Heterogeneous Photocatalytic Reduction of Cr (VI) in UV-Irradiated

Langmuir 2000, 16, 2715-2721

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Heterogeneous Photocatalytic Reduction of Cr(VI) in UV-Irradiated Titania Suspensions: Effect of Protons, Ammonium Ions, and Other Interfacial Aspects C. R. Chenthamarakshan and Krishnan Rajeshwar* Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065

Edward J. Wolfrum National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, Colorado 80401 Received August 25, 1999. In Final Form: November 19, 1999 Heterogeneous photocatalytic reactions in UV-irradiated TiO2 suspensions are comprised of two conjugate reaction pathways involving the photogenerated electrons and holes, respectively. The role of the hole reaction pathway in the dynamics of the photocatalytic reduction of Cr(VI) is a major focus of this study. It is shown that proton supply plays a crucial role in this reduction reaction. Thus, the Cr(VI) photoreduction kinetics switch from first order to zero order as the proton concentration is systematically increased in the aqueous suspensions. Ammonium ions are also shown to exert a dramatic accelerating influence on Cr(VI) reduction in media of initial pH 6. This new observation is rationalized by considering that these species act as hole scavengers. The consequent improvement in quantum yield combines with the facile proton generation upon NH4+ photooxidation, to result in the observed rate enhancement. Other interfacial aspects (i.e., adsorption) are also discussed.

Introduction That a variety of organic and inorganic substrates can be both photooxidized and photoreduced in UV-irradiated TiO2 aqueous suspensions is now well-established.1-5 Much less is known about how the photocatalytic reduction of metal ions at TiO2/solution interfaces is influenced by the “conjugate reactions”6 involving photogenerated holes and by the presence of various solution additives. Dioxygen starvation is known to be a limiting factor in the heterogeneous photocatalytic oxidation of organic substrates at TiO2/solution interfaces.7 Does co-reactant supply play a limiting role in the corresponding photoreduction pathway (at least in some instances) at these interfaces? In this paper, we address these issues using the reduction of Cr(VI) to Cr(III),

Cr2O72- + 14H+ + 6e- a 2Cr3+ + 7H2O

(1)

E° ) 1.232 V (SHE)

photoreduction of the latter. Co-added ammonium ions have a remarkable accelerating effect on the rate of reduction of Cr(VI). Finally, proton supply to the TiO2/ solution interface is shown to play a crucial role in the rate of photoreduction of Cr(VI). Aside from fundamental considerations, there is also a practical incentive for studying reaction 1. The toxicity of chromium has prompted a severe curtailment in its technological use and the search for environmentally more benign substitutes. Despite this, however, new strategies for remediation of water streams bearing this element continue to be important, either because complete replacement of Cr(VI) is not yet feasible or because of problems with leaching from old dump sites into aquifers. Thus, there have been many studies on the heterogeneous photocatalytic reduction of Cr(VI), both in our own laboratory and in others.8-16 The present study departs from this body of work in emphasizing the interfacial aspects, including the roles of medium pH and ionic additives.

as a candidate metal ion substrate. We show below that organic additives such as formate displace adsorbed Cr(VI) species from the TiO2 surface but yet accelerate

Experimental Section The photoreactor and the UV light source have been described elsewhere;14,17 the setup included provisions for

* Author to whom correspondence should be addressed. Phone: (817) 272-3810. Fax: (817) 272-3808. E-mail: [email protected].

