Environ. Sci. Technol. 1996, 30, 2484-2494
Surface-Catalyzed Chromium(VI) Reduction: Reactivity Comparisons of Different Organic Reductants and Different Oxide Surfaces BAOLIN DENG† AND ALAN T. STONE* Department of Geography and Environmental Engineering, G. W. C. Whiting School of Engineering, The Johns Hopkins University, Baltimore, Maryland 21218
CrVI reduction by low molecular weight organic compounds with various functional groups was examined in the presence and absence of oxide surfaces. Surface-catalyzed CrVI reduction has been demonstrated with R-hydroxyl carboxylic acids (glycolic acid, lactic acid, mandelic acid, and tartaric acid) and their esters (methyl glycolate, methyl lactate, and methyl mandelate), with R-carbonyl carboxylic acids (glyoxylic acid and pyruvic acid), with oxalic acid, and with substituted phenols (salicylic acid, 4-methoxyphenol, and resorcinol). Both goethite (R-FeOOH) and aluminum oxide (γ-Al2O3) exert an appreciable catalytic effect, although somewhat less than that observed with titanium dioxide. This study indicates that surfacecatalyzed CrVI reduction may occur in soils, sediments, aquifers, and other aquatic environments rich in mineral surfaces.
Introduction Rates of CrVI reduction by naturally-occurring organic compounds in aquatic environments depend not only on the constituents directly participating in the stoichiometric reaction but also on other naturally-occurring constituents that may catalyze or inhibit the reaction. The first paper in this series (1) has demonstrated that CrVI reduction by mandelic acid is dramatically catalyzed by TiO2 surfaces. Benzoylformic acid and benzaldehyde are the principal products of mandelic acid oxidation. The surface-catalyzed reaction is proportional to [CrVI]ads but zeroth-order with respect to mandelic acid. Replacing mandelic acid with its methyl ester (methyl mandelate) yields a lower rate of surface-catalyzed CrVI reduction and a shift to a fractionalorder dependence on the reductant concentration. The surface-catalyzed reaction is best characterized as “saturation kinetics” (2). The objective of the present work is to obtain information that allows us to evaluate whether the surface-catalyzed * Corresponding author e-mail address: dog
[email protected]. jhu.edu. † Present address: AL/EQC, 139 Barnes Drive, Suite 2, Tyndall AFB, FL 32403-5323.
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CrVI reduction by naturally-occurring organic compounds is an important and widespread phenomenon. To do this, we must assess the ability of a wider selection of naturallyoccurring reductants and mineral surfaces to bring about surface-catalyzed CrVI reduction. Numerous organic compounds exist in aquatic environments that may reduce CrVI. Formaldehyde, acetaldehyde, and glyoxylate have been found to be produced in a wide variety of natural waters under sunlight irradiation (3). Acetic, fumaric, lactic, malonic, oxalic, phthalic, pyruvic, succinic, and valeric acids were detected in unpolluted stream waters (4). Malonic, oxalic, succinic, acetic, tartaric, vanillic, and p-hydroxybenzoic acids, many of which are released by organisms and plants or produced by the breakdown of large polymers, have been found in surface soil horizons at concentrations as high as 1 × 10-51 × 10-4 M, with oxalate concentrations being the highest (5). In several climatically and geographically diverse soils, fungi exude oxalic acid or oxalate in sufficient abundance to cause the precipitation of calcium oxalate (6). In anaerobic environments, various thiols have been identified such as glutathione, cysteine, methane thiol, and 3-mercaptopropionate (7, 8). Furthermore, readily identifiable organic compounds represent only a small portion of the total organic carbon content in most environments (9). For this reason, the contribution of humic substances to reductant capacity and reductant reactivity must also be evaluated. Because of the exceedingly large number of organic compounds found in aquatic environments, it is not feasible to examine the reactivity of every individual compound. Instead, we rely upon the hypothesis that reactivity toward CrVI and other higher valent metal ions (e.g., MnIII,IV, FeIII, and CoIII) arises from a small set of reductant moieties. Carboxylate, carbonyl, alcoholate, and phenolate groups are the most abundant functional groups in natural organic matter (9, 10). For this reason, R-hydroxyl carbonyl, R-hydroxyl carboxylate, R-carbonyl