Dumsile W Nyembe, Victor Wepener, Bhekie B Mamba and Ndeke Musee
Multiwalled carbon nanotubes (MWCNTs) are among the nanomaterials earmarked for water purification, yet they have been reported to cause toxicity in fresh water species. This study was conducted to investigate the histopathological and physiological effects of MWCNTs in freshwater fish (Poecilia reticulata). Literature reports that physicochemical properties of MWCNTs could be linked to their toxicological effects, hence the MWCNTs used in this study were characterized prior to (for physical and chemical properties) and during testing (for chemical interactions). Fourier transmission infra-red spectroscopy (FT-IR) revealed additional functional groups in oxidized multiwalled carbon nanotubes (o-MWCNTs) that were absent in the pristine multiwalled carbon nanotubes (p-MWCNTs). Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) also revealed amorphous carbon and residual Fe catalyst only in p-MWCNTs. The fish were exposed for 96 h in suspensions of p-MWCNTs and o-MWCNTs prepared by sonicating in moderately hard reconstituted water to test toxicity. Zeta potential measurements revealed an impartation of a negative charge onto MWCNT surfaces by HA, whilst hydrodynamic size measurements revealed aggregation/ disaggregation occurring alternately over the 96-h exposure time. Apart from only fish hyperactivity, no mortality was observed for 96-h acute tests for up to 300 mg/L MWCNT concentration. Histopathology revealed that MWCNTs aggregates were deposited mainly in the gut where they caused inflammation, hemorrhaging and oedema which were characterized by fragmented fibrin. Cell lyses also occurred in o-MWCNT exposed fish as evidenced by nuclei clusters among o-MWCNT aggregates. Statistical analysis by 2-way ANOVA followed by Bonferroni posttests showed that significant (p<0.05) reactive oxygen species (ROS) was induced as indicated by depleted antioxidant capacity (TCA), superoxide dismutase (SOD), lipid peroxidase (MDA) and protein carbonyl (PC) content between 3.25 and 15.0 mg/L of MWCNTs. These effects were attributed to the MWCNTs’ physicochemical properties as predetermined by functionalization (acid oxidation). The obtained results indicate that indeed MWCNTs cause toxic effects in freshwater fish, and although depuration was evident, there is a risk of MWCNT trophic transfer to fish eaters.
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