<p>Metal sulfide ion exchangers (MSIEs) are promising for radioactive wastewater treatment due to their soft S²⁻-containing frameworks and ion-exchange capabilities. However, their fine powder form and susceptibility to competing ions limit practical application in complex water matrices. Herein, we developed a novel MSIE-based composite, K₂ZnSn₂S₆ nanosheets/cationic polyacrylamide (KZTS-NS/CPAM) flocs, integrated with an improved accumulative countercurrent adsorption (iACA) process for effective Cs⁺, Sr²⁺, and Co²⁺ removal. The composite flocs exhibited good settling performance, achieving complete sedimentation within 40 s. Their maximum adsorption capacities (<i>q</i>_m) reached 106.7, 43.54, and 40.54 mg/g for Cs⁺, Sr²⁺, and Co²⁺, respectively, close to or even higher than those of KZTS-NS. The adsorption kinetics were rapid, reaching equilibrium within 40 min. Furthermore, the flocs showed broad pH tolerance (removal efficiencies ⩾86.93% for Cs⁺ at pH 4–11, ⩾95.10% for Sr²⁺ at pH 3–12, and ⩾96.17% for Co²⁺ at pH 1–8) and ultrahigh selectivity for Co²⁺ (distribution coefficients up to 6.63×10⁶ mL/g under competitive conditions). When integrated with the iACA process, &gt;93% of Cs⁺ and &gt;99% of Sr²⁺ and Co²⁺ were removed from tap water. Notably, 93.76% of Co²⁺ was removed from seawater without microfiltration, benefiting from both the composite’s strong selectivity and excellent settling behavior. The average turbidity of the effluents was as low as 28.6 NTU. Thus, no additional solid-liquid separation step was needed. Mechanistic studies confirmed three key pathways: (1) ion exchange between Cs⁺/Sr²⁺/Co²⁺ and interlayer K⁺ in KZTS-NS; (2) amide complexation by CPAM for Sr²⁺ and Co²⁺; and (3) surface precipitation for Co²⁺ forming Co_1.2Zn_2.8S₄. Overall, this material-process integration provides an innovative strategy for radionuclide remediation in actual and high-salinity water environments, offering new insights into the design of processable composite adsorbents and advanced treatment processes.</p>