carboxylate, phenolate, and other reductant moieties containing these functional groups merit special attention (11). Once the reactivity of different reductant moieties has been assessed, it is important to further investigate how electronic and steric interactions arising from neighboring portions of each molecule affect reactivity. An approach of this kind has been previously used to investigate the reduction of manganeseIII,IV oxides (12, 13) and CrVI (14, 15). In the upper earth’s crust, Si, Al, and Fe are by far the most abundant metal elements; primary aluminosilicates such as feldspars and weathering products such as clays and hydrous oxides dominate mineral-water interactions in many aquatic systems. In order to understand the processes occurring on natural mineral surfaces, hydrous oxides of Si, Al, Fe, Mn, and Ti have frequently been selected in research because of their widespread natural distribution and their similarity to more complex aluminosilicates and other minerals (16-18). In addition, synthetic forms of these minerals possess well-defined and reproducible physical and chemical properties. We will therefore use pure oxides as representative mineral surfaces. In most cases where surface-catalyzed reaction has been reported, at least one reactant has been shown to adsorb
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(1, 16-18). CrVI adsorption occurs on the surfaces of aluminum oxides (19, 20), goethite (21-23), amorphous iron oxide (24), and titanium oxide (25). The adsorption of carboxylic acids and other organic compounds on hydrous oxides has also been observed (26-29). The reactant adsorption observed in those studies serves as an indication that CrVI reduction by some organic compounds may be subject to surface catalysis. Different hydrous oxides, however, may demonstrate quite different catalytic reactivity. Torrents and Stone (18) have shown that the hydrolysis of phenyl picolinate ester is catalyzed by TiO2 and FeOOH but not by Al2O3 and SiO2. For the surface-catalyzed MnII oxygenation reaction, Davies and Morgan (17) have observed that goethite (R-FeOOH) and lepidocrocite (γ-FeOOH) possess much stronger catalytic efficiency than silica (SiO2) and alumina (δ-Al2O3), even when differences in MnII adsorption are accounted for. It is reasonable, therefore, to expect substantial differences in the ability of various oxides and mineral surfaces to catalyze CrVI reduction reactions.
Materials and Methods All solutions and suspensions were prepared from distilled deionized water with resistivity of 18 MΩ‚cm (DDW, Millipore Corp.). Glassware was cleaned in 5 N HNO3 and thoroughly rinsed with DDW prior to use. Stock potassium dichromate and other inorganic salt solutions were prepared from analytical-grade reagents (J. T. Baker) and filtered through 0.2-µm membrane filters (Nuclepore Corp.) prior to use. Organic reagents were obtained from T. J. Baker (phenol), Sigma Chemical Co. (benzaldehyde), and Aldrich (glycolic acid, lactic acid, mandelic acid, atrolactic acid hemihydrate, tartaric acid, methyl glycolate, methyl lactate, methyl mandelate, glyoxylic acid, pyruvic acid, sodium benzoylformate, oxalic acid, 4-methoxyphenol, resorcinol, and salicylic acid) in the highest possible purity and used as received. Aluminum oxide (γ-Al2O3, type C) was obtained from Degussa Corporation and possesses a BET surface area of 90.1 m2/g, a CrVI adsorption site density of 0.67 site/nm2 (this study), and a pHzpc of 8.9 as measured by acid/base titration (18). Titanium dioxide (TiO2, P-25, mainly anatase) was also obtained from Degussa Corporation and possesses a BET surface area of 40.5 m2/g, a CrVI adsorption site density of 0.59 site/nm2 (this study), and a pHzpc of 6.5 (18). Goethite (R-FeOOH) was synthesized and characterized as discussed in Coughlin and Stone (30), following the procedure of Atkinson et al. (31). A 6.5-L mixture of 0.5 M Fe(NO3)2 and 0.21 M HNO3 was slowly added to 12.6 L of 1.55 M KOH while sparging with 20% O2/80% N2. The resulting slurry was then aged for 36 h in a 70 °C oven and was washed nine times with DDW. The 39.8 g/L goethite stock suspension was stored at 4 °C in a polyethylene bottle. X-ray diffraction analysis confirmed that goethite was the only mineral phase. Transmission electron microscopy analysis showed that goethite existed as acicular crystals approximately 1 µm long by 0.1 µm wide with no other morphologies present. The sample possesses a BET surface area of 47.5 m2/g (30) and an adsorption site density of 1.6 sites/nm2 (this study). Goethite preparations synthesized using the same procedure yield a pHzpc of 7.9 (18). The experimental procedure for CrVI adsorption has previously been described in detail (1). The extent of CrVI adsorption onto oxide surfaces was calculated as the difference between total added adsorbate concentration
