Toxicity of cyanide, iron-cyanide complexes, and a blast furnace effluent to the banana prawn, Penaeus monodon

Banana prawns were exposed to cyanide and its iron complexes for 96 h; dissolved oxygen, pH, salinity and temperature were monitored daily before and after solution renewal. The 96 h LC50 was calculated. A reference toxicity test with sodium dodecyl sulphate was carried out. At pH 8, 96 h LC50 values were 0.11, 9.1 and 60.8 mg cyanide per litre for sodium cyanide, potassium ferricyanide and potassium ferrocyanide, respectively. At pH 9, the values for both complexes fell to the range 2.41-2.70 mg cyanide per litre. The toxicity of the complexes at pH 8 was due to their slight dissociation to free cyanide. The mechanism was more complex at pH 9 and was probably the result of the organisms absorbing hydroxyl ion, hydrogen cyanide and perhaps iron. The toxicity of a complex blast-furnace effluent, which was also tested, was ascribed to its free cyanide content.