Labile complexes of trace metals in aquatic humic substances: Investigations by means of an ion exchange-based flow procedure

The lability/inertness of heavy metals bound in aquatic humic substances (HS) has been characterized by means of ligand exchange with cellulose-immobilized triethylenetetramine-pentaacetic acid (TETPA) applying a flow system. On the basis of high metal distribution coefficients, Kd of 103 to 104 (ml/g) on cellulose TETPA even in slightly acidic HS solutions, labile and inert metal fractions in HS are characterized by their different kinetics and degree of phase exchange in small TETPA columns. For traces of metals bound to dissolved HS, the lability order Cd ≈ Mn(II)>Zn>Pb>Co>Ni>Cu is revealed. Systematic variation of environmentally relevant parameters shows the strong influence of the pH value and the ratio of metal loading/complexing capacity on the metal lability in HS. Surprisingly, in the case of freshly formed HS/Ni and HS/Cu complexes, slow transformation processes occur which lower their initial lability by one order of magnitude and supposedly increase their thermodynamic stability.