Photo-transformation of graphene oxide in synthetic and natural waters

Graphene oxide (GO) is widely employed due to its outstanding properties, leading to an increasing release into the environment and natural waters. Although some studies have reported on the photo-transformation of GO, its behavior in complex natural waters remains inadequately explored. This study demonstrates that different types of ions may promote the photoreduction of GO in the order of Ca2+ > K+ > NO3- > Na+ by interacting with the functional groups on the surface of GO, and the photoreduction is enhanced with increasing ion concentrations. Additionally, natural organic matter (NOM) can inhibit the photoreduction of GO by scavenging reactive oxygen species. However, with increasing NOM concentrations (≥ 5 mgC/L), more NOM adsorb onto the surface of GO through hydrogen bonding, Lewis acid-base interactions, and π-π interactions, thereby enhancing the photoreduction of GO. On this basis, our results further indicate that the combined effects of different ions, such as Ca2+, Mg2+, NOM, and other complex hydrochemical conditions in different natural waters can promote the photoreduction of GO, resulting in a reduction in oxygen functional groups and the formation of defects. This study provides a theoretical basis for assessing the long-term transformation and fate of GO in natural waters.

A Meta-Analysis to Revisit the Property-Aggregation Relationships of Carbon Nanomaterials: Experimental Observations versus Predictions of the DLVO Theory

Contradicting relationships between physicochemical properties of nanomaterials (e.g., size and ζ-potential) and their aggregation behavior have been constantly reported in previous literature, and such contradictions deviate from the predictions of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. To resolve such controversies, in this work, we employed a meta-analytic approach to synthesize the data from 46 individual studies reporting the critical coagulation concentration (CCC) of two carbon nanomaterials, namely, graphene oxide (GO) and carbon nanotube (CNT). The correlations between CCC and material physicochemical properties (i.e., size, ζ-potential, and surface functionalities) were examined and compared to the theoretical predictions. Results showed that the CCC of electrostatically stabilized carbon nanomaterials increased with decreasing nanomaterial size when their hydrodynamic sizes were smaller than ca. 200 nm. This is qualitatively consistent with the prediction of the DLVO theory but with a smaller threshold size than the predicted 2 μm. Above the threshold size, the material ζ-potential can be correlated to CCC for nanomaterials with moderate/low surface charge, in agreement with the DLVO theory. The correlation was not observed for highly charged nanomaterials because of their underestimated surface potential by the ζ-potential. Furthermore, a correlation between the C/O ratio and CCC was observed, where a lower C/O ratio resulted in a higher CCC. Overall, our findings rationalized the inconsistency between experimental observation and theoretical prediction and provided essential insights into the aggregation behavior of nanomaterials in water, which could facilitate their rational design.

Colloidal stability of nanosized activated carbon in aquatic systems: Effects of pH, electrolytes, and macromolecules

Nanosized activated carbon (NAC) is a novel adsorbent with great potential for water reclamation. However, its transport and reactivity in aqueous environments may be greatly affected by its stability against aggregation. This study investigated the colloidal stability of NAC in model aqueous systems with broad background solution chemistries including 7 electrolytes (NaCl, NaNO₃, Na₂SO₄, KCl, CaCl₂, MgCl₂, and BaCl₂), pH 4-9, and 6 macromolecules (humic acid (HA), fulvic acid (FA), cellulose (CEL), bovine serum albumin (BSA), alginate (ALG), and extracellular polymeric substance (EPS)), along with natural water samples collected from pristine to polluted rivers. The results showed that higher solution pH stabilized NAC by raising the critical coagulation concentration from 28 to 590 mM NaCl. Increased cation concentration destabilized NAC by charge screening, with the cationic influence following Ba²⁺ > Ca²⁺ > Mg²⁺ >> Na⁺ > K⁺. Its aggregation behavior could be predicted with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with a Hamaker constant (A_CWC) of 4.3 × 10^-20 J. The presence of macromolecules stabilized NAC in NaCl solution and most CaCl₂ solution following EPS > BSA > CEL > HA > FA > ALG, due largely to enhanced electrical repulsion and steric hindrance originated from adsorbed macromolecules. However, ALG and HA strongly destabilized NAC via cation bridging at high Ca²⁺ concentrations. Approximately half of NAC particles remained stably suspended for ∼10 d in neutral freshwater samples. The results demonstrated the complex effects of water chemistry on fate and transport of NAC in aquatic environments.

Can the properties of engineered nanoparticles be indicative of their functions and effects in plants?

