Environ. Sci. Technol. 2005, 39, 9541-9546
Photoirradiation of Dissolved Humic Acid Induces Arsenic(III) Oxidation JOHANNA BUSCHMANN,* SILVIO CANONICA, URSULA LINDAUER, STEPHAN J. HUG, AND LAURA SIGG Swiss Federal Institute of Aquatic Science and Technology (Eawag), P.O. Box 611, CH-8600 Du ¨ bendorf, Switzerland
The fate of arsenic in aquatic systems is influenced by dissolved natural organic matter (DOM). Using UV-A and visible light from a medium-pressure mercury lamp, the photosensitized oxidation of As(III) to As(V) in the presence of Suwannee River humic acid was investigated. Pseudofirst-order kinetics was observed. For 5 mg L-1 of dissolved organic carbon (DOC) and 1.85 mEinstein m-2 s-1 UV-A fluence rate, the rate coefficient k°exp was 21.2 ( 3.2 10-5 s-1, corresponding to a half-life 4 min, 50 mM, pH 8.2. UV/vis spectra of Fe(II)(phen)3 (11 000 L mol-1 cm-1) were recorded at λ ) 510 nm with a Kontron Instruments Uvikon 930 spectrophotometer using plastic cuvettes of 1 cm path length. Furfuryl alcohol and 4-nitroanisole were analyzed using high-performance liquid chromatography (HPLC) as previously described (11). Photoirradiations. The photoirradiation experiments were carried out using quartz tubes and a merry-go-round photoreactor (MGRR) DEMA (Hans Mangels GmbH, Bornheim-Roisdorf, Germany) Model 125 equipped with a TQ718 (700 W) medium-pressure mercury lamp (Heraeus Noblelight GmbH, Hanau, Germany). The exact procedure is described in ref 11. The photon fluence rate at 366 nm was determined by the 4-nitroanisole/pyridine actinometric method (15). Dark controls were done for each series of experiments. Analysis of Experimental Data. Linear regression of logarithmic concentration values determined at different irradiation times was used to determine pseudo-first-order rate coefficients, kexp (eq 1):
-
d[As(III)] [As(III)] ) k exp[As(III)] f ln ) -kexpt dt [As(III)]o
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TABLE 1. Chemical Properties of Natural Water Samples: Greifensee Water (GSW), Furtbach Water (FBW), River Wyre Water (RWW), Rice Field Water (RFW) from Bangladesh, and Synthetic Groundwater (SGW)a water sample
date
collected
filtered
[DOC]/ mg L-1
abs coeff 280 nm/m-1
abs coeff 350 nm/m-1
GSW
9/4/03
yes
GSW
2/11/04
FBW FBW RWW RWW RWW RFW SGW
9/3/03 9/5/03 4/20/04 4/21/04 4/22/04 4/18/05
2.5 m depth, deepest point 0 m, near shore 0.2 m depth 0.2 m depth 0.2 m depth 0.2 m depth 0.2 m depth surface
8.68
5.1
2.91
0.65
20
no
8.20
1.7b
2.16
0.57
20
yes yes yes yes yes no no
8.16 7.48 7.95 7.98 8.30 7.70 7.0-7.3
2.71 2.03 31.36 21.26 17.68 18.00 0.01
0.8 0.58 12.88 8.07 6.68 5.22 0.02
25 25 3300 2800 2400 2800 1000
pH
2.1 1.6 8.3 6.5 5.7 8.1 98% As(V)), while all other water samples had negligible As background concentrations. Absorption coefficients are denoted as abs coeff, dissolved organic carbon as DOC, and total organic carbon as TOC. b TOC
where [As(III)]o is the total concentration of As(III) at time zero. All kinetic results are based on the net disappearance of As(III). The value of kexp was corrected for inner filter effects (light absorbance by DOC) by integral-calculated correction factors which were 0.963, 0.927, 0.832, and 0.703 for SRHA containing solutions with 1, 2, 5, and 10 mg L-1 DOC, respectively (16). This corrected value is, in the following, denoted as k°exp (near-surface rate coefficient). The rate coefficient k°exp reflects the sum of the rate coefficient for direct photolysis of light-absorbing As(III) species, k°direct, and for photosensitized transformation of all As(III) species, k°sens (eq 2):
k°exp ) kdirect + k°sens
(2)
Equation 1 can also be expressed as follows (eq 3):
-
d[As(III)] dt
)
∑k°
pi[As(III)i]
i
+
∑k° ij
OXij[OXj]ss[As(III)i]
(3)
where k°pi is the first-order rate coefficient for the oxidation of an As(III) species i, [OXj]°ss is the steady-state concentration of a given photooxidant j, e.g. OH radical, 1O2, HO2 radical/ superoxide, 3DOM*, and other DOM-derived oxidants, and k°OXij is the second-order rate coefficient for the oxidation of an As(III) species i by the photooxidant j.
