Release Of Multi-walled Carbon Nanotubes Research Articles

Effects of multi-walled carbon nanotubes in soil on earthworm growth and reproduction, enzymatic activities, and metabolomics

Increased production and environmental release of multi-walled carbon nanotubes (MWCNTs) increase soil exposure and potential risk to earthworms. However, MWCNT toxicity to earthworms remains unclear, with some studies identifying negative effects and others negligible effects. In this study, to determine whether exposure to MWCNTs negatively affects earthworms and to elucidate possible mechanisms of toxicity, earthworms were exposed to sublethal soil concentrations of MWCNTs (10, 50, and 100 mg/kg) for 28 days. Earthworm growth and reproduction, activities of cytochrome P450 (CYP) isoforms (1A2, 2C9, and 3A4) and antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), and glutathione-s-transferase (GST)), and metabolomics were determined. Effects of MWCNTs on earthworms depended on exposure concentration. Exposure to MWCNTs did not significantly affect growth and reproduction of individual earthworms. Exposure to 50 mg/kg MWCNTs significantly increased activities of CYP2C9, CYP3A4, SOD, CAT, and GST but clearly reduced levels of L-aspartate, L-asparagine, and glutamine. With exposure to 100 mg/kg MWCNTs, toxic effects on earthworms were observed, with significant inhibition in activities of CYP isoenzymes and SOD, significant reductions in L-aspartate, L-asparagine, glutamine, and tryptophan, and simultaneous accumulations of citrate, isocitrate, fumarate, 2-oxoglutarate, pyruvate, D-galactose, carbamoyl phosphate, formyl anthranilate, hypoxanthine, and xanthine. Results suggest that toxicity of MWCNTs to earthworms is associated with reduced detoxification capacity, excessive oxidative stress, and disturbance of multiple metabolic pathways, including amino acids metabolism, the tricarboxylic acid cycle, pyruvate metabolism, D-galactose metabolism, and purine metabolism. The study provides new insights to better understand and predict the toxicity of MWCNTs in soil.

Humic acid non-covalent functionalized multi-walled carbon nanotubes composite membrane and its application for the removal of organic dyes

It was the first time that humic acid (HA) was used to non-covalent functionalize multi-walled carbon nanotubes (MWCNTs). The MWCNT and HA-MWCNT was characterized by X-ray photoelectron spectroscopy (XPS), infrared spectra (IR) and transmission electron microscopy (TEM). Then, a HA non-covalent functionalized multi-walled carbon nanotubes (HA-MWCNTs) membrane was prepared by vacuum filtration and used to remove organic dyes. To investigate the removal mechanism of organic dyes by HA-MWCNTs membrane, the dynamic filtration effectiveness of HA-MWCNTs membrane was compared with the adsorption ability of HA-MWCNTs. The results proved that HA-MWCNTs membrane had a higher effectiveness for the removal of dyes as it had shorter filtration time compared with the static adsorption experiment. Especially, the HA-MWCNTs membrane had a higher removal efficiency for cationic dye methylene blue (MB) than the raw MWCNTs (P-MWCNTs) membrane. The electrostatic interaction played an important role on the removal of MB, and the functionalization of MWCNTs with HA increased the surface electronegativity of MWCNTs membrane, and promoted the removal of MB during filtration process. As for RB19, in addition to π–π stacking interaction, electrostatic interaction also influenced its removal by MWCNTs membrane. Compared with P-MWCNTs membrane, HA-MWCNTs membrane slightly decreased the effectiveness of RB19 removal during filtration due to the increased electrostatic repulsion. Generally, the dynamic membrane filtration based on MWCNTs composite membrane could avoid the ecological risk caused by the release of MWCNTs. It provided a new idea for the further innovation of low-pressure membrane technology in the field of dye wastewater treatment.

Quantifying Mechanical Abrasion of MWCNT Nanocomposites Used in 3D Printing: Influence of CNT Content on Abrasion Products and Rate of Microplastic Production.

Manufactured nanomaterials (MNMs) are incorporated as "nanofillers" into consumer products to enhance properties of interest. Multiwalled carbon nanotubes (MWCNTs) are known for their unique properties and have many applications in polymers. However, the release of MWCNTs during the nanoenabled product life cycle is concerning. During the use phase, mechanical stresses can produce fragmented materials containing MNMs. The degree of MNM release, the resulting exposure to these materials, and the potential impacts of their release are active research topics. In this study, we describe methodological improvements to study the abrasion of plastics containing MNMs (nanocomposites) and report on characteristics of abrasion products produced and rates of microplastic production. The abrasion device developed for this work allows for the measurement of power inputs to determine scaled release rates. Abrasion rates for plastics used in 3D printing were found to be 0.27 g/m2/s for the PETG polymer and 0.3 g/m2/s for the 2% MWCNT-PETG nanocomposite. Embedded and protuberant MWCNTs appeared to impact the particle size, shape, hydrophobicity, and surface charge of the microplastics, while the inclusion of MWCNTs had a small effect on microplastic production. Measurements of power input to the abrasion process provided a basis for estimating microplastic production rates for these nanocomposites.

