Transport and Retention of Selected Engineered Nanoparticles by

Jan 24, 2013 - Column experiments were conducted to investigate the transport of aqueous C60 (aqu-nC60), fullerol, silver nanoparticles (NPs) coated w...
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Transport and Retention of Selected Engineered Nanoparticles by Porous Media in the Presence of a Biofilm Yao Xiao†,‡ and Mark R. Wiesner*,†,‡ †

Department of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, North Carolina 27708, United States ‡ Center for the Environmental Implications of Nanotechnologies (CEINT) and the International Consortium for the Environmental Implications of NanoTechnology (iCEINT), Durham, North Carolina, United States S Supporting Information *

ABSTRACT: Column experiments were conducted to investigate the transport of aqueous C60 (aqu-nC60), fullerol, silver nanoparticles (NPs) coated with polyvinylpyrrolidone (Ag-PVP) and stabilized by citrate (Ag-CIT) in biofilm-laden porous media. Gram-negative Pseudomonas aeruginosa (PA) and Gram-positive Bacillus cereus (BC) biofilm-laden glass beads were selected to represent the bacterial interfaces NPs might encounter in the natural aquatic environment. The biomass distribution, extracellular polymeric substances (EPS) components, electrokinetic property, and hydrophobicity of these interfaces were characterized, and the hydrophobicity was found to correlate with the quantity of proteins in EPS. The retention of NPs on glass beads coated with bovine serum albumin (BSA) and alginate were also studied. Except for Ag-PVP, the affinity of NPs for porous medium, indicated by attachment efficiency α, increased in the presence of biofilms, BSA and alginate. For hydrophobic aqunC60, the larger the proteins/polysaccharides ratio, the larger the α, suggesting the hydrophobic interaction determines the attachment of aqu-nC60 to the collector surface. Uncharged PVP stabilized Ag-PVP by steric repulsion, and the attachment to glass beads was not enhanced by biofilm. The presence of divalent ion Ca2+ significantly hydrophobized biofilm, BSA, and alginate-coated glass beads and further retarded the mobility of aqu-nC60, fullerol, and Ag-CIT; while Ag-PVP was again sterically stabilized.

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INTRODUCTION

exposure and eventually the environmental impact in natural and engineered systems. Packed columns with granular materials such as quartz sand or glass beads have often been employed to study porous media such as groundwater aquifers and sand filters, and these methods have been applied to the study of ENPs.7,8 However, an important element of many environmentally relevant porous media is the presence of a biofilm. Biofilms are typically multilayer coatings of bacterial cells that accumulate at a living or inert surface and are surrounded by a matrix of extracellular polymeric substances (EPS).9 They are ubiquitous in natural

1−3

Increasingly used in numerous commercial applications, engineered nanoparticles (ENPs) will inevitably find their way into aquatic environments.4,5 Where nanomaterials accumulate in aquatic systems reflects in part the physical chemistry of interactions between nanoparticles (NPs) and the surfaces they encounter. A consideration of these interactions is important for understanding not only the fate of nanomaterials in the environment but also how they may be used in environmental applications. For example, aquifers or sediments contaminated by halogenated organic compounds may be treated by intentionally injecting ENPs such as nanoscale zerovalent iron (NZVI).6 Therefore, understanding the attachment of ENPs to relevant environment interfaces they may interact with and their transport behavior in the subsurface system that is composed of those interfaces is crucial for assessing their © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2246

November 4, 2012 January 20, 2013 January 24, 2013 January 24, 2013 dx.doi.org/10.1021/es304501n | Environ. Sci. Technol. 2013, 47, 2246−2253