Results and Discussion Rates of Humic Acid-Mediated Photooxidation of As(III). Figure 1a shows typical data obtained for As(III) oxidation in the presence of SRHA under photoirradiation and in the dark. These reactions followed close to pseudo-first-order kinetics (R2 was between 0.97 and 0.99). At any time during the reaction, the oxidation process could be stopped by transferring the sample to dark conditions. Compared to the background medium buffered with 1 mM phosphate, pH 7.2 (k°medium ) (2.3 ( 1.0) ×10-5 s-1), 5 mg L-1 DOC (SRHA) accelerates the reaction significantly (k°exp ) (21.2 ( 3.2) × 10-5 s-1). Thus, the first-order rate coefficient for photoinduced transformation, k°exp, is calculated as follows (eq 4):
k°exp ≈ k°exp - k°medium
(4)
Direct photolysis and the impact of impurities in the medium seem to play a minor role because k°medium in the ionic medium is small compared to that in the SRHA-containing solution. Therefore, k°sens is about the same as k°exp. As photon fluence rate varied by about (20% over the whole series of experiments (mainly due to aging of the filter solution in the 9542
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FIGURE 1. (a) Oxidation of As(III) in the presence of SRHA (O) and without SRHA (]) under photoirradiation and dark control (b). Photoirradiation was stopped at 30 min (2) and 197 min (4), respectively. (b) Species concentration as a function of time; [As(OH)3] (b), [HAsO42-] (O), and the sum of both (1). Experimental conditions in a and b: 5 mg L-1 DOC (SRHA), pH 7.2, 1 mM phosphate buffer, [As(III)]o ) 0.40 µM, 25 °C. photoreactor), all k°exp values are normalized to a photon fluence rate of 1.85 mEinstein m-2 s-1 at 366 nm. Estimated overall quantum yield for 5 mg L-1 DOC (SRHA) was 3 × 10-6 molAs(III) molphotons-1. Decreases in As(OH)3 were balanced by corresponding increases in HAsO42- (Figure 1b). Measuring As(total) led to mass balances of 92 ( 5% for all experiments. The rate coefficient k°exp is linearly related to the DOC concentration of SRHA (R2 ) 0.9998) (Figure 2). Photoirra-
FIGURE 2. (a,b) Pseudo-first-order rate coefficients, k°exp (10-5 s-1), for [As(III)]o ) 0.40 µM, 25 °C, as a function of DOC (SRHA, pH 7.2) (b), GSW filtered, 9/4/03 (0, a), GSW unfiltered, 2/11/04 (1, a), FBW filtered ([, a), RFW filtered ([, b), RFW unfiltered (0, b), RFW unfiltered with 2.5 mg L-1 Fe (1, b), RWW filtered (]), SGW (4), SGW with 2.5 mg L-1 Fe (9), SRHA with 2.5 mg L-1 Fe (O, b). RFW with 2.5 mg L-1 Fe and 50 µM citrate had an initial k°exp of 5.5 × 10-3 s-1. Error bars indicate standard deviations (n ) 3) for SRHA (b), otherwise they indicate 95% confidence intervals. diation of As(V) in the presence of SRHA did not lead to the formation of As(III) (data not shown). Dark Reactions. Dark controls with initial spiking of As(III) in the presence of either SRHA or acetate (as a surrogate for carboxylic entities in humics), both experiments without phosphate buffer, did not show any decrease in As(III) concentration during 26 days, whereas 50% As(III) was oxidized in the presence of phosphate and DOM during the same time frame. However, dark reactions with initial spiking of As(V) showed a reduction to As(III) (90 ( 5%) within 2 days in the presence of either SRHA or acetate, while the addition of phosphate inhibited the reduction completely for up to 26 days (data not shown). The As(V) reduction took place after a lag phase of 19 h. After this time, bacterial growth was observed. In the presence of 0.15 mM NaN3, the same experiments did not show any As(V) reduction. Therefore, it is concluded that the observed reduction of As(V) is microbially catalyzed. According to Inskeep and McDermott, microbial As(V) reduction can not only occur due to anaerobic dissimilatory respiration or detoxification, but also under oxic conditions (17). Evaluation of Various Photooxidants. Photooxidants such as 1O2, H2O2, OH radicals, HO2 radicals, 3DOM*, and other DOM-derived oxidants are produced during photoirradiation of DOM (18, 19). They may be involved in the
