Citrate Concentrations and Ratio on the

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Effect of Fe(III)/Citrate Concentrations and Ratio on the Photoproduction of Hydroxyl Radicals: Application on the Degradation of Diphenhydramine Xiaonan Feng,† Zongping Wang,† Yong Chen,*,† Tao Tao,† Feng Wu,‡ and Yuegang Zuo§ †

School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, People’s Republic of China ‡ School of Resources and Environmental Science, Wuhan University, Wuhan, 430079, People’s Republic of China § University of Massachusetts-Dartmouth, 285 Old Westport Road, North Dartmouth, Massachusetts 02747-2300, United States S Supporting Information *

ABSTRACT: Fe(III)-citrate complexes are photoreactive and ubiquitous in natural waters. In this study, the effect of Fe(III)/ citrate concentrations and ratio on the photoproduction of •OH as a function of pH (3−9) was systematically investigated. The • OH formation mechanism was elucidated according to the pH-dependent formation rate of •OH and the speciation distribution analysis of Fe(III) species. At high Fe(III)-to-citrate ratio (10:50), the •OH photoproduction increased with decreasing pH. In contrast, the •OH photoproduction increased in the order of pH 9.0 < 3.0 < 7.0 < 6.0 < 5.0 at low Fe(III)-to-citrate ratios (10:100−10:300). At identical Fe(III)-to-citrate ratio (1:10), high concentration of Fe(III)-citrate complexes rendered a downward trend for •OH production with increasing pH. FeOHcit− is the predominant reactive species responsible for the •OH formation at high pH. The optimal pH for •OH production was governed by the amount of O2• − and the stability of Fe(II) species in the Fe(III)-cit solution. The Fe(III)-cit-induced photodegradation of diphenhydramine verified the pH-dependent trend for •OH production. By GC-MS and LC-ESI-MS analyses, the photoproducts of diphenhydramine were identified and the degradation pathway was proposed.

1. INTRODUCTION Both citric acid and iron are naturally occurring substances, and they are ubiquitous in natural waters. Fe(III)-cit complexes can undergo ligand-to-metal charge transfer (LMCT) process and the subsequent oxygen-related radicals cycling to generate Fe(II) and reactive oxygen species (ROS) such as HO2•/O2• −, H2O2, and •OH.1,2 The photoinduced •OH is significant for the elimination of organic substances from natural or anthropogenic sources in surface waters.3 Several studies have reported the photoreduction ability and photoreactivity of Fe(III)-citrate complexes due to their environmental significances.1,4−9 However, most of these studies were focused on the photoreactivity of Fe(III)-citrate in weak acid circumstances. The photoproduction of •OH and the corresponding photodegradation of organic pollutants at near neutral pH are not highlighted. Balmer and Sulzberger10 reported that the photodegradation rate of atrazine followed the order of pH 7.5 < 5.6 < 3.2 < 4.3 in Fe(III)-oxalate solution at Fe(III)-to-oxalate ratio of 6:18 (μM), whereas it became pH 7.9 < 3.2 < 4.6 ≈ 5.4 at Fe(III)to-oxalate ratio of 6:180 (μM) in the Fe(III)-oxalate solution. A similar result was obtained for the photodegradation of 2,4-D,11 indicating that the Fe(III)-to-oxalate ratio affected the optimal pH for the photodegradation of substrates. In the photoFenton system, the presence of carboxylate also enhanced the treatment efficiency of pollutants at circumneutral pH.12,13 Therefore, it is crucial to investigate the effect of Fe(III)-toligand molar ratio on the photoreactivity of Fe(III)-carboxylate complexes. The study of photoreactivity, in turn, contributes to © 2012 American Chemical Society

elucidate the underlying photochemical mechanism of the complexes. In our previous study, the photoproduction of •OH was assessed in the Fe(III)-cit system.6 It was observed that •OH production increased with decreasing pH within the pH range of 3.0−7.0, and the production sharply decreased at pH >4.0 when the Fe(III)-to-citrate ratio was 30:30 (μM).6 Likewise, for the other Fe(III)-carboxylate complexes (e.g., Fe(III)-oxalate, pyruvate, tartaric), a similar phenomenon was observed.14−16 Despite the high photoreactivity of these Fe(III)-carboxylate complexes, it is still difficult to use them at the ratios studied in near-neutral pH range. It is well-known that, in the Fenton reaction, the efficient pH for •OH production is within the range of 2−4, because of the negligible concentration of Fe2+ at pH >4. For Fe(III)carboxylate complexes, •OH is finally formed via a Fenton or Fenton-like reaction. Therefore, it is undoubtedly reasonable that the optimal pH falls in the same range (pH 2−4) in the Fe(III)-carboxylate system, assuming the •OH production is governed by the Fenton reaction. However, Fe(II) is capable of being present as the Fe(II)-carboxylate species at pH >4 in the presence of enough carboxylate ligand, which outcompetes hydroxide ions for complexation with Fe(II) in aqueous solution. Therefore, Fenton-like reactions can occur in the Received: Revised: Accepted: Published: 7007

