Adsorption and sequential degradation of polybrominated diphenyl ethers with zerovalent iron

Journal of Hazardous Materials 260 (2013) 844–850

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Adsorption and sequential degradation of polybrominated diphenyl ethers with zerovalent iron Yu-Huei Peng, Mei-kuei Chen, Yang-hsin Shih ∗ Department of Agricultural Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan, ROC

h i g h l i g h t s • • • • •

A detailed analysis in the removal mechanism of PBDEs by MZVI was conducted. 13% of DBDE and 24% of BDE-3 in average were adsorbed on the surface of MZVI. The adsorbed PBDEs on MZVI were also confirmed by FTIR spectroscopy. MZVI continues the reductive debromination of PBDEs even after one month. MZVI has great longevity to adsorb and degrade PBDE in the environment.

a r t i c l e

i n f o

Article history: Received 6 March 2013 Received in revised form 5 May 2013 Accepted 30 May 2013 Available online 6 June 2013 Keywords: Polybrominated diphenyl ethers Microscale zerovalent iron Adsorption Degradation

a b s t r a c t The widely used flame retardants, polybrominated diphenyl ethers (PBDEs), have been regulated owing to their persistence and toxicity. However, the high and increasing accumulation amount of PBDEs in the environment raises a big concern for public safety. In this study, the removal processes of decabromodiphenyl ether (BDE-209) and monobromodiphenyl ether (BDE-3) with microscale zerovalent iron (MZVI) were investigated to get better understandings for the removal mechanism based upon adsorption and degradation. The removal kinetics of both compounds was analyzed and revealed two-step kinetics: a fast removal step at the beginning of the reaction and a follow-up slow removal step. By-products generated during the entire process followed a stepwise sequence. The content of brominated compounds on the surface of MZVI was measured. About 10–20% of BDE-209 and 15–30% of BDE-3 were adsorbed on MZVI. The adsorption of BDE-209 and BDE-3 on MZVI was confirmed through the Fourier transform infrared spectroscopy. Surface adsorption of PBDEs on MZVI dominates the removal mechanism in the beginning and further debromination with MZVI was found. Finally, about 70% of BDE-209 and 60% of BDE-3 was degraded by MZVI within about one month. Our findings provide evidences for understanding the removal mechanism of PBDEs with MZVI and its great longevity on the PBDE degradation, which can facilitate the remediation design. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Polybrominated diphenyl ethers (PBDEs) are widely used flame retardants which have been integrated into many industrial and daily use products for over three decades. Since PBDEs are not chemically bound with other components in the products, they are prompt to leak out during the process of manufacture, usage or discard. Therefore, they were detected in many kinds of environmental samples, such as dusts, water bodies and sediments all over the world [1–3]. PBDEs are lipophilic with low solubility in water and hence easily accumulate in biota through the food chain [4–7]. They

∗ Corresponding author. Tel.: +886 2 33669443; fax: +886 2 33669443. E-mail address: [email protected] (Y.-h. Shih). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.05.057

are harmful for the endocrine system, liver, neuron development, immune system and reproduction system in humans and mammals [8–12]. Although the usage of most congeners was regulated, their accumulation in the environment still exponentially increases because of the long-time utilization [6,13]. Therefore, PBDEs constitute a major concern for human health and one main target in environmental remediation. PBDEs can be degraded by physical, biological or chemical ways. The photodegradation efficiency is pretty good [14–17]; however, the feasibility of solar irradiation to pollutants in soils and sediments can be a limitation. The biodegradation of PBDEs to less brominated congeners has been reported for sediment microcosms, mixed consortia or pure microbial strains [18–22]. Although naturally evolved biodegradation mechanisms are cheaper than other remediation processes, their efficiencies are relatively lower and

Y.-H. Peng et al. / Journal of Hazardous Materials 260 (2013) 844–850

the type of target compounds is restricted. Recently, micro- or nano-scale ZVI (MZVI or NZVI) have been found can reductively degrade PBDEs under anoxic conditions [23–25]. Electrons offered from iron can reduce the PBDE compounds through reductive debromination. The reaction could be expressed by the following equation:

