Influence of agglomeration of cerium oxide nanoparticles and speciation of cerium(III) on short term effects to the green algae Chlamydomonas reinhardtii

Acute toxicity of cerium to neonatal Daphnia magna: Responses of antioxidant systems, influence of environmental factors and development of a biotic ligand model

The bioavailability of cerium (Ce) and its toxic effects on aquatic organisms are still unclear, which limits the toxicity prediction and pollution control for this element. Here, the acute toxicity of Ce to Daphnia magna neonates and the responses of the antioxidant system were investigated, and the quantitative relationships between the toxicity of Ce and environmental factors were determined. The 24 and 48 h EC50Ce-D values based on the dissolved concentration of Ce in Daphnia magna were 60.6 and 10.9 μM, respectively, and the EC50Ce3+ values were 23.4 and 3.73 μM, respectively. After Ce exposure at environmentally relevant concentrations (0.5-3.5 μM), significant increases in superoxide dismutase activity and malondialdehyde content were observed in Daphnia magna, while significant decreases in catalase activity and H2O2 content occurred. Low levels of Ce cause oxidative damage to Daphnia magna and adverse impacts on the antioxidant system; however, further molecular-based studies are needed. The addition of Ca2+ or Na+ reduced the acute toxicity of Ce to Daphnia magna. In contrast, Mg2+ (MgSO4) promoted Ce toxicity, which is a new finding related to the interaction effects between cations and rare earth elements on biological ligands; however, the effects of SO42+ could not be distinguished. Complexation with organic ligands could significantly reduce the toxicity of Ce to Daphnia magna; however, complexes of Ce with citric acid and malic acid might be bioavailable to Daphnia magna. In the absence of organic ligands and competing metals, the binding constant of Ce3+ to Daphnia magna at toxic concentrations was 5.83. The log K values for the competitive effects of Ca2+ and Na+ were 3.73 and 2.59, respectively, while the log K value for the protective effect of fulvic acid was 3.76. These results contribute to understanding the toxicity of Ce and will help predict the toxicity of Ce in freshwater.

Interlaboratory comparison of centrifugal ultrafiltration with ICP-MS detection in a first-step towards methods to screen for nanomaterial release during certification of drinking water contact materials

A key requirement for evaluating the safety of nano-enabled water treatment devices is measuring concentrations of insoluble nanomaterials released from devices into water that may be ingested by consumers. Therefore, there is a need for simple technique that uses commonly available commercial laboratory techniques to discriminate between nanoparticles and dissolved by-products of the nanomaterial (e.g., ionic metals). Such capabilities would enable screening for particulate or dissolved metals released into water from nanomaterial-containing drinking water contact materials (e.g., paint coatings) or devices (e.g., filters). This multi-laboratory study sought to investigate the use of relatively inexpensive centrifugal ultrafilters to separate nanoparticulate from ionic metal in combination with inductively-coupled plasma mass spectrometry (ICP-MS) detection. The accuracy, precision, and reproducibility for the proposed method were assessed using mixtures of nanoparticulate and ionic gold (Au) in a standard and widely utilized model water matrix (NSF International Standard 53/61). Concentrations for both ionic and nanoparticulate gold based upon measurements of Au mass in the initial solutions and Au permeating the centrifugal ultrafilters. Results across different solution compositions and different participating labs showed that ionic and nanoparticulate Au could be consistently discriminated with ppb concentrations typically resulting in <10 % error. A mass balance was not achieved because nanoparticles were retained on membranes embedded in plastic holders inside the centrifuge tubes, and the entire apparatus could not be acid and/or microwave digested. This was a minor limitation considering the ultrafiltration method is a screening tool, and gold concentration in the permeate indicates the presence of ionic metal rather than nanoforms. With further development, this approach could prove to be an effective tool in screening for nanomaterial release from water-system or device materials as part of third-party certification processes of drinking water compatible products.

The microalgae-based wastewater treatment system coupled with Cerium: A potential way for energy saving and microalgae boost

The microalgae-based system attracts more attention in wastewater treatment for high quality effluent, low carbon emission, and resource utilization. Light is the key factor for algae growth, but the light masking in sewage will cause low efficiency of the system. This study designed laboratory scale experiments with Chlorella to investigate the influence of cerium on the nutrient removal by algae wastewater treatment system under different light intensities. The best removal rates of NH₄-N, TP, and COD were 72.43%, 88.87%, and 68.08% under 50 µmol/(m ²·s) light intensity and 1 mg/L Ce. Low concentration of Ce could activate protein synthesis, electron transfer, and antioxidase, while excessive Ce might cause toxicity which could be relieved by strong light for energy supply and further activating superoxide dismutase (SOD) and catalase (CAT). Comparing to other similar experiences, this system reached an equal or greater performance on nutrients removal with better efficiency in light utilization. It might provide a new idea for microalgae-based system development.

Evaluation of Apical and Molecular Effects of Algae Pseudokirchneriella subcapitata to Cerium Oxide Nanoparticles

Cerium oxide engineered nanoparticles (nCeO₂) are widely used in various applications and are, also, increasingly being detected in different environmental matrixes. However, their impacts on the aquatic environment remain poorly quantified. Hence, there is a need to investigate their effects on non-target aquatic organisms. Here, we evaluated the cytotoxic and genotoxic effects of <25 nm uncoated-nCeO₂ on algae Pseudokirchneriella subcapitata. Apical (growth and chlorophyll a (Chl a) content) and genotoxic effects were investigated at 62.5-1000 µg/L after 72 and 168 h. Results demonstrated that nCeO₂ induced significant growth inhibition after 72 h and promotion post 96-168 h. Conversely, nCeO₂ induced enhanced Chl a content post 72 h, but no significant changes were observed between nCeO₂-exposed and control samples after 168 h. Hence, the results indicate P. subcapitata photosynthetic system recovery ability to nCeO₂ effects under chronic-exposure conditions. RAPD-PCR profiles showed the appearance and/or disappearance of normal bands relative to controls; indicative of DNA damage and/or DNA mutation. Unlike cell recovery observed post 96 h, DNA damage persisted over 168 h. Thus, sub-lethal nCeO₂-induced toxicological effects may pose a more serious threat to algae than at present anticipated.

Humic acid-mediated reduction in toxicity of Co3O4 NPs towards freshwater and marine microalgae in surfactant mixed medium

The ever-increasing applications of Co3O4 nanoparticles (NPs) have posed a serious concern about their discharge in the aquatic environment and ecotoxic implications. Being toxic towards aquatic species, the impact of other aquatic components such as dissolved organic matter (DOM), salinity, and surfactants are not studied sufficiently for their effect on the stability and ecotoxicity of Co3O4 NPs. The present study aims at the influence of humic acid (HA) on the toxicity of Co3O4 NPs in freshwater (C. minutissima) and marine (T. suecica) microalgae under surfactants mixed medium. The measure of % reduction in biomass and photosynthetic pigment were used as toxicity endpoints. Among various tested concentrations of HA, 25 mg/L HA was found suitable to minimize the NP's toxicity with or without the presence of surfactants. Co3O4 NPs mediated reduction in biomass of C. minutissima was significantly minimized by the cumulative effect of HA with T80 (51.68 ± 4.55%) followed by CTAB (46.23 ± 5.62%) and SDS (42.60 ± 2.46%). Similarly, HA with T80 (26.93 ± 6.38%) followed by SDS (17.02 ± 6.64%) and CTAB (13.01 ± 3.81%) were found to minimize the growth inhibitory effect of Co3O4 NPs in T. suecica. The estimation of chlorophyll - a content also indicated that microalgae treated with HA could maintain their photosynthetic ability more than control even in the co-presence of surfactants. Also, the reduced toxicity of Co3O4 NPs were attributed to an increase in hydrodynamic sizes of HA-treated Co3O4 NPs in both marine media (f/2) and freshwater media (BG11) due to increased aggregation and faster sedimentation of Co3O4 NPs.

