With an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase makes it difficult to recover data from an SSD.
Even though Apple states that you really don’t need to perform a Secure Erase on an SSD Drive, Peter Cohen put together a great tutorial on how to do just that over at the BackBlaze Blog. For those who take security seriously, Peter’s “Better safe than sorry” article is worth the read.