Selective Removal of Cs+, Sr2+, and Ni2+ by K2xMgxSn3–xS6 (x = 0.5–1) (KMS-2) Relevant to Nuclear Waste Remediation

137Cs and 90Sr, both byproducts of the uranium and plutonium fission processes, make up the majority of high-level waste from nuclear power plants. 63Ni is a byproduct of the erosion-corrosion process of the reactor components in nuclear energy plants. The concentrations of these ions in solution determine the Waste Class (A, B, or C); thus, their selective removal in the presence of large excesses of nonradioactive ions is necessary to reduce waste volume and cut disposal costs. We report the new material K2xMgxSn3–xS6 (x = 0.5–1, KMS-2) and its application for the ion exchange of Cs+, Sr2+, and Ni2+ in varying conditions. This compound crystallizes in the hexagonal space group P63/mmc with cell parameters a = b = 3.6749(8) Å and c = 16.827(4) Å. The difference in crystal structure between KMS-2, the previously reported Mn analog K2xMnxSn3–xS6 (KMS-1), and their parent SnS2 is also described. Distribution coefficients for KMS-2 are high for Cs+ (7.1 × 103 mL/g) and Sr2+ (2.1 × 104 mL/g) at neutral pH (∼ 6 ppm, V/m ∼1000 mL/g). We also report on the comparative study of Ni2+ ion exchange with both KMS-1 and KMS-2. Additional competitive reactions using Cs+, Sr2+, and Ni2+ in high concentrations of salt solution and at different pH values are reported.