Divalent cations speciation with three phosphonate ligands in the pH-range of natural waters

AMP and HEDP complexation constants for seven divalent cations (Ca, Cu, Cd, Fe, Ni, Pb and Zn) have been determined by acid-base titrations at 25 +/- 0.5 degrees C, at constant ionic strength (0.1 M KNO3) with Martell and Motekaitis's computer programs. Speciation diagrams enable us to compare AMP, HEDP and TPP complexing properties. AMP complexes more strongly all seven divalent cations than HEDP and TPP, which have similar ligand behavior. Among the divalent cations studied here, the ligands have lowest affinity with calcium and usually higher with copper. In all the cation/ligand systems, the major species are ML and MKL which are charged species. The uncharged ionic species Ca2Y0, CaH4X0 and Cd2Y0 (H6X = AMP and H4Y = HEDP) which can potentially exist in solid phase, cannot be neglected. Moreover hydroxide complexes have to be taken into consideration in the complexation constant determinations and in the environmental impact. (C) 1997 Elsevier Science B.V.