Secure Erase Freespace on El Capitan

After upgrading to El Capitan, Disk Utility was changed and "Erase" is greyed-out over Macintosh HD. I used to perform erase free space on the disk as that would empty out space for me. I checked to see my storage just now and I had 19 GB left on my 500 GB drive. I emptied my 100 GB trash only to find a 2GB increment on the remaining disk space. How do you perform the function again as I'm running out of space?

TLDR QUESTIONS:

1. How does one perform erase free space on El Capitan?

2. If it is completely unavailable, what is an alternative I can pursue to achieve the same outcome?

EDIT:

I've seen this link http://www.macworld.com/article/1139688/os-x/erasefreespace.html multiple times in similar discussions. I followed the instructions to first get list:

diskutil list

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *500.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage Macintosh HD 499.4 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1 (internal, virtual):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD +499.0 GB disk1

Logical Volume on disk0s2

5B5D0FC5-7709-4DC1-8D89-F36DFCC4739A

Unencrypted

/dev/disk2 (disk image):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_partition_scheme +18.1 MB disk2

1: Apple_partition_map 32.3 KB disk2s1

2: Apple_HFS Flash Player 18.1 MB disk2s2

and then perform the erase:

diskutil secureErase freespace 1 /dev/disk1

except this error comes up and stops the process:

rror: -69847: Couldn't create temporary file

Underlying error: 1: POSIX reports: Operation not permitted