The secure erase free space feature has been removed form Disk Utility in El Capitan (10.11) and later because it isn't useful for SSDs. It can still be accessed from the command line, like this...
diskutil secureErase freespace
Where "level" is a number from 0 to 4 indicating the type of erasing to do... 0 = Single pass zero fill erase. 1 = Single pass random fill erase. 2 = US DoD 7 pass secure erase. 3 = Gutmann algorithm 35 pass secure erase. 4 = US DoE algorithm 3 pass secure erase.
And "drivename" is the path to the drive, like /Volumes/Macintosh\ HD. Note the escaping of the space in the drive name above. You can see the mounted disks using the distil list command.
IMPORTANT: Don't forget the "freespace" option to make sure the whole disk isn't erased!
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