Ruey-an Doong*ab,
Sandip Sahaa,
Cheng-hsien Leeb and
Hong-ping Linc
aInstitute of Environmental Engineering, National Chiao Tung University, Hsinchu, 30010, Taiwan. E-mail: radoong@nctu.edu.tw
bDepartment of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, 30013, Taiwan
cDepartment of Chemistry, National Cheng Kung University, Tainan, 700, Taiwan
First published on 9th October 2015
In this study, mesoporous silica (SiO2) microspheres were hydrothermally synthesized in the presence of gelatin for the immobilization of bimetallic Pd/Fe nanoparticles (Pd–Fe/SiO2) to enhance the dechlorination efficiency and dechlorination rate of tetrachloroethylene (PCE) under anoxic conditions. Scanning electron microscopy images and elemental mapping showed that the distribution of nanoscale zerovalent iron (NZVI, Fe) on SiO2 was uniform and the density of Fe increased as the iron loading increased from 10 to 50 wt%. The optimized 30 wt% Fe/SiO2 particles were used to fabricate Pd–Fe/SiO2 microspheres by the electrochemical reduction of Pd ions to Pd0 for the enhanced dechlorination of PCE under anoxic conditions. The dechlorination efficiency and rate of PCE by Fe/SiO2 was significantly enhanced in the presence of 0.5–3 wt% Pd and the pseudo-first-order rate constant (kobs) for PCE dechlorination by the mesoporous Pd–Fe/SiO2 microspheres increased by 2–3 orders of magnitude when compared with that of Fe/SiO2 alone. Ethane was found as the only end product with a carbon mass balance of 95–100%, showing that hydrodechlorination was the main reaction mechanism for PCE dechlorination by the Pd–Fe/SiO2 microspheres. Column experiments showed the good mobility and permeability of the mesoporous Pd–Fe/SiO2 microspheres when compared with that of pure NZVI alone. The results obtained in this study clearly show that the immobilization of bimetallic Pd–Fe nanoparticles on the mesoporous SiO2 microspheres not only increases the reactivity for the dechlorination of PCE but also enhances the mobility and permeability for in situ remediation of chlorinated hydrocarbons in porous media.
Several technologies have been developed for the detection6 and dechlorination of chlorinated hydrocarbons.7–10 Permeable reactive barriers (PRBs) packed with zerovalent metals have been demonstrated to be an effective method for the removal of organic and inorganic pollutants under anoxic conditions.11–13 Various laboratory-scale and field applications have found that nanoscale zerovalent iron (NZVI) and bimetallic iron-based nanoparticles including Fe/Pd and Fe/Ni can transform the chlorinated compounds into less-chlorinated homologues and non-chlorinated end products through reductive dechlorination and hydrodechlorination.14–18 Several studies have shown that chlorinated hydrocarbons can be rapidly dechlorinated into non-chlorinated hydrocarbons by using nanoscale bimetallic Pd/Fe particles.18,19 However, the aggregation of NZVI decreases the dechlorination efficiency and dechlorination rate of chlorinated compounds. Several studies have demonstrated the effectiveness of using a stabilizer or support to homogeneously disperse NZVI nanoparticles to enhance their stability and mobility.20–28 Various organic polymers including carboxymethyl cellulose, polyacrylic acid (PAA) and electrospun PAA/polyvinyl alcohol nanofibers have been used to immobilize Pd/Fe for enhanced dechlorination efficiency and dechlorination rate of chlorinated compounds.22,23 In addition, the organic polymer-immobilized iron-based nanoparticles have high mobility to facilitate the nanoparticles’ transport through soil columns.24,25
Another strategy for increasing the stability and reactivity of nanoparticles is the immobilization on supports such as membranes and carbon materials.26–31 Parshetti and Doong28 have immobilized bimetallic Ni/Fe nanoparticles in PVDF and nylon 66 membranes in the presence of polyethylene glycol for the dechlorination of TCE and found that the immobilization of the Fe/Ni nanoparticles in the hydrophilic membrane can retain the longevity and high reactivity of the nanoparticles towards TCE dechlorination. In addition, Su et al. used activated carbon-supported zerovalent iron (ZVI/AC) for the adsorption and dechlorination of TCE.27 The rate constant for TCE dechlorination by ZVI/AC was 7 times higher than that of AC only. More recently, silica has been used as a support to maintain the reactivity and stability of iron-based nanoparticles.32–34 The use of mesoporous SiO2 microspheres as the support has several advantages including the prevention of NZVI agglomeration, environmental friendliness and increased reactivity and stability. Ensie and Samad have immobilized NZVI onto a SiO2@FeOOH core using a reduction method to remove nitrate from drinking water and found that greater than 99% of the nitrate could be removed by the nano SiO2@FeOOH@Fe core–shell material at pH 3.34 In addition, Xie et al. compared four different iron-based nanoparticles for the dechlorination of polybrominated diphenyl ethers.32 The results showed that the core–shell type SiO2@FeOOH@Fe materials had higher usability and stability and a low Fe2+ leaching rate when compared to the other iron-based materials. It is noteworthy that a mesoporous SiO2 microsphere is stable, environmentally friendly and can be transported with water in porous media. However, the use of mesoporous SiO2 microspheres as a support for bimetallic Pd/Fe nanoparticles for the dechlorination of chlorinated hydrocarbons has received less attention. In addition, little information is available on the characterization and dechlorination efficiency of bimetallic Pd–Fe nanoparticles on mesoporous silica.