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Table 1. Characteristics of Nanoparticle Suspensions NPs

sizea/nm

sizeb/nm

Aqu-nC60 fullerol Ag-CIT Ag-PVP

102 ± 29 105 ± 29 27 ± 13 40 ± 11

88 97 19 36

± ± ± ±

41 39 4 9

EPMc/(10‑8 m2 /V·s) −2.480 −4.527 −3.329 −0.765

± ± ± ±

0.024 0.031 0.121 0.020

ζ-potential/mV

hydrophobicity

source

−31.66 ± 0.31 −57.79 ± 0.39 −42.49 ± 1.54 −9.77 ± 0.25

RBd, 0.24e NBf, 0.16g NB, 0.08 RB, 0.08

lab prepared lab prepared CEINTh CEINT

The second order average hydrodynamic diameter given by DLS ± standard deviation. bSize determined by TEM image analysis, all values are means ± standard deviation. cAll values are means ±95% confidence interval (n = 3). dSurface hydrophobicity measured by Rose Bengal (RB) adsorption (method description in the SI). eThe slope of PQ (SI) of RB versus total surface area; larger slope suggests more hydrophobic surface. f Surface hydrophobicity measured by Nile Blue (NB) adsorption (method description in the SI). gThe slope of PQ (SI) of NB versus total surface area; smaller slope suggests more hydrophobic surface. hNPs suspension was obtained from sample store of Center for the Environmental Implications of Nano-Technology (CEINT, Durham, NC, USA). a

the influence of divalent ions (i.e., Ca2+) in solution on ENP retention by each of the porous media considered was also investigated. The ENPs studied covered a range of surface hydrophobicities and included carbon-based and silver-based nanoparticles with or without polymeric coatings.

and engineered environments and are commonly found in soils and at water-sediment interfaces as coatings surround particles.10 Surprisingly, little attention has been paid to studying the impact that biofilms may have on the fate and transport of ENPs in a biofilm-laden porous media.11−15 In the handful of studies conducted to date, Pseudomonas aeruginosa (PA) biofilm was found to reduce the mobility of ENPs in the case of functionalized polystyrene latex nanoparticles observed by Tripathi et al.,11 nanosized laponite clay particles used in work by Leon-Morales et al.,12 and NZVI particles studied by Lerner et al.14 The transport of fullerene C60 nanoparticles was also retarded by biofilms formed by Escherichia coli (E. coli).13 Reduced mobility of ENPs in these systems could not be predicted by the surface potential alone,11,13 suggesting that factors other than electrical double layer (EDL) interactions were in play. Hydrophobic interactions13 as well as steric and bridging interactions by macromolecules on ENPs surfaces14 were proposed as candidates that affect the attachment of nanoparticles to the collector surface. EPS, responsible for 85% of the biofilm mass,9 mediate attachment of bacteria to surfaces and aid in the formation and integrity of biofilm structure.16 The main components of EPS are proteins, polysaccharides, and humic substances,17 but the composition of biofilm (i.e., type of macromolecules, concentration of macromolecules) can be completely different for different species of bacteria and growth conditions.9 For instance, the primary polysaccharides constituting the EPS matrix produced by Gram-positive and Gram-negative bacteria are known to differ.18 The amount of polysaccharides and the ratio of cells/EPS may vary depending on whether the biofilm is grown in a sand column or on agar plates.12 The composition of EPS, especially the ratio of proteins/polysaccharides, has been shown to correlate with the deposition of nanoparticles on biofilm-coated surfaces.12,19 Morrow et al. 20 revealed that quantum dots preferentially colocalized with extracellular protein rather than other components or cell surface in a PA biofilm by confocal laser scanning microscopy. Though the affinity of nanoparticles to EPS may not be, on its own, a predictor for nanoparticle attachment to the biofilm,11 studying the interaction between nanoparticles and EPS components is nonetheless important for understanding how biofilm affect the transport of nanoparticles. In this study, the mobility of selected ENPs in the granular porous media in the presence of biofilms was investigated in column experiments using glass beads coated with gramnegative and gram-positive bacteria. In addition, bovine serum albumin (BSA) and alginate coatings of glass beads were employed as surrogates for proteins and polysaccharides to evaluate their impact on the transport behavior of ENPs, and

