Securely overwrite the free space and inodes of the partition where the specified directory resides. More information: https://manned.org/sfill.
sfill /path/to/mounted_disk_directory
sfill -l -v /path/to/mounted_disk_directory
sfill -ll -v /path/to/mounted_disk_directory
sfill -I /path/to/mounted_disk_directory
sfill -i /path/to/mounted_disk_directory