Reductive Degradation of Tetrabromobisphenol Using Iron–Silver and Iron–Nickel Bimetallic Nanoparticles with Microwave Energy

Abstract

In this work, a novel combined system by Fe-Ag or Fe-Ni nanoparticles and microwave (MW) radiation were used for the debromination of tetrabromobisphenol A (TBBPA) in aqueous solutions. Core-shell structure bimetallic nanoparticles were prepared by replacement reaction in liquid phase and then characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy techniques. MW radiation can enhance the degradation of TBBPA by Fe-Ag or Fe-Ni observably. The rate of reduction reactions for bimetallic nanoparticles under MW were first compared with those under conventional heating conditions. Compared with nano-iron, the deposition of Ag or Ni also accelerated the debromination, and Fe-Ag was more reactive than Fe-Ni toward TBBPA reduction. Removal efficiencies increased with increasing Fe-Ag dosage and MW energy level. Major reduction products of TBBPA identified by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) were tri-BBPA, di-BBPA, mono-BBPA, and BPA, which indicated a stepwise debromination process. It provides an effective technology for TBBPA laden wastewater treatment.

Introduction

Microwave (MW) radiation has been extensively investigated for assisting and accelerating the degradation of many wastes (Giguere et al., 1986; Bram et al., 1990; Jones et al., 2002) because of its advantages of swiftness, high efficiency, and no pollution to environment. In a large number of cases, MW is simply a heat source. Compared with the conventional heating processes, the characteristic of MW effect via internal molecule vibration induces selective heating. The alignment and reorientation of molecules in the applied MW field bring about molecular movement and subsequent heat generation (Michael et al., 1991). Because of avoidance of thermal gradients and heat transfer, MW heating shows higher energy efficiency (Kingston and Jassie, 1988). However, MW is not merely a heat source. Some investigations assumed its nonthermal effect resulted from polarizing parts of macromolecules aligning with the poles of the electromagnetic field (Cigdem et al., 2007).

MW-assisted treatment could prove beneficial to the rapid decomposition of chlorinated substances (Jou and Tai, 1999; Coss and Cha, 2000; Wada et al., 2000; Jou, 2008). For example, Jou (2008) has reported the combination of MW with zerovalent iron (Fe⁰) for pyrolysis of pentachlorophenol, which displayed higher removal efficiency than Fe⁰ reduction alone. Fe⁰ absorbed MW energy to induce electronic vibration and the friction among molecules led to the generation of thermal energy. The advantage of MW-enhanced degradation processes stimulates us to utilize a combined system of Fe⁰ or iron-based bimetal with MW for the elimination of pollutants. It is well known that Fe⁰ has a great potential for treating a large number of environmental contaminants including halogen-containing organics (Gillham and O'Hannesin, 1994; Kim and Carraway, 2000). In recent years, many modified materials such as bimetallic particles (e.g., Fe-Ni, Fe-Pd, and Fe-Ag) have been synthesized to enhance the activity of Fe⁰ (Grittini et al., 1995; Xu and Zhang, 2000; Zhang et al., 2006).

Tetrabromobisphenol A (TBBPA) is a kind of reactive flame retardant, applied extensively in the manufacture of printed circuit boards, textiles, plastics, upholstery, and many other consumer goods (WHO, 1995). TBBPA and its dimethylated derivative have been detected in environmental samples around the world (Sellström and Jansson, 1995; de Wit, 2000; Oberg et al., 2002; Akutsu et al., 2003). Because of the specific characteristics of TBBPA, such as lipophilicity, bioassimilability, persistency, and observed toxicity, it has become an issue of interest to many research and governmental bodies. It is obvious that the treatment of TBBPA in contaminated systems is necessary and meaningful.

In this study, we examined the reductive debromination of TBBPA in solution over Fe-Ag and Fe-Ni bimetallic nanoparticles coupled with MW energy. The roles of second metal deposited on the surface of nano-iron and MW radiation were illuminated; the discrepancy between Fe-Ag and Fe-Ni nanoparticles were also described herein. Efforts were made to identify reaction intermediates and final products, and then we assessed the degradation pathways of TBBPA. The experimental factors affecting the debromination efficiency, such as nanoparticle addition and MW power level, were investigated.

