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

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 23.8mg/g for graphene oxide (GO) sponge to 93.8mg/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 30min, corresponding to a large k₂ value of pseudo-second order model (0.061mg/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.