and the supernatant concentration measured 12 h after CrVI addition. Experiments examining CrVI reduction employed the following concentrations: 20 µM CrVI, 200 µM organic reductant, 5.0 mM acetic acid/sodium acetate buffer, and 0.10 M NaClO4 supporting electrolyte. Experiments examining the catalytic effect of oxides employed 1.0 g/L TiO2, 1.0 g/L Al2O3, or 0.20 g/L FeOOH. Because of the 10-fold excess in organic reductant, changes in organic reductant concentrations were small. Samples were filtered (0.2 µm pore-diameter filters, Nuclepore Corp.) prior to the determination of CrVI concentration in the filtrate ([CrVI]aq). Changes in [CrVI]aq provided the basis for monitoring reaction progress. Lower FeOOH loading was needed to ensure that sufficient CrVI remained in solution to monitor reaction progress. A number of observations regarding the TiO2-CrVImandelic acid system (1) are pertinent here. An initial, rapid decrease in [CrVI]aq is caused by adsorption, while a subsequent, gradual decrease in [CrVI]aq is caused by surface-catalyzed reduction. When excess mandelic acid is employed, plots of ln[CrVI]aq versus time are linear, indicating that surface-catalyzed reduction is first-order with respect to [CrVI]aq
-d[CrVI]aq/dt ) kobs[CrVI]aq
(1)
Integration of this equation yields
[CrVI]aq ) B exp(-kobst)
(2)
where kobs is the pseudo-first-order rate constant (in h-1) and B is [CrVI]aq at t ) 0 (in mol/L). When mandelic acid is replaced by other organic reductants, the pseudo-first-order behavior just described may or may not apply. The quantities kobs and B, which can be obtained by nonlinear fit of experimental data using eq 2, still provide a useful base for comparison, and will be reported. In the mandelic acid experiments, subtracting B from the amount of CrVI added corresponds to the amount of CrVI initially adsorbed. Several of the organic reductants included in this study are known to adsorb onto oxide surfaces to a significant extent (27) and can therefore be expected to compete with CrVI for available surface sites. It would be desirable, therefore, to have an independent means of measuring the extent of CrVI adsorption in the presence of organic reductants. Unfortunately, attempts to recover adsorbed CrVI quantitatively through the addition of 10 mM sulfate and phosphate proved unsuccessful within the pH range examined (4.7 < pH < 10). The duration of the CrVI reduction experiments and our ability to analytically distinguish small changes in [CrVI]aq place a lower limit on values of kobs that can be determined. For this reason, only values greater than 5.0 × 10-4 h-1 will be reported (corresponding to a 10% decrease in [CrVI]aq during 200 h of reaction). CrVI was analyzed by the diphenylcarbazide colorimetric method (32). Organic reductants may interfere with the diphenylcarbazide test in two ways: (i) the reductant may absorb at the test wavelength, and (ii) a reductant that did not reduce CrVI in the original sample may reduce CrVI at the low pH of the test, preventing color development of the diphenylcarbazide reagent. Blank experiments performed in the absence of added surfaces confirmed that interference
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FIGURE 1. Postulated stoichiometries for the oxidation of representative organic compounds examined in this study. pKa values are derived from values cited by Smith et al. (42) under the conditions of 25 °C and 0.10 M ionic strength.
did not take place. For a selected number of experiments employing mandelic acid, concentrations of the parent compound and its oxidation products (benzaldehyde, benzoylformic acid, benzoic acid) were determined in supernatant solutions using HPLC. A µ-Bondapak C18 column (Waters Corp.) and a UV detector were employed. For experiments employing Al2O3, supernatant solutions were obtained by filtration through 0.2-µm filters (Nuclepore Corp.), and dissolved aluminum concentrations were determined using graphite furnace atomic absorption spectroscopy (Perkin-Elmer Model 2380 with HGA 300 programmable graphite furnace and AS 40 autosampler).
