Mechanisms And Influencing Factors Of The Toxicity Of Carbon Nanotubes To Chlorella Nvrenoidosa

The widespread production and application of carbon nanotubes (CNTs) create the potential for CNTs being discarged into aquatic environment, which may pose a threat to aquatic ecosystem. Many researches have reported the toxicity of CNTs toward aquatic organisms. The toxic effects as influenced by the properties of CNTs and environmental factors, and their underlying mechanisms however remain in dispute, which merit more investigation. This research investigated the effect of dissolved organic matter (DOM) and surface oxidation on the toxicity of CNTs toward Chlorella Pyrenoidosa, and the mechanisms underlying the toxicity are analyzed at the cellular, subcellular and molecular levels. The main conclusions are as follows:(1) Effects of two synthetic surfactants (sodium dodecylbenzenesulfonate-SDBS, octyl phenoxy polyethoxyethanol-TX100) and a natural originated humic acid (HA) on the toxicity of multiwalled CNTs (MWCNTs) were compared.According to the result of algal growth assay, The 96 h IC50 determined for MWCNTs was 14,5 ± 1.1 mg/L,which was significantly lowered by the addition of 5 mg/L SDBS and TX100 to 10.0 ± 1.7 mg/L and 12.6 ± 0.6 mg/L,respectively. In contrast,the presence of HA (5 mg/L) markedly increased the IC50 to 23.1 ± 0.9 mg/L. The results suggested that SDBS and TX100 could increase the toxicity of MWCNTs by promoting internalization of MWCNTs and triggering higher oxidative stress; while HA was capable of limiting internalization of MWCNTs and reducing oxidative stress, and thereby alleviating the toxicity.(2) Oxidized MWCNTs (o-MWCNTs) were prepared using mixtures of concerntrated H2SO4 and HNO3 solutions. Increased surface area, disclosed tube ends, defects on the sidewalls, disruption of the electronic structure, and removal of metal catalysts and amorphous carbon were observed for the o-MWCNTs. The oxidation mainly occurred on the MWCNT surfaces. After oxidation, the oxygen content of MWCNTs increased from 2.82% to 5.22-7.25%, and the oxidation degree increased with increasing oxidation time. And carboxyl and lactonic groups made up the majority of the total content of acidic groups on the MWCNTs. The dissociation of functional groups on the MWCNT surface increased the electronegativity of the o-MWCNTs and facilitated the aqueous stabilization of o-MWCNTs through electrostatic repulsions, which consequently led to increased interactions between MWCNTs and algal cells and their toxicity. The toxicity of o-MWCNTs increased as oxidaition degree increased. The 96 h IC50 values of o-MWCNTs were 3.8-6.6 mg/L.(3) The o-MWCNTs would absorb onto the cell surfaces, and subsequently be internalized by algal cells via direct puncturing and endocytosis. Algal cells would initiate a cascade of protective responses including antioxidant defense,enhanced cell division and pentose phosphate pathway, formation of polyphosphate bodies,accumulation of neutral lipids, and regulation of osmotic pressure and cell membrane fluidity, to resist MWCNT-stress. When these self-defense responses were overwhelmed, damaging responses, including membrane damage, organelle damage,denaturation of biomacromolecule, and disturbance in energy metabolism, would arise,which ultimately resulting in cell death.