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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20

Removal of Pb (II), Cd (II), Cu (II) and trichloroethylene from water by Nanofer ZVI a

Mahmoud M. Eglal & Amruthur S. Ramamurthy

a

a

Department of Building Civil and Environmental Engineering, Concordia University, Montreal, Canada Published online: 10 Jun 2015.

Click for updates To cite this article: Mahmoud M. Eglal & Amruthur S. Ramamurthy (2015) Removal of Pb (II), Cd (II), Cu (II) and trichloroethylene from water by Nanofer ZVI, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 50:9, 901-912 To link to this article: http://dx.doi.org/10.1080/10934529.2015.1030277

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Journal of Environmental Science and Health, Part A (2015) 50, 901–912 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2015.1030277

Removal of Pb (II), Cd (II), Cu (II) and trichloroethylene from water by Nanofer ZVI MAHMOUD M. EGLAL and AMRUTHUR S. RAMAMURTHY

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Department of Building Civil and Environmental Engineering, Concordia University, Montreal, Canada

Zero-valent iron nanoparticle (Nanofer ZVI) is a new reagent due to its unique structure and properties. Images of scanning electron microscopy/electron dispersive spectroscopy (SEM/EDS), transmission electron microscopy and X-ray diffraction revealed that Nanofer ZVI is stable, reactive and has a unique structure. The particles exhibited a spherical shape, a chain-like structure with a particle size of 20 to 100 nm and a surface area between 25–30 m2g¡1. The time interval for particles to agglomerate and settle was between 4–6 h. SEM/EDS Images showed that particle size increased to 2 mm due to agglomeration. Investigation of adsorption and oxidation behavior of Nanofer ZVI used for the removal of Cu(II), Pb(II), Cd(II) ions and trichloroethylene (TCE) from aqueous solutions showed that the optimal pH for Pb(II), Cu(II), Cd(II) and TCE removal were 4.5 and 4.8, 5.0 and 6.5, respectively. Test data were used to form Langmuir and Freundlich isotherms. The maximum contaminant loading was estimated as 270, 170, 110, 130 mg per gram of Nanofer ZVI for Cu(II), Pb(II), Cd(II) and TCE respectively. Removal of metal ions is interpreted in terms of their hydrated ionic radii and their electronegativity. TCE oxidation followed the dechlorination pathway resulting in nonhazardous by-products. Keywords: Freundlich isotherm, heavy metals, Langmuir isotherms, Nanofer ZVI, TCE.

Introduction Nano–zero-valent iron (nZVI) is a new and innovative nanomaterial that is used for water and groundwater treatment. The application of engineered nanomaterials for environmental clean-up emerged when a small amount of nZVI was used to reduce trichloroethylene and polychlorinated biphenyls.[1] Later, intensive studies have focused on the degradation of water and groundwater contaminants by nZVI. Weile[2] reviewed and classified more than three hundred publications dealing with the use of nZVI for the removal of contaminants from environmental media. The classification included studies linked to removal of halogenated organic compounds, arsenic, radionuclides and heavy metals. However, with increasing attention for field applications for both in situ groundwater and soil treatment, research efforts are focused on enhancing the removal efficiency, stability, mobility, particle separation, and long-term eco-impact.[3] Address correspondence to Mahmoud M. Eglal, Building Civil and Environnemental Engineering, Concordia University, 1455 de Maisonneuve Blvd. West, Montreal, Quebec H3G 1M8, Canada; E-mail: [email protected] Received December 23, 2014. Color versions of one or more figures in this article can be found online at www.tandfonline.com/lesa.

Although there is an intense research focus resulting in a huge number of publications on the nZVI application to remove contaminants, some aspects of the fundamental knowledge of nanoparticles, surface morphology, physicochemistry, interaction with contaminants and the change of nZVI properties due to environmental media warrants further investigation.[4–6] Mueller et al.[4] reviewed the practical application of nZVI and found that the application of bimetallic nZVI to remove chlorinated compounds such as TCE and PCB was challenging due to the dynamic change of reactivity of nZVI caused by the presence of other contaminants. Consequently, many uncertainties concerning the fundamental aspect of the use of nZVI in permeable reactive barrier (PRB) technology remain to be probed. Recent studies of Li and Zhang[7] dealing with the core shell of ZVI nanoparticle suggested that the passivation of the oxide layer surrounding the nanoparticle surface is an important factor for particle stability and reactivity. It is capable of absorbing contaminants forming coordinative bonds and remain permeable to electron and mass transports. Nanofer ZVI is the new generation of nZVI produced by Nanoiron Ltd. in 2012. The material was developed to overcome the problems faced by the coated and non-coated nZVI.[6,8] The Nanofer ZVI was first covered by polyvinylpyrrolidone (PVP), which served as the anchor polymer for silica deposition using tetraethyl