photosensitized transformation process and are evaluated using the following methods. 1. Singlet Oxygen. To evaluate the contribution of 1O2 to the sensitized photooxidation of As(III), experiments were performed in D2O. D2O is a weaker quencher for 1O2 than H2O resulting in a lifetime 13 times longer of 1O2 in D2O than in H2O (20). However, k°exp values were determined to be similar: (13.1 ( 3.0) × 10-5 s-1 and (7.7 ( 0.3) × 10-5 s-1 for D2O and H2O, respectively ([DOC] ) 1.5 mgL-1 (SRHA), pD ) 7.2, pH 7.2, 1 mM phosphate buffer). This indicates that 1O is not the prevailing photooxidant in the system studied. 2 2. Hydroxyl Radicals. Photoirradiation of humic acids leads to the formation of OH radicals (21, 22), which are known to react fast with As(OH)3 (k ) 8.5 × 109 M-1 s-1) (23). To verify the importance of OH radicals, experiments with SRHA in the presence and absence of the OH radical scavenger 2-propanol (5 mM, 35 °C and 10 mM, 25 °C) were performed (at both concentrations, 2-propanol scavenges >99% of the OH radicals (24, 25)). At 25 °C, the oxidation rates of As(III) did not change significantly, and at 35 °C the reaction with 5 mM 2-propanol was 0.75 times slower than that without 2-propanol (Supporting Information, Figure S1). Therefore, the role of OH radicals in As(III) oxidation seems to be negligible at pH 7.2 in the present system. 3. Hydrogen Peroxide. As H2O2 is a weak oxidant (26), its contribution to the overall oxidation rate of As(III) should be negligible. The oxidation rate found in the system studied is about 120 000 times higher than the reported rate for As(III) oxidation by H2O2 in the dark, at pH 7.2 and 1 µM H2O2 (27). Additionally, direct photolysis of H2O2 under terrestrial sunlight has been shown not to be a relevant source of OH radicals (28). 4. Hydroperoxyl Radical and Superoxide. Although superoxide, O2-, has been reported to be the main oxidant for As(III) in the TiO2 photocatalysis (9), its one-electron standard reduction potential should be lower than 0.21 V (calculated from ref 29). Also the hydroperoxyl radical, HO2 (pKa ) 4.8 ( 0.1 (29)), is a weak oxidant (standard reduction potential of 0.75 V (30)). The reaction should be faster at pH 7, and/or by increased production of photooxidants with increasing pH. In the presence of 100 µM 4-cyanophenol (pKa ) 8.0) and 50 µM 3′-MAP, which leads to the formation of 4-cyanophenoxyl radicals, an increase in the oxidation rate by a factor of 10 was observed between pH 4 and 8 (Supporting Information, Figure S4). A linear correlation of k°exp versus percentage of deprotonated 4-cyanophenol was found (Figure S4). As humics exhibit phenolic entities with various pKa values (range 7-10) and various standard oxidation potentials (38), the 10-fold increase in rate between pH 4 and 8 is in agreement with the suggestion that phenoxyl radicals are the prevailing photooxidants for the oxidation of As(III) in the presence of SRHA. The photoinduced As(III) oxidation in the presence of SRHA has a low Arrhenius activation energy of 15.4 kJ mol-1, which is typical for reactions of excited species in aqueous solutions (39). Alternatively, this low activation energy could be explained by assuming an As(III) species bound to humics that is involved in an intramolecular electron-transfer process. This is much less than the apparent energy of activation of 96.6 kJ mol-1 reported by Pettine et al. for the reaction of H2O2 with As(III) at pH 8.1 and 8.5 (27). The influence of the ionic strength on the reaction rate is negligible for experiments with I ) 0.001 up to 2 (NaCl) in the presence and absence of SRHA (Supporting Information, Figure S5). This stands in contrast to the results found for the As(III) oxidation by H2O2 (27), where the variation of NaCl concentration resulted in 10 times higher rates for 1 M than for 0.01 M NaCl, although the change in speciation due to the formation of chloro arsenic complexes is very small (40).
