| Abstract: |
Metal contaminations in groundwater systems may present a major environmental problem. If mobilised, metals may reach the groundwater, which represents an important source for drinking water, or they may become bioavailable and thus possibly toxic for plants and groundwater fauna. Therefore it is very important to understand under which conditions they are mobilised or immobilised.
In this work, the influence of organic ligands on heavy metal adsorption (copper, cadmium, and nickel) was studied in batch laboratory experiments using goethite as a solid phase. On the one hand, simple organic ligands (oxalate, pyromellitate (1,2,4,5-benzenetetracarboxylate), and salicylate), and on the other hand a natural unfractionated DOC from an infiltration groundwater of the river Glatt (Switzerland) were used.
Experiments were performed using untreated groundwater with DOC in its natural concentrations (1.5 – 2.3 mg/L), and metal concentrations close to environmental conditions. Complexation
of the metals by natural ligands was determined by voltammetric methods and by an ion selective electrode and was described by a model defining the minimum necessary number of discrete ligands. Complexation of copper and cadmium was described by assuming the presence of a strong ligand with a low concentration and additional weaker ligands with higher concentrations, whereas complexation of nickel within a limited concentration range was described with only one strong ligand with low concentration. In the presence of the natural ligands, copper, cadmium, and nickel adsorption was decreased at high pH values. Metal adsorption at low total metal concentrations was suppressed, as could be shown by adsorption isotherms at pH 7.35.
The three synthetic ligands showed quite different results. Salicylate had no major effect on the adsorption of copper, and nickel, and only a slight enhancement of cadmium adsorption at pH > 6 could be observed. In the presence of oxalate, the pH adsorption edge of copper was shifted to higher pH, and nickel adsorption was decreased at pH values above 7. Cadmium adsorption in the oxalate system was weakly increased between pH 6 and 7. In the presence of pyromellitate, adsorption of copper, cadmium, and nickel was stronger than expected according to the complexation capacity of pyromellitate in solution.
Modelling of the metal-ligand-goethite systems was performed by a surface complexation model for metal adsorption, with a double layer model for the electrostatic interactions, to which three modules were applied step by step. These modules take into account competition reactions between the surface functional groups and the aqueous ligands, electrostatic attraction of the metal by the adsorbed ligand, and formation of ternary surface complexes of type A (surface-metal-ligand) or B (surface-ligand- etal).
In the case of salicylate, a model including competition between surface and solution complexation was sufficient to describe the experimental data for cadmium and nickel, whereas ternary surface complexes of type A had to be defined for copper. In the case of oxalate, competition between surface and solution complexation was able to describe the cadmium data, but gave unsatisfactory results for copper and nickel. Ternary surface complexes of type B were assumed for the nickel-oxalate-goethite system. In the case of pyromellitate, formation of ternary surface complexes of type B had to be included to describe the copper and cadmium data, which is in agreement with the favourable pyromellitate structure.
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