Research paperPolysaccharide coating on environmental collectors affects the affinity and deposition of nanoparticles
Graphical abstract
Introduction
Over the past 15 years, the production and application of manufactured nanomaterials in commercial products has increased, as well as the environmental concern regarding the release of nanoscale residues, i.e., nanoparticles (NPs) (Kiser et al., 2009); (Klaine et al., 2012); (Westerhoff et al., 2011). NPs may be released during the production, use or disposal of the original nanomaterials, and they appear in soil, water, and air and may be harmful to the environment and humans upon exposure (Klaine et al., 2008). Understanding the transport and fate of NPs in the environment is a key way to assess the exposure-driven risk (Petosa et al., 2010). In soil and water systems, the fate of NPs is largely determined by transport and deposition mechanisms. The deposition or attachment of NPs on mineral collector surfaces in porous media or in suspensions via heteroaggregation has been widely studied (Tufenkji and Elimelech, 2004); (Fatisson et al., 2009); (Solovitch et al., 2010); (Praetorius et al., 2014); (Labille et al., 2015). This phenomenon is influenced by many factors such as the particle size, the shape and surface chemistry, the roughness of the collector surface, and the physical chemistry of the solution (Wiesner et al., 2006). The distribution of NPs on the surface can be affected by steric particle-particle interactions or particle-substrate interactions (Brouwer et al., 2003). The NP surface functionalization also influences the adhesion to the substrate, often decreasing deposition and increasing transport due to higher colloidal stability (Sirk et al., 2009); (Phenrat et al., 2010).
In addition, in aqueous environment such as soil and sediments, most of the surfaces can be colonized by microbial biofilms, made of extracellular polymeric substances, EPSs, embedding the organisms. The EPS matrix is a porous and heterogeneous structure (De Beer et al., 1994) mostly composed of polysaccharides (Ikuma et al., 2015). It also includes proteins and lipids. EPSs often behave in a similar way as hydrogels and display swelling and deswelling properties (Hall-Stoodley et al., 2004). The permeability of such biogels for NPs determines the NP mobility and exposure to a large extent (Peulen and Wilkinson, 2011); (Ikuma et al., 2015). This gel acts as a filtration layer between NPs and living organisms (Lieleg and Ribbeck, 2011). The mobility of NPs within the biogel was shown to depend on steric constraints, i.e. porosity and their affinity with the gel components (Fatin-Rouge et al., 2003); (Golmohamadi et al., 2013). In addition, the mobility is influenced by many factors such as the NP charge (Golmohamadi et al., 2013) and aggregation (Stewart, 1998); (Choi et al., 2010), and gel hydrophobicity (Xiao and Wiesner, 2013) and structure (Labille et al., 2007). In particular, the binding capacity of biofilms has been shown to initially limit the NP mobility (Kurlanda-Witek et al., 2015). Moreover, polysaccharides, as the main constituent of the biofilm matrix, were shown to strongly control the diffusion of solutes (Zhang et al., 2011) and to influence the interaction with NPs (Lawrence et al., 2007); (Ikuma et al., 2014); (Miller et al., 2013). Hydrogen bonding, ionic interactions and dehydration of polar groups were shown to be important factors which influence NPs and polysaccharide interactions (Zeng et al., 2012). Attractive interactions between NPs and polysaccharides tend to limit the NP diffusion in biofilms, while repulsive interactions may favour mobility if steric constraints allow it. The affinity of NPs for polysaccharides thus appears to be a key parameter to evaluate in order to assess the fate and exposure of NPs.
The interaction of natural organic matter (NOM) with NPs, and its effects on the NPs fate, was studied extensively and was shown to depend on many physicochemical factors (Chen and Elimelech, 2006); (Sani-Kast et al., 2017, under review); (Peralta-Videa et al., 2011); (Cornelis et al., 2013); (Franchi, 2000); (Pelley and Tufenkji, 2008); (Thio et al., 2011); (Kroll et al., 2014); (Pachapur et al., 2015). Generally, a reduced aggregation is observed due to steric or electrostatic stabilization mechanisms (Phenrat et al., 2010). Otherwise, bridging flocculation may take place, depending on the NOM type, relative concentration, pH, ionic strength… (Baalousha et al., 2008). However, in most of these cases, NOM was used as initially free molecules in solution, which strongly favours interaction with NPs. This is not the case in biofilms where NOM is rather immobile on the colonized collector surface. In these conditions, NOM reconformation is very limited and the interaction with NPs may be altered. The deposition of NPs on collectors coated with NOM has not been extensively studied (Chen and Elimelech, 2008); (Ikuma et al., 2015); (Jiang et al., 2010); (Adamczyk et al., 2010). The limited studies indicate that the deposition is mostly controlled by electrostatic interactions, but that additional factors at the collector surface scale, such as heterogeneity or the extent of deposited NP, may play an important role on the deposition mechanism (Ikuma et al., 2014).
