Environ Sci Pollut Res (2014) 21:6228–6233 DOI 10.1007/s11356-014-2553-0
RESEARCH ARTICLE
Photodegradation of hexabromocyclododecane (HBCD) by Fe(III) complexes/H2O2 under simulated sunlight Danna Zhou & Yao Wu & Xiaonan Feng & Yong Chen & Zongping Wang & Tao Tao & Dongbin Wei
Received: 5 November 2013 / Accepted: 12 January 2014 / Published online: 1 February 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Hexabromocyclododecane (HBCD) is a globally produced brominated flame retardant used primarily as an additive flame retardant in polystyrene and textile products. Photodegradation of HBCD in the presence of Fe(III)-carboxylate complexes/H2O2 was investigated under simulated sunlight. The degradation of HBCD decreased with increasing pH in the Fe(III)-oxalate solutions. In contrast, the optimum pH was 5.0 for the Fe(III)-citrate-catalyzed photodegradation within the range of 3.0 to 7.0. For both Fe(III)-oxalate and Fe(III)-citrate complexes, the increase of carboxylate concentrations facilitated the photodegradation. The photochemical removal of HBCD was related to the photoreactivity and speciation distribution of Fe(III) complexes. The addition of H2O2 markedly accelerated the degradation of HBCD in the presence of Fe(III)-citrate complexes. The quenching experiments showed that ·OH was responsible for the photodegradation of HBCD in the Fe(III)-carboxylate complexes/H2O2 solutions. The results suggest that Fe(III) Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-2553-0) contains supplementary material, which is available to authorized users. D. Zhou College of Material Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, China Y. Wu : X. Feng : Y. Chen (*) : Z. Wang (*) : T. Tao School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China e-mail:
[email protected] e-mail:
[email protected] D. Wei State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100084, People’s Republic of China
complexes/H2O2 catalysis is a potential method for the removal of HBCD in the aqueous solutions. Keywords HBCD . Photodegradation . Fe(III)-carboxylate complexes . H2O2
Introduction Hexabromocyclododecane (HBCD) is the principal flame retardant used in extruded and expanded polystyrene, upholstery textiles, and electrical equipment housings (Marvin et al. 2011; Covaci et al. 2006). Due to the bioaccumulation, longrange transport, environmental persistence, and potential toxicity, HBCDs have been recently listed as persistent organic pollutants (POPs) in the Stockholm Convention at its sixth meeting (Zhu et al. 2013). HBCDs have been widely detected in effluent of wastewater treatment plants (WWTPs), sediments, soil, air, freshwater and marine fish, aquatic invertebrates, birds, and even in human adipose tissue, milk, and blood (Carignan et al. 2012; Klosterhaus et al. 2012; Li et al. 2012; Wang et al. 2013; Munschy et al. 2013; Gorga et al. 2013; Zhang et al. 2013). Nevertheless, to our best knowledge, there are few studies involving the removal of HBCDs (Zhou et al. 2012). Progress in the chemical treatment of pollutants in water and wastewater led to the development of advanced oxidation processes (AOPs). They mainly rely on the formation of reactive and oxygen-containing intermediates such as hydroxyl radicals (·OH). Although these processes often have high capital and operating costs, they are the only viable treatment methods for refractory, toxic, and nonbiodegradable pollutants. The current studies on POPs removal are mainly focused on the UV/TiO2-based techniques. Fe(III)-carboxylate catalysis is based on the utilization of solar energy and expected to be a promising technique for
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Materials Hexabromocyclododecane, isopropanol, and chloroform were purchased from Sigma and used as received. Ferric chloride (99 %), oxalate (ox), and trisodium citrate (cit) dehydrate (99 %) were obtained from Fengchuan Chemical Corporation (Tianjin, China). Stock solution of HBCD was prepared using acetonitrile and diluted to the desired concentration by deionized water for photolysis experiments. All chemicals used were of at least analytical reagent grade. Photolysis experiments The photolysis experiments were performed in a 60-mL capped cylindrical Pyrex vessel (40 mm i.d., containing 50-mL solution) under 150-W Xenon short-arc lamp. The light of wavelengths less than 300 nm was filtered with the Pyrex glass to simulate the sunlight. Lamp output was monitored over time by ferrioxalate actinometry (Hatchard 1956). The ·OH and O2·− quenching experiments were carried out with addition of 50 mM isopropanol and chloroform, respectively (Latch et al. 2003; Huang et al. 2013). All solutions for the photolysis experiments were freshly prepared prior to irradiation. Aliquots of samples were withdrawn at various intervals, and substrate decay was measured by gas chromatography.
