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Removal of hexavalent chromium from groundwater using sodium alginate dispersed nano zero-valent iron
Journal of Environmental Management 244 (2019) 33–39
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Research article
Removal of hexavalent chromium from groundwater using sodium alginate dispersed nano zero-valent iron
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Zihan Lia, Shuyuan Xua, Guanghui Xiaob, Limin Qiana, Yun Songa,c,∗ a
Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, Beijing, China College of Life Science, Shaanxi Normal University, Xi'an, China c Beijing Key Laboratory of Industrial Land Contamination and Remediation, Beijing, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nano zero-valent iron Sodium alginate Hexavalent chromium Groundwater Removal rate
Hexavalent chromium (Cr), one of the most common heavy metals, is widely found in contaminated soil and groundwater. Nano zero-valent iron (nZVI) is used to treat Cr(VI) in polluted groundwater. However, due to agglomeration, rapid sedimentation, and limited mobility of nanoparticles in the aquatic environment, nZVI is not widely used in groundwater treatment. In this study, we used sodium alginate (SA) to modify nZVI to generate dispersed SA-nZVI. SA-nZVI particles were found to embed in the polymer material and exist as an amorphous state with a diameter less than 100 nm. Compared with traditional nZVI and carboxymethyl cellulose (CMC)-nZVI, SA-nZVI had better stability and higher absolute zeta potential. The presence of SA enhanced mobility of nZVI and effectively prevented sedimentation and aggregation. Furthermore, SA-nZVI had a higher Cr(VI) removal rate than (CMC)-nZVI under both acidic and alkaline conditions. XPS analysis showed that Cr(VI) was reduced to Cr(III) and formed Cr(OH)3 as precipitates after treatment with SA-nZVI. In addition, NO3− had no effect on the final removal rate of Cr(VI) by SA-nZVI. These results suggest that SA-nZVI has high penetration and a high removal rate in Cr(VI) removal and can be used to stabilize nZVI to remediate Cr(VI)-contaminated groundwater in the future.
1. Introduction Groundwater is a valuable resource that is essential for human life, agriculture, and industry (Bhowmick et al., 2018). Heavy metal pollution is widespread in groundwater and poses a great threat to human life and health (Huang et al., 2018; Kurwadkar, 2017; Zhou et al., 2017; Saleh et al., 2018). Chromium (Cr), a heavy metal, is widely used in industrial processes and is found in industrial waste, especially in chromium slag (Gebru and Das, 2018; Dokou et al., 2017). Water-soluble sodium chromate and acid-soluble calcium chromate found in chromium slag or petroleum contain large amounts of hexavalent chromium (Cr(VI)), which can be transferred from the soil to the groundwater, leading to groundwater pollution (Bae et al., 2017; Shen et al., 2018). Currently, Cr(VI) contamination in groundwater is a common and serious problems in China (Zhao et al., 2016a). The conventional pump and treat technique, one of the most important exsitu technologies, was used in early groundwater treatment. Due to high cost and low efficiency, this approach is not widely used in modern industry (Lee et al., 2009). Nanomaterials have rapid action in wastewater treatment because
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nanoparticles have unique physical and chemical properties such as large surface-area-to-volume ratio and high interfacial reactivity (Li et al., 2006). Due to the characteristics of fast response, high reduction potential, and low cost, nano zero-valent iron (nZVI) is one of the most effective materials for in-situ remediation of wastewater (Vidmar et al., 2018; El-Temsah et al., 2016). Compared with traditional zero-valent iron (ZVI) particles, nZVI has a higher specific surface area, stronger surface activity, and broad applicability (Zhao et al., 2016b; Tesh and Scott, 2014). nZVI is widely used in in situ remediation of Cr(VI) contaminated groundwater because it is lower cost, faster, higher efficiency, and more environmentally friendly than traditional ZVI, ferrous sulfate, and calcium polysulfide (Yirsaw et al., 2016; Di Palma et al., 2015; Chrysochoou et al., 2012). However, due to the large specific surface area and ultra-high surface energy of nanoparticles, combined with the van der Waals force and magnetic properties, nZVI particles often form aggregates, which largely reduces their transmission in contaminated groundwater and soil (Xie and Cwiertny, 2010; Noubactep et al., 2012). Also, oxidation often occurs when nZVI is not in contact with the target contaminant, resulting in deactivation of nZVI (Lee et al., 2014).
Corresponding author. Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, Beijing, China. E-mail address: [email protected] (Y. Song).
https://doi.org/10.1016/j.jenvman.2019.04.130 Received 17 November 2018; Received in revised form 23 April 2019; Accepted 30 April 2019 Available online 17 May 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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(UV-2550, Daojin) at a wavelength of 540 nm. Cr(IV) concentration in solution was quantified using a standard curve method and the removal rate was calculated.
Many attempts have been made to enhance stability and prevent aggregation of nZVI (Wu et al., 2015). Among them, carboxymethylcelluose (CMC) was successfully used to modify nZVI particles and was thus further used for in-situ groundwater remediation (Fatisson et al., 2010). CMC modified nZVI (CMC-nZVI) prevented particle aggregates and formed stable suspensions (De Lima Fontes et al., 2018; Fan et al., 2015; Johnson et al., 2013). Sodium alginate (SA) is commonly used to remove heavy metals and toxic substances from groundwater and soil (Gopi et al., 2018; Huang et al., 2018). SA has many carboxyl groups on its molecular chain, which can adsorb metal ions (Cho et al., 2018). In addition, SA is an ideal dispersant and has been used to modify nanomaterials to improve the immobilization of biological enzymes (Yu et al., 2014). However, modification of nZVI with SA is not in widespread use and thus there are no reports of SAnZVI being used for wastewater remediation. The objectives of this study were to prepare SA- and CMC-nZVI particles, determine their characteristics, compare the Cr(VI) removal rates of both nZVI particles, and investigate pH and initial concentrations of Cr(VI) and nZVI during Cr(VI) removal. This study developed a new material and optimized conditions for Cr(VI) removal. This nanomaterial has the potential to be widely used to remediate Cr(VI)-contaminated wastewater in the future.