The extensive applicability of engineered nanoparticles (ENPs) in various fields such as environment, agriculture, medicine or biotechnology has mostly been attributed to their better physicochemical properties as compared with conventional bulk materials. However, functions and biological effects of ENPs change across different scenarios which impede the progress in their risk assessment and safety management. This review thus intends to figure out whether properties of ENPs can be indicators of their behavior through summarizing and analyzing the available literature and knowledge. The studies have indicated that size, shape, solubility, specific surface area, surface charge and surface reactivity constitute a more accurate measure of ENPs functions and toxic effects in addition to mass concentration. Effects of ENPs are also highly dependent on dose metrics, species and strains of organisms, environmental conditions, exposure route and duration. Searching correlations between properties and functions or biological effects may serve as an effective way in understanding positive and negative impacts of ENPs. This will ensure safe design and sustainable future use of ENPs.

The mechanisms and environmental implications of engineered nanoparticles dispersion

Dispersion of engineered nanoparticles (ENPs) has drawn special research attentions because the environmental behavior, risks, and applications of ENPs are greatly dependent on their dispersing status. This review summarizes the latest research progress of dispersion mechanisms, environmental applications in contaminants adsorption, and toxicity of ENPs dispersed in liquid and in solid matrix (3D-ENPs). Dispersion mechanisms of ENPs, including steric hindrance, electrostatic repulsion and "micelle wrapping" are well understood in single dispersing agent, however, the prediction of ENPs dispersion in real environments is not straightforward because of the diversity of structures, components, and properties of natural organic molecule mixtures. The adsorption characteristics, depending on the exposed surface areas in liquid, are significantly different between dispersed and aggregated ENPs. Comparing with the aggregated ENPs, the toxicity of dispersed ENPs is generally enhanced due to the increased uptake, released metal ions, carried contaminants, and induced ROS. 3D-ENPs not only inherit the excellent adsorption performance of ENPs dispersed in liquid, but also are beneficial to the separation and recycle from aqueous solutions due to their 3D rigid structures. However, the adsorption mechanisms as affected by environmental conditions are still unclear. Additionally, the potential risks of 3D-ENPs should be paid more attentions, with an emphasis on free radicals and stability of 3D structure.

Aggregation and stability of nanoscale plastics in aquatic environment

The widespread use and release of plastics in nature have raised global concerns about their impact on public health and the environment. While much research has been conducted on macro- and micro-sized plastics, the fate of nanoscale plastics remains unexplored. In this study, the aggregation kinetics and stability of polyethylene and polystyrene nanoscale plastics were investigated over a wide range of aquatic chemistries (pH, salt types (NaCl, CaCl_2, MgCl₂), ionic strength) relevant to the natural environment. Results showed that salt types and ionic strength had significant effects on the stability of both polyethylene and polystyrene nanoscale plastics, while pH had none. Aggregation and stability of both polyethylene and polystyrene nanoscale plastics in the aquatic environment followed colloidal theory (DLVO theory and Schulze-Hardy rule), similar to other colloidal particles. The critical coagulation concentration (CCC) values of polyethylene nanoscale plastics were lower for CaCl₂ (0.1 mM) compared to NaCl (80 mM) and MgCl₂ (3 mM). Similarly, CCC values of polystyrene nanospheres were 10 mM for CaCl₂, 800 mM for NaCl and 25 mM for MgCl₂. It implies that CaCl₂ destabilized both polyethylene and polystyrene nanoscale plastics more aggressively than NaCl and MgCl₂. Moreover, polystyrene nanospheres are more stable in the aquatic environment than polyethylene nanoscale plastics. However, natural organic matter improved the stability of polyethylene nanoscale plastics in water primarily due to steric repulsion, increasing CCC values to 0.4 mM, 120 mM and 8 mM for CaCl_2, NaCl and MgCl₂ respectively. Stability studies with various water conditions demonstrated that polyethylene nanoscale plastics will be fairly stable in the natural surface waters. Conversely, synthetic surface water, wastewater, seawater and groundwater rapidly destabilized polyethylene nanoscale plastics. Overall, our findings indicate that significant aqueous transport of nanoscale plastics will be possible in natural surface waters.

Establishing structure-property-hazard relationships for multi-walled carbon nanotubes: the role of aggregation, surface charge, and oxidative stress on embryonic zebrafish mortality

Increasing use of carbon nanotubes (CNTs) in consumer and industrials goods increases their potential release, and subsequent risks to environmental and human health. Therefore, it is becoming ever more important that CNTs are designed to reduce or eliminate hazards and that hazard assessment methodologies are robust. Here, oxygen-functionalized multi-walled CNTs (O-MWCNTs), modified under varying redox conditions, were assessed for toxic potential using the zebrafish (Danio rerio) embryo model. Multiple physicochemical properties (e.g., MWCNT aggregate size, morphology, and rate; surface charge and oxygen concentration; and reactive oxygen species (ROS) generation) were characterized and related to zebrafish embryo mortality through the use of multivariate statistical methods. Of these properties, surface charge and aggregate morphology emerged as the greatest predictors of embryo mortality. Interestingly, ROS generation was not significantly correlated to observed mortality, contrary to prior predictions by nanotoxicology researchers. This suggests that the mechanism of MWCNT-induced mortality of embryonic zebrafish is physical, driven by electrostatic and shape effects, both of which are related to nanomaterial aggregation. This raises the importance of rigorously considering aggregation during aqueous-based nanotoxicology assays as nanomaterial aggregation can affect perceived nanomaterial toxicity. As such, future nanotoxicity studies relying on aqueous media must sufficiently consider nanomaterial aggregation.

