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Photoinduced Transformation of γ-HCH in the Presence of Dissolved Organic Matter and Enhanced Photoreactive Activity of Humate-Coated r-Fe2O3 Hongbo Fu, Xie Quan,* Zhaoyang Liu, and Shuo Chen School of Environmental and Biological Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, Liaoning Province, P. R. China Received December 23, 2003. In Final Form: March 14, 2004 This study examined phototransformation of γ-hexachlorocyclohexane (γ-HCH) in different solutions. The presence of dissolved organic matter (DOM) inhibited the phototransformation of γ-HCH. This phenomenon could be correlated to the binding interaction between γ-HCH and DOM. R-Fe2O3 promoted the transformation of γ-HCH. The humate-coated R-Fe2O3 revealed a slight, however significant, favorable effect compared to the bare one. Fourier transform infrared spectroscopy (FTIR) offered the direct evidence that humate-coated R-Fe2O3 could form surface Fe(III)-carboxylate complexes by ligand exchange. Additional experiments demonstrated that the photocorrosion of R-Fe2O3 coated by DOM was much more acute than that of the bare one. These combined results suggested that the transformation of γ-HCH on humate-coated R-Fe2O3 is more related to a surface complex and not to a semiconductor-assisted photoreaction. In the humate-coated R-Fe2O3, absorption of a photon results in an excited ligand-to-metal charge-transfer state of the complexes, and a rich variety of free radical reactions ensue, which is concurrently accompanied by the dissolution of the iron oxide. Such reactions may generate reactive transients such as superoxide and hydroxyl radicals, which may be expected to transform γ-HCH.
1. Introduction Over the recent decade, heterogeneous photocatalysis involving photoinduced redox reactions at the surface of semiconductor minerals is a rapidly developing field of investigations.1-3 Iron oxides are widespread in the natural environment and are important components of suspended materials in natural water.4,5 Most of the iron oxides and oxyhydroxides show semiconductor properties. Photochemical transformations catalyzed by iron oxides have also been extensively investigated.6-12 R-Fe2O3 is stable in the transformation of sulfites,6 oxalates, chlorophenols,1 aminophenols,7,8 azo dyes,9 bisbiphenyl,10 atrazine,11 and halogenated acetic acids.12 Our recent survey displays that total iron contents in natural sediments or soil have remarkable effect on photodegradation of γ-HCH, and a significant positive correlation was found between iron oxide contents and the photodegradation rate constant of the target pollutant.13 Dissolved organic matter (DOM), such as fulvic and humic acids, is another prevalent constituent of natural * Corresponding author. Fax: 86-411-4706-263; tel.: 86-4114706-140; e-mail:
[email protected]. (1) Bandara, J.; Mielczarski, J. A.; Lopez, A.; Kiwi, J. Appl. Catal., B 2001, 34, 321. (2) Hyunjoo, L.; Wonyong, C. Environ. Sci. Technol. 2002, 36, 3872. (3) Zaleska, A.; Hupka, J.; Wiergowski, M.; Biziuk, M. J. Photochem. Photobiol., A 2000, 135, 213. (4) Waite, T. D.; Morel, F. M. M. Environ. Sci. Technol. 1984, 18, 860. (5) Voelker, M. B.; Francols, M. M.; Morel, Sulzberger, B. Environ. Sci. Technol. 1997, 31, 1004. (6) Faust, B. C.; Hoffmann, M. R.; Bahnemann, D. W. J. Phys. Chem. 1989, 93, 6371. (7) Pulgarin, C.; Kiwi, J. Langmuir 1995, 11, 519. (8) Andreozzi, R.; Caprio, V.; Marotta, R. Water Res. 2003, 37, 3682. (9) Bandara, J.; Mielczarski, J. A.; Kiwi, J. Langmuir 1999, 15, 7680. (10) Janet, M. K. T.; Stephan, J. H. Environ. Sci. Technol. 1999, 33, 3171. (11) Marion, L.; Reinhard, N. Environ. Sci. Technol. 2002, 36, 5342. (12) Simo, O. P.; Ronald, L. S.; Michael, R. H. Environ. Sci. Technol. 1995, 29, 1215. (13) Quan, X.; Niu, J. F.; Chen, J. W. Chemosphere 2003, 52, 1749.
