Discarded watermelon rinds were used to prepare porous biochars, which act as regenerable adsorbents for treating both synthetic and real Tl-containing wastewater. The primary biochar prepared under pyrolysis temperature of 500 degrees C was found to be the most effective for Tl(I) removal. The primary biochar had the best Tl(I) removal efficacy over a wide pH range (4-12), followed by the KOH-modified and HCl-modified biochar. Maximum Tl(I) adsorption capacity reached 178.4 mg/g, which is superior to that of other biochar. Strong resistance to the interference from co-existing cations and organics on Tl(I) removal was also observed during treatment of complex synthetic and real industrial wastewater. Characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, high resolution transmission electron microscope (HRTEM), and X-ray photoelectron spectroscopy (XPS) reveal that the K- and Cl-rich primary biochar acts as regenerable amphibious ion exchange resins to perform reversible adsorption and desorption of Tl(I). The biochar derived from watermelon rinds exhibits effective Tl(I) removal performance and strong regenerability, and is a promising adsorbent for Tl(I) removal with excellent application prospects. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.