Humic Acid-Sensitized Photoreduction of Cr(VI) on ZnO Particles

The percent reduction of Cr(VI) obtained with 0.2 g L-1 ZnO is comparable to that obtained in a .... (9). Nakabayashi, S.; Kawai, T. Electron Transfer...
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Environ. Sci. Technol. 1996, 30, 598-604

Humic Acid-Sensitized Photoreduction of Cr(VI) on ZnO Particles ELENA SELLI,† ALESSANDRA DE GIORGI,‡ AND G I O V A N N I B I D O G L I O * ,‡ Commission of the European Communities, Joint Research Centre, Environment Institute, I-21020 Ispra (Va), Italy, and Dipartimento di Chimica Fisica ed Elettrochimica, Universita` di Milano, Via Golgi 19, I-20133 Milano, Italy

The possibility that humic acids act as sensitizers in the photoinduced reduction of Cr(VI) to Cr(III) in aqueous suspensions containing ZnO particles was verified. Equilibrium sorption studies showed that Cr(III) is totally adsorbed on ZnO, while Cr(VI) adsorption on ZnO is very low. Solar irradiation of the ZnO suspensions induced the photoreduction of Cr(VI), leading to a marked increase of the amount of chromium adsorbed on the mineral oxide. The yield of the reaction is enhanced both in the presence of humic acids and under anoxic conditions. Humic acids adsorbed onto the oxide surface sensitize Cr(VI) photoreduction and the competitive photoreduction of molecular oxygen leading to hydrogen peroxide.

Introduction Synergic processes of chemical, biological, and photochemical transformations may modify the chemical form of contaminants in natural aquatic systems and thus their mobility and biological availability. Recently, much attention has been devoted to heterogeneous light-induced processes occurring at mineral-water interfaces (1, 2). When the mineral has semiconducting properties, the absorption of light of energy equal to or greater than its band gap causes the formation of an electron-hole pair, by promoting one electron from the valence band to the conduction band (3-6). These charge carriers can migrate to the mineral surface and react with species having suitable redox potentials. The absorptive range of the system can be extended if substances, generally dyes of high extinction coefficients, are adsorbed on the semiconductor. These species, which can be excited by radiation of energy even lower than the band gap, are able to transfer either energy or electrons to activate the semiconductor (7-11). A favorable energetics of the electron-donating (or hole-accepting) level of the sensitizer is essential for the occurrence of the photosen* Corresponding author fax: +39-332-78.93.28; e-mail address: [email protected]. † Universita ` di Milano. ‡ Commission of the European Communities, Joint Research Centre.

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sitization process. Also an analogue of chlorophyll pigment has been successfully used to photosensitize TiO2 colloidal particles in a nonaqueous medium (12). Humic substances derived from the decomposition of plants and animal organisms are present in natural aquatic systems in rather high concentrations. They can absorb a significant portion of the solar radiation, leading to the formation of electronically excited states capable of participating in a variety of reactions with aquatic pollutants (13-16). Time-resolved absorption spectroscopy revealed that photoexcited humic substances lead to a steady-state concentration of hydrated electrons in natural water bodies under sunlight irradiation (17, 18). Humic substances can play a role also in heterogeneous photoprocesses, such as manganese oxide dissolution (19) and Fe(II) release from ferric hydroxides (20). The ability of fulvic acid to sensitize colloidal ZnO has been demonstrated by fluorescence emission and transient absorption measurements in a mixed alcohol-water mixture (21). By visible light irradiation, long-lived electrons can be trapped at the semiconductor surface and utilized as charge carriers for the reduction of other substrates and environmental pollutants. Consequently, humic substances might sensitize photoredox transformations of contaminants at mineral-water interfaces in aquatic environments. The aim of the present paper was to verify if photosensitization of heterogeneous light-induced processes can occur in natural systems and, in particular, if humic acids adsorbed on a semiconductor mineral oxide, such as ZnO, are able to sensitize the photocatalytic reduction of Cr(VI) to Cr(III) in the neutral to alkaline pH range. Aqueous chromium, a common component of industrial wastes (22), is a significant environmental contaminant, being notoriously toxic, mutagenic, and carcinogenic (23-25). However, its mobility and toxicity depend on its oxidation states. Photoreduction on semiconductors (TiO2, ZnO, WO3, CdS, ZnS) has been demonstrated to be an efficient process (2630) to convert Cr(VI), usually occurring as highly soluble and highly toxic chromate anions (31), into Cr(III), which exhibits little or low toxicity (32) and is much less mobile in the environment due to its very low solubility and high adsorbability onto mineral surfaces (33, 34). Although thermodynamic calculations predict that Cr(VI) should be the predominant species in oxygenated surface waters (35), Cr(III) is always present in measurable concentrations both in seawater and in freshwater lakes and estuaries, being sometimes the major form in near-shore surface waters (36-40). The discovery of a diurnal cycle in the oxidation state of chromium in estuarine water (37) is a clear evidence of a sunlight-induced photochemical redox process. This has been found to occur in the presence of particulate mineral oxides and dissolved organic matter and postulated to be a homogeneous aqueous phase process. In the present work, the adsorption equilibria involved in the aqueous system containing ZnO particles and chromium ions with or without humic acids (HA) were investigated first. The effects induced by solar irradiation both in the presence and in the absence of ZnO particles and of HA adsorbed on the semiconductor were then studied in detail.

