Humic Colloid-Borne Natural Polyvalent Metal Ions:  Dissociation Experiment

Abstract

The natural association nature of the humic colloid-borne trace elements is investigated. Rare earth elements (REE) Th and U are chosen as naturally occurring representatives and chemical homologues for actinides of different oxidation states present in nuclear waste. Tri- and tetravalent elements in two investigated Gorleben groundwaters (Gohy-532 and -2227) almost exclusively occur as humic or fulvic colloid-borne species. Their desorption behavior from colloids is examined in the unperturbed groundwater (pH approximately 8) under anaerobic conditions (Ar/1% CO2) by addition of a chelating cation exchanger resin. Particularly, the dissociation process of naturally occurring Eu(III) in the groundwater is compared with the Eu(III) desorption from its humate complex prepared with purified Aldrich humic acid in a buffered aqueous solution at pH approximately 8. The Eu(III) dissociation from the groundwater colloids is found to be considerably slower than found for the humate complex synthesized in the laboratory. This suggests that under natural aquatic conditions the Eu(III) binding in colloids is chemically different from the simple humate complexation as observed in the laboratory experiment. The colloid characterization bythe size exclusion chromatography (SEC) and the flow field-flow fractionation (FFFF) indicates that natural colloid-borne trace elements are found predominantly in colloids of larger size (>15 nm in size), while Eu(III) in its humate complex is found mainly in colloids of hydrodynamic diameters <5 nm. The slower desorption kinetics and the larger colloid size suggest that the polyvalent metal ion binding in natural humic colloids is associated to polynucleation with other co-present trace metal ions. Radiotracer experiments reveal that isotopic equilibria with the naturally colloid-borne trace elements are not attained within a period of more than 100 days, indicating irreversible binding of at least a part of colloid-borne polyvalent trace elements. The different kinetic properties of colloid-bound Eu(III) are discussed taking the aqueous speciation based on thermodynamic data into account.

Competitive effect of iron(III) on metal complexation by humic substances: Characterisation of ageing processes

“…Binding of higher-valent metals to humic substances has been reported to be accompanied by time-dependent transformations that entail a decreasing availability to competing ligands in dissociation experiments (Van den Bergh et al, 2001;Geckeis et al, 2002). Such findings are of some importance in view of current modelling strategies, which are based on thermodynamic equilibrium constants.…”

Investigation of the effect of pH, ionic strength, foreign ions, temperature, soil humic substances on the sorption of 152+154Eu(III) onto NKF-6 zeolite

“…ions and the Table 1) are shown in Fig. 5b [54,55]. The results demonstrate that Eu(III) presents in the forms of Eu 3?…”

Potential Use of Ultrafiltration for Groundwater Remediation and Aqueous Speciation of 60Co and 137Cs from A Contaminated Area

“…They have reported sorption coefficients K d which were 1-2 orders of magnitude higher with tracers than their counterpart in contaminated groundwater. Geckeis, Rabung, Ngo, Kim, and Beck (2002) have reported that naturally present Eu in groundwaters (associated with colloidal material) behaved differently from the Eu added from artificial solutions. Desorption of naturally occurring Eu was slow (>100 d and still not finalized), whereas desorption of artificially added Eu was fast.…”