Right-click the partition and Select Wipe data, or select in the main menu Partitions > Wipe data;
Set the number of times to wipe the unallocated space in Wipe data window (The wipe algorithm is to alternately overwrite the unallocated space by 0x00 and 0xFF according to the wipe times);
By Clicking OK, you will add a pending operation of wiping unallocated space.