Grantee Research Project Results
Final Report: Agglomeration, Retention, and Transport Behavior of Manufactured Nanoparticles in Variably-Saturated Porous Media
EPA Grant Number: R833318Title: Agglomeration, Retention, and Transport Behavior of Manufactured Nanoparticles in Variably-Saturated Porous Media
Investigators: Jin, Yan , Xiao, John
Institution: University of Delaware
EPA Project Officer: Aja, Hayley
Project Period: March 1, 2007 through February 28, 2011
Project Amount: $399,035
RFA: Exploratory Research: Nanotechnology Research Grants Investigating Environmental and Human Health Effects of Manufactured Nanomaterials: a Joint Research Solicitation-EPA, NSF, NIOSH, NIEHS (2006) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Nanotechnology , Safer Chemicals
Objective:
The production of significant and increasing quantities of synthetic nanomaterials and our very limited knowledge on their potential environmental and health effects have caused increasing public concerns. The overall objective of the project is to develop an understanding of the fate of nanoparticles released into the subsurface environments. We hypothesize that nanoparticles are likely to be mobile and have the potential to contaminate water resources either as contaminants themselves or by facilitating the transport of other toxic substances. We propose to conduct a comprehensive study to systematically investigate the major processes that control the movement of nanoparticles (NPs) in the subsurface under environmentally relevant conditions. Our specific objectives are to (1) determine agglomeration behavior of nanoparticles under different solution chemistry (pH, ionic strength, and presence of dissolved humic material), (2) measure mobility of nanoparticles in model porous media under both saturated and unsaturated flow conditions; and (3) experimentally elucidate the attachment and retention mechanisms of nanoparticles at various interfaces at the pore scale.
Summary/Accomplishments (Outputs/Outcomes):
- During the first year of this project, we focused on evaluating the agglomeration potential of magnetite nanoparticles (NPs) in batch experiments, in line with the project objective #1.
Magnetite particles were synthesized using a co-precipitation method with 0.5 M FeCl3•6H2O and 1 M FeCl2•4H2O followed by addition of 0.7 M NH4OH. The precipitated particles were washed with DI water and stabilized using surfactant TMAH. The average size of the resulting magnetite NPs is 58.0±0.3 nm, measured by dynamic light scattering (DLS). The structure and morphology of the particles were examined using XRD and TEM. These particles were used in batch experiments (year 1) and column experiments (year 2).
Through a series of aggregation kinetic experiments by DLS, we demonstrated that the presence of humic acid (HA) can both stabilize and destabilize NP suspensions by altering the NP surface charge status. Zeta potential measurements showed that the presence of HA led to a notable reduction of the PZC value of the magnetite NPs by either neutralizing or increasing, depending on solution pH and the loading of HA, the negative surface charge of magnetite NPs. The critical concentration of HA for complete particle surface charge neutralization was found to be around 3 - 4 mg L−1 for magnetite NPs. The observed aggregation behavior can be explained clearly via the analysis of NP surface charge status. Kinetic study on NP stability, showing a 36% decrease in attachment efficiency in the presence of 2 mg L−1 HA and increasing the critical coagulation concentration (CCC) values with the increase HA loading, further quantified the stabilization potential of HA as a function of ionic strength. DLVO interaction energy calculations, using experimentally determined values of the Hamaker constant, provided consistent support to the experimental results. The concentration effect of HA was further proved via the observed changing of secondary minimum and energy barrier in DLVO profiles with the increase of HA introduction.
Real-time AFM observations indicated that HA acted similarly to surfactant molecules where they could form micelles while in solution alone and then were absorbed onto magnetite surfaces upon mixing with the NP suspension as coatings, resulting in changes in particle surface charge hence their suspension stability. Our results clearly showed that as natural organic matter (NOM), HA, even at a low concentration, can have a great impact on the stability and aggregation/deaggregation behavior of engineered magnetite NPs by changing particle surface charge status. These results have important implications on estimating the engineered magnetite NPs environmental hazards and understanding NPs environmental remediation functions in natural subsurface environments where NOM is in general abundant.
- During the second year of this project, we measured magnetite mobility in model porous media (e.g., sand) under different solution chemistry (pH and ionic strength) and at different HA concentrations, in line with project objective #2.
