Application Of Membrane Filtration In Removing Radioactive Contaminant Strontium

The recent accident at the Fukushima Daiichi Nuclear Power Plant caused by the Great East Japan Earthquake reminds people of the serious hazards of radioactive contaminants. Strontium-90(90Sr), as a representative radionuclides, is generated from the nuclear fuel cycle and the production of radioisotopes.90Sr is harmful to human health, natural resources and the environment. Internal exposure to90Sr is linked to bone cancer, cancer of the soft tissue near the bone, and leukemia. Non-radioactive strontium has relatively low toxicity, but excessive exposure may lead to bone growth problems in children.Membrane processes, especially nanofiltration (NF) and reverse osmosis (RO), are very promising technologies to remove strontium from water, but the actual performance is strongly affected by feed water quality, operating conditions and membrane fouling.Firstly, this study investigates the impact of membrane type, recovery, and pH on strontium retention from drinking water samples. Six NF/RO membranes (BW30, BWLE, NF90, NF270, TFC-SR2, TFC-SR3) are tested in a dead-end stirred cell. Results show that strontium is rejected by the membrane mostly due to size exclusion. RO has a higher strontium retention than NF. Recovery affects the retention by affecting concentration polarization. The pH affects the retention by changing strontium speciation. Secondly, this study investigates the interactions between strontium and natural organic matter (NOM) and the impact of such interactions on strontium retention. NOM contains high content of oxygen-containing functional groups which allow it to form complexes with metal ions. The transport and retardation of strontium in the environment is strongly affected by Sr-NOM interactions. The influencing factors include pH, concentration, and competing cations.In bench-scale experiments, synthetic strontium solution and NF270membrane are used. Humic acid is added as the representative of NOM. Interactions of strontium and NOM are modelled by the Visual MINTEQ software package. Results show that pH affects the solubility of both NOM and strontium species. While NOM is more soluble at high pH, strontium ions are more soluble at low pH. At pH7-9strontium forms complexes with NOM due to the full dissociation of carboxylic groups which enhances the attraction of strontium to NOM. Consequently, strontium is effectively removed together with NOM.In pilot-scale experiments, a small crossflow RO machine is used. A natural groundwater containing high strontium (seasonal variation1.2-10.4mg L-1) and high NOM (total organic carbon46.6-107.0mg L-1) is treated at a wide pH range (pH3-12) by three NF/RO membranes (Desal AG, TFC-ULP, TFC-SR2). Results show that the role of NOM on strontium speciation is screened by sulphate during the whole pH range. At pH above10, strontium commences to precipitate in the form of SrCO3and SrSO4which causes serious membrane scaling.This study with naturally occurring strontium indicates that the removal of strontium is best at neutral pH. The results are applicable to the treatment of waters polluted by radioactive strontium.