Energy window-guided molecular adsorbent for highly selective cesium capture: Breaking the Rb+/Cs+ separation bottleneck

APPW adsorbent designed via 4-aminopyridine (4-AP) tuning of the adsorption energy window. • Achieves ultra-high Cs + selectivity with an Rb + /Cs + separation factor of 11,122. • Enables one-step Cs + separation without Rb + interference. • Shows strong resistance to coexisting ions in salt lake brines and ore leachates. • Establishes a new strategy of adsorption energy window modulation for ion separation. Efficient separation of Cs + from Rb + remains a critical bottleneck due to their similar ionic properties. This work fabricates a 4-aminopyridinium phosphotungstate (APPW) adsorbent by substituting conventional NH₄ + in phosphotungstic acid (HPW) with tailored4-Amino pyridinium ion (4-AP + ) cations to construct a tunable adsorption energy window. DFT calculations confirm the binding energy order ΔE (Rb + -PW)  > ΔE (4-AP + -PW)  > ΔE (Cs + -PW) , which thermodynamically enables exclusive exchange of 4-AP + by Cs + and suppresses Rb + adsorption. APPW delivers an ultrahigh Rb + /Cs + distribution coefficient of 11,122, outperforming previously reported adsorbents. At Rb + /Cs + molar ratio of 10:1 mimicking natural brine, APPW still achieves selective Cs + capture with negligible Rb + uptake and stable performance across pH 2–11. Coexisting Li + , Na + , K + , Ca 2+ , Mg 2+ barely interfere with Cs + selectivity. Adsorption follows Langmuir monolayer chemisorption and pseudo-second-order kinetics, and the endothermic process proceeds spontaneously. After five regeneration cycles with 4-aminopyridine eluent, APPW retains 87.9% initial Cs + capacity. Practical treatment of oilfield brine achieves 81.91% Cs + recovery under high-salinity complex matrices. This study overcomes the poor Cs + selectivity limitation of traditional heteropolyacid materials and establishes a universal molecular design strategy via adsorption energy window modulation, offering a recyclable high-performance adsorbent for one-step Cs + separation from salt lake brines, ore leachates and radioactive wastewater.