0013-936X/96/0930-0598$12.00/0

 1996 American Chemical Society

Experimental Section Materials. The ZnO powder used in this study was purchased from Fluka. The specific surface area, measured by N2 adsorption at 77 K, was found to be 5.1 m2 g-1. ZnO suspensions were prepared by adding a weighed amount of powder to 0.1 M NaClO4 aqueous solutions, which were successively ultrasonicated for 30 min. Suspensions containing 2 g L-1 ZnO were used in standard conditions. Only basic solutions could be used for pH adjustment, because the addition of even small volumes of acid caused the partial dissolution of ZnO particles. The amorphous SiO2 (Aerosil 200, Degussa) employed in blank experiments had a surface area of 200 m2 g-1. The humic acid (HA) used in most of the experiments was obtained from Aldrich as a sodium salt. A humic acid extracted from groundwater (identification number Gohy573) was also used (41). Stock HA solutions (500 ppm) were prepared by dissolving 0.25 g of material in 2 mL of a 0.1 M NaOH aqueous solution under nitrogen atmosphere, followed by sonication, dilution with distilled water to 500 mL, pH adjustment to 7, and filtration through a 0.45-µm pore diameter Millipore membrane. Stock solutions were stored at 4 °C in the dark. Cr(III) and Cr(VI) solutions employed in the preliminary adsorption analyses as well as Cr(VI) solutions employed in some photoreduction studies contained the γ-emitter 51Cr (half-life 28 days). The radiotracer purchased from Amersham, U.K., as chromium(III) chloride was oxidized to Cr(VI), if necessary, by hydrogen peroxide in alkaline solution (0.1 mL of 30% H2O2 + 1 mL 0.1 M NaOH). After 24 h, the solution was evaporated almost to dryness in order to eliminate the excess H2O2. A check of the oxidation state of the 51Cr tracer was made by ion chromatography. The employed Dionex column retained Cr(III), while the anionic Cr(VI) eluted rapidly. A further oxidation step was carried out if necessary. The solution was then diluted as desired. Potassium chromate (Merck) was used to prepare nonradioactive solutions. Analytical Methods and Experimental Procedures. The ζ potential of ZnO suspensions was measured in a microelectrophoresis cell (Zeta-Meter, New York), equipped with an automated sample-transfer system, in order to limit the thermal overturn occurring with suspensions of high specific conductance. Each reported value is the average of at least 10 determinations. The equilibrium adsorption on ZnO of the Cr(VI) and Cr(III) species was investigated by adding the mixture of stable and radioactive Cr(VI) or Cr(III) to 8 mL of ZnO suspension in Pyrex tubes with Teflon caps (Corning), which were then shaken for 3 days in the dark. After centrifugation at 5000 rpm for 30 min and filtration through 0.45-µm pore diameter Millipore filters, the supernatant was analyzed for the residual chromium content by γ-counting of 51Cr with a NaI crystal detector connected to a PW4800 Philips analyzer. The efficiency of the separation procedure was checked by measuring the total zinc concentration in the filtrates. This was found to be less than 0.1% of the total ZnO at pH 7.2 and below 0.02% above pH 8. The pH of the equilibrium solution was measured immediately after phase separation. The radiochemical method of analysis was chosen for equilibrium adsorption determinations due to its very high sensitivity. In chromium speciation determinations, Cr(VI) analysis in the supernatant was carried out spectrophotometrically

