Cu(II) and Cd(II) removal from aqueous solution with LDH@GO-NH₂ and LDH@GO-SH: kinetics and probable mechanism

Two novel adsorbents (LDH@GO-NH₂ and LDH@GO-SH) were successfully synthesized by grafting thiol- or amino-functionalized GO onto LDH and their adsorption capacities for heavy metal ions (Cu(II) and Cd(II)) were significantly enhanced. Characterization experiments illustrated that the thiol group (-SH) or amino group (-NH₂) was grafted onto LDH@GO-NH₂ or LDH@GO-SH. Adsorption isotherms were satisfactorily fitted by both Langmuir and Freundlich models. The maximum adsorption capacity of Cd(II) on LDH@GO-SH at 308 K was 102.77 mg/g, which was about triple that of LDH@GO-NH₂. The enhancement in adsorption capacity of LDH@GO-SH was due to the cooperative effect of LDH and GO-SH. The kinetic experimental data for LDH@GO-NH₂ and LDH@GO-SH were found to be in good agreement with the pseudo-second-order model. The thermodynamic parameters calculated from the temperature-dependent adsorption isotherms indicated that the adsorption was spontaneous and an endothermic process. The possible adsorption mechanisms comprising formation of precipitation, isomorphic substitution of Mg(II), and formation of complexation with amino groups or thiol groups were proposed. Desorption experiments put into evidence that LDH@GO-NH₂ and LDH@GO-SH may be promising suitable candidates for the remediation of metal ions from aqueous solutions in real work in the near future.