Bio-chars with high adsorption capacity derived from rice-husks and corncobs were prepared at different retention times (RTs) in a pyrolysis reactor. At a fixed pyrolysis temperature, the pyrolysis RT is a key factor influencing the surface areas and functional group contents of the bin-chars, and further influencing their adsorption capacities. The results indicate that the bio-char prepared at RT of 1.6 s exhibits a higher phenol adsorption capacity (589 mg g(-1)) than other bio-chars and many activated carbons reported in the literature. An adsorption mechanism based on acid-base interaction and hydrogen binding between phenol and the functional groups was proposed to elucidate the adsorption process. An economic evaluation of the use of bio-chars as adsorbents was made. (C) 2011 Elsevier Ltd. All rights reserved.