Degradation of the polybrominated diphenyl ethers by nanoscale zero-valent metallic particles prepared from steel pickling waste liquor

Desalination 267 (2011) 34–41

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Degradation of the polybrominated diphenyl ethers by nanoscale zero-valent metallic particles prepared from steel pickling waste liquor Zhanqiang Fang ⁎, Xinhong Qiu, Jinhong Chen, Xiuqi Qiu School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China

a r t i c l e

i n f o

Article history: Received 11 March 2010 Received in revised form 3 September 2010 Accepted 3 September 2010 Available online 28 September 2010 Keywords: Nano zero-valent iron Nanoparticles Polybrominated diphenyl ethers Removal Pickling waste liquor

a b s t r a c t Due to the increasing levels of polybrominated diphenyl ethers (PBDEs) in the environment and their persistent toxicity, methods of removing PBDEs from the environment have become a necessary. Nanoscale zero-valent metallic particles (S-NZVI) were prepared from steel pickling waste liquor by chemical deposition and used to remove BDE209 in a water/tetrahydrofuran (4/6, v/v) solution. Nanoscale zero-valent iron particles (NZVI), nanoscale zero-valent and Ni/Fe particles were also prepared. These particles were characterized by BET, TEM, SEM, XRD, and EDS. The crystalline structure of S-NZVI was different from NZVI. However, the BET surface area of S-NZVI was the same as that of NZVI. The degradation rate of BDE209 by SNZVI followed a pseudo-first order kinetics. The removal efficiency increased with increasing metal dosage but decreased with increasing initial BDE209 concentration. High reaction rate was observed at more water content solvent, indicating that hydrogen ion was the driving force of reaction. Comparing different nanoscale Fe-based materials, the removal of BDE209 by S-NZVI was found more effective than NZVI. By evaluating the cost and the significance of reclaimable iron resource, S-NZVI was found to give better compensation compared to other metals. Thus, the degradation of BDE209 by S-NZVI is both feasible and efficient. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Chemical reduction of persistent pollutants using nanoscale zerovalent iron (NZVI) and iron-based bimetallic nanoparticles has received much attention due to the large specific surface area, environmental benignity, and high activities of these materials. Many studies have shown that a wide variety of toxic contaminants, such as chlorinated organic substances [1], polybrominated diphenyl ethers (PBDEs) [2], nitrate [3], azo dye [4], heavy metals [5], and so on, could be rapidly degraded by NZVI. Meanwhile, an increasing number of field tests in pilot- and full-scale remediation sites have been implemented by using NZVI [6]. As such, nanoscale metallic particles have the potential for future use in the degradation of persistent organic pollutants (POPs). To date, there are two main approaches for preparing nanoparticles. One is physical methods, such as inert gas condensation [7] and high-energy ball milling [8]. The other is through chemical methods, like liquid-phase reduction [9] and reverse micelle [10]. Among these methods, the liquid-phase reduction method has been most widely used for its simplicity and productivity. In this method, borohydride is employed to reduce iron ions in order to synthesize NZVI. Many chemicals (e.g., ferrous sulfate, ferrous chloride, etc.) are usually consumed during the preparation, which results in high production

Abbreviations: S-NZVI, nanoscale zero-valent metallic particles from steel pickling waste liquor; PBDE, Polybrominated diphenyl ether; NZVI, nanoscale zero-valent iron. ⁎ Corresponding author. Tel.: +86 20 39310250; fax: +86 20 39310187. E-mail address: [email protected] (Z. Fang). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.003

costs and limits the engineering application of nanoscale metals. On the other hand, pickling waste liquor discharged from steel industry during pickling process is highly attractive and reclaimable resource containing many iron and nickel ions, whose contents are up to 122 g/L and 17 mg/L, respectively. These waste liquor could be used to prepare NZVI. However, traditional methods of neutralization and precipitation for pickling waste liquor generate large volumes of waste [11]. Although many possible applications of NZVI have been reported, the feasibility of preparing nanoscale zero-valent metallic particles by utilizing steel pickling waste liquor (S-NZVI) to decrease the production cost has not been demonstrated, and the removal of POPs by S-NZVI has not yet been studied. Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants in various industrial products, and have recently become a major environmental concern [12]. Their high hydrophobicity, persistence, and bioaccumulation [13–17] have made PBDEs ubiquitous in the environment, as well as in human and animal bodies [18–21]. Human exposure to PBDEs has been found to affect the balance of the thyroid and cause neurotoxicity [22]. The US Environmental Protection Agency (EPA) has classified deca-BDE as a possible human carcinogen [23]. Hence, exploring a feasible way to eliminate PBDEs contamination is becoming an urgent matter. Gerecke et al. [24] and He et al. [25] have reported the microbial debromination of PBDEs and determined the debromination pathway. However, the biotic reductive debromination method is inefficient and needs an additional disposal step. Photochemical debromination of PBDEs using ultraviolet light [26] and titanium dioxide [27] has also

