Hexavalent chromium biosorption by raw algae is always accompanied with significantly high organic leaching. In this study, hydrochloric acid, sodium hydroxide, calcium chloride, formaldehyde, and glutaraldehyde were used for modification of raw Sargassum sp. seaweed (RSW), in order that the modified seaweed (MSW) has a lower organic leaching while the metal biosorption capacity is comparable to the RSW. The result shows that the chemical modification by 0.2% formaldehyde achieves such goals. The biosorption of both RSW and MSW is highly pH dependent. At the optimal pH of 2.0, the maximum biosorption capacities of MSW and RSW are 1.123 and 0.601 mmol g(-1), respectively. The surface treatment improves the reduction capacity of the biosorbents. The instrumental analysis demonstrates that the Cr(VI) biosorption is controlled by redox, ion exchange and coordination reactions, of which alcohol, carboxyl, amino and sulphonic groups play important roles. The complete uptake of hexavalent chromium is achieved in 20 h. The chemical reduction for Cr(VI) to Cr(III) is pH dependent and controls the overall chromium removal kinetics. (c) 2007 Elsevier Ltd. All rights reserved.