Carboxylation as an effective approach to improve the adsorption performance of graphene materials for Cu2+ removal

Abstract Graphene materials are high-performance adsorbents for water and soil remediation, whose oxygen containing groups bind to metal ions intensely. In this study, we prepared carboxylated graphene oxide (GO-OCH2COOH) sponge and investigated the adsorption behaviors of Cu2+ on it by both experimental and computational approaches. Carboxylation largely improved the adsorption capacity from 23.8 mg/g for graphene oxide (GO) sponge to 93.8 mg/g for GO-OCH2COOH. The efficient adsorption was due to the strong interaction between Cu2+ and carboxyl groups (especially in –OCH2COOH form) according to the density functional theory calculation, while epoxy and hydroxyl groups contributed lowly. The fast adsorption process was achieved within 30 min, corresponding to a large k2 value of pseudo-second order model (0.061 mg/g/min). The adsorption was spontaneous and exothermic according to thermodynamics analyses. The binding strength of Cu2+ on GO-OCH2COOH was so strong that pH and ionic strength had mild impact. The strong binding sites were not recyclable, but the weaker ones (more than 40%) could be regenerated by simple washing. Our results highlighted the importance of chemical design in graphene adsorbents and the potential of GO-OCH2COOH in heavy metal fixation from water and soil.