February 9, 2012 April 19, 2012 May 6, 2012 May 7, 2012 dx.doi.org/10.1021/ie300360p | Ind. Eng. Chem. Res. 2012, 51, 7007−7012

Industrial & Engineering Chemistry Research

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temperature, 320 °C; drying gas (N2) flow, 10 L/min; nebullizer pressure, 40 psi; trap drive, 30.0; skimmer, 40.0 V; Octopole RF amplitude, 143.6 V; capillary exit, 110.1; maximum accumulation time, 200 ms; ion charge control (ICC), 200 000. 2.5. Equilibration calculation. The fraction of Fe(III) species in Fe(III)-cit solution was calculated according to the equilibrium constants data from Medusa, which is a chemical equilibrium calculation program (version: 18 Feb. 2004).

Fe(III)-carboxylate system in the presence of enough carboxylate ligand. Therefore, the solution pH for •OH production can be extended to near neutral. Fe(III)-cit complexes has higher cumulative stabilization constants (log β = 14.29), compared to Fe(III)-oxalate (log β = 5.52),17 thereby rendering it more likely to outcompete hydroxide ions for Fe(III)/Fe(II) at high pH. In the present study, the pH-dependent •OH photoproduction was determined in the presence of Fe(III)-citrate complexes by adjusting the Fe(III)-to-cit molar ratio or the concentration of the complexes with identical Fe(III)-to-citrate molar ratios. Diphenhydramine, which is a widely used amine drug,18 was chosen as the model pollutant to probe the corresponding effect of the Fe(III)-to-citrate ratio on the photodegradation. Moreover, the photoproducts of diphenhydramine were identified by GC-MS and LC-MS analysis.

3. RESULTS AND DISCUSSION 3.1. •OH Production at Constant Concentration of Fe(III) or cit. The photoproduction of •OH was determined in the Fe(III)-cit system with the addition of benzene as an •OH scavenger in the solution within the pH range of 3.0−9.0 (see Figure S1 in the Supporting Information). The Fe(III)-to-cit molar ratio was varied by increasing the concentration of Fe(III) or cit at fixed concentration of cit or Fe(III), respectively. As shown in Figure 1a, the •OH photoproduction increased in the order of pH 9.0 < 3.0 < 7.0 < 6.0 < 5.0 at the Fe(III)-to-cit ratios of 10:100, 10:200, and 10:300. Similar results have been reported in the Fe(III)-oxalate system by

2. MATERIALS AND METHODS 2.1. Chemicals. Diphenhydramine hydrochloride (99%) was purchased from Acros and used as received. Ferric chloride (99%), trisodium citrate dehydrate (cit, 99%), and benzene (99%) were obtained from Fengchuan Chemicals Corporation (Tianjin, PRC). All solutions were prepared with deionized water. 2.2. Photochemical experiments. The photochemical experiments were performed in a 60-mL capped cylindrical Pyrex vessel (40 mm i.d., containing 50 mL solution). The light source was a 150-W xenon short arc lamp. The light, which had wavelengths of 4. From the UV−vis spectra of Fe(III)-cit complex (see Figure S3 in the Supporting Information), the precipitation of Fe(III) was not observed, although ferric oxyhydroxide is thermodynamically expected at pH >7.0. It was readily understood that the downward trend of •OH production with increasing pH, since, for the Fenton reaction, the optimal pH is 2−4. At pH >4, Fe2+ is rapidly oxidized into Fe3+ to cause Fe(III) precipitation in the absence of coordination ligand. At high Fe(III)-to-cit ratio (e.g., 10:50), the depletion of cit during LMCT processes (Figure 2) resulted in the loss of ligand. Subsequently, Fe3+ is present as forms of Fe(III)−OH complexes. Accordingly, lower pH facilitated •OH production at high Fe(III)-to-cit ratio in the Fe(III) system. At low Fe(III)-to-cit ratio (e.g., 10:100−10:300), the concentration of cit is far excessive. Although a depletion of cit occurs 7009