Toxicity mechanism of cerium oxide nanoparticles on cyanobacteria Microcystis aeruginosa and their ecological risks

The extensive application of cerium oxide nanoparticles (CeO₂ NPs), a type of rare earth nanomaterial, led to pollution into aquatic environments. Cyanobacteria, a significant component of freshwater ecosystems, can interact with CeO₂ NPs. However, little attention has been paid as to whether CeO₂ NPs will have adverse effects on cyanobacteria. In the present study, Microcystis aeruginosa (FACHB-942) was exposed to different concentrations (0, 1, 10, and 50 mg/L) of CeO₂ NPs. Results showed 50 mg/L CeO₂ NPs inhibited algal growth (11.48% ± 5.76%), suppressed photosynthesis and induced the generation of reactive oxygen species (ROS) after 72 h exposure. The toxicity mechanism is the adsorption of CeO₂ NPs on cell surface, the ROS formation and the intracellular Ce. Additionally, the intracellular microcystins (MCs) content was significantly induced (11.84% ± 1.47%) by 50 mg/L CeO₂ NPs, while no significance was found in 1 and 10 mg/L CeO₂ NP treatments. Results indicated high concentrations of CeO₂ NPs could be toxic to algae through the adverse effects on algal growth and photosynthesis. Moreover, the promoted MCs production could also pose a threat to freshwater ecosystems due to the possible release into the environment.

An integrated ICP-MS-based analytical approach to fractionate and characterize ionic and nanoparticulate Ce species

Cerium dioxide nanoparticles (CeO₂ NPs) are widely used in various fields, leading to concern about their effect on human health. When conducting in vivo investigations of CeO₂ NPs, the challenge is to fractionate ionic Ce and CeO₂ NPs and to characterize CeO₂ NPs without changing their properties/state. To meet this challenge, we developed an integrated inductively coupled plasma-mass spectrometry (ICP-MS)-based analytical approach in which ultrafiltration is used to fractionate ionic and nanoparticulate Ce species while CeO₂ NPs are characterized by single particle-ICP-MS (sp-ICP-MS). We used this technique to compare the effects of two sample pretreatment methods, alkaline and enzymatic pretreatments, on ionic Ce and CeO₂ NPs. Results showed that enzymatic pretreatment was more efficient in extracting ionic Ce or CeO₂ NPs from animal tissues. Moreover, results further showed that the properties/states of all ionic and nanoparticulate Ce species were well preserved. The rates of recovery of both species were over 85%; the size distribution of CeO₂ NPs was comparable to that of original NPs. We then applied this analytical approach, including the enzymatic pretreatment and ICP-MS-based analytical techniques, to investigate the bioaccumulation and biotransformation of CeO₂ NPs in mice. It was found that the thymus acts as a "holding station" in CeO₂ NP translocation in vivo. CeO₂ NP biotransformation was reported to be organ-specific. This is the first study to evaluate the impact of enzymatic and alkaline pretreatment on Ce species, namely ionic Ce and CeO₂ NPs. This integrated ICP-MS-based analytical approach enables us to conduct in vivo biotransformation investigations of CeO₂ NPs.

GO-CeO₂ nanohybrid for ultra-rapid fluoride removal from drinking water

The presence of excess fluoride (F^- > 1.5 mg/L) in drinking water affects more than 260 million people globally and leads to dental and skeletal fluorosis among other health problems. This study investigated fluoride removal by graphene oxide-ceria nanohybrid (GO-CeO₂) and elucidated the mechanisms involved. The nanohybrid exhibited ultra-rapid kinetics for fluoride removal and the equilibrium (85% removal, 10 mg F^-/L initial concentration) was achieved within 1 min which is one of the fastest kinetics for fluoride removal reported so far. Fluoride removal by the nanohybrid followed Langmuir isotherm with a maximum adsorption capacity of 8.61 mg/g at pH 6.5 and that increased to 16.07 mg/g when the pH was lowered to 4.0. Based on the experimental results and characterization data, we have postulated that both electrostatic interaction and surface complexation participated in the fluoride removal process. The O^2- ions present in the CeO₂ lattice were replaced by F^- ions to make a coordination compound (complex). While both Ce⁴⁺ and Ce³⁺ were present in ceria nanoparticles (CeO₂ NPs), Ce³⁺ participated in fluoride complexation. During fluoride removal by GO-CeO₂, the GO sheets acted as electron mediators and help to reduce Ce⁴⁺ to Ce³⁺ at the CeO₂ NPs-GO interface, and the additional Ce³⁺ enhanced fluoride removal by the nanohybrid.

The preparation temperature influences the physicochemical nature and activity of nanoceria

Cerium oxide nanoparticles, so-called nanoceria, are engineered nanomaterials prepared by many methods that result in products with varying physicochemical properties and applications. Those used industrially are often calcined, an example is NM-212. Other nanoceria have beneficial pharmaceutical properties and are often prepared by solvothermal synthesis. Solvothermally synthesized nanoceria dissolve in acidic environments, accelerated by carboxylic acids. NM-212 dissolution has been reported to be minimal. To gain insight into the role of high-temperature exposure on nanoceria dissolution, product susceptibility to carboxylic acid-accelerated dissolution, and its effect on biological and catalytic properties of nanoceria, the dissolution of NM-212, a solvothermally synthesized nanoceria material, and a calcined form of the solvothermally synthesized nanoceria material (ca. 40, 4, and 40 nm diameter, respectively) was investigated. Two dissolution methods were employed. Dissolution of NM-212 and the calcined nanoceria was much slower than that of the non-calcined form. The decreased solubility was attributed to an increased amount of surface Ce4+ species induced by the high temperature. Carboxylic acids doubled the very low dissolution rate of NM-212. Nanoceria dissolution releases Ce3+ ions, which, with phosphate, form insoluble cerium phosphate in vivo. The addition of immobilized phosphates did not accelerate nanoceria dissolution, suggesting that the Ce3+ ion release during nanoceria dissolution was phosphate-independent. Smaller particles resulting from partial nanoceria dissolution led to less cellular protein carbonyl formation, attributed to an increased amount of surface Ce3+ species. Surface reactivity was greater for the solvothermally synthesized nanoceria, which had more Ce3+ species at the surface. The results show that temperature treatment of nanoceria can produce significant differences in solubility and surface cerium valence, which affect the biological and catalytic properties of nanoceria.

The stochastic association of nanoparticles with algae at the cellular level: Effects of NOM, particle size and particle shape

Association of nanoparticles (NPs) with algae likely plays a critical role in their transfer in aquatic food chains. Although our understanding of the ecotoxicity and fate of NPs in the environment is increasing, it is still unclear how the physicochemical properties of NPs influence their interaction with algae at cellular levels and how this is reflected at a population level. This is due to the limitation in the existing analytical techniques to quantify the association of NPs with cells. To fill this data gap, we applied the novel technique of single-cell inductively coupled plasma mass spectrometry to quantify the cellular association of gold (Au)-NPs with algal cells (Pseudokirchneriella subcapitata) as a function of particle size, shape (spherical 10 nm, spherical 60 nm, spherical 100 nm, rod-shaped 10 × 40 nm, and rod-shaped 50 × 100 nm), and surface chemistry [citrate and natural organic matter (NOM) coating] on a cell-by-cell basis. The association of Au-NPs with algal cells was found to be a random probability following a so-called stochastic process; after 72 h of exposure, less than 45% of the cell population accumulated NPs on their surface. The number of Au-NPs per cell was found to be heterogeneously distributed as some cells were associated with a significantly higher number (e.g. up to 600 spherical 10 nm particles per cell) of Au-NPs than other cells present in the medium. The presence of NOM on the surface of the particles decreased the percentage of cells containing NPs except for the spherical 60 nm Au-NPs. We conclude that some algae within a population can accumulate NPs on their surface and this accumulation is influenced by the size, shape, and surface chemistry of NPs. It is important to understand how NPs may enter aquatic food chains to assess the possible risk.

