Potential toxicity of carbonaceous nanomaterials on aquatic organisms and their alleviation strategies: A review

Carbonaceous nanomaterials (CNMs) are widely used in electronics, biomedicine, agriculture, environmental remediation, and catalysis due to their excellent biocompatibility, high reactivity, and high specific surface area. However, the extensive applications of CNMs cause their inevitable release into water, which may result in toxic effects on the aquatic ecological environment and organisms. CNMs can cause lipid peroxidation damage and neurotoxicity in aquatic organisms, affecting embryo hatching and larval morphology. The effects of CNMs on aquatic organisms vary depending on their structures and physicochemical properties, as well as the species, age, and tolerance of the tested organisms. The above uncertainties have increased the difficulty of exploring the impact of carbonaceous nanomaterials on the toxicity of aquatic organisms to a certain extent. Solving these issues is of great significance and reference value for promoting the research and safe utilization of carbon nanomaterials. Therefore, a systematic review of the effects of potential toxicity of carbonaceous nanomaterials on aquatic organisms and their alleviation strategies is needed. This paper firstly summarizes the toxic effects of commonly used CNMs (i.e., carbon nanotubes, graphene, and fullerene) on different aquatic organisms, which include developmental toxicity, behavioral and metabolic toxicity, reproductive toxicity, and organ toxicity. Then the main mechanisms of CNMs to aquatic organisms are further explored, and the methods to reduce the toxicity of CNMs are also summarized. Finally, the current challenges and future perspectives for studying CNM toxicity to aquatic organisms are proposed.

Effects of Mixtures of Engineered Nanoparticles and Cocontaminants on Anaerobic Digestion

The widespread application of nanotechnology inevitably leads to an increased release of engineered nanoparticles (ENPs) into the environment. Due to their specific physicochemical properties, ENPs may interact with other contaminants and exert combined effects on the microbial community and metabolism of anaerobic digestion (AD), an important process for organic waste reduction, stabilization, and bioenergy recovery. However, the complicated interactions between ENPs and other contaminants as well as their combined effects on AD are often overlooked. This review therefore focuses on the co-occurrence of ENPs and cocontaminants in the AD process. The key interactions between ENPs and cocontaminants and their combined influences on AD are summarized from the available literature, including the critical mechanisms and influencing factors. Some sulfides, coagulants, and chelating agents have a dramatic "detoxification" effect on the inhibition effect of ENPs on AD. However, some antibiotics and surfactants increase the inhibition of ENPs on AD. The reasons for these differences may be related to the interactive effects between ENPs and cocontaminants, changes of key enzyme activities, adenosine triphosphate (ATP) levels, reactive oxygen species (ROS) production, and microbial communities. New scientific opportunities for a better understanding of the coexistence in real world situations are converging on the scale of nanoparticles.

Adsorption of antibiotics onto low-grade charcoal in the presence of organic matter: Batch and column tests

Antibiotics contaminate diverse ecosystems and threaten human health. In ecosystems including water, sediment, and soil, the amount of antibiotics present is tiny compared to the amount of natural organic matter. However, most studies have ignored the co-presence of natural organic matter in the adsorption of target antibiotics. In this study, we quantitatively evaluated the effect of co-presenting natural organic matter on the adsorption of sulfamethazine (SMZ) through batch and column experiments using low-grade charcoal, an industrial by-product. SMZ was used as a model antibiotic compound and humic acid (HA) was used to represent natural organic matter. The co-presence of 2000 mg/L HA (400 times the concentration of SMZ) lowered the adsorption rate of SMZ from 0.023 g/mg·min to 0.007 g/mg·min, and the maximum adsorption capacity from 39.8 mg/g to 15.6 mg/g. HA blocked the charcoal's pores and covered its surface adsorption sites, which dramatically lowered its capacity to adsorb SMZ. Similar results were obtained in the flow-through column experiments, where the co-presence of natural organic matter shortened the lifetime of the charcoal. As a result, the co-presence of a relatively high concentration of natural organic matter can inhibit the adsorption of SMZ and likely other antibiotic compounds, and thus the presence of natural organic matter should be accounted for in the design of adsorption processes to treat antibiotics in water.

