Fe (III) Ratio Induced by Periodic

Mar 31, 2005 - Science, Institute of Chemistry, Chinese Academy of Sciences, ... Different from that, an oscillation in Fe(II)/Fe(t) ratio induced by ...
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Environ. Sci. Technol. 2005, 39, 3121-3127

Photochemical Oscillation of Fe(II)/Fe(III) Ratio Induced by Periodic Flux of Dissolved Organic Matter WENJING SONG, WANHONG MA, JIAHAI MA, CHUNCHENG CHEN, AND JINCAI ZHAO* Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China YINGPING HUANG Research Center for Eco-Environmental Sciences, China Three Gorges University, Yichang, Hubei 443002, China YIMING XU

complexes with hydroxyl, sulfate, and other ligands (eq 1) (22, 23, 27-29). hν

Fe(III)L 98 Fe(II) + L•

(1)

Naturally abundant DOM facilitates this process by scavenging the photoproduced free radicals (eq 2) (24) or by forming sunlight-answering complexes with Fe(III) (eq 3) (23), enhancing consequently the Fe(II) production.

DOM + L• f products

(2)

hν

Fe(III)(DOM) 98 Fe(II) + DOM+•

(3)

In addition, the photogenerated superoxide/hydroperoxide radicals (eq 4-6) (12, 30, 31), which is an important source of Fe(II) upon reducing Fe(III) at neutral pHs (25) but act as the sink of Fe(II) species in acid media (12, 31-34), are responsible for the solar light dependency of iron species.

DOM•+ + O2 f products + O2•-/HO2•

Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China

(4)

The variation in iron(II)/total iron [Fe(II)/Fe(t)] ratio, as hν

Variation of iron species in the UV-irradiated aqueous solution was examined in the presence of various dissolved organic matter (DOM). Under the irradiation at constant light intensity, a regular oscillation in the ratio of Fe(II) to total iron, Fe(II)/Fe(t), was observed when DOM was periodically added into the solution. In each cycle, the Fe(II)/Fe(t) ratio increased initially and then decreased with concomitant degradation of DOM. The Fe(II)/Fe(t) ratio approached a constant value after the DOM was completely mineralized. The period and amplitude of the oscillation were dependent on DOM structure and its initial concentration, but the ultimate photosteady state was not affected by DOM. It was revealed that both DOM and photoreactive Fe(III) species were indispensable for the fluctuation in Fe(II)/Fe(t) ratio. The ultimate photosteady state originated from the equilibrium between Fe(III) photoreduction and aerobic Fe(II) photooxidation induced simultaneously by UV irradiation. It was the DOM that disturbed these two opposite processes, leading to the oscillation in Fe(II)/ Fe(t) ratio under UV irradiation.

Introduction Iron cycling plays an important role in environmental and chemical systems. It couples with the generation and depletion of reactive oxygen species (1-5), oxidation of S(IV) to S(VI) (6, 7), As(III)/As(V) and Cr(III)/Cr(VI) redox cycling (8-11), and the degradation of dissolved organic matter (DOM) (12-17) in natural aquatic bodies. It is also involved in the regeneration process of iron-containing catalysts in many chemical and biological systems (18-21). It has been confirmed that solar light irradiation is a vital factor that controls the iron species in natural aquatic bodies (22-26). This is primarily attributed to the sunlight-induced LMCT (ligand to metal charge transfer) reactions of Fe(III) * Corresponding [email protected].

