The present investigation studies the regeneration of waste engine oil by active Ca-bentonite powder. The porous adsorbents were prepared by acidification in the presence of nitric and sulfuric acid solutions at different residence times. The methylene blue adsorption test was applied to evaluate the acidification. The characteristics of normal and treated powders were also characterized by ICP, XRD, FTIR and BET techniques. The optimum condition was determined in which the methylene blue adsorption reached the maximum value. In this condition, the specific surface area of clays activated in the presence of nitric and sulfuric acids were determined to be 109.0 and 89.0 m(2) g(-1), respectively. The change in specific surface area was found to be due to formation of new nano-pores in the range of 10-12 nm, according the N-2 adsorption isotherms. The feasibility of active powders for removal of contaminants from waste oil has been studied in the next part. The modified powders had higher adsorption capacity for removal of contaminants from waste oil. It was demonstrated that the adsorption of methylene blue by active clay yields the valuable information about minimizing the powder content in recovery of waste oil by semi-treated process. (C) 2014 Elsevier B.V. All rights reserved.