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Environmental context: Carbon nanotubes are added to polymers such as polycarbonate and epoxy to form nanocomposites with enhanced material properties. Environmental factors including temperature, UV light exposure and pH have the potential to degrade these composites and increase the release of toxic polymer-associated chemicals. This study investigates how carbon nanotube loading decreases the release of known endocrine-disrupting compounds, bisphenol A and 4-tert-butylphenol, from polymer nanocomposites under simulated weathering environments. Nanoparticles such as carbon nanotubes are increasingly added to polymer matrices to improve tensile strength and electrical and thermal conductivity, and to reduce gas permeability. During use and after disposal, these plastic nanocomposites (PNCs) are degraded into microplastics by physical and chemical processes including mechanical abrasion, UV light exposure, hydrolysis and oxidation. Such polymers have the potential to enter aquatic environments and release potentially hazardous polymer-associated chemicals and transformation products. This work identifies and quantifies polymer-associated chemicals leached from polymers and nanocomposites during simulated environmental exposure. Epoxy and polycarbonate PNCs containing single-walled carbon nanotube (SWCNT) loadings ranging from 0 to 1 wt-% were exposed to water for 5 days, and the release of the chemicals bisphenol A (BPA) and 4- tert -butylphenol (TBP) was measured. The role of UV exposure, pH, temperature and natural organic matter in regulating chemical release was also investigated. Temperature, pH and UV light were found to be the most significant factors influencing release of TBP and BPA from PNCs. Additionally, increasing carbon nanotube loading in both polycarbonate and epoxy composites was found to decrease the release of these phenolic chemicals. A 0.3 % higher SWCNT loading decreased the release of BPA 45 ± 18 %, and a 1 % SWCNT loading decreased chemical release from epoxy by 48 ± 26 % for BPA and 58 ± 8 % for TBP. This information provides important data that can be used to help assess the risks posed by SWCNT polymer nanocomposites in aqueous environments, particularly as they age and are transformed. Environmental context. Carbon nanotubes are added to polymers such as polycarbonate and epoxy to form nanocomposites with enhanced material properties. Environmental factors including temperature, UV light exposure and pH have the potential to degrade these composites and increase the release of toxic polymer-associated chemicals. This study investigates how carbon nanotube loading decreases the release of known endocrine-disrupting compounds, bisphenol A and 4-tert-butylphenol, from polymer nanocomposites under simulated weathering environments. [ABSTRACT FROM AUTHOR]

In the present work, we have studied the flame-retardant properties of phosphorus-functionalized multi-walled carbon nanotubes (MWCNTs) created by treating oxidized MWCNTs (O-MWCNTs) with phosphoric acid. These phosphorus MWCNTs (P-MWCNTs), along with pristine MWCNTs and O-MWCNTs, were incorporated into polystyrene (PS) and poly (methyl methacrylate) (PMMA) by solution blending at lower concentrations (0.5-10 mass%) than conventional organophosphorus flame retardants (5-30 mass%). Thermal properties of the PS and PMMA polymer nanocomposites were subsequently investigated through thermogravimetric (TG) analysis and microcombustion calorimetry. Scanning electron microscopy was used to evaluate P-MWCNT dispersion in each polymer matrix. P-MWCNTs were well-dispersed in PS, improved PS thermal stability during nitrogen pyrolysis in proportion to the phosphorus loading, and led to an improvement in PS flammability properties. In contrast, the P-MWCNTs were more aggregated in PMMA and only moderately improved PMMA thermal stability during pyrolysis. In both PS and PMMA, P-MWCNTs were more effective than either MWCNTs or O-MWCNTs at improving each polymer's thermal oxidative stability during combustion in air. Characterization of the P-MWCNT nanoparticles, the PS and PMMA nanocomposites, and TG residues indicated that P-MWCNT flame retardants primarily act in the condensed phase, encouraging char formation of the dispersed MWCNT network as a route to protecting the polymer. By comparing results of polymer composites containing MWCNTs, O-MWCNTs or P-MWCNTs at comparable loadings, data from the current study indicate that phosphorus incorporation can improve the fire-retardant properties of MWCNTs, although optimizing P-MWCNT dispersion in different polymers remains critical to optimizing the flame-retardant effect. [ABSTRACT FROM AUTHOR]

