Relations between metal ion characteristics and adsorption performance of graphene oxide: A comprehensive experimental and theoretical study

Abstract

The adsorption performance of graphene oxide (GO) towards metallic hard (Na+, Mg2+), soft (Cd2+ and Pb2+) and borderline ions (Ni2+, Cu2+, Zn2+, Co2+) was studied experimentally giving the sequence of maximum Langmuir adsorption capacities Qm, mmol/g, as Na+ < Mg2+ < Co2+ < Cd2+ < Zn2+ < Ni2+ < Cu2+ < Pb2+. The results showed that the adsorption capacities Qm of hard ions were substantially lower than that of soft and borderline ones. The relationship between the Qm values of metal ions and their characteristics was studied using relative correlation analysis. The ion characteristics include atomic number AN, ionic radius r, change in ionization potential ΔIP, electronegativity Xm, atomic weight AW, covalent index Xm2r, absolute difference between electrochemical potential of the ion and that of its first stable reduced state ΔE0, absolute value of the first hydrolysis constant decimal logarithm |lgKOH|, atomic radius AR, atomic radius to atomic weight ratio AR/AW, polarization force or ionic index Z2/r and softness index σp. The Qm value exhibited positive correlation with r, △IP, Xm, Z2/r, AN/△IP, AW, Xm2/r and AN, while negative correlation was observed with △E0, |lgKOH|, σp, AR and AR/AW. The results allow the Qm values being predicted by the equation, the correlation coefficient R2 of which exceeds 0.885:Qm=0.338ΔIP-0.110|lgKOH|+0.114Z2/r+26.934σp-1.479AR-1.162Xm2r+0.115AN-3.059Density functional theory (DFT) was adopted to study the adsorption mechanism of metal ions at GO, taking Na+, Cu2+ and Pb2+ ions as examples. The results showed the adsorption energy Ead of GO-Na+ bond being negative indicating Na+ adsorption less likely resulting possibly in relatively low adsorption capacity of GO towards hard ions. The Ead values of GO bonds with Cu2+ and Pb2+ comprised the rows GO(O)-Cu2+ > GO(COOH)-Cu2+ > GO(OH)-Cu2+, and GO(COOH)-Pb2+ > GO(OH)-Pb2+ > GO(O)-Pb2+, respectively. The results provide guidelines in predicting the adsorption ability of GO adsorbent towards metal ions.