dx.doi.org/10.1021/ie300360p | Ind. Eng. Chem. Res. 2012, 51, 7007−7012


Article

photodegradation of diphenhydramine in the Fe(III)-cit solution followed pseudo-first-order kinetics (see Figure S4 in the Supporting Information). Figure 4 illustrates the observed rate constant of photodegradation of diphenhydramine as a function of Fe(III)-to-cit ratios at different pHs. The trend of photodegradation is in good agreement with that of •OH photoproduction, suggesting that •OH is the predominant reactive oxygen species responsible for photodegradation of diphenhydramine in the Fe(III)-cit solution. In addition, the increase of photodegradation did not show linear relationship with the increase of Fe(III)-cit concentration. This can be interpreted by the fact that cit is the quencher of •OH in the reaction solution.21 Although the •OH reaction rate constant of diphenhydramine is higher than that of cit, the increasing cit concentration disfavored the •OH oxidation of diphenhydramine. 3.4. Effect of Sequential Addition of cit on Photodegradation. The discussion above shows that the Fe(III)-tocit ratios and concentrations exhibited significant effect on the photoproduction of •OH and degradation of diphenhydramine at different pH values. To further elucidate the ratios and concentrations effect, the photodegradation of substrate was studied in the Fe(III)-cit solution with equal amount of concentration of Fe(III) and cit but the different addition methods of cit. As shown in Figure 5a, the photodegradation of

during LMCT processes (Figure 2), the remaining cit was capable to outcompete hydroxide ions for complexation with Fe(III) or Fe(II) at higher pH (pH >4). As shown in Figure 3 and Figure S2 in the Supporting Information, Fe(III) species are mainly present as FeOHcit− and Fe2(OH)2(cit)22− at pH >4. In the Fe(III)-cit-photo system, the formation of •OH is strongly related to H2O2, which is produced by the reaction between Fe(II) and O2• −/HO2•. In comparison with HO2•, O2• − are more favorable for the generation of H2O2.11 The pKa for HO2•/O2• − equilibrium is 4.8,24 suggesting that O2• − accounts for 99% in the solution at pH >6.8. Thus, higher pH (until pH 6.8) facilitates the formation of O2• −. However, as the increase of solution pH, Fe(II)/Fe(II)-cit are more easily oxidized into Fe(III) by the dissolved oxygen in the aqueous solutions.27 Therefore, on one hand, the increase of solution pH favored the production of O2• − until pH >6.8; meanwhile, it is unfavorable for the stable presence of reduction-state Fe(II) and Fe(II)-cit. The contrary contribution of solution pH to the Fenton or Fenton-like reaction led to an optimal pH of 5.0 for the production of •OH in Fe(III)-cit solution with low Fe(III)to-cit ratios (e.g., 10:100−10:300). For the identical Fe(III)-to-cit ratio (1:10), higher concentration of Fe(III)-cit complexes led to a higher fraction of photoinert Fe2(OH)2(cit)22− (see Figure S2 in the Supporting Information). For example, the fraction of FeOHcit − decreased from 86.7% to 52.2% when the concentration of Fe(III)-cit complexes increased from the ratio 10:100 to 100:1000 (see Figure S2 in the Supporting Information). For Fe(III)-to-cit ratio 100:1000, more cit is present as photoinert Fe2(OH)2(cit)22−, and thus the relative amount of cit in FeOHcit− decreases. Therefore, even at identical Fe(III)-to-cit ratio, increase of concentration of complexes disfavored the formation of •OH at higher pH in the Fe(III)-cit solution. 3.3. Photodegradation of Diphenhydramine. The bimolecular rate constant for the reaction between diphenhydramine and •OH was determined to be (7.1 ± 0.2) × 109 M−1 s−1,28 indicating that diphenhydramine reacts at near diffusioncontrolled limit with •OH. In addition, diphenhydramine did not undergo direct photodegradation under simulated sunlight. It was thus chosen as model pollutant to demonstrate the effects of Fe(III)-to-cit ratios on •OH photoproduction in the Fe(III)-cit solutions. Representative ratios including 10:50, 10:300, 100:300, and 100:1000 were examined (Figure 4). The

Figure 5. Concentration decay of diphenhydramine (a) and cit (b) during irradiation in Fe(III)-cit solutions at pH 6.0: (▲) [Fe(III)]0/ [cit]0 = 10/300 μM; (●) [Fe(III)]0/[cit]0 = 10/50 μM with the addition of cit at the time interval of 30 min until the total amount of cit added was 300 μM.

diphenhydramine was markedly more rapid at the initial Fe(III)-to-cit ratio 10:300 than that at the initial ratio 10:50 with a six-times sequential addition of cit. Although the total amount of cit added in the solution was finally equal to 300 μM, the sequential addition was unfavorable for the photodegradation. However, as the decay of cit in the Fe(III)-cit solution at Fe(III)-to-cit ratio 10:300 and the accumulation of cit due to the sequential addition (Figure 5b), the rate