Environmental concentrations of copper nanoparticles affect vital functions in Ankistrodesmus densus

Nanoparticles (NPs) have unique properties, leading to their widespread application in industry, consequently increasing their concentration in aquatic ecosystems. Although environmentally significant concentrations are still low, they tend to increase because of the intense use, posing into risk microalgae communities. Microalgae are primary producers that support food chains in aquatic ecosystems; thus factors that interfere with their physiology can be propagated throughout the food web. The present research investigated the effects of copper nanoparticles (Cu-NPs) in the physiology of a cosmopolitan green microalgae, Ankistrodesmus densus. Here, we focused on environmental NPs levels, so an ample Cu-NPs range was used, 0.3-635 μg L^-1. Considering that NPs dissolve into the medium releasing their constituent material, free Cu²⁺ ions were determined and considered as surrogate for NPs concentration, which varied from 2.1 × 10^-9 to 8.4 × 10^-9 mol L^-1. The experiment was based in 72 h Cu-NPs exposure, and to access the physiology of A. densus, we monitored population growth, photochemistry of photosynthesis and the content of cell biomolecules (total proteins, carbohydrates and lipids). The results showed that 2.1 × 10^-9 mol L^-1 free Cu²⁺ was enough to decrease growth rate, but 2.5x higher Cu was necessary to affect the photosynthetic parameters. Inorganic carbon fixation rate calculated by absolute electron transport rates was affected. Considering cell biomolecules, total proteins accumulated at 6.5 × 10^-9 and kept increasing up to 8.4 × 10^-9 mol L^-1 free Cu²⁺. Because this was not related to biomass formation, we suggest a possible association with cell detoxification mechanisms. The most clear finding that emerged from this study is that environmental Cu-NPs concentrations affect vital functions in the green microalgae A. densus. An implication of this is the possibility of facing problems related to a increase of NPs in aquatic ecosystems in the near future.

Mitigating the toxic effects of CdSe quantum dots towards freshwater alga Scenedesmus obliquus: Role of eco-corona

The extensive use of semiconducting nanoparticles such as quantum dots in biomedical and industrial products can lead to their inadvertent release into the freshwater system. Natural exudates in the aquatic system comprising extracellular polymeric substance (EPS) and protein-rich metabolites can eventually adsorb onto the quantum dots (QDs) surface and form an eco-corona. The alterations in the physio-chemical and toxicological behavior of CdSe/ZnS QDs under the influence of eco-corona in the freshwater system have not been explored yet. In the present study, lake water medium conditioned with exudate secreted by Scenedesmus obliquus was utilized as an eco-corona forming matrix. The time-based evolution of the eco-corona on the differently charged CdSe/ZnS QDs was analyzed using transmission electron microscopy and dynamic light scattering. Aging of amine-QDs in algal exudate for 72 h showed enhanced aggregation (Mean Hydrodynamic Diameter- 1969 nm) as compared to carboxyl-QDs (1543 nm). Further, eco-coronation tends to impart an overall negative charge to the QDs. The fluorescence intensity of amine-QDs was quenched by 84% due to the accumulation of higher eco-corona. An integrative effect of surface charge and accumulated eco-corona layer influenced the Cd²⁺ ion leaching from the QDs. An enhancement in the algal cell viability treated with carboxyl - CdSe/ZnS (90%) and amine- CdSe/ZnS QDs (94%) aged for 72 h suggested that eco-corona can effectively mitigate the inherent toxicity of the QDs. The oxidative stress markers in the algal cells (LPO, SOD, and CAT) were in correlation with the cytotoxicity results. The algal photosynthetic efficiency depended on the deposition of eco-coronated QDs on the cell surface. Cellular uptake results indicated low Cd²⁺ concentration of nearly 13.9 and 11.5% for carboxyl- and amine- CdSe/ZnS QDs respectively. This suggests that eco-coronation directly influences the bioavailability of engineered nanoparticles.

Ecotoxicity Responses of the Macrophyte Algae Nitellopsis obtusa and Freshwater Crustacean Thamnocephalus platyurus to 12 Rare Earth Elements

Due to unique chemical properties, rare earth elements (REEs) are increasingly used in versatile technological applications. They are considered emerging environmental contaminants, since they become mobile instead of being bound in rocks. At present, the information on REE effects to aquatic biota is scarce and contradictory. This study aims to explore the ecotoxicity of 11 lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, and Lu) and yttrium (Y) to charophyte algae Nitellopsis obtusa and microcrustaceans Thamnocephalus platyurus. Median lethal concentrations (LC50) were assessed in characean cells at 8, 12, 16, 20, and 24 days of exposure, and 24-h LC50s were determined in shrimps. According to the EU−Directive 93/67/EEC hazard classification scheme and 24-day LC50 values generated for N. obtusa, REE effects were assigned from “harmful” to “very toxic” (Gd), while 24-h LC50s for T. platyurus were classified as “harmful” or “toxic” (based on nominal concentrations) and as “toxic” or “very toxic” (based on REE free ion concentrations calculated with CHEAQS Next software). The data obtained for algae showed correlations with the REE atomic numbers (r = −0.68, p < 0.05) and ionic radii (r = 0.65, p < 0.05) at the most extended 24-day exposure only. The analysis of the trends of concentration−response (c–r) curves obtained at increasing exposure durations (8–24 days), alongside the 24-day LC50s ranging within almost two orders of magnitude, allowed a more-toxic heavy REE group to be distinguished, and somewhat different modes REE actions to be envisioned for N. obtusa.

Effects of CeO2 Nanoparticles on Microcystis aeruginosa Growth and Microcystin Production

The interaction between metal oxide nanoparticles and toxin-producing cyanobacteria is relatively unknown. The present work exposed Microcystis sp.7806 to different concentrations of cerium oxide nanoparticles (CeO₂ NPs) (1 mg/L, 10 mg/L and 50 mg/L), and evaluated the growth, photosynthetic activity, reactive oxygen species level, and the extra-(intra-) cellular microcystin-LR (MC-LR) contents. The particle size, zeta potential and cerium ions released into the medium were analyzed. Results showed 10 mg/L NP treatment promoted algae growth but slightly inhibited the photosynthetic yield of algae, and the 50 mg/L treatment reduced algae biomass. The algal cells remarkably responded to oxidative stress at higher concentrations (10 mg/L and 50 mg/L). CeO₂ NPs largely increased the intracellular MC-LR content at 50 mg/L, and significantly reduced the extracellular MC-LR content at any concentration. This demonstrates CeO₂ NPs may pose an ecological risk potential during harmful algal blooms by stimulating toxin production.

Physical Properties of Carbon Nanomaterials and Nanoceria Affect Pathways Important to the Nodulation Competitiveness of the Symbiotic N2 -Fixing Bacterium Bradyrhizobium diazoefficiens

The pathogenicity and antimicrobial properties of engineered nanomaterials (ENMs) are relatively well studied. However, less is known regarding the interactions of ENMs and agriculturally beneficial microorganisms that affect food security. Nanoceria (CeO2 nanoparticles (NPs)), multiwall carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), and carbon black (CB) have been previously shown to inhibit symbiotic N2 fixation in soybeans, but direct rhizobial susceptibility is uncertain. Here, Bradyrhizobium diazoefficiens associated with symbiotic N2 fixation in soybeans is assessed, evaluating the role of soybean root exudates (RE) on ENM-bacterial interactions and the effects of CeO2 NPs, MWCNTs, GNPs, and CB on bacterial growth and gene expression. Although bacterial growth is inhibited by 50 mg L-1 CeO2 NPs, MWCNTs, and CB, all ENMs at 0.1 and 10 mg L-1 cause a global transcriptomic response that is mitigated by RE. ENMs may interfere with plant-bacterial signaling, as evidenced by suppressed upregulation of genes induced by RE, and downregulation of genes encoding transport RNA, which facilitates nodulation signaling. MWCNTs and CeO2 NPs inhibit the expression of genes conferring B. diazoefficiens nodulation competitiveness. Surprisingly, the transcriptomic effects on B. diazoefficiens are similar for these two ENMs, indicating that physical, not chemical, ENM properties explain the observed effects.

