A novel protonated graphite carbon nitride and acid-activated montmorillonite (g-C3N4/Mt) composite was prepared and evaluated its removal efficiency for PO43- and Pb(II) from aqueous media. The as-prepared composite was characterized by XRD, FT-IR, TG, XPS, SEM, BET and Zeta potential. Batch experiments were carried out under various conditions such as the amount of adsorbent, initial adsorbate concentration (C-o), solution pH, temperature, and contact time. 2.0%-g-C3N4/Mt presented maximum adsorption capacity for PO43- and Pb(II) as compared to single g-C3N4, Mt or other synthesized composites. The removal efficiency of the as-prepared composite (2.0%-g-C3N4/Mt) was significantly elevated owing to the synergistic effects and the saturated adsorption capacities of PO43- and Pb(II) on 2%-g-C3N4/Mt was 5.06 mg/g and 182.7 mg/g respectively. While the saturated adsorption capacity of bare g-C3N4 was only 1.56 mg/g and 22.3 mg/g for PO43- and Pb(II) respectively, and bare Mt showed adsorption capacity of 0.65 mg/g and 124.2 mg/g for PO43- and Pb(II) respectively. The adsorption process for both contaminants showed well-fitting with the pseudo-second-order kinetic model, and the adsorption isotherm for both contaminants followed the Langmuir model. The thermodynamic study suggested that the adsorption of Pb(II) was a spontaneous and endothermic process. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.