- Email: [email protected]
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution
Journal of Environmental Sciences 20(2008) 257–261
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution XIE Li1,2,∗, SHANG Chii2 , ZHOU Qi1 1. State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China. E-mail: [email protected] 2. Department of Civil Engineering, Hong Kong University of Science and Technology, Hong Kong 852, China Received 9 May 2007; revised 10 August 2007; accepted 30 September 2007
Abstract Humic substances are ubiquitous redox-active organic compounds of environment. In this study, experiments were conducted to determine the reduction capacity of humic acid in the matrix of bromate and Fe(III) solutions and the role of Fe(III) in this redox process. The results showed that the humic acid regenerated Fe(II) and reduced bromate abiotically. The addition of Fe(III) could accelerate the bromate reduction rate by forming humic acid-Fe(III) complexes. Iron species acts as electron mediator and catalyst for the bromate reduction by humic acid, in which humic acid transfers electrons to the complexed Fe(III) to form Fe(II), and the regenerated Fe(II) donate the electrons to bromate. The kinetics study on bromate reduction further indicated that bromate reduction by humic acid-Fe(III) complexes is pH dependent. The rate decreased by 2-fold with the increase in solution pH by one unit. The reduction capacity of Aldrich humic acid was observed to be lower than that of humic acid or natural organic matter of Suwanne River, indicating that such redox process is expected to occur in the environment. Key words: humic acid; Fe(III) ion; bromate reduction; electron mediator
Introduction Bromate (BrO3 − ) formation via ozonation process in waters containing bromide was observed with concentrations up to 127 µg/L (Krasner et al., 1993). Bromate is of particular concern because it is a carcinogenic disinfection by-product and its current maximum contaminant level in drinking water is set at 10 µg/L by USEPA (1998). Bromate is a strong oxidant that can be reduced to bromide (Br− ) in water by redox reactions with reduced iron species, such as Fe(II), or Fe(0) (Siddiqui et al., 1994; Xie and Shang, 2007), which is indicated by the following equations: BrO−3 + 6Fe2+ + 6H+ −→ Br− + 6Fe3+ + 3H2 O BrO−3
−
+ 2Fe + 3H2 O −→ Br + 2Fe 0
3+
(1) −
+ 6OH
(2)
Laboratory studies showed that factors affecting the activity of available iron species, such as humic substances, which are ubiquitously present in soils, surface water, and groundwater, could affect the chemical reduction reactions. In a study of bromate reduction by active iron species, Xie and Shang (2004) observed that humic substances form complex with Fe(III) and Fe(II) species and adsorb on the iron surface and thereby lower the bromate removal efficiency within the reaction time of 60 min. Humic substances are redox-active organic compounds. It was * Corresponding author. E-mail: [email protected].
reported to play an important role in electron transfer in the presence of microorganisms to facilitate the reduction of Fe(III) to Fe(II) (Lovley et al., 1996; Royer et al., 2002). Humic substances has been demonstrated to participate in the oxidation and reduction of iron as a factor controlling the iron speciation biotically and abiotically (Voelker et al., 1997; Chen et al., 2003). Chemical reduction of hexachloroethane by humic substances has also been reported recently (Kappler and Haderlein, 2003). The electron-transfering property of humic substances has been proposed to be related to the presence of functional groups such as quinone, phenolic, and carboxylate moieties (Scott et al., 1998; Tipping, 2002). Our previous study based on FTIR and ESR spectrum analyses has indicated that the functional quinone-phenol groups present in the humic substances may contribute to the complexed Fe(III) reduction (Xie and Shang, 2005). In addition, the redox activity of humic substances was attributed to the formed humic substance-Fe(III) complexes (Struyk and Sposito, 2001). With the growing understanding of redox capability of humic substances, more studies are still needed to understand the complexed iron species and the contributions of humic subtances to redox reactions, and therefore, the regeneration of oxidized iron to active species is of great interest. It is also not clear from previous studies as to which factors were actually responsible for the stimulation of the reduction rates in the presence of humic substances. The purpose of this research was therefore to determine
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the comlexed iron species in these redox reactions and the factor controlling the reduction process. Redox capacities from different humic substance sources were also discussed.