Potential Hazard of Lanthanides and Lanthanide-Based Nanoparticles to Aquatic Ecosystems: Data Gaps, Challenges and Future Research Needs Derived from Bibliometric Analysis

Lanthanides (Ln), applied mostly in the form of nanoparticles (NPs), are critical to emerging high-tech and green energy industries due to their distinct physicochemical properties. The resulting anthropogenic input of Ln and Ln-based NPs into aquatic environment might create a problem of emerging contaminants. Thus, information on the biological effects of Ln and Ln-based NPs is urgently needed for relevant environmental risk assessment. In this mini-review, we made a bibliometric survey on existing scientific literature with the main aim of identifying the most important data gaps on Ln and Ln-based nanoparticles' toxicity to aquatic biota. We report that the most studied Ln for ecotoxicity are Ce and Ln, whereas practically no information was found for Nd, Tb, Tm, and Yb. We also discuss the challenges of the research on Ln ecotoxicity, such as relevance of nominal versus bioavailable concentrations of Ln, and point out future research needs (long-term toxicity to aquatic biota and toxic effects of Ln to bottom-dwelling species).

Current methods to monitor microalgae-nanoparticle interaction and associated effects

Widespread use of nanoparticles for different applications has diffused their presence in the environment, particularly in water. Many studies have been conducted to evaluate their effects on aquatic organisms. Microalgae are at the base of aquatic trophic chains. These organisms which can be benthic or pelagic, meaning that they can enter into interaction with all kinds of particulate materials whatever their density, and constitute an interesting model study. The purpose of this review was to gather more than sixty studies on microalgae exposure to the different nanoparticles that may be present in the aquatic environment. After a brief description of each type of nanoparticle (metals, silica and plastic) commonly used in ecotoxicological studies, techniques to monitor their properties are presented. Then, different effects on microalgae resulting from interaction with nanoparticles are described as well as the parameters and techniques for monitoring them. The impacts described in the literature are primarily shading, ions release, oxidative stress, adsorption, absorption and disruption of microalgae barriers. Several parameters are proposed to monitor effects such as growth, photosynthesis, membrane integrity, biochemical composition variations and gene expression changes. Finally, in the literature, while different impacts of nanoparticles on microalgae have been described, there is no consensus on evidence of nanomaterial toxicity with regard to microalgae. A parallel comparison of different nanoparticle types appears essential in order to prioritize which factors exert the most influence on toxicity in microalgae cultures: size, nature, surface chemistry, concentration or interaction time.

Simultaneous Removal and Recovery of Metal Ions and Dyes from Wastewater through Montmorillonite Clay Mineral

The main objective of this work was to evaluate the potential of Montmorillonite nanoclay (Mt), readily and inexpensively available, for the simultaneous adsorption (and removal) of two classes of pollutants: metal ions and dyes. The attention was focused on two "model" pollutants: Ce(III) and crystal violet (CV). The choice is due to the fact that they are widespread in wastewaters of various origins. These characteristics, together with their effect on human health, make them ideal for studies on water remediation. Moreover, when separated from wastewater, they can be recycled individually in industrial production with no or simple treatment. Clay/pollutant hybrids were prepared under different pH conditions and characterized through the construction of the adsorption isotherms and powder X-ray diffraction. The adsorption behavior of the two contaminants was revealed to be significantly different: the Langmuir model reproduces the adsorption isotherm of Ce(III) better, thus indicating that the clay offers a unique adsorption site to the metal ions, while the Freundlich model proved to be the most reliable for the uptake of CV which implies heterogeneity of adsorption sites. Moreover, metal ions do not adsorb at all under acidic conditions, whereas the dye is able to adsorb under all the investigated conditions. The possibility to modulate the adsorption features by simply changing the pH conditions was successfully employed to develop an efficient protocol for the removal and separation of the different components from aqueous solutions mimicking wastewaters.

Role of extracellular polymeric substances on the behavior and toxicity of silver nanoparticles and ions to green algae Chlorella vulgaris

The effect of extracellular polymeric substances (EPS), vital organic matters and nutrient elements in the natural environment, on the behavior and toxicology of silver nanoparticles (AgNPs) and ions remains ambiguous. In this study, the role of EPS on the toxicity of AgNPs and dissolved silver ions (from AgNO₃) to a green algae Chlorella vulgaris was investigated. After the removal of EPS, algae accumulated more silver, about 7.41- and 1.25-fold of those in the algae with EPS for AgNPs and AgNO₃ treatments, respectively. The large amount of accumulated silver was bound to the algal cell surface for AgNPs treatment and was internalized in the algae for AgNO₃ treatment, irrespective of the presence of EPS in algae. After exposure to AgNPs, the ruffles in the surfaces of algal cells were filled by AgNPs, and almost invisible. FTIR showed that for both AgNPs and AgNO₃, the aldehyde groups on the cell surface were oxidized to carboxyl groups by silver ions, irrespective of the presence of EPS in algal cells, indicating that silver ions were released from the oxidization of AgNPs and reacted with algal cells. The content of chlorophyll showed that AgNPs depressed algal growth more remarkably than did AgNO₃, independent of the presence of EPS in algae, suggesting that AgNPs had greater toxic effects on algae than did silver ions. The findings suggest that the barrier effect of EPS gave nanoparticles an extraordinary edge over ions, but EPS had no discerning effect on the interaction of algal cells with the silver ions released from AgNPs and AgNO₃, and also on the effect of AgNPs and AgNO₃ on algal growth.

Combined Effects of Test Media and Dietary Algae on the Toxicity of CuO and ZnO Nanoparticles to Freshwater Microcrustaceans Daphnia magna and Heterocypris incongruens: Food for Thought

The chemical composition of the test medium as well as the presence of algae (microcrustaceans' food) affects the bioavailability and thus the toxicity of metal nanoparticles (NP) to freshwater microcrustaceans. This study evaluated the effect of the addition of algae (Rapidocelis subcapitata at 7.5 × 10⁶ cells/mL) on the toxicity of CuO (primary size 22⁻25 nm) and ZnO NP (10⁻15 nm) to planktic Daphnia magna and benthic Heterocypris incongruens in artificial (mineral) and natural freshwater (lake water). The toxicity of ionic controls, CuSO₄ and ZnSO₄, was evaluated in parallel. When algae were added and the toxicity was tested in mineral medium, 48 h EC₅₀ of CuO and ZnO NP to D. magna was ~2 mg metal/L and 6-day LC₅₀ of H. incongruens was 1.1 mg metal/L for CuO and 0.36 mg metal/L for ZnO. The addition of algae to D. magna test medium mitigated the toxicity of CuO and ZnO NP 4⁻11-fold when the test was conducted in natural water but not in the artificial freshwater. The addition of algae mitigated the toxicity of CuSO₄ (but not ZnSO₄) to D. magna at least 3-fold, whatever the test medium. In the 6-day H. incongruens tests (all exposures included algae), only up to 2-fold differences in metal NP and salt toxicity between mineral and natural test media were observed. To add environmental relevance to NP hazard assessment for the freshwater ecosystem, toxicity tests could be conducted in natural water and organisms could be fed during the exposure.

Metallic nanoparticles influence the structure and function of the photosynthetic apparatus in plants

The applications of nanoparticles continue to expand into areas as diverse as medicine, bioremediation, cosmetics, pharmacology and various industries, including agri-food production. The widespread use of nanoparticles has generated concerns given the impact these nanoparticles - mostly metal-based such as CuO, Ag, Au, CeO₂, TiO₂, ZnO, Co, and Pt - could be having on plants. Some of the most studied variables are plant growth, development, production of biomass, and ultimately oxidative stress and photosynthesis. A systematic appraisal of information about the impact of nanoparticles on these processes is needed to enhance our understanding of the effects of metallic nanoparticles and oxides on the structure and function on the plant photosynthetic apparatus. Most nanoparticles studied, especially CuO and Ag, had a detrimental impact on the structure and function of the photosynthetic apparatus. Nanoparticles led to a decrease in concentration of photosynthetic pigments, especially chlorophyll, and disruption of grana and other malformations in chloroplasts. Regarding the functions of the photosynthetic apparatus, nanoparticles were associated with a decrease in the photosynthetic efficiency of photosystem II and decreased net photosynthesis. However, CeO₂ and TiO₂ nanoparticles may have a positive effect on photosynthetic efficiency, mainly due to an increase in electron flow between the photosystems II and I in the Hill reaction, as well as an increase in Rubisco activity in the Calvin and Benson cycle. Nevertheless, the underlying mechanisms are poorly understood. The future mechanistic work needs to be aimed at characterizing the enhancing effect of nanoparticles on the active generation of ATP and NADPH, carbon fixation and its incorporation into primary molecules such as photo-assimilates.