Co-transport of multi-walled carbon nanotubes and sodium dodecylbenzenesulfonate in chemically heterogeneous porous media

Multi-walled carbon nanotubes (MWCNTs) are increasing used in commercial applications and may be released into the environment with anionic surfactants, such as sodium dodecylbenzenesulfonate (SDBS), in sewer discharge. Little research has examined the transport, retention, and remobilization of MWCNTs in the presence or absence of SDBS in porous media with controlled chemical heterogeneity, and batch and column scale studies were therefore undertaken to address this gap in knowledge. The adsorption isotherms of SDBS on quartz sand (QS), goethite coated quartz sand (GQS), and MWCNTs were determined. Adsorption of SDBS (MWCNTs » GQS > QS) decreased zeta potentials for these materials, and produced a charge reversal for goethite. Transport of MWCNTs (5 mg L−1) dramatically decreased with an increase in the fraction of GQS from 0 to 0.1 in the absence of SDBS. Conversely, co-injection of SDBS (10 and 50 mg L−1) and MWCNTs radically increased the transport of MWCNTs when the GQS fraction was 0, 0.1, and 0.3, especially at a higher SDBS concentration, and altered the shape of retention profile. Mathematical modeling revealed that competitive blocking was not the dominant mechanism for the SDBS enhancement of MWCNT transport. Rather, SDBS sorption increased MWCNT transport by increasing electrostatic and/or steric interactions, or creating reversible interactions on rough surfaces. Sequential injection of pulses of MWCNTs and SDBS in sand (0.1 GQS fraction) indicated that SDBS could mobilize some of retained MWCNTs from the top to deeper sand layers, but only a small amount of released MWCNTs were recovered in the effluent. SDBS therefore had a much smaller influence on MWCNT transport in sequential injection than in co-injection, presumably because of a greater energy barrier to MWCNT release than retention. This research sheds novel insight on the roles of competitive blocking, chemical heterogeneity and nanoscale roughness, and injection sequence on MWCNT retention and release.

Release of radiolabeled multi-walled carbon nanotubes (14C-MWCNT) from epoxy nanocomposites into quartz sand-water systems and their uptake by Lumbriculus variegatus

Once plastic products containing multi-walled carbon nanotubes (MWCNT) are disposed, UV-light exposure may destabilize their structural integrity, leading to a release of MWCNT into the environment that can possibly be taken up by organisms and cause adverse effects. The aim of this study was to quantify the released amount of embedded 14C–MWCNT epoxy (E) nanocomposites. Furthermore, bioaccumulation of the released material was investigated in Lumbriculus variegatus.The release of radioactivity (RA) from irradiated (+SSR) E/14C-MWCNT composites was quantified after a series of mechanical treatments and in a quartz sand-water system. Non-irradiated composites served as control group (-SSR). Unlabeled ±SSR-nanocomposites were analyzed by means of electron microscopy. The exposure of blackworms to the released material and the amount of RA in the water phase, the quartz sand, and the faeces was quantified.About 0.1% of the embedded RA was released from +SSR-nanocomposites by mechanical treatment and exposure in the quartz sand-water system, which was about 23 times higher compared to -SSR composites. This corresponds to around 3 mg 14C-MWCNT m−2 for both approaches. Based on the released mass per composite area, +SSR plates set free a 50-fold higher amount of RA. A detectable amount of 7.3 ± 1.8 ng of 14C-MWCNT was also found in the water phase of the quartz sand-water systems. Electron microscopy revealed an enhanced surface degradation of the composites after SSR and mechanical treatments. Released polymer particles also contained protruding MWCNT. An amount of 2.4 ± 1.8% of released RA was taken up by blackworms after 2 d and about 66% of ingested material was eliminated again after 24 h.The results of this study show that SSR leads to an enhanced release of mostly E-polymer associated MWCNT from nanocomposites that are bioavailable for sediment-dwelling organisms, although the absolute amount of released material is low. Follow-up studies examining toxicological effects after uptake are essential for the environmental risk assessment of MWCNT and E/MWCNT-composites.

Colloidal stability and ecotoxicity of multiwalled carbon nanotubes: Influence of select organic matters.

