Analytical fractionation of labile metals in selected groundwater humic substances by means of chelating ion-exchanger

Summary Trace metals remaining in humic substances (HUS) after their acidic isolation (XAD 2) from aquatic sources may preferably be bound in inert form. In the present study, the reactivity (lability) of such a trace metal fraction (e.g. Cu, Fe, Mn, Zn) in selected groundwater HUS (BOC 1 and 3 from the DFG Versuchsfeld Bocholt, FRG) is characterized by its different separation behaviour towards a chelating ion-exchanger (e.g. Hyphan) using a time-controlled sequential batch procedure (96 h). Under standardized conditions, the kinetics and the degree of the ion-exchange reaction serve as parameters for the operational evaluation of metal lability in the above mentioned HUS. Surprisingly, according to the above ion-exchange procedure, about 50% of the total Fe, 81 (95)% Cu, 65 (97)% Mn, 69 (97)% Ni and 82 (95)% Zn in BOC 1 (BOC 3) proved to be bound in labile form. However, 20 to 30% of the labile metal fraction only react following very slow first order kinetics (half-time 24 h). In contrast, trace metals freshly bound to BOC 1 are quantitatively recovered by the collector Hyphan within 1 to 2 h, with the exception of Fe. Moreover, in this way HUS samples can be purified for molecular spectroscopy investigations (e.g. NMR, ESR, fluorescence) which are highly sensitive to metal interferences.