Z. Fang et al. / Desalination 267 (2011) 34–41

been reported. Photocatalytic degradation can rapidly debrominate BDE209, but the UV light requirement limits its engineering application in environmental remediation. Prior studies concerning the debromination of PBDEs by nanoscale metallic particles are limited [28]. In this study, we synthesized nanoscale metallic particles from steel pickling waste liquor (S-NZVI) and investigated the possible application of S-NZVI in the degradation of BDE209. To evaluate the degradation efficiency of BDE209 by S-NZVI, the removal efficiency, transformation and degradation kinetics of BDE209 using iron powder, NZVI and nano Ni/Fe particles prepared from chemical reagents were also measured. 2. Experimental 2.1. Chemicals A standard solution of decabromodiphenyl ether was purchased from Cambridge Isotope Laboratories (CIL, Andover, U.S.) and used to establish the standard curve. Decabromodiphenyl ether (98%, chemical reagent) was purchased from Aladdin (Shanghai, China) and used as the degradation sample. Ferrous sulfate (FeSO4·7H2O N 99%), nickel chloride (NiCl2·6H2O N 99%), sodium borohydride (NaBH4 N 98%), polyvinylpyrrolidone (PVP K-30), and ethanol (99.7%) were supplied by Tianjin Damao Chemical Agent Company (Tianjin, China). Methanol (HPLC grade) was supplied by Tianjin Kermel Chemical Reagents Company. Commercial iron powder with an average diameter of 69 μm and a specific surface area of 3 m2/g was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). The powder was soaked in 5 M hydrochloric acid solution for 10 min, then washed with deionized water, and dried in a vacuum box prior to use. Pickling waste liquor obtained from Jinlai steel plant in Guangzhou (China) was diluted or adjusted to the desired pH value using sodium hydroxide solution. The concentration of the metal element in the raw waste liquor was measured using an ICP-AES instrument (IRIS Intrepid II XSP, Thermo Elemental Company, USA). The results are shown in Table 1. 2.2. Preparation of nanoscale metallic particles S-NZVI was synthesized through the sodium borohydride (NaBH4) reduction method [29]. Briefly, 100 mL of diluted pickling waste liquor (CFe = 0.1 M) and the appropriate mixture of polyvinylpyrrolidone (Fe/ PVP, w/w = 1:1) acting as surface active agent were reacted in a 250 mL three-necked flask. Under the nitrogen gas and vigorous stirring, 0.3 M NaBH4 solution was rapidly added into the pickling solution and stirred for 5 min until the mixture turned black. Subsequently, the black particles formed were magnetically separated. In order to remove excess amounts of NaBH4, the nanoparticles were consecutively rinsed three times with deoxygenated water and ethanol. Since there were Fe2+ and Fe3+ complexes in the waste liquor, the nanoscale metal was synthesized according to the following reactions: 2þ

