A settling curve modeling method for quantitative description of the dispersion stability of carbon nanotubes in aquatic environments

Fate of weathered multi-walled carbon nanotubes in an aquatic sediment system

The widespread application of carbon nanotubes (CNT) in various consumer products leads to their inevitable release into aquatic systems. But only little is known about their distribution among aquatic compartments. In this study, we investigated the partitioning of radiolabeled, weathered multi-walled CNT (¹⁴C-wMWCNT) in an aquatic sediment system over a period of 180 days (d). The applied nanomaterial concentration in water phase was 100 μg L^-1. Over time, the wMWCNT disappeared exponentially from the water phase and simultaneously accumulated in the sediment phase. After 2 h incubation just 77%, after seven days 30% and after 180 d only 0.03% of applied radioactivity (AR) remained in the water phase. The respective values for the disappearance times DT₅₀ and DT₉₀ were 3.2 d and 10.7 d. Further, minor mineralization of ¹⁴C-wMWCNT to ¹⁴CO₂ was observed with values below 0.06% of AR. In addition, a study was carried out to estimate the deposition of wMWCNT in the water phase with and without sediment in the test system for 28 d. We found no influence of a sediment phase on the sedimentation behavior of wMWCNT in the water phase: After 6.5 d and 7.3 d 50% of the applied wMWCNT subsided in the presence and absence of sediment, respectively. The slow removal of wMWCNT from the water body by deposition into sediment implies that in addition to sediment-dwelling organisms, pelagic organisms are also at risk of exposure to nanomaterials and prone for their take-up.

A trophic transfer study: accumulation of multi-walled carbon nanotubes associated to green algae in water flea Daphnia magna

Carbon nanotubes (CNT) are promising nanomaterials in modern nanotechnology and their use in many different applications leads to an inevitable release into the aquatic environment. In this study, we quantified trophic transfer of weathered multi-walled carbon nanotubes (wMWCNT) from green algae to primary consumer Daphnia magna in a concentration of 100 μg L^-1 using radioactive labeling of the carbon backbone (¹⁴C-wMWCNT). Trophic transfer of wMWCNT was compared to the uptake by daphnids exposed to nanomaterials in the water phase without algae. Due to the rather long observed CNT sedimentation times (DT) from the water phase (DT₅₀: 3.9 days (d), DT₉₀: 12.8 d) wMWCNT interact with aquatic organisms and associated to the green algae Chlamydomonas reinhardtii and Raphidocelis subcapitata. After the exposition of algae, the nanotubes accumulated to a maximum of 1.6 ± 0.4 μg ¹⁴C-wMWCNT mg^-1 dry weight^-1 (dw^-1) and 0.7 ± 0.3 μg ¹⁴C-wMWCNT mg^-1 dw^-1 after 24 h and 48 h, respectively. To study trophic transfer, R. subcapitata was loaded with ¹⁴C-wMWCNT and subsequently fed to D. magna. A maximum body burden of 0.07 ± 0.01 μg ¹⁴C-wMWCNT mg^-1 dw^-1 and 7.1 ± 1.5 μg ¹⁴C-wMWCNT mg^-1 dw^-1 for D. magna after trophic transfer and waterborne exposure was measured, respectively, indicating no CNT accumulation after short-term exposure via trophic transfer. Additionally, the animals eliminated nanomaterials from their guts, while feeding algae facilitated their excretion. Further, accumulation of ¹⁴C-wMWCNT in a growing population of D. magna revealed a maximum uptake of 0.7 ± 0.2 μg mg^-1 dw^-1. Therefore, the calculated bioaccumulation factor (BAF) after 28 d of 6700 ± 2900 L kg^-1 is above the limit that indicates a chemical is bioaccumulative in the European Union Regulation REACH. Although wMWCNT did not bioaccumulate in neonate D. magna after trophic transfer, wMWCNT enriched in a 28 d growing D. magna population regardless of daily feeding, which increases the risk of CNT accumulation along the aquatic food chain.