Adsorption behaviors and influencing factors of antibiotic norfloxacin on natural kaolinite-humic composite colloids in aquatic environment

Particles are ubiquitous and abundant in natural waters and play a crucial role in the fate and bioavailability of organic pollution. In the present study, natural mineral (kaolinites, KL), organic (humic/fulvic acid, HA/FA) and their composite particles were further separated into particles fractions (PFs, >1 μm) and colloidal fractions (CFs, 1 kDa-1 μm) by cross-flow ultrafiltration (CFUF). This research demonstrated the role of kaolinite-humic composite colloids on the adsorption of fluoroquinolone norfloxacin (NOR). The Freundlich model satisfactory described adsorption curves, showing strong affinity of NOR to CFs, with sorption capacity (K_F) between 8975.50 and 16638.13 for NOR. The adsorption capacities of NOR decreased with the particle size increasing from CFs to PFs. In addition, composite CFs showed excellent adsorption capacity, which was mainly attributed to the larger specific surface area of composite CFs and electronegativity and numerous oxygen-containing functional groups on the surfaces of the complexes, and electrostatic attraction, hydrogen bond and cation exchange could dominate the NOR adsorption onto the composite CFs. The best pH value under adsorption condition of composite CFs varied from weakly acidic to neutral with the increase of load amount of humic and fulvic acids on the surface of inorganic particles. The adsorption decreased with higher cation strength, larger cation radius and higher cation valence, which depended on the surface charge of colloids and the molecular shape of NOR. These results provided insight into the interfacial behaviors of NOR on the surfaces of natural colloids and promoted the understanding of the migration and transport of antibiotics in environmental systems.

Toxic effects and molecular mechanisms of sulfamethoxazole on Scenedesmus obliquus

The antibiotic sulfamethoxazole (SMX) is a pollutant that is widely distributed in the global water environment.This substance has toxic effects on various aquatic organisms. Previous studies on SMX have focused on its acute toxicity towards algae and the changes induced at biological and cellular levels, rather than its biotoxicity and mechanisms at the molecular level. In this study, we investigated the effects of SMX on Scenedesmus obliquus as the model organism by performing transmission electron microscopy and transcriptome sequencing analyses. Exposure to SMX promoted gene expression, resulting in changes to algal cell ultrastructure. The cell walls became blurred, the chloroplast structure was seriously damaged, and the number and volume of mitochondria per cell increased. These changes were related to the inhibition of cell growth, decrease in chlorophyll content, increase in cell membrane permeability, and increased production of reactive oxygen species, which led to increased amounts of the lipid peroxidation product malondialdehyde, and higher activities of antioxidant enzymes. Our results suggest that SMX affects gene expression by influencing non-coding RNA metabolic processes, leading to changes in nuclear structures. Abnormally expressed long non-coding RNAs extensively regulate downstream gene expression through various mechanisms, such as chromatin recombination, thereby promoting tumor occurrence, invasion, and metastasis. This abnormal expression may be an important mechanism underlying the carcinogenic effects of SMX.

The role of solvents and oxygen-containing functional groups on the adsorption of Bisphenol A on carbon nanotubes

The wide application of endocrine disruptors (EDs) has recently created great public concerns because of their toxicities. Previous studies have stated that the effect of oxygen-containing functional groups of carbon nanotubes (CNTs) for Bisphenol A (BPA) sorption, but no study has been quantified the exact contribution of the oxygen-containing functional groups. Moreover, the role of solvents on the adsorption of BPA should be considered. Considering the well properties of CNTs, graphitized (MG), carboxylated (MC) and hydroxylated (MH) multi-walled CNTs were selected as model adsorbents, BPA was used as model adsorbate. Solubility and single point adsorption coefficient (logK_d) of BPA were n-hexadecane > water > methanol, suggesting that hydrophobic interaction was the main mechanism for BPA sorption on CNTs. For different functional groups of CNTs, π-π interaction between MH and BPA may be stronger than that of MC, and thus the sorption of BPA on MH was higher than that of MC. Moreover, hydrogen bond resulted in the higher adsorption of BPA on MH when compared with MC. The oxygen-containing functional groups of CNTs played a key role for BPA sorption in methanol because the values of contribution were 20%-45% for -OH and were 5%-25% for -COOH. In n-hexadecane, other factors such as hydrophobic interactions should be considered because the contribution percentages of -OH were ca.15% and the values for -COOH were ca.10%. The results are expected to provide important information on the interaction of EDs and CNTs.