Qualitatively, the photoinduced oxidation of As(III) follows the kinetic pattern already observed for Sb(III) (Supporting Information, Figure S6 a,b,c) (11). As(III) tends to react more slowly than Sb(III), at pH 7.2 and 5 mg L-1 of DOC, by a factor of 3.5. This factor increases at lower pH values and at higher DOC concentrations. Both As(III) and Sb(III) photoinduced oxidation reactions have low Arrhenius activation energies and the ionic strength has a negligible influence on the rate (11). Natural Waters, Influence of Fe. RFW samples from Bangladesh (Table 1) were spiked with 0.4 µM As(III). The photoinduced oxidation followed pseudo-first-order kinetics and did not differ significantly for filtered and unfiltered samples (Figure 2b) (dark controls were 23 times slower). The addition of 45 µM Fe(III) (2.5 mg L-1; equilibration time 0.5 h; 16 times the neutral Fe content of RFW) led to an increase in the light-induced reaction rate by a factor of 1.9 (dark controls were 35 times slower). Adding 45 µM Fe(III) and 50 µM citrate to RFW, however, showed a 20-fold increase in the initial rate compared to the unspiked RFW. To compare these findings to those of our laboratory system, 45 µM Fe(III) added to a 5 mg L-1 DOC SRHA solution (pH 7.2) accelerated the photoinduced oxidation of As(III) by a factor of 1.4 (Figure 2b), similar to the finding for RFW. Adding 45 µM Fe(III) to SGW led to an As(III) oxidation rate that was twice the rate observed with 5 mg L-1 DOC (SRHA) only (Figure 2b). The relatively modest increase in the oxidation rates with iron addition (except in the presence of citrate) can be explained by the Fe(III) speciation: At neutral pH, almost no mononuclear Fe(III) is present. The quantum yields for the formation of oxidizing radicals and Fe(II) (ΦFe(II)) from polynuclear Fe(III)-hydroxo species have not been determined, but they are much lower than those for mononuclear Fe(III)(OH)2+ (ΦFe(II) ) 0.017 at 366 nm) (8). Iron is also partly complexed to DOC (41, 42), which can have three effects. (i) SRHA reduces Fe(III) and releases less strongly complexed Fe(II) (42), which, in turn, is oxidized back to Fe(III) by dissolved O2 (t1/2 < 1 h (14)). For 45 µM Fe(III) we found 62% reduction after 0.5 h (pH 7.2, dark) in the presence of 5 mg L-1 DOC (SRHA). Some of the As(III) is oxidized in parallel to Fe(II) by reactive intermediates formed in this process (13). (ii) Fe(III)-SRHA complexes are photolyzed to Fe(II) and reactive radicals (43). (iii) Fe(III) binds to reactive sites on SRHA and outcompetes As(III) and/or reduces the production of photooxidants. Most likely, effects (i) and (ii) of these processes contribute to the 1.4 times faster oxidation of As(III) in the presence of Fe(III) and SRHA, whereas effect (iii) rather slows down the As(III) oxidation. Moreover, the ΦFe(II) for Fe(III)-SRHA complexes appears to be low. In contrast, citrate keeps a fraction of the Fe(III) in solution as mononuclear Fe(III)-citrate (8), which is photolyzed with high quantum yields (ΦFe(II) ) 0.2-0.4 at 366 nm and pH 5-7) (44). In SGW (Table 1), Fe(III) can only form polynuclear Fe(III)-hydroxo complexes which seem to be slightly more active in forming oxidizing species in the absence of DOC (6, 43). Natural waters with low levels of Fe, like GSW or FBW, generally show oxidation rates that can be mimicked quite well with the DOC concentration of SRHA (Figure 2a). In the case of RWW, reaction rates are underestimated (Figure 2b). The approximately 5 times faster photoinduced reaction rate for RWW is probably not only due to higher Fe concentrations compared to GSW or FBW, but also due to DOC with high aromaticity (11). Environmental Significance. Because the [As(III)]o has an influence on the oxidation rate (Figure 3) and the range of As concentrations in natural waters is large (1), two pseudofirst-order rate coefficients for the photoinduced oxidation of As(III) are estimated for environmentally relevant condi-
