Behavior of Ag nanoparticles in soil: Effects of particle surface coating, aging and sewage sludge amendment

Highlights

• Silver nanoparticle coating affects fate in unamended soils.

• Citrated coated silver nanoparticles could be found in pore water for up to six months.

• Pre-incubation of silver nanoparticles in sewage sludge negated effects of surface coating.

• Weathered or reprecipitated particles found in pore water for up to two months in sludge amended soils.

• Particle surface coating, sewage sludge amendment and aging all have important impacts.

Abstract

This study addressed the relative importance of particle coating, sewage sludge amendment, and aging on aggregation and dissolution of manufactured Ag nanoparticles (Ag MNPs) in soil pore water. Ag MNPs with citrate (CIT) or polyvinylpyrrolidone (PVP) coatings were incubated with soil or municipal sewage sludge which was then amended to soil (1% or 3% sludge (w/w)). Pore waters were extracted after 1 week and 2 and 6 months and analyzed for chemical speciation, aggregation state and dissolution. Ag MNP coating had profound effects on aggregation state and partitioning to pore water in the absence of sewage sludge, but pre-incubation with sewage sludge negated these effects. This suggests that Ag MNP coating does not need to be taken into account to understand fate of AgMNPs applied to soil through biosolids amendment. Aging of soil also had profound effects that depended on Ag MNP coating and sludge amendment.

Introduction

The growth of nanotechnology has raised public concern about potential environmental and human health effects of manufactured nanoparticles (MNPs) released to the environment (Helland et al., 2006, Wiesner et al., 2006). Production of consumer products containing MNPs continues to increase despite the lack of sufficient knowledge concerning how they may affect the environment (Luoma, 2008, Project on Emerging Nanotechnologies, 2008). The lack of detection and in situ characterization capabilities for nanomaterials in complex biological and environmental matrices also hinders the development of regulations for MNPs in the environment (von der Kammer et al., 2012, Weinberg et al., 2011). One major class of MNPs currently being used is Ag MNPs, due to their antimicrobial properties (Levard et al., 2012). Ag MNP containing products, such as paint and textiles have already been shown to release Ag MNPs through normal use (Benn and Westerhoff, 2008, Kaegi et al., 2010).

In some instances particles may directly enter terrestrial environments as they are shed from Ag MNP containing products such as paints (Kaegi et al., 2010). However, a large fraction of Ag MNPs are predicted to enter wastewater treatment plants (WWTP) via sewage streams where they are likely to efficiently partition to the sewage sludge and be sulfidized (Gottschalk et al., 2009, Kaegi et al., 2011, Kim et al., 2012, Lombi et al., 2013). In the United States and elsewhere, the majority (about 60% in the US) of sewage sludge is applied to agricultural lands as biosolids (EPA, 1995). Because of this, we expect agricultural soils to be a major repository for MNPs and a source of Ag MNPs to aquatic environments through erosion and runoff. It has been shown that aggregation and dissolution behavior of Ag MNPs can have important implications for environmental fate and toxicity (Bone et al., 2012, Unrine et al., 2012).

Ag MNP behavior in soil has not been widely investigated (Coutris et al., 2012, Sagee et al., 2012), in part due to difficulties associated with tracking MNPs in the complex soil matrix (Blaser et al., 2007, Kaegi et al., 2011). Conclusions drawn from aquatic studies may have limited relevance to terrestrial systems. Behavior of MNPs in soil may differ from aquatic systems due to the unique biological, physical and chemical characteristics of soil that differ from aqueous systems. These characteristics vary widely among soil types and through time and space. For example, aggregation of the particles onto the solid phase of the soil is a complicating factor that cannot be taken into account in simple aqueous studies. To our knowledge, few previous studies have been published that attempt to characterize Ag MNP aggregation and dissolution behavior in soil pore water. A recent study investigated Ag MNP dissolution in soil (Cornelis et al., 2012); however, this study did not investigate the role of particle coating, aging or sludge amendment.

Changes in Ag MNP behavior due to modification by the manufacturer (e.g., surface coating) or transformations in the environment via contact with naturally occurring minerals or organic matter (NOM), as well as other ligands such as sulfide or chloride further increases difficulties associated with assessing the risk of MNPs to human health and the environment. Differences in surface coating alone can affect Ag MNP aggregation and dissolution behavior under differing environmental conditions (Bone et al., 2012, El Badawy et al., 2010, Kittler et al., 2010, MacCuspie, 2011, Unrine et al., 2012). Likewise, environmental constituents such as NOM have been shown to promote particle stability for both MNPs and naturally occurring particles (King and Jarvie, 2012), in some cases by coating the particle surface (Bertsch and Seaman, 1999, Fabrega et al., 2009, Hotze et al., 2010, Zhang et al., 2009). In wastewater treatment plants, Ag MNPs are sulfidized which significantly reduces Ag solubility and results in decreased toxicity (Hirsch, 1998, Kaegi et al., 2011, Kim et al., 2010, Ratte, 1999, Reinsch et al., 2012). However, little is known of Ag MNP behavior following application of sewage sludge to agricultural soils including the influence of sulfidation and surface coating.

The objective of this study was to determine the aggregation and dissolution behavior of Ag MNPs in soil pore water as a function of surface chemistry, sewage sludge biosolids pre-incubation and amendment rate as well as soil aging time. To observe effects of Ag MNP surface coating and biosolids pre-incubation, we aged soils containing Ag MNPs having different surface coatings and under controlled laboratory conditions with various incubation times between 1 week and 6 months. The Ag MNPs were introduced to the soil either directly or through amendment of sewage sludge containing the Ag MNPs which were pre-incubated for 1 week. Previous studies have shown rapid transformation (within minutes) of AgNPs both when incubated with processed biosolids (Colman et al., 2013) or in pilot wastewater treatment plants (Lombi et al., 2013). We expected differences in surface coating to result in dissimilar Ag MNP behavior, with sterically stabilized polyvinylpyrrolidone (PVP) Ag MNPs being more stable against aggregation than the low molecular weight organic acid (citrate, CIT) coated particles which would be subject to removal of coating through desorption as well as screening of surface charge by divalent cations in soil solution. Further, we expected sulfidation to negate the effects of manufactured coating (Levard et al., 2011, Lombi et al., 2013) within pore waters and aging to yield a decline in pore water Ag because of the very low solubility of Ag₂S. We adapted techniques developed and used in our previous research to track the chemical and physical changes of Ag MNPs in the soils including asymmetrical flow field-flow fractionation (AF4) (Unrine et al., 2012) and extended X-ray absorption fine structure spectroscopy (EXAFS) (Shoults-Wilson et al., 2011a, Shoults-Wilson et al., 2011b, Unrine et al., 2012). We extracted pore water from soils and analyzed them using AF4 coupled to static/dynamic multi-angle laser light scattering (MALLS/DLS) and an inductively coupled plasma mass spectrometer (ICP-MS) to assess Ag particle size distribution. Ultracentrifugation followed by ICP-MS analysis was also used to assess the proportion of dissolved Ag in pore waters. EXAFS of soil solids and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) of AF4 fractions were used to examine chemical speciation and physical form of Ag in soils and soil pore water, respectively.