In this special issue, we invited primary scientists on the leading edge in this field with recent high impact research works to share their expertise and perspectives. The collected papers cover various topics, such as the measurement and characteristics of engineered nanoparticles in the environment, toxicities of nanoparticles to organisms, complex behaviors of nanoparticles with contaminants in the environment, and fate, transformation, and transport of nanoparticles in the environment.

Engineered nanomaterials (ENMs) are being discharged into the environment and to agricultural fields, with unknown impacts on crop species. In this paper, we review the literature on ENMs uptake, translocation/distribution, and generational transmission in various crop species, as well as potential material trophic transfer. Previous studies reveal that ENM-exposed crops exhibit adaptive processes in response to stress, including endocytosis/endosome activities, production of antioxidant enzymes, regulation of genes related to cell division/extension and membrane transport. Some agronomic traits of crops are compromised during the adaption response, including photosynthesis, fruit yields, nutritional quality and nitrogen fixation. Cultivation of crops in ENMs-contaminated environments has unknown implications for food safety and quality. Notably, mechanisms underlying ENMs phytotoxicity and bioavailability are unclear. Additional investigations focused on developing novel techniques for in vivo identification/characterization of ENMs are critically needed. Given the abundance of uncertainty in the literature, it is clear that more research is urgently needed in the area of ENMs-crop interactions; only then can one accurately assess exposure, risk, and overall implications for food safety and also enable guidance development for the sustainable implementation of nanotechnology in agriculture and food production/manufacturing.

With the extensive applications of metallic-based nanomaterials (MNs), concerns are growing of their potential impact on aquatic organisms. Unlike traditional metal pollutants, MNs have different surface properties and compositions, which may modify their impact on aquatic environments as well as their bioavailability to aquatic organisms. Kinetic processes of MNs, such as dissolution, stabilization, aggregation, and sedimentation, are important in determining their bioavailability and subsequent toxicity to aquatic organisms. Among all of the physicochemical kinetics, the dissolution of MNs attracts the most attention, due to their potential toxicity generated by dissolved ions. This review summarizes the dissolution behavior of three common MNs, i.e., ZnO nanoparticles (ZnO-NPs), Ag nanoparticles (Ag-NPs), and TiO₂ nanoparticles (TiO₂-NPs), in toxicological studies. A kinetic model was developed to evaluate the contribution of dissolved ion on the total MN accumulation. Finally, toxicological data of the MNs to algae, zooplankton, and fish are summarized and interpreted based on their kinetics. Different dissolution rates were observed for ZnO-NPs, Ag-NPs, and TiO₂-NPs, and their solubility also varied during different toxicological studies, leading to a variable but increasing waterborne ion concentration during exposure. The bioavailability of these MNs and corresponding ions also varied for different aquatic organisms (e.g., algae, zooplankton, and fish). Specifically, the MNs appeared to be more bioavailable to daphnids, rendering a minor contribution of ion during short-term exposure. Generally, dissolved ion contributed partially to toxicity of ZnO-NPs and Ag-NPs, while the toxicity of TiO₂-NPs was mainly due to the generated reactive oxygen species (ROS). Additionally, the role of dissolved ion in both MN bioaccumulation and toxicity intensified during chronic exposure as a result of dissolution, thus it is critical to monitor the dissolution of MNs in toxicological studies. This review emphasizes the importance of integrating physicochemical kinetics and uptake kinetics in evaluating the bioavailability and toxicity of both MNs and dissolved ions.

Titanium dioxide nanoparticles (TiO₂-NPs) are common components used in sunscreens, cosmetics, industrial applications, and many other products. Concerning their high production and widespread applications, characterization and quantification of TiO₂-NPs in various matrixes is a topic of great interest for researchers studying their potential environmental and health impacts. Validated and easily applicable analytical tools are required to develop and implement regulatory frameworks and an appropriate risk assessment for engineered nanoparticles (ENPs). Herein, we provide a critical review of the current knowledge available on world-wide production and measured environmental concentrations as well as on available techniques to measure and characterize these ENPs in the environment.