water. The photolysis of aquatic organic contaminants can be influenced by the presence of DOM in several ways. On one hand, direct photolysis can be inhibited because of the competitive absorption for available photos by colored DOM.14 On the other hand, ultraviolet irradiation (UV) also induces a variety of photochemical changes in DOM and leads to production of reactive oxygen species (e.g., singlet oxygen, peroxy radicals).4,5 These reactive chemical species could oxidize the organic contaminants. Owing to the twofold role of DOM, photochemical fate of chemical contaminants in the presence of DOM may differ significantly.14 Therefore, there is a need to expand the number of organic pollutants whose photodegradation is examined in the presence of DOM, and this was one of the purposes of this study. On the other hand, many observations have demonstrated that DOM could interplay with iron oxide to form a surface complex.15 It is clear that the structure of the complex influences the photochemical behavior of the oxide bulk. Faust and co-workers6 used laboratory experiments to define that there are at least two potential chromophores in surface photochemical reactions, both the bulk solid and surficial complexes, when S(IV) complexes onto the surface of hematite. Both iron oxides and DOM are the prevalent constituent of natural waters. In addition, iron oxides are likely to be coated by DOM prior to an influx of contaminates in many instances. A thorough investigation of photoreactivity of humate-coated iron oxides is warranted. Organochlorine compounds constitute an important category of water pollutants. Among these, γ-HCH, marketed under the generic name of lindane, had been most extensively applied to control many leaf-eating insects throughout the world during the last century.16 Because of its toxicity and nonbiodegradability, its use (14) Zeng, K.; Hwang, H.; Yu, H. Int. J. Mol. Sci. 2002, 3, 1048. (15) Gu, B.; Schmit, J.; Chen, J. Environ. Sci. Technol. 1994, 28, 38. (16) Font, J.; Marsal, A. J. Chromatogr., A 1998, 811, 256.
10.1021/la0364486 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/06/2004
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has been restricted by most countries, whereas the past decade or more saw the existence of γ-HCH in the surface of water in the north of China.17 So far, only a few studies in a laboratory scale have reported the photochemical degradation of γ-HCH involving TiO2.3,18,19 To our knowledge, there is a dearth of information concerning phototransfromation of γ-HCH catalyzed by iron oxides and the influence of DOM on this photochemical process. On the basis of these considerations, the principal objective of this paper is the study of phototransformation of γ-HCH by iron oxide coated by DOM to inspect the effect of the Fe(III)-humate complexes likely formed on the surface of iron oxide. The influence of DOM on transformation of γ-HCH is the other topic. The present study was conducted using a well-defined crystalline iron oxide phase (R-Fe2O3) and two DOM samples. FTIR was employed to study the interaction mechanism between DOM and R-Fe2O3. 2. Experimental Section 2.1 Chemicals and Materials. R-Fe2O3 was synthesized according to a previously published procedure.20 X-ray diffraction, which was obtained with a Philips PW 1820 X-ray diffractometer, was identical with R-Fe2O3 standards and rounding of the baseline spectra indicated the presence of trace amounts of amorphous iron oxides. Its BET specific surface area was 9.8 m2 g-1 and a pHpzc near 8.4. γ-HCH (ACS reagent grade, purity g99.0%) was purchased from National Station of Environmental Monitoring of China without further purification. A primary stock of 4000 mg/L was prepared in acetone and a substock of 5 mg/L was gained by dilution of the primary stock solution in high-purity water and was used after 2 days to achieve almost complete dissolution/ equilibration. Acetone has been known to be resistant to degradation in reactions involving hydroxyl radical.3 The stock solutions were prepared monthly and stored in the dark at 277 K in a refrigerator. 2.2 Collection and Preparation of DOM Samples. The humic acid (HA) was gained as a gift, which was extracted from the fallen needle-leave soils. The sample sites were situated in Jilin province, China. The fulvic acid (FA) was extracted from leonaradite in Xinjiang province, China. The FA extraction procedure followed the protocol of the International Humic Substance Society (IHSS), with some modifications.21 2.3 Preparation of the Humic-Coated R-Fe2O3. The humate-coated R-Fe2O3 samples were prepared by adding the desired amount of humate solution (10 mg/L) to 1 g R-Fe2O3. The solution was subsequently adjusted to the desired pH by the addition of 0.1 mol/L HCl or NaOH. After gentle shaking on a rotator for 6 h to gain the equilibrium, the mixtures were filtrated by 0.1 µm filter. The desired amount of filtrate was resuspended by deionized water preadjusted to the corresponding pH. 2.4 Photochemical Experiments. Sample solutions (300 mL) were magnetically stirred and exposed to UV light from a 300 W high-pressure mercury lamp, which was positioned within the inner part of the photoreactor. Cooling water was circulated through a quartz jacket surrounding the lamp. The lamp emitted a wide band of radiation in the full wavelength of UV lamp and did not employ additional filters or monochromators. All experiments were performed at room temperature. The working solutions were adjusted by 0.1 mol/L HCl or NaOH. The reactor (17) Gawlik, B. M.; Platzer, B.; Muntau, H. Final Report: INCODC; Contract No. IC 18 CT 970166; Muntau, Europe Comission, Joint Research Center, Environmental Institute, EUR 15931 EN 2000. (18) Vidal, A. Chemosphere 1998, 36, 2593. (19) Dionysiou, D. D.; Khodadoust, P. A.; Kern, M. A.; Suidan, T. M. Appl. Catal., B 2000, 24, 139. (20) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63, 509. (21) Ussiri, A. D.; Johnson, C. E. In Humic Substances: Structures, Models and Functions; Ghabbour, E. A., Davies, G. R. Soc. Chem.; 2001; Vol. 4, p 281.
