Organic complexation of copper in surface waters of Galveston Bay
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abstract
The importance of colloidal ligands in the organic complexation of Cu was elucidated in the estuarine waters of Galveston Bay by use of a number of instrumental techniques. Ultrafiltration was conducted to isolate the ultrapermeate fraction from the filter-passing fraction and competitive ligand equilibration/adsorptive cathodic stripping voltammetry measurements were conducted by use of varying amounts of catechol as the competitive ligand. Greater than 99.9% of Cu was bound by strong organic ligands with log[K (M-1)] values of 12.3 0.15 and 11.1 0.29 in the filter-passing and ultrapermeate fractions, respectively. The concentrations of Cu complexing ligands were lower in the ultrapermeate than in the filter-passing fraction, and the calculated K values for the colloidal ligands (log K = 12.9) were 60 times greater than those for the ultrapermeate fraction, suggesting that good separation of the colloidal fraction during ultrafiltration was achieved. Total Cu concentrations in the filter-passing fraction ranged from 2.27 to 12.9 nM and were fairly constant at salinity <20 but decreased at salinity >20. The calculated free Cu concentrations (0.05-0.49 pM) showed an overall increasing trend with salinity, possibly resulting from decreasing concentrations of Cu complexing ligands toward the seawater endmember. The concentration of Cu complexing ligands (21-54 nM) in the estuarine regions of Galveston Bay was approximately equal to the concentration of total reduced sulfur species (20-60 nM) in the filter-passing fraction, suggesting that reduced sulfur species could account for most of the Cu-complexing ligands in these waters.
