Investigation of the metal species in seawater by sorption of the metal ion on complexing resins with different sorbing properties

The distribution of some metal ions in seawater samples among different species was investigated by the resin titration (RT) procedure, based on the separation of the metal ion on resins with different sorbing properties. Two complexing resins were used for the titration of four metal ions (Cu(II), Cd(II), Ni(II) and Mn(II)): Amberlite CG 50 and Chelex 100. The first one is a weak resin, able to sorb only the metal ion combined in weak complexes, or free, while Chelex 100 is a strongly sorbing resin, able to sorb also the metal ion combined in strong complexes. The concentration of the total metal ion completely sorbed and/or in equilibrium with the resin is obtained by RT. Also the reaction coefficient, i.e. the fraction of total to free metal ion in the solution phase, in the presence of the resin, can be calculated. By comparing the titration with the two resins, three groups of species for each metal ion can be detected. For each of these, the reaction coefficients are evaluated, or at least their limiting values. For example, the complexes detected with Chelex 100 in the seawater samples here examined had very high reaction coefficients: in the case of copper in one Ligurian Sea sample at pH = 7.1, log α(M(I)) = 9.7 was found, for a complex concentration corresponding to 62% of the total metal ion. The rest of Cu(II) was linked to complexes with log α(M(I)) near to 1, weak enough to be dissociated and sorbed on Amberlite CG 50. The proportion of the groups of complexes was different in the different samples examined, while the reaction coefficients were similar.