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adsorption passivated the iron surface and decreased the rate of bromate reduction by Fe(0). On the other hand, in the long run, the reduced functiona...
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Environ. Sci. Technol. 2005, 39, 1092-1100

Role of Humic Acid and Quinone Model Compounds in Bromate Reduction by Zerovalent Iron LI XIE AND CHII SHANG* Department of Civil Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China

Experiments were conducted to examine the role of humic acid and quinone model compounds in bromate reduction by Fe(0). The reactivity of Fe(0) toward bromate declined by a factor of 1.3-2.0 in the presence of humic acid. Evidence was obtained that the quick complexation of humic acid with iron species and its adsorption passivated the iron surface and decreased the rate of bromate reduction by Fe(0). On the other hand, in the long run, the reduced functional groups present in humic acid were observed to regenerate Fe(II) and reduce bromate abiotically. Compared with the case of humic acid only, the simultaneous presence of Fe(III) and humic acid significantly increased the bromate removal rate. Fe(III)/Fe(II) acted as a catalyst in the oxidation of humic acid by bromate. Anthraquinone-2,6-disulfonate (AQDS) and lawsone did not cause any significant effect on the bromate reduction rate by Fe(0). However, the redox reactivity of lawsone in the presence of Fe(III) was evident, while AQDS did not show any under the tested conditions. The difference was attributable to the presence/ absence of reducing functional groups in the model compounds. The electron spin resonance further demonstrated that the redox functional groups in humic acid are most likely quinone-phenol moieties. Although the bromate reduction rate by regenerated Fe(II) is a few times slower than that by Fe(0), the reactive Fe(II) can be, alternatively, reductively formed to maintain iron surface activation and bromate reduction to prolong the lifetime of the zerovalent iron.

Introduction Bromate is a carcinogenic disinfection byproduct, resulting from ozonation of bromide-containing water. The USEPA has established the maximum contaminant level (MCL) of 10 µg/L for bromate (1). Surveys around the world have shown that, in drinking water, the bromate concentration ranges from 0 to 127 µg/L, depending upon the bromide concentration in raw water and the disinfection process used (2, 3). Reduced iron species, such as ferrous ions (Fe(II)) and zerovalent iron (Fe(0)), have been utilized as chemical reductants for bromate removal (4-6). In our previous study, bromate removal by Fe(0) was described as a process controlled by surface-mediated reactions (6). Any factor that alters the iron surface reactivity influences the rate of bromate reduction. Natural organic matter (NOM), ubiquitously * Corresponding author phone: (852)2358 7885; fax: (852)2358 1534; e-mail: [email protected]. 1092