tions (pH 7, 5 mg L-1 of DOC, sunlight, summer, cloudless, noon), assuming that the kinetics are 10 times slower under natural sunlight than in the MGRR (45). For a moderate As(III) concentration of 66 nM (5 µg L-1) the rate coefficient would be 3.6 ( 1.7 day-1 (half-life 0.2 days); for 8 µM (600 µg L-1) it would be 0.22 ( 0.024 day-1 (half-life 3.2 days). Averaging over day and night and considering cloud coverage and seasonal changes, these rate coefficients may be smaller by a factor of 10 (11), resulting in half-lives of 2 and 34 days, respectively. Such half-lives are important for the environmental fate of arsenic, but are too large to be relevant for drinking water treatment by sunlight irradiation due to pathogen formation. Johnson and Pilson studied As(III) oxidation in Sargasso Sea water (46). They determined zero-order kinetics with k ) 0.0206 µM day-1 in the dark ([As(III)]o ) 0.93 µM, pH 8.3, salinity 36‰, 23 °C). Exposed to sunlight, the reaction was 5-10 times faster (0.206 µM day-1). Estimated half-lives, [As(III)]0/2k, would be 0.16 days (for 66 nM) and 19 days (for 8 µM), respectively, which is similar to our findings. Although the conditions in Sargasso Sea water are different from our system, the lower DOC concentration (0.8 mg L-1 DOC (47)) is probably compensated by the higher pH with respect to the rate. Kocar et al. determined the photoinduced As(III) oxidation in natural water from Hyalite Canyon (pH 5, 10 mg L-1 DOC, 25 ( 2 °C) spiked with 18 µM Fe(III) 1 h before photoirradiation (6). For 18 µM As(III) initial concentration, they determined zero-order kinetics with k ) 5.6 µM h-1 under natural sunlight. The half-life would then be 0.7 h for 8 µM As(III). Compared to our system spiked with 45 µM Fe(III) (pH 7.2), this half-life is shorter by a factor 80. This may be due to the higher DOC concentration and, more importantly, due to the lower pH, which favors iron-catalyzed processes (13, 48). In lakes with high photosynthetic production, reduction of As(V) by algae is probably faster than photochemical oxidation of As(III) (49-51). However, in less productive waters with high concentrations of humic acids, photosensitized oxidation of As(III) may be an important process. Therefore, it should be considered in As(III) speciation.
Acknowledgments We thank Thomas Ru ¨ ttimann for his help with HG-AFS and Se´bastien Meylan for DOC measurements. Adrian Ammann is acknowledged for analysis with IC-ICP MS and David Kistler for laboratory work. We thank Emily Unsworth and Stefanie To¨pperwien for providing natural water samples and the reviewers for their recommendations.
Supporting Information Available Pseudo-first-order rate coefficients of As(III) oxidation in the presence of 2-propanol, 3′-methoxyacetophenone, and 4-cyanophenol, the influence of ionic strength, the comparison of reactivity of As(III) and Sb(III) (Figures S2-S6 a,b,c), and the model for photooxidation kinetics. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review August 12, 2005. Revised manuscript received October 5, 2005. Accepted October 7, 2005. ES051597R