Adsorption of Sulfamethazine Drug onto the Modified Derivatives of Carbon Nanotubes at Different pH

The sulfamethazine drug interaction with carbon nanotubes was investigated with the aim of improving the adsorption capacity of the adsorptive materials. Experiments were performed to clarify how the molecular environment affects the adsorption process. Single-walled carbon nanotubes have a higher removal efficiency of sulfamethazine than pristine or functionalized multi-walled carbon nanotubes. Although the presence of cyclodextrin molecules improves the solubility of sulfamethazine, it reduces the adsorption capacity of the carbon nanotube towards the sulfamethazine drug and, therefore, inhibits the removal of these antibiotic pollutants from waters by carbon nanotubes.

Graphene modified anatase/titanate nanosheets with enhanced photocatalytic activity for efficient degradation of sulfamethazine under simulated solar light

Graphene modified anatase/titanate nanosheets (G/A/TNS) synthesized through hydrothermal treatment were used for solar-light-driven photocatalytic degradation of a typical pharmaceutically active compound, sulfamethazine (SMT). The optimal material was synthesized with 0.5 wt% of graphene loading (G/A/TNS-0.5), which could efficiently degrade 96.1% of SMT at 4 h. G/A/TNS-0.5 showed enhanced photocatalytic activity compared with the neat anatase and unmodified anatase/titanate nanosheets (A/TNS). UV-vis diffuse reflection spectra indicated that G/A/TNS-0.5 had a lower energy band gap (E_g) of 2.8 eV than A/TNS (3.1 eV). The grafted graphene acted as an electron transfer mediator after photoexcitation, resulting in inhibition on rapid recombination of electron-hole pairs. More importantly, architecture of graphene and titanate nanosheets both with two-dimensional structures greatly facilitated the photoexcited electron transfer. •OH and ¹O₂ were the primary reactive oxygen species (ROS) to SMT degradation. Fukui index (f^-) derived from density functional theory (DFT) calculation predicted the active sites on SMT molecule, and then SMT degradation pathway was proposed by means of intermediates identification and theoretical calculation. Furthermore, G/A/TNS-0.5 could be well reused and 90.5% of SMT was also degraded after five runs. The developed new photocatalysts show great potential for degradation of emerging organic contaminants through photocatalysis under solar light.

The missing link between carbon nanotubes, dissolved organic matter and organic pollutants

Ternary interactions between carbon nanotubes (CNTs), dissolved organic matter (DOM) and small organic molecules (namely low molecular mass organic pollutants) are of great importance since they can affect the reactivity and fate of all involved compartments in the environment. This review thoroughly assesses existing knowledge on the adsorption of DOM and small organic molecules by CNTs, while giving special attention to (i) the complex nature of DOM, (ii) the ternary rather than binary interactions between CNTs, DOM and the small organic molecules and (iii) the DOM-organic molecule interactions. We discuss in detail the main factors influencing DOM adsorption by CNTs and attempt to differentiate between the role of DOM composition and conformation. We then outline how the presence of DOM influences the adsorption of small organic molecules by CNTs, considering the introduction stage of DOM and the impact of the organic molecule's properties. DOM adsorption by CNTs is highly dependent on its composition and is governed by the size, hydrophobicity and aromaticity of DOM. DOM adsorption was found to alter the assembly of the CNTs, resulting in changes in the distribution of adsorption sites. Small organic molecules may adsorb to residual surface area on the CNTs, to DOM-coating the CNTs or remain in solution, possibly complexed with DOM. This results in their suppressed or enhanced adsorption in comparison to DOM-free media. The physicochemical properties of the organic molecules (hydrophobicity, size, structure and charge) also play a major role in this process. We present knowledge gaps that need clarification such as the extent of DOM desorption from CNTs, the amount of co-adsorbed DOM during competition with small organic molecules for adsorption sites on the CNTs and the behavior of CNTs under realistic conditions. More data generated from experiments using natural DOM rather than dissolved humic substances are required to improve our understanding of the interactions between CNTs and small organic molecules in realistic environmental scenarios. This review provides conclusions and research directions needed to evaluate the nature of interactions between CNTs, DOM and organic pollutants in aquatic systems affected by anthropogenic activities.

