Environ. Sci. Technol. 2008, 42, 7628–7633
Deposition Kinetics of Bacteriophage MS2 on a Silica Surface Coated with Natural Organic Matter in a Radial Stagnation Point Flow Cell B A O L I N G Y U A N , †,‡ M A I P H A M , † A N D T H A N H H . N G U Y E N * ,† Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, Illinois 61801, and Department of Environmental Science and Engineering, Fuzhou University, Minhou, Fujian, 350108 P. R. China
Received April 10, 2008. Revised manuscript received July 16, 2008. Accepted July 25, 2008.
A quartz crystal microbalance (QCM) coupled with a radial stagnation point flow (RSPF) cell was used to study deposition kinetics of bacteriophage MS2 on silica surface coated with Suwannee River natural organic matter (SRNOM). Three stocks of MS2 stored in 1 mM NaHCO3, deionized (DI) water or phosphate buffer saline (PBS) solution were studied. MS2 stored in PBS solution were found to aggregate at all studied ionic strengths from 3 mM to 200 mM, while MS2 stored in DI water and bicarbonate solutions remained monodispersed. Isoelectric points of MS2 stored in PBS solution were lower than for those stored in DI water and 1 mM NaHCO3 solution. Nonrepulsive deposition rates of MS2 on silica surface coated with poly-L-lysine (PLL) were independent of ionic strength. In contrast, MS2 deposition rates on bare silica surface or silica surface coated with SRNOM increased gradually and stabilized at an ionic strength of 60 mM. MS2 deposition rates on bare silica surface were higher than those on silica surface coated with SRNOM at low ionic strengths. Deposition rates on these two surfaces were similar at high ionic strengths. Experimental data suggest that electrostatic and steric interactions were the two main deposition mechanisms of MS2 on either bare silica or silica surface coated with SRNOM.
Introduction Surface water has been found to be especially susceptible to virus contamination (1, 2). This is of significant concern because contaminated surface water can enter groundwater aquifers through percolation, direct injection of partially treated wastewater, and leaky sewage pipes. It is therefore not surprising that viruses have also been found in well water (3) and groundwater from a confined bedrock aquifer (4). The survivability of viruses in groundwater and surface water depends on a number of environmental factors including chemistry of the water and virus association with solid surfaces present in the water such as natural organic matter (5, 6). Mobility of viruses in the aquatic environment is directly * Corresponding author phone: (217) 244-5965; fax: (217) 3336968; e-mail:
[email protected]. † University of Illinois at Urbana-Champaign. ‡ Fuzhou University. 7628
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controlled by the viruses’ ability to attach to solid surfaces. Therefore, understanding the mechanisms of virus attachment to solid surfaces is of great interest. Compared to micrometer-sized colloidal particles and biological materials such as bacteria, nanometer-sized virus particles have been studied much less due to the challenges presented by experimental methods for detection, separation, and characterization. Nonpathogenic bacteriophages such as PRD1 and MS2 have been used as model viruses for subsurface transport studies. MS2 has been shown to adsorb irreversibly to the surface of zero valence iron (7). PRD1 virus attachment to ferric oxyhydroxide-coated quartz surfaces has been found to depend on solution composition and characteristics of the porous media (8-10). Electrostatic and steric repulsion have been suggested as the dominant interactions for MS2 and λ phages with silica surface (11) and MS2 to natural organic matter (12). Hydrophobic effects from organic matter were shown to be significant in attachment of PRD1 and MS2 to modified silica surface (13, 14). Natural organic matter (NOM) is ubiquitous in the environment. Adsorption of NOM to MS2 has been suggested to prevent MS2 adsorption to soil minerals and subsequently increase MS2 mobility in the