after reaction with diphenylcarbazide (DPC) (42). The sensitivity limit of this method is ∼5.0 × 10-7 M. In Cr(VI) analyses of irradiated suspensions, the absorption of hydrogen peroxide produced by photocatalyzed oxygen reduction was taken into account on the basis of calibration curves experimentally obtained after the addition of known amounts of H2O2. The Cr(VI) amount on the solid oxide, spectrophotochemically analyzed after phase separation and dissolution of ZnO in acids, was always below the sensitivity of the method. The overall chromium content in each of the two phases after separation was determined by ICP atomic absorption spectroscopy. Hydrogen peroxide concentrations were determined by measuring the absorbance at 475 nm of the red complex formed after the addition of Mohr salt and KSCN to the supernatant (43, 44). The detection limit of this method was in the order of 5.0 × 10-7 M. H2O2 standard solutions were analyzed iodometrically. Also the amount of HA adsorbed on ZnO particles in aqueous suspensions containing no chromium was determined spectrophotometrically by measuring the decrease in absorbance at 240 nm of the supernatant after equilibration and phase separation. In the photoreduction experiments, the Pyrex tubes (internal diameter ∼0.8 cm) containing the ZnO aqueous suspensions equilibrated with Cr(VI) for 3 days in the dark were irradiated in a Solarbox (Cofomegra, Italy) by a xenon lamp (1500 W m-2) having a spectral energy distribution similar to solar radiation. Following preliminary kinetic tests, all tubes were irradiated for 30 min. Under these conditions, the photoreduction of Cr(VI) was not complete, as found instead with longer irradiation times for both the sensitized and the unsensitized reactions. The irradiation time of 30 min, therefore, proved to be more convenient to evidence sensitization effects due to humic acids. A continuous water recirculation around the samples assured both temperature control (20 ( 1 °C) during irradiation and filtering of infrared light. Many samples belonging to the same series were irradiated simultaneously, thus attaining a higher reproducibility of the procedure. In standard conditions, the suspensions were not stirred during irradiation. Some irradiations were also carried out in magnetically stirred cells. As polychromatic radiation was used in the present study, the fraction of light absorbed by chromate solutions was evaluated by integration over the emission spectrum of the lamp and the absorption spectrum of chromate. Similarly, a mean polychromatic radiation flux of 10-4 Einstein s-1 L-1 was estimated by ferrioxalate actinometry (45), by taking into account the emission spectrum of the lamp and the dependence on irradiation wavelength of the quantum yield of the actinometer. Quantum yields of photoreduction in different experimental conditions were estimated as the ratio between the amount of Cr(III) photoproduced after 30-min irradiation and the total amount of light effectively reaching the semiconductor surface during the same time. In order to test the effect of oxygen, some suspensions were purged with nitrogen for 24 h prior to irradiation and then accurately sealed in the Pyrex tubes. In this case, all sorption experiments were conducted in a metallic chamber under nitrogen. This was continuously recirculated through high-temperature Cu/Pd catalysts in order to remove all possible oxygen contaminations.

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FIGURE 1. Percent Cr(III) and Cr(VI) adsorption on ZnO particles (2 g L-1) from 5.0 × 10-6 M aqueous solutions. Adsorption density of Cr(III) in the investigated pH range (4.9 × 10-7 mol m-2) and of Cr(VI) at pH 7.6 (3.2 × 10-8 mol m-2). The results obtained in the presence of 5 ppm HA are identified by the open and closed circles for respectively Cr(VI) and Cr(III).