Fu et al. was characterized using a potassium ferrioxylate actinometer as described by John et al.22 Standard preparation techniques were applied to obtain TEM of the colloidal particles. A drop of colloidal sol was mounted on a carbon-coated copper grid and evaporated at room temperature. The grid was studied using a Hitachi-600 transmission electron microscope. Aqueous iron is quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkins-Elmer Plasma 40 Emission Spectrophotometer). 2.5 Analytical Methods. Two mL of the sample were extracted from the reactor. The samples were sequentially added to 10 mL of acetone:petroleum ether (1:1; v/v) and immediately sealed to avoid any loss of the analyte, agitated on a rotary for 20 min, followed by sonicating for 30 min. The sample was then transferred into 30-mL separatory funnel. The organic phase was collected, and the aqueous phase was further extracted twice with 5-mL petroleum ether. The collected organic phase was combined and dried with a small amount of anhydrous Na2SO4 and finally analyzed by gas chromatograph (GC). The recovery of γ-HCH at three levels 0.2, 0.5, and 1 mg/L is 78%, 89%, and 92%, respectively. These levels were selected because they covered the range of this study. A Hewlett-Packard (Hewlett-Packard Co., Avondale, PA) 6890 GC equipped with 63Ni electron capture detector, a split/splitless injector operated in the splitless mode, and a fused-silica capillary HP-5 column (30 m × 0.32 mm × 0.25 µm, Hewlett-Packard Co.) was used. Operating conditions were as follows: initial column temperature 403 K (1 min), increased at 10 K/min to 513 K (11 min); injector temperature 523 K; detector temperature 623 K; carrier gas helium at a flow-rate 1 mL/min; injection volume 1 µL. Duplicate measurements were made for each sample. An HP chemstation A.05.02 software was used for instrument control and data process. 2.6 FTIR Study. The sample was prepared by a mixture of DOM and R-Fe2O3 at the desired solution conditions, stirring for 6 h, followed by drying directly. All samples were stored in a desiccator until analyzed by FTIR spectrometer. Analytical grade KBr was first heated at 398 K to remove the adsorbed water, and then the oxide samples were mixed with KBr after cooling in a ratio of 1 to 100, followed by compressing the mixture to pellets and investigating spectroscopically by the FTIR spectrometer (Nicolet, 20DXB). FTIR spectra were obtained for a wavenmuber range of 4000-400 cm-1.
3. Results 3.1 Phototransformation of γ-HCH. Effect of DOM. γ-HCH was first irradiated in aqueous solution in the presence of DOM; samples for quantitation of γ-HCH were collected over a period of 12 h. Experiments were conducted with 1 mg/L initial γ-HCH concentration and 10 mg/L concentration of both the HA and the FA. The results are shown in Figure 1. The presence of DOM slowed the transformation of γ-HCH. The FA produced less amount of rate reduction than the HA. The experiments were also performed with 1 mg/L initial γ-HCH but with various concentrations of the DOM samples. The data were plotted as the natural log of γ-HCH concentration (ln [γ-HCH]) versus irradiation time. All experiments produced linear plots of ln(C0/C) versus t (C0 is the initial concentration, C is the concentration at time t), indicating that the photochemical degradation of γ-HCH was fitted for pseudo-first-order kinetics. In all cases, the presence of DOM slowed the transform rate. For example, in the HA, the following rate constants were available: 0.076, 0.059, 0.049, and 0.043 h-1 for solutions with 0, 5, 10, and 20 mg/L HA, respectively. Figure 2 shows the rate data from these experiments. One can see that increasing DOM concentration decreases the slope of the rate plots, indicating a decreasing reaction rate. (22) Bachman, J.; Patterson, H. H. Environ. Sci. Technol. 1999, 33, 874.
