Silver uptake by the green alga Chlamydomonas reinhardtii in relation to chemical speciation: Influence of chloride

Short-term (<1 h) silver uptake by the green alga Chlamydomonas reinhardtii was measured in the laboratory in defined inorganic media over a range of silver and chloride concentrations. For a low, fixed, free Ag+ concentratio n (e.g., 8 nM), silver uptake increases markedly (up to <similar to>4X) as a function of the chloride concentration (5 muM-->4 mM Cl-); the free-ion m odel would have predicted a constant silver uptake rate. No evidence could be found for the passive diffusion of the neutral AgCl0 complex or for the facilitated uptake of the anionic AgCl2- complex. The enhanced uptake obser ved in the presence of chloride is related to the very high silver uptake r ates demonstrated by the test alga, which lead to diffusion limitation in t he boundary layer surrounding the algal cell. In such a situation, metal ac cumulation is proportional to the total metal concentration (i.e:, to the c oncentration gradient between the bulk solution and the algal surface). At higher silver concentrations (e.g., greater than or equal to 10(-7) M), dif fusion in the phycosphere is no longer rate limiting, the chloride stimulat ion disappears, and silver uptake is sensitive to the free-ion concentratio n. However, such a high concentration of silver is not likely to be encount ered in the environment, even in wastewater effluents.