Dittmarite-type magnesium phosphates for highly efficient capture of Cs+

The presence of cesium ions (Cs+) in radioactive wastewater has attracted considerable attention owing to their extreme toxic effects. Thus, there is an urgent need to develop adsorbents for Cs+ with high adsorption capacities (q). While phosphate-based adsorbents have advantages for their disposal, previous adsorbents have shown limited q because of their limited capacity for ion exchange, despite showing high theoretical q values. In this study, two dittmarite-type magnesium phosphates, KMgPO4·H2O (KMP) and NH4MgPO4·H2O (NMP), were synthesized because of their ability to contain readily exchangeable cations in their interlayers. KMP and NMP demonstrated remarkable adsorption capacities for Cs+ (qeKMP = 630 mg g-1 and qeNMP = 711 mg g-1), which were the highest among all reported adsorbents and are ∼84 % of their theoretical values. Their distribution coefficients in waters with high divalent ion concentrations were low, which limits their use for the adsorption of Cs+ from such environments. After adsorption, KMP and NMP were structurally transformed into struvite-type CsMgPO4·6H2O (CsMP), which has two different stacking structures, either cubic or hexagonal, depending on the pH of the solution. The high q values of KMP and NMP enable them to reduce the volume of radioactive waste for disposal.