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Figure 1. Phototransformation of γ-HCH in the presence of both the HA and the FA induced by UV light, pH ) 4.3. Direct phototransformation (1); 10 mg/L FA (b); 10 mg/L HA (9).
Figure 2. Plots of ln[C0/C] vs irradiation time for γ-HCH phototransformation experiments were carried out in the presence of the HA. The concentration of the HA: 5, 10, and 20 mg/L, pH 4.3. Direct phototransformation (9); 5 mg/L HA (b); 10 mg/L HA (2) and 20 mg/L HA (1).
Effect of the Humate-Coated R-Fe2O3. To inspect the effect of humate-coated R-Fe2O3, a series of photochemical experiments were performed in various solutions. Figure 3 showed the removal of γ-HCH in UV, UV/R-Fe2O3, and UV/humate-coated R-Fe2O3 systems at different pH (2.8, 3.7, and 4.8). At a given pH, for example, at pH 2.8 (shown in Figure 3), there are several obvious trends. The presence of R-Fe2O3 promoted the transformation of γ-HCH, compared to the direct phototransformation. Although 41% of γ-HCH was removed after 6 h of direct phototransformation, more than 56.4% was removed in the UV/R-Fe2O3 system. Dark control experiments were carried out along R-Fe2O3 light activated runs. No degradation was observed during runs in the dark in the preequilibrated suspensions (R-Fe2O3/ γ-HCH). This indicates that the presence of R-Fe2O3 results in an enhanced phototransformation. Interestingly, comparing the humate-coated R-Fe2O3 and the bare one result, the removal rate was slightly larger for the humate-coated R-Fe2O3 in all cases. For example, the result indicates that the loss of γ-HCH was 57.2% and 59.5% for R-Fe2O3 coated by the HA and the FA, respectively, after 6 h UV light exposure. This result clearly demonstrated that the humate-coated R-Fe2O3 could phototransform γ-HCH more
Figure 3. The phototransformation of γ-HCH (1 mg/L) in the presence of R-Fe2O3 (250 µM/L) coated by both the HA and the FA at pH 2.8, 3.7, and 4.8. Direct phototransformation (b); R-Fe2O3 (9); R-Fe2O3 coated by the FA (2) and coated by the HA ([).
rapidly. It was also found that R-Fe2O3 coated by the FA transformed γ-HCH at the faster rate than the one coated by the HA. Curves similar to those at pH 2.8 are obtained at pH 3.7 and pH 4.8. Clearly, the effect of R-Fe2O3 on the phototransformation of γ-HCH was pH-dependent. Increasing pH from 2.8 to 4.8 decreased the transformation of γ-HCH. The transformation rate of γ-HCH by the humate-coated R-Fe2O3 was also reduced but still higher than that of the bare R-Fe2O3 system. 3.2 Interaction of r-Fe2O3 and DOM Studied by FTIR. The FTIR was employed to explore the surface
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Table 1. Assignment of FTIR Absorption Bands of the DOM Samples DOM samples
wavenumber (cm-1)
peak
FA
3406 2919 1717 1606 1399 1217
broad band small peak small peak broad peak small peak broad peak
H-bonds; OH groups C-H asymmetric; C-H stretch of -CH; CdO of COOH CdO of COOH CdC in aromatic structure; COOCOO- (sym-streching) C-O stretch of aliphatic OH
HA
3295 2923 1659 1384 1211
broad band two peaks broad band small peak small peak
H-bonds; OH groups C-H asymmetric; C-H stretch of -CH; CdO of COOH CdC in aromatic structure; CdO of COOH (asym-streching) COO- (sym-streching) C-O stretch of aliphatic OH
assignment
Figure 4. FTIR spectra of the FA before and after interplay with R-Fe2O3 at pH 2.8 (A1, A2), pH 3.7 (B1, B2), and pH 4.8 (C1, C2).
