Water pollution caused by highly toxic Cd(II), Pb(II), and Cr(V1) is a serious problem. In the present work, a green and low-cost adsorbent of g-C3N4 nanosheets was developed with superior capacity for both cationic and anionic heavy metals. The adsorbent was easily fabricated through one-step calcination of guanidine hydrochloride with thickness less than 1.6 nm and spedfic surface area of 111.2 m(2).g(-1). Kinetic and isotherm studies suggest that the adsorption is an endothermic chemisorption process, occurring on the energetically heterogeneous surface based on a hybrid mechanism of multilayer and monolayer adsorotion. The tri-s-triazine units and surface N-containing groups of g-C3N4 nanosheets are proposed to be responsible for the adsorption process. Further study on pH demonstrates that electrostatic interaction plays an important role. The maximum adsorption capacity of Cd(II), Pb(II), and Cr(VI) on g-C3N4 nanosheets is 123.205 mg.g(-1), 136.571 mg.g(-1), and 684.451 mg.g(-1), respectively. The better adsorption performance of the adsorbent than that of the recently reported nanomaterials and low-cost adsorbents proves its great application potential in the removal of heavy metal contaminants from wastewater. The present paper developed a promising adsorbent which will certainly find applications in wastewater treatment and also provides guiding significance in designing adsorption processes. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.