Carbon nanoparticles (CNPs) are novel manufactured materials with unique properties and have potential for a variety of applications. Adsorption of organic contaminants by discharged CNPs may affect the fate and transport of organic contaminants in the environment. This review summarizes the present research progress regarding organic contaminant adsorption on CNPs, and provides important information for the evaluation of the environmental behavior of organic contaminants and the risks associated with the use of CNPs. The main adsorption mechanisms involve hydrophobic interactions, π-π interactions, hydrogen bonds, and electrostatic interactions. These interactions may exist simultaneously, while the controlling adsorption mechanism differs depending on the properties of both the organic contaminants and the CNPs along with environmental conditions. The status of CNPs in the environment greatly affects or even controls their characteristics for adsorption of organic contaminants. The mobility and transport of dispersed CNPs and CNP-adsorbed organic contaminants could be promoted in natural aqueous environments, potentially increasing the spread of various organic contaminants and their associated environmental risks. Investigating the adsorption mechanisms and impact parameters is vital in predicting the environmental behaviors of both organic contaminants and CNPs and their associated risks.

Titanium dioxide nanoparticles (TiO₂ NPs) are used in cosmetics, sunscreens, paints, and toothpaste, among other applications. These NPs are very stable and can be transported and dispersed in wastewater and biosolids. Animal species have shown negative reactions to TiO₂ NPs. However, little is known about their toxicity in plants, specifically the possibility of genotoxic effects. In this study, we used a random amplified polymorphic DNA (RAPD) technique to study the genotoxic effects of TiO₂ NPs on hydroponically cultivated zucchini (Cucurbita pepo) plants. Seeds were allowed to germinate for 7 d and plants were selected at random for individual and population studies. Four plants were selected for the individual study and 18 for the population study. RAPD profiles of TiO₂ NPs treated plants showed differences in band intensity, loss of bands, or appearance of new bands, compared to untreated plants. To the authors’ knowledge, this is the first report of the genotoxic potential of TiO₂ NPs in zucchini.

Understanding the dispersion and aggregation of carbon nanotubes (CNTs) in the aqueous environment are critical for the fate, bioavailability, and the environment and health risk assessment of them because the better suspended CNTs display a higher mobility and could transfer to a longer distance in the environment to possibly pose greater ecological and environmental risks. In this study, we have found that bulk single-walled carbon nanotubes (SWCNTs) could not be dispersed and stably suspended in water and sodium dodecylbenzene sulfonate (SDBS) solution by shaking at 140 r/min, although they could be stably suspended in SDBS solution by sonication. Even through sonication, SWCNTs suspended in SDBS solution do not remain stable at the presence of environmentally relevant cations (e.g., Na⁺, K⁺, Ca²⁺, and Mg²⁺) after dilution. These observations suggest that SWCNTs will not travel long distances in significant concentrations in the natural environment to pose great ecological and environmental risks. We also observed that the re-aggregation of suspended SWCNTs in the presence of cations was dependent on the SDBS concentration rather than the SWCNT concentration in the suspension. Both SDBS and sonication play important roles in the dispersion of SWCNTs, with sonication breaking down large aggregates of SWCNTs, while SDBS adsorbed on the SWCNTs inhibits the coagulation and aggregation by steric/electrostatic repulsion to maintain the stability of the suspension in water.

Natural organic matter (NOM) has a profound effect on the colloidal stability of discharged C₆₀ nanoparticles in the water environment, which influences the environmental behaviors and risks of C₆₀ and therefore merits more specific studies. This study investigates the effects of humic acid (HA), as a model NOM, on the aqueous stabilization of C₆₀ powder and the colloidal stability of a previously suspended C₆₀ suspension (aqu/nC₆₀) with variations of pH values and ionic strengths. Our results reveal that HA could disperse C₆₀ powder in water to some degree, but was unable to stably suspend them. The aqu/nC₆₀ could remain stable at pH>4 but was destabilized at lower pH values. However, the colloidal stability of aqu/nC₆₀ in the presence of HA was insensitive to pH 3–11, owing to the adsorption of HA onto nC₆₀ and the increased electrosteric repulsions among nC₆₀ aggregates. The colloidal stability of aqu/nC₆₀, with and without HA, decreased as we increased the valence and concentration of the added cations. HA was found to mitigate the destabilization effect of Na⁺ on the colloidal stability of aqu/nC₆₀ by increasing the critical coagulation concentration (CCC) of Na⁺, while HA lowered the CCCs of Ca²⁺ and La³⁺ probably by the bridging effect of nC₆₀ with HA aggregates formed through the intermolecular bridging of the HA macromolecules via cation complexation at high concentrations of cations with high valences.

