Metal sorption to ferrihydrite : phosphate effects, X-ray absorption spectroscopy and surface complexation modelling

Sammanfattning: Phosphorus affects the sorption of many metals and arsenate to iron (hydr)oxides. This may influence the mobility and bioavailability of metals and arsenate. Phosphorus therefore plays an important role in, e.g., determining the ecotoxicological risk of contaminants in soils. The overall aim of this thesis was to improve the understanding of lead(II), copper(II), cadmium(II) and arsenate binding to iron (hydr)oxides. The focus was on how phosphate affects the sorption on ferrihydrite and soils in which ferrihydrite is an important constituent. The effect of phosphate on the sorption of lead(II), copper(II), cadmium(II) and arsenate was determined by batch experiments. X-ray absorption spectroscopy (XAS) was performed to identify the binding mechanisms. Geochemical models were developed based on the XAS results and knowledge about the mineral or soil properties. Phosphate enhanced the sorption of lead(II), copper(II) and cadmium(II) to ferrihydrite. The increased sorption was best explained by the formation of ternary complexes including the ferrihydrite surface, the metal and the phosphate ion. Phosphate competed strongly with arsenate for sorption sites on ferrihydrite. The competition was even stronger on poorly crystalline aluminium hydroxide. Zero-valent iron that is mixed into soil rapidly oxidises to ferrihydrite that can adsorb contaminants. It was shown that the immobilisation of copper and arsenic in soils that had been stabilised by zero-valent iron is long-lasting. Copper immobilisation was most effective at high pH (>6) and at low organic matter content. Competition with phosphate needs to be taken into account when modelling arsenate sorption in soils. Otherwise the latter may be greatly overestimated. Metal sorption to some podzolised soils was investigated in batch experiments. Despite the large influence on metal sorption in the pure ferrihydrite systems the addition of phosphate did not affect lead(II), copper(II) or cadmium(II) sorption to the B and C horizons of podzolised soils. The reasons may be strong metal binding to organic matter combined with a relatively small addition of phosphorus in the experiments. In conclusion this thesis shows that phosphate greatly affects the sorption of lead(II), copper(II), cadmium(II) and arsenate to iron (hydr)oxides. To determine the impact of this effect in more complex matrices such as soils, more research is needed.

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