In this work, bimetallic Pd/Fe nanoparticles were immobilized onto mesoporous SiO2 microspheres for PCE dechlorination under anoxic conditions. The mesoporous SiO2 microspheres were hydrothermally fabricated in the presence of gelatin and then the bimetallic Pd/Fe nanoparticles were immobilized onto the surface of the mesoporous SiO2. The properties of the microstructures of the Pd–Fe/SiO2 microspheres including morphology, specific surface area, pore texture, and chemical species were characterized. The effect of the Fe and Pd loadings on the dechlorination efficiency and dechlorination rate of PCE was optimized. In addition, the longevity and stability of the mesoporous Pd–Fe/SiO2 microspheres were investigated.
Pd2+ + Fe0 → Pd0 + Fe2+ | (1) |
The column experiments were performed by using a 15 mL glass burette packed with 0.4–0.6 mm glass beads to investigate the mobility and water permeability of the Pd–Fe/SiO2 microspheres and NZVI. The column diameter was 1.5 cm and the total pore volume was 11.9 cm3. The columns were saturated with deoxygenated deionized water before the addition of 2 g L−1 NZVI or Pd–Fe/SiO2 suspensions. The flow rate of the feeding solution was 4.8 mL min−1.
The crystallite size and crystal phase of the Pd–Fe/SiO2 microspheres were examined using a Scintag X1 advanced X-ray diffractometer (XRD) with Ni-filtered Cu Kα radiation (λ = 0.1541 nm). The XRD patterns were obtained from 20° to 70° 2θ with a sampling step width of 0.05° and a step time of 0.5 s. The specific surface area, pore volume and pore size distribution were determined by the N2 adsorption–desorption isotherms at 77 K using a ASAP 2020 surface area and porosimetry system manufactured by Micromeritics Co. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas (SBET) using the adsorption data in a relative pressure (P/P0) range from 0.02 to 0.2. By using the Barrett–Joyner–Halenda (BJH) model, the pore volumes and pore size distributions in the mesopore range (>2 nm) could be derived from the adsorption branches of the isotherms, and the total pore volumes (Vt) were calculated from the adsorbed amount at a relative pressure of 0.995.
The surface morphology of the mesoporous Pd–Fe/SiO2 microspheres was observed using a JOEL field-emission scanning electron microscope (FE-SEM). The TEM images and particle sizes of Pd–Fe/SiO2 were obtained using a Hitachi H-7500 transmission electron microscope at an acceleration voltage of 80 kV by suspending one drop of the microsphere suspension on a 300-mesh carbon coated Cu grid. The X-ray photoelectron spectroscopic (XPS) measurements were performed on a ESCA PHI 1600 photoelectron spectrometer using an Al Kα X-ray source (1486.6 eV). The binding energies of the photoelectrons were determined by assuming that the carbon 1s electron has a binding energy of 284.8 eV. During the data acquisition, the pressure in the sample chamber did not exceed 2.5 × 10−9 Torr.