Results and Discussion Comparison among Representative Organic Reductants. Reductant. The organic reductants included in this study fall into six functional group categories, as shown in Figure 1. Oxidation products for each group have been postulated, based upon products observed in the TiO2-CrVI-mandelic acid system (1) and upon a recent review of the reactivity of various organic functional groups toward reaction with metal oxidants (11). No effort has been made to identify oxidation products in the present work. Group I consists of R-hydroxyl carboxylic acids, Group II consists of their esters, and Group III consists of R-keto acids. Oxidation by CrVI in strongly acidic aqueous solutions has been observed to form the products listed (33-35). In the TiO2-catalyzed oxidation of R-hydroxyl carboxylic acids (e.g., mandelic acid), similar kinds of products are formed (1). Group IV consists of oxalic acid, a reductant oxidized to CO2 through more than one reaction pathway (36). Group V consists of benzaldehyde, which is believed to be a reaction intermediate in the oxidation of mandelic acid
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(1). Group VI consists of phenolic compounds; the uncatalyzed reaction of phenols with CrVI has recently been extensively examined (14, 15). Direct and TiO2-Catalyzed CrVI Reduction. Under the experimental conditions selected for our experiments (20 µM CrVI, 200 µM reductant, pH 4.7), none of the 16 organic compounds listed in Figure 1 reduced significant amounts of CrVI; kobs values are significantly less than 5.0 × 10-4 h-1. In subsequent surface-catalyzed CrVI reduction experiments, the solution-phase reduction of CrVI can be ignored. Results from TiO2-catalyzed CrVI reduction experiments with the 16 representative organic compounds are presented in Figure 2. kobs and B values were determined using nonlinear fits to eq 2, as discussed previously. The smooth curve in each plot, corresponding to this pseudo-first-order fit, matches the experimental data reasonably well for all 16 compounds. For 14 of the 16 organic reductants, an initial, nearly instantaneous drop in [CrVI]aq can be clearly distinguished from a subsequent, more gradual drop in [CrVI]aq. Extensive examination of the reaction with mandelic acid (1) indicates that this behavior is caused by rapid CrVI adsorption followed by gradual reduction of adsorbed CrVI. For reaction with each organic reductant, the extent of CrVI adsorption ([CrVI]aq) reported in Table 1 was calculated by subtracting the y-intercept B from total added CrVI. In order to compare the reactivity of various organic reductants, it is necessary to correct the pseudo-first-order rate constant kobs for differences in [CrVI]ads. The following equation was derived for this purpose in our earlier work (1):
ksurf )
kobs[CrVI]aq [CrVI]ads
(3)
This equation is only applicable when the pH and fraction of reductant adsorbed are held constant and when the CrVI concentration is within the linear range of the Langmuir adsorption isotherm (1). These conditions are met by the experiments discussed in this section; since the pH is held constant by the 5.0 mM acetate buffer, the reductant concentration and extent of reductant adsorption are kept constant using a 10-fold excess in reductant, and the CrVI concentration employed is well within the linear range. TiO2-catalyzed CrVI reduction by 4-methoxyphenol and resorcinol is so fast that it is difficult to quantitatively distinguish adsorption and redox reactions. As a consequence, values of ksurf will not be reported. As shown in Table 1, Group I compounds (R-hydroxyl carboxylic acids) decrease CrVI adsorption in the order: tartaric acid > glycolic acid > lactic acid > mandelic acid. The effect of mandelic acid on CrVI adsorption is small: [CrVI]ads in the presence of mandelic acid (5.45 mM) is only 10% less than [CrVI]ads in reductant-free suspension (6.0 µM) (1). In contrast, tartaric acid decreases CrVI adsorption by 60%. Tartaric acid has two free carboxylic acid groups, which may contribute to its stronger effect than compounds possessing only one free carboxylic acid group. Group II compounds (the esters of R-hydroxyl carboxylic acids) exhibit negligible adsorption onto the surfaces due to their lack of free carboxylic acid groups, as demonstrated in the case of methyl mandelate (1). For this reason, CrVI adsorption is not decreased by any of the esters. The effect of Group III compounds (R-carbonyl
