Biochar, a by-product from the production of biofuel and syngas by gasification, was tested as a material for adsorption and fixation of U-VI from aqueous solutions. A batch experiment was conducted to study the factors that influence the adsorption and time-dependent fixation on biochar at 20 degrees C, including pH, initial concentration of U-VI and contact time. Uranium (U-VI) adsorption was highly dependent on pH but adsorption on biochar was high over a wide range of pH values, from 4.5 to 9.0, and adsorption strength was time -dependent over several days. The experimental data for pH > 7 were most effectively modelled using a Freundlich adsorption isotherm coupled to a reversible first order kinetic equation to describe the time-dependent fixation of U-VI within the biochar structure. Desorption experiments showed that U-VI was only sparingly desorbable from the biochar with time and isotopic dilution with U-233(VI) confirmed the low, or time -dependent, lability of adsorbed U-238(VI.) Below pH 7 the adsorption isotherm trend suggested precipitation, rather than true adsorption, may occur. However, across all pH values (4.5-9) measured saturation indices suggested precipitation was possible: autunite below pH 6.5 and either swartzite, liebigite or bayleyite above pH 6.5. (C) 2016 Elsevier B.V. All rights reserved.