Article pubs.acs.org/est
Key Factors Controlling the Transport of Silver Nanoparticles in Porous Media Amro M. El Badawy,† Ashraf Aly Hassan,‡ Kirk G. Scheckel,‡ Makram T. Suidan,† and Thabet M. Tolaymat‡,* †
Department of Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221, United States Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, Ohio 45224, United States
‡
S Supporting Information *
ABSTRACT: The current study investigated the mobility of four silver nanoparticles (AgNPs) stabilized using different capping agents and represent the common stabilization mechanisms as well as surface charging scenarios in reactive and nonreactive porous media. The AgNPs were (1) uncoated H2−AgNPs (electrostatically stabilized) and (2) citrate coated AgNPs (Citrate-AgNPs) (electrostatically stabilized), (3) polyvinylpyrrolidone coated AgNPs (PVP-AgNPs) (sterically stabilized), and (4) branched polyethyleneimine coated AgNPs (BPEI-AgNPs) (electrosterically stabilized). The porous media were (1) quartz sand (QS), (2) ferrihydrite-coated sand (FcS), and (3) kaolin-coated sand (KcS). The H2−AgNPs and Citrate-AgNPs were readily mobile in QS but significantly retained in FcS and KcS with more deposition achieved in the KcS media. The deposition of the H2−AgNPs and Citrate-AgNPs followed the order of KcS > FcS > QS. The PVP-AgNPs breakthrough occurred more rapid as compared to the H2−AgNPs and Citrate-AgNPs but the deposition of PVP-AgNPs followed the same order of the electrostatically stabilized AgNPs (KcS > FcS > QS). The BPEI-AgNPs were readily mobile regardless of the porous media reactivity. Physicochemical interactions were the dominant filtration mechanism in the majority of the investigated cases but straining played the major role in the deposition of the electrostatically stabilized H2−AgNPs and Citrate-AgNPs in the KcS media. The results highlight the importance of both the stabilization mechanism and capping agent chemistry as key factors governing the transport of AgNPs in the environment.
1. INTRODUCTION Silver nanoparticles (AgNPs) are widely used in a variety of scientific, industrial and medical applications in addition to consumer products.1−3 This is a result of their unique sizedependent physical and chemical properties and strong antimicrobial properties.1−3 Currently, there is a growing concern with regards to their potential release into the environment and their potential adverse environmental impacts.4,5 The released AgNPs, depending on their mobility, could accumulate in the food chains of the living organisms.6 Research studies reported that AgNPs can be toxic to a wide range of living cells such as microorganisms and mammalian and human cells.7−9 Thus, investigating the transport of AgNPs in the environment is needed in order to assess the risks associated with these nanoparticles. Furthermore, this may also assist in the evaluation of mechanisms to remove AgNPs in water and wastewater treatment filters and the prediction of contaminant facilitated transport in subsurface environments.10,11 Many factors, including particle-specific properties (e.g., size, shape, surface charge, and capping agent) and the surrounding solution chemistry, impact nanoparticles’ aggregation, and consequently, their fate and transport in environmental systems.12,13 Without aggregation, other factors also influence © 2013 American Chemical Society
nanoparticles’ transport and deposition in porous media. These factors include Brownian diffusion, interception, gravitational sedimentation, and nonphysicochemical removal mechanisms (e.g., porous media charge heterogeneities).10,11,14−16 Furthermore, transport of nanoparticles in porous media will also be influenced by the reactivity of the media. A number of studies investigated the mobility of various nanoparticles in porous media under different environmental conditions.6 Lin et al.17 studied the transport of uncoated AgNPs (synthesized using sodium borohydride as the reducing agent) in glass beads (GB) and hematite-coated glass beads (FeO-GB) media. The results showed that at pH lower than the point of zero charge (PZC) for hematite, the affinity of the AgNPs to the FeO-GB was significantly higher than that to the GB. On the other hand, the difference in the affinity to both media was insignificant at pH > PZC of hematite. Lecoanet et al.,18 found that the mobility of a range of metallic and carbon-based nanoparticles in columns filled with glass beads varied greatly as a Received: Revised: Accepted: Published: 4039
December 9, 2011 February 21, 2013 March 22, 2013 March 22, 2013 dx.doi.org/10.1021/es304580r | Environ. Sci. Technol. 2013, 47, 4039−4045
Environmental Science & Technology
Article
other impurities, such as excess capping agents, from the AgNPs suspensions. Conductivity of the AgNPs suspensions was measured using an Orion STAR meter equipped with a conductivity probe (Orion 013010MD). 2.2. Porous Media. Quartz sand (J.T. Baker, Phillipsburg, NJ) with an average grain diameter of 360 μm and specific gravity of 2.65 g cm−3 was repeatedly washed with 0.1 M HNO3, rinsed seven times with Milli-Q water until the pH was equilibrated and finally oven-dried at 60 °C and stored in a sealed container. Portions of the cleaned quartz sand were coated with ferrihydrite and aluminosilicate (kaolin) according to the methods reported by Grafe et al.23 and Jerez et al.,24 respectively. Details of the coating procedures for both types of coated sands are presented in the SI. The porosity of the sand was experimentally determined and presented in the SI. In order to measure the zeta potential (ζ) of the sand types, colloidal-size particles were isolated from the clean sand soaked in a pH 7.0 and 5 mM NaNO3 solution (experimental pH and ionic strength) using high power probe sonication (30 W) for 2 min.6 A Zetasizer Nanoseries (Malvern Instruments) analyzer was used to measure the ζ potential of the isolated sand colloids. 