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902 orthosilicate (TEOS).[6] Various tests related to z potential and particle settling revealed that dipping Nanofer ZVI in TEOS increases its stability for subsurface applications and reduces agglomeration. Further detailed studies are needed to investigate the reactivity of this new coated material with different organic and inorganic water contaminants such as class A metals and TCE. The mechanisms of heavy metal removal using nZVI depends on the standard redox potential (Eo) of the metal contaminant.[9] For instance, Cd which is more electronegative than Fe0 gets removed by adsorption to the iron hydroxide shell. On the other hand, metals with E0 which is much more positive than Fe0 (e.g., Cu) are more readily removed by reduction and precipitation. However, Wilkin and McNeil[10] found that the adsorption mechanism is the main dominant factor for Cu uptake. Pb (II), which is slightly more electropositive than Fe0 gets removed by reduction and adsorption. In general, metal removal is also linked to oxidation and co-precipitation, which in turn depends on prevailing chemical conditions (pH, initial concentration and speciation of contaminant metals). Earlier studies on different forms of nZVI and ZVI impregnated with other metals have focused on the reductive dehalogonation of organic compounds such as TCE and BCP, as well as the reductive precipitation of hexavalent chromium and arsenic.[8] The number of studies examining the specific functions of nZVI specially the metalcore and the oxide-shell on metal adsorption are limited. The effect of solution chemistry dealing with pH, contaminant concentration and other related aspects have received less attention and are not widely discussed in literature. The objective of the present study is to determine the effect of solution chemistry dealing with pH, contaminant concentration, dosage and other related aspects on the stability and reactivity of the new coated Nanofer ZVI towards different contaminants [Pb (II), Cd (II), Cu (II) and TCE] on the basis of its nanostructure using SEM/ EDS, TEM and XRD. These contaminants are specifically different from each other in terms of electrochemical properties and are used to probe various reactive pathways permissible with nZVI applications. Further, the reactivity and stability of the selected Nanofer ZVI to reduce, adsorb and transform contaminants and its potential in treating diverse groups of water contaminants will also be examined.

Eglal and Ramamurthy Table 1. Selected organic and inorganic chemicals used in the study. Reagent Inorganic Cu(II) Cd(II) Pb (II) Organic TCE PVP TEOS

Formula

Source

CuCl2 CdCl2 PbCl2

Fisher* Fisher* Fisher*

C2HCl3 (C6H9NO)n SiC8H20O4

Fisher* Sigma Aldrich** Sigma Aldrich**

*Fisher Scientific, Waltham, MA, USA. **Sigma Aldrich, St. Louis, MO, USA.

ZVI nanoparticles For the study, Nanofer ZVI was obtained from NANOIRON Ltd. (Czech Republic). Following the procedure suggested by Lenka et al.[6] the iron particles were disintegrated using ultrasound (Hielsher UP 200S, 18 kHz. After 15 min of sonification, the particles were mixed and stirred (2 h) with polyvinylpyrrolidone (PVP). Later, the suspension of (20 mL) was mixed with 30 mL of ethanol and 1 mL of tetraethyl orthosilicate (TEOS). The resulting mixture was then heated to 50 C. After adding 2 mL of NH4OH, it was stirred for 15 min, the resulting Nanofer ZVI was stored for later use. Particle characteristics Images of the newly synthesized and coated Nanofer ZVI particles were recorded at 200 kV using the transmission electron microscope (TEM, JEOL Ltd., Japan the JEOL 2000 FX). In the TEM, the particle structure was examined by passing a beam of electrons through the specimen. The image appeared as a shadow of the specimen. SEM was operated using the Hitachi S-3400N that was equipped with the energy- dispersive X-ray spectroscope (EDS) system. The EDS was conditioned at 55 kV, dead time 35%. Philips Xpert Pro multipurpose X-ray diffractometer (PANalytical, Almelo, the Netherlands) was used with Bragg-Vrentano geometry and CuKa radiation column. The particles were placed in a glass holder and scanned from 20 to 75 . Table 2. Metal ion characteristics. Metals

Materials and methods Chemicals All chemicals were obtained from Fisher Scientific and they were classed as pure grade (99.9%). Tables 1 and 2 provide additional details related to different chemical solutions [CuCl2, CdCl2, PbCl2,C2HCl3, (C6H9NO)n and SiC8H20O4] used in the study.