To better understand the nanoparticle (NP) transport in the environment, the agglomeration and sedimentation of Al₂O₃, SiO₂, and TiO₂ NPs were evaluated after being treated with bovine serum albumin (BSA) and a commercial humic acid (HA). The morphology of NP agglomerates was examined through a transmission electron microscope (TEM), and the agglomeration kinetics was evaluated using established time-resolved dynamic light scattering techniques. BSA treatments decreased the hydrodynamic diameters (d_H) of the three NPs in both NaCl and CaCl₂ electrolytes due to their steric repulsive forces caused by the BSA globular architecture. The treatments using HA induced the smallest d_H of NPs in NaCl electrolyte, but the largest d_H of NPs was found in CaCl₂ electrolyte, because the HA bound to each other via calcium complexation and thereby enhanced the NP agglomeration. The zeta potentials of NPs were not the dominant factor to affect agglomeration. The NP sedimentation kinetics were studied through measuring the suspension optical absorbance. It was shown that the BSA treatments retarded the sedimentation in most situations; however, HA treatments accelerated the sedimentation greatly in CaCl₂ electrolyte, which was consistent with the measured changes in the d_H values. The smallest d_H of HA-treated NPs in NaCl electrolyte did not result in the lowest sedimentation rate, which indicated that the agglomeration size was not the only factor to affect the NP sedimentation.

The environmental applications and implications of functionalized carbon nanotubes (CNTs) have received much attention. In this study, the adsorption interactions of graphitized multi-walled carbon nanotubes (G-MWCNTs) and functionalized MWCNTs, including hydroxylated MWCNTs (OH-MWCNTs), carboxylated MWCNTs (COOH-MWCNTs), and aminated MWCNTs (NH₂-MWCNTs), for tetracycline in the aqueous solution were systemically investigated, and the effects of Cu(II) and Ni(II) (two metal ions commonly present in aquatic environments) on MWCNTs-tetracycline interactions were examined. Results showed that the adsorption affinities followed an order of G-MWCNTs>OH-MWCNTs>COOH-MWCNTs>NH₂-MWCNTs, indicating that the adsorptive interactions between MWCNTs and tetracycline can be largely affected by the types and abundance of functionalities on the MWCNTs surfaces. Both Cu(II) and Ni(II) had a negligible effect on the adsorption of tetracycline to G-MWCNTs, but varied effects of the metal ions were observed for the three functionalized MWCNTs. In general, Cu(II) exhibited a more pronounced effect for the adsorption of tetracycline than Ni(II), due to the degree of complexing capability.

The increasing release of silver (Ag) nanoparticles (NPs) into the environment highlights the importance of exploring the interactions between Ag NPs and plants, which are the basis of most ecosystems. In this study, two plant species, Cucumis sativus L. (cucumber) and Triticum aestivum L. (wheat) were exposed to Ag NPs and Ag⁺ (added as AgNO₃) at the germination and vegetative growth stages. Above certain concentrations, Ag NPs and Ag⁺ were toxic to the two plants. However, stimulatory effects were observed on root elongation for the cucumbers that were exposed to Ag NPs at concentrations below 200 mg/L, and Ag⁺ at concentrations below 5 mg/L. The two plants were more susceptible to the toxicity of Ag NPs at the vegetative growth stage than the germination stage. Ag was accumulated in the roots and was subsequently translocated to the shoots after the exposure to Ag NPs. To assess the role of released Ag⁺, we measured the dissolution of Ag NPs in exposure solutions. About 0.03% and 0.01% of Ag NPs were dissolved into a hydroponic solution at the germination stage for cucumber and wheat, respectively; while 0.17% and 0.06% at the vegetative period for cucumber and wheat, respectively. Cysteine, a strong chelating ligand of Ag⁺, could completely eliminate the effects of Ag NPs on cucumber and wheat, suggesting that the phytotoxicity of Ag NPs was possibly caused by the release of Ag⁺.

The removal of arsenic from aqueous solution is crucial to human health and environmental pollution. Herein, flower-like α-Fe₂O₃ nanostructures were synthesized via a template-free microwave-assisted solvothermal technique, and were applied as adsorbents for the removal of arsenic (As(V)) from aqueous solutions. The results indicated that the synthesized flower-like α-Fe₂O₃ showed excellent sorption properties and had a maximum sorption capacity of 47.64 mg/g for As(V). Meanwhile, the experimental results of photodegradation of methylene blue (MB) indicated that the as-synthesized flower-like α-Fe₂O₃ exhibited very high photocatalytic performance for the photodegradation of MB and that the as-obtained flower-like α-Fe₂O₃ nanostructures were suitable materials in wastewater treatment.