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DOM 98 DOM*

(5)

DOM* + O2 f DOM•+ + O2•-/HO2•

(6)

shown above, is dependent on the light intensity. Different from that, an oscillation in Fe(II)/Fe(t) ratio induced by periodic flux of various DOM at a constant light intensity was observed in the present study. The photochemical interconversion between Fe(II) and Fe(III), especially the process of Fe(II) (photo)oxidation, was examined in detail. Both DOM and Fe(III) species were revealed to be responsible for the observed oscillation in Fe(II)/Fe(t) ratio: the former participated in the reduction of Fe(III), as well as the oxidation of Fe(II); The latter not only produced Fe(II) but also catalyzed the oxidation of Fe(II) by dioxygen under UV light irradiation. The oscillation was attributed to the disturbance of DOM on photoreduction of Fe(III) and photooxidaiton of Fe(II). After DOM was completely mineralized, the equilibrium between the two processes was established, displaying the photosteady state of the irradiated system.

Experimental Section Materials. Iron(III) perchlorate hydrate (low chloride), iron (granule, 99.999%), and perchloric acid (70%, 99.999%) were purchased from Aldrich. The solution of iron(II) was prepared from the reaction of iron granule with an excess amount of perchloric acid. Sulforhodamine B (SRB), oxalic acid, benzoic acid, salicylic acid, 2,4-dichlorophenol, citric acid, formaldehyde solution, sodium sulfate, sodium perchlorate, sodium dihydrogenphospate, ammonium fluoride, and ascorbic acid were of reagent grade and used without further purification. Barnstead UltraPure water (18.3 MΩ) was used throughout the study. Procedure and Analysis. The photochemical reactions were performed using a 100 W Hg lamp (Toshiba SHL-100UVQ-2) as the irradiation source. All the irradiation experiments were carried out in cylindrical Pyrex vessels (60 mL capacity, 3.8 cm diameter) that were 10 cm from the light center. Pyrex vessels were used as reactors to eliminate any irradiation below 300 nm. The average path length was calculated to be 2.98 cm (πd/4) (8). The incident light intensity VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Changes in the (a) Fe(II)/Fe(t) ratio (b), (b) [SRB]/[SRB]0 ratio (9), and (c) TOC/TOC0 (2) with irradiation time. The initial solution was a mixture of 1.0 × 10-4 M Fe(III) and 1.0 × 10-5 M SRB at pH 2.5. on the solution was measured with ferrioxalate actinometry (50 mL, 6 mM in 0.1 N HCl). An average ferrioxalate quantum yield of 1.19 was used. The light intensity for 50 mL of the solution was ∼1.9 × 10-7 einstein s-1. Unless otherwise noted, the ionic strength (I) of the reaction solution was adjusted to 0.04 by 0.1 M NaClO4. The solution pH was adjusted by 1 M HClO4. The concentrations of Fe(II) and total iron were determined spectrophotometrically on a Hitachi U-3100 spectrophotometer by a modified 1,10-phenanthroline method (35, 36). For this analysis, 1 mL of sodium acetate/acetic acid buffer (pH 5.5), 0.5 mL of 1,10-phenanthroline solution (5.0 mM), and 0.5 mL of ammonium fluoride solution (0.1 M) were premixed, followed by addition of 1 mL of the sample solution. The absorption of the resulting solution was read at 509 nm using a 1 cm quartz cell on a Hitachi U-3100 spectrophotometer. The total iron concentration was measured at given time intervals by the same procedure except that ascorbic acid (0.1 M), instead of ammonium fluoride, was added for Fe(III) reduction. The concentration of total iron was almost unchanged within the experimental duration. The Fe(III)/Fe(II) ratio was monitored in situ by a potential method (Pt electrode as the working electrode and SCE as the reference electrode). Total organic carbon (TOC) was determined on a Tekmar Dohrmann Apollo 9000 TOC analyzer.

