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 Cu 2+ from water and soil. The pores in GO-COOH/CS allowed the internal diffusion of Cu 2+ 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 Cu 2+ fixation. The partition coefficient (PC) for Cu 2+ adsorption onto GO-COOH/CS spheres was 4.2 mg/g/μM at C e of 0.48 mg/L and q e of 31 mg/g, while the PC value decreased to 0.096 mg/g/μM at C e of 91.4 mg/L and q e of 78 mg/g. At initial Cu 2+ concentrations of 120 mg/L and lower, the fixation efficiencies were all higher than 99% and the corresponding free Cu 2+ concentrations in leachates were lower than 1.0 mg/L. The Cu 2+ 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 Cu 2+ . 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 Cu 2+ to reduce Cu 2+ bioaccumulation in plants.