In light of the widespread Cs pollution that may follow nuclear disasters, the decontamination of post-accident soil has earned much attention due to the difficulty of Cs removal for its super-retention in micaceous minerals. Herein, we successfully used solvated Mg2+ to desorb Cs from Cs-saturated vermiculitized biotite (VB) to clarify the microscale mechanism underlying a Cs removal process, and we applied this new method to actual radioactive soil to validate the practical remediation effect. The results revealed that sorbed Cs was uniformly fixed in the collapsed interlayers of VB crystals and thus was poorly and slowly desorbed by ambient treatment with Mg2+ regardless of the treating concentration, duration time, and number of cycles. However, almost all Cs was effectively and efficiently removed by hydrothermal treatment at 250 degrees C. Further characterizations of treated Cs-VB confirmed that Mg2+ indeed diffused into Cs collapsed interlayers from the edge-side to the interior central region with the increase of the treating temperature, and the substituted anhydrous Cs+ ensuing interlayer decollapsed. Similarly, a negligible amount of Cs was removed from the radioactive soil by treatment with Mg2+ at 60 degrees C, whereas nearly complete Cs removal was achieved at 250 degrees C, which clearly validated the desorption process as achieved for Cs-VB and definitely reinforced the applicability of the current reported pathway for practical Cs decontamination with soil remediation.