The aim of this study was to determine whether multiwalled carbon nanotubes (MWNCT) are taken up by and are toxic to human intestinal enterocytes using the Caco-2 cell model. Caco-2 cells were exposed to 50 μg/ml MWCNT (oxidized or pristine) for 24 h, and experiments were repeated in the presence of 2.5 mg/L natural organic matter. Cells displayed many of the properties that characterize enterocytes, such as apical microvilli, basolateral basement membrane, and glycogen. The cell monolayers also displayed tight junctions and electrical resistance. Exposure to pristine and oxidized MWCNT, with or without natural organic matter, did not markedly affect viability, which was assessed by measuring activity of released lactate dehydrogenase (LDH) and staining with propidium iodide. Ultrastructural analysis revealed some damage to microvilli colocalized with the MWCNT; however, neither type of MWCNT was taken up by Caco-2 cells. In contrast, pristine and oxidized MWCNT were taken up by the macrophage RAW 264.7 line. Our study suggests that intestinal enterocytes cells do not take up MWCNT. [ABSTRACT FROM AUTHOR]

To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphene sheets that comprise CNT surfaces are often modified to tailor properties, such as dispersion. In this article, we provide a critical review of the techniques used to explore the chemical and structural characteristics of CNTs modified by covalent surface modification strategies that involve the direct incorporation of specific elements and inorganic or organic functional groups into the graphene sidewalls. Using examples from the literature, we discuss not only the popular techniques such as TEM, XPS, IR, and Raman spectroscopy but also more specialized techniques such as chemical derivatization, Boehm titrations, EELS, NEXAFS, TPD, and TGA. The chemical or structural information provided by each technique discussed, as well as their strengths and limitations. Particular emphasis is placed on XPS and the application of chemical derivatization in conjunction with XPS to quantify functional groups on CNT surfaces in situations where spectral deconvolution of XPS lineshapes is ambiguous. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The properties and commercial viability of biodegradable polymers can be significantly enhanced by the incorporation of carbon nanotubes (CNTs). The environmental impact and persistence of these carbon nanotube/polymer nanocomposites (CNT/PNCs) after disposal will be strongly influenced by their microbial interactions, including their biodegradation rates. At the end of consumer use, CNT/PNCs will encounter diverse communities of microorganisms in landfills, surface waters, and wastewater treatment plants. To explore CNT/PNC biodegradation under realistic environmental conditions, the effect of multi-wall CNT (MWCNT) incorporation on the biodegradation of polyhydroxyalkanoates (PHA) was investigated using a mixed culture of microorganisms from wastewater. Relative to unfilled PHA (0% w/w), the MWCNT loading (0.5–10% w/w) had no statistically significant effect on the rate of PHA matrix biodegradation. Independent of the MWCNT loading, the extent of CNT/PNC mass remaining closely corresponded to the initial mass of CNTs in the matrix suggesting a lack of CNT release. CNT/PNC biodegradation was complete in approximately 20 days and resulted in the formation of a compressed CNT mat that retained the shape of the initial CNT/PNC. This study suggests that although CNTs have been shown to be cytotoxic towards a range of different microorganisms, this does not necessarily impact the biodegradation of the surrounding polymer matrix in mixed culture, particularly in situations where the polymer type and/or microbial population favor rapid polymer biodegradation. [ABSTRACT FROM AUTHOR]

