Technetium-99 decontamination from radioactive wastewater by modified bentonite: batch, column experiment and mechanism investigation

Abstract Technetium-99 (99Tc), as one of the important high-yield fission products, poses a significant threat to the environment and human health due to its radiotoxicity, long half-life, high solubility and mobility in the environment. Thus, removal of 99TcO4− from radioactive wastewater is considerably imminent for waste disposal and environment conservation. In this work, Hexadecylpyridinium Chloride Monohydrate modified bentonite (HDPy-bent) was synthesised and used as an adsorbent to remove 99TcO4−. The adsorption behaviours of ReO4− as an analogue for TcO4− were investigated as functions of contact time, solution pH, initial concentration, and competing anions using batch techniques. It is demonstrated that the HDPy-bent showed an excellent adsorption ability for ReO4− at the pH range of 2–11, fast adsorption kinetic (reaching equilibrium within 3 min at room temperature), and maintains desirable ReO4− adsorption in the presence of concomitant anions at an excessive concentration of 500 times (Cl−, HCO3−, NO3−, CO32−, SO42− and PO43−). As for adsorption mechanism, we propose the anion exchange process at low concentration of Re(VII) and precipitation reaction at high concentration, while the driving force of the high ReO4− adsorption ability is the interaction between ReO4− anion and pyridinium ring of HDPy+, which is confirmed by density functional theory (DFT) simulations. Additionally, DFT studies also shed light on the adsorption structure of HDPy+/ReO4− and the origin of adsorption selectivity. Moreover, four adsorption-desorption cycles suggest that the HDPy-bent has good durability and reusability. Furthermore, ReO4− can be removed completely and recovered fractionally by using the chromatography column packed with HDPy-bent from simulated groundwater samples. The work presented herein demonstrates a low-cost adsorbent which can efficiently remove 99Tc from contaminated water.