Voltammetric study of the speciation of cadmium(II) with L-aspartic acid in sea water

Abstract

The complexation of Cd(II) with L-aspartic acid has been studied in model solutions of ionic strength 0.7 M, in artificial and genuine sea water by linear sweep anodic stripping voltammetry (LASV) at a low overall Cd(II) concentration of 9×10 −8 M. The side reactions of the Cd 2=+ ion and of the organic ligand L in the sea water medium have been take into account and their effects on the conditional stability constant of the Cd(II)—L aspartic acid complexes have been clarified. Cd(II) forms two complexes of the type CdL and CdL 2, which behave reversibly under polarographic conditions. Using the DeFord—Hume approach, we have evaluated the conditional stability constants from the dependence of the half-wave potential shift on the ligand concentration. A more precise equation for the half-wave potential shift in a multi-ligand system has been derived taking into account the mean coordination number n̄ of the metal ion. The stability constants of Cd—L aspartic acid complexes are of the same order of magnitude as those obtained earlier for the Cd—glycine complexes. Taking into account the low dissolved organic matter (DOM) content of the open sea, it can be concluded that amino acids of this type cannot contribute significantly to the chelation of cadmium. Yet in estuarine waters and in interstitial waters of sediments, the often substantially higher DOM level may cause a certain degree of chelation of the dissolved trace metal Cd(II) by these amino acids.