subsurface (12, 15, 16). In such studies, NOM is typically viewed as a competitor with viruses for sorption sites on soil mineral surfaces. In the context of virus inactivation, it is usually suggested that virus association with NOM and particles inhibits virus inactivation, because NOM interferes with the access of disinfectant to the virus (17, 18). Recent research has demonstrated, however, that solar inactivation of MS2 by singlet oxygen (1O2) increases significantly in the presence of NOM (19, 20), which, it has been suggested, acts as an exogenous photoinactivation sensitizer because 1O2 is produced inside the NOM supramolecular assembly (19-21). As 1O2 radicals have a lifetime in the range of microseconds (22), these radicals formed by NOM can damage the virus only when NOM is closely associated with the virus. Thus, NOM plays an important role in deposition, transport, and inactivation of viruses in the aquatic environment. Previous studies on viruses are based on results from experiments using chromatographic columns (10, 11, 13), batch systems (13), and field experiments (8, 9). Attachment has been quantified by using either radioactive labeling of viruses or the plaque-forming unit (PFU) assay for viruses present in the effluent solution. Attachment efficiency (R), a crucial parameter in modeling virus transport in porous media, has been derived from column breakthrough curves for MS2 and λ phages in quartz sand (11), but has not been reported for surfaces coated with NOM. The objective of this paper is to study the deposition kinetics of MS2 on bare silica and Suwannee River NOM (SRNOM)-coated silica surfaces in the presence of monovalent electrolytes (NaCl) using a quartz crystal microbalance (QCM) coupled with a radial stagnation point flow (RSPF) cell. We investigated the influence of MS2 storing solutions (i.e., NaHCO3, deionized (DI) water or phosphate buffer saline (PBS)) on hydrodynamic diameters, and isoelectric points. Based on the measured deposition kinetics data, we suggest deposition mechanisms for MS2 on bare and SRNOM-coated silica surface. An improved understanding of how MS2 adsorbs onto SRNOM-coated silica surfaces will ultimately aid our ability to better control waterborne viral pathogenic contaminants. 10.1021/es801003s CCC: $40.75
2008 American Chemical Society
Published on Web 09/06/2008
Materials and Methods Materials and Solution Preparation. Analytical grade NaCl, NaHCO3, phosphate salts, poly-L-lysine (PLL) hydrobromide (molecular weight of approximately 150 kDa), and HEPES buffer were purchased from Sigma. All electrolyte, SRNOM and HEPES buffer solutions were filtered through a 0.22 µm sterile cellulose acetate filter (Corning Inc., Corning, NY) and kept at 4 °C until use. The solutions were sonicated for 30 min to remove air bubbles, and kept at 27 °C until use in QCM experiments. Details on SRNOM preparation are in the Supporting Information (SI). Briefly, the SRNOM sample was purified and isolated by the IHSS using cation exchange resins to desalt and reverse osmosis to concentrate NOM (23). Total organic carbon (TOC) of the filtered SRNOM solution was 21.83 ( 0.13 mg/L (duplicate measurements of 21.92 and 21.74 mg/L). All solutions were made using DI water with a resistivity of at least 18 MΩ. MS2 Preparation. Method for MS2 propagation and purification is in the SI. Briefly, MS2 phage stock was propagated with Escherichia coli ATCC 15597 and purified using sequential filtering. Virus enumeration was conducted using a standard double agar layer procedure plaque forming unit (PFU) test (24). The stock solutions were then stored at 4 °C until being used for characterization or deposition experiments. The stock concentration in 1 mM NaHCO3, 10 mM PBS, and DI water was determined to be 3 × 1011, 7 × 1010, and 8 × 1010 pfu/mL, respectively. pH of the storing solutions were 7.5 for 1 mM NaHCO3, 7.0 for 10 mM PBS, and 5.8-6.0 for DI water. Hydrodynamic Diameter Measurement for MS2. A dynamic light scattering (DLS) instrument, Zetasizer Nano ZS90 (Malvern Instruments, Southborough, MA), was