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present in soils, surface water, and groundwater, has been reported to form complexes with iron and iron oxide through various types of carboxylate, phenolic, and carbonyl functional groups in NOM (7, 8). The adsorption of NOM is generally dominant over contaminants for available active sites on the iron surface (9, 10), thereby lowering the contaminant removal efficiency. The accumulation of NOM at the iron-water interface may also adversely affect contaminant reduction through the inhibition of metal corrosion (11). On the other hand, NOM may act as an electron-transfer mediator in the chemical reduction of organic pollutants. It enhances the rate of chemical reduction of nitro aromatic or halogenated compounds by sulfide or ferrous ions (12, 13) or facilitates the reduction of Fe(III) to Fe(II) in the presence of microorganisms (14, 15). NOM has been demonstrated to participate in the oxidation/reduction of iron as a factor controlling the iron speciation (16). Recently, oxidation of various types of NOM in the abiotic reduction of Fe(III) to Fe(II) (17) or in the reduction of hexachloroethane has been reported (18). The electrontransfer property of NOM has been proposed to be related to the formation of Fe(III)-NOM complexes (19) and the presence of functional groups such as quinone, phenolic, and carboxylate moieties in NOM (7, 20, 21). In particular, quinone moieties in NOM have been speculated to play an important role in electron transfer (12, 13). Tratnyek et al. (10) have found that quinone model compounds such as juglone, lawsone, and anthraquinone-2,6-disulfonate (AQDS) can enhance the reduction rate of carbon tetrachloride by Fe(0). Electron spin resonance (ESR) spectroscopy has been used to measure the semiquinone radical concentration such that the quinone moieties in humic substances have been confirmed as electron-transfer mediators in biotic systems (21). Different redox capacities of NOM have been reported (19, 21), which are attributable to the different structural and functional groups in NOM (17). Moreover, in addition to the quinone groups, complexed Fe(III) in NOM has been hypothesized to participate in redox reactions as an electrontransfer mediator (19). Owing to the limited information available in the literature, more studies are needed to understand the contributions of NOM and complexed metal ions in the redox reactions and the mechanisms of these reactions. This study was, therefore, conducted to evaluate the roles of humic acid (representing NOM) and quinone model compounds, such as lawsone and AQDS, in bromate reduction by Fe(0) and to assess the redox reactivity of these compounds on the reduction of bromate and Fe(III). The role of Fe(III)/Fe(II) in the reaction cycle was investigated. Fourier transform infrared (FTIR) and ESR spectroscopic analyses were also conducted to substantiate the findings with respect to the identification of the functional groups of humic acid that complex with the iron surface and the redox functional groups that participate in the reduction of bromate.

Experimental Section Chemicals. All chemical stock solutions were prepared using doubly distilled deionized (DDDI) water and stored at 4 °C. Sodium bromate was obtained from Nacalai Tesque. Ferrous sulfate and ferric nitrate were from Riedel-deHae¨n. Lawsone and AQDS (reagent grade) were from Acros Organics and Sigma, respectively. Irons purchased from Connelly-GPM (catalog no. 1004) were sieved to obtain iron grains with sizes of 425-1000 µm. The iron grains were pretreated with sonication and acid washing prior to the experiments in the same manner as described in ref 6. Humic acid (representing 10.1021/es049027z CCC: $30.25

 2005 American Chemical Society Published on Web 01/15/2005

NOM) was obtained from Aldrich, which has been applied directly or after pretreatment in numbers of studies on the role of humic substances as an electron-transfer mediator (15, 22, 23). In this study, the humic acid stock solution was prepared by dissolving 1 g of humic acid powder in 1 L of DDDI water and filtering the solution through a 0.45 µm filter paper (Advantec MFS) without further purification. The major impurities of concern are Fe and Cu, and their concentrations under the test conditions were less than 200 and 1.2 µg/L, respectively, according to the report from the supplier. Experimental Procedures. Batch experiments were performed in 43-mL lined, capped glass bottles (Wheaton) at room temperature (20 ( 1 °C) in the dark under abiotic conditions (filtered solutions and autoclaved bottles). The tests were initiated by turning on an end-over-end rotator at 47 rpm immediately after the additions of reactants into the bottles. The time period between the two events was within 5 s. The test solutions were not buffered against pH change to prevent any potential interference. Samples were taken out at regular intervals, and these were subjected to bromate, bromide, dissolved iron, and dissolved organic carbon (DOC) measurements after filtration. A series of batch experiments were conducted to evaluate the inhibitive effects of humic acid on bromate reduction by Fe(0) and Fe(II) and to study the redox reactivity of humic acid, lawsone, and AQDS on bromate reduction. In the first case, two different scenarios with respect to the mode of addition of humic acid were considered. First, the humic acid was added at different concentrations (0-35 mg/L as DOC) simultaneously with a fixed concentration of bromate (5 mg/L) into the reactors, each filled with 1 g of preweighed irons. In the second scenario, to study the effect of accumulation of humic acid on bromate reduction by Fe(0) in the long-term run, irons were presoaked in humic acid solutions of various concentrations for 24 h and then the humic acid-amended irons were vacuum freeze-dried and used for bromate reduction under conditions similar to those in the first case, but without humic acid addition. In both the scenarios, no adjustment to the solution pH was made. The experiments on the inhibitive effect of humic acid on bromate reduction by Fe(II) were conducted in the same manner, but instead of using Fe(0) powders, 2 mL of a freshly prepared ferrous stock solution was pipetted into the bottle to get a target concentration of 20 mg/L. The effects of lawsone and AQDS on bromate reduction by Fe(0) were evaluated in the same manner as the first scenario described above, except that the humic acid was replaced with either lawsone or AQDS at predetermined concentrations. In the case of evaluating the redox reactivity of humic acid, lawsone, or AQDS on bromate reduction in the absence of Fe(0), the batch experiments were conducted with solutions containing bromate (5 mg/L) and humic acid/lawsone/AQDS (45 mg/L each as DOC). The experiments were conducted with or without the presence of Fe(III), and the initial solution pH was adjusted between 3.2 and 8.7 by the addition of H2SO4 or NaOH. The concentration of Fe(III) was maintained at 20 mg/L. The reduction of bromate with variations in humic acid concentrations (0, 10, 30, and 45 mg/L) and 20 mg/L Fe(III) at pH 3.2-3.9 was also tested. Fe(III) reduction by humic acid alone was studied with solutions containing humic acid (45 mg/L as DOC) and 20 mg/L Fe(III). The pH was maintained at 3.9, and no bromate was added. Analytical Methods. Bromate and bromide concentrations were measured with an ion chromatograph (Dionex 500) using an AS-9HC analytical column. The total soluble iron concentration in the filtered samples was determined with an atomic absorption spectrometer (SpectrAA 220FS). Humic acid, lawsone, and AQDS were quantified as dissolved organic carbon by a TOC analyzer (TOC-5000A, Shimadzu).

