An acid-resistant magnetic Nb-substituted crystalline silicotitanate for selective separation of strontium and/or cesium ions from aqueous solution

•Magnetic Nb-CST, which can be separated easily by an external magnetic field, is able to eliminate Sr2+ and Cs+ at ppb level.•Besides excellent selectivity towards Sr2+ and Cs+, it is able to separate them from each other by adjusting pH.•Even in real seawater, magnetic Nb-CST can remove as high as 94.19% of Cs+ at ppb level.•Its high radiation stability and acid-resistance benefit the treatment of radioactive liquid wastes.