Synthesis of a thiacalix[4]arenetetrasulfonate-functionalized reduced graphene oxide adsorbent for the removal of lead(ii) and cadmium(ii) from aqueous solutions

Abstract

A thiacalix[4]arenetetrasulfonate-functionalized reduced graphene oxide (TCAS–rGO) adsorbent was synthesized and used as an adsorbent for the removal of lead(II) and cadmium(II) from aqueous solutions. TCAS–rGO was characterized by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and thermal gravimetric analysis (TGA). The adsorption of Pb(II) and Cd(II) onto TCAS–rGO as a function of pH, contact time, metal ion concentration, and temperature was investigated. Results showed that the adsorption of Pb(II) and Cd(II) onto TCAS–rGO was greatly influenced by the pH. TCAS–rGO shows excellent selective adsorption for Pb(II) and Cd(II) compared with other metals Na(I), K(I), Mg(II) and Sr(II) at neutral pH. The adsorption reached equilibrium within 4 h and the kinetics could be described well by a pseudo-second-order model. The adsorption isotherms for Pb(II) and Cd(II) were fitted by the Langmuir model, and the maximum adsorption capacities of the adsorbent were 230 mg g⁻¹ for Pb(II) and 128 mg g⁻¹ for Cd(II). Thermodynamic studies indicate that the adsorption was a spontaneous and endothermic process in nature. The chloride and nitrate anions may barely influence the adsorption. The TCAS–rGO can be reused as an adsorbent following 4 cycles of adsorption/desorption. Possible adsorption mechanisms are also analysed. This paper provides a potential application of the TCAS–rGO composite in heavy metal treatment.