Combined toxicities of copper nanoparticles with carbon nanotubes on marine microalgae Skeletonema costatum

To investigate the combined toxicities of copper nanoparticles (nano-Cu) with carbon nanotubes (CNTs) on marine microalgae Skeletonema costatum, algal growth inhibition tests were carried out. Toxicities of nano-Cu with CNTs and without CNTs on microalgae were determined, respectively. Chlorophyll content and photosynthetic efficiency (ΦPSII) were determined to compare negative effects of nano-Cu with CNTs and without CNTs on photosynthesis. The concentration of Cu²⁺ released by nano-Cu into the medium was determined, and interactions between nano-Cu and CNTs were analyzed to study toxic mechanisms of combined toxicities of nano-Cu with CNTs. It was found that both nano-Cu and CNTs could inhibit the growth of the microalgae; however, the toxicity of CNTs on the microalgae was far lower than that of nano-Cu. The maximum growth inhibition ratio (IR) of nano-Cu on the microalgae was 86% appearing at 96 h under 1.0 mg/L nano-Cu treatment, while the maximum IR of CNTs on the microalgae was 58% at 96 h under 200 mg/L CNT treatment. CNTs could reduce the toxicity of nano-Cu on the microalgae in processes of growth and photosynthesis. Adsorption of Cu²⁺ on CNTs and aggregate between Cu and CNTs in the medium were main reasons for attenuation of toxicity of nano-Cu with adding CNTs.

Toxicity of Co nanoparticles on three species of marine microalgae

Cobalt nanoparticles (CoNPs) are being used in wide range of applications and may enter aquatic environments where they pose a potential threat to aquatic organisms. Algal growth inhibition tests were conducted to explore the potential toxicity of CoNPs on marine microalgae, Platymonas subcordiforus, Chaetoceros curvisetus and Skeletonema costatum. This is one of the first time to explore toxicity of CoNPs on marine algae systematically. The results showed that CoNPs induced toxicity on the three algae. The CoNP toxicity on three species microalgae was partly attributed to the Co²⁺ released by CoNPs in the f/2 seawater medium. The particle size distribution of CoNPs in seawater revealed that CoNPs were agglomerated in the seawater. The shading effect of CoNPs and scanning electron microscope (SEM) images also showed the aggregating of CoNPs and microalgae, which influenced the photosynthetic utilization and inhibited the growth of the three algae. The order of toxic sensitivity of CoNPs on the three algae was as follows: Platymonas subcordiforus < Chaetoceros curvisetus < Skeletonema costatum.

Uptake and effects of cerium(III) and cerium oxide nanoparticles to Chlamydomonas reinhardtii

Cerium (Ce) and cerium oxide nanoparticles (CeO₂ NP) are increasingly used in different applications. Upon their release into the aquatic environment, the exposure of aquatic organisms becomes likely. In this study, the uptake of CeO₂ NP and Ce³⁺ into the wild type and cell wall free mutant of Chlamydomonas reinhardtii was examined upon short term exposure. Separation of CeO₂ NP and Ce³⁺ not taken up or loosely bound to the cells was performed by washing algae with EDTA. Despite a concentration and time dependent increase of cellular Ce upon exposure to CeO₂ NP with the maximal calculated Ce concentration corresponding to 1.1 CeO₂ NP per cell, an internalization of CeO₂ NP with a mean size of 140 nm in C. reinhardtii was excluded. In contrast, dissolved Ce³⁺ (1 and 10 μM) was taken up both in the wild type and cell wall free mutant of C. reinhardtii, with a linear increase of cellular Ce within 1-2 h and maximal cellular Ce of 6.04 × 10^-4 mol L_cell^-1 (wild type) and 9.0 × 10^-5 mol L_cell^-1 (cell wall free mutant). Based on competition with Ca²⁺ for Ce³⁺ uptake, on the comparison of the wild type and the cell wall free mutant and on inhibition of photosynthetic yield, we suggest that no efficient uptake routes for Ce³⁺ are available in C. reinhardtii and that a fraction of the cellular Ce in the wild type strongly sorbs to the algal cell wall.

Toxicity, uptake, and accumulation of nano and bulk cerium oxide particles in Artemia salina

Although the toxicological impact of metal oxide nanoparticles has been studied for the last few decades on aquatic organisms, the exact mechanism of action is still unclear. The fate, behavior, and biological activity of nanoparticles are dependent on physicochemical factors like size, shape, surface area, and stability in the medium. This study deals with the effect of nano and bulk CeO₂ particles on marine microcrustacean, Artemia salina. The primary size was found to be 15 ± 3.5 and 582 ± 50 nm for nano and bulk CeO₂ (TEM), respectively. The colloidal stability and sedimentation assays showed rapid aggregation of bulk particles in seawater. Both the sizes of CeO₂ particles inhibited the hatching rate of brine shrimp cyst. Nano CeO₂ was found to be more toxic to A. salina (48 h LC₅₀ 38.0 mg/L) when compared to bulk CeO₂ (48 h LC₅₀ 92.2 mg/L). Nano CeO₂-treated A. salina showed higher oxidative stress (ROS) than those treated with the bulk form. The reduction in the antioxidant activity indicated an increase in oxidative stress in the cells. Higher acetylcholinesterase activity (AChE) was observed upon exposure to nano and bulk CeO₂ particles. The uptake and accumulation of CeO₂ particles were increased with respect to the concentration and particle size. Thus, the above results revealed that nano CeO₂ was more lethal to A. salina as compared to bulk particles.

Potency of (doped) rare earth oxide particles and their constituent metals to inhibit algal growth and induce direct toxic effects

Use of rare earth elements (REEs) has increased rapidly in recent decades due to technological advances. It has been accompanied by recurring rare earth element anomalies in water bodies. In this work we (i) studied the effects of eight novel doped and one non-doped rare earth oxide (REO) particles (aimed to be used in solid oxide fuel cells and gas separation membranes) on algae, (ii) quantified the individual adverse effects of the elements that constitute the (doped) REO particles and (iii) attempted to find a discernible pattern to relate REO particle physicochemical characteristics to algal growth inhibitory properties. Green algae Raphidocelis subcapitata (formerly Pseudokirchneriella subcapitata) were used as a test species in two different formats: a standard OECD201 algal growth inhibition assay and the algal viability assay (a 'spot test') that avoids nutrient removal effects. In the 24h 'spot' test that demonstrated direct toxicity, algae were not viable at REE concentrations above 1mgmetal/L. 72-hour algal growth inhibition EC₅₀ values for four REE salts (Ce, Gd, La, Pr) were between 1.2 and 1.4mg/L, whereas the EC₅₀ for REO particles ranged from 1 to 98mg/L. The growth inhibition of REEs was presumably the result of nutrient sequestration from the algal growth medium. The adverse effects of REO particles were at least in part due to the entrapment of algae within particle agglomerates. Adverse effects due to the dissolution of constituent elements from (doped) REO particles and the size or specific surface area of particles were excluded, except for La₂NiO₄. However, the structure of the particles and/or the varying effects of oxide composition might have played a role in the observed effects. As the production rates of these REO particles are negligible compared to other forms of REEs, there is presumably no acute risk for aquatic unicellular algae.