FIGURE 2. ζ potential of ZnO suspensions (0.2 g L-1) as a function of pH, without HA at different ionic strength (NaClO4) (() 0.001 M, (0) 0.01 M, (9) 0.1 M and in the presence of (4) 5 ppm HA and (b) 20 ppm HA at ionic strength 0.01 M.

Results and Discussion Adsorption of Cr and HA on ZnO Particles. The adsorption behavior of Cr(III) and Cr(VI) from 5.0 × 10-6 M aqueous solutions [adopted as standard initial Cr(VI) concentration in the photoreduction experiments] on ZnO particles (2 g L-1) under dark conditions is illustrated in Figure 1. While Cr(III) is totally adsorbed on ZnO in all the investigated pH range, the adsorption of Cr(VI) is very low, especially at high pH values, being 6% at pH ∼7.5. Similar adsorption data were obtained also for chromium concentrations in the range 2.0 × 10-7-4.0 × 10-4 M. In the presence of a 10-fold higher amount of semiconductor (20 g L-1), higher percent amounts of Cr(VI) were adsorbed on the oxide, depending on Cr(VI) concentration in the aqueous phase. Our results are comparable to those obtained by Dome`nech and Mun ˜ oz (28), who employed however different experimental conditions. The expected increase of Cr(VI) adsorption at lower pH (46, 47) could not be checked experimentally because the addition of acid to the suspension caused immediate partial dissolution of the zinc oxide. The results of Figure 1 are consistent with the adsorption behavior of chromium on other mineral oxides (33, 34, 48) and with the surface properties of ZnO, which were investigated by microelectrophoresis. Figure 2 shows the ζ potential values of ZnO suspensions at different ionic strength and as a function of pH. The isoelectric point, i.e., the pH value where the particles are electrokinetically uncharged (49), is ∼9.3. This is in agreement with the values of point of zero charge (around pH 9.0-9.5) obtained by acid-base titration (50, 51). Figure 3 shows that HA adsorption on ZnO decreases with increasing pH, due to the electrostatic repulsion between the ionized functional groups of HA and the oxide surface, which becomes negatively charged at pH >9.3. As shown in Figure 2, the surface potential is considerably influenced by the presence of HA. A humic acid concentration as low as 5 ppm reduces the ζ potential to about -50 mV for any ionic strength and pH value. No further change is observed by increasing the HA concentration to 20 ppm. Binding of HA to the surface exposes ionized functional groups, thus imposing a net negative charge at the plane of shear. Similar observations have been reported for other mineral oxides (52-54).

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FIGURE 3. pH dependence of HA adsorption on ZnO particles (2 g L-1) from aqueous suspensions initially containing 5 ppm HA.

In order to check how these modifications of surface properties affect the partitioning of chromium between the two phases, the adsorption tests shown in Figure 1 were repeated in the presence of HA. In any case, the adsorption behavior of Cr(VI) was not affected by the presence of HA over a time period of 3 days, and no significant variation in the adsorption of Cr(III) was observed in the presence of 5 ppm of HA. For relatively high HA concentrations (50 ppm), a constant 15-20% decrease of Cr(III) adsorption was measured in the entire pH range. This might be due to blocking of ZnO active surface sites by HA. However, aqueous-phase Cr(III) complexation with HA may also occur (55). In the standard conditions of the present study (2 g L-1 ZnO, 5 ppm HA), all Cr(III) was adsorbed on the oxide, whereas practically all Cr(VI) remained in the aqueous phase. Therefore, the presence of chromium on the solid adsorbent can be taken as a proof of Cr(VI) reduction. This observation proved to be very useful in the present investigation, as it was adopted as a tool for determining chromium speciation. Indeed, available speciation techniques do not allow a direct determination of the oxidation state of chromium in heterogeneous systems at concentrations of the order of only a few ppm. Photoreduction of Cr(VI) and HA Sensitization. The irradiation of ZnO suspensions, previously equilibrated with

FIGURE 4. Percent chromium adsorption on ZnO particles (2 g L-1) after 30-min irradiation. Initial Cr(VI) concentrations and, in the brackets, corresponding range of Cr(III) adsorption densities: (4) 2.0 × 10-7 M (1.6-1.9 × 10-8 mol m-2); (9) 5.0 × 10-6 M (1.9-4.0 × 10-7 mol m-2); (O) 2.5 × 10-5 M (0.9-1.7 × 10-6 mol m-2); (() 4.0 × 10-4 M (2.2-3.8 × 10-6 mol m-2).

