The aim of this work is to investigate the ability of a new functionalized graphene oxide 3-amino-5-phenylpyrazole (F-GO) in the adsorption and removal of Hg2+ from aqueous solution. Both untreated graphene oxide (GO) and F-GO were characterized using FT-IR, EDX, FE-SEM, XRD and TGA analysis. The effects of three operational variables (pH, adsorbent dose and initial metal ion concentrations) on Hg2+ adsorption capacity of F-GO were investigated by central composite design. This technique aims to find a simple way to optimize the adsorption process and to analyze the interaction between the significant parameters. A quadratic model suggested for the analysis of variance found that the adsorption of metal ions heavily depend upon pH of the solution. The adsorption mechanism has been determined by pseudo-first-order kinetic models and the adsorption behavior was modeled by Freundlich isotherm. Results demonstrated that the adsorption capacities of F-GO for removal of Hg2+ were generally higher than those of GO, which is attributed to a decrease in the agglomeration of graphene layers due to the presence of amino-functional moieties with their bulky phenyl groups. Thermodynamic data indicated that the functionalization significantly affects the thermostability of the GO precursor materials. The desorption study demonstrated favorable regenerability of the F-GO adsorbent, even after three adsorption–desorption cycles.

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.