(8) Miyake, M.; Yoneyama, H.; Tamura, H. Bull. Chem. Soc. Jpn. 1977, 50, 1492. (9) Yoneyama, H.; Yamashita, Y.; Tamura, H. Nature 1979, 282, 817. (10) Domenech, J.; Munoz, J. Electrochim. Acta 1987, 32, 1383. (11) Xu, Y.; Chen, X. Chem. Ind. 1990, 15, 497. (12) Domenech, J.; Munoz, J. J. Chem. Technol. Biotechnol. 1990, 47, 101. (13) Prairie, M. R.; Evans, L. R.; Stange, B. M.; Martinez, S. L. Environ. Sci. Technol. 1993, 27, 1776. (14) Lin, W.-Y.; Wei, C.; Rajeshwar, K. J. Electrochem. Soc. 1993, 140, 2477. (15) Gimenez, J.; Aguado, M. A.; Cervera-March, S. J. Mol. Catal. A: Chem. 1996, 105, 67. (16) Hongxiang, F.; Gongxuan, Lu; Shuben, L. Adsorpt. Sci. Technol. 1998, 16, 117. Hongxiang, F.; Gongxuan, Lu; Shuben, L. J. Photochem., Photobiol A: Chem. 1998, 14, 81.

(1) Pichat, P. Catal. Today 1994, 19, 313. (2) Blake, D. M. Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air; NREL/ TP-430-22197, January, 1997. (3) Rajeshwar, K.; Ibanez, J. G. Environmental Electrochemistry; Academic Press: San Diego, 1997. (4) Kamat, P. V.; Vinodgopal, K. In Molecular and Supramolecular Photochemistry; Ramamurthy, V., Schanze, K., Eds.; Marcel Dekker: New York, 1998; Chapter 7, p 307. (5) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC, Inc.: Tokyo, 1999. (6) Rajeshwar, K.; Ibanez, J. G. J. Chem. Educ. 1995, 72, 1044. (7) Shin, E.-M.; Senthurchelvan, R.; Munoz, J.; Basak, S.; Rajeshwar, K.; Benglas-Smith, G.; Howell, B. C., II J. Electrochem. Soc. 1996, 143, 1562.

10.1021/la9911483 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/12/2000

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Figure 1. Temporal profiles for Cr(VI) reduction in the presence of various dissolved gases. The initial Cr(VI) concentration was 400 µM, the TiO2 dose was 2 g/L, and the initial medium pH was ∼6. The medium contained no other additives. A firstorder kinetics model fit to the data is shown for the N2 case.

sparging the Cr(VI)-laden solutions with N2, O2, or air and for solution sampling. Titanium dioxide (Degussa, P-25) was used as the photocatalyst; the nominal dose was 2 g/L. The Cr(VI) concentration was spectrophotometrically determined at 352 nm using a Beer’s law plot constructed from standard solutions.14 The total chromium concentration was determined by flame atomic absorption spectroscopy (FAAS), using an analytical wavelength of 357.87 nm. For X-ray photoelectron spectroscopy (XPS), the photocatalyst after use was filtered from the suspension, dried, and then cast on a glass plate. Potassium dichromate and ammonium dichromate were from Mallinckrodt; these (and the other chemicals) were used without further purification. Ammonium and nitrate ion levels in the medium were potentiometrically determined using ion-selective electrodes (ISE) (Orion model 93-18 and 93-07, respectively) and Ag/AgCl/3 M KCl as a reference. Other instrumentation details pertinent to this study are given elsewhere.14,17,18 In all the photocatalysis experiments described below, 400 µM Cr(VI) was added to the TiO2 suspensions that were agitated by the gas supply (usually N2). These suspensions were initially maintained under agitation in the dark for 30 min to allow for adsorption-desorption equilibria to be established. The UV lamp was then turned on, and aliquots were syringed out periodically for spectrophotometric (or other) analyses. These solutions were filtered through a 0.45 µm PTFE syringe filter to remove the suspended TiO2 particles. Results and Discussion Influence of Dissolved O2 and Organic Additives. Figure 1 contains data from three photocatalysis experiments in a pH 6 aqueous medium where N2, air, or O2 was used as the sparge gas. The Cr(VI) at zero time (ca. 325 µM) affords a measure of the amount adsorbed on the TiO2 particle surfaces, recalling that an initial concentration of 400 µM was used in each case (see Experimental Section). Rather surprisingly, the reaction progress up to at least ∼30 min is not significantly influenced by the (17) Lin, W.-Y.; Rajeshwar, K. J. Electrochem. Soc. 1997, 144, 2751. (18) Bose, C. S. C.; Rajeshwar, K. J. Electroanal. Chem. 1992, 333, 235.