Figure 4. Photodegradation of diphenhydramine (30 μM) at different Fe(III)-to-cit ratios as a function of pH. Error bars indicate 95% confidence intervals for n = 3. 7010



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difference gradually reduced after 60 min. This result confirmed the significant effect of Fe(III)-to-cit ratio and concentration on photodegradation. Meanwhile, supplementary addition of cit was necessary to maintain high photoreactivity and continuous degradation of substrates. 3.5. Photoproducts and Degradation Pathway. In order to get further insight into the photodegradation of diphenhydramine at near neutral pH, photoproducts of diphenhydramine in Fe(III)-cit solution were identified by GC-MS and LC-ESI-MS analysis. The products for photodegradation of diphenhydramine were similar at the reaction time of 30, 60, and 90 min (data not shown). Figure S5 in the Supporting Information illustrates the HPLC chromatograms and (+)-ESI-MSn spectra of diphenhydramine and its photodegradation products. Four products were identified with retention time 6.0, 6.9, 7.7, and 9.9 min, respectively. At 6.0 min, the m/z of the photoproduct was 304 with the fragment ion of 290. This product was proposed to result from cleavage and oxidation of the benzene ring by the attack of •OH. For the products at 6.9, 7.7, and 9.9 min, they have the same molecular weight with m/z 272. As shown in Figure S5 in the Supporting Information, the products at 6.9 and 9.9 min exhibited similar mass spectra of fragment ions with m/z 183 and 90, whereas only one fragment ion was found for the product at 7.0. Therefore, the molecular structures for products 6.9 and 9.9 are similar. According to the fragment ions analysis, the •OH addition for the two products occurred in the benzene rings, while for the product 7.7 min the •OH likely attacked at the branched chain of diphenhydramine. The low-molecular-weight photoproducts are easily evaporated and detected by GC-MS. Those intermediates formed in Fe(III)-cit system were shown in Table S1 of the Supporting Information. All of these compounds were unequivocally identified using the NIST98 library database, and the comparison of mass spectra between standard compounds and the photoproducts is illustrated in Figure S6 of the Supporting Information. Those products were attributed to the further oxidation followed by the •OH addition reactions. As shown in Figure 6, the photodegradation in Fe(III)-cit solution proceeded by the •OH addition reactions at the benzene rings and branched chain of diphenhydramine. Further attack of the • OH led to cleavage of the branched chain and benzene rings of diphenhydramine, resulting in the generation of some benzene series compounds such as hydroquinone, 3-hydroxybenzoic acid, benzene-1,2,3-triol, 4-hydroxybenzoic acid, 1,2-benzenedicarboxylic acid, and 1,4-benzenedicarboxylic acid. The further oxidation of those benzene series compounds gave rise to lowmolecular-weight carboxylic acids.

Figure 6. Photoproducts and the proposed pathway for photodegradation of diphenhydramine in the Fe(III)-cit solutions.

efficiently degraded in Fe(III)-cit solution at near-neutral pH. Photodegradation of diphenhydramine occurred via •OH addition reactions, followed by cleavage of branching chain and benzene rings to generate low-molecular-weight carboxylic acids.

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ASSOCIATED CONTENT

S Supporting Information *

Six figures and one table on the determination of hydroxyl radicals, the fraction of Fe(III) species, UV−vis spectra of Fe(III)-cit complexes, photodegradation kinetics of diphenhydramine, and determination of the photoproducts. This material is available free of charge via the Internet at http:// pubs.acs.org.

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4. CONCLUSIONS The •OH photoproduction in Fe(III)-cit solutions at various Fe(III)-to-cit ratios was pH-dependent. The •OH photoproduction increased with decreasing pH within the range of 3.0−9.0 at high Fe(III)-to-cit ratio (10:50). In contrast, •OH production increased in the order of pH 9.0 < 3.0 < 7.0 < 6.0 < 5.0 at low Fe(III)-to-cit ratios (10:100−10:300). At identical Fe(III)-to-cit ratio (1:10), high concentration of Fe(III)-cit complexes rendered a downward trend for •OH production with increasing pH. FeOHcit− is the predominant reactive species responsible for the •OH photoproduction in Fe(III)-cit solution at high pH. The optimal pH for •OH production was governed by the amount of O2• − and the stability of Fe(II) species in the Fe(III)-cit solution. Diphenhydramine was

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-27-87792406. Fax: +86-27-87792101. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21007018 and 51078161), Natural Science Foundation of Hubei Province (No. 2010CDB01104), Chenguang Youth Found of Wuhan (No. 201050231074), and Research Fund for the Doctoral Program of Higher Education of China (No. 20100142120004). 7011



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Citrate Concentrations and Ratio on the

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