In the last few years, the release of multiwalled carbon nanotubes (MWCNTs) into the environment has raised serious concerns regarding their fate and potential impacts. Aquatic organisms constitute an important pathway for their entrance and transfer throughout the food web, and the current demand for standardization of methodologies to analyze the interactions of MWCNTs with them requires aquatic media that represent natural systems. However, the inherent hydrophobicity of MWCNTs and the substances present in natural waters may greatly affect their stability and bioavailability. The present study analyzes the influence of the most referenced synthetic and natural organic matters (Sigma-Aldrich humic acid and Suwannee River natural organic matter) in the agglomeration kinetics and ecotoxicity of MWCNTs, with the aim of determining their suitability to fulfill the current standardization requirements. Natural organic matter provides increased colloidal stability to the MWCNTs' dispersions, which results in higher adverse effects on the key invertebrate organism Daphnia magna. Furthermore, the results obtained with this type of organic matter allow for observation of the important role of the outer diameter and content impurities of MWCNTs in their stability and ecotoxicity on daphnids. Sigma-Aldrich humic acid appeared to alter the response of the organisms to carbon nanotubes compared with that observed in the presence of natural organic matter.

Heteroaggregation of multiwalled carbon nanotubes with sediments

In this study, the effect of ion type and concentration on the heteroaggregation and release of multiwalled carbon nanotubes (MWCNTs) and hydroxylated MWCNTs (OH-MWCNTs) with sediments is reported. Means of the first-order heteroaggregation rate constant (khet) for MWCNTs and OH-MWCNTs and a freshwater sediment in 0.4mM CaCl2 (0.21 and 0.17d−1) are significantly higher than those in 10mM NaCl (0.096 and 0.094d−1). All sediment-electrolyte combinations exhibit first-order heteroaggregation rates during the initial time period (≤5 days) as evidenced by the linearity of the ln concentration vs time plots, and for some low ionic strength treatments heteroaggregation is first-order over the duration of the entire experimental period. However, faster heteroaggregation rate treatments deviate from linearity at longer time periods due to the formation of large, settleable aggregates (homoaggregation). Heteroaggregation attachment efficiency (αhet) values for both MWCNTs and OH-MWCNTs increase mostly linearly with ionic strength and are greater for the sediment higher in Al hydroxide content. Both nanotube heteroaggregation with sediments and deposition on model environmental surfaces is irreversible under constant background solution conditions. When background solution ionic strength is reduced, nanotube release from sediments and a model surface is dependent on the initial solution chemistry, with nanotubes deposited from NaCl solutions released much more readily than those deposited from CaCl2 solutions. The lowest fractional release observed was for OH-MWCNTs deposited from CaCl2 solutions indicating potential OH-Ca-NOM bridging effects.

Measuring Nanomaterial Release from Carbon Nanotube Composites: Review of the State of the Science

Hazard studies of “as-produced” nanomaterials are increasingly available, yet a critical gap exists in exposure science that may impede safe development of nanomaterials. The gap is that we do not understand what is actually released because nanomaterials can change when released in ways that are not understood. We also generally do not have methods capable of quantitatively measuring what is released to support dose assessment. This review presents a case study of multi-walled carbon nanotubes (MWCNTs) for the measurement challenge to bridge this gap. As the use and value of MWCNTs increases, methods to measure what is released in ways relevant to risk evaluation are critically needed if products containing these materials are to be economically, environmentally, and socially sustainable. This review draws on the input of over 50 experts engaged in a program of workshops and technical report writing to address the release of MWCNTs from nanocomposite materials across their life cycle. The expert analyses reveals that new and sophisticated methods are required to measure and assess MWCNT exposures for realistic exposure scenarios. Furthermore, method requirements vary with the materials and conditions of release across life cycle stages of products. While review shows that the likelihood of significant release of MWCNTs appears to be low for many stages of composite life cycle, measurement methods are needed so that exposures from MWCNT-composites are understood and managed. In addition, there is an immediate need to refocus attention from study of “as-produced” nanomaterials to coordinated research on actual release scenarios.

