Transport and deposition of stabilized engineered silver nanoparticles in water saturated loamy sand and silty loam
Introduction
As production and application of engineered nanoparticles (ENPs) are growing and the potential of their release into the environment is becoming more evident, the question of ENP mobility and fate in the environment is of increasing significance (Klaine et al., 2012). Regarding the safety of drinking water resources, it is crucial to understand transport and deposition processes of ENP in soils and aquifers. In contaminant transport studies ENPs represent a special case because the behavior of particles in the environment differs from that of conventional contaminants that usually occur in dissolved form. According to particle filtration theory, transport and deposition processes of colloids and nanoparticles in porous media are dominated by physicochemical interactions between the particle surfaces and the porous media (collector) surfaces on the one hand, but also to hydrodynamic processes on the other hand (Yao et al., 1971, Torkzaban et al., 2007).
Studies with ENP and natural colloids showed deviations of experimental observations from filtration theory, especially in environmentally relevant systems. For instance, under conditions that are generally considered as unfavorable for attachment, deposition may occur due to surface charge heterogeneities providing local favorable attachment sites (Lin et al., 2011). The occupation of these sites is eventually related to hydrodynamic drag forces allowing particles to travel along a collector surface until possibly reaching an attachment site (Bradford et al., 2011, Li et al., 2005). Moreover in natural porous media retention may occur due to mechanical straining in small pore throats and at locations of high surface roughness (Jaisi and Elimelech, 2009, Kasel et al., 2013a, Li et al., 2004, Wang et al., 2012). On the other hand, transport may be enhanced by stabilizing agents causing short range repulsive forces (El-Badawy et al., 2013, Xiao and Wiesner, 2013). The number of available attachment sites is subject to physicochemical factors and may change with the environmental conditions.
Silver nanoparticles (AgNPs) are the most commonly used nanomaterial and they are already widely applied in consumer products because of their strong antibacterial activity (WWICS, 2014). Because of the inevitable release of AgNPs into the environment (Geranio et al., 2009, Lorenz et al., 2012) and the high toxicity potential (Reidy et al., 2013) ENP environmental implication research has been particularly focused on AgNPs.
Studies with AgNPs in artificial porous media under well-defined conditions have demonstrated that AgNP transport is sensitive to physicochemical factors such as solution ionic strength (IS), presence of divalent cations, grain size and hydrodynamics as is generally suggested by filtration theory (Liang et al., 2013a, Lin et al., 2011, Taghavy et al., 2013, Thio et al., 2011). The behavior of AgNPs has been shown to also strongly depend on the application of stabilizing agents that eventually enhance the mobility due to increased repulsion between AgNPs and AgNPs and porous media surfaces (El-Badawy et al., 2013, Liang et al., 2013a, Thio et al., 2011). Additionally, Liang et al. (2013a) observed concurrent deposition of surfactants to quartz sand surfaces, resulting in nonmonotonic deposition profiles of AgNPs in sand columns.
Experimental studies on the transport behavior of AgNPs in natural porous media are scarce. Transport experiments in natural soils (Cornelis et al., 2013, Liang et al., 2013b, Sagee et al., 2012) and sandstones (Neukum et al., 2014a) showed that Ag nanoparticle transport is not only affected by the physicochemical parameters discussed above, but also by texture or grain size of the porous medium and the mineralogical composition. Heteroaggregation of AgNPs with natural colloids (Cornelis et al., 2012, Cornelis et al., 2013), and subsequent colloid facilitated transport of the AgNPs (Liang et al., 2013b, Neukum et al., 2014b) have been observed, demonstrating an important pathway for AgNP migration in soils and aquifers.
In addition to the scarcity of studies with natural porous media, many experimental studies lack the application of adequate analytical techniques, which do not only consider the chemical but also the physical form of the nanomaterials (von der Kammer et al., 2012). The distinction between particulate and dissolved species or the characterization of transport relevant parameters such as particle size distribution and aggregation state are needed in order to understand the processes that occur during the transport of nanoparticles. While in many studies the nanoparticle samples are thoroughly characterized prior to the experiments, their physical state is not monitored during or after the experiments. Exceptions are for example Lin et al. (2011) who monitored the particle size distribution of AgNPs in the effluent of transport experiments by dynamic light scattering and found no change in particle size during transport experiments or Taghavy et al. (2013) who separated particulate Ag from dissolved Ag by centrifugation to monitor the dissolution of AgNPs and found that dissolution did occur during transport through the porous media.
The purpose of this study was to gain more insight into the transport and retention processes of AgNPs in natural porous media. Column experiments were carried out with two soils, a loamy sand and a silty loam, and furthermore with glass beads as a reference medium to investigate the effect of different transport velocities and chemical compositions of the background solution on the transport behavior of AgNPs, which was then described with mathematical models. While in comparable studies with saturated porous media AgNP transport is investigated only at relatively high flow velocities that are not necessarily relevant for natural soil or aquifer systems (e.g. Cornelis et al., 2013 (8.14 m d− 1); Sagee et al., 2012 (9.5 m d− 1); Taghavy et al., 2013 (7.6 m d− 1)) in this study, a wider scope of expected groundwater velocities in comparable natural systems was covered ranging from 0.05 m d− 1 to 2.88 m d− 1. Another step towards more realistic conditions in soil would be the investigation under unsaturated conditions, which has partly been implemented for AgNPs by Liang et al. (2013b), who applied a water saturation of 90% in undisturbed loamy sand columns. Flow field-flow fractionation (FlFFF) was employed as analytical technique to characterize AgNP concentration and particle size distribution before and after the soil passage. This enabled not only the detection of changes in particle size distribution during transport through the soil but also to account only for particulate Ag in the breakthrough curves because dissolved silver is not detected in the FlFFF measurement. In the case of some selected experiments the depth profile of AgNP retention in the soil columns was also analyzed and modeled providing further insights into the respective transport and deposition processes.
Section snippets
Porous media and solution chemistry
Glass beads with mean diameters between 300 and 400 μm were used as inert reference medium. In order to remove metal oxides and obtain completely hydrophilic surfaces of the glass beads they were cleaned using a procedure that was modified by Han et al. (2006) after Mehra and Jackson (1958) (see supporting information). Two soils were sampled from the upper 30 cm of agricultural field sites in Germany. The soils have already been used for other publications (Unold et al., 2009, Kasel et al., 2013b
Characterization of the experimental system
The AgNP suspensions were very stable in terms of dissolution and aggregation. Visual examination of AgNPs diluted to a concentration of 60 mg l− 1 in solutions with electrolyte concentrations of up to 500 mM NaNO3 and 100 mM Ca(NO3)2, indicated no signs of aggregation or sedimentation over a time period of weeks. Also the measurement of our standard sample used for the quantitative calibration of FlFFF showed a stable particle size with a mean diameter of 54 nm and a standard deviation of 2 nm and
Conclusions
The transport and deposition behavior of stabilized AgNPs was examined in glass beads and in two natural soils, a loamy sand and a silty loam. In glass beads the transport of the AgNPs was characterized by time dependent blocking and by the formation of nonmonotonic RPs in the packed columns, which are probably related to concurrent deposition of the stabilizing agents and/or reversible deposition of the AgNPs due to secondary minimum interactions. The AgNPs showed a relatively high mobility in
Acknowledgments
This work was funded by the German Ministry of Education and Research (BMBF) within the WING/NanoNature program under contract 03X0077A. The authors are solely responsible for the content of this publication.
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