2.4. Column experiments for mobility Two glass columns (diameter 4 cm and height 40 cm) were filled with 20–40 mesh quartz sand. The metal gauze was fixed with a silicone plate on the top and bottom of the cover to reduce disturbance of the water. The same initial concentrations of CMC-nVZI, SA-nVZI, and nVZI without any dispersant solutions flowed from bottom to top through a quartz-filled glass column with a peristaltic pump at a fixed flow rate of 50 mL/min. For the breakthrough curves experiment, three different nVZI solutions flowed through the glass column filled with quartz sand. The volumes of each influent and effluent were measured. 2.5. SEM and XPS analyses The freshly prepared CMC- and SA-nVZI samples were dried in a vacuum oven, washed with ethanol and dried with nitrogen. The phase composition of two nVZI samples was characterized using a fieldemission scanning electron microscope (ZEISS Merlin Compact). Reaction products of Cr(VI) and CMC- or SA-nVZI were characterized by X-ray photoelectron spectroscopy (XPS) (VG Multilab 2000; Thermo VG Scientific) (Yoon et al., 2011). The reaction products were quantified by determining the position, height, width, and shape of each peak.
2. Materials and methods 2.1. Materials and chemicals
2.6. Statistical analysis
All chemicals used in this study, including potassium borohydride, ferrous sulfate, sodium alginate (SA), sodium carboxymethyl sulfate, sodium hydroxide, hydrochloric acid, absolute ethyl alcohol, diphenylcarbazide, acetone, sulfuric acid, and nitric acid were of analytical grade. All solutions were prepared with deionized water generated from a Milli-Q water system (Merck Millipore Corporation). Cr(VI) stock solutions were prepared by dissolving sodium dichromate into deionized water. The pH of all solutions was adjusted using sodium hydroxide or hydrochloric acid solution, and then measured with a pH meter (VSTAR80, Thermo Fisher).
All the experiments were repeated in triplicate. 3. Results and discussion 3.1. Characteristics of SA- and CMC-nZVI The SEM images showed that CMC-nZVI was spherical with a diameter of about 100 nm and was clearly well-dispersed (Fig. 1a). nZVI was wrapped in the surface of CMC mediums with a micelle shape (Fig. 1b). In the presence of CMC, the magnetic effect of nZVI is effectively overcome, and nZVI is effectively dispersed by electrostatic repulsion and steric hindrance (De Lima Fontes et al., 2018; Johnson et al., 2013). nZVI dispersed by SA was spherical with a diameter of less than 100 nm (Fig. 1c). nZVI particles were “glued” together by SA (Fig. 1d) and unstabilized nZVI was stacked and agglomerated due to magnetic forces (Honetschlägerová et al., 2015). The TEM images also showed that SA-nZVI particles were coated with polymer and the size of them were about 100 nm (Fig. 1e and f). SA-nZVI particles are not easy to agglomerate duo to the adsorption of functional groups such as –COOH and –OH in SA (Fig. 1c; Huang et al., 2016). When SA was added to the ferric trichloride solution to synthesize a magnetic microsphere, the ferric ion replaced the sodium ion in SA and iron hydroxide was embedded in the polymer network. nZVI formed by reduction was not simply agglomerated and stacked but was embedded in a polymer network (Finotelli et al., 2004). These results indicate that SA can reduce the agglomeration of nZVI. XPS analysis was used to investigate surface compositions of CMCnZVI and SA-nZVI, particularly for the oxidation state of Fe on the surface of as-prepared material spectra. Both CMC-nZVI and SA-nZVI had two peaks at binding energies of nearly 710 eV and 724.3 eV, characteristic of Fe-ox and FeOOH, respectively (Fig. 2a and c). The peak of Fe0, which was at the binding energy 706.2 eV, was very weak. This confirms the core-shell structure of nZVI, with an iron oxide film shell on the surface (Ling and Zhang, 2017). The XPS technique is widely used for surface-level detection (Xie and Cwiertny, 2010) and nZVI in this study was coated with organic polymer materials, CMC or SA, which led to the failed escape of Fe0 photoelectrons located in the
2.2. Preparation of CMC- and SA-nZVI composites nZVI particles were synthesized from a boroyhdride reduction using ferrous iron (Dong et al., 2018). CMC and SA were used to modify nZVI particles. In detail, 12% (w/w, CMC/Fe) CMC and 15% (w/w, SA/Fe) SA solutions (150 ml) were obtained and then stirred continuously for 15 min in an N2 environment. A 0.005 M Fe2SO4 solution (50 mL) was prepared and then stirred continuously for 15 min in an N2 environment. Subsequently, the ferrous sulfate solution was added to the CMC or SA solution with continuous agitation for 30 min in an N2 environment to ensure the formation of CMC-Fe2+ or SA-Fe2+ complexes and prevent oxidation. A 0.01 M KBH4 solution (50 mL) was prepared based on the molar ratio of BH4−/Fe2+ 2.0, and then stirred continuously for 15 min in an N2 environment to discharge oxygen. Finally, KBH4 solution was dropped into the CMC-Fe2+ or SA-Fe2+ mixed solution at a rate of 2 drops per second. The liquid prepared above was stored in a black-brown bottle filled with N2 above the liquid. 2.3. Batch experiments for Cr (VI) removal The effects of initial Cr(VI) concentration, solution pH, and nZVI concentration (loading) on Cr(VI) removal were investigated in this study. Quantification of the concentration of nZVI was performed as previous report (Johnson et al., 2013). A Cr(VI) standard curve was prepared to calculate Cr(VI) concentration. During the Cr(VI) removal process, the dispersed nZVI and Cr(VI) solution was mixed and stirred continuously for the indicated time according to the experimental design. Cr(IV) absorbance was determined using a spectrophotometer 34
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Fig. 1. SEM and TEM image of CMC- and SA-nZVI particles at different resolution ratios. a, CMC-nZVI image magnified 50,000 times (scale bar = 100 nm); b, CMCnZVI image magnified 10,000 times (scale bar = 1 μm); c, SA-nZVI image magnified 50,000 times (scale bar = 100 nm); d, SA-nZVI image magnified 10,000 times (scale bar = 1 μm). e, TEM image of SA-nZVI particles magnified 10,000 times (scale bar = 500 nm); f, TEM image of SA-nZVI particles magnified 50,000 times (scale bar = 100 nm).