Redox Conversion of Arsenite and Nitrate in the UV/Quinone Systems

Whether superoxide radical anion (O₂^•-) was a key reactive species in the oxidation of arsenite (As(III)) in photochemical processes has long been a controversial issue. With hydroquinone (BQH₂) and 1,4-benzoquinone (BQ) as redox mediators, the photochemical oxidation of As(III) and reduction of nitrate (NO₃^-) was carefully investigated. O₂^•-, singlet oxygen (¹O₂), H₂O₂, and semiquinone radical (BQH^•) were all possible reactive species in the irradiated system. However, since the formation of As(IV) is a necessary step in the oxidation of As(III), taking the standard reduction potentials into account, the reactions between the above species and As(III) were thermodynamically unfavorable. On the basis of radical scavenging experiments, hydroxyl radical (^•OH) was proved as the key species that led to the oxidation of As(III) in the UV/BQH₂ system. It should be noted that the ^•OH radicals were generated from the photolysis of H₂O₂, which came from the disproportionation of O₂^•- and the reaction of O₂^•- with BQH₂. Both the photoejected e_aq^- from ¹(BQH₂)* and the direct electron transfer with ³(BQH₂)* contributed to the reduction of NO₃^- in the UV/BQH₂ process. No synergistic effect was observed in the redox conversion of As(III) and NO₃^-, further demonstrating that the role of BQH^• was negligible in the studied systems. The results here are helpful for a better understanding of the photochemical behaviors of quinones in the aquatic environment.

Effects of acetylacetone on the photoconversion of pharmaceuticals in natural and pure waters

Acetylacetone (AcAc) has proven to be a potent photo-activator in the degradation of color compounds. The effects of AcAc on the photochemical conversion of five colorless pharmaceuticals were for the first time investigated in both pure and natural waters with the UV/H₂O₂ process as a reference. In most cases, AcAc played a similar role to H₂O₂. For example, AcAc accelerated the photodecomposition of carbamazepine, oxytetracycline, and tetracycline in pure water. Meanwhile, the toxicity of tetracyclines and carbamazepine were reduced to a similar extent to that in the UV/H₂O₂ process. However, AcAc worked in a way different from that of H₂O₂. Based on the degradation kinetics, solvent kinetic isotope effect, and the inhibiting effect of O₂, the underlying mechanisms for the degradation of pharmaceuticals in the UV/AcAc process were believed mainly to be direct energy transfer from excited AcAc to pharmaceuticals rather than reactive oxygen species-mediated reactions. In natural waters, dissolved organic matter (DOM) played a crucial role in the photoconversion of pharmaceuticals. The role of H₂O₂ became negligible due to the scavenging effects of DOM and inorganic ions. Interestingly, in natural waters, AcAc first accelerated the photodecomposition of pharmaceuticals and then led to a dramatic reduction with the depletion of dissolved oxygen. Considering the natural occurrence of diketones, the results here point out a possible pathway in the fate and transport of pharmaceuticals in aquatic ecosystems.

Water chemistry controlled aggregation and photo-transformation of silver nanoparticles in environmental waters

The inevitable release of engineered silver nanoparticles (AgNPs) into aquatic environments has drawn great concerns about its environmental toxicity and safety. Although aggregation and transformation play crucial roles in the transport and toxicity of AgNPs, how the water chemistry of environmental waters influences the aggregation and transformation of engineered AgNPs is still not well understood. In this study, the aggregation of polyvinylpyrrolidone (PVP) coated AgNPs was investigated in eight typical environmental water samples (with different ionic strengths, hardness, and dissolved organic matter (DOM) concentrations) by using UV-visible spectroscopy and dynamic light scattering. Raman spectroscopy was applied to probe the interaction of DOM with the surface of AgNPs. Further, the photo-transformation and morphology changes of AgNPs in environmental waters were studied by UV-visible spectroscopy, inductively coupled plasma mass spectrometry, and transmission electron microscopy. The results suggested that both electrolytes (especially Ca(2+) and Mg(2+)) and DOM in the surface waters are key parameters for AgNP aggregation, and sunlight could accelerate the morphology change, aggregation, and further sedimentation of AgNPs. This water chemistry controlled aggregation and photo-transformation should have significant environmental impacts on the transport and toxicity of AgNPs in the aquatic environments.