Seven species of brown, green, and red seaweeds were examined for their abilities to sequester cadmium ions from aqueous solution. Although all the seaweed types investigated were capable of binding appreciable amounts of cadmium, considerable variability in their biosorption performance was observed. Maximum cadmium uptake capacities at pH 5 ranged from the highest value of 0.74 mmol/g for the brown seaweed Sargassum baccularia to the lowest value of 0.16 mmol/g for the red seaweed Gracilaria salicornia, representing a 363% difference. In general, brown seaweeds were found to exhibit the best overall cadmium ion removal. Additional experiments were conducted to evaluate the biosorption characteristics of the brown seaweed S. baccularia. The equilibrium uptakes of cadmium were similar within the pH 3-5 range but decreased significantly when the solution pH was reduced to pH 2. The presence of background cations such as sodium, potassium, and magnesium and anions such as chloride, nitrate, sulphate, and acetate up to a concentration of 3.24 mmol/l was found to have no significant effect on the equilibrium uptake of cadmium. However, the biosorbent uptake of cadmium was markedly inhibited in the presence of calcium ions at 3.24 mmol/l. Kinetic studies revealed that cadmium uptake was fast with 90% or more of the uptake occurring within 30-40 min of contact time. (C) 2003 Published by Elsevier B.V.