combined with translucent solution and many particles suspended in the middle and lower parts of the bottle. CMC-nZVI and SA-nZVI did not show significant agglomeration or sedimentation: they were highly dispersed in a dark black solution (Fig. 3a). After being left to stand for 3 h, nZVI particles were suspended in a yellow solution, indicating that nZVI was oxidized. In the CMC-nZVI bottle, some black particles settled at the bottom and the solution began to turn green, indicating that CMC-nZVI was partial oxidized. No obvious sedimentation particles were observed in the SA-nZVI bottle (Fig. 3b). After being left to stand for 3 h, SA-nZVI had no sedimentation or oxidation. These results suggest that, compared with nZVI and stabilized CMC-nZVI, SA-nZVI has better dispersibility and less agglomeration. Zeta potential is a key indicator of the stability of colloidal
shell core of CMC-nZVI and SA-nZVI particles. XPS analysis revealed that C–OOOH and C–OH, characteristic functional groups of CMC and SA, were found in these nZVI (Fig. 2b and d), which also suggested that the surface of the synthetic nZVI was surrounded by CMC or SA (Fig. 1). 3.2. Stability and mobility of SA-nZVI and CMC-nZVI Although nZVI can effectively remove a variety of pollutants, it is easy for nZVI to agglomerate and grow into larger size particles (Honetschlägerová et al., 2015). Therefore, we then assessed the stability of synthetic SA-nZVI and CMC-nZVI using the settlement experiment and zeta potential analysis. The newly synthetic nZVI particles were left to stand for 30 min, and there was obvious sedimentation
Fig. 2. The X-ray photoelectron spectroscopy (XPS) analysis of CMC- and SA-nZVI particles. a, Fe 2p XPS spectra of SA-nZVI; b, 1s XPS spectra of SA-nZVI; c, Fe 2p XPS spectra of CMC-nZVI; d, C 1s XPS spectra of CMC-nZVI. 35
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Fig. 3. Comparison of stability and zeta potential of different nZVI particles. a, stability of nZVI, CMC- nZVI, and SA-nZVI after synthesis for 30 min; b, stability of nZVI, CMC- nZVI, and SA-nZVI after synthesis for 3 h; c, zeta potential of 12% CMC-nZVI particles in different pH conditions; d, zeta potential of 12% SA-nZVI particles in different pH conditions. Reaction conditions: CMC-nZVI, 15 mg/L; SA-nZVI, 15 mg/L; nZVI, 15 mg/L. Error bars represent SD for three independent experiments.
in Fig. 4A. For CMC-Fe and nZVI, the black particles settled down at the bottom of column. SA-stabilized nZVI almost reached the top of the column with the flow (Fig. 4a). The breakthrough rates showed that when the pore volume reached 1.5, transport of both CMC-nZVI and nZVI no longer increased, whereas the transport of SA-nZVI increased: C/Co at 2.5 pore volume was 0.9 in SA-nZVI, much higher than CMCnZVI and nZVI (Fig. 4b). These findings suggest that the presence of dispersant SA enhances mobility of nZVI and effectively prevents sedimentation and aggregation. In the presence of natural organic matter, mobility of nZVI was significantly increased (Johnson et al., 2009).
dispersions and a higher absolute zeta potential confers more stability (Hanaor et al., 2012). Usually, colloids with an absolute zeta potential above thirty are basically stable (Clogston and Patri, 2011). We detected the zeta potential under alkaline conditions as nZVI is more likely oxidized under acidity, which is ineffective in remediation. As shown in Fig. 3c, when pH was over 9, the absolute value of zeta potential of both SA-nZVI and CMC-nZVI exceeded 30 mV. The zeta potential of SA-nZVI has a higher absolute value (Fig. 3d). These data indicate that SA increases the electrostatic repulsion between nZVI and disperses the particles by steric hindrance to achieve better stability, which is consistent with the settlement experiment (Fig. 3). Column experiments were used to evaluate the mobility of dispersed nZVI. nZVI and CMC-nZVI migrated and stopped a quarter of the way through the column and the breakthrough curves were plotted as shown
Fig. 4. Column migration experiment (a) and breakthrough rate (b) of nZVI, CMC-nZVI, and SA-nZVI. Reaction conditions: CMC-nZVI, 15 mg/L; SA-nZVI, 15 mg/L; nZVI, 15 mg/L pH = 6.5. Error bars represent SD for three independent experiments. 36
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Fig. 5. Effect of pH on removal rate of Cr(VI) using CMC- and SA-nZVI (a, b). The comparison of removal rate of Cr(VI) with CMC- and SA-nZVI particles under different pH conditions (a, b) and different nZVI content (c). Reaction conditions: CMC-nZVI, 15 mg/L; SA-nZVI, 15 mg/L; Cr(VI), 4 mg/L. Error bars represent SD for three independent experiments.
Fig. 6. Effect of NO3− on Cr(VI) removal by SA-nZVI particles. Reaction conditions: the concentrations of NO3−, Cr(VI), and SA-nZVI were 5 mg/L, 4 mg/L and 15 mg/L, respectively; pH 8.5. Error bars represent SD for three independent experiments.
