| Abstract: |
Transport of lead(II) and copper(II) ions in soil is affected by the soil phosphorus status. Part of the explanation may be that phosphate increases the adsorption of copper(II) and lead(II) to iron (hydr)oxides in soil, but the details of these interactions are poorly known. Knowledge about such mechanisms is important, for example, in risk assessments of contaminated sites and development of remediation methods. We used a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and surface complexation modeling with the three-plane CD-MUSIC model to study the effect of phosphate on sorption of copper(II) and lead(II) to ferrihydrite. The aim was to identify the surface complexes formed and to derive constants for the surface complexation reactions. In the batch experiments phosphate greatly enhanced the adsorption of copper(II) and lead(II) to ferrihydrite at pH < 6. The largest effects were seen for lead(II). Based on interpretation of the EXAFS spectra edge-sharing bidentate copper(II) or lead(II) complexes predominated in the single-sorbate systems with ferrihydrite. Lead(II) EXAFS spectra suggested a distinct change of coordination in the presence of phosphate, i.e. the signal from edge-sharing complexes diminished and a longer Pb···Fe distance appeared at about 4 Å. A similar, but less pronounced, pattern was observed for copper(II). Based on the results from interpretation of EXAFS spectra and surface complexation modeling with the CD-MUSIC model the enhanced sorption in presence of phosphate was most satisfactorily explained by the appearance of ternary metal–phosphate complexes in which the metal interacts directly with the surface.In conclusion, geochemical models used for simulating trace element behavior in acidic environments seem to require ternary metal–phosphate surface complexes to properly describe partitioning of metals between solution and the solid phase. |