FTIR spectra of humic acid and complexes were recorded by a Fourier transform infrared spectrometer (Bio Rad FTS 6000) equipped with a DTGS detector. The spectral resolution was 4 cm-1. Purified humic acid powders were mixed with an aliquot amount of spectroscopic grade KBr. The mixture was finely ground with an agata ball and mill. Pellets were prepared using a pressing machine for further FTIR analysis. The reflectance Fourier transform spectrometer equipped with a Bio-Rad UMA500 IR microscope was used for the analysis of the iron surface adsorbed with or without NOM after freeze-drying, with 128-time scanning. The ESR spectra were obtained with a spectrometer (JEOL JES-TE ESR) at room temperature. Solid samples, including Fe(III) salts, humic acid, lawsone, and AQDS powders, and Fe(III)-humic acid complexes after vacuum freeze-drying, were packed in quartz ESR tubes (Wilmad Glass710-SQ-250M) and placed into the ESR cavity. The ESR instrument was operated with the following parameters: a microwave frequency of 9.45 GHz, a microwave power of 1.25 mW, and a modulation frequency of 100 kHz. Data Analysis. Pseudo-first-order reaction kinetics was used to fit bromate reduction by Fe(0) since Fe(0) was used in excess:

-

d[BrO3-] ) kobs[BrO3-] dt

(1)

where [BrO3-] is the bromate concentration at time t and kobs is the observed rate constant.