Fig. 1 shows the SEM and TEM images of 10–50 wt% Fe/SiO2 prepared by adding 50 mL of 0.2 M NaBH4 into the mesoporous SiO2 solutions containing ferrous ions. It is clear that the particle number as well as the size of the NZVI on the SiO2 microspheres increased with the increase in Fe loading. No obvious aggregation of NZVI was observed when the loading of Fe was lower than 30 wt% (Fig. 1a and b). However, the NZVI aggregated into large particles at 50 wt% Fe and some NZVI nanoparticles were agglomerated into long chains outside of the SiO2 microspheres (Fig. 1c). The EDS analysis indicated that the atomic ratios of Fe to Si were 0.122 for 10 wt% Fe/SiO2, 0.316 for 30 wt% Fe/SiO2 and 0.575 for 50 wt% Fe/SiO2. In addition, the ICP-MS analysis showed similar results and the Fe concentration was 12.5 wt% for 10 wt% Fe/SiO2, 30.3 wt% for 30 wt% Fe/SiO2, and 45.1 wt% for 50 wt% Fe/SiO2. These results clearly indicate the nearly complete reduction of ferrous ions to NZVI on the surface of mesoporous SiO2 by NaBH4.
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Fig. 1 The SEM and TEM images of various iron loadings of Fe/SiO2 microspheres. (a–c) SEM images of 10, 30, and 50 wt% Fe/SiO2, (d–f) TEM image of 10, 30 and 50 wt% Fe/SiO2. |
Similar to the SEM images, the TEM images clearly showed the formation of discrete NZVI nanoparticles on the surface of the mesoporous SiO2 microspheres when the Fe loadings were in the range 10–30 wt% (Fig. 1d and e). In contrast, the agglomeration of long-chained NZVI was clearly observed at 50 wt% Fe/SiO2 (Fig. 1f). Fig. 2 shows a histogram of the particle sizes of NZVI on Fe/SiO2 at various iron loadings of 10–50 wt%. The particle size distribution of NZVI at 10 wt% Fe/SiO2 was in the range of 60–160 nm, and then increased to 120–240 nm at 50 wt% Fe/SiO2. In addition, the average particle sizes of NZVI were 85 ± 27 nm, 150 ± 21 nm and 165 ± 30 nm for 10, 30, and 50 wt% Fe/SiO2, respectively. No obvious difference in the mean diameter of NZVI between 30 and 50 wt% Fe/SiO2 reflected the fact that 30 wt% could be the optimal iron loading for the mesoporous SiO2 microspheres.
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Fig. 2 The particle size distribution of NZVI on the Fe/SiO2 microspheres at various iron loadings ranging from 10 to 50 wt%. |
Fig. 3 shows the XRD patterns and specific surface areas of the Fe/SiO2 microspheres with various Fe loading. As shown in Fig. 3a, the XRD pattern of pure NZVI showed peaks centered at 44.6° and 65.0° 2θ, which can be assigned to the (110) and (200) planes of body-centered cubic Fe (JCPDS 06-0696) respectively.35 The broad peak at 20–25° 2θ for the Fe/SiO2 microspheres was mainly attributed to the amorphous characteristics of SiO2. In addition, the small and broad peak at 44.6° 2θ appeared when the iron loading increased from 10 to 50 wt%, indicating the successful immobilization of NZVI onto the surface of SiO2.
Fig. 3b shows the BET surface area of the 10–50 wt% Fe/SiO2 microspheres. Similar to the pure mesoporous SiO2 microspheres, the N2 adsorption–desorption isotherms of Fe/SiO2 with varying Fe loadings followed a type IV physisorption isotherm with a H3 hysteresis loop in the P/P0 range of 0.4–0.95, which is mainly attributed to capillary condensation in the mesopores of the silica substrate.36 The specific surface areas of 10 and 30 wt% Fe/SiO2 were in the range of 196–209 m2 g−1 and then decreased to 143 m2 g−1 when the Fe loading increased to 50 wt%. In addition, the pore size distributions of 10–50 wt% Fe/SiO2 were similar and the average pore diameters were in the range of 5.7–6.0 nm, which indicate that the decrease in the specific surface area of 50 wt% Fe/SiO2 is mainly attributed to the formation of large and excess NZVI nanoparticles onto and outside of the mesoporous SiO2 microspheres. It is noteworthy that the average pore diameter of the Fe/SiO2 microspheres is smaller than that of the pure SiO2 microspheres, presumably attributed to the formation of NZVI on the surface of SiO2 microspheres.
