secure erasing empty space without destroying the disk I know I can use Disk Utility to secure erase a disk. 

My question is: I have a disk half empty. I would like to know if there's a way to secure erase the empty part of the disk. I mean, an app/command that can identify those empty sectors of the disk and give them a secure erase treatment. 

thanks.
If you are referring to a partition, then you can also do it with Disk Utility:
First, select the partition on the list on the left, go to the "Erase" tab on the right and use "Erase Free Space ...". On the dialog that shows now you can choose the level of security (i.e., how many times the data should be overwritten) and then click "Erase Free Space".

If you are talking about space on a disk with no active partition then I think you need to create a partition first that you can then erase.