Instrument Corporation, Norcross, GA, USA). A mixture of 22 PBDE congeners (TBDE-79X) containing five to nine bromine atoms was obtained from Wellington Laboratories (Guelph, Canada). Diphenyl ether (DE, MW = 170.2, 99% purity) was obtained from Acros Organics (Geel, Belgium). Hexachlorobenzene purchased

O

O

+ Br x

845

Br y

n/2 Fe

o

+ Br x- n

2+

+ n Br

-

Br y

The removal by NZVI was much faster than that by equal amount of MZVI due to its larger surface area. The reactivity of NZVI to BDE-209, decabrominated diphenyl ether, was higher in an acid condition as compared to that under different initial pH conditions ranged from 5 to 10 [23]. However, NZVI may not be more reactive than MZVI on a surface area normalized basis for BDE-209 [23] and other compounds [26] may due to the overestimated NZVI surface area in water. Furthermore, NZVI has more impacts on ecosystems than MZVI and was unstable. Bare NZVI interrupted the transduction of energy and enhanced generation of reactive oxygen species, resulting in bactericidal effect [27]. It also inhibited biodegradation through affecting the expression of responsible enzymes [28]. In addition, bare NZVI easily aggregated under environmental condition and thus the reactivity reduced [29,30]. In contrast, MZVI is cheaper than NZVI and was ready-for-use and easily to be handled [31,32]. It also showed the synergistic effect when combined with biodegradation mechanisms in treating PBDEs or other contaminants [33–36]. Therefore, there were still many advantages for choosing MZVI as a remediation material. The efficiency of ZVI treatment on halogenated contaminants is highly dependent on the properties of the metal surface because reduction reaction mainly occurs on the surface of iron. The surface of ZVI in water usually coated with a mixed-valent iron oxides, which have been reported as contaminant sorbents [37,38]. We have reported that BDE-209 removals by adsorption on NZVI after 2 hr were 18% under initial pH 5 and 48% under initial pH 8 [23]. Due the rapid reactions of BDE-209 and water with NZVI, aqueous pH generally increased and less degradation and more adsorption processes were observed in NZVI. The adsorption of PBDEs on iron could be simplified as the following equation: Fe (or iron oxides) + PBDEs → Fe (or iron oxides) − PBDEs

n/2 Fe

(2)

The responsible adsorption and debromination processes of PBDEs with MZVI during a long time of incubation have not yet been identified. The objective of this study was to demonstrate the contribution of MZVI in the removal of two of PBDEs, BDE-209 and BDE-3, through adsorption and degradation. Kinetic experiments for both brominated compounds were performed. The adsorption amount and identity were measured and the portion of degradation was calculated. Our findings help to evaluate the feasibility of ZVI technology for remediating PBDEs in the environment. 2. Materials and methods 2.1. Chemical and reagents BDE-209 (98% purity) was purchased from Fluka Chemical Co. (Buchs, Switzerland). BDE-3 (purity >99%) was purchased from Sigma–Aldrich (Milwaukee, WI, USA). Commercially reduced iron particle (purity >90%, 325 mesh, Hayashi Pure Chemical Ind., Ltd.) was chosen in this experiment. The Brunauer Emmett Teller (BET) surface area was determined to be 0.95 m2 g−1 on the basis of nitrogen adsorption by an ASAP2020 analyzer (Micrometrics

(1)

from Sigma-Aldrich (Milwaukee, WI) was used as an internal standard. Methanol, ethyl acetate and n-hexane of ultra resianalyzed grade were purchased from JT Baker Chemical Co. (Phillipsburg, NJ). Water used in this study was double-distilled and then de-ionized with a Milli-Q water purification system (Millipore). 2.2. Degradation of BDE-209 and BDE-3 by MZVI Batch experiments were conducted to investigate the reactivity of MZVI toward both PBDEs under pH 7. In a 100-mL amber serum bottle including around 1.83 mg/L BDE-209 or 50 mg/L BDE-3 solutions, 5 or 25 g/L MZVI were added in according to previous study without any headspace left [33]. The BDE-209 solution was diluted from a stock solution in which BDE-209 was dissolved in an ethyl acetate/methanol mixture (1:1, v/v). The BDE-3 solution was also diluted from a stock solution in which methanol was the solvent. The final concentration of methanol was about 3 vol.%. Both solutions contained 50 mM HEPES in order to maintain the pH values [39] and therefore keep the reactivity of ZVI [40]. The pH values in this study were stable through the whole reaction. The bottles were horizontally shook at 150 rpm under 27 ◦ C in the orbital shaker incubator and kept in the dark (Yih Der LM-530R, Taipei, Taiwan). Controls without iron powder were prepared according to the same procedures. All experiments were duplicated. The pseudo-first-order model was employed to describe the degradation kinetics of PBDEs: C = C0 e−kt