−

3þ

−

0

Fe ðH2 OÞ6 þ 2 BH4 → Fe ↓ þ 2 B ðOHÞ3 þ 7H2 ↑

method with polyvinylpyrrolidone [30]. Finally, all the nanoparticles were dried in vacuum overnight at 50 °C before use. 2.3. Characterization of nanoscale metallic particles Brunnaer–Emmett–Teller (BET) surface area analysis of S-NZVI, NZVI, nano Ni/Fe particles, and Fe powder were performed using an ASAP2020M surface analyzer (Micromeritics Instrument, USA) and employing the nitrogen adsorption method. The size and morphology of these synthesized nanoparticles were characterized with a transmission electron microscope (TEM, H-3000, Hitachi, Japan). After dispersion by an ultrasonicator, the nanoparticles/ethanol mixture was deposited as several droplets on a carboncoated Cu-grid. After the ethanol has completely evaporated, the samples were introduced into a vacuum chamber. The crystalline phase of these nanoparticles was determined using an X-ray diffractometer (Y-2000, Dandong, China) employing Cu Kα radiation. The accelerating voltage and applied current were 30 kV and 20 mA, respectively (Fig. 1). The morphology of the nanoparticles was viewed using a S-3700N (Hitachi, Japan) scanning electron microscope (SEM) at 20 kV and a magnification of 70,000. The surface elemental composition of the nanoscale metal was determined by energy dispersive spectroscopy (EDS, HORIBA EMAX, Japan). And, the content of Fe, Ni and Zn in the S-NZVI were measured using an ICP-AES instrument (IRIS Intrepid II XSP, Thermo Elemental Company, USA), the procedure was as follows: firstly, steel pickling waste liquor was pipetted to synthesize S-NZVI. Secondly, concentrated hydrochloric acid (10 mL) was added into a porcelain crucible containing S-NZVI, and the crucible was placed on a hot plate till the S-NZVI was completely digested. Finally, the solution in the crucible was transferred into a 100 mL measuring flask and brought to volume by deionized water, then the content of Fe, Ni and Zn were measured by ICP-AES. 2.4. Experimental procedures 2.4.1. Preparation of the BDE209 solution It is well known that BDE209 with high hydrophobicity is difficultly soluble in water [12,13]. In order to verify the feasibility of using new method to remove the BDE209 from solution, many researchers chose organic solvent in theirs experiment. For example, Li et al. [2] used THF as a stock solution to dissolve the BDE209 and removed the BDE209 in a acetone/water by NZVI. In 2007, Sun et al.

ð1Þ

0

Fe ðH2 OÞ6 þ 3 BH4 þ 3 H2 O → Fe ↓ þ 3 B ðOHÞ3 þ 10:5 H2 ↑

ð2Þ

NZVI and nano Ni/Fe particles prepared from the chemical reagents were also synthesized through a previously described Table 1 Concentration of metal element in steel pickling waste liquor. Metallic element

Fe

Ni

Zn

Cr

Mn

Ca

Mg

Concentration (mg/L)

122 × 103

17

4

88

300

131

18

35

Fig. 1. XRD patterns of the synthesized particles and the commercial Fe powder.