Characteristics Hydrated radius (A ) Ionic radius (A ) electronegativity Polarization Electron configuration

Cu (II) 4.19 0.72 1.8 6.1 4s 3d 1B

Pb (II) 4.01 1.2 1.6 6.8 6s 4f 5d 6p 4A

Cd (II) 4.26 0.97 1.5 7.2 5s 4d 2B

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The BET isotherm was the basis for determining the extent of nitrogen adsorption on the particle surface. The nitrogen physio-sorption with Coulter SA 3100 analyzer (Barrett-Joyner-Hanleda) was used in this study. The sorption and desorption of nitrogen provided the important information related to surface area characteristics. The pH versus zeta potential (z) diagram was employed to determine the Iso-electric point (IEP) for the Nanofer ZVI. The solution pH was adjusted using 2.0 N NaOH. The titration was begun after the iron nanoparticles were suspended with de-ionized water for 30 min to allow the solution to reach equilibrium. The diagram of pH vs. z yielded the IEP data at pH D 4.3. The coating of Nanofer ZVI with TEOS exhibits z potential of § 85 mV. This occurs in pH range of 6.5 to 11, which is comparable with the normal pH of groundwater. More information regarding the determination of IEP and z potential are provided in recent related references.[6,11] Batch equilibrium experiments Stock solutions were prepared using the indicated chemicals (Table 1). Equilibrium tests were performed in closed, 1000-mL glass serum bottles. An appropriate volume of the stock solution was diluted with de-ionized water to 1000 mL. The solutions were mixed using a magnetic mixer and left over night. After the mixing, the amount of nZVI and stock solution were added to a 40-mL bottle. Following this, the bottles were closed and the cap was sealed with a Teflon liner to prevent leakage. The bottles were agitated on a mechanical shaker at 250 rpm. After 24 h, the reaction was stopped by separating the solution and the particles with a vacuum filtration, using 0.2 mm filter paper (42 Whatman). The particles were dried and stored in a N2-filled glove box ahead of solid phase analysis. The solution was acidified with HNO3 to pH less than two and stored at 4 C before solution analysis. A control was performed under identical conditions in parallel, except for the case where no iron nanoparticles were added. For experiments involving volatile compounds (e.g., TCE), 250-mL serum bottles containing 40 mL aqueous solutions were used. After adding specific amounts of nZVI, the bottles were capped with Teflon Mininert valves and placed on a mechanical shaker at 250 rpm at 25 C. Both pH and temperature were measured before and after adding the nZVI to each bottle. All bottles were cleaned using 0.1 M HCl and washed to insure that no residual contaminants were present. Information regarding the determination of the effect of time and kinetics related to Nanofer ZVI and metals as well as TCE are provided in a recent related reference.[11] Total aqueous concentrations of metals were measured with a Perkin Elmer atomic absorption (AAS) spectrometer (A Analyst 100). For analysis of each species, a fivepoint calibration curve was obtained with solutions prepared from the respective AAS standards (1000 mg L¡1

903

standards purchased from Fisher). Sample concentrations exceeding linear concentration range of the respective wavelength were diluted accordingly. The set up and selected wavelength for each metal were taken from the AAS spectrometer manual. The stock solution for the calibration curve was used to insure consistency after each 5th reading. After completing the reading, samples were storage in 4 C. The concentrations of TCE were measured by a Varian GC analyzer (3800) equipped with a flame ionization detector (FID) and a SUPELCO SPB 624 capillary column. The injection port temperature was set at 180 C. The oven temperature was set to 50 C and ramped to 200 C at 10 C min¡1. The detector temperature was 300 C. Helium gas was used as the carrier gas. A 5-point calibration curve was obtained with solutions prepared from the respective standards (1000 mg L¡1 standards purchased from Fisher). The method used for the analysis selected followed the GC Varian 3800 manual for liquid. After completing the analysis, all samples were capped and storage in 4 C. Solution pH and Temperature were measured with bench pH-mV meter Hanna pH 209. Three-point calibration was performed daily using pH 4.0, 7.0, and 10.0 standard pH buffers (Fisher). Oxidation-reduction potential (ORP) was measured using the same analyzer equipped with an ORP electrode (Pt band with Ag/AgCl/Saturated KCl reference cell). One-point ORP calibration was performed using a Zobel standard solution (Sigma), which gives a reading of ~ C230 mV at 20 C. ORP reading can be converted to the standard hydrogen electrode potential (Eh) by adding C197 mV at 25 C. Temperature was recorded daily before and after the analysis. For quality control, three replicate readings were obtained for each sample, and the average results are reported.

Results and discussions Particle characteristics Primarily studies were devoted to characterize Nanofer ZVI using images recorded by (XRD), (SEM), and (TEM). The specific surface area of the Nanofer ZVI was determined by BET analysis. Figure 1a shows that the average particle size was 50 nm (range 20–100 nm). In Figure 1b, the TEM image shows that Nanofer ZVI exhibits a chain-like structure due to the inherent magnetic interaction between particles.[6,12] The enlarged image (insert, Fig. 1b) also shows a dark area, which is possibly the core of a single Nanofer ZVI particle surrounded by a thin film of oxide shell.[11] XRD analysis (Fig. 2) indicated two distinct peaks corresponding to ferrite (a-Fe, 97.9%) and magnetite (Fe3O4, 2.1%). The increase of pure iron (ferrite) is caused by the coating of Nanofer ZVI with TEOS. However, Lenka

