Simultaneous determination of stability constants of humate complexes with various metal ions using multitracer technique

Stability constants of humic acid complexes of 19 elements, Be, Ca, Sc, V, Cr, Mn, Fe, Co, Zn, Ga, Sr, Y, Ba, Ce, Eu, Gd, Tm, Yb, and Lu, were determined simultaneously using the multitracer technique. Since the results were obtained under an identical method, model, and sample, we could compare the stability constants of humate with various metal ions. As a whole, humate complex of ions having more charge and smaller ionic radii is more stable, showing the ‘hard’ character of humic acid as a complexing ligand. Interpretation of the results of stability constants in terms of simple electrostatic model suggests that a bidentate carboxylate site is responsible for the humate formation with most of those trace ions. Among these elements studied, it is estimated that humate formation has the most substantial influence on the environmental behavior of rare earth elements as compared with that on the other elementsʹ behavior.