The influence of natural organic matter on the toxicity of multiwalled carbon nanotubes

Abstract

Engineered carbon nanostructures, such as multiwalled carbon nanotubes (MWNTs), are inherently hydrophobic and are not readily stable in aqueous media. However, the aqueous stability and bioavailability of these nanotubes may be influenced by the water quality parameters such as ionic strength, pH, and natural organic matter (NOM). Natural organic matter adsorbs onto the surface of MWNTs, effectively covering the hydrophobic surface and resulting in increased aqueous stability. This enhanced stability is likely to lead to an increased residence time in the water column and increased exposure times for pelagic organisms. In the current study, NOM from three different river systems in the southeast United States increased the stability of MWNT suspensions. The effects of these suspensions were evaluated using acute and chronic bioassays with Daphnia magna and Ceriodaphnia dubia. The 96-h LC50 for D. magna exposed to MWNTs suspended in Suwannee River (USA) NOM was approximately 2.0 mg/L and was not significantly influenced by NOM concentrations ranging from 1.79 to 18.5 mg/L DOC. However, there were differences in 96-h LC50 values among different sources of NOM (Suwannee, Black, and Edisto Rivers, USA). Daphnid growth was reduced in both D. magna and C. dubia, whereas reproduction was reduced in C. dubia. Characterization of the different NOM sources and MWNT suspensions was conducted. Visual inspection using transmission electron microscopy (TEM) and gut elimination observations suggested that the toxicity was attributable to ingested MWNTs clogging the gut tract of D. magna. The TEM micrographs indicated that MWNTs can disaggregate within the gut tract, but single MWNTs are unable to absorb across the gut lumen.

Methodological considerations for testing the ecotoxicity of carbon nanotubes and fullerenes: Review

“…The role that these surface coatings have in CNP toxicity is not yet well understood, but limited numbers of studies have investigated this topic using Daphnia [69,75]. Multiwalled carbon nanotubes wrapped with different types of NOM possessed significantly different LC50 values [69].…”

“…The role that these surface coatings have in CNP toxicity is not yet well understood, but limited numbers of studies have investigated this topic using Daphnia [69,75]. Multiwalled carbon nanotubes wrapped with different types of NOM possessed significantly different LC50 values [69]. Grafting PEI coatings onto MWNTs to give the CNTs positive, negative, or neutral surface charges increased the toxicity of the MWNTs compared with unmodified nanotubes, but the toxicity did not show a clear trend with regard to the surface charge [75].…”

“…However, chemical derivatization using fluorine containing tags/molecules combined with XPS is a far more accurate approach for identification of specific functional groups rather than deconvoluting the C(1s) spectra given the close proximity of different functional groups, the limited resolution of the spectra, and the possibility for different fitting combinations to give equally good results [67,68]. Fourier transform infrared spectroscopy has also been used to analyze the functional groups of CNTs [69], but this approach rarely provides quantitative functional group concentrations, and it may be challenging to differentiate between small peaks and the background [68]. For additional information about characterization of CNTs surfaces, see the recent review article by Wepasnick et al [68].…”

Paradigms to assess the environmental impact of manufactured nanomaterials

“…These include nanomaterial measurement and characterization, running appropriate controls, and assessing the appropriate endpoints. For example, several investigators have concluded that the toxicity of carbon nanotubes was not due to some biochemical interaction; rather, the toxicity was due to clogging the intestinal tract and interfering with food processing and assimilation [16]. The evolution of ecotoxicity testing methods can be viewed as a refinement in the current toolbox of methods.…”

Cell Rescue by Nanosequestration: Reduced Cytotoxicity of An Environmental Remediation Residue, Mg(OH)2 Nanoflake/Cr(VI) Adduct

“…For example, the adsorption of natural organic matter to multiwalled carbon nanotubes increases the aqueous stability of hydrophobic nanotubes and toxicity of the adducts to daphnia. 22 The tributyltin/nano-TiO 2 adduct is more toxic to abalone embryos than tributyltin alone. 23 Similar synergistic toxicities to aquatic organisms have also been found for As, Pb, Cu, and Cd in the presence of nano-Al 2 O 3 , nanoFe 2 O 3 , and nano-TiO 2 .…”