| Radiostrontium is generally found in the waste from nuclear reactors, which has persistent radioactivity and biological toxicity. In this paper, hydraulic pellet co-precipitation microfiltration(HPC-MF) process was developed to treat the strontium-containing wastewater.Calcium carbonate seed crystal was self-prepared. Calcite was main component, while a small amount of aragonite existed. Seed crystal had large particle size, and the maximum diameter was 50.11 μm. A hydraulic rotational flow stirring reactor(it was abbreviated to hydraulic reactor) was developed to substitute hydraulic mixing for mechanic mixing during the co-precipitation process. Hydraulic reactor can simultaneously realise chemical reactions, pelleting and precipitation role. In the trial operation stage, tap water was raw water and sodium carbonate was precipitant agent. The result showed that the hydraulic reactor ran steadily when temperature was 20 °C, calcium carbonate was seed crystal, the raw water was simultaneously pumped into a tangential entry at the bottom of hydraulic reactor, and, the diameter was 1.2 mm.HPC-MF process was developed for the treatment of radioactive wastewater. Strontium ion concentration was 5 mg/L when sodium carbonate, ferric chloride and calcium carbonate were used as the precipitating agent, flocculating agent and seed crystal, respectively. Strontium was removed by forming compact crystal particles, which improved the particle sedimentation performance and overall performance of HPC-MF process. The effects of hydraulic retention time, operation temperature and seed crystal concentration on strontium removal were investigated. The best result can be obtained when the hydraulic retention time was 60 min, operation temperature was 17~29 °C and seed crystal concentration was 0.340 g/L. When 1800 L wastewater was treated, the mean and highest decontamination factors(DFs) were 842 and 1000, respectively. The concentration factor(CF) of the HPC-MF process was 2790.Strontium removal stability was studied, when 3900 L raw wastewater was treated. Strontium ion concentration in the effluent maintained 9.874±0.640 μg/L during the stable period when the operation temperature was 23 °C and seed crystal concentration was 0.170 g/L. The maximum diameter of the precipitate in the hydraulic reactor achieved 590.23 μm and DF was 540. The result showed that the hybrid process had great stability and reproducibility. The cake layer will reduce the effluent strontium concentration to a certain extent. However, when the cake layer thickness increased to a certain value, strontium ion concentration no longer varied with the volume of treated water significantly.Strontium removal mechanism at low temperature and ambient temperature was investigated, when the temperature was 12 °C and 22 °C, respectively. At low temperature, the role of hydraulic reactor was not significant, while the membrane separator led a major role in strontium removal. The effluent strontium ion concentration decreased when the small particle blocked the membrane hole. The membrane filtration fit the intermediate blockage law. At ambient temperature, the transfer of Sr2+ to the solid phase was primarily completed in the hydraulic reactor. Microfiltration process was essential to not only ensure stable separation efficiency but also further improve DF. Sr2+ was transferred from the liquid phase into the solid phase by being incorporated into the calcium carbonate crystalline lattice, which formed a strontianite solid solution that was removed by subsequent solid-liquid separation. Strontium concentration exhibited an exponential decrease. A higher seed crystal concentration resulted in faster precipitation kinetics, a shorter equilibrium time and a lower effluent strontium concentration. The Sr CO3, with large particle size, belongs to the same crystal system with aragonite, which can be substituted for Ca CO3 and used as seed crystal. |
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