Results and Discussion Oscillation in Fe(II)/Fe(t) Ratio by Periodic Introduction of DOM. Figure 1 shows the changes in the ratio of Fe(II)/ Fe(t), the concentration of DOM, and TOC under continuous UV light irradiation in ferric ion/Sulforhodamine B (SRB) system. Due to the easy detection by UV-vis spectrometer, SRB was employed here as a representative DOM. Upon UV irradiation, the Fe(II)/Fe(t) ratio increased notably in the first 30 min and then decreased at a relatively slow rate. The former was due to the rapid photoreduction of Fe(III), and the latter was indicative of the Fe(II) reoxidation. When the Fe(II)/Fe(t) ratio reached its maximum value, the SRB disappeared almost completely. At this point, however, TOC was only decreased by 20%. Further irradiation led to a continuous decrease in TOC, in parallel to the reoxidation of the photoproduced Fe(II). The oxidation of Fe(II) slowed with the decrease in TOC, implying that a photosteady state would be reached after the DOM present in the system was completely mineralized, as shown in the following sections. Periodic addition of SRB to the reaction system at given time intervals led to a regular increase and decrease in Fe(II)/ 3122

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FIGURE 2. Periodic changes in the (a) Fe(II)/Fe(t) ratio (b) and (b) [SRB]/[SRB]0 ratio (9) of the solution upon periodic addition of SRB under UV light irradiation. The initial solution (pH 2.5) was (A) 1.0 × 10-4 M Fe(III) and (B) 1.0 × 10-4 M Fe(II). At given time interval 1.0 × 10-5 M SRB was added into the irradiated solution (The SRB solution was added in a small volume to keep a constant concentration of iron).

FIGURE 3. Periodic changes in the ratio of Fe(III)/Fe(II) (depicted by the potential) in the presence of various DOM measured by in situ potential method: (a) 1.0 × 10-4 M oxalic acid was added stepwise, and 1.0 × 10-4 M Fe(III) was the initial iron species; (b) benzoic acid from 1.0 × 10-5 to 5.0 × 10-5 M was added stepwise, and 1.0 × 10-4 M Fe(III) was the initial iron species; (c) 1.0 × 10-5 M SRB was added stepwise, and 1.0 × 10-4 M Fe(II) was the initial iron species. Fe(t) ratio, demonstrating the oscillation phenomena of the Fe(II)/Fe(t) ratio (Figure 2A). Such an oscillation in Fe(II)/Fe(t) ratio was also observed in the Fe(II)/SRB initiated reaction system (Figure 2B). The only difference between the Fe(II)- and Fe(III)-initiated systems was the induction period for the first cycle of Fe(II) to Fe(III). From the second cycle, both systems exhibited similar behavior. It was found that the Fe(II)/Fe(t) ratio remained almost unchanged after the light was turned off at any point in the cycle curve, implying the necessity of UV irradiation for the oscillation. Similar oscillation was also observed in the presence of two other kinds of organic substrates, oxalic acid and benzoic acid, as shown in Figure 3. However, the rate in the Fe(II) photoproduction and its depletion appeared quite different, following the decreasing order of oxalic acid . benzoic acid > SRB. In addition, the period and amplitude of the oscillation were also affected by DOM concentration, as presented by benzoic acid (curve b). Both parameters increased with the

