How can I secure-wipe free space? Any way that I can securely wipe free internal storage on Android? I am not talking about secure deleting, but just wiping whatever is possible to recover.
The most reliable method is to manually copy large non-sensitive files (such as Linux images or the LibreOffice Installer Setup File) to the partition that you wish to erase the free space from, delete and repeat it around THREE to FIVE times. Ensure the files you copy fill up the drive as much as possible, i.e. start copying small files when the large ones are too big to fit in the last remaining MB / KB of free space.

If you don't want to mess around finding files to write, you can use a utility called Dummy File Creator: http://www.mynikko.com/dummy/

This will allow you to create files of a specific size containing random data (ensure the Random File Content option is selected).