Figure 5. FTIR spectra of the HA before and after interplay with R-Fe2O3 at pH 2.8 (A1, A2), pH 3.7 (B1, B2), and pH 4.8 (C1, C2).
characteristics of humate-coated R-Fe2O3. These spectra were compared with the ones of unloaded oxide samples to determine any changes observed, which included the emergence of new peaks, intensity variations of peaks originally present, and wavelength shifts. The spectra A1 (in Figure 4 and Figure 5) are typical of those generally shown for humic substances extracted from soil. The features of the band for these matters are at 3200-3600 cm-1 for phenolic O-H stretching and at 1720-1700 cm-1 for carboxylic CdC stretching. These bands, which might represent binding sites, were clearly observed in the spectrum of the DOM samples. Table 1 shows the main absorbance bands in FTIR spectra of the DOM samples. At pH ) 2.8, both the carboxylic acid and carboxylate groups were jointly present, as shown by the spectra A1 in Figure 4. The following assignment was straightforward: ν(CdO) ) 1717 cm-1, νasym(COO) ) 1606 cm-1, and νsym(COO) ) 1399 cm-1. When the FA interplay with R-Fe2O3, shown by the spectra A2 in Figure 4, the intensity of the peak of 1717 cm-1 became lower and the one of the peak around 1600 cm-1 became higher, the stretching band of carboxylate COO- shifted from 1606 to 1615 cm-1, and synchronously, the peak at 1399 cm-1 disappeared whereby a new strong and pointed band at 1384 cm-1 appeared, which indicated that a part of carboxylic acid was deprotonated by the ligand exchange with the oxide surface species ()-FeOH2+, )-FeOH) and ultimately was transformed to carboxylate. This assignment was supported by the fact that a strong absorption band at about 1400 cm-1 appeared when DOM interacted with Fe atoms to
form the complexes.15 Also, similar changes of the spectra could be found at pH 3.7 (Figure 4, B1 and B2) and 4.8 (Figure 4, C1 and C2). The FA spectra in the absence of iron oxides display a decrease in intensity at the band around 1700 cm-1 and a small increase in intensity at the bands around 1600 and 1400 cm-1, as may illuminate that more carboxylic acid was deprotonated to carboxylate. Additionally, the binding of the FA by R-Fe2O3 was pHdependent, and an increase in pH was accompanied by a decreased intensity of the peak at 1384 cm-1, suggesting that the amount of the Fe(III)-humate complexes became lower, as reported in the literature.15 In the HA (Figure 5), the same general trends are observed. 4. Discussion 4.1 Effect of DOM on Phototransformation of γ-HCH. The rate constants clearly show that the presence of DOM slows the transformation of γ-HCH. It was postulated that the decrease in the rate of transformation could be due either to DOM competing with γ-HCH for the available photons or to binding between γ-HCH and the humate molecular. Competing photochemical reactions were initially investigated as the cause of the measured rate effect. Figure 6 shows the change of UVvis spectrum of the HA before and after exposure to light. It is clear that DOM does react in the presence of light (Figure 6). Additionally, it has been reported elsewhere that DOM can react photochemically.14 However, the amount of light needed for γ-HCH reaction is small relative to the total flux from the lamp. For example, at the highest HA concentration used (20 mg/L), it can be determined
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Table 2. Relevant Reactions Involved in the Phototransformation of γ-HCH in the present studya no.
-+
hvb+
ref
T1 T2 T3 T4 T5 T6 T7 T8
R-Fe2O3 + hν f ecb hν > R-Fe2O3 energy band gap ecb- + hvb+ f heat OHads + hvb+ f ‚OH H2Oads + hvb+ f ‚OH + H+ O2ads + ecb- f O2ecb- + )-Fe(III) f )-Fe(II) )-Fe(II) + hvb+ f Fe3+ )-Fe(II) + O2ads f Fe3+
T9 T10 T11 T12 T13
DOM + hν f DOM* *DOM T 3DOM* 3DOM* + P f DOM + P 3DOM*f DOM + heat 3DOM* + Q f DOM
T14 T15 T16
passive effect of DOM on the transformation of γ-HCH L + DOM f L-DOM L-DOM + hν f L*-DOM L*-DOM f L-DOM*
T17 T18
)-FeOH2+ T )-FeOH + H+ )-FeOH T )-FeO- + H+
T19 T20 T21 T22 T23 T24 T25 T26
related reaction of humate-coated R-Fe2O3 )-Fe(III) - DOM + O2 + hν f )-Fe(III) - DOM* )-Fe(III) - DOM*f )-Fe(III)* - DOMox f )-Fe(II) - DOMox )-Fe(II) - DOMox f Fe2+ - DOMox DOM + O2 + hν f DOMox + HO2/O22HO2/O2- f H2O2 + O2 H2O2 + hν f OH + OHHO2/O2- + Fe(II) f Fe(III) + H2O2 2HO2/O2- + Fe(III) f Fe(II) + O2
T27
FeOH2+ + hν f Fe2+ + OH
T28 a
generation of charge carriers and semiconductor
L + OH f Product
k ) 1.1 × 107
9 8
E° ) -0.33 V
2 7
k ) 6.0 × 10-9 s-1
DOM sensitization Eg < 250 kJ/mol
reaction of surficial sites of R-Fe2O3
iron-related reactions phototransformation of γ-HCH
k ) 6.70 k ) 10.4
24 24 24 24 24 23 22
30 30
32 k ) 1.40 × 10-10 (pH ) 3) φ (yield %) ) 47.6 k ) 49.8 M-1 s-1 (pH ) 3)
5 5 5
φ ) 0.017 (360 nm) k ) 1.2 × 10-13 cm3 s-1
31
), P, and L are represented as the solid surface, pollutant, and γ-HCH, respectively.