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Fig. 1. Nanofer particle characteristics; a: SEM image b: TEM image of Nanofer ZVI with the oxide shell. Tip of arrow indicates zoom location.[11]

et al.[6] found that the percentage of a-Fe for uncoated ZVI 25 was 60% and 70% for coated ZVI 25. Further, Nurmi et al.[13] investigated the hydrogen reduction of iron oxides. Sun et al.[14] reduced Fe3C by borohydride to form zero-valent nanoparticles. It was found that nZVI contains two phases. The percentage of a-Fe ranged from 30 to 70% and the surrounding oxide layer (Fe3O4) ranged from 70 to 30%. Broad peaks were observed by Sun et al.[14] in the XRD images for a-Fe and FeO. This contrasts with the slim peak for ferrite (98%) in the image for the new Nanofer ZVI (Fig. 3) which indicates that it is a very highly reactive adsorbent. The particle surface area was determined to be 27.5 § 2 m2g¡1based on N2-BET analysis. The surface area of Nanofer 25 (uncoated) was found to be 20 § 1 m2 g¡1.[6] It is believed that the increase in surface area between the (uncoated) Nanofer 25 and the new Nanofer ZVI is caused by the coating of particle, which prevents particle

agglomeration. For nZVI, coated with Pd, Li et al.[15] found that the particle size was 70 nm and the surface area ranges from 30 to 35 m2 g¡1. The oxide shell surrounding the core nanoparticle was less reactive due to metal coating. While studying dechlorination of chlorinated compounds by Pd/Fe after aging, Zhu and Lim[16] found that the reactivity and stability of particle decreases significantly after their prolonged use in the aqueous phase. The iso-electric point (IEP) for the Nanofer ZVI was found to be at pH of 4.3. However, increasing pH from 7.5 to 11, nZVI displayed a z-potential higher than § 85 mV which is suitable for groundwater remediation.[14] Lenka et al.[6] has reported similar IEP results for nZVI with a different film coating. However, they found that the z-potential for uncoated nZVI 25 was below § 30 mV, at the pH range of 6 to 10. Weile[2] detected that reductionoxidation potential for Pd-ZVI was § 230 mV. The palladium in this case made nZVI more stable and mobile.



Fig. 2. XRD Nanofer ZVI of a-Fe the particle size: 50–100 nm and high content of iron range of 70–90%, (λ D 1.5418 A, U D 40 Kv, I 30 D 30 mA).[11]

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Fig. 3. Effect of pH on the removal of metal ions and TCE using Nanofer ZVI star, Dose: 10 mg, Conc: 0.01 mM, T: 20–22 0C, a: Cu II, b: Pb II, c: Cd II and d: TCE.

However, the cost associated with the production of PdZVI is considered to be expensive. Effect of pH Batch experiments were carried out to analyze the sorption, reduction and surface complexation of Pb (II), Cu (II), Cd (II) and degradation of TCE by Nanofer iron as a function of pH. For the removal of heavy metals, pH is an important parameter. The results of pH effects related to the removal of Pb (II), Cu (II) II, Cd (II) and TCE are presented in Figure 3. In the present study, the rate of removal of Pb (II), Cu (II) and Cd (II) ions by Nanofer ZVI is mainly controlled by pH of the solution. The experiments showed that the bulk of contaminant removal generally occurs over a relatively narrow pH range of two units. This pH range moves to a higher pH as the initial metal concentration increases. Such features have also been noted as the characteristics of metal ion adsorption, complexation or reduction in several previous studies.[15,17] In present studies, the optimal pH values for Pb (II), Cu (II) and Cd (II) removal were 4.5 and 4.8 and 5.0 respectively. At higher pH, metals were precipitated due to the formation of metal hydroxides and consequently the removal due to sorption and reduction was very low. At a low pH, the concentration of protons was high and metal binding sites became positively charged repelling the metal cations. With an increase in pH, the negative charge density on the Nanofer

ZVI increases due to deprotonation of the metal binding sites, thus permitting increased metal sorption especially for both Pb (II) and Cd (II). These results are in agreement with previous studies.[17-25] These results also showed that the optimum pH for metal removal was in the range of 4.0 to 7.0. Various reasons might be attributed to the metal removal behavior of the Nanofer ZVI relative to solution pH. Since the Nanofer ZVI contains large number of active sites, the pH dependence of metals removal can largely be related to these active sites and also to the chemistry of the solution. The solution chemistry of transition metal is complicated due to the hydrolysis phenomenon. As pH increases from an acid to neutral, various hydrolyzed species exist and the affinities of these species to the adsorbent surface varies.[26] Figure 3c demonstrates the removal of Cd (II) using Nanofer ZVI at the pH range of 2 to 9 units. It is apparent that the adsorption/complexation is quite low at pH from 2 to 4. However, with the increase in pH from 5 to 7, significant enhancement in metal removal is reached. The optimum pH for Cd (II) removal was 5.5 with a removal efficiency of 99.9%. Beyond pH 6, Cd (II) adsorption capacity leveled off at the maximum value and above this pH, Cd (II) ions got precipitated. It is also interesting to note that at any pH, removal of all metals is very much greater through adsorption than through hydroxide precipitation. At pH  8.0, the dominant species is Cd (OH)2. However, at pH  8.0, Cd (II) will form Cd (OH)C.[27] At lower pH, there is a competition between the hydronium (H3OC) ions and

