Physicochemical transformation of Fe/Ni bimetallic nanoparticles during aging in simulated groundwater and the consequent effect on contaminant removal
Graphical abstract
Introduction
Bimetallic iron nanoparticles (BNPs), comprised of nanoscale zero-valent iron (nZVI) as the base metal and a small amount of noble metal as catalyst additives, have been widely studied for the potential application in groundwater and soil remediation (Han and Yan, 2014). Since Fowkes and Anderson first reported Fe/Al bimetallic particles in 1970 (Fowkes et al., 2002), various types of BNPs (e.g., Fe/Ni, Fe/Cu, Fe/Pd and Fe/Pt) have been synthesized successfully (Leistner et al., 2006, Cao et al., 2011, Fang et al., 2011, Nie et al., 2013, Lin et al., 2015). Compared with monometallic iron nanoparticles, BNPs show much better performance in site remediation of contaminants (Lien and Zhang, 2007, Choi et al., 2008, Lin et al., 2015). In the BNPs system, Fe0 has a lower redox potential and acts as an electron donator, which is sacrificed to be oxidized. The added noble metal usually acts as a catalyst to accelerate the production of atomic hydrogen or hydride, thus increasing the reductive degradation and preventing formation of toxic byproducts (Zhu and Lim, 2007, Lee and Sedlak, 2008, Chun et al., 2010, Greenlee et al., 2012).
However, the reactivity of BNPs could be gradually reduced when exposed to water (Liu et al., 2015, Dong et al., 2016a). Since the BNPs have large specific area and high reactivity, they tend to corrode in water environment (Guan et al., 2015, Dong et al., 2017a). And then, the composition and structure of BNPs could be transformed during the corrosion process (Greenlee et al., 2012). Previous studies showed that different kinds of iron oxides (maghemite, magnetite and lepidocrocite) were generated with time of aging when Fe0 nanoparticles were exposed to water (Reinsch et al., 2013, Dong et al., 2016a, Dong et al., 2017b, Xie et al., 2017a, Dong et al., 2017b). Correspondingly, various kinds of structures (spherical, needle-like, sheet-like and plate-like) were found during the aging time (Greenlee et al., 2012, Liu et al., 2015, Dong et al., 2016a, Dong et al., 2017b). The differences in morphology and composition could be attributed to the geochemical conditions (solution composition, pH, ORP), the preparation method of nanoparticles (FeBH and FeH2, produced respectively by the borohydride reduction method and the hydrogen reduction method) and the aging time (Kim et al., 2012, Reinsch et al., 2013, Han and Yan, 2014, Liu et al., 2015). Hence, it is important to trace the physicochemical transformation of nanoparticles during short-time and long-time aging in order to better understand their fate and long-term reactivity (Wiesner et al., 2006, Reinsch et al., 2013). However, most of the previous studies focused on the reactivity of fresh BNPs toward contaminants (Nie et al., 2013, Kuang et al., 2015), there were little study systematically probed the physical and chemical transformation of BNPs during aging in groundwater and the consequent effect on the removal efficiency of target contaminant.
Therefore, in this study we prepared Fe/Ni BNPs and systematically investigated their short-term (in hours and days) and long-term aging (in months) in simulated groundwater, and the performance of the aged Fe/Ni BNPs in contaminant removal. The specific objectives of this study were to (1) probe the corrosion kinetics of Fe/Ni BNPs in simulated groundwater; (2) investigate the morphological, compositional and structural transformation of Fe/Ni BNPs in simulated groundwater during short-term aging (within 5d) and long-term aging (up to 90 d); and (3) study the removal efficiency of target contaminant by the aged Fe/Ni BNPs. Tetracycline (TC), a symbol of antibiotics which were commonly detected in the environments and pose potential risks to human health and the environment (Storteboom et al., 2010, Lu et al., 2017), was chosen as the target contaminant in this study.
Section snippets
Materials and chemical agents
The chemical agents used in the experiments, including FeCl3·6H2O, NiCl2·6H2O, NaBH4, ethanol, HCl, NaOH, NaCl, NaHCO3, Na2SO4, and CaCl26H2O, were all of reagent grade and obtained from the local reagent suppliers of Changsha. Tetracycline (TC, 98% purity) was bought from Hefei Bomei Biotechnology Co., Ltd. The stock solutions were all prepared by using deoxygenated ultrapure water (purged with N2 for at least 30 min before use to guarantee an anaerobic environment). Suwannee River humic acid
Corrosion of Fe/Ni BNPs during short-term aging
In the field application of iron-based nanoparticles (NPs), the variation of pH, ORP and dissolved iron concentration in the aqueous environment are commonly monitored to indirectly demonstrate the occurrence of corrosion reactions of the iron-based NPs (Reinsch et al., 2013, Guan et al., 2015, Dong et al., 2017b). The information is of great importance for the further determination of the life-time and fate of the NPs in the subsurface environment. It was reported that the corrosion of
Conclusions
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In the short-term aging of Fe/Ni BNPs, the pH, ORP and concentration of soluble iron changed drastically in the first few hours and kept stable in the following aging time up to 5 d. This demonstrates that the corrosion of Fe/Ni BNPs in simulated groundwater occurred rapidly (within hours). The results indicate that Fe/Ni BNPs are indeed efficient reductant but may lose their reactivity rapidly due to fast electron loss from corrosion.
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Physicochemical transformations of the corrosion products
Acknowledgments
This research was supported by the National Natural Science Foundation of China (51409100, 51521006), the Fundamental Research Funds for the Central Universities, (531107040788) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17).
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