Which of the following are poor anion choices for a separation scheme?

OH-
Cl-
S2-
(CO3)2-
(PO4)3-

Um... I'm not familiar with what you are calling 'separation scheme', but my guess is that you are looking for an anion that you can easily separate?

Are you talking about electrolysis perhaps?

Then, OH- is the most easily obtained (ie a good anion choice). You'll get it in the form of oxygen gas though.

4OH^- - 4e^- \rightarrow 2H_2O + O_2

More details would be appreciated :)