Effect of groundwater pH and ionic strength on strontium sorption in aquifer sediments: Implications for ~(90)Sr mobility at contaminated nuclear sites

摘要

Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of ~(90)Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of ~(90)Sr in sediment-water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (<0.01molL ~(-1)), pH variation is the main control on sorption. The sorption edge for ~(90)Sr was observed between pH 4 and 6 with maximum sorption occurring (K _d~10 ~3Lkg ~(-1)) at pH 6-8. At ionic strengths above 10mmolL ~(-1), and at pH values between 6 and 8, cation exchange processes reduced 90Sr uptake to the sediment. This exchange process explains the lower ~(90)Sr sorption (K _d~40Lkg ~(-1)) in the presence of artificial Magnox tank liquor (IS=29mmolL ~(-1)). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr ~(2+) in either HCO _3-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ~9 O atoms at 2.58-2.61? after 10days. This is equivalent to the Sr ~(2+) hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365days incubation. Sequential extractions performed on sediments after 365days also found that ~80% of solid associated ~(90)Sr was exchangeable with 1M MgCl _2 in all experiments. These results suggest that over long periods, ~(90)Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of ~(90)Sr is to be expected.