Observed equilibrium partition and second-order kinetic interaction of quantum dot nanoparticles in saturated porous media
Introduction
Quantum dots (QDs) are new emerging engineered nanoparticles (NPs) with diameters typically ranging between 2 and 10 nm. QDs have been used in diverse areas such as solar energy conversion, medical diagnostics, drug delivery, and computing systems (Luque et al., 2007; Torkzaban et al., 2013). Due to their extremely small sizes, QD nanoparticles (QDNPs) exhibit unique properties that are not shared by their bulk counterparts with the same chemical compositions (Hu et al., 2016). The production and use of QDNPs will inevitably cause them to enter into subsurface environments including soil and sediments (Lin et al., 2010; Lowry et al., 2012; Bundschuh et al., 2018). However, commercial QDNPs can release toxic heavy metal ions (e.g., Cd2+ and Se2+) that may cause substantial cytotoxic effects to living organisms (Mahendra et al., 2008; Domingos et al., 2011; Tong et al., 2017). The toxicity effects of QDNPs can be further enhanced by polymer coatings used to encapsulate the QDs (Priester et al., 2009; Grabowska-Jadach et al., 2016). Therefore, it is critical to investigate fate and transport of QDNPs in the subsurface environments such as soil to evaluate their environmental risks (Holbrook et al., 2008; Navarro et al., 2009; Darlington et al., 2009).
The transport of nanoparticles in porous media has been examined over two decades (Petosa et al., 2010; Phenrat et al., 2010; Wang et al., 2008; Wang et al., 2016a, Wang et al., 2016b; Meng and Yang, 2019). Surprisingly, only very limited attention has been paid to investigating the transport of QDNPs in porous media to date (Quevedo and Tufenkji, 2009, Quevedo and Tufenkji, 2012; Quevedo et al., 2013; Torkzaban et al., 2010, Torkzaban et al., 2013; Yu et al., 2020). These reports revealed that the deposition of QDNPs on collector surfaces increased with increasing solution ionic strength (IS) and cation valence. The results are consistent with the prediction by the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory because increasing IS decreases repulsive energy barrier and accordingly increases deposition in primary minima (Shen et al., 2007, Shen et al., 2018). The secondary minimum deposition should have a minor influence on the retention of QDNPs because the depths of secondary minima are extremely shallow for the QDNPs (Shen et al., 2011, Shen et al., 2012a, Shen et al., 2012b, Shen et al., 2012c; Bradford et al., 2012; Wang et al., 2012; Godinez et al., 2013).
The colloid filtration theory (CFT) has been used to describe QDNPs deposition in saturated quartz sand loamy sand (Quevedo and Tufenkji, 2012). The CFT considers that particle deposition in primary minima is an irreversible immobilization process that follows the first-order kinetic interaction (Ryan and Elimelech, 1996; Shen et al., 2020). However, Torkzaban et al. (2013), using saturated column experiments, showed that a fraction of deposited QDNPs can be spontaneously detached from the sand collector surfaces without perturbation of any system condition. This indicates that the QDNPs deposition is a reversible process. Quevedo and Tufenkji (2009) and Torkzaban et al. (2013) showed that almost all QDNPs that were deposited in the presence of monovalent cations can be detached by introducing deionized (DI) water to decrease IS. The rate of detachment was very fast, which was comparable to that of deposition. These results underline that an equilibrium partition between the solid and solution phases may be established for the QDNPs although the assumption of equilibrium partition for colloids including nanoparticles and QDNPs has been argued (Praetorius et al., 2014; Cornelis, 2015; Dale et al., 2015). Therefore, the mechanisms controlling the deposition and transport of QDNPs in porous media remain to be elucidated to date.
By integrating batch adsorption experiments and theoretical calculations, this study specifically explored the kinetic and equilibrium interaction of CdSeS/ZnS alloyed QDNPs (6 nm) with sand grains. We revealed that the deposition of QDNPs followed first- and second-order kinetic interactions at low and high ISs, respectively. The deposition is a reversible process, which resulted in equilibrium partition of the QDNPs between the sand surfaces and solution. These findings indicate that the QDNPs behave more likely solutes instead of colloids, and the CFT that assumes irreversible first-order kinetic interaction is not appropriate for modeling the adsorption of QDNPs. As such, adsorption and desorption (rather than deposition or attachment and detachment) were used to describe the retention and release of QDNPs, respectively, later in the paper. To our knowledge, our study was the first to show that the order of kinetic interaction of NPs with collectors in porous media changed with IS. While the equilibrium partition for colloids (e.g., QDNPs) has been argued in the literature (Praetorius et al., 2014; Cornelis, 2015; Dale et al., 2015; Shen et al., 2020), we unambiguously provided experimental evidences on existence of the equilibrium partition of the QDNPs.
Section snippets
QDNPs
The 6 nm CdSeS QDs with a carboxyl (-COOH) functionalized ZnS (1-octadecylamine) capping were purchased from Sigma Aldrich (Saint Louis, United States). The manufacturer reports that the fluorescence emission wavelength and mass concentration of the QDNPs in the stock suspension are 540 nm and 1 mg mL−1, respectively. The method from Treumann et al. (2014) was used to prepare QDNP suspensions for batch equilibrium and kinetic experiments. Briefly, NaCl solutions with different solution ISs
Characterization of QDNPs and quartz sand
Table 1 presents sizes of QDNPs and zeta potentials of the QDNPs and quartz sand at different ISs. The sizes of QDNPs increased with increasing IS, indicating that aggregation occurred under these chemical conditions. The tendency towards aggregation for QDNPs in solution has also been reported in previous studies (Quevedo and Tufenkji, 2009, Quevedo and Tufenkji, 2012; Torkzaban et al., 2010). The zeta potentials of the QDNPs were less negative at higher IS due to compression of the
Conclusions
Through conducting batch experiments, our work unambiguously showed that the QDNPs behave more likely solutes. Specifically, the adsorption of QDNPs in sand was a reversible process even without any disturbance of system condition. This is because the primary minimum depths of QDNPs were comparable to the average kinetic energy of a colloid at all ISs and the QDNPs adsorbed at the shallow primary energy wells can escape by Brownian diffusion. The reversible adsorption resulted in equilibrium
Author statement
We declare that our work described has not been published previously, that it is not under consideration for publication elsewhere, that its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder.
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.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
Acknowledgments
We acknowledge the financial support provided by the National Natural Science Foundation of China (41922047), National Key Research and Development Program of China (2017YFD0800301), and 948 Project of Ministry of Agriculture of the People's Republic of China (2015-Z32, 2016-X44).
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