Abstract
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 KF 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.
Highlights
Dye pollution is one of the most serious environmental pollutions that is usually released from textile, printing, and pharmaceutical factories [1,2,3]
The as-prepared graphene oxide (GO) sponge and GO-COOH sponge were characterized by infrared spectroscopy (IR) (Magna-IR 750, Nicolet, SpectraLab Scientific Inc., Alexandria, VA, USA), Raman, thermogravimetric analysis (TGA) (Q500, TA Instruments, New Castle, DE, USA), X-ray photoelectron spectroscopy (XPS) (Kratos, London, UK), scanning electron microscope (SEM) (Quanta 200FEG, FEI, Eindhoven, The Netherlands), and transmission electron microscope (TEM) (JEM-200CX, JEOL, Tokyo, Japan)
For GO-COOH sponge, the stacking and wrinkles still presented (Figure 1b), while the sheets of GOFor GO-COOH sponge, the stacking and wrinkles still presented (Figure 1b), while the sheets of COOH sponge had some pores on the surface
Summary
Dye pollution is one of the most serious environmental pollutions that is usually released from textile, printing, and pharmaceutical factories [1,2,3]. To improve the adsorption capacity of graphene sponge for cationic pollutants, there are two possible approaches, namely increasing the oxygen content and introducing other groups that have stronger affinities to the pollutants. A better choice is to introduce groups of higher affinity to the pollutants Following this strategy, we previously reduced and doped GO with cysteine, and found that the resulting S-doped graphene possessed high binding strength with heavy metal ions [23]. Sun et al [27] investigated the removal of U6+ by GO-COOH experimentally and theoretically with a maximum adsorption capacity of 103.09 mg/g Based on these findings, we speculated that the strategy of converting hydroxyl groups and the ether bonds into carboxyl groups could improve the performance of GO sponge as well. The implication to the application of GO-COOH sponge in water treatment is discussed
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