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MATERIALS AND METHODS Preparation and Characteristics of Nanoparticle Suspensions. Four different engineered nanoparticle aqueous suspensions were studied in this paper: aqueous nC60 (aqunC60), fullerol, nanosilver stabilized by citrate (Ag-CIT), and nanosilver coated with polyvinylpyrrolidone (Ag-PVP) (Table 1). Aqu-nC60 suspension was prepared by the extend stirring technique.21 80 mg/L of fullerol powder was added to nanopure water (Barnstead Nanopure Diamond system) to obtain the fullerol suspension. Characteristics of the nanoparticles used in this study are summarized in Table 1. Hydrodynamic sizes were determined by dynamic light scattering (DLS) using ALV CGS-3 system (ALV-GMBH, Langen, Germany). Nanoparticles sizes were measured by transmission electron microscopy (TEM, FEI Tecnai G2 Twin, SMIF facility at Duke University, Durham NC). Zeta Sizer Nano ZS (Malvern, Bedford, MA) were employed to measure electrophoretic mobility (EPM), with which the ζ-potentials were calculated accordingly. Surface hydrophobicity was characterized by organic dye adsorption methods which are also briefly described in the Supporting Information (SI).22 Because the macroscale contact angle measurement and solute-scale partition coefficient measurement are not suitable for nanoparticles,22 the organic dye adsorption methods were selected to characterize the hydrophobicity of nanoparticles. Preparation of Porous Media. Spherical silicate glass beads (Potters Industries Inc., Berwyn, PA) were used as the porous media and were cleaned following a procedure established previously to remove extraneous materials initially present on the glass bead surface.7 The roughness on the glass bead surface was improved by stirring them in water with metal paddles of a stirring machine, so that a better adherence of biofilm could be achieved on a roughened glass bead surface. The glass beads were sterilized by autoclaving for 30 min before wet packed into a 10 cm-long glass column (C10/10, GE Healthcare, NJ), with an inner diameter of 10 mm and porosity of approximately 0.36. Gram-negative Pseudomonas aeruginosa (PA) ATCC 7700 and Gram-positive Bacillus cereus (BC) ATCC 14579 were obtained from Dr. Claudia Gunsch Lab (Duke University, Durham NC). A single colony was transferred from a streaked plate into Lysogeny broth (LB) and incubated at 37 °C in a 2247

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Figure 1. Spatial distribution of biofilm in the column.

FEI XL30 SEM-FEG, SMIF facility at Duke University, Durham NC). The EPM and ζ-potential of glass beads were determined by Zeta Sizer Nano ZS (Malvern, Bedford, MA) using crushed glass beads. For porous media coated with biofilm, BSA, or alginate, the electrokinetic property was characterized by measuring the EPM and ζ-potential in solutions containing biofilm, BSA, or alginate. Hydrophobicity of collector surface was characterized by employing the organic dye adsorption methods as described previously.22 After biofilm was removed from glass beads by sonication, EPS extraction from the biofilm was conducted in the presence of cation exchange resin (J. T. Baker, Mansfield, MA) following a protocol described elsewhere.26 The proteins content of EPS was quantified by the bicinchoninic acid (BCA) assay27 with BCA Protein Assay kit (Thermo Fisher Scientific, Waltham, MA) using BSA as standard; while the polysaccharides content was measured by the DuBois method28 using glucose as standard. Column Experiments. The retention of nanoparticles by different collector surfaces was investigated by column experiments, and the types of collector surface evaluated were as follows: (1) roughened clean glass beads, (2) Gram-negative PA biofilm-coated glass beads, (3) Gram-positive BC biofilmcoated glass beads, (4) BSA-coated glass beads, and (5) alginate-coated glass beads. The background electrolyte solution was 1 mM of NaCl, an environmentally relevant concentration of monovalent ions.29 Nanoparticles were found to be aggregates, rather than monomers at this ionic strength. After preparing the biofilm, at least 20 pore-volumes of background electrolyte solution were pumped through the column (1 mL/min) to elute loose biofilm and residual impurities, resulting in a low suspended solid concentration in the effluent as monitored by optical density. 0.05 mL/min of nanoparticle suspension was then mixed with the electrolyte solution (1 mL/min) and introduced into the column for 5−6 pore volumes, followed by background electrolyte elution for another 10 pore volumes. The flow direction was upward. Samples from the effluent were collected intermittently at a rate of 0.5 mL/min, and the nanoparticle concentrations were quantified afterward. The aqu-nC60 concentration was measured by liquid−liquid extraction coupled with HPLC-UV/vis spectroscopy.30 UV/vis spectroscopy was used to quantify the concentration of fullerol.31 The concentrations of silver

