Mild Ethanol–Water Hydrothermal Modification of Cu‐Based Prussian Blue Analogs for Highly Selective Cs + Capture in High‐Salinity Complex Systems

The selective capture of trace Cs + from high‐salinity water remains challenging because of intense ionic competition and limited accessibility of internal ion‐exchange sites. Here, a mild ethanol–water hydrothermal treatment was employed to regulate the pore structure and defect environment of a Cu‐based Prussian blue analog (Cu‐PBA). The sample treated at 100 °C (CHF‐100) retained the original framework while exhibiting a more open porous structure and showing a possible increase in [Fe(CN) 6 ] vacancies, with its brunauer–emmett–teller (BET) specific surface area increasing from 92.1 to 137.3 m 2 g −1 . CHF‐100 reached near‐equilibrium within approximately 10 min, compared with approximately 30 min for pristine CHF, and its pseudo‐second‐order rate constant increased from 0.0534 to 0.1143 g mg −1 min −1 . In simulated seawater, the Cs + removal efficiency increased from 3.8% for pristine CHF to 73.7% for CHF‐100. In addition, CHF‐100 maintained a Cs + removal efficiency above 90% at Na + or Mg 2+ concentrations of up to 0.4 mol L −1 and at K + concentrations of up to 0.1 mol L −1 . The enhanced performance is associated with improved pore accessibility, defect‐related local structural changes, and facilitated Cs + mass transfer and K + /Cs + ion exchange.