carboxylic acids) follows the order: glyoxylic acid g pyruvic acid > benzoylformic acid. Oxalic acid in Group IV is an another compound possessing two carboxylate groups, as with tartaric acid, and it appears to compete effectively with CrVI for adsorption sites. The change of [CrVI]ads arising from benzaldehyde or phenol is less than 10%, and salicylic acid, a phenolic compound with a carboxylic acid group suitably spaced for chelating, decreases CrVI adsorption nearly 30%. Reactivity of different organic reductants will be evaluated according to ksurf, the rate constant corrected for the differences in CrVI adsorption. All Group I compounds have both a OH group and a H group at the R-carbon. The H group is important, since replacing this H with a CH3 group results in a compound (e.g., atrolactic acid) that is not capable of reducing CrVI (1). The ksurf value for tartaric acid is 2.25 times higher than for glycolic acid and 1.77 times higher than for lactic acid. This is not surprising, since tartaric acid has two R-hydroxyl acid reductant functional groups, while glycolic acid and lactic acid have only one. For compounds possessing a single R-hydroxyl acid functional group, changing the R substituent can have a significant effect on the CrVI reduction rate. Mandelic acid (R ) C6H5), as indicated by ksurf, is 5.88 times as reactive as lactic acid (R ) CH3) and 7.45 times as reactive as glycolic acid (R ) H). Rate constants for direct CrVI reduction by Group I compounds under acidic conditions (pH 3.0, 6.0 × 10-4 M CrVI, 0.10 M reductant) have been reported (37). In close agreement with our experiments on the surface-catalyzed reaction, mandelic acid (R ) C6H5) reacts 10.0 times faster than glycolic acid (R ) H). It is interesting to note that direct reduction by lactic acid (R ) CH3) is 3.9 times faster and direct reduction by tartaric acid (R ) CH(OH)COOH) is 41 times faster than direct reduction by glycolic acid (R ) H). Thus, differences in reactivity among these compounds are less pronounced for the surface-catalyzed reaction than for the uncatalyzed reaction. As Table 1 indicates, surface-catalyzed reaction with methyl glycolate is 1.74 times slower, with methyl lactate is 2.88 times slower, and with methyl mandelate is 6.55 times slower than reaction with the corresponding free carboxylic acids examined in Group I. The R substituent effect is smaller for Group II compounds than for Group I compounds. ksurf for methyl mandelate (R ) C6H5) is only 1.98 times of the ksurf for glycolic acid (R ) H). The ksurf value for methyl lactate (R ) CH3) is comparable to ksurf for glycolic acid (R ) H). Direct reduction of CrVI by one of the esters of the R-hydroxyl carboxylic acids has been reported (37). Employing acid conditions (pH 3.0, 6.0 × 10-4 M CrVI, 0.10 M reductant), reaction with methyl mandelate was found to be 6.7 times slower than reaction with mandelic acid, in close agreement with our results for the surface-catalyzed reaction. Among Group III compounds (R-keto acids), pyruvic acid and benzoylformic acid yield kobs values below our cutoff value for reliable determination. In contrast, R-hydroxyl carboxylic acids with the same R substituent (i.e., lactic acid and mandelic acid) yield TiO2-catalyzed CrVI reduction rate constants that are quite substantial, as discussed earlier. The reactivity of glyoxylic acid is distinctly different from the other two keto acids. ksurf for glyoxylic acid is actually 2.3 times higher than the corresponding R-hydroxyl carboxylic acid (glycolic acid).
