Albert Barnes Commentary Nehemiah 13:3

"And it came to pass, when they had heard the law, that they separated from Israel all the mixed multitude." — Nehemiah 13:3 (ASV)

A separation like that made by Ezra, some 20 years previously (Ezra 10:15–44), seems to be intended. The pagan wives were divorced and sent back, with their offspring, to their own countries.