[apo]

The article isn't clear where the metals are coming from. It could be that the desalination process simply concentrates what's already in the seawater.

[tfha]

They add copper as part of the purification process

[apo]

What's the source? The article says nothing about it, and seawater contains copper and other metal ions all by itself:

https://web.stanford.edu/group/Urchin/mineral.html