FIGURE 4. Change in Fe(II)/Fe(t) ratio in the presence of various DOM: (a) salicylic acid (1.0 × 10-5 M) (9), (b) benzoic acid (1.0 × 10-5 M) (b), (c) 2, 4-dichlorophenol (1.0 × 10-5 M) (2), (d) citric acid (1.0 × 10-5 M) (+), and (e) formaldehyde (×) at 6.0 × 10-5 M. In all the cases, the total iron concentration was 1.0 × 10-4 M and the pH was adjusted to 2.0. increase in benzoic acid concentration. Once again, TOC was found to decreased along with both the processes of Fe(II) photoproduction and depletion (Data not shown). After DOM was completely mineralized, these reaction systems reached the photosteady states, where the potential remained constant. The fluctuation in Fe(II)/Fe(t) ratio was further investigated with an alternative irradiation procedure. DOM-free Fe(III) homogeneous solutions were first irradiated to reach their photosteady states, and then a certain amount of DOM was injected. The DOM chosen in this study included formaldehyde, citric acid, benzoic acid, 2,4-dichlorophenol, and salicylic acid, and the initial TOC of these reaction systems was adjusted to be approximately the same. As displayed in Figure 4, upon UV irradiation, all the systems experienced a similar change in Fe(II)/Fe(t) ratio, indicating the universality of such DOM-induced oscillation in the Fe(II)/Fe(t) ratio. The period and amplitude, however, were different from each other. The reasons for the DOMdependent oscillation period and amplitude will be discussed in detail in the following sections. It is worthy of being noted that despite the diversity in period and amplitude of the cycle, the ratios of Fe(II)/Fe(t) at the photosteady states were almost the same for all the cases as that in DOM-free reaction system. The effect of DOM concentration on Fe(II)/Fe(t) variation was also examined (Figure 5). In this case, salicylic acid (SA) was chosen as the model compound of naturally abundant aromatic DOM. As the concentration of SA increased from 2 to 50 µM, the period and the Fe(II)/Fe(t) maximum value increased from 1 h and 40% to 22 h and 98%, respectively. However, the concentration of SA added did not alter the ultimate photochemical steady states of these reaction systems, at which the Fe(II)/Fe(t) ratios were the same as that in DOM-free system. As shown above, the period and amplitude of the oscillation in Fe(II)/Fe(t) ratio were affected by the amount and the structure of the DOM, while the ultimate photosteady state was independent of DOM. This indicated that both DOM-dependent and -independent reactions could contribute to the photochemical cycling of iron. In DOM-free systems, no fluctuation in Fe(II)/Fe(t) ratio was observed, regardless of the initial ratio of Fe(II)/Fe(t) employed in the reaction solution (Figure 6A). When the initial ratio of Fe(II)/Fe(t) was in the region from 100% to the photosteady value, the UV irradiation always led to the conversion of Fe(II) to Fe(III), whereas the irradiation of ferric solution

FIGURE 5. Effect of SA concentration on Fe(II)/Fe(t) ratio under UV irradiation. The initial concentration of SA was (a) 50 µM (9), (b) 20 µM (b), (c) 10 µM (2), (d) 5 µM (1), and (e) 2 µM (+). Other conditions: Fe(t) ) 1.0 × 10-4 M, pH 2.0, and I ) 0.04.

FIGURE 6. Changes in Fe(II)/Fe(t) ratio in aerated solutions (A) in the absence of DOM and (B) in the presence of 1.0 × 10-5 M SA under UV irradiation at different initial Fe(II)/Fe(t) ratio of (a) 100% (9), (b) 80% (b), (c) 50% (2), (d) 30% (1), and (e) 0% (+). Fe(t) ) 1.0 × 10-4 M, pH 2.0, and I ) 0.04. merely resulted in Fe(II) production. All the reaction systems reached the photosteady state with almost identical Fe(II)/ Fe(t) ratio. Moreover, if the initial Fe(II)/Fe(t) ratio was the same as that at the photosteady state, it did not change further during the irradiation. In the presence of SA, the variation in Fe(II)/Fe(t) ratio was significantly different from that in DOM-free systems (Figure 6B). Except for the reaction system initiated with 100% Fe(II), the Fe(II)/Fe(t) ratio always increased initially and then decreased until it reached the same value as that at the photosteady state in the DOM-free system. In deaerated reaction solution, no photooxidation of Fe(II) was observed, both in the presence and absence of SA (Data not shown). This illustrated that the molecular oxygen was a necessary component for the oxidation of Fe(II) under UV light irradiation. The equilibrium between Fe(III) photoreduction and aerobic Fe(II) photooxidation by dioxygen led to the ultimate photosteady state in the absence of DOM, or after the DOM added was completely mineralized. Equation 7 represents the net reaction for the Fe(II)/Fe(III) cycle. hν

Fe(II) + 1/2O2 + H+ 798 Fe(III)(OH)

(7)

The concentration of O2 remained constant by equilibrium with atmosphere and no substantial change in pH (less than VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Oxidation of Fe(II) under UV light irradiation with initial Fe(III) percentage of (a) 0.8% (9), (b) 3.2% (b), (c) 5.7% (2), and (d) 10% (1). Fe(t) ) 1.0 × 10-4 M, pH 2.0, and I ) 0.04.