1 Materials and methods 1.1 Chemicals Bromate and Fe(III) were prepared with reagent-grade sodium bromate (Nacalai Tesque, Japan) and ferric nitrate (Riedel deHa¨en, Germany). Humic acid solution from Aldrich, Suwannee River humic acid standard, and Suwannee River aquatic natural organic matter were prepared by dissolving humic acid powders in doubly distilled deionized water and filtering through a 0.45-µm filter paper (Advantec MFS) to remove insoluble impurities. 1.2 Experimental procedures A series of batch experiments were performed in dark at room temperature. After quick mixing, the solution was divided into series of capped bottles. The reduction of bromate with an initial concentration of 5 mg/L was studied in solutions containing various humic acid concentrations with fixed concentration of Fe(III) and pH; various solutions with pH ranging from 4 to 8 adjusted by dropwise additions of H2 SO4 with fixed concentration of Fe(III) and humic acid; and Fe(III) or humic acid alone at pH 3.9. At given intervals, samples were taken out and filtered for subsequent measurements. Fe(III) reduction by humic acid was conducted in solution containing humic acid and Fe(III) at room temperature and pH 3.9. One set was irradiated by visible light; the other set was wrapped with aluminum foil to avoid the exposure of light. Controls containing only Fe(III) were set for comparison. The procedures for sampling and analyses are the same as those described above. All the experiments were conducted in duplicate. Zero-order rate constant value of Kobs on bromate for experiments conducted in duplicate is given as the confidence interval of the mean with 95% probability.
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1.3 Analytical methods Bromate and bromide concentrations were measured with an ion chromatograph (Dionex 500, USA). An Ionpac AS9-HC analytical column with an Ionpac AG9-HC guard column and a 250-µl injection loop were used for anion separation. A 9.0-mmol/L sodium carbonate solution was used as the eluent at a flow rate of 1 ml/min. Dissolved iron and ferrous ion concentrations in the filtered samples were determined by an atomic absorption spectrometer (SpectrAA 220FS, Varian, USA) and a spectrophotometer at 510 nm (MultiSpe-1501, Shimadzu, Japan), respectively. Humic acid was quantified as dissolved organic carbon (DOC) by a TOC analyzer (TOC-5000A, Shimadzu, Japan).
2 Results and discussion 2.1 Bromate reduction in solutions of humic acid and Fe(III) Bromate reduction in the presence of humic acid and Fe(III) were carried out in dark condition with Fe(III) 20 mg/L, humic acid 45 mg/L, and bromate 5 mg/L. Black complexes (in precipitates) were observed at the bottom of the bottles after quick mixing. Fig.1a gives representative examples of the concentration/time courses typically observed for the disappearance of bromate and appearance of its reduction product, bromide. Bromide concentration was observed to increase with reaction time with the decrease of bromate. Bromine mass balance was made and nearly 90% bromine recovery was achieved. Good bromine mass balance indicated that the removal of bromate should be primarily attributed to the chemical reduction, rather than adsorption onto the complex. As shown in Fig.1b, dissolved organic carbon and ferric ion concentration decreased sharply after quick mixing at the reaction time of 5 min, and their concentrations remained constant after that. The obtained results could be attributed to the formation of the complexes at the beginning. In this system, trace amount of Fe(II) was measured. The role of Fe(III) in bromate reduction was further investigated in this study.
Fig. 1 Bromate reduction and formation of its reduced products in solutions of humic acid, ferric ion, and bromate (a), and changes of dissolved humic acid and iron concentration with reaction time (b).