Chenthamarakshan et al.

Figure 2. Influence of formate ion addition on the temporal profiles for Cr(VI) reduction. The suspensions were sparged with N2, and other conditions are as in Figure 1. Two firstorder kinetics model fits are shown (dashed and solid lines).

nature of the dissolved gas. Dioxygen is a facile electron acceptor and, unlike in our earlier study on Ni(II) photoreduction,17 does not impede the Cr(VI) reduction. We interpret this trend via two factors. The O2intermediate formed as a result of the scavenging of photogenerated electrons from TiO2 is an efficient mediator for Cr(VI) reduction. On the other hand, consider the two reactions:

Ni2+ + 2e- a Ni E° ) - 0.257 (SHE) -

O 2 + e a O2

-

E° ) - 0.284 (SHE)

(2) (3)

The Ni(II) f Ni(0) potential is much more negative [relative to the Cr(VI) f Cr(III) reaction] such that the mediated reduction, which also has a much smaller driving force in the nickel case, is not effective. Second, note that the dissolved gas does exert a small but systematic effect at irradiation times longer than ∼30 min when the photocatalytic reaction slows down appreciably (for reasons discussed later). Indeed, we observed a significant decrease in the photoreduction rate in the presence of O2 in our earlier study on the TiO2/Cr(VI) system at pH 10.14 The reduction rate is sluggish in basic media relative to the acidic ones (see below). Thus, when the reaction is intrinsically fast (as in the initial stages, see Figure 1), the medium appears to play a less significant role. At times longer than ∼30 min, the conversion of Cr(VI) essentially ceases (Figure 1), suggesting either that the photocatalyst surface is irreversibly “poisoned” or that the supply of some species to the reaction zone is limiting the Cr(VI) reduction. We show below that proton starvation is the causal factor in the slowing of the photocatalysis rate. We have shown earlier6 that the rate of a targeted photocatalysis pathway (e.g., reduction) can be increased by promoting the rate of the conjugate reaction (i.e., the hole pathway). Simply put, if the photogenerated holes are rapidly scavenged from the TiO2 particles, fewer are available for recombination with the photogenerated electrons, and consequently, more of the latter are available for reduction of the targeted oxidant. Indeed, previous authors13 have shown that organic substrates such as salicylic acid promote the photocatalytic reduction of a number of metal ions, including Cr(VI). Figure 2 contains data for another organic additive (formate) chosen to be in ionic form. Again the medium

Heterogeneous Photocatalytic Reduction of Cr(VI)

Figure 3. Plot of the amount of Cr(VI) adsorbed on the TiO2 surface vs ammoniun or formate ion concentration (the latter plotted in logarithimic scale). The adsorbed amount was determined from the dark equilibration measurements (see Experimental). The lines are least-squares fits of the data points.

