Ground State Reactions of nC60 with Free Chlorine in Water

Effect of microplastics on sodium hypochlorite disinfection and changes in its toxicity on zebrafish

The presence of microplastics (MPs) in water may affect the efficacy of the disinfection process and induce toxicity changes to MPs themselves during disinfection. Therefore, this study evaluated the two-way effects of polyethylene microplastic (MP) particles in water and wastewater during sodium hypochlorite (NaClO) disinfection. On the one hand, it has been confirmed that the presence of MPs reduced the disinfection efficiency of NaClO. The required CT (concentration of the disinfection × contact time) for a 2-4-log inactivation of Escherichia coli (E. coli) in different water samples was in the order of deionized water < turbid water (1 NTU) < water with MPs (1 mg/L) < turbid water (10 NTU). On the other hand, although exposure to MPs did induce significant changes in the activities of superoxide dismutase and glutathione, compared to pristine MPs, the MPs treated by NaClO at current conditions (0.3 and 3.0 mg/L for 30 min) did not show significant changes in their toxicity on zebrafish, at an MP exposure concentration of 1 mg/L. There was no significant difference in the survival rate and weight growth rate, neither as in the activities of the oxidative stress-related enzymes (superoxide dismutase, catalase, glutathione, glutathione peroxidase, and glutathione s-transferase) in both gut and muscle tissues of the zebrafish, between exposure to the pristine and NaClO-treated MPs. It is indicated that NaClO disinfection commonly applied for water and wastewater treatment would not pose a serious concern to effluent safety in the presence of mild levels of MPs.

Chlorine disinfection elevates the toxicity of polystyrene microplastics to human cells by inducing mitochondria-dependent apoptosis

Microplastics (MPs) are ubiquitous in drinking water and pose potential threats to human health. Despite increasingly attentions on the toxicity of MPs, the deleterious effects of MPs after chlorine disinfection, which might be a more accessible form of MPs, has rarely been considered. Here, we first treated pristine polystyrene microplastics (PS-MPs) with chlorine to simulate the reactions that occur during drinking water treatment, and investigated and compared the cytotoxicity of chlorinated PS-MPs to those of pristine PS-MPs. Chlorine disinfection did not change the size of pristine PS-MPs, but increased the surface roughness. In addition, abundant carbon-chlorine bonds and persistent free radicals were generated on the surface of chlorinated PS-MPs. Compared with pristine PS-MPs, chlorinated PS-MPs markedly inhibited the cell proliferation, changed cellular morphology, destroyed cell membrane integrity, induced cell inflammatory response and apoptosis. Proteomics confirmed the difference in interactions with intracellular proteins between these particles. Furthermore, we found that the regulation of PI3K/AKT and Bcl-2/Bax pathways, oxidative stress-triggered mitochondrial depolarization, and the activation of caspase cascade were identified as the underlying mechanisms for the enhanced apoptosis ratio in GES-1 cells when exposed to chlorinated PS-MPs. This exacerbated cytotoxicity could be explained by the enhanced surface roughness and changed surface chemistry of these PS-MPs after chlorine disinfection. This work discloses the impacts of chlorine disinfection on the cytotoxicity of PS-MPs, which provides new insights for a more systematic risk assessment of MPs.

Transformation of bromophenols by aqueous chlorination and exploration of main reaction mechanisms

Bromophenols (BPs) are ubiquitous phenolic contaminants and typical halogenated disinfection byproducts (DBPs) that are commonly detected in aquatic environments. The transformation of 2,4-dibromophenol (2,4-DBP) during chlorination process was fully explored in this research. It was found that active chlorine can react with 2,4-DBP effectively in a wide pH range of 5.0-11.0, with an apparent second-order rate constant (k_app) varying from 0.8 M^-1 s^-1 to 110.3 M^-1 s^-1. The addition of 5 mM ammonium ions almost completely suppressed the reaction via competitive consumption of free chlorine. With the concentration of HA increasing from 1.0 to 10.0 mg L^-1, the inhibition on the degradation of 2,4-DBP increased from 8.7% to 63.4%. By contrast, bromide ions at a concentration of 5 mM accelerated the process by about 4 times, due to the formation of hypobromous acid. On the basis of the eleven products (with eight nominal masses) identified by LC-TOF-MS, electrophilic substitution reactions and single-electron transfer reactions were mainly involved in the chlorination process. The concentration of primary chlorine-substituted products was about 4 times that of the dimer products, demonstrating that electrophilic substitution reaction was predominant during chlorination of 2,4-DBP. Density functional theory (DFT) based calculations revealed that HOCl is the dominant active oxidizing species for elimination of 2,4-DBP and coupling reaction occurs more easily at para and ortho position of hydroxyl group in the phenolic moiety. These findings could provide some new insights into the environmental fate of bromophenols during chlorine disinfection of water and wastewaters.

