Functionalized nanoparticle interactions with polymeric membranes
Introduction
Engineered nanoparticles (NPs) are finding increased use in consumer products such as clothing, children's toys, household products, and personal care products [1], [2], [3], [4]. The Woodrow Wilson International Center for Scholars found 212 nanomaterial-containing products on the market in 2006 and more than 1000 in 2009 [5]. Environmental fate models predict that many NPs will come into contact with and be conveyed by sewage and storm water [6], [7], [8]. Laboratory tests have demonstrated NP release from consumer products and during clothes washing [1], [2], and NPs have been found in wastewater treatment plant effluents [9], [10]. Discharges from NP manufacturing facilities are also likely to be an important point source of environmental NPs [11].
In addition to the potentially negative impacts noted above, NPs also have positive environmental uses when employed to remediate other contaminants. Often the NPs must be retained in the treatment system, such as when they are used as catalysts. TiO2 and zero-valent iron are two NP materials with remediation capabilities for which retention in the system is desired [12], [13].
Membrane processes are a potentially viable technology for removing or retaining engineered NPs [14], [15], [16], [17]. Much of the information available on nanoparticle (colloid) interactions with membranes comes from studies in which membrane fouling is of interest [18], [19], [20], [21], [22]. In those cases the NPs (colloids) are meant to represent naturally occurring material and are typically larger than the membrane pores. NP removal by microfiltration (MF) and ultrafiltration (UF) membranes in which the NPs are smaller than the pores has been studied for a few NP types and membrane materials. For example, rejection of silver colloids (2–20 nm, mean 8 nm) was tested on 30-, 100-, and 300-kDa polysulfone and 0.22-μm PVDF membranes [23]. Particles were well rejected by the tighter membranes, but even the looser PVDF membrane demonstrated some particle removal. Silver (8.3 nm) and gold (10 nm) colloids were shown to adsorb to porous membranes, which enabled visualization of fouling phenomena [24]. Adsorption was also important in the rejection of colloidal hematite (75, 250, and 500 nm) by 0.22-μm MF membranes [25]. Rejection decreased with several membrane backwash cycles, presumably because adsorption sites were filled in early stages. Adhesion of NPs to membranes can be strong, as demonstrated by silica colloids that bound more readily than polystyrene colloids [22]. NP aggregation is also an important mechanism of rejection, as has been demonstrated with hematite [26] and more recently with magnetic CoFe2O4 NPs [27].
NP separation using UF membranes has received attention as a means for NP characterization. For example, UF was used for differentiation between ZnO NPs and dissolved Zn2+ in toxicity studies [28]. Separation of ionic and NP forms of silver has also been reported [29]. Other groups have similarly purified Au [30] and iron oxide [31], [32] during NP synthesis. Another need in the environmental research field is to collect and concentrate environmental NPs. A review covering aquatic environmental NPs described several workers using UF and tangential-flow UF to collect NPs [33]. One group developed an automated UF device that concentrates NPs with various membranes [34]. In these reports on NP separation and collection from environmental matrices, membrane pore sizes well below the NP size are typically chosen, and little discussion of NP-membrane interactions is included.
Membrane filtration has also been used to remove viruses, which are in the nanometer size range. One group used several UF membranes between 30 and 300 kDa to remove viruses between 18 and 26 nm in size [35]. The 100-kDa membrane was deemed optimal because it effectively rejected viruses while allowing most proteins to pass.
In general, NPs with diameters larger than the membrane pore sizes can be removed, but NPs smaller than the pores can also be removed because of adsorption to membrane surfaces, electrostatic interactions, or other interactions. To fully understand and predict separation behavior, it is important to understand such interactions; however, studies demonstrating these effects are sparse. We found only one study that used a variety of membranes with varying porosity to examine rejection, and in that case the only NP used was colloidal silver, which has a fairly strong adsorption affinity [23]. A more complete set of experiments is warranted to determine the interactions between NPs with varying material properties and polymeric membranes of varying pore size.
This study evaluates the extent to which MF and UF membranes remove engineered NPs with surface coatings (e.g., carboxy or amino functional groups). Solutions containing NPs were applied to a range of polymeric membranes composed of different materials and with varying pore sizes (ranging from ∼2 nm [3 kDa molecular weight cutoff] to 0.2 μm). Potential mechanisms for NP removal by the membranes are investigated. The outcome of this research provides valuable information on the use of membranes to remove NPs from waste streams, to prevent their release into the environment, and to size-separate and characterize NPs in environmental samples.
Section snippets
Nanoparticles
Five NPs of similar hydrodynamic size but different composition and functionality were used: negatively charged silver [Ag(−)], negatively and positively charged titanium dioxide [TiO2(−) and TiO2(+)] and gold [Au(−) and Au(+)] NPs (all from Vive Nano, Toronto, ON, Canada). Negatively charged NPs were manufactured with polyacrylate such that the polymer was incorporated into the NP and carboxyl groups imparted a negative surface charge at neutral pH. Positively charged NPs were formed
Dead-end filtration and rejection over time
Four dead-end filtrations using 0.1-μm PVDF membranes performed with one-liter solutions of Ag(−) NPs had an average Ag rejection of only 4.2% after passage of 800 ml through the membrane. Data collected over time, however, showed that at the very beginning of two experiments (i.e., in the first 5 ml of collection), the membrane retained 36% and 20% of the Ag (Fig. S4). The captured Ag(−) was observed visually by a graying of the white membranes. This was also confirmed by SEM (Fig. S5) and by
Conclusions
From these experiments, five main conclusions can be drawn about the interactions of this class of functionalized NPs with polymeric MF and UF membranes.
- (1)
All of the functionalized NPs were well rejected by membranes with pores smaller than the NP size, but some were well removed by membranes with larger pores. This occurred when NP-membrane adsorption affinity was high, as in the case of positively charged NPs being electrostatically attracted to the negatively charged membranes.
- (2)
Even though
Acknowledgements
This work was supported by the NIH Grand Opportunities (RC2) program through NIEHS grant DE-FG02-08ER64613. Assistance from an undergraduate researcher (Amanda Hernandez) and technician (Marisa Masles) are greatly appreciated. Characterization of nanoparticles was conducted within the LeRoyEyring Center for Solid State Science.
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