Carboxylated multi-wall carbon nanotubes (MWCNTs-COOH) was functionalized with 3-amino-5-phenylpyrazole (MWCNTs- f) and characterized by FTIR, EDX, SEM, XRD and TGA. The MWCNTs-COOH and MWCNTs-f were used for the adsorption of Cd(II), Hg(II), and As(III) ions from aqueous solutions. Additionally, to study the influence of pH, adsorbent dose, and initial ions concentration on the adsorption process, the central composite design (CCD) was applied. The quadratic model was used for analysis of variance and indicated that adsorption of metal ions strongly depends on pH. Timedependent adsorption can be described by the pseudo-second-order kinetic model, and adsorption process was modeled by Langmuir isotherm for the adsorbents. Thermodynamic analysis showed that the adsorption of Cd(II), Hg(II) and As(III) ions were spontaneous and endothermic. Moreover, the competitive adsorption capacities of the heavy metal ions were slightly lower than noncompetitive ones. The same affinity order was observed under noncompetitive and competitive adsorption: As(III) > Cd(II) > Hg(II) in the case of MWCNTs-f. Desorption study revealed the favorable regeneration ability of adsorbents powders, even after three adsorption–desorption cycles.

Abstract
    1 Introduction
    2 Materials and methods
        2.1 Materials and characterization
        2.2 Preparation of MWCNTs-f
        2.3 Preparation of metal Stock Solution
        2.4 Adsorption procedure
        2.5 Experimental design
        2.6 Desorption studies
    3 Results and discussion
        3.1 Characterization of adsorbents
        3.2 Adsorption process modeling by CCD
        3.3 Response surface plotting
            3.3.1 Effect of pH and initial ions concentration on response parameter
            3.3.2 Effect of pH and adsorbent dose on response parameter
        3.4 Adsorption isotherms
        3.5 Adsorption kinetics
        3.6 Thermodynamic studies
        3.7 Competitive adsorption
        3.8 Desorption study
    4 Conclusions
    Acknowledgements 
    References

• Mobina Alimohammady(Faculty of Chemical, Petroleum and Gas Engineering, Semnan University)
• Mansour Jahangiri(Faculty of Chemical, Petroleum and Gas Engineering, Semnan University)
• Farhoush Kiani(Department of Chemistry, Ayatollah Amoli Branch, Islamic Azad University)
• Hasan Tahermansouri(Department of Chemistry, Ayatollah Amoli Branch, Islamic Azad University)