To further elucidate the immobilization of Pd onto the Fe/SiO2 surfaces, XPS was used to characterize the change in the chemical species of the elements in the Pd–Fe/SiO2 microspheres. Fig. 4 shows the XPS spectra of the Si 2p, Fe 2p and Pd 3d species in the 3 wt% Pd–Fe/SiO2 microspheres. The XP spectrum of Si 2p showed a broad peak at 100–105 eV, indicating the formation of SiO2 (Fig. 4a). After Ar sputtering for 1 min to remove the top 27 nm of the particle surface, the Si 2p peak still remained at the same position, indicating the stability of SiO2 as the support. The XPS of Fe 2p showed two peaks centered at 711 and 724 eV, which were the characteristic Fe 2p3/2 and Fe 2p1/2 peaks of iron oxides (Fig. 4b). Additional zerovalent iron peaks at 707 and 720 eV appeared after Ar sputtering for 1 min, clearly indicating the transformation of ZNVI to iron oxides after the addition of Pd2+. This result also supports the XRD results, that no iron species was identified in the Pd–Fe/SiO2 microspheres. In addition, the XPS peak of the iron oxides shifted from 711.8 eV to 710.4 eV. After peak deconvolution, the major species of the iron oxides changed from goethite (FeOOH) to magnetite (Fe3O4) after Ar sputtering (Fig. S5, ESI†), which reflected the fact that the reduction of Pd2+ to Pd0 changed the species of the iron oxides on the surface of NZVI. The XPS spectra of the 3 wt% Pd loaded Fe/SiO2 showed Pd 3d peaks at 334.9 and 340.2 eV, which belonged to Pd0 3d5/2 and Pd0 3d3/2, respectively (Fig. 4c). The peak positions of the Pd species did not change after Ar sputtering for 1 min, clearly indicating the formation of zerovalent Pd. In addition, no other peak related to the oxidation state of Pd2+ was observed, confirming the complete reduction of the immobilized Pd2+ to Pd0 through the electrochemical Pd2+–Fe0 reduction reaction.
The addition of second catalytic metal ions such as Pd, Ni and Pt onto the iron surface can significantly enhance the dechlorination efficiency and dechlorination rate of chlorinated compounds.35,37,38 Fig. 5a shows the PCE dechlorination efficiency of SiO2-supported Pd–Fe nanoparticles at various Pd loadings of 0.1–3 wt%. The PCE dechlorination efficiency of Pd–Fe/SiO2 increased significantly from 33% at 0.1 wt% Pd to 99.5% at 0.3 wt% Pd and nearly complete PCE dechlorination was observed with Pd loadings >0.5 wt%. Fig. 5b shows the kobs for PCE dechlorination as a function of Pd loading. The kobs values increased positively from 0.011 ± 0.004 min−1 at 0.1 wt% Pd to 0.267 ± 0.021 min−1 at 0.5 wt% Pd and then slightly decreased to 0.253 ± 0.023 min−1 when the Pd concentration increased to 3 wt%. These values are 2–3 orders of magnitude (108–2625 times) higher than that of the pure Fe/SiO2 microspheres at pH 5.5, showing that the addition of low amounts of Pd ions can significantly enhance the dechlorination efficiency and dechlorination rate of PCE by mesoporous SiO2-supported NZVI.
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Fig. 5 (a) The dechlorination of PCE by 0.1–3 wt% Pd–Fe/SiO2 under anoxic conditions and (b) pseudo-first-order rate constant for PCE dechlorination as a function of Pd loading. |
Several studies have addressed the effect of additive loadings on the dechlorination efficiency of zerovalent metals toward chlorinated hydrocarbons.39–41 An optimal mass loading often exists for a wide variety of bimetallic catalysts including Ni/Fe, Pd/Fe, Ni/Si and Ru/Fe. Lin et al. reported that the dechlorination rate of TCE by bimetallic Ru/Fe increased as the Ru loading increased from 0.25 to 1.5 wt%.41 A decrease in kobs was also observed when the Ru loading increased to 2.0 wt%. In this study, an optimal dosage of 0.5 wt% Pd ions was obtained for the dechlorination of PCE by the Fe/SiO2 microspheres. The addition of catalytic Pd ions prevents the formation of toxic products by the dechlorination of chlorinated hydrocarbons via hydrogen reduction rather than through electron transfer.35 As shown in Fig. 6, ethane was the only detected product for the dechlorination of PCE by 0.5–3 wt% Pd–Fe/SiO2 and the carbon mass balances were all greater than 95%, showing that hydrodechlorination is the major reaction mechanism for the dechlorination of PCE by the mesoporous Pd–Fe/SiO2 microspheres. In addition, several studies have reported the hydrodechlorination of gaseous PCE over Pd-based catalysts in the presence of hydrogen under high temperature conditions.42–44 Bueres et al. immobilized 1 wt% Pd catalysts on three different carbonaceous materials and found that 68–97% of the gaseous PCE was converted to ethane and trace amounts of TCE by the carbon supported Pd at 525 K after 30 h of incubation.44 This indicates that Pd only can show its dechlorination ability at high temperature and the Pd–Fe/SiO2 microspheres are excellent catalysts for the hydrodechlorination of PCE in aqueous solutions.