We conducted a large number of column experiments using the same magnetite NPs as those used in the batch experiments. For all experiments, magnetite NP concentration of 30 mg/L was used. Elliott soil humic acid (HA) standard (1S102H, International Humic Substances Society (IHSS)) was used to represent natural organic matter, which was filtered through 0.45 μm nylon membrane filters, and the pH of the filtrate was adjusted to 6.7. Zeta potential of magnetite NP suspensions at pH 5 and ionic strength 1 mM, at pH 10 and ionic strength 1, 5, 10, 15, 25 mM, and with 2, 5, 10 ppm HA were measured using a ZetaSizer Nano. Quartz sand (Accusand 40/60, Unimin Corporation, Le Sueur, MN) with diameters of 300-355 μm and an average surface zeta potential of -59.95 mV was used. The column was 10-cm long and 3.8-cm inside diameter and wet-packed. Breakthrough curves and particle depth-distribution profiles were constructed. Collision efficiencies were calculated to quantify NP retention. The major findings of the column experiments are given below.
Effect of pH. Effect of pH was evaluated at pH 5 and 10. At pH 5 magnetite particles and the sand grains possess oppositely-charged surfaces so that attachment occurs under favorable conditions. As a result, there was < 10% retention of the input magnetite particles due to strong electrostatic attraction. On the other hand, the particles and sand grains are like-3 charged at pH 10 leading to unfavorable deposition conditions; therefore, minimal retention was observed due to electrostatic repulsion.
Effect of ionic strength. Effect of ionic strength was examined at pH 10 (i.e., unfavorable attachment condition). As expected, increasing solution ionic strength increased magnetite particle retention hence decreasing transport. This was likely caused by two reasons: 1) zeta potential of magnetite particles becomes less negative as ionic strength increases reducing repulsion; and 2) agglomeration of magnetite particles increases as ionic strength increases enhancing removal by straining. Evaluation by the DLVO theory indicates that the increased magnetite removal at higher ionic strength can be attributed to increased retention in both primary and secondary energy minima.
Effect of HA. Addition of HA modifies the surface charge status of magnetite particles, and therefore, their aggregation and transport behavior. At pH 5 (favorable condition for attachment), we found that 2 ppm HA was not enough to change the nature of interaction between magnetite particles and sand; however, 10 ppm HA was sufficient to reverse the net particle charge and thus enhanced their transport considerably as compared to without HA addition or at low (2 ppm) HA concentration. At pH 10 (unfavorable condition for attachment), on the other hand, HA addition at both 2 and 5 ppm decreased magnetite retention and increased transport at any given ionic strength. In the presence of HA, increased magnetite retention at higher ionic strength was mostly due to retention in the primary minimum. While the overall transport of magnetite particles showed an increasing trend with the addition of HA, retention by straining was found to increase when HA concentration was high. Therefore, effects of HA on magnetite NP behavior are complex and can vary depending on properties of the NP, solution chemistry, as well as HA concentration. Given the complex nature of natural organic matter in the subsurface, these effects are expected to be even less predictable.
- We additionally conducted experiments with magnetite NPs to examine their potential to be taken up by plants. These experiments are beyond the scope of this project. However, as plants are an important component of the environmental and ecological systems, this study addressed another potentially important pathway of nanoparticles in the environment. We demonstrated significant uptake of magnetite (Fe3O4) NPs by pumpkin plants and their subsequent translocation and accumulation in various tissues. To the best of our knowledge, this was the first study to show that manufactured NPs can be taken up by plants as well as undergo translocation and accumulation within plant tissues. This work was published in the Journal of Environmental Monitoring as the cover article in 2008, and was reported in the Highlights in Chemical Biology (May 29, 2008).
Pumpkin (Cucurbita maxima) was selected as a model plant because of its large water uptake capacity, and was grown hydroponically in a growth medium and harvested after 20-days of growth, and tissues were analyzed for particle concentration. The Fe3O4 NPs were used because of their magnetic properties, which allowed non-intrusive tracking and quantification of the particles via magnetometry measurement with a vibrating sample magnetometer (VSM, LakeShore 7400). Results show that a significant amount of Fe3O4 particles suspended in a liquid medium can be taken up by pumpkin plants and be translocated throughout the plant tissues. Particles tend to accumulate near the roots as well as in leaves. Although this study likely represents a worst-case scenario (plants grown in a liquid medium containing high particle concentrations), it nevertheless provides convincing evidence that plant uptake is a potential transport pathway of nanoparticles in the environment.
- During the next phase of the study, we systematically examined the effects of particle concentration and size on the retention and transport of silica NPs in saturated and unsaturated porous media. This work falls under and goes beyond the extent of project objective #2.