No. 3
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution
2.2 Effect of Fe(III) on bromate reduction by humic acid Three mechanisms for bromate removal were taken into consideration, including the function of humic acid, the adsorption by Fe(III), or the reduction by the newly formed Fe(III)-humic acid complexes. Fig.2 gives representative examples of the observed disappearance of bromate in solutions containing: (1) humic acid and Fe(III), (2) humic acid alone, and (3) Fe(III) alone under the same pH condition. As shown in Fig.2, almost no bromate removal was observed in the solution containing Fe(III) alone at pH 3.2. Bromate reduction by humic acid alone was observed at pH 3.2. Compared with humic acid alone, the Fe(III)humic acid complexes formed by the addition of Fe(III) into solutions increased bromate removal rate significantly. At the reaction time of 200 h, 40% bromate was reduced in Fe(III)-humic acid solutions compared with 10% bromate reduction in humic acid solutions alone. 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). Bromate reduction at different humic acid concentrations and various pH were evaluated in our previous study (Xie and Shang, 2005). An incerease in the humic acid concentration and decrease of pH value was observed to lead to an increase in the bromate removal rate. The kinetics of bromate reduction reactions in humic acidFe(III) complexes solutions examined in this study were zero-order with respect to bromate and humic acid-Fe(III)
259
complexes concentration. Therefore, the rate of disappearance of bromate can be described as shown in Eq.(3). R=−
dCBrO−3 dt
= −kobs
(3)
and thus 0 CBrO−3 = kobs × t + CBrO − 3
(4)
where, kobs is the observed zero-order rate constant under 0 given conditions, and CBrO−3 and CBrO − are the bromate 3 concentrations at time t and time zero, respectively. The calculated rate constants under different conditions are presented in Table 1. As shown in Table 1, the solution pH affected bromate reduction rate significantly over the pH range of 3.9–6 at constant humic acid and Fe(III) concentrations. Increase in the solution pH caused a siginificant decrease by 2-fold in the rate of bromate reduction from 0.0083 mg/(L·min) at pH 3.9 to 0.0038 mg/(L·min) at pH 5. The increase of humic acid concentrations from 10 to 45 mg/L did not affect the rate of bromate reduction significantly, with the observed rate remaining constant in the range of 0.0059–0.0083 mg/(L·min) as listed in Table 1. Several possible reasons were taken into consideration to evaluate the pH dependence of bromate reduction rate. First, humic substances were observed to have higher and more stable radical concentrations than those under acid conditions (Steelink, 1966). Steelink and Fitzapatrick (1972) observed that redox functional groups in humic substances such as quinone moieties have the tendency to attract electrons to form stable free radicals even without the presence of reducing groups. Second, solution pH could greatly affect the morphology and structure of humic substances, which may be attributed to the decline in its reduction capacities as observed in our study. Finally, precipitated amorphous iron hydroxides at higher pH may be another reason for the pH dependence of the bromate reduction by humic acid-Fe(III) complexes. Therefore, in this study, the observed decline in the bromate reduction rate with increasing pH is speculated to be influenced by the above factors, and further studies on pH dependence will be further explored in future study. 2.3 Fe(III) reduction by humic acid
Fig. 2 Bromate reduction in solutions of humic acid, ferric ion, and their mixture.
The increase in Fe(II) concentration with reaction time as a result of the reduction of Fe(III) by the humic acid were observed at pH 3.9 as illustrated in Fig.3. Meanwhile, the reoxidation of the generated Fe(II) was unlikely to occur under conditions of low pH and low dissolved
Table 1 Effect of humic acid concentration and pH on bromate reduction Variable parameter
Zero-order kinetics
Humic acid concentration
CBrO− 3 CBrO− 3 CBrO− 3 CBrO− 3 CBrO− 3 CBrO−
pH
45 mg DOC/L 30 mg DOC/L 10 mg DOC/L 3.9 5 6
3
= –0.0083t + 4.70 = –0.0067t + 4.61 = –0.0059t + 4.74 = –0.0083t + 4.70 = –0.0038t + 4.84 = –0.0020t + 4.97
kobs (mg/(L·min))
R2
0.0083 0.0067 0.0059 0.0083 0.0038 0.0020
0.90 0.92 0.93 0.90 0.96 0.96
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Fig. 3 Formation of regenerated Fe(II) by humic acid vs reaction time in the presence or absence of visible light. Initial conditions: Fe(III) 20 mg/L; humic acid 45 mg/L as DOC; pH 3.9.
oxygen concentrations (less than 2 mg/L) in the tested solutions. Fe(III)-humic acid complexes (in precipitates) were observed at the bottom of the bottles. These findings suggest that humic acid contains reduced functional groups, which form complex with Fe(III) and consequently lead to the formation of Fe(II). If bromate is present in solutions, the formed Fe(II) reoxidation rate by bromate is higher than Fe(III) reduction rate by humic acid. Very low concentration of Fe(II) is maintained in the system, provided that the humic acid did not reach its full oxidation capacity. That is why only trace amounts of Fe(II) were detected in solutions containing humic acid, Fe(III), and bromate in our first study. As Fig.3 shows, visible light irradiation yielded higher dissolved Fe(II) concentrations compared with the yields obtained in the dark, indicating that the reduction of Fe(III) to Fe(II) by humic acid can be photo-catalyzed. The photoreduction of Fe(III) to Fe(II) has been observed in aqueous solutions containing humic substances (Voelker et al., 1997). Furthermore, no Fe(II) was observed in the Fe(III) solution without humic acid addition under light conditon. Therefore, the increased Fe(II) yields under irradiation of visible light suggest that the light can excitate the humic acid-Fe(III) complex and enhance the electron transfer between Fe(III) and the specific reduced moieties in humic acid.