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Fig. 6 The production of ethane and carbon mass balance during the dechlorination of PCE by the Pd–Fe/SiO2 microspheres (various Pd loadings). (a) 0.5, (b) 1, (c) 2 and (d) 3 wt% Pd. |
The long-term performance of 0.5 wt% Pd–Fe/SiO2 in the dechlorination of PCE was further investigated by repeatedly injecting 5 mg L−1 PCE into the solution containing Pd–Fe/SiO2 microspheres. Fig. 7 shows the stability and longevity of Pd–Fe/SiO2 with the repeated spiking of PCE under anoxic conditions. A fast PCE dechlorination process was observed and the Pd–Fe/SiO2 microspheres can be reused for at least 10 times, all with a stable dechlorination efficiency of >99%. Several studies have reported the repeated use of Pd/Fe nanoparticles for dechlorination. Nagpal et al. have used bimetallic Fe–Pd nanoparticles for lindane degradation under anoxic conditions and found that the dechlorination efficiency of the recycled Fe–Pd dropped slightly to 92% after the 10th cycle.45 In this study, the kobs values and initial rate for PCE dechlorination were in the range 0.207–0.271 min−1 and 8.96–11.5 μM min−1, respectively, showing that the 0.5 wt% Pd–Fe/SiO2 microspheres are a promising material for the dechlorination of chlorinated hydrocarbons. The stability of Pd–Fe/SiO2 is mainly attributed to the introduction of SiO2 as the support. Reardon et al. have proposed that the presence of silica may slow down iron corrosion by adsorbing silicate, the dissolved form of SiO2 after reaction, to the anodic surface.46 In this study, the solution pH after dechlorination in the presence of the Fe/SiO2 microspheres increased by 0.4–0.8 units which is lower than that in the presence of pure NZVI (1–1.5 units). This result supports the hypothesis that the introduction of the SiO2 microspheres could maintain the reactivity of NZVI with >99% dechlorination efficiency of PCE.
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Fig. 7 The stability and longevity of mesoporous Pd–Fe/SiO2 microspheres with repeated spiking of 5 mg L−1 PCE. |
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Fig. 8 (a) The breakthrough curves and (b) permeability of pure NZVI and mesoporous 0.5 wt% Pd–Fe/SiO2 microspheres. M/M0 represents the fraction of particles that are eluted in the effluent. |
Fig. 8b shows the water permeability of pure NZVI and Pd–Fe/SiO2 by packing 1 cm of different materials in the middle of the column. In the absence of Fe-based particles, the average water flow in the glass bead packed column was 82–90 mL h−1, and slightly decreased to 72–80 mL h−1 in the presence of the mesoporous Pd–Fe/SiO2 microspheres. However, the water flow decreased to 58–70 mL h−1 when pure NZVI was packed in the column, showing that the mesoporous SiO2 microspheres are a good support with good permeability to disperse NZVI for the remediation of chlorinated compounds in porous media.
Fig. 9 shows the possible reaction mechanism for PCE dechlorination by the mesoporous Pd–Fe/SiO2 microspheres under anoxic conditions. The negatively charged silica surface can adsorb Fe2+ ions and then convert them to NZVI in the presence of NaBH4, which can be well dispersed on the surface of the SiO2 microspheres to enhance the reactivity of Fe/SiO2. The silica-supported Pd–Fe nanoparticles were electrochemically generated when the Pd2+ ions were adsorbed onto the NZVI surfaces. The hydrodechlorination of PCE by the bimetallic Pd–Fe nanoparticles involves the oxidation of NZVI to galvanically protect the Pd metal and the Pd metal provides active sites for hydrodechlorination.48 As the corrosion of NZVI increases, protons from water are reduced to adsorbed H atoms and to molecular hydrogen at the catalytic Pd surface, resulting in the formation of ethane during the hydrodechlorination of PCE.
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Fig. 9 The (a) immobilization of Pd–Fe nanoparticles onto mesoporous SiO2 microspheres and (b) proposed mechanism for PCE dechlorination by mesoporous Pd–Fe/SiO2 microspheres. |
Footnote |
† Electronic supplementary information (ESI) available: TEM images and surface areas of mesoporous SiO2; XRD patterns, adsorption and dechlorination of PCE by Fe/SiO2; TEM images, deconvolution of XPS spectra and column experiments of Pd–Fe/SiO2. See DOI: 10.1039/c5ra15070a |
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