Results and discussion Effect of pH HBCD did not undergo noticeable direct photodegradation under simulated sunlight since it has no noticeable absorption at wavelengths above 300 nm (Fig. S1). The effect of solution pH on the photodegradation of HBCD in the presence of Fe(III)-ox and Fe(III)-cit complexes was investigated at the Fe(III)-to-carboxylate ratio of 10:500 (μM). As shown in Fig. 1, the degradation of HBCD is increasing with decreasing pH in the Fe(III)-ox solutions. In contrast, the optimum pH for the removal of substrate was 5.0 in the presence of Fe(III)-cit complexes within the range of 3.0 to 7.0. The pH trend of HBCD photodegradation was consistent with our previous studies on the other pollutants in the same system (Chen et al. 2007, 2011, 2013; Feng et al. 2012). The quantum yields of Fe(II) for the Fe(ox)2− and Fe(ox)33− complexes are 1.0 and 0.6, respectively, at 436 nm (Faust and Zepp 1993), indicating that Fe(III)-ox complexes with more ligand were less photoreactive. Figure 2 illustrates the effect of pH on the Fe(III) speciation distribution. It indicates that Fe(ox)33− predominated gradually with increasing pH in the range of 3.0–7.0 (Fig. 2a). Correspondingly, the photoreactivity of Fe(III)-ox complexes decreased when the solution pH was raised (Fig. 1), which was consistent with the previous studies. Fe(III) exists in the forms of Fecit, FeOHcit − , and Fe2(OH)2cit22− in the Fe(III)-cit solutions within the pH range of 3.0–7.0, of which the main photoreactive species are the Fecit and FeOHcit−. The main Fe(III) species complexes shift from Fecit at low pH to FeOHcit− and Fe2(OH)2cit22− at pH> 4 (Fig. 2b). It is reported that Fecit and FeOHcit − Fe(III)-ox Fe(III)-cit
0.30 0.25 0.20 -1
Materials and methods
with an initial temperature of 80 °C held for 2 min and ramp at 30 °C min−1 to 180 °C, ramp at 5 °C min−1 to 230 °C held for 10 min, and then at 20 °C min−1 to 299 °C held for 7 min.
kobs (h )
degradation of pollutants due to the relatively low cost. It is well established that Fe(III)-carboxylate complexes undergo ligand-to-metal charge transfer (LMCT) process upon irradiation, followed by generation of reactive oxygen species (ROS) such as HO2·/O2·−, H2O2, and ·OH (Zuo and Hoigné 1992, 1993, 1994). The organic compounds can be completely mineralized to carbon dioxide and water mostly by hydroxyl radicals generated in situ in the reaction solutions. The objective of this study was to investigate the photodegradation of HBCD in the presence of Fe(III)-carboxylate complexes under simulated sunlight. The effects of Fe(III)-to-carboxylate ratio, solution pH, concentration of carboxylate, and addition of H2O2 were investigated. The reaction mechanism was examined by the radical quenching experiments.
