The adsorption of Congo red (CR), an anionic dye, from aqueous solution by a cationic surfactant-modified tea waste (TW) was studied in batch experiments. Cetyl trimetyl ammonium bromide (CTAB) and cetyl pyridinium bromide (CPB) was used for the modification of TW and ground TW. CTAB-modified TW exhibited the highest adsorption capacity with respect to the other prepared adsorbents. The adsorption of CR on CTAB-TW as a function of adsorbent dosage, pH of the solution, contact time, and initial dye concentration was investigated. The optimum amount of CTAB-TW was found to be 0.2g. The equilibrium CR adsorption data on CTAB-TW were best described by the Langmuir isotherm model. The adsorption capacity of CR on CTAB-TW was found to be 106.4mg/g which is relatively high with respect to the other adsorbents. The adsorption kinetics of CR on CTAB-TW followed a pseudo-second-order model. Moreover, the intraparticle diffusion model was used to describe the kinetic data. It was found that diffusion is not the only rate controlling step. The adsorbent was characterized by the Brunauer-Emmett-Teller (BET) analysis, Fourier-transform-infrared (FTIR) spectroscopy, and scanning-electron-microscopy (SEM). The mechanism for the adsorption of CR on the surfactant modified TW may include hydrophobic interaction, van der Waals interaction, pi-pi stacking and electrostatic interaction. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.