36

Z. Fang et al. / Desalination 267 (2011) 34–41

[27] also used THF as a stock solution and removed the BDE209 in THF/acetonitrile by TiO2. And Zhao et al. [31] also used THF as solvent to prepare the stock solutions of BDE209 and test the feasibility of using wet air co-oxidation to degrade the BDE209 in THF/water system. Base on the examples cited above and other references did not listed here, it is found that using THF as a solution universally. Meanwhile, for the purpose of this paper and the reaction of S-NZVI must take place in the presence of water, so that THF was combined with an amount of water. Therefore, the stock solution (100 mg/L) of BDE209 was made by using tetrahydrofuran (THF) as solvent. And a BDE209 simulated solution with the desired concentration was prepared by spiking a known volume of the stock solution into a certain proportion of THF/water solution. 2.4.2. Batch experimental procedure After grinding with an agate mortar and pestle, a desired amount of nanoparticles was added into a three-necked flask, and then the premade BDE209 solution was added into the flask immediately. Each batch reaction took place in one three-necked flask placed in the dark at room temperature (28 ± 2 °C) and stirred at a rate of 200 rpm. For each time interval, a 1 mL sample was taken using a glass syringe and then filtered through a piece of membrane filter with a pore size of 0.45 μm. Data were obtained through three times of repeated experiments and the error bar was drawn according to the standard deviation. The control experiments were also carried out without nanoparticles. The concentration of BDE209 in these samples was determined by high performance liquid chromatography (HPLC, Shimadzu, LC10A HPLC) with a UV detector (SPD-10AV) at 240 nm. A Dikma C18 column (250 mm × 4.6 mm) was used. The mobile phase was 100% methanol delivered at a rate of 1.2 mL/min. Each sample size was 20 μL. The retention time of BDE209 in the HPLC is 16.5 ± 0.5 min. Quantification was done with a calibration curve of the BDE209 standard. 3. Results and discussion 3.1. Characterization of nanoscale particles The three characteristic peaks of Fe metal, which appeared at 44.66°, 65.16°, and 82.36° in the XRD pattern of S-NZVI, were Fe-110, Fe-200, and Fe-211 diffraction peaks, respectively. All the peaks bore a striking resemblance to commercial Fe. Also, the characteristic diffraction peaks of iron oxides or hydroxides were not detected. This indicated that the crystal structure of the nano metal prepared from the pickling waste liquor was α-Fe0. However, the structure of S-NZVI is quite different from NZVI and nano Ni/Fe particles prepared from chemical reagents in our lab. The broad peaks at 42.5° to 47.5° indicated that the NZVI particles prepared from chemical reagents were amorphous. Moreover, the X-ray diffractometer did not detect the presence of Ni in the nano Ni/Fe particles since the synthesized particles from the chemical synthesis possessed an amorphous phase, which was demonstrated by broad peaks. The morphology and structure of the nanoparticles (NZVI, nano Ni/Fe, and S-NZVI) are shown in Fig. 2 (left image shows the TEM patterns, right image shows the SEM patterns). The diameters of the synthesized nanoscale particles were lower than 100 nm. These spherical nanoparticles aggregated together to form chain structures due to magnetic interaction and the natural tendency to remain in a more thermodynamically stable state. The average sizes of NZVI, nano Ni/Fe, and S-NZVI particles were 50–80 nm, 20–50 nm, and 50– 80 nm, respectively. The average size of nano Ni/Fe particles was smaller than the NZVI and S-NZVI particles. The specific surface area of the nano Ni/Fe particles was 68 m2/g, larger than that of the NZVI and S-NZVI particles whose specific surface areas were both 35 m2/g.

The EDS analysis profile of the nano Ni/Fe particles (Fig. 3), revealed that Ni was uniformly and non-continuously distributed and attached to the Fe surface and the detected Ni loading (15.6%) of four randomly chosen regions was close to the theoretical Ni loading. But for S-ZNVI particles, no other metal besides Fe was detected by EDS. It is speculated that the other metal loadings, such as Ni and Zn, were too low in concentration to be detected on the surface of nanoparticles. Nevertheless, the contents of Ni (0.011%) and Zn (0.002%) in solid S-NZVI were detected by ICP, so they were deposited on S-NZVI. 3.2. Degradation of BDE209 by S-NZVI particles 3.2.1. Effect of S-NZVI particle addition As shown in the Fig. 4, with increasing reaction time, the removal efficiency of BDE209 increased for all samples with different amounts of S-NZVI particles added. The degradation patterns of BDE209 by various S-NZVI amounts were the same, all of them following first order kinetics. For the first 12 h of the reaction time, under the condition of different S-NZVI amounts, the removal efficiency of BDE209 steadily increased. At 12 h, under the condition of 6, 4, 3, and 2 g/L, the removal efficiency were determined to be 91.19%, 76.46%, 41.67%, and 31.81%, respectively. But after 12 h, the rate of increase slowed down, at 24 h, removal efficiency were 100%, 93.76%, 69.32%, and 39.59% for 6, 4, 3, and 2 g/L respectively. This phenomenon can be explained by the fact that the degradation process of the contaminant by Fe-based nanoparticles is an interface reaction [4,32]. Therefore, for the first 12 h of reaction, the degradation speed of BDE209 in the solution was rapid and, as the reaction proceeded, the gradual oxidation on the surface of NZVI formed a passivated layer, which covered some reaction sites. This may led to the decrease in the removal efficiency in the second half of the reaction. The residual concentration of BDE209 decreased as the amount of S-NZVI increased. At 24 h, the removal efficiency of 6 g/L S-NZVI was about 100%, which was 2.5 times higher than that at 2 g/L S-NZVI, 1.4 times higher than that at 3 g/L S-NZVI, and 1.2 times higher than that at 4 g/L S-NZVI. And the same pattern was reflected by the reaction rate constant. For example, the Kobs of 6 g/L S-NZVI was 0.239 h−1, while the Kobs of 2 g/L was only 0.026 h−1. Some researches have shown [1,32] that both the absorption and removal in the degradation system of the nano metal solution occurred on the surface of the nano metal, such that the specific surface area of the nano metal was an important factor for the removal of the contaminant. When the specific surface area of the nanoparticles was bigger, there were more corresponding reactive sites, and the absorption and removal abilities were stronger. Accordingly, the increasing amount of nano metals added meant the addition of more reactive sites, thereby improving the efficiency of removal. 3.2.2. Effect of initial BDE209 concentration The effect of initial BDE209 concentration on the reaction rate was evaluated using various initial concentrations, and the results are shown in Fig. 5. The removal efficiency of BDE209 for these different initial concentrations after 24 h reaction were 97.92%, 93.76%, and 64.39%, respectively. Moreover, the reaction rate constants decreased as the initial concentration increased, which decreased from 0.207 h−1 for the initial concentration of 1 mg/L down to 0.049 h−1 for 4 mg/L. Bokare et al. [4] suggested when fixed number of reactive sites, the competitive adsorption would affect the adsorption and degradation of contaminants on the surface of nanoparticles, and further reduce the reaction rate. In addition, different initial concentrations of the BDE209 also affected the unit removal rate. The unit removal rate increased as the initial concentration increased, which increased from 2.45 mg/g S-NZVI at 1 mg/L BDE209 to 6.44 mg/g S-NVZI at 4 mg/L BDE209. The possible reason for this increase is the corrosion of nanoparticles by dissolved oxygen or contaminant and its by-product in water of the active sites on the nano metal led to the passivation of S-NZVI as the

