How to return smart card in Terminal 2 beijing airport?

Hi, I plan to buy smart card for my 5days travel in BJ this coming November. However, I will fly of to Guangzhou afterwards using China southern airlines (checked online it is at Terminal 2 BJ airport). I plan to take subway to airport on my date of departure to T2. How can I return and get my card refund at T2? Can you advise me what is the best way? Thank you very much.