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906 Cd (II) ions to get adsorbed. Cd (II) in this case and other metals have to compete with H3OC for the available sites of Nanofer ZVI. However, as pH increases, the competing effect of H3OC ions decreases and the positively charged Cd (II) and Cd (OH)C ions hook up to the free binding sites. Hence, in the pH range from 2 to 6, the metal adsorption increases. Agrawal and Sahu[28] studied Cd (II) adsorption on manganese nodule residue and obtained similar results. The removal of Pb (II) and Cd (II) is essentially similar (Figs. 3b and 3c). One notes that the optimum pH range for Pb (II) is 4 to 6, while it is 5 to 7 for Cd (II). The removal of Pb (II) by Nanofer ZVI is shown in Figure 3b. The optimum pH to achieve 99.9% removal efficiency ranged from 4 to 6. The mechanism of (Pb II) removal in the acidic range with Nanofer ZVI is commonly traced to both reduction and adsorption.[23,29,30] The maximum percent removal of Cu (II) ions was observed at pH 4 to 5 (Fig. 3a). At low pH, HC competes for the sorption sites along with the metal ions. Hence, as stated earlier, at higher HC concentration, the nZVI surface becomes more positively charged, thus reducing the attraction between adsorbent or reactive material and metal ions However, as Christophi and Axe[31] noted, with increasing pH, more negatively charged surface become available, thus facilitating greater metal removal (adsorption, reduction and complexation). The dominant mechanism for Cu II removal by Nanofer ZVI is chemical reduction. Cu II has a greater redox potential than Fe0, which renders Cu (II) to get attracted to the surface of Nanofer ZVI, sequestering the two electrons released to form Cu0. Also due to the existence of oxygen, Cu (II) can form CuO2. At a higher pH, Cu II may precipitated as the hydroxide forming a greenish color layer, which decreases the rate of reduction. Based on XPS and XRD analysis, Ponder et al.[29] suggested that metal ions are reduced to their metallic form and possibly other insoluble phases. Similar results were noted by Lien and Zhang[30] and Li and Zhang.[7] Weile[2] studied the removal of Hg (II) by two bimetallic nanoparticle (nZVI-Ag and nZVI-Pd). It was found that the addition of silver (Ag) and palladium on nZVI improves mercury retention efficiency remarkably. The concentration of Hg (II) in the residue dropped from 50 mg L¡1 in the presences of pure nZVI compared to 5 mg L¡1 in the presences of bimetallic nanoparticles. Compared to the more costly nobel metals (Pd and Ag), coated Nanofer ZVI is quite inexpensive. Further, TEOS coating is nonhazardous and hence is harmless to the environments. In the case of TCE degradation by Nanofer ZVI, the proton availability may influence the rate of reaction. It is observed that rate of hydrogenolysis of carbon tetrachloride too increases linearly with decreasing pH.[32] However, the relationship between bulk pH and availability of protons at the metal surface of nZVI is unknown.[7,33]

Eglal and Ramamurthy Matheson and Tratnyek[34] showed that the order of reaction with respect to protons is approximately 0.15 indicating that protons are not involved in the rate-determining step but have an indirect effect in enhancing iron corrosion in aqueous solution. The optimum pH for TCE degradation ranged of 6 to 8 (Fig. 3d). Since TCE depletion in reaction vials was observed, the next step was to confirm that TCE degradation was occurring. Before sampling, the vials were visually inspected for the presence of gas bubbles. As the reaction proceeded, gas bubbles formed in the vials containing iron and they grew larger over time (2 h). The presence of gas bubbles was attributed to the increased concentration of the volatile breakdown by-products of TCE and to the production of hydrogen. The GC/FID used to determine the concentration of TCE was also calibrated for the ethene and the intermediate by-products cis-DCE, trans-DCE, 1, 1-DCE, and vinyl chloride. The presence of ethene and vinyl chloride was confirmed by GC/FID, indicating that TCE degradation was occurring. However, other intermediate byproducts were not detected as it was listed elsewhere. The confirmed presence of ethene indicated that complete dehalogenation does occur when enough reaction time is provided. The absence of any by-products in the control vials (without iron) confirmed that the iron was responsible for TCE degradation. Although the mechanistic pathway for the degradation of TCE is not precisely known, three potential pathways have been proposed by Matheson and Tratnyek[34] The first proposed pathway occurs by direct electron transfer from the surface of the iron to the alkyl halide. The second proposed pathway involves the production of Fe2C resulting from the corrosion of the metal. The Fe2C is then reduced to Fe3C through the transfer of electrons to the alkyl halide. The utilization of hydrogen produced from the corrosion of water is involved in the third pathway. The iron surface serves as a catalyst in the reaction. Matheson and Tratnyek[34] have proposed the pathways of TCE dehalogenation. Hydrogenolysis occurs, where each of the three chlorines are replaced sequentially. TCE first gets reduce to cis-1, 2-dichloroethene, trans-1, 2-dichloroethene, and 1, 1-dichloroethene. Following this, the intermediates are reduced to vinyl chloride, ethene and ethane. Beta elimination as well as replacement of two chlorides has been suggested by Roberts et al.[35] as the major pathway of TCE degradation. Both proposed pathways involve a net transfer of two-electrons. Effect of Nanofer ZVI dose It is important to fix the amount of Nanofer ZVI dose to design the optimum treatment systems. A series of batch experiments were conducted with 40-mL solutions using Nanofer ZVI doses that ranged between 2 to 80 mg. When the addition of Nanofer ZVI dose increased, the percentage removal of metal ions also increased. To keep particles