used to measure the hydrodynamic diameter of MS2. The electrolyte solution (NaCl) was prepared at different concentrations (from 1 to 200 mM) and filtered through a 0.22 µm sterile cellulose acetate filter. MS2 stocks in PBS and bicarbonate solutions were diluted five times into electrolyte solutions (pH 6) and used for measuring the size of MS2 particles. The final solution after dilution had pH 6. The temperature was maintained at 25 °C during the experiment. For each condition, three measurements were performed. The DLS experiments were conducted 6 min after mixing MS2 with electrolyte solution because 6 min is the time it takes for MS2 solution to flow to the QCMD sensor surface. Our main concern was that MS2 remained monodispersed for the deposition experiments. Electrophoretic Mobility Measurement. A Zetasizer Nano ZS90 (Malvern Instruments, Southborough, MA) was used to measure electrophoretic mobility. Similar to DLS experiments, MS2 stocks in PBS, bicarbonate solution and DI water were diluted five times in electrolyte solutions for electrophoretic mobility measurements. For the first set of measurements, electrophoretic mobility was measured for MS2 in electrolyte solutions at ionic strength in the 1-200 mM range. For the second set of measurements, MS2 stocks were diluted in 1 mM NaCl solution and solution pH was varied from 2.2 to 8 to determine the isoelectric point of the MS2 samples. For solutions with a pH lower than 5.2, 1 M HCl was used to obtain the desired pH. Background salt concentration was maintained at 1 mM Na+ for the measurements taken at each pH. Higher salt concentration was not used to avoid charge shielding. At the lowest pH of 2.7, addition of HCl increased ionic strength (IS) to 2 mM. The highest pH of 8 was achieved by using 1 mM NaHCO3. For each condition, at least three measurements were performed. We followed the method described in refs 25, 26 and used 1.5 µm silica particles coated sequentially with PLL and then SRNOM to obtain electrophoretic mobility for NOM-coated surface. Details for coating the silica particles are described
in the SI. Briefly, negatively charged bare silica particles were coated first with PLL to obtain positively charged PLL-coated particles. These PLL-coated particles were subsequently coated with SRNOM. Uncoated and coated silica particles were subjected to electrophoretic mobility measurements. Protocol for MS2 Deposition Kinetics Studies. We used a QCM-D300 system (Q-Sense AB, Gothenburg, Sweden) to measure deposition kinetics of MS2 samples on silica surface coated with SRNOM and bare silica surface. Quartz sensors were supplied by Q-Sense QSX 303 silica (batch 070112-2). The application of the QCM-D technique for determining deposition kinetics and reversibility was described in detail in our previous publication (27-29). Briefly, the QCM-D technique monitors the change in frequency of vibration due to deposition of wet mass on the quartz crystal sensor. As wet mass deposits onto the quartz sensor, the frequency of vibration decreases. This technique is used to study initial deposition rates of hydrated MS2 particles on hydrated silica surface or SRNOM-coated surface. Before each experiment, the silica sensors were soaked in 2% Hellmanex II (Hellma GmbH & Co. KG, Mu ¨ llheim, Germany) cleaning solution for 2 h (for experiment with PLL and SRNOM surfaces) or 30 min for silica surfaces, rinsed thoroughly with DI water, dried with ultra high-purity N2, and treated in an ozone/UV chamber (BioForce Nanosciences, Inc., Ames, IA) for 30 min. To ensure that experiments were performed with silica surface, we discarded the sensor after five uses. Cleaned sensors were mounted in a temperature-controlled (25 °C) chamber of the QCM-D system. All test solutions were fed into the chamber using a syringe pump (KD Scientific Inc., Holliston, MA) operating in withdrawal mode at a rate of 0.1 mL/min. This flow rate corresponds to a Re number of 1.0 and a Peclet number of 1.7 × 10-8. Thus, laminar flow is maintained and diffusion is dominant for