Figure 6. UV-visible spectrum of the HA (20 mg/L), initial solution (A), and the solution irradiated for 3 h (B).
through photometry experiments that the HA absorbed 39.5% of the total flux while a 1 mg/L solution of γ-HCH absorbs 7.8% of the total flux. Thus, in the presence of the HA there remains a sufficient amount of photons in the system for the phototransformation of γ-HCH. Under the conditions of our experiments (radiant flux, respective concentrations, reactor geometry, etc.), it is not possible to account for the total of rate decrease with a competing photochemical pathway. Alternatively, the second mechanism to describe our photochemical results has been considered. When γ-HCH and DOM existed in solution together, a certain fraction of the pollutant is possibly bound to DOM. It is believed that binding occurs predominantly through a hydrophobic partitioning mechanism.23 Representing the bound γ-HCH
as L-DOM, the equilibrium can be written as T14 in Table 2, which shows the relevant reactions involved in the present study. DOM absorbs solar energy mostly between 300 and 500 nm. Absorption of irradiation in this region can initiate several photochemical processes, producing peroxy and hydroxyl radicals as well as hydrated electrons, hydrogen peroxide, singlet oxygen, and superoxide.14 These chemical species are reactive with organic contaminants. Many investigations14 displayed that the absorption of light by DOM can lead to rapid photosensitized reactions of many pollutants via energy transfer from molecules in their triplet states (T9-T11). However, Zepp et al.24 found that they had a limitation involving photosensitized reactions of humic substances, namely, only when the pollutants with triplet state energies are less than 250 kJ/mol can these photoreactions be sensitized by humic molecular. Otherwise, 3DOM* will be deactivated by a variety of pathways, such as decay to ground state (T12) or other interaction that quenches 3DOM* (T13). In the present study, the sensitization effect of DOM was not found. Furthermore, the transformation of γ-HCH was clearly retarded by DOM. As mentioned previously, it is believed that the binding mechanism is hydrophobic partitioning. This hydrophobic binding mechanism draws the pollutant molecular into an aggregate of humic molecules. This arrangement allows the pollutant molecular transfer the excess energy resulting from the absorption of a photon (23) Nicholas, H.; Malcolm, N. J.; Edward, T. Anal. Chim. Acta 1996, 327, 191. (24) Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Environ. Sci. Technol. 1985, 19, 74.
