The effect of sodium cyanide (NaCN), iron-cyanide complexes and a blas
t furnace effluent on the growth of the unicellular marine diatom Nitz
schia closterium was investigated. Due to the volatile nature of HCN,
the free and total cyanide concentrations in ail test solutions were m
onitored dairy to ensure that average daily losses of free cyanide did
not exceed 5%. The iron-cyanide complexes were less toxic than the Na
CN, with 72-h EC50 values of 57, 127 and 275 mu g total-CN litre(-1) f
or NaCN, K3Fe(CN)(6) and K4Fe(CN)(6), respectively. The estimated lowe
st observed effect concentration (LOEC) for NaCN was 10 mu g litre(-1)
. The toxicity of the cyanide complexes was largely due to free cyanid
e (HCN + CN-) with possible additional toxicity caused by dissociation
of the complexes at the algal cell membrane. The toxicity of an efflu
ent from a blast furnace drawdown episode also correlated well with th
e toxicity of free cyanide. (C) 1997 Elsevier Science Ltd.