The biodegradation rates of carbon nanotube (CNT)/ polymer nanocomposites (PNCs) containing poly-ε-caprolactone (PCL) were investigated using Pseudomonas aeruginosa, a microorganism commonly found in the environment. CNT/PCL nanocomposite mass loss profiles revealed that the rate of PCL matrix biodegradation decreased systematically as the CNT loading increased from 0.1 to 10% w/w. Addition of even a low CNT loading (<1% w/w) caused the CNT/PCL biodegradation rate constant to decrease by more than 50%. Similar trends in biodegradation rate were observed for both pristine and oxidized multiwall CNTs embedded in PCL. During PCL matrix biodegradation, CNT accumulation was observed at the surface of CNT/PCL nanocomposites and single particle inductively coupled-mass spectrometry experiments revealed no measurable CNT release to the culture fluid. Experimental data indicated that biodegradation proceeded as a result of biofilm formation on the CNT/PCL nanocomposites and decreased as a function of CNT loading due to the cytotoxicity of CNTs toward P. aeruginosa and the physical barrier presented by the surface-accumulated CNTs to the underlying PCL substrate. As the CNT loading in the CNT/PCL nanocomposites increased, the microbial proliferation of planktonic cells in the surrounding media also decreased as did the biodegradation rate of PCL samples present in the same reactors. Results from this study demonstrate that the inclusion of CNTs into polymer matrices could increase the environmental persistence of polymers in lakes, landfills, and surface waters. [ABSTRACT FROM AUTHOR]

With production of carbon nanotubes surpassing billions of tons per annum, concern about their potential interactions with biological systems is growing. Herein, we utilize second harmonic generation spectroscopy, sum frequency generation spectroscopy, and quartz crystal microbalance with dissipation monitoring to probe the interactions between oxidized multiwalled carbon nanotubes (O-MWCNTs) and supported lipid bilayers composed of phospholipids with phosphatidylcholine head groups as the dominant component. We quantify O-MWCNT attachment to supported lipid bilayers under biogeochemically relevant conditions and discern that the interactions occur without disrupting the structural integrity of the lipid bilayers for the systems probed. The extent of O-MWCNT sorption was far below a monolayer even at 100 mM NaCl and was independent of the chemical composition of the supported lipid bilayer. [ABSTRACT FROM AUTHOR]

In many environmental scenarios, the fete and impact of polymer nanocomposites (PNCs) that contain carbon nanotubes (CNT/PNCs) will be influenced by their interactions with microorganisms, with implications for antimicrobial properties and the long-term persistence of PNCs. Using oxidized single-wall (OSWCNTs) and multi-wall CNTs (O-MWCNTs), we explored the influence that CNT loading (mass fraction ⩽0.1%-10%) and type have on the initial interactions of Pseudomonas aeruginosa with OCNT/ poly(vinyl alcohol) (PVOH) nanocomposites containing well-dispersed O-CNTs. LIVE/DEAD staining revealed that, despite oxidation, the indusion of O-SWCNTs or O-MWCNTs caused PNC surfaces to exhibit antimicrobial properties. The fraction of living cells deposited on both O-SWCNT and O-MWCNT/PNC surfaces decreased exponentially with increasing CNT loading, with O-SWCNTs being approximately three times more cytotoxic on a % w/w basis. Although not every contact event between attached microorganisms and CNTs led to cell death, the cytotoxidty of the CNT/PNC surfaces scaled with the total contact area that existed between the microorganisms and CNTs. However, because the antimicrobial properties of CNT/PNC surfaces require direct CNT-microbe contact, dead cells were able to shield living cells from the cytotoxic effects of CNTs, allowing biofilm formation to occur on CNT/PNCs exposed to Pseudomonas aeruginosa for longer time periods. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanotubes (CNTs) with different physiochemical properties were layered onto low pressure membranes and tested for antifouling properties using a natural surface water with high fouling potential. Membranes modified with the largest diameter pristine multi-walled CNTs (MWCNTs) were most effective in controlling membrane fouling, tripling the time it took for the membrane to become noticeably fouled at a CNT loading of 22 g/m2. The differences in the structure of CNT layers were an important contributing factor for antifouling properties; scanning electron microscopy imaging showed that large diameter MWCNTs formed homogeneous porous layers across the membrane surface, while less effective, small diameter MWCNTs formed heterogeneous layers. Water quality analysis showed that CNT-membranes constructed with larger diameter CNTs were more effective at removing larger organic macromolecules responsible for fouling from feedwater compared to membranes made with smaller diameter CNTs. This reduced the concentration of foulants reaching the PVDF membrane and thus helped reduce membrane fouling. Beneficial for application, increased loadings of CNTs onto the membrane surface increased resistance to fouling while only slightly reducing the clean water permeability of the modified membranes. Overall, CNT layered membranes were shown to highly resist membrane fouling with potential applications in sustainable water treatment. [Copyright &y& Elsevier]