In this work, we studied NP deposition on a surface coated with bacterial polysaccharide compared to that on a bare SiO2 mineral surface to estimate the effect of the collector colonization by bacteria on the NP fate and to better understand the underlying mechanisms. The NP coating, pH and salt concentration were varied to induce attractive or repulsive interactions of electrostatic or steric nature between the NPs and the substrate, and thus to examine how effective the NP deposition was in various cases.
NPs deposition was studied both by AFM after dipping the substrate in a NP suspension and by QCM-D in flow mode. TiO2 NPs were selected in this work since they are widely used in different applications such as sunscreens and paints. Moreover, since it is often functionalized to match the application requirements, polyacrylate-coated TiO2 (PAA-TiO2) was also selected as a case study for UV-filters used in sunscreens (Labille et al., 2010); (Botta et al., 2011); (Smijs and Pavel, 2011). Here, we show that the deposition of TiO2 NPs is not only driven by electrostatic interactions with the collector but is also affected by the heterogeneity of the collector surface caused by the polysaccharide and by the interactions between the neighbouring NPs.
Section snippets
Materials
Pure TiO2 nanoparticles were obtained from NanoAmor (anatase, with an average particle size of 5–30 nm). The TiO2 NPs are obtained as an aqueous 170 g/L dispersion with pH = 1. The average elementary size distribution of the dispersed NPs, measured by dynamic light scattering (DLS, Malvern Zetasizer Nano ZS), was centred at 12 nm (Fig. S1a). The ζ dependence with pH, which was measured using phase analysis light scattering (Malvern Zetasizer Nano ZS), revealed an isoelectric point pHIEP = 4.9 (Fig.
Approach
Favourable and unfavourable conditions for NP deposition on the substrate were obtained here by controlling the pH and salt concentration of the NP dispersing medium.
For pure TiO2 NPs with an IEP of 4.9, favourable conditions for deposition were obtained by adjusting the pH to 3. Although this condition is not environmentally relevant, our aim here was to obtain contrasting conditions that enable us to better assess the deposition mechanism. At this pH, TiO2 NP are positively charged and stable
Deposition of NPs under attractive electrostatic interactions with the substrate
The deposition of NPs under attractive electrostatic interactions with the substrate was studied using positively charged pure TiO2 NPs at pH = 3. Fig. 1 shows AFM images of the polysaccharide-coated substrate surface after deposition of pure TiO2 in NaCl 10− 3 M or in NaCl-free solution. Both the topography and phase AFM images show less deposition of TiO2 NPs in NaCl-free solution than in 10− 3 M NaCl (θAFM = 12 vs. 40%, Table 1). Fig. 2 shows AFM images of the mineral substrate after deposition of
Conclusion
The mobility and fate of NPs in the environment drive the exposure behaviour. The objective of this work was to study the effect of an organic polysaccharide coating on a collector surface on the affinity and deposition of TiO2 NPs. AFM and QCM-D were used to measure the deposition after dipping and in flow, respectively. Our results show that the physicochemical conditions strongly influence the mode of NP deposition. Under attractive electrostatic interactions, higher deposition was favoured
Acknowledgements
This work is a contribution to the Labex Serenade (no ANR-11-LABX-0064) funded by the “Investissements d'Avenir” French Government program of the French National Research Agency (ANR) through the A*MIDEX project (no ANR-11-IDEX-0001-02). The authors acknowledge Dr. Jonathan Brant, Department of Civil and Architectural Engineering, University of Wyoming, USA, for enabling the preliminary QCM-D studies.
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