Converting 'For each' to 'Parallel For each'

I have a for each loop that loops through columns of a datatable.
-Type Argument: Object
-Values: datatable.Columns

I need to change it to a parallel for each because I’m using a Form activity and wait for task activity that requires a persistence activity and it seems it doesn’t support for each.

However, I don’t know what types the argument must be in the parallel loop definition. There is error when I use the same argument types as in for each.