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Fig. 4. Effect of adsorbent on the removal of metal ions & TCE using Nanofer ZVI star, Dose: 10 mg, Conc: 0.01 mM, T: 20–22 0C, a: Cu II, B: Pb II, C: Cd II and D: TCE.

in suspension, all samples were placed in a shaker during the experimental period to insure maximum contact time. A maximum removal efficiency for 99.9% for Cu (II), 99.2% for Pb (II), 98.7% for Cd (II) and 99.6% for TCE respectively was achieved (Fig. 4). A Nanofer ZVI dose of 10 mg appears to be sufficient for optimal removal of metal ions and TCE. However, for Cu (II), the optimum dose was 30 mg. The ability of Nanofer ZVI to be effective for metal ions as well as TCE at low dosages (10 mg) can be traced to the greater availability of the exchangeable sites or surface area at higher concentrations. Christophi and Axe[31] reported similar findings for the removal of metal [Pb (II), Cu (II), and Cd (II)] ions from aqueous solutions using goethite. It can easily be inferred that the percent removal of metal ions [Cu (II), Pb (II), and Cd (II)] increases with increasing dose of Nanofer ZVI. The maximum removal efficiency is achieved at 10 mg. As stated earlier, dechlorination follows pseudo–first-order kinetics.[11] The important factor that influences the process is the surface area of Nanofer ZVI. The increase in surface area of Nanofer ZVI particles imply higher surface energies which impart more reactive sites. Also, Nanofer ZVI will form surface hydroxyl groups that can be coordinated to one, two, or three iron atoms, giving sites with very different reactivates. Thus, Nanofer particles are easier to stay suspend in solution for certain time and this helps the transport and diffusion processes.[30] Together, these factors will generate surfaces with a wide range of

adsorption capacity and reactivity. However, Chen et al.[32] found that the cleanness and impurities of particle surface are more important the surface area for the total removal efficiency. Effect of metal ion concentration The metal removal and TCE degradation by Nanofer ZVI particle’s mechanism is also dependent on concentration of metals and TCE. Initial concentrations of 0.01 and 0.25 mM of metals and TCE were selected for the comparative study at Nanofer ZVI dosage of 10 mg filled in 40 mL tubes. Figures 5a, 5b and 5c show that the effect of metal ion concentration on the removal of Pb (II), Cu (II), and Cd (II). The maximum removal of Pb (II) and Cd (II) was achieved within 60 min. The heavy metals were absorbed by specific sites provided by the acidic functional groups on nZVI. With increasing metal concentrations the specific sites get saturated.[21,36] However, as the metal concentration increases, the maximum removal efficiency decreases. This indicates that the removal of metal ions is highly dependent on the concentration of Nanofer ZVI. TCE concentration ranged from 10 ml to 100 mL per liter of deionized water and the optimum pH ranged from 6 to 8 (Fig. 5d). The maximum removal of TCE was achieved at the concentration of 10 to 40 ml L¡1 within 180 min. This is consistent with the fact that lag time was due to the hydrogen-terminated surface of Nanofer ZVI.

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Eglal and Ramamurthy

Fig. 5. Effect of ion and TCE concentration on the removal efficiency of Nanofer ZVI star. Dose: 10 mg, a: Cu II pH-4.5, b: Pb II pH-4.8, c: Cd II, pH-4.9 and D: TCE, pH-6.5, T: 20–22 C.

The effect of ionic strength on the removal of metal contaminants (Cu II, Pb II and Cd II) was tested. The concentration of NaNO3 was ranged from (0, 0.01, 0.1 and 1.0 mmol L¡1). It was found that the NaNO3 has little effect on the metal removal by Nanofer ZVI. These results are consistent with the hypothesis that the removal of metals from aqueous solution is merely a process which involves adsorption, reduction, and complexation.

