Core-shell silica-coated Fe0 nanoparticles (Fe@SiO2) were prepared in one-step synthesis by aqueous reduction combined with modified St？ber method. The as-prepared Fe@SiO2 were then used for simultaneous removal of Cr(VI) and Cd(II) from aqueous solution. Batch tests indicated that Fe@SiO2 exhibited high removal capacity toward Cr(VI) and Cd(II). Cr(VI) was removed by Fe@SiO2 through reduction rather than adsorption, while Cd(II) removal was mainly through adsorption. The removal rate increased with increasing initial Fe NPs dose and decreased with increasing initial Cr(VI) and Cd(II) concentrations. Cd(II) adsorption was also strengthened by Cr(VI) reduction with the release of OH？. The removals of Cr(VI) and Cd(II) were weakened in the presence of cations or humic acid, as a result of aggregation and less active site of Fe@SiO2. Overall, the simply prepared Fe@SiO2 were potential material for the heavy metals removed from water. 1. Introduction Most heavy metals are toxic and carcinogenic even at very low concentrations and usually cause a serious threat to the environment and the public health. For example, Cr(VI) is a toxic, carcinogenic substance to human and animals [1]. Contrarily, Cr(III) is relatively nontoxic and an essential nutrient for human [2]. Cd(II) is also a potent carcinogen causing damage to the lungs, kidneys, liver, and reproductive organs [3, 4]. Therefore, US Environmental Protection Agency [5] regulates at least ten metals, including chromium and cadmium, as primary contaminants that must be removed from drinking water. There are various conventional techniques applied for removing heavy metals from water. Electroplating wastewater usually contains various heavy metals as cocontaminants; however, few studies have been reported on simultaneous removal of Cr(VI) and Cd(II) from wastewater. Among the different treatments for removing heavy metals, adsorption has been developed as a simple, efficient, and cost-effective method. Many of adsorbents such as clays, activated carbon, sewage sludge, and plant parts have been used for heavy metal removal [6–9]. But due to extremely small particle size and large surface area, iron nanoparticles (Fe NPs) with a high adsorption capacity are found to be one of the most effective adsorbents for removing heavy metals [10–12]. Moreover, Fe NPs have shown a high chemical reduction rate on several kinds of contaminants, including toxic metal ions [13–15]. For example, Ponder et al. [13] have found that Fe NPs acting as reductants could chemically reduce Cr(VI) to Cr(III). Besides, Fe NPs can
References
[1]
M. Costa, “Potential hazards of hexavalent chromate in our drinking water,” Toxicology and Applied Pharmacology, vol. 188, no. 1, pp. 1–5, 2003.
[2]
D. Rai, B. M. Sass, and D. A. Moore, “Chromium (III) hydrolysis constants and solubility of chromium(III) hydroxide,” Inorganic Chemistry, vol. 26, no. 3, pp. 345–349, 1987.
[3]
R. K. Zalups and S. Ahmad, “Molecular handling of cadmium in transporting epithelia,” Toxicology and Applied Pharmacology, vol. 186, no. 3, pp. 163–188, 2003.
[4]
L. J？rup and T. Alfvén, “Low level cadmium exposure, renal and bone effects—the OSCAR study,” BioMetals, vol. 17, no. 5, pp. 505–509, 2004.
[5]
USEPA, Edition of the Drinking Water Standards and Health Advisories, USEPA, Washington, DC, USA, 2006.
[6]
R. D. C. Soltani, A. J. Jafari, and G. S. Khorramabadi, “Investigation of cadmium (II) ions biosorption onto pretreated dried activated sludge,” American Journal of Environmental Sciences, vol. 5, no. 1, pp. 41–46, 2009.
[7]
M.-Q. Jiang, Q.-P. Wang, X.-Y. Jin, and Z.-L. Chen, “Removal of Pb(II) from aqueous solution using modified and unmodified kaolinite clay,” Journal of Hazardous Materials, vol. 170, no. 1, pp. 332–339, 2009.
[8]
F. Boudrahem, A. Soualah, and F. A. Benissad, “Pb(II) and Cd(II) removal from aqueous solutions using activated carbon developed from coffee residue activated with phosphoric acid and zinc chloride,” Journal of Chemical & Engineering Data, vol. 56, no. 5, pp. 1946–1955, 2011.