Abstract: Six commonly used wet chemical oxidants (HNO3, KMnO4, H2SO4/HNO3, (NH4)2S2O8, H2O2, and O3) were evaluated in terms of their effects on the surface chemistry and structure of MWCNTs using a combination of analytical techniques. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDX) were used to characterize the extent of surface oxidation, while chemical derivatization techniques used in conjunction with XPS allowed the concentration of carboxyl, carbonyl, and hydroxyl groups at the surface to be quantified for each MWCNT sample. Our results indicate that the distribution of oxygen-containing functional groups was insensitive to the reaction conditions (e.g., w/w% of oxidant), but was sensitive to the identity of the oxidant. MWCNTs treated with (NH4)2S2O8, H2O2, and O3 yielded higher concentrations of carbonyl and hydroxyl functional groups, while more aggressive oxidants (e.g., HNO3, KMnO4) formed higher fractional concentrations of carboxyl groups. IR spectroscopy was unable to identify oxygen-containing functional groups present on MWCNTs, while Raman spectra highlighted the frequently ambiguous nature of this technique for measuring CNT structural integrity. TEM was able to provide detailed structural information on oxidized MWCNT, including the extent of sidewall damage for different oxidative treatments. [Copyright &y& Elsevier]

Grafting oxygen-containing functional groups onto carbon nanotubes (CNTs) by acid treatment improves their dispersion in aqueous solutions, but there is a lack of quantitative information on the colloidal properties of oxidized CNTs. We have studied the influence that pH and electrolytes have in determining the colloidal stability of oxidized multiwalled carbon nanotubes (O-MWCNTs), prepared by refluxing pristine MWCNTs in nitric acid. The acid-treated MWCNTs contained oxygen predominantly in the form of carboxyl groups. Colloidal suspensions of O-MWCNTs were prepared by low-power sonication and contained negatively charged, individual MWCNTs with an average length of ∼650 nm. Time-resolved dynamic light scattering revealed that the aggregation rate of O-MWCNTs exhibited both reaction and mass-transport limited regimes in the presence of different electrolytes and as a function of pH. Particle stability profiles constructed from aggregation rate data allowed for the determination of critical coagulation concentrations (CCC), a metric of colloidal stability. The CCC values of O-MWCNTs varied with counterion concentration and valence in a manner consistent with DLVO theory. Potentiometric measurements of surface charge correlated well with the observed pH-dependent variations in the O-MWCNT's colloidal stability. Electrophoretic mobility was also a diagnostic of particle stability, but only in neutral and acidic conditions. [ABSTRACT FROM AUTHOR]

As greater quantities of carbon nanotubes (CNTs) enter the environment, they will have an increasingly important effect on the availability and transport of aqueous contaminants. As a consequence of purification, deliberate surface functionalization, and/or exposure to oxidizing agents after release to the environment, CNTs often contain surface oxides (i.e., oxygen containing functional groups). To probe the influence that surface oxides exert on CNT sorption properties, multiwalled CNTs (MWCNTs) with varying oxygen concentrations were studied with respect to their sorption properties toward naphthalene. For pristine (as-received) MWCNTs, the sorption capacity was intermediate between that of a natural char and a granular activated carbon. Sorption data also reveal that a linear relationship exists between the oxygen content of MWCNTs and their maximum adsorption capacity for naphthalene, with 10% surface oxygen concentration resulting in a roughly 70% decrease in maximum adsorption capacity. The relative distribution of sorption energies, as characterized by Freundlich isotherm exponents was, however, unaffected by oxidation. Thus, the data are consistent with the idea that incorporated surface oxides create polar regions that reduce the surface area available for naphthalene sorption. These results highlight the important role of surface chemistry in controlling the environmental properties of CNTs. [ABSTRACT FROM AUTHOR]