Isotherms models They isotherms models are very important in describing the contaminants removal behavior on Nanofer ZVI. Two isotherm models such as Langmuir and Freundlich were selected and studied. The Langmuir isotherm[37] takes an assumption that the removal of contaminant occurs at specific homogeneous sites within the ZVI. The linear form of isotherm equation can be written as Eq. (1).      1 1 1 1 D C qc bqmax Ce qmax

(1)

Here, qmax is the maximum uptake of metals and TCE corresponding to the saturation capacity of Nanofer ZVI and b is the energy of removal mechanism. The variables qe and Ce are the amounts of contaminant on Nanofer ZVI and the equilibrium concentration in the solution respectively. The qmax constants and b are the

characteristics of the Langmuir isotherm and can be determined using Eq. (1). The data fit the Langmuir isotherms model well for single component ions and TCE (Fig. 5). The Freundlich expression is an empirical equation based on a heterogeneous surface as written in Eq. (2). 1 log qe D log kf C log Ce n

(2)

Here, the intercept log Kf is a measure of removal capacity, and the slope 1/n is the intensity of removal. The average strength of nZVI bonds with metal ions varies with removal densities. The variables qe and Ce are the amounts of contaminant and the equilibrium concentration in solution. The values of Langmuir and Freundlich parameters for the removal of both metal ions and TCE are presented in Table 3. The correlation coefficient (R2) for the removal of Pb (II), Cd (II), Cu (II) and TCE are between 0.94 and 0.98. The linearity of the plots supports the application of the Langmuir equation and confirms the monolayer formation of the sorption for metals and TCE on the surface. The values of b and qmax indicate that sorption is dependent on concentration and pH. The expression of separation factor (RL) in the dimensionless form of the Langmuir isotherm is RL D 1/ (1 C bC0), where, C0 is the initial concentration of metal ion and b is the Langmuir constant. RL can be used to predict affinity between Nanofer ZVI and metal ions in the removal system. The characteristics of RL value indicates

909

Removal of metals and trichloroethylene by Nanofer ZVI Table 3. Sorption isotherm models parameters. Langmuir parameters Components Inorganics

qmax mg mg¡1

b mL mg¡1

Cu(II) Pb (II) Cd (II) Organic TCE

0.27 0.17 0.11

45.25 15.34 8.18 mL mg¡1 13.88

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0.12

Freundlich parameters R2

RL

Kf (mg g¡1)(0.01 mM)n

n

0.94 0.98 0.98

0.69 0.87 0.92

0.53 0.32 0.24

0.32 0.06 0.16

0.97 0.99 0.97

0.96

0.88

0.65

0.76

0.96

the nature of removal as unfavorable (RL >1), linear (RL D 1), favourable (0 < RL < 1), and irreversible (RL D 0).[38] It is observed that for the selected concentration (0.01 mM) of metal ions, the separation factor (RL) is less than 1.0 (Table 3), which indicates that the removal conditions are favorable for all metals and TCE. Both isotherm models show a better fit with the data. The R2 values for both models are quite high. Generally, it has been found that the Langmuir model is more physically consistent with adsorption isotherm than the Freundlich model.[38] The Metal capacities (Table 3) are consistent with the electronegativity (Table 2). The b values indicate that the equilibrium constant for Langmuir model follows the order Pb > Cu > Cd. This trend is consistent with the corresponding hydrated ion radii data shown in Table 2 and in agreement with earlier studies.[31,39-42] Comparison of Langmuir removal capacity of the Nanofer ZVI with other adsorbent are given in Table 4. It

R2

includes data related to performance characteristics of other common adsorbents used in removal of contaminants in aqueous solutions.[42–52] Clearly, all ZVI nanoparticles have much greater removal capacity compared to conventional adsorbents used for eliminating these contaminants from polluted water. Although, Li et al.[42] developed nano-ZVI with a slightly higher surface area compared to the present Nanofer ZVI, it is more prone to agglomerate, as it is uncoated. Kim et al.[43] incorporated nano-ZVI in zeolite, which has a huge surface area and removal capacity compared with all other nZVIs. However, in field conditions, it is not applicable for direct injection to remove target groundwater contaminants. NanoZVI in zeolite is recommended for use in ex-stiu water remediation. However, for TCE degradations, there are numerous field and laboratory applications. nZVI is an applicable remedial reagent. Here, focus should be on reducing particle aggregation, since it is the main hurdle for effective TCE degradation.