FIGURE 9. Effect of foreign inorganic ligands (a) perchlorate (9), (b) sulfate (b), (c) phosphate (2), and (d) fluoride (1) on the photochemical interconversion between Fe(II) and Fe(III). (A) photooxidation of Fe(II), (B) photoreduction of Fe(III) in aerated solutions, (C) photoreduction of Fe(III) in deaerated solutions, and (D) iron redox cycling in the presence of 1.0 × 10-5 M SA. The total iron concentration was 1.0 × 10-4 M, the foreign ligands were at 1.0 × 10-2 M. pH 2.0, and I ) 0.04. All the salts were used in the sodium form. FIGURE 8. Effect of pH on the Fe(II) photooxidation. The initial concentration of Fe(II) was 1.0 × 10-4 M. (a) pH 1.0 (9), (b) pH 1.3 (b), (c) pH 1.5 (2), (d) pH 2.0 (1), (e) pH 2.5 (×). 0.1 units) was observed during the reaction. In addition, the terminate products CO2, formed from DOM degradation, has little effect on Fe(II) or Fe(III) species at the experimental pH (37). Therefor, all the reaction systems finally reached photosteady states with almost identical Fe(II)/ Fe(t) ratios. Mechanism of DOM-Independent and -Dependent Reactions Involved in the Oscillation in the Fe(II)/Fe(t) Ratio. In the absence of DOM, Fe(II) was rapidly oxidized after an obvious induction period. The induction period became shorter as the initial Fe(III) percentage in the irradiation systems increased, as shown in Figures 6A and 7. Such an accelerating effect of Fe(III) on the Fe(II) oxidation implies that there is an autocatalytic pathway for Fe(II) photooxidation. To further understand the DOM-independent photooxidation of Fe(II), the effect of pH was studied. The pH was adjusted below 3.0 to avoid any heterogeneous catalytic Fe(II) oxidation on the interfaces of Fe(III) hydroxide (38-41). The results are shown in Figure 8. As the pH increased from 1.0 to 2.5, the induction period decreased from 6 h to less than 1 h, whereas the photosteady ratio of Fe(II)/Fe(t) increased from 7% to 35%. It is well-known that the pH has a strong influence on the form of Fe(III) in aqueous solution. In the pH range from 1.0 to 3.0, the predominant Fe(III) species are [FeIII(H2O)6]3+ and [(H2O)5FeIII(OH)]2+ (6, 27). The latter is more photoreactive and its concentration 3124

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increases with the increase in pH (6, 27, 28). Thus, the shorter induction period was related to the higher photolysis efficiency of the reaction solutions. With further irradiation, the photoreduction of Fe(III) counteracted the Fe(II) oxidation, resulting in higher photosteady Fe(II)/Fe(t) ratio at higher pHs. Various inorganic ligands, such as hydroxyl ion and sulfate, have been reported to exhibit diverse effect on the (photo)redox iron cycling in environmental aquatic bodies and interfaces (28, 37). These anions also displayed great influence on the Fe(II) photooxidation (Figure 9A). The UV-vis absorption study showed that all the examined anions were able to form complexes with Fe(III), whereas their interactions with Fe(II) were relatively weak. The Fe(II) species underwent rapid oxidation in the presence of perchlorate and sulfate ions after induction periods. Moreover, the induction period and the photosteady ratio of Fe(II)/Fe(t) were observed to be longer and lower in the presence of sulfate than those in the presence of perchlorate anion. As for phosphate and fluoride anions, they inhibited strongly the Fe(II) photooxidation. The photolysis of Fe(III) in aerated and deaerated solutions of these ligands (Figure 9B,C) gave the following information: first, both hydroxyl (in the presence of perchlorate) and sulfate complexes of Fe(III) were photoreactive. The latter photolyzed much more slowly than the former (28), thus resulting in a lower ratio of Fe(II)/Fe(t) at the photosteady state in aerated solution. It was noted that the photosteady Fe(II)/Fe(t) ratios obtained were the same as those resulting from the Fe(II)-initiated reactions. In deaerated systems, the photoproduced Fe(II) significantly increased, further confirming the early conclusion that dioxygen was a necessary reagent for Fe(II) photooxidation.