Synthesis of Carboxymethyl Starch-Bio-Based Epoxy Resin and their Impact on Mechanical Properties

In the current research, we observed numerous suggestions are promoting the use of bio-based epoxy resins, replacing the petroleum-based products like Diglycidyl ether of bisphenol A type epoxy resin DGEBA. With the passage of time, the impending challenges include preparation of environmentally-friendly epoxy with minimum toxic side effect and improved properties. Therefore, we describe a very useful method for preparing new silicone-bridged dimethyl siloxane monomers in high quantity, derived from naturally occurring eugenol. By putting the methyl siloxane, computed with different chain lengths into their molecular backbone. Such epoxy monomers have different molecular structure with high purity. This dimethyl siloxane epoxy, with lower viscosity than commercial DGEBA epoxy, has superior thermal properties, which were evaluated using differential scanning calorimetry DSC. Modification of CMS increases the hydrophilicity. Bio-based epoxy (self-prepared) resin improved adhesive properties, with the help of modified CMS. This study presents a very easy and effective chemical modification to enhance interfacial adhesion composites with superior properties.

Aggregation of ferrihydrite nanoparticles: Effects of pH, electrolytes,and organics

To better understand the fate and transport of ferrihydrite nanoparticles (FNPs), which carry many contaminants in natural and engineered aquatic environments, the aggregation of FNPs was systematically investigated in this study. The pH isoelectric point (pH_IEP), surface zeta potential, and particle size evolutions of FNPs were measured under varied aqueous conditions using dynamic light scattering (DLS). The influence of pH (5.0 ± 0.1 and 7.0 ± 0.1), ionic strength (IS), electrolytes (NaCl, CaCl₂ and Na₂SO₄), and organics (humic acid, fulvic acid and CH₃COONa) on the aggregation behaviors of FNPs were explored. Meanwhile, Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to better understand the controlling mechanisms of FNP aggregation. In the presence of sulfate, the surface charge of FNPs was neutralized under varied pH and ionic strength due to ion adsorption and FNPs phase transformation to schwertmannite based on FT-IR results. This phase transformation resulted in rapid aggregation in all water chemistries tested, whereas other salt species affected the aggregation primarily by ion adsorption and charge screening. Presence of increasing concentrations of the organic acids significantly shifted the pH_IEP of FNPs (7.0 ± 0.2) to lower pH (< 4.0) due to adsorption of organics on FNPs surfaces making them negatively charged. The adsorption of HA/FA inhibited FNP aggregation significantly while CH₃COONa did not, due to different effects on steric and/or electrosteric interactions among FNPs by organics with varied pK_a values and molecular weights. After accounting for the important effects of pH, electrolytes, and organics in modifying FNPs' surface charge, DLVO calculations agreed well with measured critical coagulation concentrations (CCC) values of FNPs at both pH 5.0 ± 0.1 and 7.0 ± 0.1 in the presence of NaCl. This study will hence be useful to better predict and control the fate and transport of FNPs in the presence of electrolytes and organics with different molecular weights, as well as the fate of the associated contaminants in natural and engineered systems.

A Facile and Efficient Protocol for Preparing Residual-Free Single-Walled Carbon Nanotube Films for Stable Sensing Applications

In this article, we report on an efficient post-treatment protocol for the manufacturing of pristine single-walled carbon nanotube (SWCNT) films. To produce an ink for the deposition, the SWCNTs are dispersed in an aqueous solution with the aid of a carboxymethyl cellulose (CMC) derivative as the dispersing agent. On the basis of this SWCNT-ink, ultra-thin and uniform films are then fabricated by spray-deposition using a commercial and fully automated robot. By means of X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), we show that the CMC matrix covering the CNTs can be fully removed by an immersion treatment in HNO₃ followed by thermal annealing at a moderate temperature of 100 °C, in the ambient air. We propose that the presented protocols for the ink preparation and the post-deposition treatments can in future serve as a facile and efficient platform for the fabrication of high-quality and residual-free SWCNT films. The purity of SWCNT films is of particular importance for sensing applications, where residual-induced doping and dedoping processes distort the contributions from the sensing specimen. To study the usability of the presented films for practical applications, gas sensors are fabricated and characterized with the CNT-films as the sensing material, screen printed silver-based films for the interdigitated electrode (IDE) structure, and polyimide as a flexible and robust substrate. The sensors show a high and stable response of 11% to an ammonia (NH₃) test gas, at a concentration of 10 ppm.

