An adsorption equilibrium model was applied to simulate the sorption of an antibiotic, levofloxacin (LEV), one of the emerging contaminants, from aqueous solution by the biosorbent based on pretreated barley straw. The effects of solution pH, contact time, LEV concentration and ionic strength on LEV removal were investigated, and desorption of LEV loaded on pretreated barley straw was also examined. In addition, site energy distribution of the pretreated biosorbent for LEV molecules adsorption was estimated. The average site energy and standard deviation of the site energy distribution under various pH values were determined and applied to analyze the interaction between the biosorbent and adsorbate, and sorption site heterogeneity. With higher average site energy (28.1 kJ/mop, the pretreated barley straw at neutral pH had higher sorption affinity, thus be more favorable for the sorption reaction than the lower affinity surface at acidic or basic pH. The experimentally achieved LEV uptake of the pretreated barley straw at pH 6.88 is much higher than that of raw barley straw, and other sorbents reported in literatures. The methods of biomass pretreatment and site energy distribution analysis could be transferable to extended organics or inorganics adsorption by biosorbents. (C) 2016 Elsevier B.V. All rights reserved.