Fig. 4 Comaprison of reducing reactivity of different humic acid and NOM sources.
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 Fig.5. It shows that the Fe(III)/Fe(II) couple acts as a catalyst for bromate reduction by humic acid. The reduced functional groups in the humic acid form 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. The above reaction process is expected to occur in the environment, since the reducing capacity of Aldrich humic acid was observed to be lower than that of humic acid or NOM from Suwanne River, purchased from IHSS.
Fig. 5 Electron-transfer process from the electron donor (humic acid) to the electron acceptor (bromate) via Fe(III).
2.4 Comparision of reduction capacity of different humic acid sources
3 Conclusions
The reducing activity of humic acid from different sources was compared. Humic acid from Aldrich, Suwannee River humic acid (standard) 2S101H IHSS, and Suwanne River natural organic matter (NOM) (RO isolation) 1R101N IHSS were used for the above purpose. As Fig.4 shows, under the same condition of fixed humic acid concentration, Fe(III) concentrations, and pH, humic acid from Aldrich achieved much lower bromate reduction rate compared with humic acid or NOM from the Suwanne River, both of which has similar reducing reactivity to bromate as observed. Research is currently underway to characterize the mechanisms involved. On the basis of the results presented above, an electron-
The reduction capacity of humic acid and the effect of complexed Fe(III) in redox reaction considering bromate as target pollutant were examined in this study. The Fe(III)humic acid complexes could reduce bromate to bromide under acidic condition. The humic acid was observed to regenerate Fe(II) and reduce bromate abiotically. The addition of Fe(III) was observed to accelerate the bromate reduction rate. According to the results, an electrontransfer scheme was proposed. The reduced functional groups in the humic acid form 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. Kinetics study on bromate
No. 3
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution
reduction further indictated that pH was a controlling factor for the redox capacity of humic acid-Fe(III) complexes. The above reaction process is expected to occur in the environment, since the reducing capacity of Aldrich humic acid was observed to be lower than that of humic acid or NOM from Suwanne River. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50608056), the Hong Kong Research Grants (No. HKUST6106/03E), and the Program for Young Excellent Talents in Tongji University in part (No. 2006KJ033). They are gratefully acknowledged.
References Chen J, Gu B, Royer R A, Burgos W D, 2003. The role of natural organic matter in chemical and microbial reduction of ferric iron. Sci Total Environ, 307: 167–178. Chen Y, Schnitzer M, 1989. Sizes and shapes of humic substances by electron microscopy. In: Humic Substances II: Search of Structure (Hayes M. H. B., MacCarthy P., Malcolm R. L. et al., eds.). John Wiley & Sons: New York. Kappler A, Haderlein S B, 2003. Natural organic matter as reductant for chlorinated aliphatic pollutants. Environ Sci Technol, 37: 2714–2719. Krasner S W, Glaze W H, Weinberg H S, Daniel P A, Najm I, 1993. Formation and control of bromate during ozonation of waters containing bromide. J Am Water Works Ass, 85: 73–81. Lovley D R, Coates J D, Blunt-Harris E L, Phillips E J P, Woodward J C, 1996. Humic substances as electron acceptors for microbial respiration. Nature, 382: 445–448. Royer R A, Burgos W D, Fisher A S, Unz R F, Dempsey B A, 2002. Enhancement of biological reduction of hematite
