BINDING OF CU(II) TO THE SURFACE AND EXUDATES OF THE ALGA DUNALIELLA-TERTIOLECTA IN SEAWATER

The adsorption process and the organic matter interaction between the marine phytoplankton specie Dunaliella tertiolecta and copper ions were investigated by differential pulse anodic stripping voltammetry. Suspensions of living algae in natural Gran Canaria (Islas Canarias) seawater were titrated with Cu(ll)as a function of pH, temperature, and salinity. The acid-base properties of the surface of the alga in a 0.7 M NaCl solution were characterized and interpreted as if the surface contained carboxylic acid groups (pK(a,1) = 4.92 +/- 0.16) and amino groups (pK(a,2) = 6.28 +/- 0.09, pK(a,3) = 10.06 +/- 0.10). The binding constant for the weaker ligands of the Gran Canaria seawater increased from 8.60 +/- 0.03 to 9.30 +/- 0.12 when 2.2 x 10(7) cell L(-1) was added to 0.45-mu m filtered seawater. The rate of uptake was found to occur in two steps. The adsorption equilibrium data correlate well with a two-site model which considers the algal surface as one which possesses two major functional groups: high-affinity binding and low-affinity binding. The high-affinity constant was of the same order of magnitude as the complexing capacity of the exudates excreted by the algae, showing that extracellular ligands play an important role in decreasing the concentration of the free metal concentration. Changes in the temperature, salinity, and pH of the seawater solution modified both the adsorption of metal and the amount of complexed copper.