Preparation and Application of Carboxylated Graphene Oxide Sponge in Dye Removal

Spongy graphene is a newly developed adsorbent of high performance for water treatment. Proper functionalization is an efficient approach to improve the adsorption capacity of graphene adsorbents. In this study, we prepared graphene oxide (GO), functionalized it with carboxyl groups to produce carboxylated GO (GO-COOH) dispersion, and lyophilized the GO-COOH dispersion to obtain the GO-COOH sponge. The adsorption isotherm, kinetics, thermodynamics, influencing factors, and regeneration of the adsorption of dye methylene blue (MB) on GO-COOH sponge were evaluated in batch experiments. The adsorption capacity of GO-COOH sponge was measured as 780 mg/g, which was nearly twice that of GO sponge (446 mg/g). The adsorption isotherm could be well described by the Freundlich model with a K of 508 (L/mg)^1/n. The adsorption kinetic was nicely fitted by pseudo-first-order model with a k₁ of 0.00157·min^-1. In thermodynamics analysis, the negative ΔG indicated the spontaneous nature of adsorption on GO-COOH sponge. The adsorption process was endothermic and was driven by the increase of entropy. Higher pH benefited the removal of MB by GO-COOH sponge and the ionic strength had no meaningful effect. The regeneration was poor due to the strong electrostatic interaction between MB and the GO-COOH sponge. The results collectively suggested that carboxylation increased the adsorption performance of GO sponge.