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by electron shuttling and Fe(II) complexation. Environ Sci Technol, 36: 1939–1946. Scott D T, Mcknight D M, Blunt-Harris E L, Kolesar S E, Lovley D R, 1998. Quinone moieties act as electron acceptors in the reduction of humic substances by humic-reducing microorganisms. Environ Sci Technol, 32: 2984–2989. Siddiqui M, Amy G, Ozekin K, Zhai W Y, Westerhoff P, 1994. Alternative strategies for removing bromate. J Am Water Works Ass, 86: 81–96. Steelink C, 1966. Electron paramagnetic resonance sutdies of humic acid and related model compounds. In: Coal Science. Adances in chemistry series, No. 55, American Chemical Society, Washington DC. 80–90. Steelink C, Fitzpatrick J D, 1972. The origin of the paramagnetic species in lignin solutions. Autoreduction of 2,6-dimethoxybenzoquinone and related quinines to radicals anions in alkaline solutions. J Org Chem, 37: 762–767. Struyk Z, Sposito G, 2001. Redox properties of standard humic acids. Geoderma, 102: 329–346. Tipping E, 2002. Cation Binding by Humic Substances. Cambridge University Press: New York. USEPA 1998. National Primary Drinking Water Regulations: Disinfectants and Disinfection Byproducts. Final Rule. Fed. Reg., 63(241): 69390–69476. Voelker B M, Morel F M M, Sulzberger B, 1997. Iron redox cycling in surface waters: effects of humic substances and light. Environ Sci Technol, 31: 1004–1011. Xie L, Shang C, 2004. Effect of humic acid on bromate reduction by zero valent iron. AWWA 2004 Annual Conference and Exposition, June, Florida, USA. Xie L, Shang C, 2005. Role of humic acid and quinone model compounds in bromate reduction by zerovalent iron. Environ Sci Technol, 39: 1092–1100. Xie L, Shang C, 2007. Effects of operational parameters and common anions on the reactivity of zero-valent iron in bromate reduction. Chemosphere, 66: 1652–1659.
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution XIE Li1,2,∗, SHANG Chii2 , ZHOU Qi1 1. State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China. E-mail: [email protected] 2. Department of Civil Engineering, Hong Kong University of Science and Technology, Hong Kong 852, China Received 9 May 2007; revised 10 August 2007; accepted 30 September 2007
Abstract Humic substances are ubiquitous redox-active organic compounds of environment. In this study, experiments were conducted to determine the reduction capacity of humic acid in the matrix of bromate and Fe(III) solutions and the role of Fe(III) in this redox process. The results showed that the humic acid regenerated Fe(II) and reduced bromate abiotically. The addition of Fe(III) could accelerate the bromate reduction rate by forming humic acid-Fe(III) complexes. Iron species acts as electron mediator and catalyst for the bromate reduction by humic acid, in which humic acid transfers electrons to the complexed Fe(III) to form Fe(II), and the regenerated Fe(II) donate the electrons to bromate. The kinetics study on bromate reduction further indicated that bromate reduction by humic acid-Fe(III) complexes is pH dependent. The rate decreased by 2-fold with the increase in solution pH by one unit. The reduction capacity of Aldrich humic acid was observed to be lower than that of humic acid or natural organic matter of Suwanne River, indicating that such redox process is expected to occur in the environment. Key words: humic acid; Fe(III) ion; bromate reduction; electron mediator
Introduction Bromate (BrO3 − ) formation via ozonation process in waters containing bromide was observed with concentrations up to 127 µg/L (Krasner et al., 1993). Bromate is of particular concern because it is a carcinogenic disinfection by-product and its current maximum contaminant level in drinking water is set at 10 µg/L by USEPA (1998). Bromate is a strong oxidant that can be reduced to bromide (Br− ) in water by redox reactions with reduced iron species, such as Fe(II), or Fe(0) (Siddiqui et al., 1994; Xie and Shang, 2007), which is indicated by the following equations: BrO−3 + 6Fe2+ + 6H+ −→ Br− + 6Fe3+ + 3H2 O BrO−3
−
+ 2Fe + 3H2 O −→ Br + 2Fe 0
3+
(1) −
+ 6OH
(2)
Laboratory studies showed that factors affecting the activity of available iron species, such as humic substances, which are ubiquitously present in soils, surface water, and groundwater, could affect the chemical reduction reactions. In a study of bromate reduction by active iron species, Xie and Shang (2004) observed that humic substances form complex with Fe(III) and Fe(II) species and adsorb on the iron surface and thereby lower the bromate removal efficiency within the reaction time of 60 min. Humic substances are redox-active organic compounds. It was * Corresponding author. E-mail: [email protected].
