Complexation of Copper in Natural Waters. Determination of Complexing Capacities and Conditional Stability Constants in Natural Waters by MnO₂, and the Implications for Phytoplankton Toxicity

Abstract

<p>After a recapitulation of complexation theory, a theory is developed for the determination of copper complexation for mono- and bidentate complexes and for 2:1 complexes (CuL₂) by ion exchange. This theory can be applied in principle to other ion exchange methods that make use of weak ion exchange media with an adsorption behavior of metal ions similar to Langmuir adsorption. From this theory it has been deduced that titration with Cu²⁺ of a single ligand, in presence of a weak ion exchange medium, can be graphically represented by a straight line; slope and intercept with the Y-axis provide information on the conditional stability constant and the ligand concentration. In certain conditions similar data may be obtained for a second ligand, from a single titration of the sample with Cu²⁺ at fixed pH.</p> <p>An aged dispersion of MnO₂ is used as an ion exchange medium to determine complexing capacities of natural waters and conditional stability constants for complexes with copper ions. The method is sufficiently sensitive to apply to original water samples: no pre-concentration is necessary. Limits of detection are a minimum of 0.15 μM ligands and log conditional stability constants (log K') between 6 and 10 at pH 8, analyses may be performed at any pH between 6 and 9.</p> <p>For fulvic acid (FA) and for ligands in dystrophic rivers log K' = ~7.8 at pH 7.6, the ligand concentration is 5-30 μM; for ligands in lakes log K' ranges from 7.6 to 8.5 at pH 7.6, the ligand concentration is 0.2-3 μM. Generally only one ligand is dominantly present. From pH 5 to 9 the state of copper in natural waters is regulated by organic complexing ligands.</p> <p>Literature survey shows that conditional stability constants for complexes of humic acids (HAs) and FAs with Cu²⁺ have a constant relationship with the pH: log K' = pH -0.5, when 5 < pH < 8.5; for the same pH limits, the affinity of HAs and FAs for protons is about 3x larger than for Cu²⁺ ions. K' values for natural ligands analysed with the Mn°2 method also vary with the pH on a 1:1 basis of log K' vs. pH. Apparently a proton is exchanged for each Cu²⁺ ion bound.</p> <p>Three algal species, Anabaena cylindrica, Navicula pelliculosa and Scenedesmus quadricauda excreted 6.73, 2.86, and 0.66 μM of complexing ligands, respectively, with values for log K' of 7.7, 8.1, and 8.6, respectively, for complexes with copper. A free copper ion concentration of 10ˉ¹⁰˙³ M is calculated for each exudate, while in lakes a concentration of 10ˉ⁹˙⁸ to 10ˉ¹⁰˙² M is found. Algal exudates are similar in complexing ability to some amino acids and to ligands in lakes, and ameliorate toxicity of copper ions to the primary production of Chlorella vulgaris. From model calculation it is apparent that in presence of dissolved organic ligands toxicity or availability of Cu²⁺ to phytoplankton does not depend on pH, for 5.5 < pH < 8 and low Pco₂.</p>