FIGURE 5. Chromium speciation after 30-min irradiation of a ZnO (2 g L-1) suspension initially containing 5.0 × 10-6 M Cr(VI).

Cr(VI) for 3 days at different pH values, caused a great increase in chromium adsorption on the oxide (Figure 4). As stated above, this can occur only as a consequence of the transformation of Cr(VI), an anion which does not adsorb on ZnO particles in the investigated pH range, to the reduced species Cr(III), which has a marked affinity for the ZnO surface (Figure 1). The results of chromium speciation measurements in the irradiated system (Figure 5) indicate that a decrease of Cr(VI) in the aqueous phase is paralleled by an increase in the amount of Cr(III) adsorbed onto the mineral oxide. The Cr(III) produced in this way was likely to be present as Cr(OH)3 (56, 57) since Cr(III) concentration exceeded the solubility limit in the investigated pH range. No Cr(VI) photoreduction was observed in blank adsorption and speciation experiments conducted with non-semiconducting oxides (2 g L-1 SiO2 ). This confirms the active role of ZnO particles in the photoprocess. Adsorption densities evaluated from the data of Figure 4 reflect the fact that soluble chromates absorb light in the

FIGURE 6. Percent chromium adsorption on ZnO particles (2 g L-1) after 30-min irradiation (() in the absence of HA and in the presence of (9) 5 ppm HA-Aldrich and (0) 13.2 ppm HA Gohy 573 (groundwater origin).

visible-near-UV region. As their concentration increases, they absorb a progressively increasing portion of light. For λ e 380 nm, the inner filter effect was estimated equal to 0.08%, 2.2%, 10%, and 72% at Cr(VI) concentrations of 2.0 × 10-7, 5.0 × 10-6, 2.5 × 10-5, and 4.0 × 10-4 M, respectively. This inner filter effect together with the increasing coverage of the semiconducting surface with increasing chromium adsorption densities (see caption of Figure 4) leads to a nonlinear decrease with concentration of the percent photoreduction yield (26). Quantum yields were in the order of 10-6 for the most dilute Cr(VI) solution and around 6 × 10-4 for the most concentrated one, showing an increase but not a direct proportionality with reactant concentration. Adsorption data (Figure 4) show that chromium adsorption decreases with increasing pH. As quantitative adsorption of Cr(III) is expected under these conditions (see Figure 1), this indicates a lowering of the photoreduction yield at alkaline pH, which is confirmed by the trend shown by the speciation results of Figure 5. This is a consequence of the fact that the potential of the Cr(VI)/ Cr(III) couple shifts to anodic values with increasing pH more than the potential of the photogenerated electrons in the conduction band of the semiconductor (3, 28). The Cr(VI) photoreduction may be affected by the surface modifications of ZnO induced by HA adsorption (Figures 2 and 3). Figure 6 shows that chromium adsorption, i.e., reduction, increases in the presence of HA in an irradiated ZnO suspension containing 5.0 × 10-6 M Cr(VI). This occurs with humic acids of different origin, which could of course induce higher or lower sensitization effects depending on their nature and structure. The data reported in Figure 6 refer to humic acid solutions of comparable optical density in the range of irradiation wavelengths. A HA concentration higher than 2 ppm is needed to observe an increase in the photoreduction yield, whereas no further improvement can be observed with HA concentrations higher than 5 ppm, probably as a consequence of the saturation of the ZnO surface sites by the humic acid. Of course an inner filter effect due to HA could also play a role in this case. Around 2% of the light in the range 300-400 nm is absorbed by 5 ppm of HA in our experimental conditions.

