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

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.