Application of nanofiltration to separate strontium from seawater

Removal of radioactive substances from the seawater in the port has now been one of the most critical issues in the Fukushima Daiichi Nuclear Power Plant. To solve the issues, nanofiltration (NF) process was evaluated to selectively separate strontium (Sr) ion from the seawater. Four (4) types of NF membranes were examined to evaluate fundamental Sr removal capability and also evaluate other alkali earth metals, magnesium (Mg) and calcium (Ca) ion removal by a flat sheet membrane test with controlled operating conditions (flux, temperature, and pH). It was found that Sr can be rejected more than 90% by a certain type of NF membrane. It was also found that Mg and Ca removal rate is similar with Sr removal and it is difficult to separate Sr from Mg and Ca in the seawater. When evaluating the effect of flux, temperature, and pH for the removal of the above divalent ions, it was found that higher flux, lower temperature, and lower pH result in higher divalent cation rejection rates, which follow the theory of the membrane chemistry. The single element test and pilot test with six elements in series were also conducted with one type of NF membrane. It was confirmed that the same Mg, Ca, and Sr removal is achieved by the single element test as flat sheet membrane test result. As per the pilot test result, it was found that 2 pass system is required to achieve more than 99% Sr decontamination rate in the actual plant. (author)