Limited transport of functionalized multi-walled carbon nanotubes in two natural soils

Highlights

• Investigation of undisturbed soil columns and lysimeter.

• Transport experiments under water-unsaturated conditions.

• Retention profiles were measured and numerically modeled.

• Complete retention of MWCNT in undisturbed and repacked soil columns.

Abstract

Column experiments were conducted in undisturbed and in repacked soil columns at water contents close to saturation (85–96%) to investigate the transport and retention of functionalized ¹⁴C-labeled multi-walled carbon nanotubes (MWCNT) in two natural soils. Additionally, a field lysimeter experiment was performed to provide long-term information at a larger scale. In all experiments, no breakthrough of MWCNTs was detectable and more than 85% of the applied radioactivity was recovered in the soil profiles. The retention profiles exhibited a hyper-exponential shape with greater retention near the column or lysimeter inlet and were successfully simulated using a numerical model that accounted for depth-dependent retention. In conclusion, results indicated that the soils acted as a strong sink for MWCNTs. Little transport of MWCNTs is therefore likely to occur in the vadose zone, and this implies limited potential for groundwater contamination in the investigated soils.

Introduction

Carbon nanotubes (CNT) are tubular nanoparticles with nano-scale diameters and micro-scale lengths composed of aligned benzene rings (Iijima, 1991; Mauter and Elimelech, 2008; Petersen et al., 2011). Two types of CNT are most commonly distinguished and produced: single-walled carbon nanotubes (SWCNT) are individual graphene tubes, and multi-walled carbon nanotubes (MWCNT) are tubes within tubes consisting of more than two carbon walls (Sinnott, 2002). Due to their exceptional electrical, chemical, and physical properties, CNT are used in numerous applications (Jaisi and Elimelech, 2009; Mattison et al., 2011). This widespread use will result in their release to the environment through point sources (e.g., production facilities, landfills, or wastewater treatment plants), nonpoint sources (e.g., abrasion of materials containing CNT), accidental release (e.g., during transport), or intentional release (e.g., for groundwater remediation) (Jaisi and Elimelech, 2009; Köhler et al., 2008; Nowack and Bucheli, 2007; Pan and Xing, 2012).

To date, there is still a lack of knowledge on the fate and effects of CNT in the environment (Saleh et al., 2008). Ecotoxicological studies on CNT have revealed potential for bioaccumulation (e.g., daphnia magna) (Petersen et al., 2009a), health risks to different organisms (e.g., rainbow trout and rats) (Farré et al., 2009; Handy et al., 2008; Lam et al., 2006; Ma-Hock et al., 2009), and growth inhibition of algae due to shading and agglomeration of cells with CNT (Schwab et al., 2011). Furthermore, CNT act as strong adsorbents for organic pollutants (Chen et al., 2007; Li et al., 2012; Lu et al., 2005; Peng et al., 2003) and may therefore affect the fate and mobility of these chemicals in natural environments. A CNT facilitated transport (co-transport) of organic pollutants could lead to enhanced migration of contaminants (Cheng et al., 2005) but adsorption of organic pollutants onto CNT may also decrease bioavailability (Petersen et al., 2009b). To assess potential risks to humans and other organisms, it is important to gain knowledge on the environmental fate of CNT. Thus, information on CNT transport and deposition in natural soils is needed.

In general, CNT can be released as agglomerates or as individual particles. Recently, the release of free-standing individual CNTs from CNT-embedded nanocomposites was demonstrated (Schlagenhauf et al., 2012). The environmental behavior of nanoparticles is frequently controlled by their colloidal stability. Functionalized CNT are of special interest because the modification will not only increase their stability in aqueous suspensions but also increase their mobility in the environment (Mattison et al., 2011).

Several studies have reported on the transport of CNT in water-saturated sand columns (Jaisi et al., 2008; Liu et al., 2009; Mattison et al., 2011; Tian et al., 2010, 2012). Most of these studies have been conducted in repacked, homogeneous, coarse textured porous media in order to understand mechanisms and factors influencing CNT retention. Results indicate that CNT transport is sensitive to a diversity of experimental conditions including ionic strength (IS), pore water velocity, and collector grain size. However, mechanisms influencing the transport and deposition of MWCNT are still not completely understood even in these highly idealized systems (Mattison et al., 2011). Information on MWCNT retention profiles is still very scarce (Kasel et al., 2013; Wang et al., 2012). Determination of retention profiles provides mass balance information and gives useful insight on mechanisms controlling retention (Bradford and Bettahar, 2005).

The transport of CNT in field soils has received very limited attention (Jaisi and Elimelech, 2009). In general, soil is expected to be a more important sink for nanoparticles than sand because its chemical composition and pore size distribution are more heterogeneous (Pan and Xing, 2012). However, preferential flow paths in soils (e.g., root or earthworm channels) may enhance transport through the vadoze zone (Camobreco et al., 1996). Undisturbed soil cores and lysimeters are much closer to environmental conditions than packed sand columns because they allow consideration of preferential flow paths. To our knowledge, there is no study available on the transport and retention of MWCNT in undisturbed soil cores or lysimeters. Furthermore, there are only a few studies on CNT transport under unsaturated flow conditions (Tian et al., 2011). Unsaturated and/or variably saturated transport experiments are closer to natural conditions in the vadose zone. Transport processes are much more complicated under unsaturated than saturated conditions because of the presence of the air phase, although greater retention is expected under unsaturated conditions (Gargiulo et al., 2007b; Tian et al., 2011).

The objective of this research was to investigate transport and retention of MWCNT in natural, undisturbed soils under environmentally relevant conditions like low MWCNT concentrations and low flow rates. Two soil types (a silty loam and a loamy sand) were investigated in laboratory column experiments at water contents close to saturation. Additionally, the nanoparticles were applied to a field lysimeter filled with the loamy sand soil. For all experiments, MWCNT concentrations in the effluent and in the soils were determined. Finally, the observed data was numerically modeled. To our knowledge, this is the first study providing information on MWCNT fate in undisturbed soil and in a lysimeter.