Z. Fang et al. / Desalination 267 (2011) 34–41

37

Fig. 2. TEM and SEM patterns of nano metallic particles: (a) NZVI, (b) nano Ni/Fe, and (c) S-NZVI.

reaction time increased. The reactive sites of the stoichiometric excess would then provide the necessary surface area for the reaction [33]. Under a fixed amount of S-NZVI, when the initial BDE209 concentration was higher, this phenomenon was more obvious because the specific surface of the nanoparticles was fully used when there was a higher initial concentration. 3.2.3. Effect of solvent properties on degradation efficiency To demonstrate the importance of the proton to remove a bromine atom from BDE209, removal experiments were carried out in different solvent systems. The influence of different solvent systems on BDE209 removal by S-NZVI is shown in Fig. 6. For pure THF or a mixture of THF and ethanol as solvent, no BDE209 was removed within the set time of the experiment. The proportion of water in the solvent was one of the important factors influencing debromination efficiency. When the water content in the solution increased, BDE209 removal notably

enhanced. After 24 h of reaction time, the removal efficiency of BDE209 was only 15% for the THF/water (8/2, v/v) solution, and the removal efficiency increased to 48.35% for the THF/water (7/3, v/v) solution and 93.76% for the THF/water (6/4, v/v) solution. Catalytic hydrogenation was proposed by Shih and Tai [34] and Li et al. [2] as the major pathway for the debromination process based on their experimental data. The most fundamental steps of catalytic, reductive dehalogenation by Fe-based materials could be depicted by Eqs. (3)– (5) [35–37] (X = Cl, Br): 0

2þ

Fe → Fe

þ 2e

−

þ

Cx Hy Xz þ zH þ ze 2þ

2Fe

þ

ð3Þ −

−

→ Cx Hyþz þ zX

ð4Þ

−

ð5Þ

3þ

þ RX þ H → 2Fe

þ RH þ X

38

Z. Fang et al. / Desalination 267 (2011) 34–41

Fig. 5. Effect of initial BDE209 concentration on the removal efficiency (S-NZVI particles, 4 g/L; reaction time, 24 h; temperature, 28 ± 2 °C; pH = 6.09; THF/water = 6/4, v/v).

paper followed this trend: water (K a = 1 × 10 −14 ) N ethanol (Ka = 1 × 10−30) N THF (aprotic solvent). This means that the water can provide more hydrogen ion than other organic solvents for the reduction. Consequently, in the presence of water, particularly at 20%, 30%, and 40% water contents, the removal efficiency improved for increasing water content, as well as increasing the concentration of hydrogen ion in the solvent. But in organic solvent system, THF is the aprotic solvent, and the system cannot provide hydrogen ion for the reaction. Although ethanol is a protic solvent like water, its ionization constants (Ka) is 1016 times smaller than water's, so the solution could not provide enough protons for the reaction quickly, resulted in the reaction slowly, or even not occurring in the organic solvents. 3.3. Comparison of the degradation efficiency by different nano metallic particles

Fig. 3. EDS patterns of nano metallic particles.

