A facile technique is described here that can separate carbon nanotubes (CNTs) from corrosive CNTs/acid mixtures. The total time by flocculation method is ca. 0.5 h, which is only 5%, 20%, and .50% of that for dialysis, membrane filtration, and highspeed centrifugation. This strategy is a "green" process because our approach can recycle the "wasted acids." Transmission electron microscopy (TEM) and contact angle measurement also show that the transition of surface properties of nanotubes plays a significant role in the separation of CNTs.