reported to play an important role in electron transfer in the presence of microorganisms to facilitate the reduction of Fe(III) to Fe(II) (Lovley et al., 1996; Royer et al., 2002). Humic substances has been demonstrated to participate in the oxidation and reduction of iron as a factor controlling the iron speciation biotically and abiotically (Voelker et al., 1997; Chen et al., 2003). Chemical reduction of hexachloroethane by humic substances has also been reported recently (Kappler and Haderlein, 2003). The electron-transfering property of humic substances has been proposed to be related to the presence of functional groups such as quinone, phenolic, and carboxylate moieties (Scott et al., 1998; Tipping, 2002). Our previous study based on FTIR and ESR spectrum analyses has indicated that the functional quinone-phenol groups present in the humic substances may contribute to the complexed Fe(III) reduction (Xie and Shang, 2005). In addition, the redox activity of humic substances was attributed to the formed humic substance-Fe(III) complexes (Struyk and Sposito, 2001). With the growing understanding of redox capability of humic substances, more studies are still needed to understand the complexed iron species and the contributions of humic subtances to redox reactions, and therefore, the regeneration of oxidized iron to active species is of great interest. It is also not clear from previous studies as to which factors were actually responsible for the stimulation of the reduction rates in the presence of humic substances. The purpose of this research was therefore to determine
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the comlexed iron species in these redox reactions and the factor controlling the reduction process. Redox capacities from different humic substance sources were also discussed.
1 Materials and methods 1.1 Chemicals Bromate and Fe(III) were prepared with reagent-grade sodium bromate (Nacalai Tesque, Japan) and ferric nitrate (Riedel deHa¨en, Germany). Humic acid solution from Aldrich, Suwannee River humic acid standard, and Suwannee River aquatic natural organic matter were prepared by dissolving humic acid powders in doubly distilled deionized water and filtering through a 0.45-µm filter paper (Advantec MFS) to remove insoluble impurities. 1.2 Experimental procedures A series of batch experiments were performed in dark at room temperature. After quick mixing, the solution was divided into series of capped bottles. The reduction of bromate with an initial concentration of 5 mg/L was studied in solutions containing various humic acid concentrations with fixed concentration of Fe(III) and pH; various solutions with pH ranging from 4 to 8 adjusted by dropwise additions of H2 SO4 with fixed concentration of Fe(III) and humic acid; and Fe(III) or humic acid alone at pH 3.9. At given intervals, samples were taken out and filtered for subsequent measurements. Fe(III) reduction by humic acid was conducted in solution containing humic acid and Fe(III) at room temperature and pH 3.9. One set was irradiated by visible light; the other set was wrapped with aluminum foil to avoid the exposure of light. Controls containing only Fe(III) were set for comparison. The procedures for sampling and analyses are the same as those described above. All the experiments were conducted in duplicate. Zero-order rate constant value of Kobs on bromate for experiments conducted in duplicate is given as the confidence interval of the mean with 95% probability.
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1.3 Analytical methods Bromate and bromide concentrations were measured with an ion chromatograph (Dionex 500, USA). An Ionpac AS9-HC analytical column with an Ionpac AG9-HC guard column and a 250-µl injection loop were used for anion separation. A 9.0-mmol/L sodium carbonate solution was used as the eluent at a flow rate of 1 ml/min. Dissolved iron and ferrous ion concentrations in the filtered samples were determined by an atomic absorption spectrometer (SpectrAA 220FS, Varian, USA) and a spectrophotometer at 510 nm (MultiSpe-1501, Shimadzu, Japan), respectively. Humic acid was quantified as dissolved organic carbon (DOC) by a TOC analyzer (TOC-5000A, Shimadzu, Japan).
2 Results and discussion 2.1 Bromate reduction in solutions of humic acid and Fe(III) Bromate reduction in the presence of humic acid and Fe(III) were carried out in dark condition with Fe(III) 20 mg/L, humic acid 45 mg/L, and bromate 5 mg/L. Black complexes (in precipitates) were observed at the bottom of the bottles after quick mixing. Fig.1a gives representative examples of the concentration/time courses typically observed for the disappearance of bromate and appearance of its reduction product, bromide. Bromide concentration was observed to increase with reaction time with the decrease of bromate. Bromine mass balance was made and nearly 90% bromine recovery was achieved. Good bromine mass balance indicated that the removal of bromate should be primarily attributed to the chemical reduction, rather than adsorption onto the complex. As shown in Fig.1b, dissolved organic carbon and ferric ion concentration decreased sharply after quick mixing at the reaction time of 5 min, and their concentrations remained constant after that. The obtained results could be attributed to the formation of the complexes at the beginning. In this system, trace amount of Fe(II) was measured. The role of Fe(III) in bromate reduction was further investigated in this study.
