Determination of silver speciation in wastewater and receiving waters by competitive ligand equilibration/ solvent extraction

A competitive ligand equilibration/solvent extraction (CLE/SE) technique wa s used to examine silver complexation in the particulate (>0.45 mu m), coll oidal (<0.45 mu m), and dissolved (<10 kDa) states of wastewater effluent a nd receiving waters. Additions of silver at near ambient levels (0.1-10 nM) , followed by CLE/SE, allowed the determination of complexation sites with large stability constants (log K-AgL = 11-12) responsible for silver bindin g. Good agreement between these constants and formation constants of well-c haracterized silver sulfide complexes suggests that silver in wastewater ef fluents and receiving waters is likely to be complexed to sulfur(-II). This is supported by previously collected data on silver and sulfide concentrat ions in these waters, which showed that inorganic sulfide concentrations ar e 200 to 300 times in excess of silver concentrations. Organic ligands appe ar to be insignificant in comparison to the contribution of inorganic ligan ds to silver binding. These inorganic ligands probably consist of metal sul fides, which are stable for hours and days in oxic waters. When complexed t o these strong-affinity ligands, silver(I) is protected from photoreduction to zero-valent silver.