The reactions above show that the presence of hydrogen ion in the solvent plays the most important role in degradation of the BDE209, and the ionization constants (Ka) of different solvents using in this

Fig. 4. Effect of S-NZVI particle addition on the removal efficiency (initial concentration of BDE209, 2 mg/L; reaction time, 24 h; temperature, 28 ± 2 °C; pH = 6.09; THF/ water = 6/4, v/v).

To compare the removal efficiency of BDE209 by S-NZVI with other metals, several other nano metallic particles and Fe powder were also tested, and the results are shown in Fig. 7. The removal efficiency of BDE209 by different metals (nano Ni/Fe, S-NZVI, NZVI, and Fe powder) at the end of the reaction time was very different (100%, 93.7%, 24%, and 4.5%, respectively). The generalized reactivity for BDE209 reduction followed this trend: nano Ni/Fe N S-NZVI N NZVI N Fe powder. Since the specific surface area is an important factor affecting

Fig. 6. Effect of solvent conditions on the removal efficiency of BDE209 (S-NZVI, 4 g/L; initial concentration of BDE209, 2 mg/L; reaction time, 24 h; temperature, 28 ± 2 °C).

Z. Fang et al. / Desalination 267 (2011) 34–41

39

particle surface and that hydrogen concentration would also increase when it is exposed to water. This was an advantage for catalytic degradation. However, the removal efficiency by S-NZVI (crystalline structure) was faster than NZVI (amorphous alloy). This must be due to another metal, such as Ni (0.011%) and Zn (0.002%) existing in S-NZVI, which could be enhance the catalytic activity similar to bimetallic nanoparticles, such as nano Ni/Fe particles. 3.4. Transformation of BDE209 by Fe-based particles and kinetics

Fig. 7. BDE209 removed by different Fe-based metallic particles (metallic particles, 4 g/L; initial concentration of BDE209, 2 mg/L; temperature, 28 ± 2 °C; pH = 6.09; THF/water = 6/4, v/v).

the reaction rate and the kinetics of the system, the greater surface area, the more conducive for higher activity and degradation by the nanoscale metallic particles. Thus, BDE209 was removed faster by NZVI particles, which possess greater specific surface area than Fe powder, whose specific surface area was 3 m2/g. This behavior was reflected by the pseudo-first-order rate constants (kobs) (Table 2). For example, the kobs of NZVI particles was 15 times larger than that of the Fe powder. However, the specific reaction rate constant (kSA) of the degradation of BDE209 by NZVI particles was only 1.3 times more than that of the Fe powder. In addition, there was a striking difference between the kSA values of these nanoscale metallic particles. The kSA of the degradation of BDE209 by nano Ni/Fe, S-NZVI, and NZVI particles were 6.110 × 10−3, 9.429 × 10−4, and 2.214 × 10−4 L h−1 m−2, respectively. Therefore, the removal efficiency of BDE209 followed the order: nano Ni/Fe N S-NZVIN NZVI. This strongly suggests that the modified nanoscale zero iron can be varied effectively in order to accelerate the dehalogenation. Wang et al. [38] proposed that noble metals, such as Ni or Pd, have a void orbit and can form a transitional complex compound with the chlorinated organic compounds (COCs). When a transitional compound is formed, the dechlorination activation energy is decreased. That is why the nano Ni/Fe particles can remove the BDE209 faster. This is represented by Eqs. (6)–(9) (X= Cl, Br): þ

2þ

Fe þ 2H → Fe

Fe þ 2H2 O → Fe þ

þ H2 ðin acidic solutionÞ

2þ

ð6Þ

−

þ H2 þ 2OH ðin alkaline solutionÞ

ð7Þ

Ni

H + e → H*

ð8Þ

Ni

H* + RNn → RNn−1 + N

ð9Þ

On the other hand, Kanel et al. [39] proposed that nanoscale Fe with amorphous alloy has a number of structural defects on the Table 2 Model regression parameters for the BDE209 removal by Fe-based metal. Materials

S-NZVI NZVI Nano Ni/Fe Fe powder a

Dosage (g/L)

BDE209 concentration (mg/L)

kobs (h−1)

kSA (L h−1 m−2)

t1/2 (h)

a 2

4 4 4 4

2 2 2 2

0.132 0.031 1.662 0.002

9.429 × 10−4 2.214 × 10−4 6.110 × 10−3 1.667 × 10−4

5.1724 22.358 0.417 346.550

0.967 0.891 0.986 0.897

R2 is the linearity of the data fitting.