Hybrid multi-walled carbon nanotubes-alginate-polysulfone beads for adsorption of bisphenol-A from aqueous solution

In recent years, the potential application of carbon nanotubes (CNTs) as sorbent materials in wastewater treatment has garnered tremendous attention. Concerns that CNTs may be toxic to living organism, however, necessitate that the containment of CNTs to prevent their release into the environment in order to realize their practical application in wastewater treatment. In this study, we immobilized multi-walled carbon nanotubes (MWCNTs) within macro-calcium alginate beads. The composite beads were coated with an additional polysulfone (Psf) layer both to provide an external barrier to MWCNTs release into the effluent and also to improve the mechanical integrity of the beads. The hybrid beads were tested for their capacity to remove the bio-refractory endocrine disruptor compound bisphenol-A (BPA) using batch and packed bead column experiments. Maximum BPA removal was achieved at 22% MWCNTs, which was the dispersion limit of MWCNTs in our study. The adsorption of BPA followed Langmuir isotherm model with good correlation. The maximum adsorption capacity of the composite beads of dosage 4 g L−1 as obtained using the Langmuir model was 24.69 mg g−1. Addition of the Psf layer, together with MWCNTs, improved the bead compression performance by up to twelvefold at 40% compressive extension. This study showed that the hybrid alginate-Psf bead may serve as compartment for encapsulation of MWCNTs for removal of BPA. Improved compression performance introduced by addition of Psf layer could protect hybrid beads used, for example, in reactors subjected to extreme conditions such as high flow rates.

Influence of Solution Chemistry on the Release of Multiwalled Carbon Nanotubes from Silica Surfaces

The release of multiwalled carbon nanotubes (MWNTs) that were deposited on silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were deposited on silica surfaces at elevated NaCl and CaCl2 concentrations before being rinsed with eluents of different solution chemistries to induce their remobilization. Energetically speaking, the MWNTs were released from the primary energy minimum when the background NaCl or CaCl2 concentrations were decreased at pH 7.1. The increase in electrostatic repulsion between MWNTs and silica likely caused a reduction in the energy barrier, which enabled the release of MWNTs. The degree of release increased in a stepwise fashion when the nanotubes were sequentially exposed to eluents of decreasing electrolyte concentrations, possibly due to the heterogeneity in nanotube surface charge densities. The degree of release via a successive reduction in NaCl concentration was lower at pH 4.0 than at 7.1 due to MWNTs and silica surfaces exhibiting a less negative surface charge at pH 4.0. Most of the deposited MWNTs were released when the pH was decreased from 7.1 to 4.0 at 1.5 mM CaCl2. This was attributed to the elimination of calcium bridging between the carboxyl groups on MWNTs and silanol groups on silica surfaces.

The effect of multi-walled carbon nanotubes on soil microbial activity

Nanomaterials such as multi-walled carbon nanotubes (MWCNTs) are applied to various industrial products and thus may be released to soils, but their potential environmental impacts remain largely undetermined. We investigated the short-term effect of MWCNTs on the activity and biomass of microorganisms inhabiting two different soil types in an incubation study. Up to 5000 μg MWCNT g −1 soil was applied and the activities of 1,4-β-glucosidase, cellobiohydrolase, xylosidase, 1,4-β-N-acetylglucosaminidase, and phosphatase and microbial biomass were measured. In both soil types, most enzyme activities showed a tendency to be repressed under 500 μg MWCNT g −1 soil, and all enzymatic activities as well as microbial biomass C and N were significantly lowered under 5000 μg MWCNT g −1 soil. Our results suggest that high concentrations of MWCNTs could lower the microbial activity and biomass in soils, and they may serve as an important guideline in regulating the release of MWCNTs to the soil environment.

Monitoring Multiwalled Carbon Nanotube Exposure in Carbon Nanotube Research Facility

With the increased production and widespread use of multiwalled carbon nanotubes (MWCNTs), human and environmental exposure to MWCNTs is inevitably increasing. Therefore, this study monitored the possible exposure to MWCNT release in a carbon nanotube research laboratory. To estimate the potential exposure of researchers and evaluate the improvement of the workplace environment after the implementation of protective control measures, personal and area monitoring were conducted in an MWCNT research facility where the researchers handled unrefined materials. The number, composition, and aspect ratio of MWCNTs were measured using scanning transmission electron microscopy with an energy-dispersive x-ray analyzer. The gravimetric concentrations of total dust before any control measures ranged from 0.21 to 0.43 mg/m3, then decreased to a nondetectable level after implementing the control measures. The number of MWCNTs in the samples obtained from the MWCNT blending laboratory ranged from 172.9 to 193.6 MWCNTs/cc before the control measures, and decreased to 0.018–0.05 MWCNTs/cc after the protective improvements. The real-time monitoring of aerosol particles provided a signature of the MWCNTs released from the blending equipment in laboratory C. In particular, the number size response of an aerodynamic particle sizer with a relatively high concentration in the range of 2 to 3 μ m in aerodynamic diameter revealed the evidence of MWCNT exposure. The black carbon mass concentration also increased significantly during the MWCNT release process. Therefore, the present study suggests that the conventional industrial hygiene measures can significantly reduce exposure to airborne MWCNTs and other particulate materials in a nano research facility.