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0.15 0.10 0.05
GC analysis
0.00 2
GC analysis was carried out on a Shimadzu GC-2014C system equipped with an electron capture detector (ECD) with a WonderCap 5 capillary column (0.25 μm, 0.25 mm×30 m). The GC was operated in a temperature programming mode
3
4
5
6
7
8
pH
Fig. 1 Photodegradation of HBCD in the presence of Fe(III)-ox and Fe(III)-cit complexes at different pH. Initial conditions: [HBCD]0 = 1.0 μM and [Fe(III)]0/[carboxylate]0 =10/500 μM
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(a)
1.0
Fraction of Fe(III) species
Fe(C2O4)2
-
Fe(C2O4)3
3-
Fe(OH)4 Fe(OH)3
0.8 0.6 0.4 Fe(C2O4)
0.2 Fe(OH)2
2
4
6
8
+
10
pH
(b) 1.0 -
FeOHcit 3+
Fraction of Fe(III) species
0.8
Fe
ð8Þ
FeðIIÞcit− þ H2 O2 →FeðIIIÞcit þ ⋅OH þ OH−
ð9Þ
where cit, 3-HGA2−·, and 3-OGA2− represent the citrate ion, 3-hydroxo-glutarate radical, and 3-oxo-gulutarate, respectively. The photoproduction of ·OH was governed by the Fe(III) species, Fe(II)/Fe(II)cit− stability, and generation of O2·−. The formation of O2·− was correlated with the production of H2O2 (Eqs. 6 and 7) and thereby affecting the yield of ·OH (Eqs. 8 and 9). The higher pH facilitates the formation of O2·− and thus the photoproduction of ·OH in the Fe(III)-cit solutions within pH 6.8. Meanwhile, the Fe(II)/Fe(II)cit− was more readily oxidized by dissolved oxygen with increasing pH according to Eq. 10 (Willey et al. 2005):
+
0.0
FeðIIÞ þ H2 O2 →FeðIIIÞ þ ⋅OH þ OH− -
Fe(OH)4
-
Fecit
0.6
−
0.4
dFeðIIÞ ¼ k ½FeðIIÞpO2 ½OH− 2 dt
ð10Þ
Fe(OH)3 aq
0.2
Fe2(OH)2(cit)2
2-
+
FeHcit
Therefore, the increase of pH was unfavorable for the oxidation of Fe(II)/Fe(II)cit− by H2O2. The contrary effect on the production of ·OH led to the optimal pH 5.0 in this experiment at Fe(III)-to-cit ratio of 10:500 (μM).
0.0 2
4
6
8
10
pH
Fig. 2 The fraction of Fe(III) species based on the stability constants of Fe(III)-complexes species as a function of pH ([Fe(III)]0/[carboxylate]0 = 10/500 μM; data of stability constants is from Medusa soft, and formation of iron precipitation was omitted)
1.0
Fe(III):ox=10:50 Fe(III):ox=10:150 Fe(III):ox=10:500
photochemically produce ·OH via the following reactions (Hartwick 1957; Bielski et al. 1985; Rush et al. 1985; Deng et al. 1998; Hug et al. 2001): ⋅
⋅
LMCT
cit2− þ O2 →3 OGA2− þ CO2 þ O2 ⋅
3 HGA2− þ O2 →3 HGA2− þ O2 ⋅−
Hþ þ O2 ↔HO2
⋅
⋅− 2Hþ
FeðIIÞ þ O2 → FeðIIIÞ þ H2 O2 Hþ
FeðIIÞ þ HO2 ⋅ → FeðIIIÞ þ H2 O2
0
ð1Þ
FeOHcit þ hν → FeðIIÞ þ 3 HGA2− ⋅
0.6
0.4
⋅−
⋅−
ð2Þ
1.0
ð3Þ
0.8
ð4Þ ð5Þ
ð7Þ
80
120
160
200
Fe(III):cit=10:50 Fe(III):cit=10:150 Fe(III):cit=10:500
0.6
0.4 0
ð6Þ
40
Time (min)
C/C0
LMCT
Fecit þ hν → FeðIIÞ þ cit2−
C/C0
0.8
40
80
120
160
200
Time (min) Fig. 3 Photodegradation of HBCD (1.0 μM) in the presence of Fe(III)oxalate (pH 3.0) and Fe(III)-citrate (pH 5.0) at different Fe(III)-to-carboxylate ratios
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Fe(III)-cit-H2O2=10:150:200
1.0
1.0
Fe(III)-cit-H2O2=10:0:200 Fe(III)-cit-H2O2=10:150:0
0.8
C/C0
C/C0
0.8
0.6
Fe(III):cit:H2O2=10:150:200
0.6
Fe(III):cit:H2O2=10:150:400 Fe(III):cit:H2O2=10:150:600
0.4
Fe(III):cit:H2O2=10:150:800
0.4 0
20
40
60
80
0
20
40
Time (min)
60
80
Time (min)
Fig. 4 Photodegradation of HBCD (1.0 μM) in the presence of Fe(III) solutions at different conditions