3.3. Effects of initial pH on the removal of Cr(VI) by SA-nZVI and CMCnZVI We further investigated the effects of initial pH on Cr(VI) removal using CMC- and SA-nZVI. Previous studies showed that SA had no significant effect on the removal rate of Cr(VI) (Wang et al., 2017; Huang et al., 2016). At pH 6.0, the removal rate of Cr(VI) using CMCnZVI was 75.6% after 20 min of reaction, whereas 96.4% Cr(VI) was removed by SA-nZVI in the same conditions (Fig. 5a). In contrast with the 60.4% of Cr(VI) removed by CMC-nZVI, 85.7% of Cr(VI) was removed by SA-nZVI after 20 min of reaction at pH 9.0 (Fig. 5b). These results showed that the removal rate of Cr(VI) by SA-nZVI was much
Fig. 7. The reaction products of Cr(VI) and SA (a)- or CMC-nZVI (b).
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better stability, dispersibility, and less sedimentation. Zeta potential detection indicated that SA-nZVI had better mobility, which was further verified because SA-nZVI penetrated through and flowed out from the column, whereas traditional nZVI and CMC-nZVI only migrated about a quarter of the way through the entire column. The high performances indicated that SA-nZVI will have obvious effects on Cr(VI) contaminated groundwater. This result was supported by the fact that the removal rate of Cr(VI) by SA-nZVI was much higher than that by CMCnZVI under both acidic and alkaline conditions. The removal rate of Cr (VI) by SA-nZVI was up to 96.4% after 20 min of reaction at pH 6.0. After treatment with SA-nZVI, Cr(VI) was reduced to Cr(III) and formed Cr(OH)3 as precipitates. NO3− had minimal effects on the final removal rate of Cr(VI) removed by SA-nZVI. These results show that SA-nZVI has high penetration and a high removal rate in Cr(VI) removal and is a promising material for Cr(VI) contaminated groundwater remediation.
higher than that by CMC-nZVI under both acidic and alkaline conditions. Although the removal rate of Cr(VI) by SA-nZVI decreased to 85.7% at pH 9.0, in contrast to the 96.4% removal rate at pH 6.0, this is still a high removal rate. This result indicates that SA-nZVI is active and effective in groundwater, as groundwater is often alkaline. Similar findings were found in previous studies where the removal rate of Cr (VI) by CMC-nZVI was 97% at pH 4.5, whereas the removal rate decreased to 45.9% at pH 8.5 (Qian, 2008). At the high pH condition, the passivation of Fe0 hinders electron transfer from highly active zerovalent iron cores, thereby reducing the removal rate of Cr(VI) (Mondal et al., 2004). In addition, we explored the effect of initial concentration of iron on Cr(VI) removal. When the ratio of iron to Cr(VI) was 3, the removal rate of Cr(VI) by CMC-nZVI was only 51%, whereas the removal rate with SA-nZVI was 93% (Fig. 5c). Even when the ratio of iron to Cr(VI) was 5, the removal rate of Cr(VI) by CMC-nZVI was still lower than that by SAnZVI (3) under the ratio of iron to Cr(VI). These results further suggest that the removal rate of Cr(VI) using SA-nZVI is significantly higher than that by CMC-nZVI, which is consistent with the previous conclusion that SA-nZVI has a higher removal rate of Cr(VI) than CMC-nZVI under both acidic and alkaline conditions (Fig. 5a and b).
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3.4. Effects of NO3− on the removal of Cr(VI) Natural groundwater is rich in various ions, especially NO3−, one of the common pollutants in groundwater and nitrogen fertilizers widely used in the world (Deng et al., 2017), and the presence of these ions greatly affects the mobility and reactivity of nZVI (Hosseini and Tosco, 2015). NO3− was reported to inhibit the reactions by forming a film on the Fe0 surface after its reduction by Fe0 (Alowitz and Scherer, 2002). We thus investigated the removal rate of Cr(VI) in the presence and absence of NO3−. The result showed that after 11 min of reaction, the removal rate of Cr(VI) was 85% in the absence of NO3−, in contrast with 86% in the presence of 5 mg/L NO3− (Fig. 6), suggesting that NO3− does not affect the final removal rate of Cr(VI) by SA-nZVI. Although many reports showed that in the presence of NO3−, the reaction efficiency of nZVI was significantly decreased, a low concentration of NO3− was found to have no effect on the efficiency of trichloroethylene reduction (Liu et al., 2007). Another study showed that under the high concentration of Cr(VI), increasing NO3− concentration did not affect the response value of Cr(VI) and CMC-nZVI (Kaifas et al., 2014). These two results are consistent with our finding that NO3− does not have an effect on Cr(VI) removal using SA-nZVI. Our data also indicate that SAnZVI is effective in Cr(VI) removal from groundwater, although groundwater contains NO3−. 3.5. XPS analysis of reaction products According to the principle of nZVI reduction, Cr(VI) is reduced to less toxic Cr3+, which further forms Cr(OH)3 precipitates, or forms Cr3+-Fe3+ oxide. To verify this reaction of Cr(VI) and CMC-nZVI or Cr (VI) and SA-nZVI, we performed XPS analysis of reaction products of Cr (VI) and nZVI. As shown in Fig. 7, two distinct peaks at binding energies of nearly 577 eV and 587 eV, characteristics of CrOOH and Cr(OH)3, were observed in reaction products of Cr(VI) and CMC-nZVI (Fig. 7a) or Cr(VI) and SA-nZVI (Fig. 7b). These data indicate that both CMC-nZVI and SA-nZVI reduce Cr(VI) and form Cr(OH)3 as precipitates, which was consistent with previous results showing that products obtained from Cr(VI) and Fe0 were nearly identical to Cr(III)/Fe(III) hydroxides/ oxyhydroxides (Rao et al., 2013). 4. Conclusions In this work, we developed SA-dispersed nZVI and found that SA significantly decreased the agglomeration of nZVI. Compared with the traditional nZVI and CMC-nZVI, the newly synthesized SA-nZVI had 38
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Removal of hexavalent chromium from groundwater using sodium alginate dispersed nano zero-valent iron
T
Zihan Lia, Shuyuan Xua, Guanghui Xiaob, Limin Qiana, Yun Songa,c,∗ a
Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, Beijing, China College of Life Science, Shaanxi Normal University, Xi'an, China c Beijing Key Laboratory of Industrial Land Contamination and Remediation, Beijing, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nano zero-valent iron Sodium alginate Hexavalent chromium Groundwater Removal rate
Hexavalent chromium (Cr), one of the most common heavy metals, is widely found in contaminated soil and groundwater. Nano zero-valent iron (nZVI) is used to treat Cr(VI) in polluted groundwater. However, due to agglomeration, rapid sedimentation, and limited mobility of nanoparticles in the aquatic environment, nZVI is not widely used in groundwater treatment. In this study, we used sodium alginate (SA) to modify nZVI to generate dispersed SA-nZVI. SA-nZVI particles were found to embed in the polymer material and exist as an amorphous state with a diameter less than 100 nm. Compared with traditional nZVI and carboxymethyl cellulose (CMC)-nZVI, SA-nZVI had better stability and higher absolute zeta potential. The presence of SA enhanced mobility of nZVI and effectively prevented sedimentation and aggregation. Furthermore, SA-nZVI had a higher Cr(VI) removal rate than (CMC)-nZVI under both acidic and alkaline conditions. XPS analysis showed that Cr(VI) was reduced to Cr(III) and formed Cr(OH)3 as precipitates after treatment with SA-nZVI. In addition, NO3− had no effect on the final removal rate of Cr(VI) by SA-nZVI. These results suggest that SA-nZVI has high penetration and a high removal rate in Cr(VI) removal and can be used to stabilize nZVI to remediate Cr(VI)-contaminated groundwater in the future.
1. Introduction Groundwater is a valuable resource that is essential for human life, agriculture, and industry (Bhowmick et al., 2018). Heavy metal pollution is widespread in groundwater and poses a great threat to human life and health (Huang et al., 2018; Kurwadkar, 2017; Zhou et al., 2017; Saleh et al., 2018). Chromium (Cr), a heavy metal, is widely used in industrial processes and is found in industrial waste, especially in chromium slag (Gebru and Das, 2018; Dokou et al., 2017). Water-soluble sodium chromate and acid-soluble calcium chromate found in chromium slag or petroleum contain large amounts of hexavalent chromium (Cr(VI)), which can be transferred from the soil to the groundwater, leading to groundwater pollution (Bae et al., 2017; Shen et al., 2018). Currently, Cr(VI) contamination in groundwater is a common and serious problems in China (Zhao et al., 2016a). The conventional pump and treat technique, one of the most important exsitu technologies, was used in early groundwater treatment. Due to high cost and low efficiency, this approach is not widely used in modern industry (Lee et al., 2009). Nanomaterials have rapid action in wastewater treatment because
∗
nanoparticles have unique physical and chemical properties such as large surface-area-to-volume ratio and high interfacial reactivity (Li et al., 2006). Due to the characteristics of fast response, high reduction potential, and low cost, nano zero-valent iron (nZVI) is one of the most effective materials for in-situ remediation of wastewater (Vidmar et al., 2018; El-Temsah et al., 2016). Compared with traditional zero-valent iron (ZVI) particles, nZVI has a higher specific surface area, stronger surface activity, and broad applicability (Zhao et al., 2016b; Tesh and Scott, 2014). nZVI is widely used in in situ remediation of Cr(VI) contaminated groundwater because it is lower cost, faster, higher efficiency, and more environmentally friendly than traditional ZVI, ferrous sulfate, and calcium polysulfide (Yirsaw et al., 2016; Di Palma et al., 2015; Chrysochoou et al., 2012). However, due to the large specific surface area and ultra-high surface energy of nanoparticles, combined with the van der Waals force and magnetic properties, nZVI particles often form aggregates, which largely reduces their transmission in contaminated groundwater and soil (Xie and Cwiertny, 2010; Noubactep et al., 2012). Also, oxidation often occurs when nZVI is not in contact with the target contaminant, resulting in deactivation of nZVI (Lee et al., 2014).
Corresponding author. Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, Beijing, China. E-mail address: [email protected] (Y. Song).
https://doi.org/10.1016/j.jenvman.2019.04.130 Received 17 November 2018; Received in revised form 23 April 2019; Accepted 30 April 2019 Available online 17 May 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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(UV-2550, Daojin) at a wavelength of 540 nm. Cr(IV) concentration in solution was quantified using a standard curve method and the removal rate was calculated.
Many attempts have been made to enhance stability and prevent aggregation of nZVI (Wu et al., 2015). Among them, carboxymethylcelluose (CMC) was successfully used to modify nZVI particles and was thus further used for in-situ groundwater remediation (Fatisson et al., 2010). CMC modified nZVI (CMC-nZVI) prevented particle aggregates and formed stable suspensions (De Lima Fontes et al., 2018; Fan et al., 2015; Johnson et al., 2013). Sodium alginate (SA) is commonly used to remove heavy metals and toxic substances from groundwater and soil (Gopi et al., 2018; Huang et al., 2018). SA has many carboxyl groups on its molecular chain, which can adsorb metal ions (Cho et al., 2018). In addition, SA is an ideal dispersant and has been used to modify nanomaterials to improve the immobilization of biological enzymes (Yu et al., 2014). However, modification of nZVI with SA is not in widespread use and thus there are no reports of SAnZVI being used for wastewater remediation. The objectives of this study were to prepare SA- and CMC-nZVI particles, determine their characteristics, compare the Cr(VI) removal rates of both nZVI particles, and investigate pH and initial concentrations of Cr(VI) and nZVI during Cr(VI) removal. This study developed a new material and optimized conditions for Cr(VI) removal. This nanomaterial has the potential to be widely used to remediate Cr(VI)-contaminated wastewater in the future.
