Carboxylated graphene oxide-chitosan spheres immobilize Cu2+ in soil and reduce its bioaccumulation in wheat plants.

Due to the strong interaction with pollutants and the huge adsorption capacity, graphene adsorbents are widely applied in water decontamination. However, graphene adsorbents are seldom used in soil remediation, because the adsorptive sites on graphene would be occupied by soil components. In this study, we prepared carboxylated graphene oxide-chitosan (GO-COOH/CS) spheres for the immobilization of Cu2+ from water and soil. The pores in GO-COOH/CS allowed the internal diffusion of Cu2+ solution, while they blocked the direct contact between the solid soil and the adsorptive sites on graphene sheets. Therefore, the high adsorption capacity of GO-COOH/CS spheres (78 mg/g) was largely retained for the soil Cu2+ fixation. The partition coefficient (PC) for Cu2+ adsorption onto GO-COOH/CS spheres was 4.2 mg/g/µM at Ce of 0.48 mg/L and qe of 31 mg/g, while the PC value decreased to 0.096 mg/g/µM at Ce of 91.4 mg/L and qe of 78 mg/g. At initial Cu2+ concentrations of 120 mg/L and lower, the fixation efficiencies were all higher than 99% and the corresponding free Cu2+ concentrations in leachates were lower than 1.0 mg/L. The Cu2+ fixation on GO-COOH/CS spheres largely reduced its bioaccumulation in wheat roots from 127.8 µg/g to 51.2 µg/g. The toxicity evaluations suggested that GO-COOH/CS spheres were of low toxicity to wheat seedlings and did not amplify the toxicity of Cu2+. The implications to the design of graphene adsorbents for soil remediation are discussed. Overall, our results collectively indicated that porous GO-COOH/CS spheres were high-performance adsorbents for the immobilization of Cu2+ to reduce Cu2+ bioaccumulation in plants.

Toxicity of graphene oxide to white rot fungus Phanerochaete chrysosporium.

With the wide production and applications of graphene and its derivatives, their toxicity to the environment has received much attention nowadays. In this study, we investigated the toxicity of graphene oxide (GO) to white rot fungus (Phanerochaete chrysosporium). GO was prepared by modified Hummers method and well characterized before use. P. chrysosporium was exposed to GO at the concentrations of 0-4 mg/mL for 7 d. The fresh and dry weights, pH values of culture media, structures, ultrastructures, IR spectra and activities of the decomposition of pollutants were measured to reveal the hazards of GO to P. chrysosporium. Our results indicated that low concentrations of GO stimulated the growth of P. chrysosporium. The exposure to GO induced more acidic pH values of the culture media after 7 d. GO induced the disruption of the fiber structure of P. chrysosporium, while at 4 mg/mL some very long and thick fibers were formed. Such changes were reflected in the scanning electron microscopy investigations, where the disruption of fibers was observed. In the ultrastructural investigations, the shape of P. chrysosporium cells changed and more vesicles were found upon the exposure to GO. The infrared spectroscopy analyses suggested that the chemical compositions of mycelia were not changed qualitatively. Beyond the toxicity, GO did not alter the activities of P. chrysosporium at low concentrations, but led to the complete loss of activity at high concentrations. The implication to the ecological safety of graphene is discussed.

Facile hydrothermal preparation of recyclable S-doped graphene sponge for Cu2+ adsorption.

Graphene sponge (GS) has been widely employed for water purification, but adsorption capacity loss frequently occurs during the formation of spongy structure. In this study, we reported the hydrothermal preparation of S-doped GS for the removal of Cu(2+) with a huge adsorption capacity of 228 mg/g, 40 times higher than that of active carbon. The adsorption isotherm could be well fitted into the Freundlich model with a KF value of 36.309(L/mg)(1/n). The equilibrium adsorption could be fully achieved in the first 5 min. In the thermodynamics study, the negative ΔG indicated that the adsorption was spontaneous and physisorption in nature. The positive ΔH implied that the adsorption was endothermic. The changes of both pH and ionic strength had no apparent influence on the adsorption. S-doped GS could be easily regenerated by washing with acidic thiourea. Moreover, S-doped GS could be used for the adsorption of other heavy metal ions, too. The implication to the applications of S-doped GS in water treatment is discussed.