As indicated by Figure 1, the stoichiometry of R-keto acid oxidation is markedly different from that of R-hydroxyl acids (and their methyl esters). Hydrogen atom abstraction at the R-carbon is no longer possible. The mechanism of glyoxylic acid reaction with CrVI may be different from that of pyruvic acid and benzoylformic acid, as reflected by its higher reactivity. Reports on the direct reduction of CrVI under acidic conditions (pH 3.0, 6.0 × 10-4 M CrVI, 0.10 M reductant) indicate that glyoxylic acid is 56 times more reactive than glycolic acid and 194 times more reactive than pyruvic acid (37). This finding supports our suggestion that glyoxylic acid may react with CrVI via a mechanism that is markedly different from other R-keto acids. It is interesting to note that the large reaction rate difference between glyoxylic acid and glycolic acid in catalyst-free solution largely disappears in the TiO2-catalyzed reaction. Oxalic acid (Group IV) is a strong chelating agent and obeys a distinctly different oxidation stoichiometry in comparison to Group I and III compounds. Despite this fact, ksurf values for glycolic acid, glyoxylic acid, and oxalic acid are quite similar to one another. In comparison, the rate of CrVI reduction by oxalic acid in particle-free solution (pH 3.0, 6.0 × 10-4 M CrVI, 0.10 M reductant) is comparable to that by glycolic acid but 38 times slower than the reduction by pyruvic acid (37). Benzaldehyde (Group V), a compound with only an aldehyde functional group, did not reduce CrVI in the presence of TiO2 under the conditions employed. This is not surprising because aldehydes normally show substantially lower reactivity toward CrVI reduction compared to R-substituted carboxylic acids (37). Among four compounds in Group VI with phenolic functional groups, phenol did not measurably reduce CrVI in the presence of TiO2. The reaction with salicylic acid is detectable, while the reaction with resorcinol and 4-methoxyphenol occurs rapidly with half-lives less than 50 h. The much higher reactivity of resorcinol and 4-methoxyphenol in comparison to phenol suggests that electron-donating substituents such as OH and OCH3 on the aromatic ring enhance reactivity. Elovitz and Fish (15) examined the reactivity of substituted phenols toward CrVI reduction in homogeneous solutions at pH 2.0, where the reactivity was also found to increase with the increasing electron-donating nature of the substituents. In summary, TiO2-catalyzed CrVI reduction, first observed with mandelic acid and methyl mandelate (1), has been shown to occur with a number of other R-hydroxyl carboxylic acids and their esters as well as with R-carbonyl carboxylic acids, with oxalic acid, and with substituted phenols. Significant differences in reactivity are observed within each reductant class, although typically lower than differences observed in catalyst-free solution experiments reported in the literature. pH Effect on CrVI Reduction by Resorcinol. The effect of pH on the TiO2-catalyzed reduction of CrVI by mandelic acid was examined extensively in our previous work (1). The CrVI reduction rate increases with decreasing pH, closely matching increases in the extent of CrVI adsorption with decreasing pH. This observation, along with other evidence, led to the conclusion that CrVI adsorption is primarily responsible for the surface catalytic effect (1). This section examines the effect of pH on TiO2-catalyzed CrVI reduction by resorcinol. The chemical properties of resorcinol are quite different from those of mandelic acid,
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FIGURE 2. Plots of [CrVI]aq as a function of time for 20 µM CrVI reduction by 200 µM organic compounds at pH 4.7 in the presence of 1.0 g/L TiO2. The curve is the best fit of the data based upon pseudo-first-order decay with respect to [CrVI]aq. (Reaction conditions: 5.0 mM acetate buffer, 0.10 M NaClO4.) TABLE 1
Rate Constants for TiO2-Catalyzed CrVI Reduction by Various Organic Reductants reductants Group I glycolic acid lactic acid mandelic acid tartaric acid Group II methyl glycolate methyl lactate methyl mandelate Group III glyoxylic acid pyruvic acid benzoylformic acid Group IV oxalic acid Group V benzaldehyde Group VI phenol salicylic acid 4-methoxyphenol resorcinol
kobs (h-1)a
[CrVI]ads (µM)b
ksurf (h-1)
4.22 ( 0.69 × 10-3 6.52 ( 0.86 × 10-3 4.28 ( 0.70 × 10-2 4.77 ( 0.66 × 10-3
4.33 5.03 5.45 2.43
1.53 × 10-2 1.94 × 10-2 1.14 × 10-1 3.44 × 10-2
4.80 ( 0.72 × 10-3 3.29 ( 0.48 × 10-3 7.82 ( 0.26 × 10-3
7.05 6.55 6.21
8.81 × 10-3 6.73 × 10-3 1.74 × 10-2
6.00 ( 0.87 × 10-3
