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Conditional stability constants and binding capacities for copper (II) by dissolved organic carbon isolated from surface waters of the southeastern United States
Author(s) -
Giesy John P.,
Alberts James J.,
Evans David W.
Publication year - 1986
Publication title -
environmental toxicology and chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.1
H-Index - 171
eISSN - 1552-8618
pISSN - 0730-7268
DOI - 10.1002/etc.5620050204
Subject(s) - dissolved organic carbon , chemistry , titration , copper , ligand (biochemistry) , stability constants of complexes , metal , scatchard plot , gaussian , analytical chemistry (journal) , inorganic chemistry , ion , binding site , environmental chemistry , computational chemistry , organic chemistry , receptor , biochemistry
Dissolved organic carbon (DOC), isolated from each of nine surface waters in the southeastern United States, was fractionated into two nominal molecular size fractions, i.e., 52 Å < fraction I < 4,500 Å and 13 Å < fraction II < 52 Å. The binding capacities and overall conditional stability constants ( K̄' ) for copper were determined by selective ion electrode. The binding capacities ranged from 2.4 × 10 −7 to 1.3 × 10 −5 M Cu/mg DOC, with a mean for all locations and fractions of 2.2 × 10 −6 (SD = 6.2 × 10 −5 , n = 15). There was no significant difference (Mann‐Whitney U Test, p > 0.2) between binding capacities of fractions I and II. Sensitivity analyses showed that the overall mean binding capacity was adequate for use in simulation models. We estimated overall conditional stability constants by seven mathematical techniques, including both discrete and continuous models. The thermodynamic, geochemical simulation model GEOCHEM was used to determine the sensitivity of predictions to variation in both binding capacity and overall conditional stability constants estimated by each technique. Predictions were compared with titrations of isolated humate fractions and whole waters. The two‐component Scatchard function, a discrete model, and the Gaussian‐Scatchard function, a continuous model, gave the most accurate predictions in these simulations. We endorse the use of the Gaussian‐Scatchard function because it is flexible and has a stronger theoretical basis. Stability constants ( K' ) were calculated for measured values of free and bound metal and ligand across a metal‐organic ligand titration curve. These values were dependent on the copper/total ligand ratio and varied on a continuum of binding strength by as much as a factor of 100 for a given fraction and location. The means of maximum conditional stability constants calculated across locations were 5.2 × 10 5 (SD = 3.9 × 10 5 , n = 7) and 5.9 × 10 4 (SD = 4.05 × 10 4 , n = 7) for fractions I and II, respectively. The median, maximum conditional stability constant for fraction II was significantly weaker (Mann‐Whitney U Test, p < 0.005) than that for fraction I. The number of sites with each K' were log 10 ‐normally distributed, with proportionately more of the weaker binding sites.