Phosphate effects on copper and lead sorption to iron oxide

• Elevated concentrations of phosphate affects the mobility of copper and lead in soil. More detailed knowledge about the mechanisms behind this is of interest for example in risk assessments of contaminated soils. It is well known that the mineral pyromorphite is formed at high concentrations of lead(II) and phosphate in combination with high pH. This effect has been used to immobilize metals in remediation of contaminated soil.Sorption mechanisms, on the other hand, act also at low concentrations of metal and phosphate when no precipitates are formed. It has been shown that the sorption of lead(II) to iron oxide surfaces can be enhanced in precence of phosphate. Exactly how phosphate interacts with lead(II) and iron oxide has not yet been established, and there are seemingly contradictory results. Phosphate effects on copper(II) sorption to iron oxy(hydro)xides has been less studied.Here, batch experiments, X-ray absorption spectroscopy (EXAFS) and surface complexation modeling with the three-plane CD-MUSIC model has been used to study the effect of phosphate on sorption of copper(II) and lead(II) to ferrihydrite. The aim is to investigate what surface complexes are formed and to derive new and improved surface complexation constants for adsorption to ferrihydrite.A similar approach including batch experiments, X-ray absorption spectroscopy (EXAFS) and modelling is now applied to investigate sorption to samples of Swedish forest soils.Results show that addition of phosphate increases the sorption of copper(II) and lead(II) to ferrihydrite in batch experiments. The EXAFS results show, in agreement with earlier studies, that copper(II) and lead(II) form bidentate mononuclear complexes on ferrihydrite in systems without phosphate. However, in the presence of phosphate, there are indications of ternary complexes. The results are being used to suggest new surface complexation constants for the reactions.Results from experiments with Swedish forest soils are currently being evaluated.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

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