Results and Discussion Bromate Reduction in an Fe(0)-H2O-Humic Acid System. Bromate reduction by Fe(0) was first examined by simultaneously adding humic acid and bromate. The results of the experiment and the calculated kobs are presented in Figure 1a. As shown, an increase in the humic acid concentration led to a slight decrease in the bromate reduction rate. The initial pH of the test solutions varied in the range of 6.5-8 due to the addition of different quantities of humic acid as no buffering was provided and slight increases in pH during the experiments due to proton consumption were observed. However, such a range of pH shall have little effect on the bromate reduction rate (6). Therefore, the decrease in the bromate reduction rate should be primarily attributable to the increase in the humic acid concentration and somewhat to the increase in pH. During the reaction period, the aqueous humic acid concentrations decreased continuously with increasing reaction time. Meanwhile, the amount of dissolved irons released to the aqueous phase increased with increasing reaction time and increasing initial humic acid concentration (see the inset in Figure 1a). This result may be explained by the adsorption of humic acid on the iron surface and the complexation of humic acid with ferric and ferrous ions (20), which facilitates the detachment of metal ions from the surface to the solution (24), thereby increasing the dissolution of iron oxides and irons. Several hypotheses were taken into consideration to find out the mechanisms behind the aforementioned observation. The accumulation of adsorbed humic acid on the iron surface may depress the bromate reduction rate. The complexation between humic acid and the generated Fe(II) may inhibit bromate reduction by Fe(II) or occupy the active surface sites and inhibit iron corrosion. On the other hand, excavation of active surface sites by dissolution of passive iron oxides may accelerate the bromate reduction rate. The redox-active moieties in humic acid or in iron-humic acid complexes may act as reductants to reduce bromate to bromide or serve as electron-transfer mediators between Fe(0) and bromate. Therefore, further investigation was made to evaluate these VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effect of humic acid on bromate reduction by Fe(0), [BrO3-]0 ) 5 mg/L: (a) added simultaneously; (b) humic acid pretreatment; (c) by Fe(II). inhibitory and promotional roles of humic acid in the chemical reduction of bromate. Inhibition by Humic Acid. Considering the quick bromate reduction rate compared with the rate for humic acid adsorption as illustrated in Figure 1a, it may be concluded that the humic acid may not have sufficient time to exert its effects when added simultaneously with bromate. However, in the long run, the amount of humic acid adsorbed on the iron surface may increase, while the bromate is continuously 1094

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reduced, and this may eventually affect the rate of bromate reduction. Therefore, the inhibition role of humic acid was evaluated by presoaking Fe(0) in a humic acid solution for 24 h and then using the presoaked Fe(0) in the bromate reduction experiments. Different initial humic acid concentrations were used for this purpose, and the results and calculated kobs are presented in Figure 1b. After presoaking, the disappearance of bromate as a function of time could not be described very well by pseudo-first-order reaction

kinetics. On the other hand, the initial rates in Figure 1b seem to be the same, irrespective of humic acid loading. Only after a certain decrease in the initial bromate concentration, the reaction becomes retarded and the onset of the “slowing down” seems to depend on the initial humic acid loading. The similarity in the initial rates is hypothesized to result from the existence of specific active surface sites designated for bromate reduction, not for humic acid adsorption; with the reactions proceeding to exhaust those specific sites, the common active sites (available for both bromate reduction and humic acid adsorption) become limited. More studies need to be conducted to evaluate this two-stage trend in the future. Compared to the curves shown in Figure 1a, the 24 h presoaking period apparently facilitated the adsorption of humic acid on the iron-water interface, and its accumulation occupied the iron surface as indicated in the literature (10) to passivate the iron surface and therefore prevent electron transfer from the inner Fe(0) to bromate. The adsorption of humic acid on the iron surface led to the latter, large rate changes. On the other hand, the slight decreases in the initial bromate reduction are attributable to the formation of passive iron oxide during hydration. Meanwhile, during bromate reduction, the dissolution of iron to the aqueous solution was less significant in quantity (see the inset in Figure 1b), and no humic acid was found to release back to the solution phase. In addition, during the presoaking step, it was found that the release of dissolved iron increased from 9.4 to 34.2 mg/L with increasing concentrations of the presoaked humic acid from 20 to 35 mg/L. These findings suggest that the complexation of humic acid with Fe(II) or Fe(III) facilitates the release of dissolved iron into the aqueous phase only during the presoaking period. Since Fe(II) could be generated by iron corrosion during the oxidation of Fe(0) and the presence of humic acid promoted such a process, we felt that it was necessary to investigate the effects of humic acid on bromate reduction by Fe(II) in the aqueous phase. Batch tests were, therefore, conducted with aqueous solutions containing 20 mg/L Fe(II), 5 mg/L bromate, and two different humic acid concentrations (10-30 mg/L). The results of the tests are presented in Figure 1c. As illustrated in the figure, an increase in the humic acid concentration decreased the bromate reduction rate, and the solution pH remained relatively stable at 5.1 even though no pH buffering was provided. This further indicated that the quick complexation between humic acid and Fe(II) dominated the available active Fe(II) needed for bromate reduction. Although this set of tests was conducted in the aqueous phase, the complexation is also likely to occur at the iron-water interface and prevents the active Fe(II) from participating in bromate reduction on the surface. Redox Role of Humic Acid. Bromate reduction by humic acid alone was evaluated at various pH levels to examine the redox role of humic acid. The results of the batch experiments are presented in Figure 2a. As shown, bromate reduction by humic acid alone was possible, but the reduction was extremely slow. As a result, only a small drop in the bromate concentration could be observed after 284 h. The reduction rate decreased with increasing pH from 3.2 to 8.7, and almost no bromate reduction was observed at pH 8.7 at the end of the reaction period. In another set of experiments, the oxidation role of humic acid on bromate reduction in the presence of Fe(III) was evaluated at various pH values and humic acid concentrations. Figure 2b describes the disappearance of bromate at different pH levels with the same humic acid concentration. As shown in the figure, the bromate removal efficiency by humic acid and Fe(III) decreased with increasing pH. Nevertheless, a comparison with the case of humic acid alone shows that the addition of Fe(III) into solutions increased the bromate removal rate