(3)

where C is the concentration at a given time t, C0 is the initial concentration, k is the pseudo-first-order rate constant (day−1 ), and t is the reaction time. 2.3. Chemical analysis At the initiation of the degradation reaction, 1.5, 3, 6, 14.5 h, 7 days, 13 days, and 34 days during the reaction, the amount of BDE209 in aqueous solutions and on the surface of MZVI was measured. The concentrations of BDE-3 during the reaction were measured at 0, 5, 15, 35, 50mins, 14 h, 11 days, and 34 days. MZVI particles were separated from aqueous solutions by centrifugation at 7000 × g for 10 min at first. A powerful magnet was then used to attract MZVI at the bottom of bottles when aqueous solutions were taken out. MZVI particles were washed by pure water twice prior to the extraction. n-hexane was used to extract PBDEs in aqueous solutions (1:1, v/v) and ethyl acetate was used to extract PBDEs on MZVI (same volume with the aqueous solution). All the extractions were performed three times. Each extraction involved a 20-min vibration by the ultrasonic disruptor (Misonix Sonicator 3000, 30 W). The extracted solutions were separated with MZVI by the same method as the aqueous solutions. The three extracted solutions were combined and the concentration of PBDEs was quantified with a gas chromatograph (GC; Agilent 6890) equipped with a micro-electron capture detector (␮ECD) and a DB5-HT capillary column (length = 15 m,

Y.-H. Peng et al. / Journal of Hazardous Materials 260 (2013) 844–850

i.d. = 0.25 mm, film thickness = 0.1 ␮m). The carrier gas was nitrogen gas of chromatographic grade. The injection temperature was 250 ◦ C, the initial oven temperature was 150 ◦ C, maintained for 0.5 min, then programmed to 330 ◦ C at 25 ◦ C min−1 , and held for 5 min. The detector temperature was 340 ◦ C. The molecular mass of DE produced during degradation was determined with a GC–MS (Agilent 5975 inert MSD), using column and chromatographic conditions identical to those of the GC-␮ECD system. The quantification limits for BDE-209 and BDE-3 were 0.032 mg/L and 0.064 mg/L. The bromide ions were determined by ion chromatography (Methrohm 861 Advanced Compact IC, Herisau, Switzerland).

(A) 1.2 1 0.8

C/C0

846

0.6 0.4

blank 0.2

MZVI

0

2.4. Characteristics of MZVI after reaction

0

3

6

12

9

15

18

Time (hr)

(B)

1.2 1 0.8

C/C0

After the reaction, MZVI particles were filtered and performed further analysis without fine-grinding. Functional groups on the surface of MZVI were analyzed by Fourier transform infrared spectrometer (FT-IR; Horiba FT-720 spectrophotometer, Kyoto, Japan). The scanning wave number was in the range of 550–4000 cm−1 . X-ray absorption near edge structure (XANES) measurement was performed at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Fe K-edge spectra were recorded on beam line BL-16A1 using a Lytle detector from powder samples. Energy steps as small as 0.2 eV were employed near the absorption edges. Data analysis was carried out using the software Athena (version 0.8.058). The X-ray diffraction (XRD) patterns of MZVI powders were collected by x-ray diffractometer (PANalytical X’Pert PRO, The Netherlands) at 45 kV and 40 mA. A copper target tube radiation was used to produce X-rays with a wavelength of 0.15418 nm. Scans of 2 were performed in a range of 5–90◦ and in a scanning rate of 1◦ /min. The phase identification was analyzed by using the software PcpdfWin.

blank

0.6

MZVI

0.4 0.2 0 0

10

20 Time (day)

30

40

Fig. 1. (A) Short- and (B) long-term removal efficiency of 5 g/L MZVI toward BDE209.