shaking incubator for 20 h prior to inoculation. Then 100 mL of this preculture was mixed with 2 L of synthetic nutrient solution (composition of this nutrient solution can be found in Table S2) and recirculated through a clear PVC column filled with polyurethane foam at 20 mL/min to fast grow biofilm for 2 days. After the formation of biofilm, 200 mL of this biofilmcontaining recirculating solution was diluted in 2 L of synthetic nutrient solution and used to seed the glass beads column. The biofilm-containing solution was recirculated at a flow rate of approximately 1 mL/min, and the flow direction was switched between upward and downward every 12 h to homogenize the spatial distribution of biofilm in the column. After having been attached to the surface of glass beads, the growth of biofilm stabilized after 3 days and the bacteria concentration in the effluent became stable (monitored by measuring the optical density (OD) at 600 nm). All the columns and tubings used in this experiment were sterilized by soaking them in bleach solution and then rinsing with autoclaved nanopure water. Sterilizing-grade gas filters (Millipore, Billerica, MA) were employed to sterilize the air for aerating the synthetic nutrient solution. To coat glass beads with BSA, the BSA solution (0.5 mg/mL, Thermo Fisher Scientific, Waltham, MA) was recirculated through the glass beads column for 1 day, as this concentration of BSA and contact time would ensure that the surface of glass beads was saturated by BSA.23 For alginate, the glass beads were precoated with a layer of cationic poly-L-lysine (PLL) following a protocol described elsewhere,24 in order for alginate to attach to the glass beads. Then the alginic acid sodium salt solution (2 g/L, from brown algae, Sigma-Aldrich, St. Louis, MO) was recirculated through the column filled with PLLcoated glass beads for 1 day. Characterization of Porous Media. The spatial distribution of biofilm in the column was determined by measuring the total biomass in five separate segments. The column was divided into five 2 cm-long sections, and the total biomass in each section was calculated from the chemical oxygen demand (COD) measurements.25 After sonication and removing from glass beads, the oxidizable matter in biofilms, expressed by COD, was quantified with a Hach COD test kit (Method 10067, Hach, Loveland, CO). The biomass then was calculated by applying a conversion factor of 0.706 mg dry weight (DW)/ mg COD.25 The surface of glass beads with and without biofilm covering was observed by scanning electron microscope (SEM, 2248

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Figure 2. SEM images taken from a clean glass bead at (A) 500× and (B) 15000× and from a BC biofilm-coated glass bead at (C) 500×, (D) 15000×, and (E) 35000×.

biofilm being lower in the middle of the column compared to either end. Limited availability of oxygen inside the column likely contributed to this uneven growth of biofilm. BC is a facultative anaerobe,34 while PA is classified as an aerobic organism35 which may explain the slightly more even distribution of the BC if oxygen was limiting in the middle of the column. Nonetheless, the flow direction switching while the formation of biofilm led to a relatively homogeneous porous media in terms of the biofilm distribution. The coverage of BC biofilm on the surface of glass beads was shown by SEM in Figure 2, and the coating of PA biofilm was similar. Though a large portion of the glass beads surface was coated by biofilm, it was evident that the distribution of biofilm was not homogeneous. The bacteria were imbedded in a matrix of polymer-like substances. The EPM measurement and associated ζ-potentials calculated for different porous media are summarized in Table S3. The ζ-potentials of all the porous media investigated were negative, with clean glass beads being the most negative surface (−66.6 ± 3.8 mV). BC biofilm (−16.7 ± 2.0 mV) was less negative than PA biofilm (−38.1 ± 2.9 mV). This was as expected since the biofilms of Gram-positive bacteria produce an EPS with cationic functionality,18 offsetting part of the negative charges on bacterial surfaces. The quantitative analysis of different components of EPS in the biofilm showed (Figure S1) that there were more proteins than polysaccharides in the EPS of PA biofilm (proteins/ polysaccharides = 1.8); while in BC biofilm, EPS contained more polysaccharides (proteins/polysaccharides = 0.5). The

nanoparticles were determined by ICP-MS with HNO3/HCl predigestion as described elsewhere.32 To investigate the effect of divalent cations on the transport of nanoparticle in biofilmcoated porous media, 10 mM of CaCl2 was passed through the column for 1 h (approximately 20 pore volume) before the background electrolyte solution was applied and column experiments were carried out. Upon measurement of the breakthrough curves (BTC) for different nanoparticles, the attachment efficiency (α) was calculated by eq 17 α=−

⎛C⎞ 4rC ln⎜ ⎟ 3(1 − ε)η0L ⎝ C0 ⎠

(1)

where rC is the geometric radius of collector, ε and L are the porosity and length of the porous medium, and C/C0 is the ratio of nanoparticle concentration in the effluent over that in the influent. η0 is the single-collector efficiency, which is calculated by utilizing a correlation equation.33 The parameters used to calculate η0 are summarized in Table S1.

