WipeFreeSpace
Secure wiping/shredding of free disk space with many methods
...The following wiping methods are supported: Gutmann-like, random, 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.