Experimental Section

Materials

TBBPA was obtained from Sigma-Aldrich Company. Silver chloride (AgCl) and nickel chloride (NiCl₂) were purchased from Nanjing Chemical Company. High-performance liquid chromatography (HPLC)-grade methanol and n-hexane were obtained from Tedian Company. Milli-Q water was used throughout this study. The Fe⁰ used was iron powder (Shenzhen Junye Nano Material Co., Ltd.; >99.9%, <60 nm). The structure of TBBPA is given in Fig. 1.

FIG. 1.

Molecular structure of tetrabromobisphenol A (TBBPA).

Synthesis and characterization of bimetallic nanoparticles

Bimetallic nanoparticles with core-shell structure were prepared by reductive deposition of Ag or Ni on Fe⁰ nanoparticles as described in a previous report (Luo et al., 2010). The loading content of both Ag and Ni was 1 wt%.

Various characterization techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM) were used to analyze the fresh bimetallic samples. XRD was performed using Switzerland ARL X'TRA X-ray diffractometer (λ=1.5418 Å). XPS was obtained on a Thermo VG Scientific ESCALAB 250. The particle size and morphology of reductant were observed with TEM (JEM-1230; Jeol).

Debromination experiments

A given type of nanoscale particles were added into Erlenmeyer flasks (250 mL), which contained 100 mL TBBPA milli-Q water solution. Methanol (5 mL) was added to ensure the concentration of TBBPA was 30 mg L⁻¹. The bottles were then placed in a modified household MW oven successively and treated in various radiation durations. At the end of the heating period, samples were withdrawn and filtered through a syringe filled with a little silanized glass wool. Parallel control experiments were performed identically except for the exclusion of metal particles. All experimental points were duplicated.

Analytical methods

The concentration of TBBPA was analyzed via HPLC (Agilent 1100), with a C18 reversed-phase column (150 mm×4.6 mm, 5 μm particles; Agilent), a diode array detector, and an autosampler controlling under a Chemstation data acquisition system. Identification of degradation products was performed via liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) (Thermo LCQ Advantages, QuestLCQ Duo), with Beta Basic-C18 HPLC column (150 ×2.1 mm i.d., 5 μm; Thermo). The detailed operation conditions were provided in a previous report (Luo et al., 2010).

Results and Discussion

Characterization of bimetallic nanoparticles

XRD patterns of the fresh bimetal samples are shown in Fig. 2. The two peaks appeared at 44.66° (main peak) and 64.34° are Fe-110 and Fe-200 diffraction angles, respectively. The characteristic peaks assigned to Ag-111 and Ni-002 are several at the positions of 38.08° (Fig. 2a) and 50.09° (Fig. 2b).

FIG. 2.

X-ray diffraction of (a) Fe-Ag bimetallic nanoparticles and (b) Fe-Ni bimetallic nanoparticles.

The predominant features are that the bimetallic samples primarily consisted of nickel metal, silver metal, and iron metal. No characteristic diffraction peaks of ferric oxide and ferroferric oxide are found, indicating that major surface species of freshly prepared nanoparticles is identified as Fe⁰. XPS survey scan exhibits the presence of Ag, Ni, and Fe over the surface of the as-synthesized nanoparticles. The Fe 2p spectra of the newly synthesized samples show peaks at binding energies of 710.9 and 724.2 eV in Fig. 3a, which are assigned to the oxidized iron, suggesting that the surface of Fe⁰ is covered by a layer of oxide film, which might be formed during the drying process. Additionally, the peaks at 368.25 and 374.3 eV in Fig. 3b are binding energies of Ag 3d; the different signature of Ni 2p is found out at 855.3 eV in Fig. 3c. The morphology and structure of Fe-Ag and Fe-Ni nanoparticles are similar in the TEM images. Figure 4 shows the result of Fe-Ni and further information can be obtained in our previous study (Luo et al., 2010). As the image reveals, the shape of particles is like chains of beads and its diameter ranges from 20 to 100 nm in this form. This type of aggregation is likely due to magnetic interactions between the metallic particles and the natural tendency to remain in the more thermodynamically stable state (Cushing et al., 2004).

FIG. 3.

(a) Fe 2p, (b) Ag 3d, and (c) Ni 2p X-ray photoelectron spectrum of the fresh bimetallic nanoparticles.

FIG. 4.

Transmission electron microscopic images of Fe-Ni bimetallic nanoparticles.

