Phosphate effects on copper(II) and lead(II) sorption to ferrihydrite

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). 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.3

Molybdenum binding to soil constituents in acid soils: An XAS and modelling study

“…However, for goethite around 70% of the bound molybdate was present as corner-sharing bidentate complexes (Arai, 2010), whereas for ferrihydrite in the present study the corresponding figure was 25% or less. This difference is in line with what was found earlier for lead(II) and copper(II) (Tiberg et al 2013), and can be attributed to the more disordered structure of ferrihydrite, which increases the likelihood of edge-sharing coordination. The binding mode on amorphous Al(OH) 3 appears to be similar, with a predominance of edge-sharing complexes.…”

“…The model assumes a surface area of 650 m 2 g -1 and a point-of-zero charge at pH 8.1. Moreover the site density of singlycoordinated groups (where molybdate is allowed to sorb) is 6.25 sites nm -2 , while the site density of triply-coordinated groups is 1.55 sites nm -2 (Tiberg et al 2013). The Stern layer capacitances are set to 1.15 F m -2 (inner layer) and 0.9 F m -2 (outer layer) in accordance with Hiemstra and van Riemsdijk (2009).…”

“…The version of the CD-MUSIC model we used in this work was the one for ferrihydrite by Tiberg et al (2013). The model assumes a surface area of 650 m 2 g -1 and a point-of-zero charge at pH 8.1.…”

“…As input data we used the percent adsorbed molybdate from experiments detailed by Gustafsson (2003). Surface complexation constants for phosphate and electrolyte ions were those of Tiberg et al (2013), and these are detailed in Table 3.…”

“…However, the EXAFS data of the current study show that this is incorrect, as bidentate rather than monodentate complexes are formed. Therefore the molybdate adsorption data of Gustafsson (2003) were re-evaluated using the CD-MUSIC model for ferrihydrite (Tiberg et al 2013). Several different combinations of complexes were tested in the fitting.…”