pH was close to 6 and these experiments were carried out in flowing N2. The beneficial role of the organic additive is quite striking, nearly all of the initial Cr(VI) being reduced within ∼30 min at formate concentrations 20 mM or higher. However, there are other, more subtle effects. For example, the amount of Cr(VI) initially adsorbed on TiO2 (in the dark) is reduced as the formate ion concentration was increased. This trend is shown by Figure 3. On the other hand, at higher formate ion concentrations, the latter stages of the photocatalytic reaction are significantly accelerated (Figure 2). Clearly, whatever species are limiting the reaction progress (as mentioned earlier, we shall show below that these are protons) are being concomitantly generated during formate photooxidation. As a final point with reference to the data in Figure 2, it must be noted that the initial photooxidation of multiequivalent reducing agents (such as formate) generates radical intermediates that are capable of reducing metal ion substrates. We17,19 and others20,21 have discussed this radical-mediated photoreduction route for other metal ions such as Ni(II), Pb(II), and Zn(II). However, notwithstanding some similarities with these cases, there also exist significant points of departure. In the Zn(II) case, formate addition was found to both promote its adsorption on the TiO2 surface and its subsequent photoreduction.22 Such is clearly not the case for Cr(VI) (see Figures 1 and 2), which shows a much greater proclivity to adsorb on the TiO2 surface than Zn(II). It is also interesting to note that substrate adsorption is clearly not the single key to securing high photocatalytic conversion yields. In the case of the Cr(VI)-TiO2-formate system considered here, the Cr(VI) ions displaced from the TiO2 surface (at high (19) Chenthamarakshan, C. R.; Yang, H.; Savage, C. R.; Rajeshwar, K. Res. Chem. Intermed. 1999, 25, 861. (20) Baba, R.; Konda, R.; Fujishima, A.; Honda, K. Chem. Lett. 1986, 1307. (21) Forousan, F.; Richards, T. C.; Bard, A. J. J. Phys. Chem. 1996, 100, 18123. (22) Chenthamarakshan, C. R.; Yang, H.; Rajeshwar, K. Manuscript in preparation.

Langmuir, Vol. 16, No. 6, 2000 2717

formate levels) can still efficiently get photoreduced by the other (solution-confined) pathways discussed above. Influence of Medium pH. Figure 4 contains data illustrating the influence of medium pH on the photocatalytic reduction of Cr(VI). The pH of the medium was adjusted in the range ∼1.5-10 by addition of requisite amounts of 5 M H2SO4 or 1 N NaOH, respectively, to the TiO2 suspensions.23 The insert in Figure 4 illustrates how the half-life for the Cr(VI) photoreduction reaction (t1/2) varies with the medium pH. While the proton supply at the TiO2 particle/solution interface exerts little effect on this kinetics parameter at medium pH below ∼4, there is an abrupt increase at initial pH values higher than ∼6 with an inflection at pH 7 (Figure 4, insert). Our earlier assertion that the plateau regime in Cr(VI) conversion profiles at neutral pH or thereabouts (Figures 1 and 2) arises from proton starvation is borne out by the experiment considered in Figure 5. A photocatalysis run was initiated at an initial medium pH of ∼6; after 1 h of UV irradiation, the pH was changed to ∼2.4 by acid addition. The reaction takes an immediate acceleratory turn as shown in Figure 5; the normal (unperturbed) case is also illustrated for comparison (squares). On proton addition, the residual Cr(VI) after 1 h (∼25% of the initial concentration; Figure 5) dropped to zero within ∼15 min. The proton starvation effect is exacerbated at medium pH values in the 4-7 range. This is illustrated in Figure 6a which maps the pH change in the medium after the photocatalytic reaction had proceeded for 4 h (consuming protons, eq 1) as a function of the initial pH. At medium pH’s below ∼3, protons are in plentiful supply at an initial Cr(VI) level of ∼400 µM. Thus, the proton consumption by the latter is negligible. The basic medium pH case (pH 10) is interesting in that here also the pH change is negligible (Figure 6a). Considering that the Cr(VI) species in basic media are the monomeric form (CrO42-), the reduction scheme is likely to be14