the MS2 particles of 30 nm in diameter. The flow chamber of the QCMD is a radial stagnation point flow as shown in SI Figure 1S. For each experiment, silica sensors were equilibrated under flow conditions of 0.1 mL/min for at least 30 min with DI water. After the equilibration period, the baseline of the frequency signals was stabilized at an approximately 2 Hz change in frequency over 1 h. Subsequently, the silica sensor was equilibrated with HEPES buffer made from 10 mM N-(2hydroxyethyl) piperazine-N′-2-ethanesulfonic acid and 100 mM NaCl at an unadjusted pH of 6.0. The silica sensor was coated with SRNOM by deposition of a layer of PLL polycations onto the silica surface and subsequent deposition of a layer of SRNOM. Specifically, the PLL layer was formed by flowing 2 mL of a PLL hydrobromide solution of 0.1 g/L in HEPES buffer through the sensor chamber. At this pH condition, the PLL layer is positively charged because of protonated amine groups. The PLL layer was then rinsed with HEPES buffer for 20 min and with 10 mM NaCl for another 20 min. After a 40 min washing, the PLL layer was covered by a layer of SRNOM formed by flowing 2 mL of SRNOM solution at 21 mg/L TOC through the sensor chamber. The system was again equilibrated with electrolyte solution containing the NaCl concentration of interest. Finally, MS2 in the experimental NaCl solution was added to the system, allowing MS2 to adsorb to the SRNOM layer created on the crystal. For nonrepulsive conditions, we performed deposition experiments of MS2 on a PLL layer. For these experiments, the experimental NaCl solution was used to equilibrate the system following the second equilibration with HEPES buffer. This was then followed by MS2 deposition. Each experiment was repeated at least two to three times. The MS2 deposition rate is calculated as the initial slope of the change in frequency at the third overtone f(3) vs time curve. The attachment efficiency R for each solution comVOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Size of MS2 virus stored in either 1 mM NaHCO3 or PBS solution. For this measurement, the concentration of MS2 particles is 1011 and 1010 pfu/mL for MS2 stored in bicarbonate and PBS solution, respectively. The solutions for particle size measurement are at pH 6. The temperature is maintained at 25 °C. The average and standard deviation values of three measurements for each condition are plotted. position employed for deposition experiment is the ratio of the deposition rate of MS2 on SRNOM or silica to the favorable deposition rate of MS2 onto a PLL layer for a given electrolyte solution. We conducted control experiments for MS2 deposition on SRNOM-coated surface in 100 mM IS. For these experiments, when we doubled the MS2 concentration to 2 × 1010 PFU/mL, the initial slopes also doubled (i.e., 0.072 ( 0.015 to 0.162 ( 0.057 with N ) 3). Thus, we have a classic first order deposition kinetics of MS2 on SRNOM-coated surface and the initial slope of the frequency shift vs time curve represents the deposition kinetics of MS2. More details on frequency shift and initial slope data are found in SI Figures 2S-6S. DLVO Interaction Energy Profiles. The retarded van der Waals interaction (VVDW) was calculated using the approximate expression of Gregory (30) shown in eq 1 (in SI units): VVDW(J) ) -
Aa 6h(1 + 14h ⁄ λ)
(1)
where h is the separation distance, a is the particle radius (13.5 nm), λ is the dielectric wavelength for water (100 nm). We used the Hamaker constant (A) for the MS2-water-silica system of 4 × 10-21 J, as suggested in ref 11. Repulsive electrostatic interaction (VEDL) was calculated with the expression derived by Hogg et al. (31) for 1:1 electrolytes and surface potentials less than 60 mV (eq 2):
{
(J) ) (πaε0εr) 2ψ1ψ2ln V EDL
[
]
}
1 + e-kh + (ψ21 + ψ22)ln(1 - e-2kh) 1 - e-kh (2)
where ε0 is the permittivity of free space (8.85 × 10-12 C/mV), εr is the relative permittivity of water (78.5), and κ is the inverse Debye length. The surface potential, ψ1 and ψ2, are for the collector and MS2 particle, respectively.