significantly. Figure 2c shows the bromate disappearance in a humic acid-Fe(III) system at different humic acid concentrations at pH 3.2-3.9. As the figure illustrates, the bromate reduction rate was affected by the concentration of the humic acid in this system. An increase in the humic acid concentration increased the bromate removal rate. It is important to note that no bromate reduction was observed when the solution contained only Fe(III) at the pH level used in this experiment. The concentration of aqueous bromide, a reduction product, increased with decreasing bromate concentrations in all cases with a bromine recovery of approximately 90%. As such, the removal of bromate should be primarily attributable to the chemical reduction process. The bromate reduction rate decreased with increasing solution pH in the solutions containing humic acid only and in the humic acid plus Fe(III) solutions. There are several possible reasons for this. First, Steelink (25) observed that humic substances under basic conditions have higher and stable radical concentrations than those under acidic conditions. At higher pH, the quinone moieties have the tendency to attract electrons to form stable free radicals even without the presence of reducing groups (26). If the quinone and phenol groups are close to one another, electron-transfer reactions are possible, and thereby, radicals can be formed and stabilized in basic solutions (27). Second, the morphology and structure of humic substances are greatly influenced by pH (28). Humic acid looks like fibers or a bundle of fibers at acidic or neutral pH, while its shape becomes sheetlike at basic pH. Finally, as proposed by Chen et al. (17), the hydrolysis of Fe(III) at basic pH forms hydroxyliron(III) species. Precipitated amorphous iron hydroxides may be another reason for the pH dependence of the Fe(III) reduction by humic acid. In this study, therefore, the observed decline in the bromate reduction rate with both the increasing pH and the decreasing initial humic acid concentration is speculated to be influenced by the changes in the formation of the Fe(III)-humic acid complexes, in addition to the reducing reactivity of humic acid itself. However, only a very low concentration of Fe(II) was detected in the aqueous phase, which may be attributable to the quick reoxidation of the formed Fe(II) to Fe(III) during bromate reduction. Therefore, additional tests were conducted in a water matrix containing humic acid and Fe(III) only to verify the generation of Fe(II). An increase in Fe(II) concentration (up to nearly 2 mg/L) after 100 h as a result of the reduction of Fe(III) by humic acid was observed at pH 3.9. Fe(III)-humic acid complexes (in precipitates) were also observed at the bottom of the bottles. These findings suggest that the humic acid contains reduced functional groups that complex with Fe(III) and consequently lead to the formation of Fe(II). Visible light irradiation yielded higher dissolved Fe(II) concentrations as compared with the yields obtained in the dark, indicating the photocatalytic behavior of the reduction of Fe(III) to Fe(II) by humic acid. The abiotic photoreduction of Fe(III) to Fe(II) has been observed in aqueous solutions containing tannic acid (29) and humic substances (26). On the basis of the arguments presented above, an electron-transfer scheme is proposed to account for the faster bromate reduction rate observed in the solution containing humic acid and Fe(III); this scheme is illustrated in Figure 3a. As the figure shows, the Fe(III)/Fe(II) couple acts as a catalyst for bromate reduction by humic acid. The reduced functional groups in the humic acid complex with Fe(III) and then transfer electrons to Fe(III) to form Fe(II). The Fe(II) thus formed donates the accepted electrons to bromate, the terminal electron acceptor. A similar role of the Fe(III)/ Fe(II) redox couple as a catalyst has also been observed by Miles and Brezonik (29) in a study of oxygen depletion in humic-containing water. The above reaction process is VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Bromate reduction in the solution containing (a) humic acid under various pH conditions, (b) humic acid and Fe(III) under various pH conditions, and (c) humic acid and Fe(III) under various humic acid concentrations. expected to occur in the environment, since the reducing capacity of Aldrich humic acid was reported to be lower than that of the soil and aquatic humic acid (18). In the process of bromate reduction by Fe(0), a more comprehensive process involving Fe(III) and humic acid may also take place in addition to the direct reaction between Fe(0) and bromate. In such a process, the oxidized functional groups in the humic acid may be reduced by the bulk reductant, Fe(0), and then 1096