R

3.4.1. HPLC chromatogram of BDE209 at different reaction times The chromatograms of BDE209 solution at various reaction times are shown in Fig. 8. Based on Fig. 8 (a–c), BDE209 (I), which was eluted at the retention time of 16.3 min, was gradually reduced with increasing reaction time. Meanwhile, the intermediate product (II), which was eluted at the retention time of 13.4 min and the final product (III), which was eluted at a retention time of 10.8 min, varied with increasing reaction time. All the intermediate products were eluted from the chromatographic column before BDE209. The intermediate product (II) reached its maximum concentration at 16 h of reaction time (by S-NZVI) and at 60 min reaction time (by nano Ni/Fe), after which it was decreased. According to three papers about degradation of BDE209 by zero-valent iron, degradation of BDE209 is a stepwise dehalogenation process. For example, Keum and Li [40] used zero-valent iron to reductive the BDE209, the result showed that BDE 209 was converted into mono- to hexa-BDEs after 40 days. In 2007, Li et al. [2] confidently identified the by-product of BDE209 included nona- through tri-BDEs products. Recently, Shih and Tai [34] reported that di- and mono-BDE were detected in the end of their experiment. In a word, all the papers above have demonstrated that BDE209 will be changed into lower BDEs by zero-valent iron. The results of BET, TEM, XRD, and EDS showed that S-NZVI is a kind of Fe-based bimetallic nanoparticles, and the retention time of new peaks appeared in the liquid chromatogram, we supposed that the degradation by-product of BDE209 in this paper may be lower BDEs. It is known that bioaccumulation and toxicity of PBDEs depend on the number of bromine atoms and their substituted position. For example, penta-BDEs were reported to be the most toxic among PBDE congeners [27]. So, doubtless, it's very important and prospective to take the potential adverse effect of BDE209 degradation by-products into account when environmental benefit is discussed. Meanwhile, we also suggest that the nanoscale material itself should be kept together with the by-product when environmental benefit is discussed. For these questions, we think that comprehensive toxicity assessment, such as exposure toxicity study [41], MTT test [42,43], Ames-test [44], and luminous bacteria detection [45,46], can become a science and feasible method for environmental benefits of by-products and environmental function material. Therefore, in order to better understand this degradation mechanism and evaluate the environmental benefits of adverse effect of BDE209 degradation by-products, gas chromatography-mass spectrometry (GC-MS) analysis and comprehensive toxicity assessment are essential to further investigate. 3.4.2. Kinetics The process of reduction of BDE209 by Fe-based materials can be satisfactorily described by the following pseudo-first-order kinetic model [47]: dCBDE209 = −kobs CBDE209 = −kSA pm CBDE209 dt

ð10Þ

where CBDE209 is the concentration of the parent compound (mg/L), kobs is the pseudo-first-order rate constant (h−1), kSA is the specific reaction rate constant (L h−1 m−2), and pm is the metal surface area concentration in the solution (m2/L). The values of kobs and kSA are

40

Z. Fang et al. / Desalination 267 (2011) 34–41 Table 3 Price of chemical reagents used in the experiment to synthesize nanoparticles. Reagent

FeSO4·7H2O NiCl2·6H2O NaBH4 PVP K-30 CH3CH2OH (ARa, 500 g) (AR, 500 g) (AR, 100 g) (AR, 100 g) (AR, 500 mL)

Price (RMB) 10 a

100

135

63.5

7

AR is short for analytical reagent.