TBBPA dehalogenation

Figure 5 compares the TBBPA degradation efficiency using Fe⁰, Fe-Ag, and Fe-Ni bimetallic nanoparticles under MW radiation. For comparison of the effectiveness of TBBPA reduction with and without MW energy induction, the reactors containing Fe-Ag nanoparticles were placed directly on a rotary shaker for a certain period. The loading of Fe⁰ or bimetallic nanoparticles was 0.5 g L⁻¹. The initial TBBPA concentration was 30 mg L⁻¹ in all cases. Each data point reported in all figures represents the mean of duplicates.

FIG. 5.

Transformation of TBBPA by microwave (MW), Fe-Ag, Fe⁰/MW, Fe-Ag/MW, and Fe-Ni/MW. Metal addition=0.5 g L⁻¹; Ag or Ni content=1 wt%; [TBBPA]=30 mg L⁻¹; MW energy=800 W.

Under MW condition alone, there is less than 10.0% of TBBPA reduced in 720 s because of volatilization, which is neglectable in the whole experiment. No degradation product of TBBPA is detected during the entire experimental period. TBBPA is fairly stable under the condition of only reductant and the decrease of its concentration may attribute to adsorption of TBBPA on nanoparticles. However, Fe-Ag and Fe-Ni bimetallic nanoparticles coupled with MW radiation completely dehalogenate TBBPA from aqueous solution in 300- and 360-s periods, respectively. Fe⁰ nanopartilces show a lower degradation rate of TBBPA reduction, which clearly demonstrates that the roles of the second metal and the MW radiation are important. These findings show a high agreement with our previous studies that the Fe-Ag bimetallic nanoparticles exhibit higher activity than Fe⁰ nanoparticles for debromination of TBBPA coupled with ultrasonic radiation. The improved degradation efficiency of the bimetallic nanoparticles in this experiment may also be attributed to MW radiation and the second metal (Ag or Ni) deposited on the surface of nano-iron. The second metal not only enhances the corrosion of the iron and accelerates hydrogen evolution, but also generates and stores the adsorbed atomic hydrogen on the particle surface (Luo et al., 2010). The important role of MW radiation and the discrepancy between Fe-Ag and Fe-Ni nanoparticles will be discussed later.

The rate of TBBPA transformation is described as a pseudo-first-order kinetic model. The kinetic equation can be expanded as follows:

1

where C is the TBBPA concentration (mg L⁻¹) at time t (s), C₀ is the initial TBBPA concentration, and k_obs is the observed pseudo-first-rate constant (s⁻¹). The rate constants (k_obs) can be determined by fitting the pseudo-first-rate expression from Equation (1) to the experimental data. The calculated rate constants are 0.003 s⁻¹ for Fe⁰ nanoparticles, 0.0217 s⁻¹ for Fe-Ag nanoparticles, and 0.0182 s⁻¹ for Fe-Ni nanoparticles, respectively. On the other hand, the rate constants of parallel experiments are listed in Table 1 and analyzed by paired-sample t-test. A probability value of ≤0.05 is considered to indicate a significant difference. SPSS version 12.0 for Windows (SPSS, Inc.) was used for statistical analysis of the data. The difference between degradation of TBBPA under the condition of Fe-Ag/MW and Fe-Ni/MW is slight but statistically significant (p=0.039).

Table 1.

Observed Pseudo-First-Rate Constants of Tetrabromobisphenol A Debromination Under Various Conditions

	kobs(min−1)
Conditions	1	2	3	p
Fe-Ag/MW	0.0226	0.0197	0.0243	0.039
Fe-Ni/MW	0.0179	0.0158	0.0192
Fe-Ag/MW	0.0226	0.0197	0.0243	0.055
Fe-Ag/CH	0.0201	0.0184	0.021

Metal addition=0.5 g L⁻¹; Ag or Ni content=1 wt%; [TBBPA]=30 mg L⁻¹; MW energy=800 W.

TBBPA, tetrabromobisphenol A; MW, microwave; CH, conventional heat.

Factors affecting the degradation of TBBPA

The factors affecting the debromination efficiency of TBBPA include reductant dosage, initial concentration of TBBPA, pH of the solution, MW energy level, etc. The results and discussion about the main two factors (Fe-Ag dosage and MW energy level) are given hereinafter.

Effect of Fe-Ag dosage on debromination efficiency

It can be indicated from Fig. 6a that the degradation efficiency of TBBPA is improved with the increase of Fe-Ag additions (0.2, 0.5, 1.0, and 1.5 g L⁻¹). The reaction of TBBPA with reductant occurs at the metal–water interface; hence, the surface area of the nanoparticles can affect the degradation rate. The increase of Fe-Ag addition would be expected to increase the number of active sites and adsorptive Ag concentration, bringing the improvement of debromination efficiency. High degradation rate constant of TBBPA could be obtained when the bimetal addition is 0.5 g L⁻¹. In consideration of economization, 0.5 g L⁻¹ metal addition was chosen as the optimal dosage in debromination experiments.