Column experiments of NPs with particle sizes of 8 nm and 52 nm were conducted under both saturated and unsaturated flow conditions. We selected silica particles because they tend to be more stable in suspension so that their sizes remained constant during the experiments, allowing better evaluation of size effect. Experiments were run in model sand media with a mean diameter of 220 µm in NaCl background solution at different solution ionic strength (IS) and pH 10. Experiments followed a three-phase procedure including particle deposition, elution with background solution to flush out particles in pore water, and elution with DI 5 water to detach particles retained in secondary energy minima. Silica concentrations of effluent samples were measured by inductively coupled plasma-optical emission spectrometer (ICP-OES, Varian VISTA-MPX). The measured concentrations were corrected for background silica concentrations in controls (i.e., effluent samples of background buffer before introduction of silica NPs) and used to construct breakthrough curves. The results then were used to compute colloid attachment efficiency and surface coverage.
The major findings from these experiments are summarized as the following. At higher particle input concentration, both relative retention (C/C0, effluent concentration/influent concentration) and attachment efficiency were lower, and surface coverage was greater. On the other hand, at a given concentration, smaller NPs resulted in higher relative retention, greater attachment efficiency, and lower surface coverage. In addition, the results show that deposition was a kinetic process; deposition rate was higher at the beginning of the experiments and lower thereafter, suggesting that the number of attachment sites on the sand surface was limited. During the slow deposition stage, surface coverage increased more significantly at higher IS, indicating enhanced particle-particle interaction between the suspended NPs and with the previously attached at elevated IS.
We found that elution with DI water almost completely released the 8-nm particles deposited in phase 1, which is contrary to the prediction by the DLVO theory that deposition in primary energy minima is irreversible. This result suggests that the DLVO theory is not applicable for describing the retention and release behavior of the very small silica NPs. On the other hand, we found that the behavior of the 52-nm silica particles was consistent with that predicted by the DLVO theory. These findings clearly indicate the need for an improved understanding of the environmental characteristics/behavior of NPs at the lower size range (e.g., < 20 – 30 nm).
No significant differences were found between results from the experiments conducted under unsaturated water flow conditions and the results from saturated columns. For larger colloids or those that are hydrophobic, more retention in unsaturated media has been found. The apparent ‘discrepancy’ observed with the silica NPs was due to their very small size or the hydrophilic property, which renders mechanisms such as film straining or interactions with air-water interface much less effective in their retention.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 15 publications | 6 publications in selected types | All 6 journal articles |
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Bi C, Pan L, Xu M, Yin J, Qin L, Liu J, Zhu H, Xiao JQ. Synthesis and characterization of Co-doped wurtzite ZnS nanocrystals. Materials Chemistry and Physics 2009;116(2-3):363-367. |
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Bi C, Pan L, Xu M, Yin J, Guo Z, Qin L, Zhu H, Xiao JQ. Raman spectroscopy of Co-doped wurtzite ZnS nanocrystals. Chemical Physics Letters 2009;481(4-6):220-223. |
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Hu J-D, Zevi Y, Kuo X-M, Xiao J, Wang X-J, Jin Y. Effect of dissolved organic matter on the stability of magnetite nanoparticles under different pH and ionic strength conditions. Science of the Total Environment 2010;408(16):3477-3489. |
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Shen C, Li B, Wang C, Huang Y, Jin Y. Surface roughness effect on deposition of nano- and micro-sized colloids in saturated columns at different solution ionic strengths. Vadose Zone Journal 2011;10(3):1071-1081. |
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Wang C, Bobba AD, Attinti R, Shen C, Lazouskaya V, Wang L-P, Jin Y. Retention and transport of silica nanoparticles in saturated porous media: effect of concentration and particle size. Environmental Science & Technology 2012;46(13):7151-7158. |
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Zhu H, Han J, Xiao JQ, Jin Y. Uptake, translocation, and accumulation of manufactured iron oxide nanoparticles by pumpkin plants. Journal of Environmental Monitoring 2008;10(6):713-717. |
R833318 (2009) R833318 (Final) |
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Supplemental Keywords:
Silica nanoparticles, concentration effects, size effects, DLVO theory,, RFA, Health, Sustainable Industry/Business, Sustainable Environment, Risk Assessments, Technology for Sustainable Environment, contaminated sediments, ecological risk assessment, fate and transport, bioavailability, nanotechnology, manufactured nanomaterials, human exposure, nanomaterials, groundwater contaminationProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.