Fig. 1 Bromate reduction and formation of its reduced products in solutions of humic acid, ferric ion, and bromate (a), and changes of dissolved humic acid and iron concentration with reaction time (b).
No. 3
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution
2.2 Effect of Fe(III) on bromate reduction by humic acid Three mechanisms for bromate removal were taken into consideration, including the function of humic acid, the adsorption by Fe(III), or the reduction by the newly formed Fe(III)-humic acid complexes. Fig.2 gives representative examples of the observed disappearance of bromate in solutions containing: (1) humic acid and Fe(III), (2) humic acid alone, and (3) Fe(III) alone under the same pH condition. As shown in Fig.2, almost no bromate removal was observed in the solution containing Fe(III) alone at pH 3.2. Bromate reduction by humic acid alone was observed at pH 3.2. Compared with humic acid alone, the Fe(III)humic acid complexes formed by the addition of Fe(III) into solutions increased bromate removal rate significantly. At the reaction time of 200 h, 40% bromate was reduced in Fe(III)-humic acid solutions compared with 10% bromate reduction in humic acid solutions alone. 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). Bromate reduction at different humic acid concentrations and various pH were evaluated in our previous study (Xie and Shang, 2005). An incerease in the humic acid concentration and decrease of pH value was observed to lead to an increase in the bromate removal rate. The kinetics of bromate reduction reactions in humic acidFe(III) complexes solutions examined in this study were zero-order with respect to bromate and humic acid-Fe(III)
259
complexes concentration. Therefore, the rate of disappearance of bromate can be described as shown in Eq.(3). R=−
dCBrO−3 dt
= −kobs
(3)
and thus 0 CBrO−3 = kobs × t + CBrO − 3
(4)
where, kobs is the observed zero-order rate constant under 0 given conditions, and CBrO−3 and CBrO − are the bromate 3 concentrations at time t and time zero, respectively. The calculated rate constants under different conditions are presented in Table 1. As shown in Table 1, the solution pH affected bromate reduction rate significantly over the pH range of 3.9–6 at constant humic acid and Fe(III) concentrations. Increase in the solution pH caused a siginificant decrease by 2-fold in the rate of bromate reduction from 0.0083 mg/(L·min) at pH 3.9 to 0.0038 mg/(L·min) at pH 5. The increase of humic acid concentrations from 10 to 45 mg/L did not affect the rate of bromate reduction significantly, with the observed rate remaining constant in the range of 0.0059–0.0083 mg/(L·min) as listed in Table 1. Several possible reasons were taken into consideration to evaluate the pH dependence of bromate reduction rate. First, humic substances were observed to have higher and more stable radical concentrations than those under acid conditions (Steelink, 1966). Steelink and Fitzapatrick (1972) observed that redox functional groups in humic substances such as quinone moieties have the tendency to attract electrons to form stable free radicals even without the presence of reducing groups. Second, solution pH could greatly affect the morphology and structure of humic substances, which may be attributed to the decline in its reduction capacities as observed in our study. Finally, precipitated amorphous iron hydroxides at higher pH may be another reason for the pH dependence of the bromate reduction by humic acid-Fe(III) complexes. Therefore, in this study, the observed decline in the bromate reduction rate with increasing pH is speculated to be influenced by the above factors, and further studies on pH dependence will be further explored in future study. 2.3 Fe(III) reduction by humic acid
Fig. 2 Bromate reduction in solutions of humic acid, ferric ion, and their mixture.