2.4. Column experiments for mobility Two glass columns (diameter 4 cm and height 40 cm) were filled with 20–40 mesh quartz sand. The metal gauze was fixed with a silicone plate on the top and bottom of the cover to reduce disturbance of the water. The same initial concentrations of CMC-nVZI, SA-nVZI, and nVZI without any dispersant solutions flowed from bottom to top through a quartz-filled glass column with a peristaltic pump at a fixed flow rate of 50 mL/min. For the breakthrough curves experiment, three different nVZI solutions flowed through the glass column filled with quartz sand. The volumes of each influent and effluent were measured. 2.5. SEM and XPS analyses The freshly prepared CMC- and SA-nVZI samples were dried in a vacuum oven, washed with ethanol and dried with nitrogen. The phase composition of two nVZI samples was characterized using a fieldemission scanning electron microscope (ZEISS Merlin Compact). Reaction products of Cr(VI) and CMC- or SA-nVZI were characterized by X-ray photoelectron spectroscopy (XPS) (VG Multilab 2000; Thermo VG Scientific) (Yoon et al., 2011). The reaction products were quantified by determining the position, height, width, and shape of each peak.
2. Materials and methods 2.1. Materials and chemicals
2.6. Statistical analysis
All chemicals used in this study, including potassium borohydride, ferrous sulfate, sodium alginate (SA), sodium carboxymethyl sulfate, sodium hydroxide, hydrochloric acid, absolute ethyl alcohol, diphenylcarbazide, acetone, sulfuric acid, and nitric acid were of analytical grade. All solutions were prepared with deionized water generated from a Milli-Q water system (Merck Millipore Corporation). Cr(VI) stock solutions were prepared by dissolving sodium dichromate into deionized water. The pH of all solutions was adjusted using sodium hydroxide or hydrochloric acid solution, and then measured with a pH meter (VSTAR80, Thermo Fisher).
All the experiments were repeated in triplicate. 3. Results and discussion 3.1. Characteristics of SA- and CMC-nZVI The SEM images showed that CMC-nZVI was spherical with a diameter of about 100 nm and was clearly well-dispersed (Fig. 1a). nZVI was wrapped in the surface of CMC mediums with a micelle shape (Fig. 1b). In the presence of CMC, the magnetic effect of nZVI is effectively overcome, and nZVI is effectively dispersed by electrostatic repulsion and steric hindrance (De Lima Fontes et al., 2018; Johnson et al., 2013). nZVI dispersed by SA was spherical with a diameter of less than 100 nm (Fig. 1c). nZVI particles were “glued” together by SA (Fig. 1d) and unstabilized nZVI was stacked and agglomerated due to magnetic forces (Honetschlägerová et al., 2015). The TEM images also showed that SA-nZVI particles were coated with polymer and the size of them were about 100 nm (Fig. 1e and f). SA-nZVI particles are not easy to agglomerate duo to the adsorption of functional groups such as –COOH and –OH in SA (Fig. 1c; Huang et al., 2016). When SA was added to the ferric trichloride solution to synthesize a magnetic microsphere, the ferric ion replaced the sodium ion in SA and iron hydroxide was embedded in the polymer network. nZVI formed by reduction was not simply agglomerated and stacked but was embedded in a polymer network (Finotelli et al., 2004). These results indicate that SA can reduce the agglomeration of nZVI. XPS analysis was used to investigate surface compositions of CMCnZVI and SA-nZVI, particularly for the oxidation state of Fe on the surface of as-prepared material spectra. Both CMC-nZVI and SA-nZVI had two peaks at binding energies of nearly 710 eV and 724.3 eV, characteristic of Fe-ox and FeOOH, respectively (Fig. 2a and c). The peak of Fe0, which was at the binding energy 706.2 eV, was very weak. This confirms the core-shell structure of nZVI, with an iron oxide film shell on the surface (Ling and Zhang, 2017). The XPS technique is widely used for surface-level detection (Xie and Cwiertny, 2010) and nZVI in this study was coated with organic polymer materials, CMC or SA, which led to the failed escape of Fe0 photoelectrons located in the
2.2. Preparation of CMC- and SA-nZVI composites nZVI particles were synthesized from a boroyhdride reduction using ferrous iron (Dong et al., 2018). CMC and SA were used to modify nZVI particles. In detail, 12% (w/w, CMC/Fe) CMC and 15% (w/w, SA/Fe) SA solutions (150 ml) were obtained and then stirred continuously for 15 min in an N2 environment. A 0.005 M Fe2SO4 solution (50 mL) was prepared and then stirred continuously for 15 min in an N2 environment. Subsequently, the ferrous sulfate solution was added to the CMC or SA solution with continuous agitation for 30 min in an N2 environment to ensure the formation of CMC-Fe2+ or SA-Fe2+ complexes and prevent oxidation. A 0.01 M KBH4 solution (50 mL) was prepared based on the molar ratio of BH4−/Fe2+ 2.0, and then stirred continuously for 15 min in an N2 environment to discharge oxygen. Finally, KBH4 solution was dropped into the CMC-Fe2+ or SA-Fe2+ mixed solution at a rate of 2 drops per second. The liquid prepared above was stored in a black-brown bottle filled with N2 above the liquid. 2.3. Batch experiments for Cr (VI) removal The effects of initial Cr(VI) concentration, solution pH, and nZVI concentration (loading) on Cr(VI) removal were investigated in this study. Quantification of the concentration of nZVI was performed as previous report (Johnson et al., 2013). A Cr(VI) standard curve was prepared to calculate Cr(VI) concentration. During the Cr(VI) removal process, the dispersed nZVI and Cr(VI) solution was mixed and stirred continuously for the indicated time according to the experimental design. Cr(IV) absorbance was determined using a spectrophotometer 34
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Fig. 1. SEM and TEM image of CMC- and SA-nZVI particles at different resolution ratios. a, CMC-nZVI image magnified 50,000 times (scale bar = 100 nm); b, CMCnZVI image magnified 10,000 times (scale bar = 1 μm); c, SA-nZVI image magnified 50,000 times (scale bar = 100 nm); d, SA-nZVI image magnified 10,000 times (scale bar = 1 μm). e, TEM image of SA-nZVI particles magnified 10,000 times (scale bar = 500 nm); f, TEM image of SA-nZVI particles magnified 50,000 times (scale bar = 100 nm).
