METABOLISM-INDEPENDENT BINDING OF TOXIC METALS BY ULVA-LACTUCA - CADMIUM BINDS TO OXYGEN-CONTAINING GROUPS, AS DETERMINED BY NMR

The metabolism-independent metal binding characteristics of Ulva lactu ca were investigated using both freeze-dried thalli and cell walls str ipped of intracellular material by incubation in Triton-X followed by methanol. Biosorption of Cd, Zn, Cu and Co by freeze-dried thallus was concentration-dependent and followed Freundlich and Langmuir isotherm s. The Freundlich plot suggested that freeze-dried U. lactuca had the greatest binding affinity for Cu compared with Cd, Zn and Co. The BET (Brunauer-Emmett-Teller) plot, which indicates a more complex form of adsorption, and the Scatchard plot were not adequate models for Cu ads orption. The Scatchard plot of Cd suggested that two Cd binding sites were available on the freeze-dried thallus, with the second, lower aff inity site only becoming available at Cd loading capacities greater th an 4.9 mu mol g(-1) dry wt. Cd-113 nuclear magnetic resonance (NMR) st udies confirmed that two binding sites were available for Cd on the fr eeze-dried algal powder, though only one was available on the cell wal l, and that the affinity of the binding sites was greater for Cu than for Cd. The results of the NMR experiments suggested that Cd binds to oxygen-containing functional groups in the algal powder and on the cel l wall. It is proposed that sulphate or hydroxyl groups attached to po lysaccharide subunits are possible sites.