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donate electrons to electron acceptors. This electron-transfer process has been proposed to account for the role of naturally occurring electron mediators in the reduction of 4-aminoazobenzen by Fe(0) (30). Figure 3b shows that two kinds of electron-transfer mediators are involved, the humic acid and the Fe(III)/Fe(II) couple, after humic acid is adsorbed on the iron surface. The humic acid acts to mediate electrons from Fe(0) to the electron acceptors. Since Fe(III) has a higher

FIGURE 3. Electron-transfer process (a) from the electron donor (humic acid) to the electron acceptor (bromate) via Fe(III) and (b) from the electron donor (Fe(0)) to the electron acceptor (bromate) via humic acid and Fe(III).

FIGURE 5. Bromate reduction by lawsone and AQDS under various pH conditions.

FIGURE 4. Bromate reduction by Fe(0) in the presence of (a) lawsone and (b) AQDS. priority to accept the electrons as compared to BrO3-, Fe(II) will be formed, which will thereafter mediate the electrons to the terminal electron acceptor, BrO3-. On the other hand, when surface passivation occurs, the reactive Fe(II) can be, alternatively, reductively regenerated on the formed outer oxide layer by the reducing functional groups of humic acid. Additionally, direct electron transfer between Fe(III) and Fe(0) at the inner Fe(0)-Fe(III) interface may also occur and cannot be ruled out. Effects of Quinone Model Compounds on Bromate Reduction. Quinone moieties in humic acid have been identified recently as the electron-transfer mediator in redox reactions (15, 21). Two quinone model compounds, lawsone and AQDS, which have been commonly used to represent qunione moieties in humic substances to study the redox function of quinone moieties in humic acid (10, 18, 21), were chosen. Parts a and b of Figure 4 show the results of bromate reduction with Fe(0) when the bromate was simultaneously added with lawsone and AQDS, respectively. Changes in the concentrations of lawsone and AQDS yielded only small,