Fig. 6 Effect of H2O2 concentrations on the photodegradation of HBCD (1.0 μM) in the Fe(III)-cit-H2O2 system at pH 3.5
Effect of carboxylate concentration
irradiation, as shown in Eqs. (11–13) (Mulazzani et al. 1986; Hislop and Bolton 1999; Jeong and Yoon 2005):
The concentration of carboxylate concentrations affects the Fe(III) species and thus the photoreactivity of the Fe(III)carboxylate complexes. As shown in Fig. 3, the increase of both ox and cit concentrations facilitated the photodegradation of HBCD. Figures S2 and S3 illustrate the effect of ox and cit concentrations on the speciation distribution of Fe(III) species at the Fe(III)-to-ligand ratio of 10:50, 10:150, and 10:500 (μM). The Fe(III) exists as Fe(ox)2− and Fe(ox)33− at pH 3.0, and the proportion of Fe(ox)33− increases with increasing concentration of ox from 50 to 500 μM (Fig. S2a–c). As mentioned above, Fe(ox)2− displays higher photoreactivity compared to Fe(ox)33− (Faust and Zepp 1993). Nevertheless, the increase of ox concentration and correspondingly the fraction of Fe(ox)33− did not cause the decrease of Fe(III)-ox system. On the contrary, the degradation of HBCD increased with increasing concentration of ox (Fig. 3a). It can be explained by the fact that the ox was depleted during the photochemical processes. Like Fe(III)-cit complexes, ·OH is generated via the formation of O2·− in the Fe(III)-ox solution under
1.0
Fe(III)-cit-H2O2=10:50:200 Fe(III)-cit-H2O2=10:150:200
C/C0
0.8
Fe(III)-cit-H2O2=10:300:200
2FeðC2 O4 Þn 3 2n ⋅
⋅
LMCT
þ hν → 2Fe2þ þ ð2n 1ÞC2 O4 2− þ C2 O4 − ð11Þ
C2 O4 − →CO2 þCO2 −
⋅
⋅
ð12Þ ⋅
CO2 − þ O2 →CO2 þO2 −
ð13Þ
At the initial step, Fe(III)-ox solution undergoes ligand-tometal charge-transfer (LMCT) to yield C2O4·−, which decomposes subsequently and transfers the electron to dissolved oxygen. The ligand ox was gradually depleted in the photoreactions. Although the fraction of Fe(ox)2− was higher at the Fe(III)-to-ligand ratio of 10:50 compared to the other ratios, the decrease of ox concentration during the reaction processes rendered it unfavorable for the photoproduction of ·OH. Likewise, cit was depleted during the Fe(III) reduction and production of O2·− in the Fe(III)-cit solution (Eqs. 1–3). Accordingly, the photodegradation of HBCD increased with increasing concentration of cit in the Fe(III)-cit solutions (Fig. 3b). The depletion of ligand in both systems led to OH anions Table 1 The pseudo-first-order rate constants k obs (h−1) for the photodegradation of HBCD under different conditions
0.6
Conditions
Fe(III)-oxalate
Fe(III)-citrate
Fe(III)-citrate-H2O2
0.4
Blank CHCl3 2-propanol
0.221±0.026 0.188±0.028 0.027±0.004
0.206±0.018 0.186±0.022 0.085±0.006
0.588±0.031 0.530±0.028 0.144±0.007
0
20
40
60
80
Time (min) Fig. 5 Effect of citrate concentrations on the photodegradation of HBCD (1.0 μM) in the Fe(III)-cit-H2O2 system at pH 3.5
Results are the means of triplicate measurements. Initial conditions: [HBCD]0 =1.0 μM, [Fe(III)]0 =10 μM, [oxalate]0 =[citrate]0 =150 μM, [H2O2]0 =200 μM, pH=3.5, and the concentration of quenchers was 50 mM
