Nano-SiO2 transport and retention in saturated porous medium: Influence of pH, ionic strength, and natural organics
Graphical abstract
Introduction
Nano silica is a non-metallic inorganic material that is widely produced and has significant industrial applications (Wang et al., 2016b). They are frequently employed as catalyst carriers and abrasive agents (Zhang et al., 2020b). nSiO2 also acts as a potential adsorbent for the removal of contaminants and heavy metals like lead from aquatic ecosystems (Shukla et al., 2015). Furthermore, the negative charge on nSiO2 provides a large number of electrostatic binding sites, which enhances their utility in medicinal and biotechnological applications (Laranjeira et al., 2017).
A recent study indicated that highly dispersive silicate glass particles were discharged during the Fukushima Daiichi Nuclear Power Plant disaster enhanced the transfer of contaminants in subsurface environments, resulting in environmental pollution (Fujita and Kobayashi, 2016). Inexorable amounts of nSiO2 released into the environment may pose a threat to both aquatic and terrestrial ecosystems. Ayatallahzadeh Shirazi et al. (2016) reported that nSiO2 has toxic effects like decreased cell viability on marine microalgae Dunaliella salina. nSiO2 also impacts the animal morphology and behavioral parameters in Hydra vulgaris (Cnidaria) (Ambrosone et al., 2014). In addition, nSiO2 causes DNA damage, inflammatory reactions, and cell cycle arrest in a variety of cell types (Croissant et al., 2020). Therefore, it is important to critically analyze the toxic impacts of nSiO2 in different environmental matrices.
Highly dispersible colloidal silica improves the mobility of pollutants that are firmly adsorbed. Numerous research has highlighted the fact that silica nanoparticles facilitate the co-transport of various environmental pollutants. SiO2 was found to promote the transport of heavy metals (Pb2+ and Cd2+) in a recent study due to the significant adsorption of positively charged heavy metal ions on its negatively charged surface. In a separate study, it was reported that silica enhanced the retention of virus Escherichia coli phage in the saturated porous media (Qin et al., 2020). Whereas, under unsaturated conditions, it concomitantly facilitated the transport of virus by electrostatic repulsion of similarly charged particles. (Wang et al., 2016a) studied the co transport of nSiO2 and pesticide (Acetamiprid) in biochar amended porous media. By competing for binding sites on the biochar surfaces, the co-presence of acetamiprid and nSiO2 in the solutions improved the transport of both acetamiprid and NPs in the biochar-amended sand. The infiltration caused by rainfall or irrigation can transport the nanoparticles from the topsoil zone to the saturated zone (Babakhani et al., 2017). nSiO2 increases the stability of other metal oxide nanoparticles (TiO2, ZnO and CuO) and enhances their percolation into the soil layers of municipal waste landfills, hence reducing the effectiveness of the soil layer. Further, the transport behavior of nanoparticles is generally affected by organic matter and chemicals present in porous mediums (Wang et al., 2016b). It is important to understand that realistic exposure parameters such as pH, ionic strength, natural organic matter (NOM) can alter the physicochemical properties and hence the transport pattern of nSiO2 (Abbas et al., 2020; Amirbahman and Olson, 1995). Thorough knowledge of the fate and movement of nSiO2 in porous media is essential for designing reliable waste management and disposal methods for nSiO2 to avoid nanoparticle contamination in the environment.
Over the recent years, the research on the transport of several nanoparticles including TiO2 (Khan and Şengül, 2016), AgNPs (Cornelis et al., 2013), ZnO (Sun et al., 2015), FeO (Carstens et al., 2017), CNT (Sharma and Fagerlund, 2015), Al2O3 (Rahman et al., 2013) have received considerable attention. Previous reports suggest that different environmental factors have a strong role to play in modifying the surface characteristics of the nanoparticles. This will determine their mobility or deposition in the transport media (Bueno et al., 2022).
