The synergetic effect of sorption and photocatalysis for metal sequestration promises unique advantages in environmental remediation; however, the mechanism, especially with regard to the photocatalytic sites, is still ambiguous in carbon catalysts. Here, a metal-free three-dimensional carbon framework, named P-OGR, was fabricated via the assembly of oxidized graphene ribbons (OGRs) and polyacrylamide (PA). In the dark, the sorption capacity of Cr(VI) on P-OGR achieved 338.2 mg g⁻¹ within 2 min. In sunlight for 5 days, P-OGR could immobilize and photo-reduce 687.8 mg g⁻¹ of Cr(VI) into Cr₂O₃ nanospheres. The active sites on P-OGR were explored via modeling experiments. In the sorption, –NH₃ ⁺ was the best site due to the electrostatic attraction with Cr(VI) anions. Hydroxyls, followed by carboxyl groups, enabled weak hydrogen bonding to capture Cr(VI). During the photocatalysis, the activities of the catalytic sites were evaluated as follows: carboxyl > hydroxyl > ketone > aldehyde, which were consistent with their Lewis acidities.

In this study, the cytotoxicity and toxic mechanism of carbon quantum dots (CQDs) to E. coli were evaluated in vitro. The synthetic CQDs were extremely small in size (~2.08 nm) and displayed strong fluorescence. The results demonstrated that CQDs showed good biocompatibility with E. coli within a short culture time. However, when the exposure time exceeded 24 h, the toxicity of CQDs became apparent, and the contents of reactive oxygen species, lactate dehydrogenase, and the crystal violet absorption rate increased significantly. To further explore the cytotoxic mechanism, approaches including confocal laser scanning microscopy, scanning electron microscopy, and biological transmission electron microscopy combined with zeta potential tests, osmotic pressure measurement, and comet assays were performed. On the one hand, the CQDs altered the surface charges of cells and induced lipid peroxidation by adhesion on the surface of E. coli, leading to an increase in the permeability of the cell wall. On the other hand, when the concentration of CQDs reached 200 µg/mL, the osmotic pressure of the extracellular environment was significantly reduced. These are the main factors that lead to cell edema and death. Finally, the comet assays confirmed that CQDs could induce DNA damage, which could inhibit the proliferation of E. coli.

Although carbon nanotubes can be described as a seamlessly curled graphene nanosheet, two-dimensional graphene oxide (GO) and one-dimensional oxidized carbon nanotubes (OCNTs) have different fates and environmental risks, such as deposition, degradation and cytotoxicity. In particular, coexisting heavy metal ions (HMs) trigger distinct morphological transformations of both of these carbon derivatives. In addition, these morphological transformations can change the interface behaviors of HMs on both of these materials. In this study, the differences in the morphological changes of GO and OCNTs and the interface behaviors of Ni(II) were explored via the intrinsically microscopic structural changes of both of these typical carbon materials. Batch experiments revealed that Ni(II) sorption on GO drastically decreased with increasing ionic strength, while it was almost independent of ionic strength on the OCNTs. This phenomenon is attributed to the aggregation and wrinkling of GO sheets at higher Na⁺ concentrations, resulting in a decrease in the GO surface area and number of sorption sites. Meanwhile, the intertwining aggregations of OCNTs still ensured that the sorption sites were naked. For the first time, Ni²⁺ ions were observed to persist as inner-sphere complexes on GO even under alkaline conditions, where the Ni(OH)₂(s) phase was determined on the OCNTs. This could be attributed to the fact that the fast aggregation of GO, which fixed Ni²⁺ ions into the interlayers, inhibited the nucleation of Ni(OH)₂. Stable layered structures of GO aggregations were difficult to exfoliate, leading to a decreased release of Ni(II) from GO with increasing Ni(II) loading. For the OCNTs, naked Ni²⁺ ions could be easily and effectively released. These findings are critical to assess the mobility, transformation and cytotoxicity of nanomaterials and HMs in aquatic environments.