{"references": ["N. Unlu, M. Ersoz, Adsorption characteristics of heavy metal ions onto a\nlow cost biopolymeric sobent from aqueous solutions. J. Hazard. Mater.\nB 136, 2006, pp. 272\u2013280", "V.C. Srivastava, I.D. Mall, I.M. Misha, Characterisation of mesoporous\nrice husk ash (RHA) and adsorption kinetics of metal ions from aqueous\nsolution onto RHA. J. Hazard. Mater. B 134, 2006, pp. 257\u2013267.", "F. Boudrahem, F. Aissani-Benissad, A. Soualah, Adsorption of lead(II)\nfrom aqueous solution by using leaves of date trees as an adsorbent. J.\nChem. Eng. Data 56, 2011, pp. 1804\u20131812.", "F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters. A\nreview. J. Environ. Manage, 92, 2011, pp.407\u2013418.", "Y.S. Al-Degs, M.I. El-Barghouthi, A.A. Issa, M.A. Khraisheh, G.M\n.Walker, Sorption of Zn(II), Pb(II), and Co(II) using natural sorbents:\nEquilibrium and kinetic studies. Water Res. 40, 2006, pp. 2645\u20132658.", "L.Lv, M.P. Hor, F.Sa, X.S.Zhoo, Competitive adsorption of Pb2+, Cu2+\nand Cd2+ ions on microporous titanosilicate ETS-10.J. Colloids. Surf. Sci\n287, 2005, 178-184.", "A. Sdiri, T. Higashi, T. Hatta, F. Jamousssi, N. Tase, Evaluating the\nadsorptive capacity of montmorillonitic and calcareous clays on the\nremoval of several heavy metals in aqueous systems. Chem Eng J\n172(1), 2011, pp. 37\u201346", "K.G. Bhattacharyya, S.S. Gupta, Pb(II) uptake by kaolinite and\nmontmorillonite in aqueous medium: influence of acid activation of the\nclay. Colloids Surf. A. 277, 2006, pp.191\u2013200.", "I. Chaari, E. Fakhfakh, S. Chakroun, J. Bouzid, N. Boujelben, M. Feki,\nF. Rocha, F. Jamoussi, Lead removal from aqueous solutions by a\nTunisian smectitic clay. J. Hazard. Mater. 156, 2008, pp. 545\u2013551.\n[10] L. Khalfa, M.L. Cervera, M. Bagane, N.S. Soaad, Modeling of\nequilibrium isotherms and kinetic studies of Cr (VI) adsorption into\nnatural and acid-activated clays. Arab. J. Geosci. 9, 2016, 75.\n[11] M. Ben M'barek Jema\u00ef, , A. Sdiri, E. Errais, J. Duplay, I. Ben Saleh,\nM.F. Zagrarni, S. Bouaziz, Characterization of the Ain Khemouda\nhalloysite (western Tunisia) for ceramic industry. J. Afr. Earth Sci. 111,\n2015, pp.194\u2013201.\n[12] M. Eloussaief, M. Benzina, Efficiency of natural and acid-activated\nclays in the removal of Pb(II) from aqueous solutions. J. Hazard. Mater.\n178, 2010, pp. 753\u2013757.\n[13] A.A .Rouff, E.J. Elzinga, R.J. Reeder, The effect of aging and pH on\nPb(II) sorption processes at the calcite\u2013water interface. Environ. Sci.\nTechnol. 40, 2006, pp.1792\u20131798.\n[14] V.C. Srivastava, I.D. Mall, I.M. Misha, Removal of cadmium (II) and\nzinc(II) metal ions from binary aqueous solution by rice husk ash .\nColloid. Surf. Physicochem. Eng. Aspects. 312, 2008, pp. 172\u2013184.\n[15] F. Nekouei, S. Nekouei, I. Tyagi, V.K. Gupta , Kinetic, thermodynamic\nand isotherm studies for acid blue 129 removal from liquids using\ncopper oxide nanoparticle-modified activated carbon as a novel\nadsorbent, J. Molec. Liq. 201, 2015, pp.124\u2013133.\n[16] K.D. Belaid, S. Kacha, M. Kameche , Z. Derriche , Adsorption kinetics\nof some textile dyes onto granular activated carbon, J. Environ. Chem.\nEng. 1, 2013, pp. 496\u2013503.\n[17] Q. Peng, M. Liu, J. Zheng, C. Zhou, Adsorption of dyes in aqueous\nsolutions by chitosan\u2013halloysite nanotubes composite hydrogel beads,\nMicr. and Mes. Mater. 201, 2015, pp.190\u2013201.\n[18] A. Sari, M. Tuzen, D. Citak, M. Soylak, Equilibrium, kinetic and\nthermodynamic studies of adsorption of Pb(II) from aqueous solution\nonto Turkish kaolinite clay. J. Hazard. Mater. 149, 2007, pp. 283\u2013291.\n[19] A. Sdiri, T. Higashi, F. Jamoussi, Adsorption of copper and zinc onto\nnatural clay in single and binary systems. Int. J. Environ. Sci. Technol.\n11, 2014, pp.1081\u20131092.\n[20] M. Mazzotti, Equilibrium theory based design of simulated moving bed\nprocesses for a generalized Langmuir isotherm, J. Chromatogr. A 1126,\n2006, pp.311\u2013322.\n[21] M. Raoov, S. Mohamad, M.R. Abas, Removal of 2,4-dichlorophenol\nusing cyclodextrin-ionic liquid polymer as a macroporous material:\nCharacterization, adsorption isotherm, kinetic study, thermodynamics, J.\nHazard. Mater. 263, 2013, pp. 501\u2013 516.\n[22] V.C. Srivastava, I.D. Mall, I.M. Mishra, Equilibrium modeling of single\nand binary adsorption of cadmium and nickel onto bagasse fly ash,\nChem. Eng. J. 117, 2006, pp.79\u201391.\n[23] V.C. Srivastava, I.D. Mall, I.M. Mishra, Modeling individual and\ncompetitive adsorption of cadmium (II) and zinc (II) metal ions from\naqueous solution onto bagasse fly ash, Sep. Sci. Technol. 41, 2006, pp.\n2685\u20132710.\n[24] D. Mohan, K.P. Singh, Single and multi-component adsorption of\ncadmium and zinc using activated carbon derived from bagasse- an\nagricultural waste, Water Res. 36, 2002, pp. 2304\u20132318."]}

The aim of this work is to present a low cost adsorbent for removing toxic heavy metals from aqueous solutions. Therefore, we are interested to investigate the efficiency of natural clay minerals collected from south Tunisia and their modified form using sulfuric acid in the removal of toxic metal ions: Zn(II) and Pb(II) from synthetic waste water solutions. The obtained results indicate that metal uptake is pH-dependent and maximum removal was detected to occur at pH 6. Adsorption equilibrium is very rapid and it was achieved after 90 min for both metal ions studied. The kinetics results show that the pseudo-second-order model describes the adsorption and the intraparticle diffusion models are the limiting step. The treatment of natural clay with sulfuric acid creates more active sites and increases the surface area, so it showed an increase of the adsorbed quantities of lead and zinc in single and binary systems. The competitive adsorption study showed that the uptake of lead was inhibited in the presence of 10 mg/L of zinc. An antagonistic binary adsorption mechanism was observed. These results revealed that clay is an effective natural material for removing lead and zinc in single and binary systems from aqueous solution.