Distinctive Two-Step Intercalation of Sr2+ into a Coordination Polymer with Record High 90Sr Uptake Capabilities

？ SZ-4 can remove 90Sr 2+ with record-high efficiencies in acidic solution and seawater ？ Sr 2+ can be immobilized in the structure of SZ-4 as a new 90Sr waste form model ？ The intercalation mechanism is visualized by crystal structures during uptake process ？ Sr 2+ coordination by interlayer dimethylamine contributes to the uptake selectivit