CrO42- + 4H2O + 3e- f Cr(OH)3 + 5OH- E° ) -0.13 V (SHE) (4) Consideration of predominance diagrams for Cr(III) species as a function of pH (see Figure 6 in ref 15) also supports our assertion that the reduction of Cr(VI) to Cr(III) affords the latter either in ionic form (eq 1) or as the neutral hydroxide species (eq 4) depending on the medium pH. This in turn has consequences in terms of sequestration of the Cr(VI) reduction product within the photocatalyst phase facilitating its removal from the solution (see below). How does the medium pH influence Cr(VI) adsorption on the TiO2 particle surface? That there is indeed an influence is gauged by an examination of the zero irradiation time values for the Cr(VI) levels in solution (Figure 4). This is amplified in the plot in Figure 6b which shows the dependence of the concentration of Cr(VI) adsorbed (after 30 min equilibration in the dark, Experimental Section) on the medium pH. One causal factor in the trend in Figure 6b is considered to be the surface charge on the TiO2 particles and how it varies with the solution pH. In basic media, the negative charge on TiO2 would electrostatically repel the chromate (or dichromate) anions. On the other hand, the very low levels of Cr(VI) adsorption observed here (Figure 6b) at pH e ∼2 contrast (23) The initial pH of the TiO2 suspensions (N2-sparged) was ∼6.

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Figure 4. Influence of medium pH on the temporal profiles for Cr(VI) reduction. The medium pH was perturbed by either H2SO4 or NaOH (see text) except in the pH 6 case. The medium was N2-sparged and contained no other additives; other conditions as in Figure 1. The lines are first order or zero order kinetics model fits (see text). Data symbols: (O) pH 1.48; ([) pH 2; (+) pH 3; (9) pH 4; (2) pH 6; (×) pH 7; (b) pH 10. Insert: dependence of half-life, t1/2, on the initial pH of the medium. The line is a sigmoidal fit of the data points.

Figure 5. Manifestation of the proton starvation phenomenon on Cr(VI) photoreduction dynamics. Two separate runs are compared as discussed in the text.

with the trend in a recent study that reports ∼50% adsorption at a pH of 2.5.16 The maximum level of Cr(VI) adsorption in our hands is only ∼25% at a pH of 4 (Figure 6b). The only tangible difference in the conditions in the

two studies is the dark equilibration time (1 h in ref 16 versus 30 min in ours); P-25 Degussa TiO2 samples were used in both the instances. We ascribe the trend at low pH in Figure 6b to strong proton adsorption on the TiO2

Heterogeneous Photocatalytic Reduction of Cr(VI)

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Figure 6. The pH change after 4 h of photocatalysis mapped as a function of the initial medium pH (Figure 6a). The dependence on medium pH of the amount of Cr(VI) adsorbed in the dark on the TiO2 particle surface is shown in Figure 6b. The latter was determined from the difference between the initial Cr(VI) concentration (400 µM) and the level after 30 min equilibration. The dashed line in Figure 6a is merely for visualization of the data trend.

surface. Once again it is interesting (cf., the formate case discussed above) that strong substrate adsorption on TiO2 is not the sole factor in an optimal photocatalysis scenario. In this particular case, adsorption of a co-reactant (protons) exerts a beneficial influence (compare Figure 6b with Figure 4 above). FAAS analyses of the total chromium in solution after photocatalysis (for periods much longer than those shown, for example, in Figure 2) revealed zero levels at medium

pH values higher than ∼4 and substantial levels of chromium at lower pH values. In the latter cases, however, a subsequent pH adjustment (with lime, for example) brought about complete sequestration of the total chromium from the solution phase. As also discussed earlier,14 experiments in basic media and in those close to the neutral range resulted in a rather dramatic change in the visual appearance of the TiO2 suspension from a milkywhite hue to a greenish-blue color with progressive UV

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Chenthamarakshan et al. Table 1. Influence of Ammonium Ions on the Extent of pH Change of the Medium (∆pH) after Photocatalysisa NH4Cl concentration (mM)

∆pH

0.2 0.4 0.8 1 2 4 8

3.97 3.47 3.29 3.63 2.44 2.06 1.76

a The initial pH of the suspensions was ∼6. The final pH was measured after 1 h of UV irradiation with the other conditions as described in the Experimental Section.

