Inert copper ion complexes formed by humic acids

摘要

In the pH range 7-2.6 copper ions form inert complexes with humic acids, which may be bound to intraparticle binding sites. This is shown by emf studies using glass and Cu-sensitive electrodes at 24 degrees C and 0.1 M NaClO4 medium. Solutions of Cu-II, (0.1-0.33 mM) and humic acids (1-2 g l(-1)) were titrated with HClO4 in an oxygen-free atmosphere. At each point the data were recorded after reaching a steady state (after 6-24 h) as indicated by a redox electrode. Kinetic data indicate that the metal ion interaction involves diffusion into the humic acid particles; the rate-limiting step appears to be the diffusion through the boundary layer surrounding the particles (film diffusion). For total Cu concentrations less than or equal to 0.24 mM, Cu ions seem to be bound in four-coordinate sites, since four protons are released per Cu ion. However, somewhat fewer protons are released (greater than or equal to 3) far the highest copper concentrations (0.33 mM) as well as for the lowest humic acid concentration (1 g l(-1)). In addition, a small fraction of the copper ions forms a labile CuA complex with log beta(1) = 5.3, most likely involving the coordination of two carboxylates. (Here A is the binding site of humic acids.) The proton reactions of humic acids, studied under the same conditions, are consistent with the titration of three acid-base groups of apparent pK-values, 5.8, 3.8, and 1.6, and a HA(2),'dimer' involving hydrogen bonds of the particles. [References: 46]