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

Abstract

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.