3. Results and discussion 1.2

blank

5 g/L

25 g/L

1 0.8

C/C0

The reactivity of MZVI toward BDE-209 is demonstrated in Fig. 1, with different reaction duration of 15 h (Fig. 1A) and 34 days (Fig. 1B). About 40% of BDE-209 was very fast removed within 1 day, while the removal of additional 40% of BDE-209 took 33 days. The removal kinetics can be described well with the pseudo-firstorder model. The removal rate constants were 5.21 day−1 during the first period and 0.17 day−1 during the follow-up period. The surface area normalized rate constant during the first period was 9 × 10−3 L h−1 m −2 , which was only a little higher than that of our previous studied MZVI material from Aldrich (2.5 × 10−3 L h−1 m−2 ) toward higher concentration of BDE-209 (2.8 mg/L) [23]. Our observation is similar with the results reported by Keum and Li [25]: about 75% of BDE-209 was removed within 3 days and the removal of additional 25% of BDE-209 took 37 days. The reduction of removal efficiency was probably due to the oxidation or hydroxide layer formed on the iron surface, which may hinder the transfer of electrons to PBDEs. Similarly, the reactivity of MZVI toward BDE-3 also presents the two-step characteristics. About 35% of BDE-3 was removed by 5 g/L of MZVI within 50 min (Fig. 2A) and an additional 40% of BDE-3 was removed within the following 33 days (Fig. 2B). The removal rate constants were 40.2 day−1 during the first period and 0.08 day−1 during the follow-up period. The degradation rate increased with the concentration of MZVI. By using 1 g/L of MZVI, there was only 4% BDE-3 removed within 1 h (data not shown). The more MZVI was added, the faster BDE-3 removal efficiency was: about 80% of BDE-3 was removed within 50 min when the dosage of MZVI increased to 25 g/L (Fig. 2A). The removal rate constant for this

(A)

0.6 0.4 0.2 0 0

10

20

30

40

50

60

Time (min)

(B) 1.2 1 0.8 C/C0

3.1. Removal kinetics of BDE-209 and BDE-3 by MZVI

Blank

0.6

MZVI

0.4 0.2 0

0

10

20 Time (day)

30

40

Fig. 2. (A) Short- and (B) long-term removal efficiency of MZVI toward BDE-3.

Y.-H. Peng et al. / Journal of Hazardous Materials 260 (2013) 844–850

100%

relave content

80%

Deca Nona

60% Octa 40%

Hepta Hexa

20%

0% 0

10

20

Time T (day)

30

Fig. 3. By-products generated within 34-days of BDE-209 debromination process by 5 g/L MZVI.

condition was 113.9 day−1 . The degradation efficiency increased with ZVI dosage, which was relevant to the surface area of MZVI particles. This was consistent with previous reports, in which ZVI was also used as reducing agent for different organic compounds such as halogenated compounds [41,42], nitroaromatic compounds [43], and azo dyes [44–46]. 3.2. By-products generation profiles During the BDE-209 degradation process, the amount of by-products generated was monitored (Fig. 3). The by-product generation sequence was from higher brominated congeners to lower brominated congeners, consistent with the predicted PBDEs debromination order by reductive debromination [25,33]. The generation of by-products from reduced BDE-3, DE, in the reaction solution at the end of the long-term degradation by 5 g/L MZVI was analyzed according to the m/z profile obtained by GC/MS (Fig. 4).

847

For the degradtion system of BDE-3, the concentration of bromide ions was 1.1 mg/L when BDE-3 was reduced by MZVI for 1.5 h. According to the removal kinetics, there was about 35% reduction of BDE-3 at that time. If all of the removal amount was due to the reduction by MZVI, there should be about 17.5 mg/L bromide ion in the solution. The detected amount was not fit the mass balance and was discussed later. Due to the presence of ethyl acetate in the BDE-209 system, the ionic peak of bromide ions was interfered. Therefore it was unable to detect the amount of bromide ions released from BDE-209.