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RESULTS AND DISCUSSION Characterization of Porous Media. The spatial distribution of biofilm on glass beads in the column is shown in Figure 1. The total biomass per unit mass of glass beads in different sections of the column did not appear to be evenly distributed for the PA. Spatial variability for the BC was not statistically significant at a confidence interval of 95%. However, both PA and BC exhibited a similar pattern, with mean concentrations of 2249

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Figure 3. Attachment efficiencies of selected ENPs on clean glass beads, PA and BC biofilm, BSA- and alginate-coated glass beads.

composition of EPS likely varies with the aging of the biofilm12 or the changing of environmental conditions,36 but the proteins/polysaccharides ratio was significantly different for the PA and BC at the sampled phase of the biofilm growth. BSA-coated glass beads were found to be the most hydrophobic collector surface of all those examined in this study, followed by PA biofilm, BC biofilm, and alginate-coated glass beads (Figure S2). Adsorption of Rose Bengal onto clean glass beads was not observed, consistent with a hydrophilic glass beads surface. The presence of both hydrophilic (e.g., hydroxyl, carboxyl, and phenolic) and hydrophobic functional groups (e.g., aromatics, aliphatics in proteins, and hydrophobic regions in carbohydrates) in EPS molecules suggests that the surface of biofilm is amphiphilic,37 and the relative hydrophobicity of biofilm is controlled by the composition of the EPS.38 The hydrophobic fraction of EPS was found to mainly comprise of proteins; while polysaccharides were the dominant macromolecules in the hydrophilic fraction of EPS as consistent with previous studies.39 Transport and Retention of Nanoparticles in Biofilm Coated Porous Media. With the measured BTC and eq 1, the attachment efficiency of nanoparticles to different collector surfaces was calculated (Figure 3). Apart from Ag-PVP, more nanoparticles were retained by the biofilm than the clean glass beads as shown by the relative larger value of α, which suggests that the presence of a biofilm in natural environments would reduce mobility compared with transport in a porous medium of sand alone. Considering the fact that biofilm consists of a complex matrix of cells and biopolymers and the adherence of biofilm on glass beads was spatially heterogeneous, calculation of the particle-collector surface interaction energy to predict α using classic Derjaguin−Landau−Verwey−Overbeek (DLVO) theory is not justified, and both Tripathi et al.11 and Lerner et al.14 have shown the failure of DLVO models to quantitatively predict experimentally determined values of α in complex systems such as these. For aqu-nC60, the retention of NPs on clean glass beads was the lowest of all the porous media, possibly because the clean glass beads had the most negatively charged surface, favoring EDL repulsion (Table S3). However, the difference in surface potential could not explain the difference in α for biofilm-