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TABLE 1

Percent Chromium Adsorption after 30-min Irradiation of Stirred ZnO Suspensions in the Presence and Absence of HA (5 ppm)a

a

ZnO amount (g L-1)

pH

no HA

5 ppm HA

0.2 0.5 1.0 2.0

7.85 7.59 7.55 7.65

67 82 92 99

76 92 92 100

percent adsorption

Initial Cr(VI) concentration: 5.0 × 10-6 M.

Speciation measurements on samples irradiated in the presence of HA confirm that only Cr(III) is adsorbed on the ZnO surface and that the amount of Cr(III) in aqueous solution is below the experimental detection limit. Although complexation between Cr(III) and HA may occur, previous studies have shown that this process is rather slow and that the formation of Cr(III)-HA complexes can be detected only after a few hours (55). As the phase separation was always carried out in times shorter than 0.5 h after irradiation, we can be reasonably confident that the amount of Cr(III) in the liquid phase was negligible. As outlined in the Experimental Section, most of the photoreduction tests were performed without stirring the ZnO suspensions. However, in well-mixed reactors under identical irradiation conditions, the yield of both sensitized and unsensitized photoreduction markedly increases, attaining 100% Cr adsorption under standard conditions (2 g L-1 ZnO). A sensitizing effect of humic acids could be demonstrated only by investigating stirred suspensions containing lower amounts of ZnO (Table 1). The percent reduction of Cr(VI) obtained with 0.2 g L-1 ZnO is comparable to that obtained in a 10 times more concentrated ZnO suspension irradiated without stirring. This ensures that the yields of Cr(VI) photoreduction measured under static conditions both in the absence and in the presence of humic acids would be increased in natural systems undergoing a more or less fast mixing process. Mechanistic Studies. Fulvic acids have been reported to induce Cr(VI) reduction at low pH (36, 58-60). Moreover, humic substances can sensitize photochemical reactions in the homogeneous phase and produce hydrated electrons upon irradiation (15, 17, 18). However, speciation measurements demonstrated that no Cr(VI) reduction to Cr(III) occurred in the investigated pH range in solutions containing HA and Cr(VI) but no ZnO particles, both in the dark and after 30 min irradiation. This confirms the key role of light and of ZnO in the reduction of Cr(VI) in the presence of HA. Consequently, the observed increase in the reaction yield induced by humic acids is a photosensitization phenomenon strictly connected to the adsorption of HA on zinc oxide. Hydrogen peroxide is generated by irradiation of aqueous suspensions of semiconducting minerals, such as TiO2 or ZnO (61, 62), and also of desert sand (63). Due to the redox potentials of the electron-hole pair, H2O2 can theoretically be formed either by reduction of molecular oxygen by conduction band electrons or by water oxidation by holes (64). A recent work by Hoffmann and co-workers (65) definitively demonstrated that, in the case of irradiated aqueous suspensions of ZnO colloids, the reductive pathway

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FIGURE 7. Hydrogen peroxide concentration produced in aqueous 2 g L-1 ZnO suspensions at different pH and HA concentrations after 30-min irradiation.

FIGURE 8. Percent chromium adsorption on ZnO particles (0.2 g L-1) from 5.0 × 10-6 M aqueous solutions after 30-min irradiation under anoxic conditions.

is the one occurring with higher yield. Moreover, oxidation of water is the primary hole-consuming process in the absence of other electron donors. The results of Figure 7 confirm that H2O2 was present in the ZnO suspensions after irradiation and that it was produced in higher amount in the presence of humic acids. On the other hand, no hydrogen peroxide photogeneration could be detected in solutions containing only HA without ZnO. At acidic pH, hydrogen peroxide acts as a reducing agent for chromium, while in strong alkaline medium it behaves as an oxidant. Therefore, H2O2 might partially reduce Cr(VI) in the pH range from 7 to 9. In this case, the sensitizing effect of humic acids on Cr(VI) photoreduction would simply be a consequence of the HA-induced increase in the concentration of photogenerated H2O2 (Figure 7). As hydrogen peroxide is photogenerated by molecular oxygen reduction, a variation of the partial pressure of O2, which induces a variation in the production rate of H2O2 (63, 64), would be a stringent test on the reducing role played by H2O2. A marked decrease of Cr(VI) reduction should then occur with decreasing oxygen concentrations in the suspension. The results of experiments carried out under anoxic conditions (Figure 8) show instead a considerable increase in Cr(III) production and adsorption. This indi-

