We describe a new methodology to the selective trapping of priority pollutants that occur inherently as oxo-anions (e.g., perchlorate, chromate, arsenate, pertechnetate, etc.) or organic anions (e.g., salicylate, pharmaceuticals, and their metabolites, which are often chlorinated into potentially more harmful compounds). The typical approach to trapping anions is exchange into cationic hosts such as resins or layered double hydroxides. Both capacity and selectivity are limited by the equilibrium of the process and moreover are often subject to interference, e.g. by carbonate that is always present in water from atmospheric CO(2). Our approach takes advantage of the metastability of our cationically charged materials to instead trap by recrystallization to a new structure. Exceptionally high adsorption capacities for permanganate and perrhenate-studied as models for pertechnetate-were found for a Ag(I)-based cationic extended framework. The exchange capacity reached 292 and 602 mg/g, respectively, over five times the exchange capacity compared to conventional layered double hydroxides. Our cationic material can also selectively trap these and other toxic oxo-anions when nontoxic anions (e.g., nitrate, carbonate) were present in an over 100-fold excess concentration.