Table 4. Comparison of Langmuir sorption capacity for the sorption of Cu II, Cd II and Pb II as well as TCE by other adsorbents. Contaminant Cu II

Cd II

Pb II

CE

Adsorbent Sugar beet pulp Lignite Aminoproly silica get-(immobilized-polymer) GA-APTES-nanoparticle Nano-ZVI Al-nZVI Nanofer ZVI Nanostructured goethite Modified kaolinite clay Rice husk Nanofer ZVI Zeolite nZVI Z-nZVI A-CS (activated carbon Nanofer ZVI Virgin GAC (F-400) TiO-NP Nanofer ZVI

Surface area m2g¡1 Removal capacity mg g¡1 Not listed Not listed Not listed Not listed 30 Not listed 27 26.3 Not listed Not listed 27 1.03 12.25 80.37 900 27 745 51 27

30.90 18.9 5.08 61.07 343 95.3 270 29.1 44 21.24 110 39.1 96.2 806 29.44 170 47 45 130

Reference Pehlivan et al. [44] Pehlivan and Arslan[45] Tabakci and Yilmaz[46] Ozmen et al.[47] Li et al.[42] Karabelli et al.[48] Present study Mohapatra et al.[41] Unuabonah et al. [49] Kumar et al. [50] Present study Kim and Oh[43] Kim and Oh[43] Kim and Oh[43] Goel et al.[51] Present study Salih et al.[52] Salih et al.[52] Present study

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Eglal and Ramamurthy Table 5. SEM/EDS analysis of before and after the particle settling experiments with deionized water. Before

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Fig. 6. Settling of Nanofer ZVI in de-ionized water: a: Sample after adding the Nanofer ZVI and placed in 250 rmp shaker, (at 0.02 h) b: Sample after adding the Nanofer ZVI and placed in 250 rmp shaker, (at 24 h).

Effect of particle aging and agglomeration As stated earlier, the major problem facing application of nZVI in the subsurface is its tendency to agglomerate and rapidly precipitate in aqueous media. However, several studies have focused on developing methods of coating the surface of nZVI particles, which prevents particle agglomeration. In the present study, Nanofer ZVI particles were covered by TEOS. Both the agglomeration and the sedimentation behavior of the particles was assessed to determine the time during which they stay in suspension. However, after 2 h, it was noticed that nZVI particles started to agglomerate and precipitate (Fig. 6). In the presence of the hydroxyl group, metals share more than one particle creating a ligand which increases the particle size.[2] Figure 6b shows the precipitated Nanofer ZVI near the bottom of the tube. It took more than 24 h to settle most of the agglomerated particles. However, Figure 6b

After

Element

Weight %

Atomic %

Weight %

Atomic %

Oxygen Iron Silicon

4.89 94.27 0.84

15.29 84.48 0.23

14.68 84.52 0.80

38.23 61.16 0.21

indicates that some particles still stay in suspension even after 24 h. Figures 7a and 7b show the SEM/EDS images of particles before and after the experiments. They indicate that the particles lose their structure and morphology creating a sort of clothlike bigger particle. The EDS analysis (Table 5) shows that there is a large increase of oxygen level. Lenka et al.[6] studied the effect of agglomeration between uncoated and coated Nanofer ZVI for a period of 60 min. They too found that the coated nanoparticle suspension remained unchanged during the 60 min, while the uncoated nanoparticles of Nanofer ZVI 25 began to agglomerate as soon as they were introduced to water and sank as a precipitate in about 20 min.

Conclusions Nanofer ZVI successfully removes inorganic contaminants [Cu II, Cd II, and Pb II] as well as organic [TCE] contaminants from groundwater. This new nanomaterial coated with TEOS is environmentally friendly, inexpensive and can be used to replace the ZVI which is commonly used in PRB applications. Further, Nanofer ZVI displays high potential to overcome the setback of the old Nanofer ZVI 25 related to fast agglomeration as well as instability in the aqueous media when used through direct injection in filed applications.

Fig. 7. SEM/EDS images for Nanofer ZVI before and after the hydroxyl experiment; a: image before the experiments, b: image after 24 h of settling.

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Removal of metals and trichloroethylene by Nanofer ZVI Results indicate that metal removal follows the order Cu II > Pb II > Cd II. The test data show that there is a threshold concentration that has to be reached for TCE degradation. TCE reduction by Nanofer ZVI forms a new non-hazardous substances such as ethene and vinyl chloride. Due to the large surface area and hence the vast availability of active sites, as little as 10 mg of Nanofer ZVI is adequate to remove 0.1 mM of metal and 40 mL L¡1 of TCE. For the chosen concentration of the contaminants, the optimum Nanofer ZVI dose was found to be 10 mg. and the optimal pH for Pb (II), Cu (II), Cd (II) and TCE removal were 4.5 and 4.8, 5.0 and 7.2, respectively. The isotherm data fit very well to the Langmuir and Freundlich models. The maximum loading capacity was close to 270, 170, 110, 130 mg per gram of Nanofer ZVI for Cu (II), Pb (II), Cd (II) and TCE, respectively. The agglomeration test demonstrated that the particle can stays in suspension for a period of 4 to 6 h. The test revealed that the time for total agglomeration and sedimentation is almost 24 h. SEM/EDS images of particles before and after the experiments indicated that particles lose their structure and morphology creating a sort of clothlike bigger particles.

Acknowledgments The support of the NANOIRON Ltd. (Czech Republic) for supplying the Nanofer ZVI, the technical assistance of TMG group for the use for SEM/EDS and XRD and the accessibility to Environmental Engineering facilities at Concordia University (Montreal) are thankfully acknowledged.

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