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+  Q m = 0.338 Δ I P - 0.110 | l g K OH | + 0.114 Z 2 / r + 26.934 σ p - 1.479 A R - 1.162 X m 2 r + 0.115 A N - 3.059 Density 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.