Least Privilege for SQL Agent to run DISKPART
You are getting deep into internals here 


  Membership in the local Administrators group, or equivalent, is the
  minimum required to run DiskPart


And this SE thread points to the same. 

Now usually you want to avoid giving to much permission to the service accounts and I'm not certain if giving the SQL Server agent permissions to mount/unmount volumes and at the same time delete them would be considered best practices. 

You can probably program a system service that can do the diskpart magic and call that but this really ask the question why you would want to to this from SQL Server.