Results and Discussion Hydrodynamic Diameter of MS2 Stored in Different Solutions. To ensure that we performed deposition experiments with monodispersed MS2 sample, we measured the hydrodynamic diameter of MS2 using dynamic light scattering technique. The size of the MS2 samples stored in PBS solution and bicarbonate solution are shown in Figure 1. For MS2 sample stored in PBS solution, the hydrodynamic radius increased from 300 to 500 nm as IS of the electrolyte solutions increased from 3 to 202 mM. This means that MS2 sample 7630
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FIGURE 2. (a) Electrophoretic mobility of MS2 as a function of pH. For this measurement, the concentration of MS2 is 1011, 1010, and 1010 pfu/mL for MS2 stock stored in bicarbonate, PBS solution, and DI water, respectively. The average and standard deviation of two measurements of MS2 in PBS, DI water and 1 mM NaHCO3 solution are plotted. Lines connecting the points are drawn to show the trend, not to model prediction. (b) Electrophoretic mobility of MS2, silica, and SRNOM-coated silica particles as a function of ionic strength. To obtain the outer surface potential Ψ0, we fit the measured electrophoretic mobility to eq 1 in the SI using the Levenberg-Marquardt optimization method. Two fitting parameters are charge density N and surface softness 1/ λ (see eq 26 in ref 38.). The deviation of Ohshima’s equation at low ionic strength was shown in ref 45. The lines in Figure 2b are the fitting lines for MS2 (red dotted line), and SRNOM-coated silica (blue solid line). MS2 concentration during the measurement is 1010 pfu/ mL. stored in PBS solution aggregated, and was not suitable for deposition experiments. Lysine is one of the amino acid residues found on MS2 surface (11, 32). For pH 6.0, phosphate is expected to bind to positively charged lysine on MS2 surface and even link MS2 particles together to form aggregates of 300-400 nm. In contrast, for MS2 sample stored in 1 mM NaHCO3 solution, we observed that the hydrodynamic radius remained monodispersed at around 30 nm as IS varied from 1 to 200 mM, and in agreement with the dimensions (25-29 nm) for individual viral particles obtained with transmission electron microscopy (33). Thus, MS2 sample stored in bicarbonate solution was suitable for deposition experiments. Isoelectric Points for MS2. The isoelectric point (IEP) was found by measuring electrophoretic mobility at a range of pH from 2.2 to 8 for MS2 sample stored in DI water, PBS and NaHCO3 solutions. The IEP is determined as the pH condition at which electrophoretic mobility changes from positive to negative. As shown in Figure 2a, the IEP for MS2 stock in PBS, DI, and NaHCO3 is about 2.2, 3.3, and 3.5, respectively. The fact that IEP for MS2 stored in PBS is lower than for MS2 stored in DI water or NaHCO3 is probably because of the binding of phosphate to lysine on MS2 surface, and as a result, positively charged sites on MS2 surface are
covered with phosphate. Reported values of IEPs for MS2 are 3.5 (11, 34); 3.9 (35), and 3.8 (36). Thus, our results are in good agreement with previous research. Electrophoretic Mobility of MS2 and Surfaces. Electrophoretic mobilities as a function of IS for MS2 sample stored in NaHCO3 are shown in Figure 2b. For the whole range of IS from 1 to 200 mM and at pH 6, the electrophoretic mobility of MS2 is negative and becomes less negative with increasing IS due to an increase in charge screening by Na+. To obtain electrophoretic mobility for NOM surface, we used silica particles coated with SRNOM. Clean particles of silica were highly negatively charged and became less negatively charged with increasing ionic strength. The PLL coating of silica particles led to positively charged surface. With increasing IS, the PLL-coated silica surface became less positively charged. Specifically, the measured values of electrophoretic mobility are 6.25 ( 0.09 µm s-1/V cm-1 at 1 mM IS and decrease to 1.97 ( 0.03 µm s-1/V cm-1 at 200 mM IS (Figure 2b). The subsequent deposition of negatively charged SRNOM at the surfaces of PLL-coated particles resulted in negatively