Environ. Sci. Technol. 2010, 44, 4531–4538
Estimating Attachment of Nano- and Submicrometer-particles Coated with Organic Macromolecules in Porous Media: Development of an Empirical Model T A N A P O N P H E N R A T , †,‡ J E E E U N S O N G , †,‡ CHARLOTTE M. CISNEROS,‡ DANIEL P. SCHOENFELDER,‡ R O B E R T D . T I L T O N , †,§,| A N D G R E G O R Y V . L O W R Y * ,†,‡,§ Center for Environmental Implications of Nanotechnology (CEINT), Department of Civil & Environmental Engineering, Department of Chemical Engineering, and Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890
Received December 29, 2009. Revised manuscript received April 17, 2010. Accepted April 27, 2010.
Assessing the environmental transport and fate of manufactured nanoparticles (NPs) and potential exposure risks requires models for predicting attachment of NPs coated with organic macromolecules in porous media. The objective of this study was to determine the properties of coated nanoparticles that control their attachment behavior. Deposition data for a variety of nanoparticles with different types of anionic organic coatings, including natural organic matter (NOM)-coated latex and hematite nanoparticles, and poly(styrenesulfonate)-, carboxymethylcellulose-, and polyaspartate-coated hematite and titanium dioxide nanoparticles (80 data points), were used to develop an empirical correlation between measurable NP properties and their sticking coefficient (R) under a variety of electrolyte conditions and flow velocities. Available semiempirical correlations used to predict the attachment efficiency of electrostatically stabilized (uncoated) NPs overestimate the attachment efficiency of nanoparticles coated with NOM or synthetic polyelectrolytes because the correlations neglect electrosteric repulsions and the decreased friction afforded by such coatings that can inhibit attachment to surfaces. Adding a dimensionless parameter (NLEK) representing steric repulsions and the decreased friction force afforded by adsorbed NOM or anionic polyelectrolytes in the correlation significantly improves the correlation. This establishes the importance of including the adsorbed NOM- or polyelectrolyte layer properties for estimating the attachment efficiency of NPs in the environment. The form of NLEK suggests that limiting unintended transport and exposure to NPs could be achieved by using coatings with the
* Corresponding author e-mail:
[email protected]; phone: 412268-2948; fax: 412-268-7813. † Center for Environmental Implications of Nanotechnology (CEINT). ‡ Department of Civil & Environmental Engineering. § Department of Chemical Engineering. | Department of Biomedical Engineering. 10.1021/es903959c
2010 American Chemical Society
Published on Web 05/13/2010
smallest adsorbed mass and polymer density, shortest extended layer thickness, and largest molecular weight that would still afford the desired functionality of the coating.
Introduction The fate and transport of manufactured nanoparticles (NPs) released into the environment is of great interest due to their increasing use in consumer products and their potential risks to the environment and human health (1, 2). Most NPs are manufactured with a surface coating such as polymers or polyelectrolytes to provide specific functionality (3-7). For example, polymeric coatings are applied to quantum dots to make them biocompatible or to provide functional groups for specific receptor targeting (5). Poly(ethylene glycol) (PEG) is used to modify magnetite nanoparticles to promote intracellular uptake and targeting of specific cells for cancer therapy and diagnosis (6). Nanoscale zerovalent iron (NZVI) particles for groundwater remediation are engineered with surface coatings such as poly(styrene sulfonate), polyaspartate, carboxymethyl cellulose, or block copolymers to inhibit aggregation (3, 4), decrease their adhesion to solid surfaces (8), increase their mobility in the subsurface (9), and target specific pollutants (4). These coatings are not readily desorbed (10). Nanoparticles can also acquire a natural coating once released into the environment due to adsorption of natural organic matter (NOM) including humic and fulvic acids (11-18). Anionic surface coatings typically enhance NP mobility in porous media (9-12, 14, 15, 17, 18, 20), however, natural organic macromolecules (humic acid) at high concentrations of divalent cation (e.g., > 10 mM Ca2+) can also cause bridging and attachment and decrease mobility (19). The proposed hypothesis for enhanced mobility is the electrosteric repulsions resulting from adsorbed charged macromolecules inhibit attachment. The studies that support this hypothesis found that the attachment efficiency (R) of NOM or polyelectrolyte coated NPs was not explained by changes in zeta potential according to traditional DLVO theory (9, 18), i.e., electrostatic repulsions alone could not explain the data. Some coated NPs have R
