Layered sulfides with high selectivity for binding heavy metal ions and radionuclide ions are promising materials in effluent treatment and water purification. Here we present a rationally designed layered sulfide K-x[Bi4-xMnxS6] (x = 1.28) deriving from the Bi2Se3-structure type by targeted substitution to generate quintuple [Bi4-xMnxS6](x-) layers and K+ cations between them. The material has dual functionality: it is an attractive semiconductor with a bandgap of 1.40 eV and also an environmental remediation ion-exchange material. The compound is paramagnetic, and optical adsorption spectroscopy and DFT electronic structure calculations reveal that it possesses a direct band gap and a work function of 5.26 eV. The K+ ions exchange readily with alkali or alkaline-earth ions (Rb+, Cs+, and Sr2+) or soft ions (Pb2+, Cd2+, Cr3+, and Zn2+). Furthermore, when the ions are depleted the Mn2+ ions in the Bi2Se3-type slabs can also be replaced by soft ions, achieving large adsorption capacities. The ion exchange reactions of K-x[Bi4-xMnxS6] can be used to create new materials of the type M-x[Bi4-xMnxS6] in a low temperature kinetically controlled manner with significantly different electronic structures. The K-x[Bi4-xMnxS6] (x = 1.28) exhibits efficient capture of Cd2+ and Pb2+ ions with high distribution coefficient, K-d (10(7) mL/g), and exchange capacities of 221.2 and 342.4 mg/g, respectively. The material exhibits excellent capacities even in high concentration of competitive ions and over a broad pH range (2.5-11.0). The results highlight the promise of the K-x[Bi4-xMnxS6] (x = 1.28) phase to serve not only as a highly selective adsorbent for industrial and nuclear wastewater but also as a magnetic 2D semiconductor for optoelectronic applications.