CeO2 NPs, toxic or protective to phytoplankton? Charge of nanoparticles and cell wall as factors which cause changes in cell complexity

CeO₂ nanoparticles (CeO₂ NPs) are well-known for their catalytic properties and antioxidant potential. Recent uses in therapy are based on the Ce⁺³ ions released by CeO₂ NPs. Reactions involving redox cycles between Ce⁺³ and Ce⁺⁴ oxidation stage seem to promote scavenging of reactive oxygen species (ROS), thus protecting cells from oxygen damage. However, the internalization of CeO₂ NPs and release of Ce⁺³ could be responsible for a toxic effect on cells. The literature reports controversial results on the toxicity of CeO₂ NPs to phytoplankton. Therefore, we have tested the potential toxic effect of two CeO₂ NPs (with positive and negative zeta potential) and bulk CeO₂ (at 0.1, 1, 10, 100 and 200mg·L^-1) on three species of microalgae from different environments: marine diatom (Phaeodactylum tricornutum), marine chlorophyte (Nannochloris atomus) and freshwater chlorophyte (Chlamydomonas reinhardtii) over 72h in batch cultures. Responses measured in the microalgae population are: growth, chlorophyll a, cell size, cell complexity, percentage of ROS, and percentage of cell membrane damage. Positive zeta potential CeO₂ NPs provoked greater cell complexity (up to 78, 172 and 23 times more cell complexity than in controls found for C. reinhardtii, P. tricornutum and N. atomus respectively) than negative zeta potential CeO₂ NPs. The SSC signal detected by flow cytometry measured increases of particles entering cells, and this is related to cell viability and levels of intracellular ROS (correlation between SSC and percentage of ROS of 0.72 and 0.97 found for C. reinhardtii and P. tricornutum). When increased cellular complexity over controls is between 2 and 6 times greater, CeO₂ (in bulk or nanoparticulate form) seems to protect against ROS. When increased cellular complexity is from 7 to 23 times greater, CeO₂ does not provoke toxic responses; however, when increased cellular complexity over controls is very high, from 61 to 172 times, increased ROS production and toxic responses are found. Results show that two factors, the charge of CeO₂ NPs and cell wall structure, constitute the primary barrier to the possible accumulation of CeO₂ NPs within phytoplankton cytosol.

Protection of Photosynthetic Algae against Ultraviolet Radiation by One-Step CeO2 Shellization

Photosynthetic microalgae play an important role in solar-to-chemical energy conversion on Earth, but the increasing solar ultraviolet (UV) radiation seriously reduces the biological photosynthesis. Here, we developed a one-step approach to construct cell-in-shell hybrid structure by using direct adsorption of CeO₂ nanoparticles onto cells. The engineered CeO₂ nanoshell can efficiently protect the enclosed Chlorella cell due to its excellent UV filter property, which can also eliminate UV-induced oxidative stress. The experiments demonstrate that the resulted algae-CeO₂ composites can guarantee their biological photosynthetic process and efficiency even under UV. This study follows a feasible strategy to protect living organisms by using functional nanomaterials to improve their biological functions.

Interaction of silver nanoparticles with algae and fish cells: a side by side comparison

Background

Silver nanoparticles (AgNP) are widely applied and can, upon use, be released into the aquatic environment. This raises concerns about potential impacts of AgNP on aquatic organisms. We here present a side by side comparison of the interaction of AgNP with two contrasting cell types: algal cells, using the algae Euglena gracilis as model, and fish cells, a cell line originating from rainbow trout (Oncorhynchus mykiss) gill (RTgill-W1). The comparison is based on the AgNP behavior in exposure media, toxicity, uptake and interaction with proteins.

Results

(1) The composition of exposure media affected AgNP behavior and toxicity to algae and fish cells. (2) The toxicity of AgNP to algae was mediated by dissolved silver while nanoparticle specific effects in addition to dissolved silver contributed to the toxicity of AgNP to fish cells. (3) AgNP did not enter into algal cells; they only adsorbed onto the cell surface. In contrast, AgNP were taken up by fish cells via endocytic pathways. (4) AgNP can bind to both extracellular and intracellular proteins and inhibit enzyme activity.

Conclusion

Our results showed that fish cells take up AgNP in contrast to algal cells, where AgNP sorbed onto the cell surface, which indicates that the cell wall of algae is a barrier to particle uptake. This particle behaviour results in different responses to AgNP exposure in algae and fish cells. Yet, proteins from both cell types can be affected by AgNP exposure: for algae, extracellular proteins secreted from cells for, e.g., nutrient acquisition. For fish cells, intracellular and/or membrane-bound proteins, such as the Na⁺/K⁺-ATPase, are susceptible to AgNP binding and functional impairment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-017-0254-9) contains supplementary material, which is available to authorized users.

Molecular and physiological responses to titanium dioxide and cerium oxide nanoparticles in Arabidopsis

Changes in tissue transcriptomes and productivity of Arabidopsis thaliana were investigated during exposure of plants to 2 widely used engineered metal oxide nanoparticles, titanium dioxide (nano-titania) and cerium dioxide (nano-ceria). Microarray analyses confirmed that exposure to either nanoparticle altered the transcriptomes of rosette leaves and roots, with comparatively larger numbers of differentially expressed genes found under nano-titania exposure. Nano-titania induced more differentially expressed genes in rosette leaves, whereas roots had more differentially expressed genes under nano-ceria exposure. MapMan analyses indicated that although nano-titania up-regulated overall metabolism in both tissues, metabolic processes under nano-ceria remained mostly unchanged. Gene enrichment analysis indicated that both nanoparticles mainly enriched ontology groups such as responses to stress (abiotic and biotic), and defense responses (pathogens), and responses to endogenous stimuli (hormones). Nano-titania specifically induced genes associated with photosynthesis, whereas nano-ceria induced expression of genes related to activating transcription factors, most notably those belonging to the ethylene responsive element binding protein family. Interestingly, there were also increased numbers of rosette leaves and plant biomass under nano-ceria exposure, but not under nano-titania. Other transcriptomic responses did not clearly relate to responses observed at the organism level, possibly because of functional and genomic redundancy in Arabidopsis, which may mask expression of morphological changes, despite discernable responses at the transcriptome level. In addition, transcriptomic changes often relate to transgenerational phenotypic development, and hence it may be productive to direct further experimental work to integrate high-throughput genomic results with longer term changes in subsequent generations. Environ Toxicol Chem 2017;36:71-82. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Importance of doping, dopant distribution, and defects on electronic band structure alteration of metal oxide nanoparticles: Implications for reactive oxygen species

Metal oxide nanoparticles (MONPs) are considered to have the potency to generate reactive oxygen species (ROS), one of the key mechanisms underlying nanotoxicity. However, the nanotoxicology literature demonstrates a lack of consensus on the dominant toxicity mechanism(s) for a particular MONP. Moreover, recent literature has studied the correlation between band structure of pristine MONPs to their ability to introduce ROS and thus has downplayed the ROS-mediated toxicological relevance of a number of such materials. On the other hand, material science can control the band structure of these materials to engineer their electronic and optical properties and thereby is constantly modulating the pristine electronic structure. Since band structure is the fundamental material property that controls ROS-producing ability, band tuning via introduction of dopants and defects needs careful consideration in toxicity assessments. This commentary critically evaluates the existing material science and nanotoxicity literature and identifies the gap in our understanding of the role of important crystal structure features (i.e., dopants and defects) on MONPs' electronic structure alteration as well as their ROS-generation capability. Furthermore, this commentary provides suggestions on characterization techniques to evaluate dopants and defects on the crystal structure and identifies research needs for advanced theoretical predictions of their electronic band structures and ROS-generation abilities. Correlation of electronic band structure and ROS will not only aid in better mechanistic assessment of nanotoxicity but will be impactful in designing and developing ROS-based applications ranging from water disinfection to next-generation antibiotics and even cancer therapeutics.

Toxic effects of nano-ZnO on marine microalgae Skeletonema costatum: Attention to the accumulation of intracellular Zn

To explore toxic mechanisms of nano-ZnO on marine microalgae, algal growth inhibition test was carried out, and total Zn in the cell and total dissolved Zn in f/2 medium were determined. It was found that nano-ZnO was more toxic than bulk-ZnO on marine microalgae Skeletonema costatum. No matter under nano-ZnO or bulk-ZnO treatment, accumulation of intracellular Zn had a good linear correlation with growth inhibition ratio (IR). The total Zn content in the cell of microalgae was the dominant toxic mechanism of ZnO. Intracellular total Zn could be an indispensable parameter to indicate the toxic effects of nano-ZnO. Higher intracellular total Zn under nano-ZnO treatment than bulk-ZnO resulted in more toxicity of nano-ZnO on microalgae. Compared with Zn(2+) released by nano-ZnO into medium, lipid peroxidation (MDA level) injury, aggregation of nano-ZnO and transmembrane process of nano-ZnO also contributed to toxicity of nano-ZnO on Skeletonema costatum. The accumulation of intracellular Zn provides a new insight into toxic mechanisms of nano-ZnO.