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competitively complex with Fe(III) and thus resulted in the decrease of photoreactivity of Fe(III)-carboxylate complexes. Effect of H2O2 on the photoreactivity of Fe(III)-cit complexes The Fe(III)-cit complexes exhibited considerable photoreactivity at low Fe(III)-to-ligand ratio (e.g., 10:150– 10:500). Figure 4 illustrates the effect of H2O2 on the photodegradation of HBCD in the presence of Fe(III)-cit complexes. The addition of H2O2 in the Fe(III)-cit solution markedly accelerated the degradation. After 80 min, up to 54.8 % HBCD was removed with the addition of H2O2, which is approximately twofold of the degradation efficiency of the control experiment. Compared to Fe(III), Fe(III)-cit was more photoreactive and easily reduced to Fe(II) species. Furthermore, the redox potential of Fe(III)cit/Fe(II)cit (E0Fe(III)cit/ Fe(II)cit) is 0.34 V, which is much lower than that of Fe(III)/ Fe(II) (E0Fe(III)/Fe(II) =0.77 V) ( Pham and Waite 2008). It indicated that Fe(II)-cit is more readily oxidized compared to Fe(II) by H2O2. Accordingly, the degradation of HBCD was more rapid in the Fe(III)-cit-H2O2 than in the Fe(III)-H2O2. For the Fe(III)-cit-H2O2 system, the exogenous concentration of H2O2 was far higher than that generated by Fe(III)-cit itself (Eqs. 6 and 7). Correspondingly, the degradation of HBCD was enhanced in the Fe(III)-cit solution with the addition of H2O2 (Fig. 4). The effect of cit concentration was further examined in the Fe(III)-cit-H2O2 solutions. As shown in Fig. 5, the increase of cit concentration facilitated the photodegradation of HBCD. The trend was consistent with the Fe(III)-cit-catalyzed degradation. More cit favored the formation of Fe(III)-cit instead of Fe(III)-OH complexes before and during the photochemical reactions. Meanwhile, the effect of H2O2 concentration on the degradation of HBCD was investigated. Figure 6 indicates that the degradation was accelerated with the increase of H2O2 concentration within the range of 200 to 800 μM. The concentration effect was noticeable with the H2O2 concentration below 600 μM. The further addition of H2O2 did not promote the reaction (Fig. 6). It is probable that the reaction amount of H2O2 is enough for the Fenton and Fenton-like reactions (Eqs. 8 and 9) within 600 μM. Photodegradation mechanism The photodegradation mechanism of HBCD in the Fe(III)-ox, Fe(III)-cit, and Fe(III)-cit-H2O2 solutions was investigated. According to the equations discussed above, O2·− and ·OH were likely to be responsible for the Fe(III)-complex-catalyzed photodegradation of HBCD. 2-propanolol and CHCl3 are efficient ·OH and O2·− quenchers, respectively (Latch et al. 2003; Huang et al. 2013). As shown in Table 1, the addition of
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O 2 · − scavenger CHCl 3 did not noticeably affect the photodegradation of HBCD. In contrast, the addition of ·OH markedly inhibited the reaction in the three photochemical systems. Therefore, ·OH was largely responsible for the photodegradation of HBCD in the presence of Fe(III)-ox, Fe(III)-cit, and Fe(III)-cit-H2O2.
Conclusions HBCD was efficiently degraded in the presence of Fe(III)-ox and Fe(III)-cit complexes. The degradation of HBCD decreased with increasing pH in the Fe(III)-ox solution, while the removal increased in the order of pH 3.0