coated glass beads, as BC biofilm with a less negative surface yielded a lower value of α. This deviation from trends predicted by DLVO theory was also observed for BSA- and alginatecoated glass beads, suggesting that other interactions must be invoked to account for the particle-surface interactions in these cases. With aqu-nC60 being a moderately hydrophobic nanoparticle (Table 1), the hydrophobic interaction between aqu-nC60 and hydrophobic region of biofilm would inevitably contribute to the overall particle-surface interaction. This was confirmed by the experimental results (Figure 3) that the most hydrophobic BSA-coated glass beads achieved the highest retention, while the least hydrophobic alginate-coated glass beads yielded the lowest value of α. Since the components of PA and BC biofilms were of different combinations of proteins and polysaccharides (Figure Figure S1), it is logical that the hydrophobicity, as well as the values of α, for the PA and BC biofilms fell in between the BSA- and alginate-coated glass beads. At an ionic strength of 1 mM, the Debye length, κ−1, was calculated to be 9.61 nm, and the hydrophobic functional groups in the adsorbed protein might not have been long enough to protrude through the EDL. However, the hydrophobic interaction appears to have been effective over greater distances40 thus making the collector attractive to particles. It is reasonable to conclude that the relate predominance of hydrophobic (produced in part by proteins) and hydrophilic (produced in part by polysaccharides) regions of biofilm played a significant role in deciding the mobility of hydrophobic nanoparticles in porous media with biofilm grown inside. Thus the relative abundance of protein compared to the polysaccharides components in the biofilm could be a useful predictor to the fate and transport of hydrophobic nanoparticles. However, it should be pointed out that due to the complex nature of biofilm which resulted from different species of bacteria (e.g., Gram-positive or Gram negative), age of biofilm, or environmental condition, the proteins/polysaccharides ratio or even the hydrophobicity of extracellular proteins might vary.19,39,41 Although PA and BC biofilms, BSA, and alginate on glass beads all attenuated the mobility of fullerol in porous media to a certain degree, there was no difference in the values of α among them. A correlation between hydrophobicity of the 2250

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Figure 4. Attachment efficiencies of selected ENPs on clean glass beads, PA and BC biofilm, BSA- and alginate-coated glass beads after pretreated by Ca2+.

beads was the smallest of all porous media evaluated, and α for PA or BC biofilm was between that for BSA and alginate, in a similar way with aqu-nC60. Effect of Divalent Cations on the Transport of Nanoparticles in Biofilm-Coated Porous Media. After the pretreatment by Ca2+, the hydrophobicity of all of the glass beads with coatings was increased, although not to the same degree (Figure S3). The alginate-coated glass beads and PA biofilm became significantly more hydrophobic after pretreated by Ca2+, and the hydrophobicity of BSA-coated glass beads was also moderately enhanced. Bridging between negatively charged molecules or surfaces in the presence of divalent ions like Ca2+ might produce this “hydrophobizing effect” for polysaccharides.45 By bridging polysaccharide chains via Coulomb forces and altering the molecular conformation, a greater number of internal hydrogen bonds were formed so that the overall hydrophilicity of polysaccharides was lessened.46 Gram-negative bacteria had more anionic polysaccharides in their biofilm matrix than Gram-positive bacteria,18 and a more pronounced effect of Ca2+ was observed for the PA biofilm. As for proteins, Ca2+ has also been observed to increase the apparent hydrophobicity of proteins.47 Moreover, Ca2+ was shown to have a hydrophobizing effect on surfaces whose hydrophilicity was related to the negatively charged surface.48 By any mechanism, all of the porous media with coatings were hydrophobized by Ca2+ as illustrated in Figure S3. In the nutrient solution used for biofilm growth, the concentration of monovalent ions (i.e., K+ and Na+) was approximately 50 mM, indicating that the monovalent ions could not achieve the similar hydrophobizing effect as divalent ions did. The attachment efficiencies of nanoparticles to the Ca2+treated porous media were summarized in Figure 4. The transport of all the nanoparticles was retarded compared to that in nontreated porous media. For aqu-nC60, the difference in α was reduced in a similar pattern to the hydrophobicity of the four different biofilm- or macromolecules-coated glass beads. The most hydrophobic BSA-coated glass beads and least hydrophobic alginate-coated glass beads yielded the largest and smallest values of α, for the coated glass beads. In contrast to aqu-nC60, the change of the mobility of hydrophobic Ag-PVP was not as dramatic. Although the charge neutralization,