Second, in the presence of fluoride and phosphate, only a few Fe(II) species were produced both in the aerated and deaerated solutions, in agreement with the previous reports that their complexes with Fe(III) are of poor photoreactivity (14, 23). The above results suggested that the photolysis of Fe(III) species is pivotal for the Fe(II) oxidation by dioxygen under UV irradiation. If the Fe(III) species in the reaction systems did not undergo photolysis, Fe(II) cannot be oxidized.

attributed to the generation of organic intermediates that facilitate the Fe(II) oxidation under UV irradiation.

The inorganic anions also exhibited a different effect on the iron redox cycling in the presence of DOM, as summarized in Figure 9D. Whereas no obvious change in Fe(II)/Fe(t) ratio and SA concentration was observed in the presence of fluoride and phosphate, the Fe(II)/Fe(t) ratio fluctuated notably upon SA injection in the presence of perchlorate and sulfate, the Fe(III) complexes of which are photoreative. These results confirmed the contribution of photoreactive Fe(III) species to the fluctuation in Fe(II)/Fe(t) ratio.

HO2• + O2•- + H+ f H2O2 +

The autocatalytic characteristics of Fe(II) photooxidation, as well as the effect of pH and foreign inorganic anions, demonstrate the necessity of Fe(III) photolysis for the aerobic Fe(II) photooxidation. The pH and inorganic Fe(III) ligands could alter the photoreactivity and the concentration of Fe(III) species, thus affecting the rate of Fe(II) photooxidation and the photosteady ratio of Fe(II)/Fe(t). There are two possible pathways for Fe(II) photooxidation: (1) Dioxygen attacks intermediates [FeII···L•], formed from the photolysis of Fe(III). (2) The Fe(II) species generated primarily from the photolysis of [(H2O)5FeIII(OH)]2+ (or [(H2O)5FeIIISO4]+) are of five coordinative pyramidal configuration and these species coordinate to dioxygen much more easily than [FeII(H2O)6], producing [(H2O)5FeIIO2)]. Subsequently, intromolecular electron transfer between Fe(II) and dioxygen occurs, generating O2•- and HO2• (by fast O2•- protonating). In the lower pH region, HO2• is the dominant species, and its reactions with Fe(II) to form H2O2 is 2-3 orders of magnitude faster than the reactions with Fe(III) to form dioxygen (35, 42), Therefore, the generation of O2•-/HO2• radicals leads to rapid oxidation of Fe(II). Further experimental and theoretical studies are needed to understand the detailed mechanism.

Fe(II) + •OH f Fe(III) + OH- k ) 3.2 × 108 M-1 s-1 (44) (11)