The increase in Fe(II) concentration with reaction time as a result of the reduction of Fe(III) by the humic acid were observed at pH 3.9 as illustrated in Fig.3. Meanwhile, the reoxidation of the generated Fe(II) was unlikely to occur under conditions of low pH and low dissolved
Table 1 Effect of humic acid concentration and pH on bromate reduction Variable parameter
Zero-order kinetics
Humic acid concentration
CBrO− 3 CBrO− 3 CBrO− 3 CBrO− 3 CBrO− 3 CBrO−
pH
45 mg DOC/L 30 mg DOC/L 10 mg DOC/L 3.9 5 6
3
= –0.0083t + 4.70 = –0.0067t + 4.61 = –0.0059t + 4.74 = –0.0083t + 4.70 = –0.0038t + 4.84 = –0.0020t + 4.97
kobs (mg/(L·min))
R2
0.0083 0.0067 0.0059 0.0083 0.0038 0.0020
0.90 0.92 0.93 0.90 0.96 0.96
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XIE Li et al.
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Fig. 3 Formation of regenerated Fe(II) by humic acid vs reaction time in the presence or absence of visible light. Initial conditions: Fe(III) 20 mg/L; humic acid 45 mg/L as DOC; pH 3.9.
oxygen concentrations (less than 2 mg/L) in the tested solutions. Fe(III)-humic acid complexes (in precipitates) were observed at the bottom of the bottles. These findings suggest that humic acid contains reduced functional groups, which form complex with Fe(III) and consequently lead to the formation of Fe(II). If bromate is present in solutions, the formed Fe(II) reoxidation rate by bromate is higher than Fe(III) reduction rate by humic acid. Very low concentration of Fe(II) is maintained in the system, provided that the humic acid did not reach its full oxidation capacity. That is why only trace amounts of Fe(II) were detected in solutions containing humic acid, Fe(III), and bromate in our first study. As Fig.3 shows, visible light irradiation yielded higher dissolved Fe(II) concentrations compared with the yields obtained in the dark, indicating that the reduction of Fe(III) to Fe(II) by humic acid can be photo-catalyzed. The photoreduction of Fe(III) to Fe(II) has been observed in aqueous solutions containing humic substances (Voelker et al., 1997). Furthermore, no Fe(II) was observed in the Fe(III) solution without humic acid addition under light conditon. Therefore, the increased Fe(II) yields under irradiation of visible light suggest that the light can excitate the humic acid-Fe(III) complex and enhance the electron transfer between Fe(III) and the specific reduced moieties in humic acid.
Fig. 4 Comaprison of reducing reactivity of different humic acid and NOM sources.
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 Fig.5. It shows that the Fe(III)/Fe(II) couple acts as a catalyst for bromate reduction by humic acid. The reduced functional groups in the humic acid form 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. The above reaction process is expected to occur in the environment, since the reducing capacity of Aldrich humic acid was observed to be lower than that of humic acid or NOM from Suwanne River, purchased from IHSS.
Fig. 5 Electron-transfer process from the electron donor (humic acid) to the electron acceptor (bromate) via Fe(III).
2.4 Comparision of reduction capacity of different humic acid sources
3 Conclusions
The reducing activity of humic acid from different sources was compared. Humic acid from Aldrich, Suwannee River humic acid (standard) 2S101H IHSS, and Suwanne River natural organic matter (NOM) (RO isolation) 1R101N IHSS were used for the above purpose. As Fig.4 shows, under the same condition of fixed humic acid concentration, Fe(III) concentrations, and pH, humic acid from Aldrich achieved much lower bromate reduction rate compared with humic acid or NOM from the Suwanne River, both of which has similar reducing reactivity to bromate as observed. Research is currently underway to characterize the mechanisms involved. On the basis of the results presented above, an electron-
The reduction capacity of humic acid and the effect of complexed Fe(III) in redox reaction considering bromate as target pollutant were examined in this study. The Fe(III)humic acid complexes could reduce bromate to bromide under acidic condition. The humic acid was observed to regenerate Fe(II) and reduce bromate abiotically. The addition of Fe(III) was observed to accelerate the bromate reduction rate. According to the results, an electrontransfer scheme was proposed. The reduced functional groups in the humic acid form 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. Kinetics study on bromate
No. 3
Effect of Fe(III) on the bromate reduction by humic substances in aqueous solution
reduction further indictated that pH was a controlling factor for the redox capacity of humic acid-Fe(III) complexes. The above reaction process is expected to occur in the environment, since the reducing capacity of Aldrich humic acid was observed to be lower than that of humic acid or NOM from Suwanne River. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50608056), the Hong Kong Research Grants (No. HKUST6106/03E), and the Program for Young Excellent Talents in Tongji University in part (No. 2006KJ033). They are gratefully acknowledged.
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