irradiation. Examination of the photocatalyst after use by XPS revealed the Cr 2p1/2 signal at a binding energy of 577.81 eV, in good agreement with the value reported24 for Cr(OH)3 of 577.30 eV. Further quantitative analyses of the O 1s XPS signal (after correction for the “baseline” TiO2 level) and the Cr/O atomic concentration ratio were consistent with the presence of Cr(OH)3 on the TiO2 surface. In contrast, no color change was observed after Cr(VI) reduction in the acidic case. The kinetics model fits to the data in Figures 1, 2, and 4, also offer interesting insights. At initial pH values above ∼4, the Cr(VI) conversion profiles show first-order kinetics behavior (Figures 1, 2, and 4). On the other hand, when the initial pH of the medium is in the 1-3 range, zero order kinetics behavior is observed (Figure 4). Such a switch in kinetics behavior as a function of co-reactant concentration has not been reported before to our knowledge. The Langmuir-Hinshelwood kinetics model predicts first-order kinetics at low substrate [Cr(VI) in this case] concentrations reverting to zero order kinetics at high substrate levels in the photocatalysis medium.3 Such a pattern is rationalizable on the basis of substrate adsorption on the photocatalyst surface. On the other hand, the similar pattern seen here for the co-reactant concentration (Figure 4) signals the importance of proton adsorption (on the TiO2 surface) in reaction 1. Indeed, as mentioned earlier and judging from the trends in Figures 4 and 6b, Cr(VI) adsorption itself would appear to be a less critical factor than the proton supply at the surface! As also discussed elsewhere,14,25 changing the initial pH of the medium has the further consequence of affecting the relative disposition of the TiO2 conduction band edge and the Cr6+/3+ redox level. The rate of shift of the former with pH is Nernstian (i.e., -59 mV/pH unit), while the redox potential for Cr(VI) reduction has a steeper rate of change. Thus, the thermodynamic driving force for Cr(VI) photoreduction does become lower as the medium pH is increased. Influence of Ammonium Ions. Figure 7 illustrates the rather dramatic effect of NH4+ ions on the Cr(VI)

reduction profiles. Even levels as low as 0.2 mM have an appreciable influence especially in the latter stages within the proton starvation regime. In the absence of TiO2 and/ or UV light, no effect was noted. The initial pH of the medium (∼6) was not affected by NH4Cl addition. However, there was a substantive effect in the magnitude of pH change during photocatalysis on NH4+ addition; Table 1 illustrates this point. Clearly, whatever mechanism is exerting a beneficial influence on Cr(VI) reduction is also generating protons. As with the proton case, the Cr(VI) conversion kinetics also switches from first order at low NH4+ levels (e.g., 0.2 mM) to zero order at a 8 mM level (Figure 7). Previous authors26 have reported that ammonium ions can be photocatalytically oxidized to nitrite which, in turn, can be further oxidized to nitrate ions in TiO2 suspensions. Other studies have further shown that nitrite ions can be oxidized to nitrate on UV-irradiated TiO2 surfaces.27 In both cases, protons are generated in the oxidation reaction. This would be consistent with the trend seen in Table 1 and in the role of NH4+ addition in ameliorating the deleterious effect of proton starvation (Figure 7). To further prove that this mechanism indeed is operative here, the NH4+ and NO3- levels in the photocatalysis medium were monitored by ISE potentiometry. Unfortunately, other cations (e.g., Na+, K+) interfere with NH4+ detection. Therefore unlike in the earlier experiments in Figure 7 (where potassium dichromate and ammonium chloride were employed), the TiO2 suspension in this case contained only 400 µM (NH4)2Cr2O7. As the Cr(VI) level progressively dropped toward zero on UV irradiation, the NH4+ ion level concomitantly decreased from 730 to 640 µM (the initial dark adsorption of NH4+ on TiO2 corresponded to ∼70 µM; see below). While NH4+ ions are consumed by the conjugate photooxidation pathway, clearly not all of the Cr(VI) converted is accounted for in this manner. That is, holes are also consumed by another reaction (presumably water oxidation). Nitrate ion levels cannot be monitored by ISE potentiometry in the presence of dichromate species. However, at the end of the run after 1 h (by then all of the initial dichromate had been reduced, Figure 7), 70 µM nitrate was detected by the ISE consistent with the mechanism discussed above. Chemical reduction of Cr(VI) by the NO2species (generated from NH4+) can be ruled out; a blank run on a Cr(VI)-NO2- mixture revealed no decrease in the Cr(VI) concentration even after 1 h of stirring. The thermal reduction of Cr(VI) is apparently kinetically sluggish. As in the formate ion addition case (Figure 2), the fact that NH4+ ions can scavenge the photogenerated holes must mean that the photogenerated e--h+ pair separation