2.3. AgNPs Column Transport Experiments. The transport experiments were conducted using cast acrylic columns with an inner diameter of 1.1 and 10 cm length. The columns were equipped with Teflon bed supports, nylon mesh and Teflon tubing to minimize the interactions with the colloidal suspensions. A peristaltic pump (Ismatec IP Multichannel Pump, IDEX Corporation) was used to deliver the background electrolyte solutions with AgNPs suspensions or the tracer solution (10 mg L−1 Br− prepared using KBr salt) to the porous media in an upflow configuration. Prior to each experiment, the packed column was equilibrated by flushing the porous media with 10 pore volumes of Milli-Q water followed by 10 pore volumes of the background electrolyte solution. The aqueous AgNPs suspensions (10 mg L−1, pH 7.0 adjusted using 0.1 M HNO3 or NaOH solutions, I of 5 mM (adjusted using NaNO3 salt)) were then pumped through the packed bed at a constant flow rate of 1 mL min−1 similar to that reported elsewhere.16 It is noted that this flow rate (1 mL min−1) may not be realistic in most subsurface environments for the selected types of sand. However, it was selected to overcome practical issues when conducting the experiment. The column experiments were conducted without pH buffering, which was frequently monitored during the experiments and ranged between 6.8 and 7.2. Each breakthrough curve was repeated at least twice. In order to investigate the hydrodynamic properties of the porous media, tracer transport experiments were conducted at pH 7.0 except in the case of ferrihydrite coated sand (FcS). In that case the solution pH was raised to 9.9 in order for the Br− to behave conservatively as suggested by Jerez et al.24 2.4. Sampling and Analysis. The column effluent was monitored online using a UV-vis spectrophotometer (UV1650PC, Shimadzu Scientific Instruments) equipped with a flow through cell with an optical length of 3 mm. The hydrodynamic diameter (HDD) and zeta (ζ) potential of the AgNPs in the column influent and effluent were measured at 25 °C using a Zetasizer Nanoseries analyzer (Malvern Instruments). The total Ag concentration was measured using a PerkinElmer AAnalyst 800 atomic absorption spectrometer after acid digestion following EPA method 3015A.25 The acid digestions were performed using a microwave MARS 5 (CEM Corporation). For the conservative tracer experiments, the Br− concentration in the
result of the differences in nanoparticles’ chemical and physical properties. Tiraferri and Sethi19 reported that bare iron nanoparticles were immobile in sandy porous media while guar gum-coated iron nanoparticles were mobile in the same media regardless of the solution chemistry. Moreover, the reactivity of the porous media influenced the mobility of nanoparticles. For instance, the mobility of carboxymethyl cellulose stabilized anatase (TiO2) nanoparticles (CMC-ANTNPs) in mineral coated sand media was significantly retarded by amorphous Fe and Al hydroxides coatings on the sand surface as compared to that in uncoated quartz sand media.20 This highlights the importance of nanoparticles interactions with clay minerals and iron oxides that are commonly associated with soils and aquifer sediments.21 Although a number of studies have examined the fate and transport of nanoparticles, only a few concentrated specifically on AgNPs. Since AgNPs are the most widely used nanoparticles in consumer products,22 there is still a need for a more detailed evaluation of their fate and transport in the environment. The current study aims at investigating key factors controlling mobility of commonly utilized AgNPs in saturated nonreactive (clean quartz sand) and reactive (ferrihydrite-coated sand and aluminosilicate (kaolin)-coated sand) porous media under neutral pH and low ionic strength (5 mM NaNO3) conditions. The selected experimental pH and ionic strength conditions are environmentally relevant and help minimize particle−particle interactions before the introduction to the porous media. The selected AgNPs exhibit surface charging scenarios ranging from negative to positive. These nanoparticles also represent the common colloidal stabilization mechanisms that include electrostatic, electrosteric, and steric. The combination of reactive porous media and AgNPs used in the study would advance the current understanding of key factors that govern the mobility of these nanoparticles in natural and engineered environmental systems. Furthermore, it would potentially enable the generalization of some of the exhibited behavior herein to other AgNPs regardless of the chemical nature of their surface.
2. MATERIALS AND METHODS Based on a thorough review of AgNPs’ synthesis methods,1 four nanoparticles were selected: (1) uncoated hydrogen reduced AgNPs (H2−AgNPs), (2) citrate coated AgNPs (CitrateAgNPs), (3) polyvinylpyrrolidone coated AgNPs (PVPAgNPs), and (4) branched polyethyleneimine coated AgNPs (BPEI-AgNPs). The H2−AgNPs and Citrate-AgNPs are electrostatically stabilized, the PVP-AgNPs are sterically stabilized and the BPEI-AgNPs are electrosterically stabilized. Quartz sand (QS), ferrihydrite-coated sand (FcS), and aluminosilicate (kaolin)-coated sand (KcS) were used as the porous media in the column transport experiments. 2.1. Synthesis and Purification of AgNPs Suspensions. The nanoparticles were synthesized using modified procedures previously reported in the literature. The details of synthesis of the investigated AgNPs suspensions are presented in Supporting Information (SI). The purification (removal of residual impurities) of the synthesized AgNPs suspensions was performed using a 10 kDa polyethersulfone (PES) membrane (Spectrum Laboratories MidiKros Hollow Fiber Module (PX3−010 × 10−300−02N)).7 The purification procedure included an initial process of concentrating the AgNPs suspensions followed by a Milli-Q water (0.67 μs cm−1 conductivity) exchange process (until a suspension conductivity of
