Selective removal of Sr(II) from saliferous radioactive wastewater by capacitive deionization

⁹⁰Sr-containing radioactive wastewater during Fukushima nuclear accident (FNA) aroused extensive consideration for its disposal. Massive coexisted Na⁺ ions seriously inhibited Sr²⁺ removal, aggravating the expenditure of radioactive wastewater treatment. Herein, a chestnut shell derived porous carbon material modified with aryl diazonium salt (ADS) of sodium 4-aminoazobenzene-4'-sulfonate (SPAC) was developed as capacitive deionization electrode for selective removal of Sr²⁺ from saliferous radioactive wastewater. Based on ADS modification, the Sr²⁺ electrosorption capacity of SPAC electrode was improved to 33.11 mg g^-1 with fast ion removal rate of 2.89 mg g^-1 min^-1, comparing with only 16.10 mg g^-1 before modification. The isothermal adsorption and kinetics by SPAC electrode fitted well with Langmuir and pseudo-second-order model, achieving a maximum Sr²⁺ electrosorption capacity of 58.21 mg g^-1, superior cycling stability, and excellent charge efficiency (77.63%). Fascinatingly, the SPAC electrode exhibited superhigh Sr²⁺ selectivity of 70.65 against Na⁺ in Na⁺-Sr²⁺ mixed solution with molar ratio of Na⁺:Sr²⁺ as 20:1. Density functional theory (DFT) simulation, combining with electrochemical and spectral analyses, revealed that the high overlap of electron cloud between Sr²⁺ ion and anionic sulfonic group (-SO₃^-) provided SPAC with remarkable selectivity of Sr²⁺ ion, and illustrated the ion-swapping mechanism of Sr²⁺ selectivity.