Behavior and chronic toxicity of two differently stabilized silver nanoparticles to Daphnia magna

While differences in silver nanoparticle (AgNP) colloidal stability, surface potential, or acute aquatic toxicity for differently stabilized AgNP have often been reported, these have rarely been studied in long-term ecotoxicity tests. In the current study, we investigated the chronic toxicity of AgNP to Daphnia magna over a 21-day period with two different stabilizers (citrate and detergent), representative for charge and sterical stabilizers, respectively. This was coupled with a series of short-term experiments, such as mass balance and uptake/depuration testing, to investigate the behavior of both types of AgNP during a typical media exchange period in the D. magna test for chronic toxicity. As expected, the sterically stabilized AgNP was more stable in the test medium, also in the presence of food; however, a higher uptake of silver after 24h exposure of the charge stabilized AgNP was found compared to the detergent-stabilized AgNP (0.046±0.006μgAgμgDW(-1) and 0.023±0.005μgAgμgDW(-1), respectively). In accordance with this, the higher reproductive effects and mortality were found for the charge-stabilized than for the sterically-stabilized silver nanoparticles in 21-d tests for chronic toxicity. LOEC was 19.2μgAgL(-1) for both endpoints for citrate-coated AgNP and >27.5μgAgL(-1) (highest tested concentration for detergent-stabilized AgNP). This indicates a link between uptake and toxicity. The inclusion of additional short-term experiments on uptake and depuration is recommended when longer-term chronic experiments with nanoparticles are conducted.

CeO2-covered nanofiber for highly efficient removal of phosphorus from aqueous solution

The lowering phosphorus concentration of lakes or rivers using adsorbents has been considered to be the most effective way to prevent water eutrophication. However, the development of an adsorbent is still challenging because conventional adsorbents have not shown a sufficient phosphorus adsorption capacity (0.3-2.0mmol/g) to treat industrial, agricultural or domestic wastewater at a large scale. Herein, a novel and effective strategy to remove phosphorus efficiently with a CeO2-covered nanofiber is shown. The CeO2-covered nanofiber was synthesized through (1) amine group immobilization onto an electrospun polyacrylonitrile nanofiber and (2) adsorption of Ce(3+) on it. The CeO2-covered nanofiber played a role in catching phosphate ions in an aqueous solution by the oxidation, reduction, and ion-exchange of adsorbed Ce(3+) on the nanofiber from CeO2 to CePO4, and enabled remarkable phosphate adsorption capacity of the nanofiber (ca. 17.0mmol/g) at the range of ca. pH 2-6. Our strategy might be the most feasible method to efficiently lower the phosphorus concentration in lakes or rivers owing to the easy and inexpensive preparation of CeO2-covered nanofiber at an industrial scale, with a high phosphate adsorption capacity.

Cerium doped red mud catalytic ozonation for bezafibrate degradation in wastewater: Efficiency, intermediates, and toxicity

In this study, the performance of bezafibrate (BZF) degradation and detoxification in the aqueous phase using cerium-modified red mud (RM) catalysts prepared using different cerium sources and synthesis methods were evaluated. Experimental results showed that the surface cerium modification was responsible for the development of the catalytic activity of RM and this was influenced by the cerium source and the synthesis method. Catalyst prepared from cerium (IV) by precipitation was found to show the best catalytic activity in BZF degradation and detoxification. Reactive oxygen species including peroxides, hydroxyl radicals, and super oxide ions were identified in all reactions and we proposed the corresponding catalytic reaction mechanism for each catalyst that prepared from different cerium source and method. This was supported by the intermediates profiles that were generated upon BZF degradation. The surface and the structural properties of cerium-modified RM were characterized in detail by several analytical methods. Two interesting findings were made: (1) the surface texture (specific surface area and mesoporous volume) influenced the catalytic reaction pathway; and (2) Ce(III) species and oxygen vacancies were generated on the surface of the catalyst after cerium modification. This plays an important role in the development of the catalytic activity.

Effects of pH and fulvic acids concentration on the stability of fulvic acids--cerium (IV) oxide nanoparticle complexes

The behavior of cerium (IV) oxide nanoparticles has been first investigated at different pH conditions. The point of zero charge was determined as well as the stability domains using dynamic light scattering, nanoparticle tracking analysis and scanning electron microscopy. A baseline hydrodynamic diameter of 180 nm was obtained indicating that individual CeO2 nanoparticles are forming small aggregates. Then we analyzed the particle behavior at variable concentrations of fulvic acids for three different pH-electrostatic scenarios corresponding to positive, neutral and negative CeO2 surface charges. The presence of fulvic acids was found to play a key role on the CeO2 stability via the formation of electrostatic complexes. It was shown that a small amount of fulvic acids (2 mg L(-1)), representative of environmental fresh water concentrations, is sufficient to stabilize CeO2 nanoparticles (50 mg L(-1)). When electrostatic complexes are formed between negatively charged FAs and positively charged CeO2 NPs the stability of such complexes is obtained with time (up to 7 weeks) as well as in pH changing conditions. Based on zeta potential variations we also found that the fulvic acids are changing the CeO2 acid-base surface properties. Obtained results presented here constitute an important outcome in the domain of risk assessment, transformation and removal of engineered nanomaterials released into the environment.

Heteroaggregation of nanoparticles with biocolloids and geocolloids

The application of nanoparticles has raised concern over the safety of these materials to human health and the ecosystem. After release into an aquatic environment, nanoparticles are likely to experience heteroaggregation with biocolloids, geocolloids, natural organic matter (NOM) and other types of nanoparticles. Heteroaggregation is of vital importance for determining the fate and transport of nanoparticles in aqueous phase and sediments. In this article, we review the typical cases of heteroaggregation between nanoparticles and biocolloids and/or geocolloids, mechanisms, modeling, and important indicators used to determine heteroaggregation in aqueous phase. The major mechanisms of heteroaggregation include electric force, bridging, hydrogen bonding, and chemical bonding. The modeling of heteroaggregation typically considers DLVO, X-DLVO, and fractal dimension. The major indicators for studying heteroaggregation of nanoparticles include surface charge measurements, size measurements, observation of morphology of particles and aggregates, and heteroaggregation rate determination. In the end, we summarize the research challenges and perspective for the heteroaggregation of nanoparticles, such as the determination of αhetero values and heteroaggregation rates; more accurate analytical methods instead of DLS for heteroaggregation measurements; sensitive analytical techniques to measure low concentrations of nanoparticles in heteroaggregation systems; appropriate characterization of NOM at the molecular level to understand the structures and fractionation of NOM; effects of different types, concentrations, and fractions of NOM on the heteroaggregation of nanoparticles; the quantitative adsorption and desorption of NOM onto the surface of nanoparticles and heteroaggregates; and a better understanding of the fundamental mechanisms and modeling of heteroaggregation in natural water which is a complex system containing NOM, nanoparticles, biocolloids and geocolloids.

On the mechanism of nanoparticulate CeO2 toxicity to freshwater algae

The factors affecting the chronic (72-h) toxicity of three nanoparticulate (10-34nm) and one micron-sized form of CeO2 to the green alga, Pseudokirchneriella subcapitata were investigated. To characterise transformations in solution, hydrodynamic diameters (HDD) were measured by dynamic light scatter, zeta potential values by electrophoretic mobility, and dissolution by equilibrium dialysis. The protective effects of humic and fulvic dissolved organic carbon (DOC) on toxicity were also assessed. To investigate the mechanisms of algal toxicity, the CytoViva hyperspectral imaging system was used to visualise algal-CeO2 interactions in the presence and absence of DOC, and the role of reactive oxygen species (ROS) was investigated by 'switching off' ROS production using UV-filtered lighting conditions. The nanoparticulate CeO2 immediately aggregated in solution to HDDs measured in the range 113-193nm, whereas the HDD and zeta potential values were significantly lower in the presence of DOC. Negligible CeO2 dissolution over the time course of the bioassay ruled out potential toxicity from dissolved cerium. The nanoparticulate CeO2 concentration that caused 50% inhibition of algal growth rate (IC50) was in the range 7.6-28mg/L compared with 59mg/L for micron-sized ceria, indicating that smaller particles were more toxic. The presence of DOC mitigated toxicity, with IC50s increasing to greater than 100mg/L. Significant ROS were generated in the nanoparticulate CeO2 bioassays under normal light conditions. However, 'switching off' ROS under UV-filtered light conditions resulted in a similar IC50, indicating that ROS generation was not the toxic mechanism. The CytoViva imaging showed negligible sorption of nanoparticulate CeO2 to algal cells in the presence of DOC, and strong sorption in its absence, suggesting that this was the toxic mechanism. The results suggest that DOC in natural waters will coat CeO2 particles and mitigate toxicity to algal cells.