The fluctuation in Fe(II)/Fe(t) ratio occurred only in the presence of DOM. The increase in Fe(II) production at the first stage can be attributed to (1) that DOM and its degradation intermediates are able to form light-answering complexes with Fe(III), which are reduced to Fe(II) upon irradiation (23); (2) that they can scavenge the reactive oxygen species generated in the system so that the oxidation of Fe(II) is greatly suppressed (24). However, both the parent and intermediate DOM were degraded during irradiation. Then, the reactions for Fe(II) production decayed gradually with the decrease in TOC, and the reactions for Fe(II) depletion began to dominate in the reaction system, leading to the reverse change in Fe(II)/Fe(t) ratio. It was noticeable that when the photoproduced Fe(II) began to be reoxidized back to Fe(III), there were still some organic species in the reaction systems. In addition to facilitating the Fe(III) photoreduction, they may contribute to the Fe(II) oxidation. This could be achieved through the reaction with dioxygen to produce O2•-/HO2•, via the organic radicals generated from the LMCT process (eqs 3 and 4) (31, 32) or via the excited DOM (eqs 5 and 6) (12, 30). The O2•-/HO2• radicals were transformed into H2O2 either by reaction with Fe(II) (eq 8) or through self-disproportionation (eq 9) (35, 42); H2O2 reacted with Fe(II) to generate •OH radical, which caused further depletion of Fe(II) and degradation of the rest of the DOM (eqs 10-12). The reverse change in Fe(II)/Fe(t) ratio, in the light of this statement, could be

Fe(II) + O2•- + 2H+ f Fe(III) + H2O2

k ) 1 × 107 M-1 s-1 (42) (8a)

Fe(II) +HO2• + H+ f Fe(III) + H2 O 2

O2

k ) 1.2 × 106 M-1 s-1 (42) (8b) k ) 9.7 × 107 M-1 s-1 (42) (9a)

HO2• + HO2• f H2O2 + O2

k ) 8.3 × 105 M-1 s-1 (42) (9b) Fe(II) + H2O2 f Fe(III) + •OH + OH- k ) 76 M-1 s-1 (43) (10)

DOM + •OH f products

(12)

The oscillation period and amplitude were strongly dependent on DOM structure, as depicted in Figures 3 and 4. This dependence is rationalized to result from the diversity in the reactions of the DOM and its intermediates with Fe(III)/ Fe(II), dioxygen, and the reactive oxygen species. Usually, aromatic compounds and their degraded intermediates have hydroquinone or semiquinone structures (45-48), which could reduce Fe(III) to Fe(II) (18, 20, 45). Thus the relatively slow depletion of Fe(II) was observed in the presence of SA, benzoic acid, and 2,4-dichlorophenol. As the aliphatic compounds (oxalic acid, citric acid, and formaldehyde) were concerned, their intermediate radicals generated under UV irradiation are ready to react with dioxygen to form a series of reactive oxygen species (31, 32, 49). The photoproduced Fe(II) was reoxidized by the reactive oxygen species and returned to its photosteady concentration rapidly. In summary, the fluctuation in Fe(II)/Fe(t) ratio induced by various DOM occurs under UV light irradiation. DOM and its intermediates not only facilitate the Fe(II) photoproduction but also participate in the photooxidative depletion of Fe(II) in aerated reaction system. The effect of DOM on Fe(II)/Fe(III) interconversion depends on its concentration, structure, and reactivity. Photoreactive Fe(III) species, which catalyzes the photooxidation of Fe(II) by dioxygen, also contribute to the depletion of Fe(II) during irradiation. The oscillation in Fe(II)/Fe(t) ratio can be regarded as the disturbance of DOM on the competition between Fe(III) photoreduction and Fe(II) photooxidation. Such the disturbance of DOM gradually decays due to continuously degradation. After DOM is completely mineralized, the reaction system reaches the photosteady state, which originates from the equilibrium between Fe(II) photooxidation and Fe(III) photoreduction.

Acknowledgments Generous financial support by the Ministry of Science and Technology of China (No. 2003CB415006), by the National Science Foundation of China (No. 20133010, 50221001, No. 20371048, No. 20277038, and No. 20373074), and by the Chinese Academy of Sciences is gratefully acknowledged. We also thank the reviewers for their helpful comments and valuable suggestions. VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Received for review October 19, 2004. Revised manuscript received January 15, 2005. Accepted March 2, 2005. ES0483701

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