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to the surrounding DOM molecules, which greatly promote photoreaction of the humic molecular whereas prohibit the transformation of γ-HCH (T15 and T16).22 4.2 Effect of Humic-Coated r-Fe2O3 on Phototransformation of γ-HCH. R-Fe2O3 is seen to behave as expected as an active photocatalyst. Adsorption of a photon with energy greater than the band gap of the iron oxide leads to the formation of an electron hole pair (h+vb/e-cb) (T1). The valence band hole of R-Fe2O3 (h+vb)[EH ) 2.3 V] is a powerful oxidant (T3 and T4). The conduction band electron (e-cb) [EH ) 0.0] is a relatively poor reductant. ecb- does not react with oxygen (T5) but it may lead to dissolution of R-Fe2O3 (T6-T8). Compared with TiO2, catalytic power of R-Fe2O3 is generally poor because of the quick h+vb/e-cb recombination (T2), which is possibly because of a high density of intrinsic midband gap electronic states, internal defect induced trap states, and, to a lesser extent, surface defects.1 The increase in pH accompanied by the decrease rate may be due to the transformation of R-Fe2O3 to amorphous iron oxide, which could lead to a lower ‚OH production rate. Furthermore, the semiconductor particles are more liable to aggregation at higher pH, thus having the lower photoabsorption efficiency and the less adsorption amount of γ-HCH because of a decrease surface area of R-Fe2O3, and consequently the retarded transformation of γ-HCH. In the humic-coated R-Fe2O3, the transformation rate of γ-HCH was slightly but significantly larger. The surface Fe(III)-humate complexes may account for the enhanced photoreactive activity. The ability of FAs to sensitize colloidal ZnO and TiO2 has been demonstrated.25,26 By irradiation, long-life electrons, having the reductive power and possible contribution to the photodegradation of pollutants, can be observed at the semiconductor surface. The reason could be that FA adsorbed onto the oxide act as scavengers of hvb+, thus slowing down the recombination of the electron hole pair. In the current work, γ-HCH could be potentially transformed by the reductive route because of its polychlorinated structure and relatively high redox potential.27 However, e-cb formed by irradiating R-Fe2O3, unlike ones in TiO2 and ZnO, is a relatively poor reductant. According to the potential levels, a reaction between e-cb and O2 bound to surface is not possible (T5), as the potential necessary for the reaction O2/O2•- (E ) -0.16 eV) is higher than e-cb (E ) 0.02 eV). 7,33 Therefore, transformation of γ-HCH by a long life of e-cb is unlikely. On the other hand, DOM can form surface Fe(III)-humate complexes, which was already confirmed by FTIR results. Many observations have shown that the Fe(III)-humate complexes can undergo a ligand-to-metal charge transfer when exposed to UV (T19), which leads to the formation of excited states of the complex and reactive transients produced. This process is accompanied by the Fe(III)-Fe(II) circling (T25 and T26) and a variety of free-radical reactions (T22T24), resulting in the dissolution of the oxide (T20 and T21). Such a process could concurrently generate super(25) Selli, E.; Giorgi, A. D.; Bidoglio, G. Environ. Sci. Technol. 1996, 30, 598. (26) Selli, E.; Baglio, D.; Montanarella, L.; Bidoglie, G. Water Res. 1999, 33, 1827. (27) Liu, X.; Peng, P.; Fu, J.; Huang, W. Environ. Sci. Technol. 2003, 37, 1822. (28) Gao, H.; Zepp, G. R. Environ. Sci. Technol. 1998, 32, 2940. (29) Enriquez, R.; Pichat, P. Langmuir 2001, 17, 6132. (30) Herrera, F.; Lopez, A.; Mascolo, G.; Albers, P.; Kiwi, J. Appl. Catal., B 2001, 29, 147. (31) Brubaker, J. R.; Hites, R. A. Environ. Sci. Technol. 1998, 32, 766. (32) Fukushima, M.; Tatsumi, K.; Nagao, S. Environ. Sci. Technol. 2001, 35, 3683. (33) Bandara, J.; Tennakone, K.; Kiwi, J. Langmuir 2001, 17, 3964.
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Figure 7. Dissolution of R-Fe2O3 (250 µM/L) in different solutions at pH 3.0. R-Fe2O3 coated by the FA irradiated by UV (9); R-Fe2O3 irradiated by UV (b); R-Fe2O3 coated by the FA in the dark (2) and R-Fe2O3 in the dark (1).