cates a competition between O2 and Cr(VI) for photogenerated elecrons. The data of Figure 8 were obtained by irradiating ZnO suspensions containing a 10 times lower amount of semiconductor (0.2 g L-1) than in standard conditions (Figure 6). Moreover, no spontaneous reduction to Cr(III) took place after 3 days in anoxic ZnO suspensions kept in the dark. Although Cr(III) should be the thermodynamically stable form in the absence of oxygen, the kinetics of conversion of Cr(VI) into Cr(III) is quite slow in solution, requiring contact times much longer than 3 days (66). The results of Figure 8 demonstrate that Cr(III) is produced in very high yield under anoxic conditions, even though H2O2 photogeneration does not occur. Thus, chromium reduction by hydrogen peroxide as a main reaction path can be excluded. For a further confirmation, the extent of Cr(VI) reduction occurring 30 min after the addition of different amounts of H2O2 to 5.0 × 10-6 M aqueous Cr(VI) solutions was investigated in the pH range 7-11. Initial H2O2 concentrations were in the same range of those photogenerated in the ZnO suspensions (7.5 × 10-6-5.0 × 10-5 M). Only a 2-5% Cr(VI) reduction was observed, which is considerably lower than the percent reduction obtained in the photoreduction experiments with ZnO (Figure 6). Since the energy level of the conduction band of ZnO (-0.6 eV at the point of zero charge) (3) is less positive than the redox potential of the Cr(VI)/Cr(III) couple [∼+0.3 V for the reaction CrO42-/Cr(OH)3 at pH 9.3], photogenerated electrons can reduce Cr(VI). Under oxic conditions, however, both Cr(VI) and oxygen adsorbed on the semiconductor (62) are able to capture photogenerated electrons. The occurrence of a competition between the two species is evidenced by the concomitant formation under oxic conditions of the two reduction products, Cr(III) and hydrogen peroxide, while higher yields for Cr(VI) reduction are obtained under a nitrogen atmosphere (Figure 8). Thus, oxygen may control whether chromium reduction will occur in any particular water body. Neglect of this factor may explain why the efficiency of Cr(VI) photoreduction appears to vary greatly from one water body to another (38). As to the role played by humic acids in the photoreduction process, one should first of all observe that the pH dependence of both HA-sensitized Cr(III) production (Figure 4) and HA-sensitized H2O2 production (Figure 7) parallel that of HA adsorption on ZnO (Figure 3). Consequently, sensitization is a surface process, which may be envisaged to occur in two ways. The adsorbed humic acid, being an electron-rich compound, can easily scavenge the holes photogenerated in the semiconductor valence band, thus stabilizing the electron photopromoted in the conduction band. Moreover, humic substances adsorbed on semiconductor colloids are able to transfer an electron to the semiconductor after excitation by light of energy even lower than the band gap (21). Therefore, since the absorption of HA extends in the UV and visible region, a larger portion of the solar spectrum may be used for the photocatalyzed reduction.

Conclusions Adsorption of humic acids on minerals with semiconducting properties facilitates charge transfer reactions driven by light in the near-UV-visible region. Using ZnO, this was shown to sensitize the photoreduction of Cr(VI) to Cr(III). Photoinitiated interfacial electron transfer involving sedi-

ments coated with humic substances may be a mechanism for the reduction of metallic pollutants in the photic zone of aquatic systems.

Acknowledgments The authors are indebted to Prof. J. I. Kim for providing a humic acid sample (Gohy-573) and to Prof. I. R. Bellobono for his interest.

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Received for review May 31, 1995. Revised manuscript received September 25, 1995. Accepted September 25, 1995.X ES950368+ X

Abstract published in Advance ACS Abstracts, December 1, 1995.