Removal of strontium from liquid waste using a hydraulic pellet co-precipitation microfiltration (HPC-MF) process

• A hydraulic pellet microfiltration process was developed to efficiently remove Sr. • The majority of Sr was transferred to solid phase by a co-precipitation process. • Microfiltration process was essential to separation and decontamination efficiency. • An exponential equation of Sr content versus time and seed dose fit results well. • Higher seed crystal dose resulted in faster kinetics and lower Sr2 + concentration.

In the present work, the hydraulic pellet co-precipitation microfiltration (HPC-MF) process was developed for the treatment of radioactive wastewater. This hybrid process could efficiently remove strontium when the sodium carbonate dose was 1000 mg·L−1, the ferric chloride dose was 10 mg·L−1 and the seed crystal concentration was 0.340 g·L− 1. The mean and highest DF values were 842 and 1000, respectively. The hydraulic reactor can simultaneously realise chemical reactions, pelleting and precipitation role. The precipitate had excellent sedimentation performance with a large particle size, uniform distribution and good fluidity. The CF of the hybrid process was greater than 2650, which was better than that of the PCM process. According to the comparison of process performance, the hybrid process enabled hydraulic mixing to be substituted for mechanical mixing. Furthermore, the strontium removal mechanism of the HPC-MF process was fully revealed. First, the hydraulic reactor had the primary role in the removal of strontium. The membrane filtration process was essential to not only ensure stable separation efficiency but also further improve DF. Second, the strontium concentration exhibited an exponential decrease as the crystal phase of the precipitate transformed from calcite into aragonite. For a certain seed crystal concentration, there was an equilibrium effluent concentration. A higher seed crystal concentration resulted in faster precipitation kinetics, a shorter equilibrium time and a lower equilibrium concentration. Sr2+ was transferred from the liquid phase to the solid phase by being incorporated into the CaCO3 crystalline lattice and the formation of a strontianite solid solution, which was removed by subsequent solid–liquid separation. Sr2+ preferred being incorporated into the aragonite lattice over the calcite lattice. In conclusion, the HPC-MF process effectively solved the problems associated with mechanical equipment, radiation protection and precipitate concentration; furthermore, it achieved efficient and stable strontium removal. The HPC-MF process is of great potential for industrial application.