oxide radicals, hydrogen peroxide, or hydroxyl radicals (T22-T25). 4,5,27 Therefore, it may be expected that γ-HCH degrades primarily through reaction with these species (T32). To confirm the above hypothesis, additional investigations on the photodissolution of R-Fe2O3 in the different system irradiated by UV were conducted. The results are shown in Figure 7. The data display that almost no dissolution takes place during the time in the dark. When exposed to light, some formation of dissolution Fe was observed in R-Fe2O3. Dissolved Fe accumulated significantly when the humate-coated R-Fe2O3 system was irradiated. To elucidate the changes in crystal morphology during irradiation, the suspended particles were taken from the reactor and then examined with TEM (Figure 8). One can see that the crystallite boundaries become highly irregular and thinner (Figure 8A) compared to the original one (Figure 8B), which was very smooth and flat. Especially, as for the smaller crystals, it is difficult to identify their initial morphology. Because of the above considerations plus these facts, we have reason to assume that the complex formed on R-Fe2O3 surfaces is a candidate to promote the phototransformation of γ-HCH. This process does not involve charge separation in a photocatalyst, which is essential in a semiconductor-type photoreaction (the bare R-Fe2O3 system). The transformation of γ-HCH could selectively occur physically near the Fe(III)-humate complexes, which seem to be a center for the reactive species. However, the humate molecular adsorbed by the oxide could create a microenvironment around the oxide particles. In this microenvironment, γ-HCH molecules are thus trapped to diffuse to the bulk solution and, accordingly, would increase their probability to react with the active species photogenerated on the oxide surface. This could be the other potential reason for the enhanced rate of transformation.29 Although FeOH2+ may be formed by the dissolved Fe, it likely plays a minor, if any, role in the transformation of γ-HCH (T27), because Fe3+ ordinarily complexes with humate molecular as well as their photolysis products, thus forming FeOH2+ in insignificant yields in the current study. 4.3 The Possible Structures of the Fe(III)-Humate Complexes and the Interaction Mechanism of Iron Oxides and DOM. The FTIR spectroscopy alone proved
Photoinduced Transformation of γ-HCH
Langmuir, Vol. 20, No. 12, 2004 4873
Figure 10. Possible surface complexes formed between DOM and R-Fe2O3.
of Fe3+ with DOM; the possible structures of the complexes are shown in Figure 10. FTIR data show that the peak around 1600 cm-1, which is contributed to the asymmetric CdO stretching in COOgroups, varied slightly in frequency and intensity when the spectra of DOM were disturbed by R-Fe2O3 at different pH. This may be understood because there coexists different types of surficial complexes. Thus, it is difficult to further speculate the structure information of the complexes by FTIR only. The attempt to link their structures to the photoreactivity was especially an arduous challenge. 5. Conclusions Figure 8. The image of TEM, the original R-Fe2O3 colloids (A), and the ones coated by the FA exposed to UV for 12 h (B).
Figure 9. Possible surface group configurations for hydrolyzed iron oxide surface (ref 6).
that the interaction mechanism is conformed to the ligand exchange involving carboxylic functional groups of the DOM samples and the surfaces sites of R-Fe2O3, which has been described by many authors.6,7,15,32 The role of surface hydroxyl groups as the principal reactive sites on iron oxides surfaces has been clearly established.6,7 Even though there are several possible surface sites (Figure 9), the iron oxide surface species are noted as )-FeOH2+, )-FeOH, and )-FeO- to simplify the question. So, the surface hydrolysis reactions for R-Fe2O3 can be written as T17 and T18. When pH < pHzpc (i.e., 8.4), the surface charge is dominated by the concentration of )-FeOH2+ and )-FeOH active sites, and provided that the surface remains positively charged, the overall reaction rates are predicted to increase. However, above pH 8 (pHzpc ) 8.4), the particle surfaces have a net negative charge due to the increase in the concentration of )-FeO-.32 The carboxyl functional groups of DOM form very stable complexes with many metal cations or hydroxy metal cations.15 The complexation reactions are also supported by the studies with simple model organic compounds. The modes of ligand exchange interactions between iron oxide and DOM may thus be postulated analogous to the binding
In addition to the negative effect of DOM on the phototransformation of γ-HCH, which could be due to the complex formation as a result of binding between DOM and γ-HCH, a positive effect has been revealed when it coated on the surface of R-Fe2O3. According to the control experiments, it does not seem that this beneficial effect can be attributed to the bulk oxide. On the basis of FTIR data indicating that DOM can interact with R-Fe2O3 and form a surface complex by ligand exchange, we tentatively suggest that this favorable effect results from the Fe(III)-humate complexes. These complexes can undergo a ligand-to-metal charge transfer; thus, the formation of a series of reactive species such as superoxide and hydroxyl radicals may be expected to transform γ-HCH. Clearly, this favorable effect of the surface complex and its interpretation can be extended to other compounds whose properties are similar to γ-HCH. In the aquatic system that both DOM and iron oxide coexist, the balance between the positive effect and the negative effect of DOM on the phototransformation of the pollutant can be principally due to the amount of DOM. When the amount of DOM greatly exceeded surface site saturation of the iron oxide, DOM will mask the valuable function of the surface complexes and even show an adverse effect as a radical scavenger, an absorber for available light, or a complex formation with the pollutant molecular. If, on the other hand, abundant reactive iron is available but with little reactive organic matter, DOM will play a positive role by the surface complex formed on the oxide surface. Acknowledgment. The authors would like to thank National Natural Science Foundation (P. R. China, No. 20177003) for financial support. LA0364486