[9]
A. M. A. Pintor, C. I. A. Ferreira, J. C. Pereira, et al., “Use of cork powder and granules for the adsorption of pollutants: a review,” Water Research, vol. 46, no. 10, pp. 3152–3166, 2012.
[10]
S. R. Kanel, J.-M. Greneche, and H. Choi, “Arsenic(V) removal from groundwater using nano scale zero-valent iron as a colloidal reactive barrier material,” Environmental Science & Technology, vol. 40, no. 6, pp. 2045–2050, 2006.
[11]
X.-Q. Li and W.-X. Zhang, “Iron nanoparticles: the core-shell structure and unique properties for Ni(II) sequestration,” Langmuir, vol. 22, no. 10, pp. 4638–4642, 2006.
[12]
A. Alqudami, N. A. Alhemiary, and S. Munassar, “Removal of Pb(II) and Cd(II) ions from water by Fe and Ag nanoparticles prepared using electro-exploding wire technique,” Environmental Science and Pollution Research, vol. 19, pp. 2832–2841, 2012.
[13]
S. M. Ponder, J. G. Darab, and T. E. Mallouk, “Remediation of Cr(VI) and Pb(II) aqueous solutions using supported, nanoscale zero-valent iron,” Environmental Science & Technology, vol. 34, no. 12, pp. 2564–2569, 2000.
[14]
I. Dror, O. M. Jacov, A. Cortis, and B. Berkowitz, “Catalytic transformation of persistent contaminants using a new composite material based on nanosized zero-valent iron,” ACS Applied Materials & Interfaces, vol. 4, no. 7, pp. 3416–3423, 2012.
[15]
F. Dimin, P. A. Roberto, M. T. Bradley, et al., “Reductive sequestration of pertechnetate by nano zerovalent iron (nZVI) transformed by abiotic sulfide,” Environmental Science & Technology, vol. 47, no. 10, pp. 5302–5310, 2013.
[16]
F. He and D. Y. Zhao, “Preparation and characterization of a new class of starch-stabilized bimetallic nanoparticles for degradation of chlorinated hydrocarbons in water,” Environmental Science & Technology, vol. 39, no. 9, pp. 3314–3320, 2005.
[17]
Y. H. Xu and D. Y. Zhao, “Reductive immobilization of chromate in water and soil using stabilized iron nanoparticles,” Water Research, vol. 41, no. 10, pp. 2101–2108, 2007.
[18]
H. Park, Y.-M. Park, K.-M. Yoo, and S.-H. Lee, “Reduction of nitrate by resin-supported nanoscale zero-valent iron,” Water Science and Technology, vol. 59, no. 11, pp. 2153–2157, 2009.
[19]
B. Sunkara, J. J. Zhan, J. B. He, G. L. Mcpherson, G. Piringer, and V. T. John, “Nanoscale zerovalent iron supported on uniform carbon microspheres for the in situ remediation of chlorinated hydrocarbons,” ACS Applied Materials & Interfaces, vol. 2, no. 10, pp. 2854–2862, 2010.
[20]
A. Tiraferri, K. L. Chen, R. Sethi, and M. Elimelech, “Reduced aggregation and sedimentation of zero-valent iron nanoparticles in the presence of guar gum,” Journal of Colloid and Interface Science, vol. 324, no. 1-2, pp. 71–79, 2008.
[21]
M. M. Krol, A. J. Oleniuk, C. M. Kocur, et al., “A field-validated model for in situ transport of polymer-stabilized nZVI and implications for subsurface injection,” Environmental Science & Technology, vol. 47, pp. 7332–7340, 2013.
[22]
S. F. Wang, H. M. Cao, F. Gu, C. Z. Li, and G. J. Huang, “Synthesis and magnetic properties of iron/silica core/shell nanostructures,” Journal of Alloys and Compounds, vol. 457, no. 1-2, pp. 560–564, 2008.
[23]
Y. L. Tang, S. Liang, J. T. Wang, S. L. Yu, and Y. L. Wang, “Amino-functionalized core-shell magnetic mesoporous composite microspheres for Pb(II) and Cd(II) removal,” Journal of Environmental Sciences, vol. 25, no. 4, pp. 830–837, 2013.