Dispersion of Multi-Walled Carbon Nanotubes Stabilized by Humic Acid in Sustainable Cement Composites

Multi-walled carbon nanotubes (MWCNTs) are promising nanoreinforcing materials for cement-based composites due to their superior material properties. Dispersion of MWCNTs is key for achieving the most effective way of enhancing efficiency, which is challenging in an alkaline cementitious environment. In this study, humic acid (HA) was used to stabilize the degree of dispersion of MWCNTs in an alkaline environment. The efficiency of HA in stabilizing MWCNT dispersion in cement composites was characterized using an ultraviolet spectrophotometer. The influences of HA on the workability and mechanical properties of ordinary Portland cement (OPC) reinforced with MWCNTs were evaluated, and the results revealed that the addition of HA can improve the stability of MWCNT dispersion in an alkaline environment. A concentration of 0.12 wt.% HA/S added to MWCNT suspensions was found to perform the best for improving the dispersion of MWCNTs. The addition of HA results in a decreased workability of the OPC pastes but has little influence on the strength performance. HA can affect the mechanical properties of OPC reinforced with MWCNTs by influencing the dispersion degree of the MWCNTs. An optimum range of HA (0.05⁻0.10 wt.%) is required to achieve the optimum reinforcing efficiency of MWCNTs.

Transport of iron nanoparticles through natural discrete fractures

The transport of nano scale iron particles (NIP) in fractures is of concern for remediation of both fractured aquifers and porous aquifers when hydro-fracking and flow in preferential pathways takes place. In this study the transport of various NIP in a natural discrete fractured chalk core was investigated and their mass recoveries calculated. Four different types of NIP were tested and characterized in two ionic strength (IS) solutions at a particle concentration of 100-200 mg/l. The effect of IS, stability (sedimentation rate), particle size, solution viscosity and stabilizer were studied. NIP stability ranged from 1 to 100% following 120 min of stability tests and recoveries ranged from about 6 to 69%. The stabilizer type and concentration were shown to have significant role in NIP recoveries, especially at increased IS. It was evident that gravitational stability is the most crucial factor dominating transport of NIP. Accordingly, stability tests were shown to be a reliable indicator of NIP mobility. The high recoveries of some NIP tested, combined with the lack of clogging effect illustrates the enhanced mobility of NIP in fractures. The wide range of recoveries indicates NIP transport manipulation potential in such media. We therefore suggest that application of NIP in contaminated fractures has considerable potential as a remediation measure. In order to achieve NIP distribution in the aquifer while avoiding leakage to the environment, NIP stabilizer concentration should be adjusted according to the site-specific hydrogeochemical properties of the contaminated media.

Transport of multi-walled carbon nanotubes stabilized by carboxymethyl cellulose and starch in saturated porous media: Influences of electrolyte, clay and humic acid

This study investigated the transport behaviors of carboxymethyl cellulose (CMC) and starch stabilized multi-walled carbon nanotubes (MWNTs) through a saturated quartz sand column in the presence of electrolytes, model clays, and natural organic matter (humic acid) through column breakthrough experiments and model simulations. Both stabilizers, CMC and starch, greatly enhanced the breakthrough of MWNTs, with a full breakthrough plateau (C/C₀) ranging from 0.69 to 0.90 at ionic strength from 0.3 to 10mM. Between the two stabilizers, CMC was more effective in resisting particle deposition, and thus CMC-stabilized MWNTs were more transportable through the medium. While non-stabilized MWNTs were much less transportable and were vulnerable to electrolyte effects (especially Ca²⁺), the stabilized counterparts were much more resistant to the coagulation effects of electrolytes. The presence of colloidal clay particles showed contrasting effects on the transport of bare and stabilized MWNTs. The full breakthrough C/C₀ of bare MWNTs was suppressed by kaolinite and montmorillonite particles from 0.33 to <0.15 with 5mg/L clay, indicating that the presence of both clays enhanced the aggregation and deposition of MWNTs. However, kaolinite particles facilitated the transport of stabilized-MWNTs, while montmorillonite weakened the breakthrough of stabilized MWNTs. Humic acid had less effect on the mobility of stabilized-MWNTs than that of bare MWNTs. The advection-dispersion transport model incorporated with the filtration theory was able to simulate the breakthrough curves and quantitatively interpret the particle deposition. The results can facilitate our understanding of fate and transport of stabilized carbon nanotubes in the environment.