insignificant (within 2% of the experimental errors) changes in the bromate reduction rate. Meanwhile, the complexation and adsorption onto the iron surface caused the removal of lawsone or AQDS from the aqueous phase. The dissolved iron concentration increased with prolonged reaction time and also with increasing lawsone or AQDS concentrations. The reduction of bromate by lawsone and AQDS was also studied with or without the presence of Fe(III). These results are presented in parts a and b, respectively, of Figure 5. Without Fe(III), only lawsone at pH 4.1 demonstrated notable bromate reduction over a 168 h time span. There was no significant change in the bromate concentration in all remaining cases. When Fe(III) was added into a solution containing bromate and lawsone, the bromate reduction rate was enhanced significantly. However, the addition of AQDS, even in the presence of Fe(III), did not remove bromate over a reaction time of about 180 h. These findings are attributable to the differences in the chemical structures of lawsone and AQDS. Although both compounds contain quinone moieties, lawsone contains an additional phenolic group, which has been identified as a reductant group in NOM (7), and may donate electrons to bromate, the electron acceptor. Usually, quinones are reduced to semiquinone radicals, and the continuous electron transfer during such a reduction results in the formation of hydroquinone (31). As such, quinone is an oxidized form and does not have any reduction capability in the presence of other oxidants such as bromate or Fe(III). Only if other reductants such as HS- or Fe2+ reduce AQDS to AHQDS, it can mediate the accepted electrons to the contaminants of interest to facilitate chemical reduction (12, 13). Spectroscopic Analysis. Information on the specific functional groups of the humic acids adsorbed on the iron surface was provided by FTIR spectroscopy. Figure 6 shows the spectra of the humic acid powders (line a), of the iron VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. FTIR spectra of humic acid (a) before and (b) after adsorption on the iron surface. (c) FTIR spectrum of the iron surface without adsorption. surface after humic acid adsorption (line b), and of the iron surface without adsorbed humic acid (line c). On the basis of the band assignments obtained from the literature (3234), the following major bands are observed: 3429 cm-1 (hydrogen bonding and O-H stretching), the doublet between 2920 and 2851 cm-1 (C-H stretch of aliphatic groups), 1580 cm-1 (aromatic CdC), 1389 cm-1 (symmetric stretching of COO-, C-OH stretching of phenolic OH), and 1099/1034 cm-1 (C-O stretching of carbohydrate or alcohol). After adsorption, the observations of the shifts at stretching bands of OH, CdC, and COO- or phenolic OH, although some of which may not be conclusive, are consistent with many studies of NOM adsorption on surface minerals, especially the surfaces of iron oxides (24, 35, 36). Similar peak shifts at the sites of OH, CdC, and COO- were also observed in the spectra of the formed Fe(III)-humic acid complex (not shown). This limited FTIR evidence suggests that the aromatic COOH and hydroxyl or phenolic OH are the main functional groups that complex with and adsorb onto the iron oxide surfaces. It has been reported in the literature that the adsorption of these functional groups on the metal surface can shift the electron densities to the central metal ions at the surface to facilitate the breakage of metaloxygen lattice bonds, leading to the detachment of the central metal ion into the solution (24). ESR spectroscopy can supply useful information on the molecular structure of humic substances (27) or about the stereochemistry and the type of coordination sites of metalligand bonding (37, 38). The ESR spectral parameters such as g factors and line width, unaltered with various experimental and laboratory factors, can be compared and related to other chemical and physicochemical properties. Similar g values point out that the free radicals of different types of NOM are of similar nature (27). In this study, the ESR spectra of humic acid and lawsone, which are shown in Figure 7a, were very similar with obtained g values of 1.996 and 1.995, respectively. The ESR signal was caused by the presence of quinone hydrate from the reduction of quinone, including semiquinone and hydroquinone (27). On the other hand, no ESR signal was observed in the AQDS samples as expected since AQDS contains quinone moieties only. It has been reported in the literature that the radical concentration of AQDS increased from below a detection limit to 7 × 1018 spins/g after microbial reduction and the ESR signal completely disappeared when oxygen was introduced (21). The ESR spectra of the humic acid and lawsone in this study were in agreement with the literature that the ESR spectra of humic acid and fulvic acid are qualitatively described by 1098

