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

A competitive ligand equilibration/solvent extraction (CLE/SE) technique was used to examine silver complexation in the particulate (>0.45 mu m), colloidal (<0.45 mu m), and dissolved (<10 kDa) states of wastewater effluent and 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 binding. Good agreement between these constants and formation constants of well-characterized silver sulfide complexes suggests that silver in wastewater effluents and receiving waters is likely to be complexed to sulfur(-II). This is supported by previously collected data on silver and sulfide concentrations in these waters, which showed that inorganic sulfide concentrations are 200 to 300 times in excess of silver concentrations. Organic ligands appear to be insignificant in comparison to the contribution of inorganic ligands to silver binding. These inorganic ligands probably consist of metal sulfides, which are stable for hours and days in oxic waters. When complexed to these strong-affinity ligands, silver(I) is protected from photoreduction to zero-valent silver.