The influence of hydroxyl-functionalized multi-walled carbon nanotubes and pH levels on the toxicity of lead to Daphnia magna

• pH values could influence the acute toxicity of Pb to Daphnia magna.
• OH-MWCNTs could increase the toxicity of Pb(II) to D. magna.
• The combined effects of Pb and CNTs under different pH levels were different.

The acute toxicity of lead was examined for Daphnia magna in waters with different pH values (5.0, 6.0, 7.0, 8.0 and 9.0) and different concentrations (0.01, 0.10, 0.50 and 1.00 mg/L) of hydroxyl-functionalized multi-walled carbon nanotubes (OH-MWCNTs). The results indicated that pH values could affect the toxicity of lead. With pH values increasing from 5.0 to 9.0, the 24 h-LC50 of Pb(II) increased from 0.784 to 9.473 mg/L, suggesting that the toxicity of Pb(II) was dramatically decreased. Although OH-MWCNTs almost caused no lethal effect to D. magna at the studied four concentrations, the combination of OH-MWCNTs and Pb(II) could cause more serious toxic effects to D. magna than Pb(II) alone. This study indicated that the synergistic effect caused by CNTs and lead could not be neglected.