ) kr + ka values, instead, are always greater than the kr values determined under the same experimental conditions in the absence of TiO2 (Table 3) and clearly increase with pH (Figure 4). One should expect that, under identical irradiation conditions, the rate of U(VI) photoreduction in the aqueous phase should be lower in the presence of TiO2, which absorbs and scatters a not negligible fraction of impinging light. Thus, the fact that C4 values are always much greater than the kr values of Table 3 underlines the relevance of the adsorption of U(VI)aq (step 17 of Table 4) as a source of replenishment of the easily photoreducible U(VI)ads species. Indeed, C4 values increase with pH (Figure 4), i.e., with increasing ka values, closely following U(VI) adsorption curve on TiO2 (Figure 1(a)). As expected, both rate coefficients increase with increasing irradiation intensity I° (Figure 5), C1 values exhibiting a higher slope respect to C4. In the presence of HA, instead, a stationary state seems to be attained at the end of irradiation, i.e., A3 * 0 in eq 31. For I° values below 4 × 10-8 Einstein s-1 mL-1, the fluorescence signal decays according to a single exponential (Figure 3(c)), i.e., according to the equation F/F0 ) A
[HA] (ppm)
pH
102 × C1 (s-1)
102 × C4 (s-1)
A3
0 5 10
6.5 6.0 6.7
5.6 ( 0.4 8.3 ( 0.8 13.8 ( 1.3
0.97 ( 0.02 1.70 ( 0.04 3.6 ( 0.4
0.010 ( 0.003 0.029 ( 0.009
a
I° ) 5.5 × 10-8 Einstein s-1 mL-1.
exp(-Ct) + A3, formally identical to the one applying in case (a) above. However, C4 * 0 in the present case, as U(VI) undergoes photoreduction also in the aqueous phase. Due to the experimental scattering of fluorescence decay data, C1 and C4 values cannot be distinguished, having closer values than in the absence of HA. Indeed, with increasing HA amount a higher fraction of impinging radiation is absorbed by HA, thus reducing the fraction of light absorbed by the oxide and consequently the rate of U(VI) photoreduction at the wateroxide interface (lower C1 values). U(VI) photooxidation at the interface, instead, is less sensitive to a decrease of light absorption, as already discussed, and becomes competitive with photoreduction at the interface, thus leading to a photostationary state (A3 * 0). On the other hand, C4 values should increase in the presence of HA, if the increase of kr values (Table 3) consequent to their sensitization action in the aqueous phase overwhelms the possible decrease of ka consequent to U(VI) complexation. As shown in Table 5, the rate of U(VI) photoreduction is pH-independent in the pH range 6-7, confirming that the extent of U(VI) adsorption corresponds to a plateau in the adsorption curves (Figure 1(b-d)). The C values are comprised between the C1 and C4 values measured in the absence of HA under the same irradiation conditions. However, C values decrease and A3 values progressively increase with increasing HA amount (Table 5), indicating that in the lower range of irradiation intensity the prefilter effect of humic acids seems to be predominant in decreasing the overall U(VI) photoreduction rate. With impinging radiation intensities higher than 4 × 10-8 Einstein s-1 mL-1 the lower scatter of experimental fluorescence measurements allow to calculate all parameters of eq 31 with rather small uncertainties, as reported in Table 6. The fact that both C1 and C4 rate coefficients increase with increasing HA content demonstrates that when the light intensity reaching the semiconductor, though partly filtered by HA, is sufficient to fully excite this latter, humic acids have sensitizing effects on both homogeneous phase and heterogeneous U(VI) photoreduction. HA adsorbed on the semiconductor, being good electron donors, can easily scavenge the holes photogenerated in the valence band (eq 9 of Table 4), thus inhibiting electron-hole recombination (eq 2) and making the electrons photopromoted in the conduction band more easily available for U(VI) photoreduction at the water-oxide interface (eqs 5-6). As already VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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evidenced in the case of the photoinduced reduction of Cr(VI) on ZnO (4), this should be even more effective under solar irradiation, whose spectrum extends to wavelengths longer than 308 nm, where HA prefilter effects are much smaller.
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(16) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. (London) 1956, A235, 518-536. (17) Bidoglio, G.; Grenthe, I.; Qi, P.; Robouch, P.; Omenetto, N. Talanta 1991, 38, 999-1008. (18) Righetto, L.; Bidoglio, G.; Azimonti, G.; Bellobono, I. R Environ. Sci. Technol. 1991, 25, 1913-1919. (19) Puigdome`nech, I. Input, Sed and Predom: Computer Program Drawing Equilibrium Diagrams; Royal Institute of Technology, 12: Stockholm, 1983. (20) Eriksson, G. Anal. Chim. Acta 1979, 112, 375-383. (21) Eliet, V.; Bidoglio, G.; Omenetto, N.; Parma, L.; Grenthe, I. J. Chem. Soc., Faraday Trans. 1995, 91, 2275-2285, and references therein. (22) Pashalidis, I.; Kim, J. I.; Ashida, T.; Grenthe, I. Radiochim. Acta 1995, 68, 99-104. (23) Tipping, E.; Hurley, M. A. Geochim. Cosmochim. Acta 1992, 56, 3627-3641. (24) Tipping, E. Comput. Geosci. 1994, 20, 973-1023. (25) Higgo, J. J.; Kinniburgh, D.; Smith, B.; Tipping, E. Radiochim. Acta 1993, 61, 91-103. (26) Eliet, V.; Grenthe, I.; Bidoglio, G. Appl. Spectrosc. 2000, 54, 99105. (27) Moulin V.; Moulin Ch. Appl. Geochem. 1995, 10, 573-580. (28) Laszak, I. Ph.D. Dissertation, Paris VI University, France, 1997. (29) Ward, M. D.; Bard, A. J. J. Phys. Chem. 1982, 86, 3599-3605. (30) Handbook of Chemistry and Physics, 65th ed.; CRC Press: Boca Raton, FL, 1984-1985; p D-158. (31) Schwitzgebel, J.; Ekerdt, J. G.; Gerisher, H.; Heller, A. J. Phys. Chem. 1995, 99, 5633-5638. (32) Dodge, C. J.; Francis, A. J. Environ. Sci. Technol. 1994, 28, 13001306. (33) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. J. Phys. Chem. B 1997, 101, 4265-4275.
Received for review November 29, 1999. Revised manuscript received May 2, 2000. Accepted May 4, 2000. ES991319Q
