Suspended state heteroaggregation kinetics of kaolinite and fullerene (nC60) in the presence of tannic acid: Effect of π-π interactions
Graphical abstract
Introduction
The escalation in production and applications of engineered nanomaterials has resulted in the possibility of their direct and indirect environmental release. Recent studies have predicted the potential bio-accumulation and biomagnification of these engineered nanoparticles (ENPs) across the food web; thus, they may pose a risk of toxicity to a wide range of organisms (Garner et al., 2018; Jackson et al., 2012; Majumdar et al., 2016; Moor et al., 2016). Among nanomaterials, the potential application of fullerenes and their derivatives for photovoltaics and lubricants has raised concern regarding their imminent environmental exposure (Chakravarty and Kivelson, 1991; Lee et al., 2009). Despite arguments for their hydrophobicity-assisted reduced bioavailability and environmental toxicity, several natural parameters have the potential to mobilize fullerenes through environmental media (Ke and Lamm, 2011). Natural organic matter (NOM) and the light-induced surface functionalization of fullerenes can substantially facilitate their environmental mobility (Fortner et al., 2007; Hou and Jafvert, 2009; Hwang and Li, 2010; Lee et al., 2007). However, recent investigations have revealed that regardless of the higher environmental mobility of surface-modified ENPs including fullerene water suspension (FWS), naturally abundant primary and secondary mineral colloids can significantly impede their environmental transport and consequent bioavailability (Labille et al., 2015; Liu et al., 2015; Praetorius et al., 2014; Smith et al., 2015; Wang et al., 2015). However, the feasibility of the heteroassociation-based entrapment of ENPs including fullerene is incumbent upon their surface charge behavior, degree of NOM-induced surface modifications, and efficacy of counterions as effective coagulating agents (Huynh et al., 2012; Nabiul Afrooz et al., 2013; Wang et al., 2015).
Clay, a natural constituent, profoundly influences various environmental processes ranging from global carbon cycling to the transportation of radionuclides (Bone et al., 2017; Labille et al., 2015). Therefore, it is likely that the high surface activity and nanoscale dimensions of clay particles will critically influence the fate and transport behavior of ENPs. Previous studies have demonstrated the role of naturally abundant 2:1 montmorillonite and 1:1 kaolinite particles in colloidal heteroassociations with ENPs in environmental media (Labille et al., 2015; Wang et al., 2015). However, the shape anisotropy, lamellar structure, higher activity of the edge sites, and surface charge asymmetry of the clay particles have significantly increased the complexity of these colloidal heteroassociations with ENPs. Among clay minerals, the nearly symmetric surface charge distribution (due to high isomorphous substitution) in thin-lamellar montmorillonite results in a nearly equal attachment potential for ENPs on all facets and possibly on active edges (Essington, 2004; Kretzschmar et al., 1998; Labille et al., 2015). However, the attachment potential of ENPs including fullerene to the thick-lamellar, charge-anisotropic kaolinite particles is also complex. Unlike montmorillonite, the surface charge asymmetry between tetrahedral silica faces and pH-dependent variation in edge surface charges in kaolinite can cause an increased polarizability vector across the basal plane, resulting in self-association, especially in acidic pH (Brady et al., 1996; Radeva et al., 1989). Therefore, it is likely that surface charge anisotropy in kaolinite may lead to preferential attachment of ENPs. Moreover, NOM-induced surface modifications of both ENPs and clay colloids may pose an electrosteric barrier for these heteroassociations and facilitate the environmental mobility of ENPs (Huynh et al., 2012; Ghosh et al., 2016). Previous studies have shown that conjugated π-electron-rich systems can induce the surface functionalization of fullerene and reduce the hydrophobicity-driven 3D-clustering with polarizability enhancement (Ghosh et al., 2019). Therefore, stronger electrostatic barriers to these heteroassociations may facilitate the transport of FWS through environmental media.
In this study, the role of pH and tannic acid (TA) in the self-association behavior of kaolinite was investigated with transverse proton spin–spin relaxation measurements (T2). The suspended state homoaggregation kinetics of kaolinite and the heteroaggregation kinetics of a kaolinite and FWS binary mixture were investigated in NaCl and CaCl2 solutions both with and without TA at pH 4 and 7, respectively. Although pH 4 is not common in natural setting, but several acid soils exhibit this pH and relevant in terms of natural synthesis of kaolinite. Karathanasis and Hajek (1983) have found transformation of smectite to kaolinite in the Alabama coastal plain region acid soil environment. Previously Kretzschmar et al. (1998) have reported that point of zero charge (pzc) of K-Ga-2 kaolinite dispersed in 0.01 M NaClO4 is near pH 4.8. Therefore, higher edge charge of the kaolinite at pH 4 may contribute to electrostatic attraction with FWS. However, at pH 7, the electrostatic repulsion is dominant; hence higher salt concentrations are required to reduce the Debye length and heteroaggregate formation. Therefore, in order to determine the effect of pH both the homoaggregation and heteroaggregation studies were conducted at this two pHs. The calculated kaolinite and FWS heteroaggregation rates were compared to the homoaggregation rates of kaolinite obtained under various solution conditions. The effects of π-π electron donor-acceptor (EDA) interaction-mediated surface modifications of fullerene on its heteroassociation behavior with kaolinite was evaluated and compared to the electrophoretic mobility (μ) data. A 2 mg/L concentration of TA, similar to that prevalent in surface waters, was maintained in all the aggregation kinetics experiments (Le-Clech et al., 2006). A high kaolinite (10 mg/L) to FWS (0.5 mg/L) concentration ratio was maintained in all the heteroaggregation kinetics experiments to reflect the higher natural abundance of the former material. In situ atomic force microscopy (AFM) was employed to explore the inter-particle interactions during heteroaggregation in the presence of CaCl2. High-resolution transmission electron microscopy (HRTEM) of the kaolinite and FWS in the presence and absence of TA was conducted to ascertain the adhesion behavior of FWS on Na-kaolinite and to determine the aggregate morphology.
Section snippets
Materials
Powdered kaolinite was obtained from Aladdin (China). To remove other exchangeable cations, Na-saturated kaolinite (Na-kaolinite) was prepared according to the method described by Kretzschmar et al. (1998). Polydispersity of the suspension was minimized by removing the larger particles following the method also described by Kretzschmar et al. (1998). C60 fullerene powder was obtained from MER Corp. (Tucson, AZ). Preparation of the FWS has been described in our previous studies (Ghosh et al.,
Self-association of Na-kaolinite
The pH-induced self-association behavior of kaolinite was investigated through transverse proton (1H) spin–spin relaxation measurements. At pH 4, edge-to-face self-association of Na-kaolinite particles and subsequent loss of vicinal water molecules equivalent to the excluded volumes of the particles resulted in a diminished 1H exchange of the aggregates with low R2 (Fig. 1a). Previous studies have shown that isomorphous substitution in kaolinite produces a permanent negative surface charge on
Conclusion
Under acidic solution conditions, the adhesion of FWS on Na-kaolinite prevented self-association of the latter. The mechanism for colloidal stabilization of the binary mixture is attributed to steric stabilization. However, near a neutral pH, stronger repulsion between the EDLs associated with both kaolinite and fullerene diminished the potential for performing heteroassociation-based attenuation of fullerene. Furthermore, π-π EDA interaction-assisted surface functionalization of FWS by TA
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.
Acknowledgements
The authors are highly thankful to Su Bo and He Nan from the Kunming University of Science and Technology for their assistance in determining particle size, surface charge, and TEM imaging of heteroaggregates. This work was supported by National Natural Science Foundation of China (Grant Nos. 41573100, 41773134).
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