FIG. 6.

(a) Variation of degradation rate constants with Fe-Ag addition; (b) degradation rate constants of Fe⁰, Fe-Ag, and Fe-Ni under different loading. Ag or Ni content=1 wt%; [TBBPA]=30 mg L⁻¹; MW energy=800 W.

The degradation rate constants of TBBPA by Fe⁰ and bimetallic nanoparticles under different loading are shown in Fig. 6b. When the addition is 1.0 g L⁻¹, the discrepancy among reaction rates of nanometals is less than the discrepancy of values under 0.5 g L⁻¹, especially for Fe⁰ nanoparticles. In addition, slightly higher degradation rate constants could be obtained by adding Fe-Ag nanoparticles. These evidences reveal that the metal addition affects the reactivity of nano-iron more than that of bimetallic nanoparticles, and Fe-Ag appears more reactive toward TBBPA than Fe-Ni over the investigated loadings. In the studies of Cwiertny et al. (2006), they suggested that absorbed atomic hydrogen was responsible for the enhanced reactivity of iron-based bimetals, and they used the relative partial molar enthalpy for an infinitely dilute solution of hydrogen in a metal () as a metric of absorbed atomic hydrogen solubility in each metal additive. Hence, we infer that the discrepancy between the reactivity of Fe-Ag and Fe-Ni nanoparticles is attributed to the different ability of silver and nickel to take up atomic hydrogen into its crystal lattice. Unfortunately, failing to obtain the value of silver makes the illustration difficult. This effort needs further work.

Effect of MW power on debromination efficiency

To demonstrate the influence of MW radiation on TBBPA debromination, the results of control experiments are shown in Fig. 7a. The thermal effect of MW is considered by comparing results of MW-generated heat versus conventional heat (CH). It is recorded that the degradation rate constant of TBBPA in Fe-Ag-MW system was about 0.0217 s⁻¹ with corresponding removal of 100% in 300 s, whereas that of TBBPA in Fe-Ag-CH system is 0.0169 s⁻¹ with removal of 100% in 420 s. The degradation of TBBPA under the condition of Fe-Ag/MW and that under the condition of Fe-Ag/CH show nonsignificant difference (p=0.055; Table 1). Thus, both MW and CH have obvious accelerating effect on the reductive reactions, and the thermal effect of MW radiation is the main factor to promote the debromination of TBBPA.

FIG. 7.

(a) Transformation of TBBPA by Fe-Ag/MW and Fe-Ag/conventional heat; (b) temporal increase of temperature in an aqueous TBBPA/Fe-Ag dispersion under MW radiation. Metal addition=0.5 g L⁻¹; Ag content=1 wt%; [TBBPA]=30 mg L⁻¹; MW energy=800 W; T=100°C.

We further examined the generation of MW-induced heat at the Fe-Ag nanoparticle to probe this MW thermal effect by irradiating a sample of water (100 mL) with or without nanoparticles (1.0 g L⁻¹) at full MW power (800 W; Fig. 7b). The temperature rise in pure water is slightly slower than that in the aqueous dispersion, suggesting that Fe-Ag powders in aqueous solution increase the ability to absorb MW energy, and the high temperature of metal leads to improved debromination efficiency in the reduction system (Elsukov et al., 2008). Further, as the feed materials are polar compounds, they are highly MW-absorbing compounds. The MW energy causes polarization, which leads to electronic vibration and thus heat generation; therefore, the reaction is expected to be more efficient under MW condition (Pillai et al., 2004).

Effect of MW energy levels (400, 600, 800 W) on treatment removal efficiencies of TBBPA is shown in Fig. 8. With the MW energy maintained at 400, 600, and 800 W, the TBBPA degradation rate constants are 0.0114, 0.0152, and 0.0217 s⁻¹, respectively, and it takes shorter treatment time to reach 100% transformation efficiencies for larger MW power. Under the MW radiation, dielectric medium absorbs the MW to accelerate molecular movement and subsequently generate heat inside the medium. Therefore, more MW power applied causes higher temperature of Fe-Ag to achieve better TBBPA debromination efficiency. In addition, the TBBPA degradation rate is low in the initial 60 s and improves during the 60–240-s period. Then, it diminishes after 240 s. During the initial seconds, the temperature of aqueous solution and Fe-Ag particles are not high enough. Over time of MW radiation, system temperature becomes higher and portions of Fe-Ag emit incandescent light to show the spark effect. After that, the high temperature leads to the changes of the particles surface and thus brings about a decrease of the surface porosity or causes the surface porosity covered by impurities (Jou and Tai, 1999). Hence, the TBBPA removal rate is reduced in the later period of reaction.

