Sorption behavior of copper nanoparticles in soils compared to copper ions
Introduction
Nanoparticles (NPs) are characterized by at least one average dimension of < 100 nm and have special physical and chemical properties based on their size, distribution, morphology and phase (Christian et al., 2008, Nel et al., 2006). Therefore, NPs may differ considerably from their bulk counterparts resulting in different behaviors in environmental systems and organisms (Taylor and Walton, 1993). As a result of increasing industrial production, the release of engineered nanoparticles (ENPs) into the environment is highly probable (Biswas and Wu, 2005, Ma et al., 2010, Nel et al., 2006). Furthermore, many studies demonstrated adverse effects (toxicity) of NPs on plants and other organism (e.g. Karlsson et al., 2008, Midander et al., 2009, Mishra and Kumar, 2009, Nair et al., 2010, Navarro et al., 2008, Nowack and Bucheli, 2007). Such NP-related adverse effects are very complex and strongly depend on the physico-chemical characteristics and interrelation of these properties. Luyts et al. (2013) therefore linked specific properties of NPs separately to their toxicity. Regarding metallic NPs, additional toxicity may be caused by dissolution of metal ions from the particles or by involved redox-processes.
The present study focuses on copper oxide nanoparticles (CuO-NPs) which are widely used in industrial production (e.g. electrics, ceramics, films, polymers, inks, metallics, coatings) and have specific optical, electrical, and catalytic properties (e.g. Lee et al., 2008). Copper is an essential element for organisms but is toxic above a species-dependent tolerance limit. Several recent studies revealed differences in toxicity of copper salts or ions compared to nanoparticulate copper (Amorim and Scott-Fordsmand, 2012, Gomes et al., 2012, Griffitt et al., 2008, Meng et al., 2007). These studies also indicated that the negative effects were not fully caused by released Cu ions from the particles, but primarily induced nanoparticle-specific. Though dissolved metal ions may contribute to toxicity, more stable particles can accumulate and persist inside an organism (Midander et al., 2009). Nair et al. (2010) summarized the effects of metallic NPs (including CuO-NP) on plant growth and suggested that an aggregation/agglomeration of NPs may block pores and channels resulting in higher phytotoxicity of the metal ions which are more mobile within plants. Nevertheless, studies of Lee et al. (2008) and Stampoulis et al. (2009) indicated that comparatively high concentrations of copper nanoparticles are needed to cause visible effects on plant vitality where plant species was an important influencing factor.
However, there is a lack of knowledge about mobility and sorption behavior of metallic NPs in soils as crucial accumulation and transfer zone as well as potential source for NPs in ecosystems (Klaine et al., 2008, Ma et al., 2010). The main problems of investigating metallic NPs in the complex medium soil are the separation of natural NPs (colloids) from ENPs and the implementation of an appropriate experimental set-up (homogeneous mixing, prevention of aggregation, etc.). Additionally, there is still no information available on dissolution and transformation processes of metallic NPs after addition to a test medium which considerably will influence their fate and effects in the terrestrial environment (Klaine et al., 2008). In general, soils provide a large and reactive sink for substances with high surface reactivity. Strong sorption processes of metallic NPs in soils are therefore conceivable (Klaine et al., 2008). Several studies assessed the behavior of metallic NPs in porous or artificial soil media and demonstrated the dependency of ENPs-mobility from properties of the nanoparticles, test media and test conditions like solution pH and ionic strength (Christian et al., 2008, Fang et al., 2009, Jones and Su, 2012, Lecoanet et al., 2004, Nowack and Bucheli, 2007). However, porous media cannot sufficiently represent complex soil systems where sorption processes on heterogeneous soil constituents occur. Fang et al. (2011) examined the copper transport in soil columns in the presence and absence of TiO2-NPs and derived pedotransfer functions (Freundlich and Langmuir) to describe the sorption processes. Collins et al. (2012) demonstrated in a field study that Cu- and ZnO-NPs are not completely adsorbed to soil constituents but are mobile in agricultural soils. The sorption of metallic NPs on soil colloids (e.g. clay, organic matter, iron oxide, other minerals) or incorporation into such colloids may be of particular relevance for metal transport through soil profiles (Gilbert et al., 2009, Klaine et al., 2008).
In the presented study we examined the sorption behavior of copper oxide nanoparticles in comparison to copper ions in different soils by batch experiments. Batch experiments are a common method to assess the sorption behavior of heavy metals in soil where the effect of contaminant concentration on sorption processes can be included by conduction tests with varying spike concentrations (Krupka et al., 1999). Besides ionic strength and pH value of the batch solution, which affect the position of the sorption isotherm (Utermann et al., 2005) considerably, the size distribution (aggregation/agglomeration) will influence the results of the experiments with nanoparticles (Bian et al., 2011). However, it has to be considered that batch experiments provide the liquid/solid partitioning at equilibrium (KD value) but contain no information on behavior in flow conditions and on metal bonding forms (ion exchange, chemisorptions, bound to complexes and/or precipitates). The main questions in our study were: (1) are there differences in sorption behavior of Cu2 + ions and CuO-NPs? (2) Can sorption isotherms for both Cu2 + and CuO-NP describe sorption processes? (3) Which soil parameters influence Cu2 +/CuO-NP sorption?
Section snippets
Soil samples
In this study, six different top soils from agricultural managed sites in Hesse/Germany were analyzed. The sampling strategy and information about local characteristics are given in Zörner (2010). The main parameters for the selected soils are listed in Table 1. The dried (40 °C) soil samples were sieved for 2 mm and stored at room temperature. The particle size distribution was analyzed by Köhn-pipette procedure (German standard DIN 18123). Soil pH values were determined in 0.01 M CaCl2
Results and discussion
The background copper concentrations of the six test soils (Table 1) were within the typical range for arable soils in Germany (Scheffer et al., 2002: 18 mg kg− 1; Thiele and Leinweber, 2001: 12.3 mg kg− 1; Zörner, 2010: 10.0–36.3 mg kg− 1). The EDTA-extractable concentrations reached 22–33% of the total Cu contents indicating an elevated potential of mobilization.
Conclusion
The sorption behavior of Cu ions and CuO nanoparticles was considerably different in the tested soils. The results of batch experiments indicate much stronger sorption of CuO-NP to the soil compared to Cu2 +. For nanoparticles, the occupation of other bonding sites is suspected to lead to a predominantly specific bonding, but less unspecific in the observed Cu concentration range. The Cu2 + as well as the CuO-NP sorption could be sufficiently described by Freundlich isotherms. The Freundlich
Acknowledgment
Many thanks are due to Sascha Setzer and Heike Weller from the Institute of Landscape Ecology and Resources Management/Justus-Liebig University of Giessen for ICP-MS analysis of batch samples and extracts. We thank Dr. Michael Bunge from the Institute of Applied Microbiology/Justus-Liebig University of Giessen for conducting the Nanoparticle Tracking Analysis of our copper oxide nanoparticles.
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