Environmental transformation of graphene oxide in the aquatic environment

In recent years, research on graphene oxide (GO) has developed rapidly in both academic and industrial applications such as electronic, biosensor, drug delivery, water treatment and so forth. Based on the large amount of applications, it is anticipated that GO will inevitably find its own way to the environment, if used are not restricted to prevent their release. Environmental transformation is an important transformation process in the natural environment. In this review, we will summarize the recent developments on environmental transformation of GO in the aquatic environment. Although papers on environmental transformation of graphene-based nanomaterials can be found, a systematic picture describing photo-transformation of GO (dividing into different irradiation sources), environmental transformation of GO in the dark environmental, the environmental toxicity of GO are still lacking. Thus, it is essential to summarize how different light sources will affect the GO structure and reactive oxygen species generation in the photo-transformation process, how GO will react with various natural constituents in the aquatic environment, whether GO will toxic to different aquatic organisms and what will be the interactions between GO and the intracellular receptors in the intracellular level once GO released into the aquatic environment. This review will arouse the realization of potential risk that GO can bring to the aquatic environment and enlighten us to pay attention to behaviors of other two-dimensional GO-like nanomaterials, which have been intensively applied and studied in recent years.

Potential concerns in fullerene application to water treatment related to transformation, cellular uptake and intracellular catalysis

Fullerene (C₆₀) exhibits versatile properties that shows great potential for improving water treatment technologies. However, the probable transformation of C₆₀ during water treatment, which consequently changes the physicochemical properties and toxicity of the parent compound, may introduce doubt concerning its application. Our results demonstrated that the C₆₀ aggregate (nC₆₀) was transformed to a more oxidized form under common water disinfection processes (i.e., ultraviolet irradiation and photochlorination). The light-irradiated product (UV_nC₆₀) exhibited lower cytotoxicity toward macrophage J774A.1 cells relative to nC₆₀, whereas the photochlorinated product (UV/Cl_nC₆₀) increased the toxic effect. Particularly, the internalization of nanoparticles and the mimetic superoxide dismutase (SOD) activity resulted in the selective accumulation of intracellular hydrogen peroxide. Thus, sequential exposure to a nonlethal dose of nanoparticles followed by 5 μM copper ions (which is a much lower level than the EPA-regulated level of 20 μM in drinking water) led to the significant production of hydroxyl radicals inside cells. The uptake and SOD-like activity were highly structure-related, with the most noteworthy activity obtained for UV/Cl_nC₆₀. These results emphasize that environmental transformation-induced property changes should be given adequate consideration in the risk assessment of C₆₀.

Water disinfection processes change the cytotoxicity of C60 fullerene: Reactions at the nano-bio interface

The environmental transformation of nanoparticles results in significant changes in their structure, properties, and toxicity, which are imperative for assessing their environmental impact and health risks. Little is known about the toxicity alteration of fullerene nanoparticles (C₆₀) after water disinfection processes considering their potential application in antimicrobial control in water treatment ultimately ending in sewage treatment plants. We showed that C₆₀ aggregates (nC₆₀) were converted to more oxidized forms via commonly used water disinfection processes (i.e., phototransformation and photochlorination treatment). The light-irradiated nanoparticles (UV_nC₆₀) exhibited mitigated cytotoxicity relative to nC₆₀, whereas photochlorinated nC₆₀ (UV/Cl_nC₆₀) showed an exacerbated outcome. We revealed a distinct toxic mechanism occurring at the nano-bio interface, for which electrons were shuttled by C₆₀ nanoparticles from membrane-bound NADPH oxidase to extracellular molecular oxygen, resulting in the production of various extracellular reactive oxygen species (ROS). UV/Cl_nC₆₀ showed the highest electron-shuttling activity due to its high carbonyl content, and more than 2.4-fold higher level of extracellular hydroxyl radicals were detected relative to that in untreated cells. Although UV_nC₆₀ possessed a somewhat higher carbonyl content than nC₆₀, it showed a weaker adhesion to the cell membrane, which compromised the electron-transfer process. The intrinsic ROS generation/quenching capabilities and oxidative potentials of the various nanoparticles were also systematically compared. Overall, this report highlights the importance of understanding environmental transformations in risk assessment and uncovers an overlooked mechanism through which nC₆₀/derivatives can modulate the electron transfer process at the nano-bio interface via acting as electron shuttles.

Unveiling the Susceptibility of Functional Groups of Poly(ether sulfone)/Polyvinylpyrrolidone Membranes to NaOCl: A Two-Dimensional Correlation Spectroscopic Study

A clear understanding of membrane aging process is essential for the optimization of chemical cleaning in membrane-based facilities. In this study, two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (CoS) analysis was first used to decipher the sequential order of functional group changes of NaOCl-aged poly(ether sulfone)/polyvinylpyrrolidone (PES/PVP) membranes. The synchronous maps showed 12 major autopeaks in total. Based on the asynchronous maps, a similar aging sequence of membrane groups was clearly identified at three pHs (i.e., 6, 8, and 10): 1463, 1440, and 1410 (cyclic C-H structures) > 1662 (amide groups) > 1700 (succinimide groups) > 1320, 1292 (S═O asymmetric) > 1486, 1580 (aromatic structures) > 1241 (aromatic ether bands) > 1105, 1150 cm^-1 (O═S═O symmetric). Among them, membrane chlorination occurred at 1241, 1410, and 1440 cm^-1. Moreover, the initial degradation of PVP and the subsequent transformation of PES could be highly responsible for the increased water permeability and the enlargement of membrane pores, respectively, both leading to serious fouling with humic acid filtration. In summary, the 2D-FTIR-CoS analysis is a powerful approach to reveal the interaction mechanisms of NaOCl-membrane and could be also useful to probe the process of membrane fouling and chemical cleaning.