FIG. 8.

Variation of degradation rate constants with MW power. Metal addition=0.5 g L⁻¹; Ag content=1 wt%; [TBBPA]=30 mg L⁻¹.

Identification of intermediates and products of TBBPA

To well understand the possible degradation pathways, identification of degradation intermediates and final products was carried out by LC-MS/MS analysis. Displayed in Fig. 9a, all the intermediates identified by LC-MS/MS were recorded in the total ion chromatogram and corresponding mass spectra were denoted in the literature (Luo et al., 2010). LC-MS/MS analysis indicates that TBBPA (t_R=4.14 min) degradation using Fe-Ag bimetallic nanoparticles generates tri-BBPA, di-BBPA, mono-BBPA, and BPA, corresponding to several peaks in HPLC with retention time (t_R) at 3.31, 2.74, 2.37, and 2.14 min, respectively. The results of HPLC are shown in Fig. 9b.

FIG. 9.

(a) Liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) analysis of TBBPA degradation intermediates and final products; (b) high-performance liquid chromatography analysis of the disappearance of TBBPA and the appearance of intermediates and final products.

Figure 10 depicts the simultaneous dehalogenation of TBBPA and conversion to intermediates during the 720-s degradation of TBBPA using Fe-Ag bimetallic nanoparticles. Because of lack of the standard materials of the above intermediates, it was difficult to determine intermediates quantitatively, so the distributions of all the debrominated intermediates were examined by comparing their peak intensities to that of the parent compound. It is speculated that the degradation of TBBPA might conform to a stepwise debromination from the gradual transformation into lower brominated compounds. The concentration of tri-BBPA increases continuously to about 36% after 90 s and then decreases to 0%. Peak belonging to di-BBPA (60%) is observed at about 120 s. The mono-BBPA concentration increases promptly after 90 s and then decreases slowly after 300 s. BPA accounts for 80% of the TBBPA lost. The mass balance of carbon (calculated as the sum of all organic species measured) is about 97% of the calculated initial TBBPA concentration. Losses of some of the volatile organics might have occurred by experimental errors during sampling. A similar result was observed for Fe⁰ nanoparticles. The reaction pathway of TBBPA debromination was also described in the literature (Luo et al., 2010).

FIG. 10.

Temporal disappearance of TBBPA and appearance of byproducts by Fe-Ag bimetallic nanoparticles coupled with MW radiation. Metal addition=0.5 g L⁻¹; Ag content=1 wt%; [TBBPA]=30 mg L⁻¹; MW energy=800 W.

Conclusions

In this article, the Fe-Ag and Fe-Ni bimetallic nanoparticles were synthesized and characterized successfully. The results showed that TBBPA (30 mg L⁻¹) was rapidly debrominated by Fe-Ag and Fe-Ni bimetallic nanoparticles (Ag or Ni content=1 wt%) in 300 and 360 s, respectively. Compared with Fe⁰ nanoparticles, bimetals presented higher reaction efficiency. These results indicated that both the second metal and MW radiation played important roles in the activity of MW–bimetal system, and the rate of TBBPA reduction by Fe-Ag was slightly greater than that observed in the Fe-Ni system. Additionally, many intermediates and products of TBBPA, such as tri-BBPA, di-BBPA, mono-BBPA, and BPA, were identified by LC-MS/MS, which revealed that the degradation mechanism proceeded through a stepwise debromination from n-bromo- to (n-1)-bromo-BPA. With increasing nanoparticle addition and MW power, the degradation rate constants also increased. The effect of metal addition on the activity of Fe⁰ nanoparticles was greater than that of bimetallic nanoparticles.

Acknowledgments

The authors greatly acknowledge the National Natural Science Foundation of China (50938004 and 20707009) and the National Major Project of Science and Technology Ministry of China (Grant No. 2008ZX07421-002) for financial support.

Author Disclosure Statement

The authors declare that there are no completing financial interests.