Nanoparticulate iron oxyhydroxides, such as ferrihydrite (Fe10O14(OH)2), are used in a range of processes to remove contaminants (e.g. U and 90Sr) from industrial effluents and natural waters. Ferrihydrite has been used by the nuclear industry for many years to sequester radioactive contaminants, including at the Fukushima Daiichi nuclear power plant, Japan. The Enhanced Actinide Removal Plant (EARP) at the Sellafield site, UK, is a key effluent treatment facility for the removal of radionuclides via precipitation of iron oxyhydroxides. Neutralisation of an acidic effluent solution containing Fe(III) induces the formation of ferrihydrite, a poorly ordered, high surface area phase, which removes radionuclides (e.g. U) via sorption to the nanoparticles. However, Sr sorption to iron oxyhydroxides has previously been shown to be poor, furthermore the sorption mechanism of Sr to ferrihydrite is not well defined. In this study we investigated the uptake of Sr to ferrihydrite in the presence of phosphate under effluent treatment conditions. X-ray absorption spectroscopy in conjunction with ATR-FTIR and PHREEQC chemical modelling were employed to investigate the removal mechanism of strontium by the ferrihydrite phase. Sr removal by ferrihydrite without phosphate present is minimal (&lt;10%). Sr uptake increases significantly with increasing phosphate concentration up to 95% removal by pH 9 at 165 ppm PO43-. In addition, the initial pH of the solution significantly affects Sr uptake, with lower starting pH (&lt;1) leading to a reduced uptake, due to the higher final ionic strength. The Sr sorption mechanism was determined via a combination of XAS and FTIR analysis. These data indicated that the enhanced Sr uptake is due to the formation of a ternary surface complex with PO43- on the ferrihydrite surface, which has not previously been characterised. This shows that increased levels of phosphate may provide a route to enhance 90Sr removal from radioactive effluents.

Enhanced removal of strontium using iron(III) (oxyhydr)oxides via the addition of phosphate: underpinning radioactive effluent treatment

ACS Spring 2022

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