Characterization of uranium in the extracellular polymeric substances of anaerobic granular sludge used to treat uranium-contaminated groundwater

Anaerobic granular sludge (AnGS) has been proven to be long-term effective for U(VI) removal and can be used as an inoculum for permeable reactive barriers, which is an innovative technology for remediation of uranium-contaminated groundwater. Considering their great ability in biosorption and bioreduction to common metal ions, extracellular polymeric substances (EPS) should play an important role in U(VI) removal and also in maintaining bioactivity of the AnGS due to toxicity accompanied with uranium. However, the roles of the EPS of AnGS in the uranium immobilization process are not clear. In this study, batch experiments were carried out by treating synthetic uranium-contaminated groundwater with AnGS, and uranium in EPS was extracted using four different methods. Moreover, speciation of uranium in EPS by filtration and inductively coupled plasma with mass spectroscopy and the reaction between isolated EPS and uranyl sulphate solution in a NaHCO3 medium were investigated. The results showed that about 12–16% of the total uranium immobilized by AnGS (extracted by the cation exchange resin (CER) method at 600 rpm for 1 h) was found to be present in EPS in its soluble ionic and particulate forms. For EPS-associated uranium obtained by the CER method, particulate uranium was proven to be the main form with sizes ranging from 24.7 nm to 171.3 nm. In the process of uranium immobilization using EPS isolated from non-reacted AnGS, both biosorption and bioreduction were involved. The findings of this study imply the important roles of EPS in the immobilization of uranium in groundwater using AnGS.