Elevated U(VI) concentrations are widely documented in alkaline saline lakes in arid regions. Monohydrocalcite (MHC), a metastable hydrated carbonate phase, commonly forms as an authigenic mineral in these environments. Although recent studies suggest that MHC can host U(VI), the mechanism by which it controls U(VI) solid-liquid partitioning remains unclear. Here, we systematically investigate U(VI) sorption on MHC using macroscopic sorption experiments, EXAFS spectroscopy, and compare its behavior with that of calcite and aragonite. Sorption isotherms demonstrate that MHC exhibits the highest sorption affinity and capacity among the three carbonates on a mass basis. EXAFS analysis reveals that U(VI) adsorbed onto aragonite, and previously reported U(VI) adsorption complexes on calcite, retain the uranyl triscarbonato coordination structure consistent with the aqueous species under alkaline conditions. In contrast, U(VI) associated with MHC shows significant structural distortion, indicating that the uptake involves more than simple surface adsorption. Given the metastable and relatively soluble nature of MHC, U(VI) uptake is best explained by incorporation during dynamic dissolution-reprecipitation processes. pH-edge experiments further show that U(VI) sorption decreases markedly with increasing pH and alkalinity, likely due to reduced availability of dissolved Ca required for reprecipitationdriven incorporation. Kinetic sorption experiments conducted with and without Mg2* demonstrated that U(VI) is retained while MHC persists but is released during transformation to calcite in Mg2*-free systems. In contrast, U (VI) removal increases during transformation to aragonite in Mg2*-bearing systems. These contrasting behaviors reflect structural compatibility: calcite is incompatible with the uranyl triscarbonato complex, whereas aragonite provides a coordination environment that preserves its geometry, as confirmed by EXAFS. Sequential extraction analyses of suspended matter from Boon Tsagaan Lake (Mongolia) identify MHC as the principal host phase for U(VI). However, the low apparent solid-water distribution coefficient between lake water and MHC indicates limited net removal under high pH and alkalinity conditions, which is quantitatively consistent with the laboratory pH-edge measurements. Furthermore, transformation of MHC to calcite during sedimentation observed in Boon Tsagaan Lake may not contribute significantly to U(VI) removal, as suggested by laboratory kinetics experiments. Collectively, these results demonstrate that the mineralogical fate of metastable carbonates exerts primary control on U(VI) partitioning and explains the conservative behavior of U(VI) in alkaline saline lakes.