Efficiency and mechanism of adsorption of low concentration uranium in water by extracellular polymeric substances

Extracellular polymeric substances (EPS) of uranium adsorbent was first extracted from the aerobic activated sludge of municipal wastewater treatment plant as raw material. The structure and surface morphology of EPS was characterized by FTIR, SEM-EDX, 3D-EEM, and XPS. The 3D-EEM spectra of EPS revealed that there are Tryptophan-like protein and Humus which can adsorb uranium in the EPS. The results of XPS indicated that the EPS surface contained active functional groups (COOH,CONH2,-H2PO4,OH,NH2 and so on) which all react with uranium, and the C, N, O elements play an important role in the reaction. The static batch test was used to study the adsorption behavior of uranium on the EPS, and the effects of pH, dosage of EPS and initial concentration of the solution on the removal of uranium by EPS were investigated. The adsorption isotherm, thermodynamics and kinetic models were used to match the mechanism of the interaction between EPS and uranium. Batch adsorption experiments revealed that the pH value had a great influence on the adsorption effect of EPS, and the optimal solution pH for uranium adsorption was around 6.0 with the removal efficiency of uranium was about 93% in the condition of neutral. Freundlich (R2 ≈ 0.997) and Langmuir (R2 ≈ 0.9931) models can get a good fitting effect, indicating that the adsorption of uranium by EPS had both monolayer adsorption and multilayer adsorption. EPS and uranium were combined disorderly and ion exchange mechanism could be involved. In this study, the active groups on the surface of EPS were also involved in the chemisorption process of uranium adsorption. The maximum adsorption capacity of EPS by Langmuir fitting was 333.3 mg/g. We conclude EPS is a potential adsorbent for radionuclide treatment.