The toxicity order of CNTs was ranked: SWCNTs > MWCNTs > OH-MWCNTs > COOH-MWCNTs.
All CNTs enhanced the toxicity of Cd to Daphnia magna.
The toxicity-increasing of SWCNTs and MWCNTs was caused by metal impurities.
The adsorption of Cd onto OH- and COOH-MWCNTs led to the enhanced toxicity.