collector surfaces and the measured attachment efficiencies, as indicated for the case of aqu-nC60, was not observed. Considering the hydrophilic property of fullerol (Table 1), the lack of hydrophobic interaction between nanoparticles and collectors was not surprising. The enhanced affinity might be attributed to the hydrogen bonding the hydroxyl groups on the surface of fullerol formed with the collector surface or the capability of large polysaccharides to “collect” small nanoparticles.7 Despite being of different core material, the transport of AgCIT in porous media consisting of clean glass beads or biofilmgrown glass beads followed a similar trend to that of the fullerol; the values of α for clean glass beads were smaller than any other porous media tested. Like fullerol, the increased retention of Ag-CIT by biofilm or macromolecules did not appear to be the result of hydrophobic interactions as Ag-CIT was not hydrophobic (Table 1). Similar to the origins of fullerol’s affinity for the biofilm surface linked to hydroxylation of the C60, citrate on the Ag-CIT NPs may have provided sites for hydrogen bonding or other associations with macromolecules in the biofilm. In contrast to Ag-CIT, the presence of a biofilm or alginate did not reduce the mobility of Ag-PVP, though these collectors were less negative than the clean glass beads. Given that AgPVP nanoparticles were wrapped by a layer of PVP polymers and there were abundant biopolymers in biofilm, steric repulsion between PVP and biopolymers likely stabilized the Ag-PVP. Numerous studies have revealed the prominent role steric interaction played in influencing the interaction between two polymer-covered surfaces.42−44 Lerner et al. reported that under certain ionic strength, the steric interaction could turn into attractive in the form of bridging between polymers;14 nevertheless in this study, the ionic strength was not large enough for this phenomenon to occur. In spite of the steric repulsion, α for BSA-coated glass beads was noticeably larger than that for clean glass beads, which might be attributed to the hydrophobic interaction. This attractive interaction between slightly hydrophobic Ag-PVP (Table 1) and proteins attenuated the effect of repulsive steric interaction and made the attachment of Ag-PVP to collector surface a synergic result of both interactions. As a matter of fact, α for alginate-coated glass 2251

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NSF Cooperative Agreement EF-0830093, Center for the Environmental Implications of NanoTechnology (CEINT). Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not been subjected to EPA review, and no official endorsement should be inferred.

increasing hydrophobicity, and the bridging effect brought about by Ca2+ could potentially reduce the influence of steric interaction,42 it was still the major barrier for a stable attachment between two polymer-coated surfaces. Pretreating porous media with coatings greatly elevated α for both fullerol and Ag-CIT, either by the aforementioned charge neutralization effect of Ca2+ or due to the gel formation initiated by the addition of Ca2+.7

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ENVIRONMENTAL IMPLICATIONS The presence of a biofilm was generally but not always observed to increase the affinity of nanoparticles for a porous medium. The relative amounts of protein and polysaccharide in the biofilm appear to be important predictors of the degree of affinity for the porous medium with a biofilm, with simple coatings of these two materials bracketing the observed values for the affinity coefficients. Nanoparticles that are more hydrophobic exhibited a higher affinity for the biofilm, while nanoparticles that were sterically stabilized by uncharged macromolecules can be anticipated to show little to no enhanced attachment to the pour medium when a biofilm is present. The presence of divalent ions such as calcium further increased nanoparticles attached to biofilm, protein, and polysaccharide-coated media, with again the exception of nanoparticles that were sterically stabilized. In the present study, two typical yet distinct kinds of biofilm (i.e., Gram-negative and Gram-positive) were selected to represent biofilms that engineered nanoparticles might encounter in aqueous systems. However, the enormous number of species of bacteria and the various phases of biofilm growth as well as different environmental conditions might further complicate the scenarios we investigated here. The effect of divalent ions on the surface chemistry of biofilm and the attachment of nanoparticles to the biofilm was also a good example of the challenge we face in better understanding the fate of a transport of engineering nanoparticles in biofilm-laden porous media and, furthermore, the natural aquatic environment.