charged surface (Figure 2b). Because the zeta potential concept was developed for hard particles, zeta potential values derived from the measured values of electrophoretic mobility using the Smoluchowski equation may not accurately represent surface potential of MS2 and SRNOM surfaces (34, 37). We applied Ohshima’s electrokinetic theory to determine surface potential of MS2 and SRNOM surfaces (38). Specifically, electrophoretic mobility data was fitted with Ohshima’s electrokinetic equation for soft particles as shown in Figure 2b. Details for fitting to Ohshima’s equation are provided in the SI. Briefly, we obtained good fitting for electrophoretic mobility measured at IS higher than 10 mM as shown in Figure 2b. For soft particles with permeable outer layer, electrophoretic mobility depends on the characteristic hydrodynamic penetration lengths (1/ λ) of this layer as shown in ref 34. For silica particles coated with PLL or SRNOM, it is expected that conformation of PLL or SRNOM layers is more compact at high IS and as a result, the coating layers have lower 1/ λ at high IS. For MS2, decrease in 1/ λ at higher IS was shown in ref 34. and was explained by conformation changes in the FG loops of the MS2 protein capsid. Outer surface potential was then calculated using the fitting values of charge density and the characteristic hydrodynamic penetration lengths. Outer surface potential values will be used to calculate DLVO energy profiles as shown later. Deposition of MS2 on Positively Charged Layer: Nonrepulsive Deposition. MS2 deposition rate on PLL coated silica surface as a function of IS, as shown in Figure 3a, was calculated as the initial rate of frequency shift as a function of time and expressed as Hz/min. Deposition rates were measured for 20, 40, 60, 100, and 200 mM NaCl, and remained nearly constant over the studied IS (Figure 3a). Specifically, the rates varied slightly from 4.6 to 4.0 Hz/min at 20 and 200 mM IS, respectively. Elimelech (39) has suggested that IS has a significant effect on the deposition rate of particles under nonrepulsive interaction and recommends a critical IS of 1 mM. In solutions with IS lower than this critical value, deposition rate increases as IS decreases due to an expanding electrostatic double layer. When IS exceeds this critical value, the attractive electrostatic double layer interaction is compressed substantially or even resembles a van der Waals interaction. As a result, deposition rates remain constant for Brownian particles (i.e., particles less than 1 µm in size) or are slightly reduced for bigger particles. In our case, we measured the deposition rates of MS2 particles on PLL surfaces at IS greater than the critical IS suggested by Elimelech (39). Our observation that deposition rates of Brownian particles of MS2 onto PLL remain constant over
FIGURE 3. (a) Deposition kinetics of MS2 onto silica, PLL-coated silica and NOM-coated silica at different ionic strengths. Temperature was maintained at 25 °C. (b) Deposition rates of MS2 on bare and SRNOM-coated silica in a QCM-D radial stagnation point flow cell. MS2 concentrations during the measurements were 1010 pfu/ml. The dashed line shows detection limit, which was derived from the drift of the frequency shift signals during the equilibration step immediately before the deposition step. See also Figure 2S in the SI. 20-200 mM is in agreement with Elimelech’s studies (39, 40). Similar behavior was observed with nonmotile Pseudomonas aeruginosa (25). Deposition of MS2 on Bare or SRNOM-Coated Silica Surface: Repulsive Deposition. The complete coverage of PLL layer by SRNOM was proven by the fact that at 20 mM IS, deposition data on PLL were more than 2 orders of magnitude higher than the deposition rate on SRNOM. If the PLL layer had not been covered, much more deposition on the SRNOM layer would have been observed. Figure 3a shows that MS2 deposition rates on either clean or SRNOM-coated silica stored in NaHCO3 at different IS increased gradually with IS and stabilized at an IS of 60 mM. Similar to deposition rate, the attachment efficiency of MS2 onto either clean or SRNOM-coated