(24) Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division: Eden Prairie, MN, 1979. (25) Mun˜oz, J.; Domenech, J. J. Appl. Electrochem. 1990, 20, 578.

(26) Low, G. K.-C.; McEvoy, S. R.; Matthews, R. W. Environ. Sci. Technol. 1991, 25, 460. (27) Hori, Y.; Nakatsu, A.; Suzuki, S. Chem. Lett. 1985, 1429.

Figure 7. Influence of ammonium ion addition (in the form of chloride salt) on the temporal profiles for Cr(VI) reduction. The initial medium pH was ∼6 and the medium was N2-sparged. Other conditions as in Figure 1. The lines are first order or zero order kinetics model fits of the 0, 0.2, and 8 mM data sets.

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Langmuir, Vol. 16, No. 6, 2000 2721

is also enhanced; i.e., the quantum yield is improved. Comparing the efficacy of formate and NH4+ ion addition on the Cr(VI) photoreduction kinetics (cf. Figures 2 and 7), it can be seen that a 0.2 mM level of NH4+ has the same net effect on the diminution of Cr(VI) concentration as 20 mM addition of formate ions. On the other hand, Figure 3 reveals that the effect of NH4+ on Cr(VI) adsorption (on TiO2) is essentially different from the formate case. Notwithstanding possible differences in the hole capture kinetics by NH4+ and formate ions, the rather marked differences in the sensitivity of Cr(VI) reduction kinetics toward the two additives can be rationalized by arguments related to proton supply:

NH4+ + 8h+ + 3H2O f NO3- + 10H+

(5)

HCO2- + 2h+ f CO2 + H+

(6)

Per the stoichiometry of Reactions 5 and 6, 10 times as many protons are generated by NH4+ oxidation than via formate ion oxidation. On the other hand, each mole of Cr(VI) species consumes seven protons (reaction 1). Taking these factors as a whole, the trend in Table 1, the fact that the pH change during photocatalysis in the presence of formate (∆pH) is ∼4.0, and that the medium does become

alkaline in all the cases, would appear to be reasonable, especially given that the hole pathway is not wholly accounted for by reactions 5 and 6. Finally, it must be noted that aside from the ionic species discussed above (protons, formate ions, and NH4+ ions), none of the other co-ions involved (e.g., Na+, K+, Cl-, HSO4-) had an appreciable influence on the Cr(VI) photoreduction profiles. Concluding Remarks Notwithstanding the rather extensive database8-16,25 on the heterogeneous photocatalytic reduction of Cr(VI) in UV-irradiated TiO2 suspensions, the present study has contributed (at least in our opinion) new understanding and insights especially on aspects related to adsorption, additives, and proton supply. Further studies on this interesting system and on the reduction of other metal ions, are continuing. Acknowledgment. This research was funded, in part, by a grant from the U. S. Department of Energy, Office of Basic Energy Sciences. We thank Ms. Gloria Madden and Ms. Rita Anderson for assistance in the preparation of this manuscript. The reviewers are also thanked for constructive criticisms of an earlier manuscript version. LA9911483