[24]
S. E. Fendorf, G. M. Lamble, M. G. Stapleton, M. J. Kelley, and D. L. Sparks, “Mechanisms of chromium(III) sorption on silica. 1. Cr(III) surface structure derived by extended X-ray absorption fine structure spectroscopy,” Environmental Science & Technology, vol. 28, no. 2, pp. 284–289, 1994.
[25]
Y. J. Oh, H. Song, W. S. Shin, S. J. Choi, and Y.-H. Kim, “Effect of amorphous silica and silica sand on removal of chromium(VI) by zero-valent iron,” Chemosphere, vol. 66, no. 5, pp. 858–865, 2007.
[26]
M.-L. Yuan, J.-H. Tao, G.-J. Yan, M.-Y. Tan, and G.-Z. Qiu, “Preparation and characterization of Fe/SiO2 core/shell nanocomposites,” Transactions of Nonferrous Metals Society of China, vol. 20, no. 4, pp. 632–636, 2010.
[27]
T. I. Yang, R. N. C. Brown, L. C. Kempel, and P. Kofinas, “Controlled synthesis of core-shell iron-silica nanoparticles and their magneto-dielectric properties in polymer composites,” Nanotechnology, vol. 22, no. 10, Article ID 105601, 2011.
[28]
Y. C. Li, Z. H. Jin, T. L. Li, and Z. M. Xiu, “One-step synthesis and characterization of core-shell Fe@SiO2 nanocomposite for Cr (VI) reduction,” Science of the Total Environment, vol. 421-422, pp. 260–266, 2012.
[29]
G. N. Glavee, K. J. Klabunde, C. M. Sorensen, and G. C. Hadjipanayis, “Chemistry of borohydride reduction of iron(II) and iron(III) ions in aqueous and nonaqueous media. Formation of nanoscale Fe, FeB, and Fe2B powders,” Inorganic Chemistry, vol. 34, no. 1, pp. 28–35, 1995.
[30]
APHA-AWWA-WEF, Standard Methods for Examination of Water and WasteWater, American Public Health Association, Washington, DC, USA, 20th edition, 1998.
[31]
Y.-P. Sun, X.-Q. Li, J. S. Cao, W.-X. Zhang, and H. P. Wang, “Characterization of zero-valent iron nanoparticles,” Advances in Colloid and Interface Science, vol. 120, no. 1–3, pp. 47–56, 2006.
[32]
H. K. Boparai, M. Joseph, and D. M. O'carroll, “Cadmium (Cd2+) removal by nano zerovalent iron: surface analysis, effects of solution chemistry and surface complexation modeling,” Environmental Science and Pollution Research, vol. 20, no. 9, pp. 6210–6221, 2013.
[33]
B. Geng, Z. H. Jin, T. Li, and X. H. Qi, “Preparation of chitosan-stabilized Fe0 nanoparticles for removal of hexavalent chromium in water,” Science of the Total Environment, vol. 407, no. 18, pp. 4994–5000, 2009.
[34]
S. Chen, W. Chen, and C. Shih, “Heavy metal removal from wastewater using zero-valent iron nanoparticles,” Water Science and Technology, vol. 58, no. 10, pp. 1947–1954, 2008.
[35]
J.-P. Gallas, J.-M. Goupil, A. Vimont et al., “Quantification of water and silanol species on various silicas by coupling IR spectroscopy and in-situ thermogravimetry,” Langmuir, vol. 25, no. 10, pp. 5825–5834, 2009.
[36]
M. Ladd, Introduction to Physical Chemistry Press, Cambridge University, Cambridge, UK, 3rd edition, 2012.
[37]
X. Li and W. Zhang, “Sequestration of metal cations with zerovalent iron nanoparticles. A study with high resolution X-ray photoelectron spectroscopy (HR-XPS),” Journal of Physical Chemistry C, vol. 111, no. 19, pp. 6939–6946, 2007.
[38]
L. N. Shi, Y. Zhou, Z. L. Chen, M. Megharaj, and R. Naidu, “Simultaneous adsorption and degradation of Zn2+ and Cu2+ from wastewaters using nanoscale zero-valent iron impregnated with clays,” Environmental Science and Pollution Research, vol. 20, pp. 3639–3648, 2013.
[39]
Y. C. Li, Z. H. Jin, and T. L. Li, “A novel and simple method to synthesize SiO2-coated Fe nanocomposites with enhanced Cr (VI) removal under various experimental conditions,” Desalination, vol. 288, pp. 118–125, 2012.
