The adsorption of perchlorate on activated carbon was studied. A total of 10 types of commercial activated carbons were tested for perchlorate adsorption characteristics using pH as the master variable. The activated carbons were made from different base materials such as wood, bituminous coal, and lignite coal and thus expressed different surface characteristics such as specific surface area and surface charge, e-g. pH(zpc). Adsorption isotherms were obtained in the pH range from 3 to 10 and perchlorate concentration from 0.01 to 1.0 mM. It was found that the adsorption isotherms followed the Langmuir type. Results showed that under the experimental conditions of the present study, surface charge not specific surface area was the most important factor governing perchlorate removal. All wood based activated carbons that had large variation in specific surface area, e.g. 3-fold, but very close pH(zpc) (surface charge) exhibited similar perchlorate adsorption capacity. On the other hand, the activated carbons having pH(zpc) > 8 exhibited higher adsorption capacity than those with low pH(zpc) of 2-3. This provided strong evidence of electrostatic forces as responsible for perchlorate adsorption. However electrostatics alone failed to explain the better perchlorate removal compared to other anions such sulfate and phosphate. The perchlorate adsorption on activated carbon was described by surface complexation model. Results of XPS and FT-IR studies showed that perchlorate adsorption at least partially occurs through specific interactions of perchlorate with surface functional groups. Results also demonstrated that specific chemical interactions between perchlorate and surface functional groups in combination with electrostatic forces were the major mechanism for perchlorate adsorption on activated carbon. (C) 2009 Elsevier B.V. All rights reserved.