WipeFreeSpace
Secure wiping/shredding of free disk space with many methods
..., Schneier, DoD.
Additionally, WipeFreeSpace can finalize wiping the filesystem by writing blocks of zeros, which is friendly for sparse files, virtual systems and other places where zeroed-out space is not physically allocated.
See the project homepage https://wipefreespace.sourceforge.io and the project Wiki in the menu above.
To prevent clear-text data from being left on the drive in the first place, you can use LibSecRm (https://libsecrm.sourceforge.io), which wipes the data on-the-fly.