3.3. The kinetics of adsorption and degradation The amount of BDE-209 and BDE-3 remained in the reaction solution and adsorbed on the surface of MZVI was measured. The degradation fraction was calculated by the initial concentration minus the concentrations of residual and adsorption. In Fig. 5, about 78% of BDE-209 remained and 19% of BDE-209 was adsorbed on MZVI at hr 6. Only about 2% of BDE-209 was degraded. Through the increase of incubation time, the amount of BDE-209 resided in the solution and on the MZVI surface decreased: about 17% and 11% of BDE-209 was detected in the solution and on the surface of MZVI at day 34, i.e. about 70% of BDE-209 was degraded during this period. Similarly, about 71% of BDE-3 remained and 27% of BDE-3 was adsorbed on MZVI at hr 14. Only about 3% of BDE-3 was degraded. This was coincided with the concentration of bromide ions measured at hr 1.5 (1.1 mg/L), which was equal to 2.2% of BDE-3 been degraded. The mass balance should take the adsorbed part as count. At the end of reaction, about 28% and 13% of BDE-3 was detected in the solution and on the surface of MZVI at day 34, i.e. about 59% of BDE-3 was degraded during the whole reaction. Therefore, adsorption of PBDEs on the iron surface dominates the removal mechanism in the beginning and debromination with MZVI further proceeded. The oxidation layer on the iron surface may contribute to the adsorption of PBDEs, because the

Fig. 4. Mass spectrum of degradation products of BDE-3 by 25 g/L MZVI after 34 days experiment identified as diphenyl ether.

848

Y.-H. Peng et al. / Journal of Hazardous Materials 260 (2013) 844–850

relave content (%)

(A)

100 90 80 70 60 50 40 30 20 10 0

residue adsorpon degradaon

6 hrs

14.5 hrs

7 days

13 days

34 days

Time

relave content (%)

(B)

100 90 80 70 60 50 40 30 20 10 0

residue adsorpon degradaon

0.5 hr

14 hrs

11 days

Fig. 7. X-ray diffractograms of microscale ZVI before and after reaction with BDE-3.

could be degraded by MZVI after one month of incubation. This indicated that PBDEs in the adsorption layer still can access the electrons from the remaining elementary iron core of MZVI particles to become reduced.

34 days

Time Fig. 5. The residual, degradation and adsorption portions of (A) BDE-209 and (B) BDE-3 with MZVI.

adsorption amount was little at the beginning of reaction if acid washed MZVI was used (data not shown). Besides zero-valent iron (the shoulder region around 7115 eV), Fe2 O3 was detected (peak at 7131 eV) on the MZVI reacted with BDE-209 for 34 days by XANES (Fig. 6 and similar data for BDE-3 was not shown). The chemical composition of MZVI was also analyzed by XRD (Fig. 7). The main peaks of hematite and goethite were also indicated in Fig. 7 for comparison [47,48]. Three reflections located at 2 = 44.7◦ , 65.0◦ , and 82.4◦ indicated a body-centered cubic (bcc) structure, consistent with the bcc Fe pattern (JCPDF No.06-0696), indicating the predominance of elementary iron in MZVI even after the reaction for one month. Not enough amount of the crystalline iron (hydro)oxides can be quantified by XRD but, in general, amorphous iron (hydro)oxides could be formed after the oxidation of MZVI. Although a part of PBDEs were adsorbed on the MZVI, they still

Fig. 6. X-ray absorption near-edge structure of MZVI after reaction with BDE-209.

3.4. Characterization of adsorbed PBDEs on the surface of MZVI The functional groups on the surface of MZVI were detected by FT-IR (Fig. 8). The peak high at 2325 cm−1 was generally different due to the difference of background CO2 concentration during the detection processes. A small peak at 3400 cm−1 , O H stretching, detected on the reacted MZVI, indicated that the adsorption of water or possible iron hydrooxides (data not shown). Since not enough IR peak assignments for PBDEs available in the literature, we referred to FTIR textbook [49] and available data for

Fig. 8. FTIR pattern of MZVI before and after the reaction with (A) BDE-209 and (B) BDE-3.

