Pb2+ interactions with the marine phytoplankton Dunaliella tertiolecta

Abstract

Metal ions become adsorbed to algal surface groups and complexed to organic material released by algae at all growth stages influencing their distribution in the seawater. In this study, the differential pulse anodic stripping voltammetry (DPASV) technique was used to evaluate the adsorption and the interaction between the exudates excreted by the marine algae Dunaliella tertiolecta and Pb2+. The adsorption process has been studied in seawater as a function of pH (2 to 8), temperature (5 to 45 °C), salinity (5 to 36), biomass and the presence of a second metal, copper. The rate of adsorption was found to occur in two steps: first a relatively fast adsorption step (10 min) and second, a slow, diffusion-controlled uptake into the cells. A two-site model which considers the presence of two major functional groups, namely: high-affinity binding and low-affinity binding was used to fit the experimental data. The values for the stability constants of Pb2 on these two groups were log ∗KH,1S = 8.40 ± 0.18 and log ∗KH,2S = 7.25 ± 0.31. The high-affinity constant similar to the complexing capacity of the exudates (log ∗KMe2+ = 8.40 ± 0.05) produced by the alga showing that extracellular ligands play an important role in buffering the concentration of the free metal concentration. A decrease in the temperature and salinity result in lower adsorption of Pb2+ to the algae and a higher complexation of lead with the exudate. The presence of copper decreases the lead complexation to the algal surface by 70%.