Comparison of the accumulation and metabolite of fluoxetine in zebrafish larva under different environmental conditions with or without carbon nanotubes

Few studies have focused on the influence of environmental conditions on the bioavailability of pollutants interacted with nanomaterials in organisms. In this study, we primarily compared the influence of multiwalled carbon nanotubes (MWCNTs) on the bioavailability of fluoxetine in zebrafish (Danio rerio) larva under different environmental conditions: natural organic matter (NOM) and salinity. The results showed that fluoxetine accumulated in the larvae and then transformed into the metabolite norfluoxetine, with the metabolic rates from 2.8 to 3.5. Following co-exposure to MWCNTs, the accumulation of fluoxetine and norfluoxetine were further enhanced, suggesting a superior carrier of MWCNTs for fluoxetine, especially the functional MWCNTs. The consistent increase in the fluoxetine and norfluoxetine accumulation highlights the bioavailability of absorbed fluoxetine on MWCNTs in zebrafish larvae. The presence of NOM promoted the accumulation of fluoxetine and norfluoxetine in zebrafish, but alleviated the carrier effects of MWCNTs, acting as a natural antidote. Salinity negatively influenced the bioavailability of fluoxetine in the larvae, and further reversed the enhancements caused by MWCNTs. These findings provide a new insight into the influence of environmental conditions on the interactions between nanomaterials and pollutants in organisms.

Adsorption of bisphenol A on dispersed carbon nanotubes: Role of different dispersing agents

Although the adsorption of organic contaminants on carbon nanotubes (CNTs) and the dispersion of CNTs have been extensively investigated separately, the adsorption behavior of organic contaminants on dispersed CNTs, which may be a missing link to understanding their environmental behavior and risks, remain unclear yet rarely studied. In this study, the effect of the dispersing agent structure on the adsorption characteristics of BPA (a representative organic contaminant) on dispersed CNTs were investigated using tannic acid (TA), sodium dodecylbenzenesulfonate (SDBS), and gallic acid (GA) as model dispersing agents. Our results showed that at low dispersing agent concentrations, adsorption of TA could lead to higher CNT suspension than adsorption of SDBS and GA due to greater steric hindrance. However, the presence of TA reduced the adsorption of BPA due to strong competitive adsorption on dispersed CNTs. At high concentrations of TA, the suspension of CNTs was reduced by the "bridging effect," in which adjacent dispersed CNTs form hydrogen bonds and re-aggregate. However, the adsorption of BPA dramatically increased due to the enhanced partition of BPA into pseudomicelles of TA on dispersed CNTs, as indicated by the significantly increased index of heterogeneity at high TA concentration. Transmission electron microscopy images confirmed the formation of TA pseudomicelles. This study highlights the key role of the dispersing agent structure on CNT dispersion and adsorption of organic contaminants. The high mobility and transport of CNT-adsorbed contaminants may lead to higher environmental risks compared with aggregated CNTs.