silica increased with IS as shown in Figure 3b. At pH 6, MS2, bare silica and SRNOM-coated silica are negatively charged. With increasing IS, electrostatic doublelayers of MS2 and SRNOM-coated silica surface are compressed to allow a higher deposition rate. Thus deposition of MS2 on SRNOM-coated surface is controlled by electrostatic interaction rather than hydrophobic interaction. Penrod et al. (11) used breakthrough curve data to find that the attachment efficiency for MS2 on quartz sand is 0.1 at IS of 100-300 mM. This value is close to our data at similar IS (Figure 3b). The presence of repulsive steric interactions was suggested previously in ref 11. to explain the low value of attachment efficiency for MS2 on quartz sand at IS of 100-300 mM. Above 60 mM IS, deposition rate for MS2 on clean silica or SRNOM-coated silica stabilized. Moreover, attachment efficiencies of MS2 onto clean silica were comparable with VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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those for MS2 and SRNOM. At IS lower than 60 mM, attachment efficiency of MS2 to clean silica was higher than that to SRNOM-coated silica. For example, at 20 mM IS, deposition of MS2 on silica is 15 times that of MS2 on SRNOMcoated silica. The difference in deposition on silica surface and SRNOM surface is explained in the following section. DLVO interaction energy profiles. DLVO interaction energy calculations (eqs 1 and 2) were used to elucidate the effect of electrostatic double layer compression on deposition of MS2 on clean or SRNOM-coated silica surface, as shown in SI Table 1S and Figure 7S. For deposition of soft particles on solid surface (MS2 on bare silica), we performed the energy profile calculation for two cases: (1) apparent zeta potentials for both silica and MS2; (2) apparent zeta potentials for silica and outer surface potentials for MS2 particles. The calculated DLVO interaction energy profiles between MS2 and bare silica surface using zeta potential show a significant energy barrier up to 40 mM, ranging from 23 kT at 1 mM to 2 kT at 40 mM. This energy barrier vanishes at 60 mM IS. The energy barrier disappears at 20 mM when the energy profiles are calculated using zeta potential for solid silica and outer surface potential for soft MS2 (Figure 7S in the SI). As shown in Figure 3b, the critical deposition concentration (CDC) for MS2 on silica is 60 mM. Thus, the energy barrier profiles that use the zeta potential for MS2 are better for predicting CDC for “soft” MS2 on bare silica than those that use the outer surface potential of MS2. For deposition of soft particles on soft surface (MS2 on SRNOM-coated silica), interaction energy profiles were calculated using both zeta potentials and outer surface potentials for the particles and substrate as shown in SI Table 1S. The IS at which the calculated energy barriers vanished differed significantly depending upon whether the zeta potential or outer surface potential for MS2 particles or SRNOM-coated silica was used. The three IS values at which the energy barriers vanish are: (1) 20 mM IS when using outer surface potential for MS2 and zeta potential or outer surface potential for SRNOM-coated silica; (2) 40 mM IS when using zeta potential for MS2 and outer surface potential for SRNOM-coated silica; (3) 60 mM IS when using zeta potential for MS2 and SRNOM-coated silica. The fact that the energy barrier vanishes at 20 mM implies that MS2 should have higher attachment efficiency at this IS. In contrast, no measurable deposition was detected at 20 mM IS by the QCM system used in our experiment. Use of the outer surface potential for soft particles and soft surface film leads to overprediction of CDC for the MS2-SRNOM system. As shown in Figure 3b, the CDC for MS2 is found at 60 mM IS. This observation is consistent with energy profiles calculated using zeta potential for MS2 and SRNOM-coated silica or MS2 and silica. Above 60 mM IS, DLVO energy profiles show no energy barrier and predict maximum value for an attachment coefficient of 1.0. Measured attachment efficiencies for MS2 on bare silica or