A novel engineered biochar with clay particles distributed on carbon surfaces within the biochar matrix has been successfully developed as a low-cost adsorbent for environmental applications. Three biomass feedstocks were pretreated with montmorillonite or kaolinite suspensions and then pyrolyzed at 600 degrees C for 1 h in a N-2 environment. Physicochemical characterizations including microscopy and X-ray analyses confirmed clay particles were present on the surface of biochar during the annealing process. Laboratory adsorption experiments indicated that the presence of clay particles on carbon surfaces had dramatically increased (about 5 times) the biochars' adsorption ability to methylene blue (MB). In addition, the clay-biochar composites could be recycled and reused after dye adsorption with a stable capacity of around 7.90 mg g(-1). The experimental results also showed that the sorption of MB on the clay-biochar composites is mainly controlled by the ion exchange (with clay) and electrostatic attraction (with biochar) mechanisms. Findings from this work indicate that engineered biochar, prepared from two low-cost materials (clay and biochar), is a valuable adsorbent for removing contaminants from aqueous solutions. (C) 2013 Elsevier B.V. All rights reserved.