Insight into the heterogeneous adsorption of humic acid fluorescent components on multi-walled carbon nanotubes by excitation-emission matrix and parallel factor analysis

The heterogeneous adsorption behavior of commercial humic acid (HA) on pristine and functionalized multi-walled carbon nanotubes (MWCNTs) was investigated by fluorescence excitation-emission matrix and parallel factor (EEM- PARAFAC) analysis. The kinetics, isotherms, thermodynamics and mechanisms of adsorption of HA fluorescent components onto MWCNTs were the focus of the present study. Three humic-like fluorescent components were distinguished, including one carboxylic-like fluorophore C1 (λ_ex/λ_em= (250, 310) nm/428nm), and two phenolic-like fluorophores, C2 (λ_ex/λ_em= (300, 460) nm/552nm) and C3 (λ_ex/λ_em= (270, 375) nm/520nm). The Lagergren pseudo-second-order model can be used to describe the adsorption kinetics of the HA fluorescent components. In addition, both the Freundlich and Langmuir models can be suitably employed to describe the adsorption of the HA fluorescent components onto MWCNTs with significantly high correlation coefficients (R²> 0.94, P< 0.05). The dissimilarity in the adsorption affinity (K_d) and nonlinear adsorption degree from the HA fluorescent components to MWCNTs was clearly observed. The adsorption mechanism suggested that the π-π electron donor-acceptor (EDA) interaction played an important role in the interaction between HA fluorescent components and the three MWCNTs. Furthermore, the values of the thermodynamic parameters, including the Gibbs free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°), showed that the adsorption of the HA fluorescent components on MWCNTs was spontaneous and exothermic.

Sorption mechanisms of sulfamethazine to soil humin and its subfractions after sequential treatments

Sorption mechanisms of an antibiotic sulfamethazine (SMT) to humin (HM) isolated from a peat soil and its subfractions after sequential treatments were examined. The treatments of HM included removal of ash, O-alkyl carbon, lipid, and lignin components. The HF/HCl de-ashing treatment removed a large amount of minerals (mainly silicates), releasing a fraction of hydrophobic carbon sorption domains that previously were blocked, increasing the sorption of SMT by 33.3%. The de-O-alkyl carbon treatment through acid hydrolysis greatly reduced polarity of HM samples, thus weakening the interaction between sorbents with water at the interfaces via H-bonding, leaving more effective sorption sites. Sorption of SMT via mechanisms such as van der Waals forces and π-π interactions was enhanced by factors of 2.04-2.50. After removing the lipid/lignin component with the improved Soxhlet extraction/acid hydrolysis, the organic carbon content-normalized sorption enhancement index E_oc was calculated. The results demonstrated that the E_oc-lipid for SMT (16.9%) was higher than E_oc-lignin (10.1%), implying that removal of unit organic carbon mass of lipid led to a higher increase in sorption strength than that of lignin. As each component was progressively removed from HM, the sorption strength and isotherm nonlinearity of the residual HM samples for SMT were gradually enhanced. The K_oc values of SMT by HM samples were positively correlated with their aromatic carbon contents, implying that π-π electron donor-acceptor interactions between the benzene ring of sorbate and the aromatic domains in HM played a significant role in their interactions.

Adsorption of Emerging Ionizable Contaminants on Carbon Nanotubes: Advancements and Challenges

The superior adsorption capacity of carbon nanotubes has been well recognized and there is a wealth of information in the literature concerning the adsorption of unionized organic pollutants on carbon nanotubes. Recently, the adsorption of emerging environmental pollutants, most of which are ionizable, has attracted increasing attention due to the heightened concerns about the accumulation of these emerging contaminants in the environment. These recent studies suggest that the adsorption of emerging ionizable contaminants on carbon nanotubes exhibit different characteristics than unionized ones. For example, a new charge-assisted intermolecular force has been proposed for ionizable compounds because some adsorption phenomenon cannot be easily explained by the conventional force theory. The adsorption of ionizable compounds also displayed much stronger dependence on solution pH and ionic strength than unionized compounds. This article aims to present a brief review on the current understanding of the adsorption of emerging ionizable contaminants to carbon nanotubes and discuss further research needs required to advance the mechanistic understanding of the interactions between ionizable contaminants and carbon nanotubes.