Untangling the biological effects of cerium oxide nanoparticles: the role of surface valence states

Cerium oxide nanoparticles (nanoceria; CNPs) have been found to have both pro-oxidant and anti-oxidant effects on different cell systems or organisms. In order to untangle the mechanisms which underlie the biological activity of nanoceria, we have studied the effect of five different CNPs on a model relevant aquatic microorganism. Neither shape, concentration, synthesis method, surface charge (ζ-potential), nor nominal size had any influence in the observed biological activity. The main driver of toxicity was found to be the percentage of surface content of Ce(3+) sites: CNP1 (58%) and CNP5 (40%) were found to be toxic whereas CNP2 (28%), CNP3 (36%) and CNP4 (26%) were found to be non-toxic. The colloidal stability and redox chemistry of the most and least toxic CNPs, CNP1 and CNP2, respectively, were modified by incubation with iron and phosphate buffers. Blocking surface Ce(3+) sites of the most toxic CNP, CNP1, with phosphate treatment reverted toxicity and stimulated growth. Colloidal destabilization with Fe treatment only increased toxicity of CNP1. The results of this study are relevant in the understanding of the main drivers of biological activity of nanoceria and to define global descriptors of engineered nanoparticles (ENPs) bioactivity which may be useful in safer-by-design strategies of nanomaterials.

Potential of carbon nanotubes in algal biotechnology

A critical mass of knowledge is emerging on the interactions between plant cells and engineered nanomaterials, revealing the potential of plant nanobiotechnology to promote and support novel solutions for the development of a competitive bioeconomy. This knowledge can foster the adoption of new methodological strategies to empower the large-scale production of biomass from commercially important microalgae. The present review focuses on the potential of carbon nanotubes (CNTs) to enhance photosynthetic performance of microalgae by (i) widening the spectral region available for the energy conversion reactions and (ii) increasing the tolerance of microalgae towards unfavourable conditions occurring in mass production. To this end, current understanding on the mechanisms of uptake and localization of CNTs in plant cells is discussed. The available ecotoxicological data were used in an attempt to assess the feasibility of CNT-based applications in algal biotechnology, by critically correlating the experimental conditions with the observed adverse effects. Furthermore, main structural and physicochemical properties of single- and multi-walled CNTs and common approaches for the functionalization and characterization of CNTs in biological environment are presented. Here, we explore the potential that nanotechnology can offer to enhance functions of algae, paving the way for a more efficient use of photosynthetic algal systems in the sustainable production of energy, biomass and high-value compounds.

Linking the chemical speciation of cerium to its bioavailability in water for a freshwater alga

Over the past decade, researchers have begun to use metals of the lanthanide family for numerous applications, including liquid crystal display (LCD) screens, optical fibers, and laser technology. Unfortunately, little is presently known about their bioavailability or the mechanisms by which they might cause toxicity. The present study focuses on cerium (Ce), one of the most widely used lanthanides, and on validating the biotic ligand model as a means to predict Ce bioaccumulation. Short-term exposures to Ce were performed using the unicellular alga Chlamydomonas reinhardtii, to better relate Ce bioavailability to its chemical speciation in solution. Maximum uptake fluxes (Jmax ) and affinity constants for the binding of Ce to the biological uptake sites (KS ) were established at pH 5.0 and pH 7.0. An apparent affinity constant of 1.8 × 10(7) M(-1) was observed at pH 5.0, with a larger value obtained at pH 7.0 (6 × 10(7) M(-1) ), albeit under conditions where equilibrium could not be confirmed. By evaluating Ce speciation using centrifugal ultrafiltration and single-particle inductively coupled plasma spectrometry, it could be concluded that very little (∼30%) Ce was truly dissolved at pH 7.0, with the majority of the metal being present in colloidal species. Speciation was also monitored by fluorescence to evaluate Ce complexation by natural organic matter (NOM). The presence of NOM decreased Ce bioaccumulation in line with free Ce concentrations. Finally, competition with calcium for the metal uptake sites was shown to result in a decrease in Ce uptake by C. reinhardtii.

Freshwater dispersion stability of PAA-stabilised cerium oxide nanoparticles and toxicity towards Pseudokirchneriella subcapitata

An aqueous dispersion of poly (acrylic acid)-stabilised cerium oxide (CeO₂) nanoparticles (PAA-CeO₂) was evaluated for its stability in a range of freshwater ecotoxicity media (MHRW, TG 201 and M7), with and without natural organic matter (NOM). In a 15 day dispersion stability study, PAA-CeO₂ did not undergo significant aggregation in any media type. Zeta potential varied between media types and was influenced by PAA-CeO₂ concentration, but remained constant over 15 days. NOM had no influence on PAA-CeO₂ aggregation or zeta potential. The ecotoxicity of the PAA-CeO₂ dispersion was investigated in 72 h algal growth inhibition tests using the freshwater microalgae Pseudokirchneriella subcapitata. PAA-CeO₂ EC₅₀ values for growth inhibition (GI; 0.024 mg/L) were 2-3 orders of magnitude lower than pristine CeO₂ EC₅₀ values reported in the literature. The concentration of dissolved cerium (Ce(3+)/Ce(4+)) in PAA-CeO₂ exposure suspensions was very low, ranging between 0.5 and 5.6 μg/L. Free PAA concentration in the exposure solutions (0.0096-0.0384 mg/L) was significantly lower than the EC10 growth inhibition (47.7 mg/L) value of pure PAA, indicating that free PAA did not contribute to the observed toxicity. Elemental analysis indicated that up to 38% of the total Cerium becomes directly associated with the algal cells during the 72 h exposure. TOF-SIMS analysis of algal cell wall compounds indicated three different modes of action, including a significant oxidative stress response to PAA-CeO₂ exposure. In contrast to pristine CeO₂ nanoparticles, which rapidly aggregate in standard ecotoxicity media, PAA-stabilised CeO₂ nanoparticles remain dispersed and available to water column species. Interaction of PAA with cell wall components, which could be responsible for the observed biomarker alterations, could not be excluded. This study indicates that the increased dispersion stability of PAA-CeO₂ leads to an increase in toxicity compared to pristine non-stabilised forms.

Effect of hydration repulsion on nanoparticle agglomeration evaluated via a constant number Monte-Carlo simulation

The effect of hydration repulsion on the agglomeration of nanoparticles in aqueous suspensions was investigated via the description of agglomeration by the Smoluchowski coagulation equation using constant number Monte-Carlo simulation making use of the classical DLVO theory extended to include the hydration repulsion energy. Evaluation of experimental DLS measurements for TiO2, CeO2, SiO2, and α-Fe2O3 (hematite) at high IS (up to 900 mM) or low |ζ-potential| (≥1.35 mV) demonstrated that hydration repulsion energy can be above electrostatic repulsion energy such that the increased overall repulsion energy can significantly lower the agglomerate diameter relative to the classical DLVO prediction. While the classical DLVO theory, which is reasonably applicable for agglomeration of NPs of high |ζ-potential| (∼>35 mV) in suspensions of low IS (∼<1 mM), it can overpredict agglomerate sizes by up to a factor of 5 at high IS or low |ζ-potential|. Given the potential important role of hydration repulsion over a range of relevant conditions, there is merit in quantifying this repulsion energy over a wide range of conditions as part of overall characterization of NP suspensions. Such information would be of relevance to improved understanding of NP agglomeration in aqueous suspensions and its correlation with NP physicochemical and solution properties.

Dissolved cerium contributes to uptake of Ce in the presence of differently sized CeO2-nanoparticles by three crop plants

Cerium uptake into plants in the presence of CeO₂ nanoparticles occurs not only in nanoparticulate form, but also as dissolved ions.