Three competing ligand methods were compared to determine characteristics of Ag(I) complexation by dissolved and colloidal ligands present in three rivers and one sewage treatment plant effluent. Iminodiacetate groups on Chelex-100 resin (used in batch and column experiments) and diethyldithiocarbamate (DDC) were used as competing ligands. Results of batch Chelex and DDC competition experiments show good agreement with regard to relative extent of Ag binding by natural ligands among the three river systems. Results of both methods also show a possible correlation between extent of Ag(I) complexation and organic matter concentration and/or Fe concentration. Fraction of Ag(I) associated with Chelex in both batch and column Chelex experiments was similar in each of the four systems tested, indicating that lability of Ag complexes does not chanUe siguficantly on time scales ranging from seconds to 24 h. Results of Chelex and DDC competition were compared using a model based on a hypothetical single natural ligand. Under the experimental conditions used, this model quantified Ag(I) complexes with log K{sup}(cond) values from approximately 12 to 14. For the three rivers studied, ligands with silver-association characteristics similar to those of reduced sulfur groups (log K = 14-16) present at subnanomolar concentrations likely dominate Ag(I) speciation in these systems. A weaker ligand (e.g., log K{sup}(cond) 0.7 nM dominated Ag(I) speciation in the treatment plant effluent This may result from elevated concentration of metals that compete for reduced sulfur groups rather than from a lower total concentration of these groups.
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