Low-cost activated carbon was produced from alligator weed by phosphoric acid activation. The cost of production of alligator weed-activated carbon (AWAC) was CN(Sic) 2.1/kg and the content of activated carbon (AC) in AWAC was 35.33%. AWAC was used as an adsorbent to adsorb cephalexin (CEX) from aqueous solution. Scanning electron microscopy (SEM) analysis of AWAC revealed a highly porous structure with a rough surface. AWAC featured a high surface area of 736.3 m(2)/g and an average pore size of 4.05 nm, which were responsible for its excellent adsorption ability. The maximum CEX adsorption capacity of AWAC was similar to 45 mg/g. The Langmuir isotherm (R-2 = 0.9967) gave the best correlation with the experimental data at 308 K, indicating monolayer adsorption. The kinetic model data on AWAC fit well to the pseudo-second-order kinetic equation, which is considered as the rate limiting factor. Fourier transform infrared spectroscopy analysis revealed the presence of phosphorus-containing groups and C-H, C-O, and C=C moieties on the surface of AWAC. Following adsorption, the intensity of the spectral peaks of AWAC decreased owing to coverage of the CEX species or reaction with the functional groups of CEX. (C) 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.