Y.-H. Peng et al. / Journal of Hazardous Materials 260 (2013) 844–850 Table 1 Evaluation of the FT-IR spectrum of PBDEs adsorbed on MZVI. Absorption frequency (cm−1 )

Approximate assignments

BDE-209 a b c d e

1350, 1080, 754 1350 1070 964, 711 711

C C C C C

BDE-3 f g h i j k

1577, 1481 1481, 1236, 1162, 752 1236, 1162 1068 1008 867

l m

827 690

C C stretching C H in-plane-bending Symmetric C O stretching C Br stretching, ring stretch Trigonal ring breathing Asymmetric C O stretching, ring skeleton vibration C H stretching C H out-of-plane bending

H in-plane-bending C stretching Br stretching, ring stretch H out-of-plane bending H torsion

4,4 -dibromodiphenyl ether (BDE-15) [50] to identify IR peaks for different moieties of BDE-209 and BDE-3 from their own IR spectra in Fig. 8 (Table 1). Several typical absorption peaks of BDE-209 were detected on the surface of MZVI after reacted with BDE-209 (listed in Table 1 and indicated in Fig. 8A). The presence of C C bond, C Br bond, and ring structures indicated the adsorption of BDE-209 on MZVI. The C H bond may be contributed from the reduced byproducts of BDE-209. The typical absorption peaks of BDE-3 were also detected on the surface of MZVI (Table 1 and Fig. 8B). The presence of major IR peaks of BDE-3 indicated the adsorption of BDE-3 on MZVI. Some of C H bonds could be resulted from the reduced by-products, DE. Due to not high IR intensity of MZVI, the interaction of PBDEs with MZVI was not significantly observed by IR but the adsorption of PBDEs on MZVI was confirmed. Taking the results together, the reduction reaction of PBDEs took place on the surface of MZVI. PBDEs might first either adsorb on the thin surface layer of iron oxide (Eq. (2)) or be reduced by ZVI beneath the surface layer (Eq. (1)); the former reaction was predominant since the degradation part was small in the beginning (Fig. 5). During the long-term incubation, subsequent degradation occurred that electrons transferred from the core of MZVI reduced the adsorbed PBDEs, leading to more surface oxidation. Both PBDEs and their degraded by-products could be adsorbed on the amorphous oxidized surface of MZVI with the proportion to their concentration in the solution. Further PBDE can nevertheless adsorb and still access electrons from the ZVI core area or unreacted ZVI sites, leading to further reduction of BDE-209 or BDE-3 into lower brominated BDEs. Our results infer the concurrent removal mechanisms of adsorption and debromination for PBDEs by MZVI and support the long-term utilization of MZVI in the remediation. 4. Conclusion Although the degradation of PBDEs by NZVI was faster than MZVI, the surface area normalized rate constant of MZVI is higher than that of NZVI (1.3 × 10−3 L h−1 m−2 ) in our previous study [23]. Furthermore, considering the impacts of NZVI on ecosystem and biodegradation system, its stability and cost, MZVI has some advantages to treat polyhalogenated aromatic compounds in the environment. The study shows that BDE-209 and BDE-3 in solution can be removed by MZVI and reached maximally achievable degradation efficiency within about one month. About 10–20% of BDE-209 and 15–30% of BDE-3 could be adsorbed on the surface of MZVI. The adsorbed amount was the largest in the begining and decreased later with time. The adsorbed PBDEs still could access the electrons from the remaining elementary iron core or unreacted

849

sites of MZVI particles to become reduced. The removal kinetics presented a faster rate in the beginning and then continuted to aound 80% for both BDEs; consequently, about 70% of BDE-209 and 60% of BDE-3 were degraded by MZVI after one month of incubation. Our study demostrated adsorption and debromination by MZVI played roles in the removal of PBDEs, which can offer great longevity to remediate polyhalogenated aromatic compounds in the environment.

Acknowledgement The authors gratefully acknowledge the financial support of the National Science Council of Taiwan, ROC.

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