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(1) Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P. Assessing the Risks of Manufactured Nanomaterials. Environ. Sci. Technol. 2006, 40 (14), 4337−4345. (2) Jain, P.; Pradeep, T. Potential of Silver Nanoparticle-Coated Polyurethane Foam as an Antibacterial Water Filter. Biotechnol. Bioeng. 2005, 90 (1), 59−63. (3) Satoh, M.; Takayanagi, I. Pharmacological Studies on Fullerene (C60), a Novel Carbon Allotrope, and Its Derivatives. J. Pharmacol. Sci. 2006, 100 (5), 513−518. (4) Lowry, G. V.; Casman, E. A. Nanomaterial Transport, Transformation, and Fate in the Environment: A Risk-based Perspective on Research Needs. In Risk, Uncertainty and Decision Analysis for Nanomaterials: Environmental Risks and Benefits and Emerging Consumer Products; Linkov, I., Stevens, J., Eds.; Springer Verlag: 2009; pp 125−139. (5) Mueller, N. C.; Nowack, B. Exposure Modeling of Engineered Nanoparticles in the Environment. Environ. Sci. Technol. 2008, 42 (12), 4447−4453. (6) Sirk, K. M.; Saleh, N. B.; Phenrat, T.; Kim, H.-J.; Dufour, B.; Ok, J.; Golas, P. L.; Matyjaszewski, K.; Lowry, G. V.; Tilton, R. D. Effect of Adsorbed Polyelectrolytes on Nanoscale Zero Valent Iron Particle Attachment to Soil Surface Models. Environ. Sci. Technol. 2009, 43 (10), 3803−3808. (7) Espinasse, B.; Hotze, E. M.; Wiesner, M. R. Transport and Retention of Colloidal Aggregates of C60 in Porous Media: Effects of Organic Macromolecules, Ionic Composition, and Preparation Method. Environ. Sci. Technol. 2007, 41 (21), 7396−7402. (8) Song, J. E.; Phenrat, T.; Marinakos, S.; Xiao, Y.; Liu, J.; Wiesner, M. R.; Tilton, R. D.; Lowry, G. V. Hydrophobic Interactions Increase Attachment of Gum Arabic- and PVP-Coated Ag Nanoparticles to Hydrophobic Surfaces. Environ. Sci. Technol. 2011, 45 (14), 5988− 5995. (9) Smirnova, T. A.; Didenko, L. V.; Azizbekyan, R. R.; Romanova, Y. M. Structural and Functional Characteristics of Bacterial Biofilms. Microbiology 2010, 79 (4), 413−423. (10) van Hullebusch, E. D.; Zandvoort, M. H.; Lens, P. N. L. Metal Immobilisation by Biofilms: Mechanisms and Analytical Tools. Rev. Environ. Sci. Biotechnol. 2003, 2 (1), 9−33. (11) Tripathi, S.; Champagne, D.; Tufenkji, N. Transport Behavior of Selected Nanoparticles with different Surface Coatings in Granular Porous Media coated with Pseudomonas aeruginosa Biofilm. Environ. Sci. Technol. 2012, 46 (13), 6942−6949. (12) Leon-Morales, C. F.; Strathmann, M.; Flemming, H.-C. Influence of Biofilms on the Movement of Colloids in Porous Media. Implications for Colloid Facilitated Transport in Subsurface Environments. Water Res. 2007, 41 (10), 2059−2068. (13) Tong, M.; Ding, J.; Shen, Y.; Zhu, P. Influence of Biofilm on the Transport of Fullerene (C60) Nanoparticles in Porous Media. Water Res. 2010, 44 (4), 1094−1103. (14) Lerner, R. N.; Lu, Q.; Zeng, H.; Liu, Y. The Effects of Biofilm on the Transport of Stabilized Zerovalent Iron Nanoparticles in Saturated Porous Media. Water Res. 2012, 46 (4), 975−985. (15) Leon-Morales, C. F.; Leis, A. P.; Strathmann, M.; Flemming, H.C. Interactions between Laponite and Microbial Biofilms in Porous Media: Implications for Colloid Transport and Biofilm Stability. Water Res. 2004, 38 (16), 3614−3626. (16) Hall-Stoodley, L.; Costerton, J. W.; Stoodley, P. Bacterial Biofilms: From the Natural Environment to Infectious Diseases. Nat. Rev. Microbiol. 2004, 2, 95−108.

ASSOCIATED CONTENT

S Supporting Information *

Additional methodology details regarding the characterization of the hydrophobicity of nanoparticles by Rose Bengal and preparation of synthetic nutrient solution. SI figures and tables include parameters used to calculate η0, EPM and ζ-potential of porous media, biofilm composition, and hydrophobicity measurement for biofilm-coated porous media. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 1-919-660-5292. Fax: 1-919-660-5219. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Gignac for her assistance with SEM analysis and S. Marinakos, A. Gondikas, Y. Liu, T. Morse, J. Lin, and M. Deshusses for experimental assistance. This material is based upon work supported by the National Science Foundation (NSF) and the Environmental Protection Agency (EPA) under 2252

dx.doi.org/10.1021/es304501n | Environ. Sci. Technol. 2013, 47, 2246−2253

Environmental Science & Technology

Article

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