combined with translucent solution and many particles suspended in the middle and lower parts of the bottle. CMC-nZVI and SA-nZVI did not show significant agglomeration or sedimentation: they were highly dispersed in a dark black solution (Fig. 3a). After being left to stand for 3 h, nZVI particles were suspended in a yellow solution, indicating that nZVI was oxidized. In the CMC-nZVI bottle, some black particles settled at the bottom and the solution began to turn green, indicating that CMC-nZVI was partial oxidized. No obvious sedimentation particles were observed in the SA-nZVI bottle (Fig. 3b). After being left to stand for 3 h, SA-nZVI had no sedimentation or oxidation. These results suggest that, compared with nZVI and stabilized CMC-nZVI, SA-nZVI has better dispersibility and less agglomeration. Zeta potential is a key indicator of the stability of colloidal
shell core of CMC-nZVI and SA-nZVI particles. XPS analysis revealed that C–OOOH and C–OH, characteristic functional groups of CMC and SA, were found in these nZVI (Fig. 2b and d), which also suggested that the surface of the synthetic nZVI was surrounded by CMC or SA (Fig. 1). 3.2. Stability and mobility of SA-nZVI and CMC-nZVI Although nZVI can effectively remove a variety of pollutants, it is easy for nZVI to agglomerate and grow into larger size particles (Honetschlägerová et al., 2015). Therefore, we then assessed the stability of synthetic SA-nZVI and CMC-nZVI using the settlement experiment and zeta potential analysis. The newly synthetic nZVI particles were left to stand for 30 min, and there was obvious sedimentation
Fig. 2. The X-ray photoelectron spectroscopy (XPS) analysis of CMC- and SA-nZVI particles. a, Fe 2p XPS spectra of SA-nZVI; b, 1s XPS spectra of SA-nZVI; c, Fe 2p XPS spectra of CMC-nZVI; d, C 1s XPS spectra of CMC-nZVI. 35
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Fig. 3. Comparison of stability and zeta potential of different nZVI particles. a, stability of nZVI, CMC- nZVI, and SA-nZVI after synthesis for 30 min; b, stability of nZVI, CMC- nZVI, and SA-nZVI after synthesis for 3 h; c, zeta potential of 12% CMC-nZVI particles in different pH conditions; d, zeta potential of 12% SA-nZVI particles in different pH conditions. Reaction conditions: CMC-nZVI, 15 mg/L; SA-nZVI, 15 mg/L; nZVI, 15 mg/L. Error bars represent SD for three independent experiments.
in Fig. 4A. For CMC-Fe and nZVI, the black particles settled down at the bottom of column. SA-stabilized nZVI almost reached the top of the column with the flow (Fig. 4a). The breakthrough rates showed that when the pore volume reached 1.5, transport of both CMC-nZVI and nZVI no longer increased, whereas the transport of SA-nZVI increased: C/Co at 2.5 pore volume was 0.9 in SA-nZVI, much higher than CMCnZVI and nZVI (Fig. 4b). These findings suggest that the presence of dispersant SA enhances mobility of nZVI and effectively prevents sedimentation and aggregation. In the presence of natural organic matter, mobility of nZVI was significantly increased (Johnson et al., 2009).
dispersions and a higher absolute zeta potential confers more stability (Hanaor et al., 2012). Usually, colloids with an absolute zeta potential above thirty are basically stable (Clogston and Patri, 2011). We detected the zeta potential under alkaline conditions as nZVI is more likely oxidized under acidity, which is ineffective in remediation. As shown in Fig. 3c, when pH was over 9, the absolute value of zeta potential of both SA-nZVI and CMC-nZVI exceeded 30 mV. The zeta potential of SA-nZVI has a higher absolute value (Fig. 3d). These data indicate that SA increases the electrostatic repulsion between nZVI and disperses the particles by steric hindrance to achieve better stability, which is consistent with the settlement experiment (Fig. 3). Column experiments were used to evaluate the mobility of dispersed nZVI. nZVI and CMC-nZVI migrated and stopped a quarter of the way through the column and the breakthrough curves were plotted as shown
Fig. 4. Column migration experiment (a) and breakthrough rate (b) of nZVI, CMC-nZVI, and SA-nZVI. Reaction conditions: CMC-nZVI, 15 mg/L; SA-nZVI, 15 mg/L; nZVI, 15 mg/L pH = 6.5. Error bars represent SD for three independent experiments. 36
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Fig. 5. Effect of pH on removal rate of Cr(VI) using CMC- and SA-nZVI (a, b). The comparison of removal rate of Cr(VI) with CMC- and SA-nZVI particles under different pH conditions (a, b) and different nZVI content (c). Reaction conditions: CMC-nZVI, 15 mg/L; SA-nZVI, 15 mg/L; Cr(VI), 4 mg/L. Error bars represent SD for three independent experiments.
Fig. 6. Effect of NO3− on Cr(VI) removal by SA-nZVI particles. Reaction conditions: the concentrations of NO3−, Cr(VI), and SA-nZVI were 5 mg/L, 4 mg/L and 15 mg/L, respectively; pH 8.5. Error bars represent SD for three independent experiments.