Table 4 Economic cost of Fe-based materials. Fe-based material

Economic cost (1 kg, RMB)

Economic cost to remove 1 kg of BDE209 (RMB)

S-NZVI NZVI Nano Ni/Fe Fe powder

3365 3465 4580 52

727,862 1,369,351 923,616 230,455

times greater than that of Fe powder. We can therefore conclude that the degradation of BDE209 by S-NZVI is both feasible and efficient. 3.5. Economic evaluation

Fig. 8. HPLC chromatograms of BDE209 at different reaction times: (a) S-NZVI, (b) NZVI, (c) nano Ni/Fe, and (d) Fe powder (metal concentration, 4 g/L; initial concentration of BDE209, 2 mg/L; temperature, 28 ± 2 °C; pH = 6.09; THF/water = 6/4, v/v).

shown in Table 2. Since we could not keep adding nitrogen during the reaction time, the surface passivation layers or precipitation of meta hydroxides on the surface of Fe were allowed to form, and the reaction could not fit the pseudo-first-order reaction kinetics very well [48]. The kobs of the degradation of BDE209 followed the order: nano Ni/Fe N S-NZVI N NZVI N Fe powder. The kobs of S-NZVI was 4 times greater than that of NZVI and 66 times greater than that of Fe powder, while the kSA of S-NZVI was 4 times greater than that of NZVI and 5

Aside from the efficient performance and environmentally friendly aspect, economic cost can also determine whether a new approach can be applied to the environment or not. Therefore, we calculated the chemical reagent costs of several Fe-based materials. The price of all the chemical reagents (analytical grade, the prices are shown in Table 3) was based on the market prices in Guangzhou, China (Dec.2009). The results are shown in Table 4. According to Tables 2 and 4, there is no doubt that the cost and the degradation efficiency of S-NZVI is better than NZVI. Even more importantly, S-NZVI can produce a beneficial effect to the economy and the environment because the raw material is steel pickling waste liquor. In addition, the high performance and suitable cost of nano Ni/Fe is offset by the environmental hazard of Ni which limits its use in the environment. The cost of Fe powder is cheaper than the nanoparticles, but it requires more time to remove BDE209 compared to the other materials. But for the practical application, this new material need to improve by two methods since the PBDEs tend to bind to the organic fractions in sediments and soils. One is that immobilize the nano zero-valent iron into a functional carrier so that it can be suitable for different requirement. For example, Choi et al. [49] have developed a new strategy, employed a series of adsorptive granular activated carbon (GAC) composites incorporated with reactive ZVI, and Pd bimetallic nanoparticles (GAC/ZVI/Pd). This system can synergistically combine the adsorptive capacity of GAC for the contaminants and the Pd/Fe for degradation of the contaminants. Another is modifying nano zerovalent iron with stabilizer for directly injecting zero-valent iron (ZVI) nanoparticles. Because the NZVI can migrate through the saturated zone to reach the contaminant. And the stabilizer also can enhance the steric or electrostatic repulsions of particles to prevent their aggregation and makes them effective for transport through soil to the contaminate point. Like He et al. [50] used carboxymethyl cellulose (CMC) as a stabilizer and demonstrated nano zero-valnet iron with CMC can be readily dispersed in various saturated porous media including soils. 4. Conclusion A new method of reusing the steel pickling waste liquor to prepare the nanoscale zero-valent metallic particles was developed in this paper. The results of characterization indicated that the S-NZVI is a kind of Fe-based materials. And degradation experiment results

Z. Fang et al. / Desalination 267 (2011) 34–41

suggest that the nanoscale zero-valent metal particles prepared from steel pickling waste liquor are effective in removing BDE209 from a THF/water solution at ambient temperature and pressure. The degradation of BDE209 followed a pseudo-first order kinetics model, and the removal efficiency of BDE209 benefited from an increase amount of S-NZVI, a decrease in the contamination level, and proper THF/water system. Comparing its removal efficiency with that of different Fe-based materials and evaluating the cost, it was found that the removal of BDE209 by S-NZVI is more effective than NZVI under the same conditions, and the economic cost is better in comparison to other metals. Meanwhile, evidence obtained here strongly suggested that nanoscale metal from steel pickling waste liquor transformed BDE209 via catalytic degradation and proton plays an important role in the reaction. Acknowledgment The authors are grateful for the financial support provided by the National Science and Technology Major Projects of Water Pollution Control and Management of China (2009ZX07011).

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