SRNOM-coated silica are approximately 0.2. This observation suggests that the DLVO model overpredicts deposition rate of MS2 particles onto either bare or SRNOM-coated silica surface, and therefore, non-DLVO interactions should be considered. Similar observations have been reported for MS2 deposition on quartz sand (11). The slope of the stability curve for IS from 20 to 60 mM for MS2 deposition on SRNOM is 4 × 10-7. The DLVO model would predict a much higher slope of the stability curve and smaller deposition rate at low IS than observed data. This discrepancy is likely a result of charge heterogeneity for both SRNOM substrate and MS2 particles. As a porous selfassembly of polypeptide chains (41), the MS2 surface has heterogeneous charge distribution. Specifically, glutamic acid, aspartic acid, and lysine are on the surface of MS2. At pH 6, glutamic acid and aspartic acid are negatively charged, 7632
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whereas lysine is positively charged (11, 32). SRNOM should also be considered as a substrate with heterogeneous charge distribution (42). In addition to electrostatic interactions, MS2 surface proteins are likely to give rise to steric repulsion when MS2 particles approach bare or SRNOM-coated silica surface because of FG loops connecting two R helices and two β sheets (SI Figure 8S). While MS2 virus does not show detectable swelling at low IS (43), the FG loops have extended and flexible conformation (41). Repulsive steric interaction due to these FG loops has been suggested in ref 11. to explain lower than expected MS2 deposition on quartz sand. The conformation of adsorbed NOM on membrane surface was studied by Hong and Elimelech (44), who suggested that at high IS, the adsorbed layer of NOM has a more compact conformation than at low IS. In our case, we suggest that above 60 mM IS, the adsorbed layer of SRNOM has a more compact conformation than below 60 mM IS. As a result, steric repulsion between MS2 and a compact layer of SRNOM is similar to that between MS2 and flat silica surface. Consequently, similar steric repulsion leads to the same deposition as observed (Figure 3b). In contrast to high IS, below 60 mM IS, SRNOM surface has more extended and less compact conformation. Steric repulsion between MS2 and this SRNOM layer surface is greater than that between MS2 and flat silica surface. As a result, more deposition was observed on flat silica surface. We conclude that electrostatic repulsion and steric repulsion control MS2 deposition onto either bare silica surface or SRNOM-coated silica surface due to the change in conformation of SRNOM surface and MS2 capsid.
Acknowledgments Baoling Yuan is supported by a postdoctoral scholarship provided by the China Scholarship Council. We thank Professor Menachem Elimelech for discussion and Marty Page and Leonardo Gutierrez for helping with MS2 sample preparation. Deposition experiments were conducted by Baoling Yuan MS2 sample preparation and characterization were performed by Mai Pham. Thanh H. Nguyen assisted with experimental planning, data interpretation, and manuscript preparation. This work was supported by the WaterCAMPWS, a Science and Technology Center of Advanced Materials for the Purification of Water with Systems under National Science Foundation agreement number CTS0120978.
Supporting Information Available QCM setup (Figure 1S); Frequency shift divided by the third harmonic number (∆f3) and associated dissipation shifts (∆D3) as a function of time for the MS2 deposition experiment on SRNOM and PLL at different IS (Figure 2S-6S); DLVO energy profiles calculated for MS2 deposition on silica and silica coated with SRNOM (Figure 6S); MS2 protein capsid based on structure determined by X-ray and 3-D structure of a single unit of MS2 capsid (Figure 8S); Energy barrier in unit kT for the deposition of MS2 on clean or SRNOM-coated silica surface (Table 1S); SRNOM solution preparation, MS2 preparation, and protocol for bead coating. This material is available free of charge via the Internet at http://pubs.acs.org.
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