Fujita and Kobayashi (2016) investiagated the effect of charge on the transport of colloidal silica in quartz sand. A narrow range of pH 5 to 7 was selected to perform the study. The effect of alkaline pH on the transport phenomenon was not considered. In another study, the effect of humic acid on the transport of colloidal silica was examined under different hydrochemical conditions (Zhou et al., 2017). Collidal silica was passed through the sand column suspended in 10 mg/L of humic acid solution. The effect of varying the humic acid concentrations on nSiO2 transport was not analyzed here. The transport and retention of various engineered metal oxide nanoparticles (Al2O3, TiO2, and SiO2) in quartz sand was emphasized by Bayat et al. (2015). The transport behavior was evaluated considering only a single influent concentration of 50 mg/L nSiO2. The influence of particle concentrations on the nanoparticle retention and transport in the porous media is another factor that requires more attention. Overall, numerous research gaps remain to be addressed regarding transport of nano SiO2 in saturated porous media.
To the best of our knowledge, this is the first-ever work that systaematically investigated the transport behavior of nSiO2 in the saturated porous medium at different pH, ionic strength, NOM, and influent particle concentrations, and validated the results with the different real water systems. In the present study, it was hypothesized that various physicochemical parameters would alter the surface chemistry of both nSiO2 and the collector (quartz sand), which would eventually impact its mobility. Our findings indicate that interactive forces that cause interparticle attraction or repulsion determine the fate of nSiO2 in the porous media. DLVO interaction energies and CFT was used as the mathematical model to validate the observed BTCs. Additionally, XRD, FTIR, TEM, and EDAX were performed to examine the crystallographic and morphological structure, size, surface functional group, and elemental composition of nSiO2. The findings of this study will form a basis for a better understanding of the transport and release behavior of nSiO2 in the subsurface environment.
Section snippets
Chemicals and materials
SiO2 nanoparticles were synthesized by the Centre for Fire, Explosive and Environment Safety (CFEES), as per protocol based on a previous study by Saxena et al. (2010). Sodium Chloride (NaCl) extra pure AR was obtained from SRLIndia Pvt. Ltd. (CAS Number 7617–14–5). Humic acid (CAS Number 14808–60–7) and white quartz sand (particle size 50–70 mesh) used in the column transport experiment were purchased from Sigma Aldrich (CAS Number 14808–60–7). Glass chromatography columns 20 × 1.5 cm
Characterization of nSiO2
The XRD analysis of nSiO2 is shown in Fig. 1A. The XRD spectra indicated the existence of amorphous nSiO2 due to peak broadening at 2θ: 22°. Diffraction peaks at 2θ: 20.88°, 36.65°, 39.37°, 40.25°, 42.40°, and 45.80° corresponding to the reflection from crystal planes (100), (110), (102), (111), (200) and (201) respectively [JCPDS Card: 850335], clearly indicated the presence of nSiO2. The primitive lattice having lattice parameters were a = b = 4.913 Å and c = 5.405 Å which confirmed a
Conclusion
In the present study, the transport and retention behavior of nSiO2 in the presence of environmentally relevant physicochemical parameters were investigated. The presence of natural organic matter and modifying the pH of the nSiO2 suspensions promoted their mobility and transport. The key findings of this study suggest that NOM-induced repulsive forces stabilized the nanoparticles resulting in improved transport of nSiO2 in the porous media. Further, it can be concluded that salt and particle
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We wholeheartedly thank Sh. Rajiv Narang (Outstanding Scientist and Director), Centre for Fire, Explosive and Environment Safety- Defence Research and Development Organisation (CFEES-DRDO) for providing the nanomaterials and financial support for carrying out this research work (Sanction No. CFEES/TCP/EnSG/CARS(P)/DG(SAM)/FTS-ERAF/VIT-VELLORE). We also acknowledge Vellore Institute of Technology, Vellore, India, for Transmission electron microscopy (TEM) facility used in this study.
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