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FIGURE 7. ESR spectra of (a) humic acid and lawsone and (b) ferric and humic acid-Fe(III) complexes. a sharp and narrow resonance at g ≈ 2 (39), which is consistent with semiquinone radical units possibly conjugated to aromatic rings (40). On the basis of the chemical structure of lawsone, the similar ESR signals for lawsone and humic acid should be attributable to the same nature of radicals, which would be quinone-phenol groups. Steelink and Tollin (41) proposed that the quinone-phenol groups in hydroanthraquinones, tannies, lignins, solid humic, and fulvic acids account for their similar ESR properties. No ESR signal was observed in a solution containing glycine (42) and in solid hydroquinone and 2,3-dihydroxybenzoic acid used as the references of phenolic and/or carboxylate groups in our study. It should be noted that although iron porphyrin has been studied in other studies as an electron mediator (43, 44), the ESR analyses in the current study showed the presence of quinone-phenol groups only. Iron porphyrin is commonly recognized as a functional group in hemoproteins (45, 46), but it was reported to be a trace component in national organic matter with concentrations of sub-µg/L in natural water (47). The ESR spectra of Fe(III) and Fe(III)-humic acid complexes were also recorded (Figure 7b). A broad resonance signal centered near g ) 2, which is an envelope of several resonances from Fe(III), and a little shoulder at g ) 4 were obtained in the Fe(III)-humic acid complex. Usually, the magnetic parameters of the Fe(III)-NOM complexes have been observed to exist at a g value of 2 or at a g value of 4 (38). Since the ESR spectra of Fe(III)-humic acid complexes mainly center at a g value of 2, this indicated that most Fe(III) is bound to phenolic and/or carboxylic groups at octahedral sites with little or no axial distortion from the cubic symmetry ligand field, where Fe(III) has been reported

to be easily reduced (48). The little shoulder at g ) 4 indicates that there is a small amount of Fe(III) coordinating with humic acid in the tetrahedral or octahedral sites in a low-symmetry ligand field, where Fe(III) is hard to be reduced (48). Environmental Implications. In this study, batch tests show that humic acid has relatively little inhibitory effect on the rates of bromate reduction by Fe(0), wherein the rates are only retarded by a factor of about 1.3-2.0, not by orders of magnitude, due to the adsorption of humic acid. The adsorption of humic acid also inhibits iron corrosion, thereby prolonging the lifetime of the zerovalent iron. On the other hand, the adsorbed humic acid can transfer electrons from the inner Fe(0) to Fe(III) to reduce bromate in solution. The Fe(III)-humic acid complexes formed on the outer oxide layer or in solution can regenerate reactive Fe(II) to reduce bromate; however, at a much slower rate (kobs of approximately 0.01 min-1, based on the data and conditions in Figure 1c). Thus, it is expected that when the bromate reduction rate by Fe(0) is reduced to a certain extent due to both humic acid and iron oxide coverage, there will be a turning point at which such bromate reduction by regenerated Fe(II) becomes prevalent. If the quantity of the Fe(III)humic acid complexes is sufficient, the amount of Fe(II) regenerated may be high enough to maintain iron surface activation and reduction of bromate at a typical concentration of 0-127 µg/L in drinking water to prolong the lifetime of the zerovalent iron. In the remediation of halogenated or nitro aromatic compound contaminated groundwater using Fe(0), the electron-transfer function of humic acid shall be more significant due to the refractory nature of these pollutants.

Acknowledgments This study was supported in part by the Hong Kong Research Grants Council under Competitive Earmarked Research Grant HKUST6106/03E.

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Received for review June 28, 2004. Revised manuscript received November 11, 2004. Accepted November 24, 2004. ES049027Z