3.3. Effects of initial pH on the removal of Cr(VI) by SA-nZVI and CMCnZVI We further investigated the effects of initial pH on Cr(VI) removal using CMC- and SA-nZVI. Previous studies showed that SA had no significant effect on the removal rate of Cr(VI) (Wang et al., 2017; Huang et al., 2016). At pH 6.0, the removal rate of Cr(VI) using CMCnZVI was 75.6% after 20 min of reaction, whereas 96.4% Cr(VI) was removed by SA-nZVI in the same conditions (Fig. 5a). In contrast with the 60.4% of Cr(VI) removed by CMC-nZVI, 85.7% of Cr(VI) was removed by SA-nZVI after 20 min of reaction at pH 9.0 (Fig. 5b). These results showed that the removal rate of Cr(VI) by SA-nZVI was much
Fig. 7. The reaction products of Cr(VI) and SA (a)- or CMC-nZVI (b).
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better stability, dispersibility, and less sedimentation. Zeta potential detection indicated that SA-nZVI had better mobility, which was further verified because SA-nZVI penetrated through and flowed out from the column, whereas traditional nZVI and CMC-nZVI only migrated about a quarter of the way through the entire column. The high performances indicated that SA-nZVI will have obvious effects on Cr(VI) contaminated groundwater. This result was supported by the fact that the removal rate of Cr(VI) by SA-nZVI was much higher than that by CMCnZVI under both acidic and alkaline conditions. The removal rate of Cr (VI) by SA-nZVI was up to 96.4% after 20 min of reaction at pH 6.0. After treatment with SA-nZVI, Cr(VI) was reduced to Cr(III) and formed Cr(OH)3 as precipitates. NO3− had minimal effects on the final removal rate of Cr(VI) removed by SA-nZVI. These results show that SA-nZVI has high penetration and a high removal rate in Cr(VI) removal and is a promising material for Cr(VI) contaminated groundwater remediation.
higher than that by CMC-nZVI under both acidic and alkaline conditions. Although the removal rate of Cr(VI) by SA-nZVI decreased to 85.7% at pH 9.0, in contrast to the 96.4% removal rate at pH 6.0, this is still a high removal rate. This result indicates that SA-nZVI is active and effective in groundwater, as groundwater is often alkaline. Similar findings were found in previous studies where the removal rate of Cr (VI) by CMC-nZVI was 97% at pH 4.5, whereas the removal rate decreased to 45.9% at pH 8.5 (Qian, 2008). At the high pH condition, the passivation of Fe0 hinders electron transfer from highly active zerovalent iron cores, thereby reducing the removal rate of Cr(VI) (Mondal et al., 2004). In addition, we explored the effect of initial concentration of iron on Cr(VI) removal. When the ratio of iron to Cr(VI) was 3, the removal rate of Cr(VI) by CMC-nZVI was only 51%, whereas the removal rate with SA-nZVI was 93% (Fig. 5c). Even when the ratio of iron to Cr(VI) was 5, the removal rate of Cr(VI) by CMC-nZVI was still lower than that by SAnZVI (3) under the ratio of iron to Cr(VI). These results further suggest that the removal rate of Cr(VI) using SA-nZVI is significantly higher than that by CMC-nZVI, which is consistent with the previous conclusion that SA-nZVI has a higher removal rate of Cr(VI) than CMC-nZVI under both acidic and alkaline conditions (Fig. 5a and b).
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3.4. Effects of NO3− on the removal of Cr(VI) Natural groundwater is rich in various ions, especially NO3−, one of the common pollutants in groundwater and nitrogen fertilizers widely used in the world (Deng et al., 2017), and the presence of these ions greatly affects the mobility and reactivity of nZVI (Hosseini and Tosco, 2015). NO3− was reported to inhibit the reactions by forming a film on the Fe0 surface after its reduction by Fe0 (Alowitz and Scherer, 2002). We thus investigated the removal rate of Cr(VI) in the presence and absence of NO3−. The result showed that after 11 min of reaction, the removal rate of Cr(VI) was 85% in the absence of NO3−, in contrast with 86% in the presence of 5 mg/L NO3− (Fig. 6), suggesting that NO3− does not affect the final removal rate of Cr(VI) by SA-nZVI. Although many reports showed that in the presence of NO3−, the reaction efficiency of nZVI was significantly decreased, a low concentration of NO3− was found to have no effect on the efficiency of trichloroethylene reduction (Liu et al., 2007). Another study showed that under the high concentration of Cr(VI), increasing NO3− concentration did not affect the response value of Cr(VI) and CMC-nZVI (Kaifas et al., 2014). These two results are consistent with our finding that NO3− does not have an effect on Cr(VI) removal using SA-nZVI. Our data also indicate that SAnZVI is effective in Cr(VI) removal from groundwater, although groundwater contains NO3−. 3.5. XPS analysis of reaction products According to the principle of nZVI reduction, Cr(VI) is reduced to less toxic Cr3+, which further forms Cr(OH)3 precipitates, or forms Cr3+-Fe3+ oxide. To verify this reaction of Cr(VI) and CMC-nZVI or Cr (VI) and SA-nZVI, we performed XPS analysis of reaction products of Cr (VI) and nZVI. As shown in Fig. 7, two distinct peaks at binding energies of nearly 577 eV and 587 eV, characteristics of CrOOH and Cr(OH)3, were observed in reaction products of Cr(VI) and CMC-nZVI (Fig. 7a) or Cr(VI) and SA-nZVI (Fig. 7b). These data indicate that both CMC-nZVI and SA-nZVI reduce Cr(VI) and form Cr(OH)3 as precipitates, which was consistent with previous results showing that products obtained from Cr(VI) and Fe0 were nearly identical to Cr(III)/Fe(III) hydroxides/ oxyhydroxides (Rao et al., 2013). 4. Conclusions In this work, we developed SA-dispersed nZVI and found that SA significantly decreased the agglomeration of nZVI. Compared with the traditional nZVI and CMC-nZVI, the newly synthesized SA-nZVI had 38
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