Influence of salinity and mineralization on trace metal sorption to cyanobacteria in natural waters

The equilibrium distribution of Cd, Cu and Pb between the cyanobacterium Anabaena spp. and natural lake water as a function of salinity and degree of algal decomposition, was studied using batch experiments. Adsorption isotherms could be described with the Freundlich equation. Distribution coefficients (Kd) for Anabaena spp. were largest for Pb, followed by Cd and Cu. The variation in Kd values for Cd was explained by a model accounting for metal complexation to the algal surface and complexation by dissolved ligands of variable concentration. The phytoplankton/water distribution of Cd in waters of different salinity appears to be regulated by the free Cd2+ activity in solution. Mineralization of Anabaena spp. for 42 d resulted in a decrease of the solid to water ratio by a factor of 12, and an increase in Kd for Cd (factor four), Cu (factor eight) and Pb (factor 10–20). The increase in Kd is attributed to a higher affinity of the metals for the adsorbent. The use of constant Kd values for mineralizing phytoplankton and detritus in trace metal fate models, may result in underestimations of bound fractions. The extent to which this fraction is underestimated, depends on the time course of the distribution coefficient and the solid to water ratio during the mineralization process.