Co-transport of U(VI) and colloidal biochar in quartz sand heterogeneous media

Biochar has attracted much attention for remediating the sites contaminated with heavy metals and radionuclides due to its lowcost and high adsorption affinity. However, little is known about howcolloidal biochar influences U(VI) transport in the environment. In this study, column experiments were conducted to investigate the individual and co-transport of U(VI) and biochar colloids (BC) in quartz sand heterogeneous media. Results showed that the transport of U(VI) in the individual transport system was pH-dependent and insensitive to ionic strength, whereas the individual BC transportwasmore sensitive to the changes in ionic strength compared to those in pH, indicating that electrostatic interaction plays a major role during BC transport but chemical interaction dominates U(VI) transport. In the presence of BC, the transport of U(VI) was significantly facilitated because of U(VI) adsorption on BC. The existence of low concentration of U(VI) (2.5 x 10(-6) M), however, did not affect the breakthrough curves (BTCs) of BC, except for the co-transport at relatively high ionic strength (100 mM) where BC transport was impeded due to the decrease of colloid suspension stability. Colloid size exclusion effect was evidenced by the evolution of particle size and zeta potential of the effluents. The transport of BC in both the individual and co-transport systems could be described by a two-site kinetic attachment/detachment model. This work implies that a risk assessment of BC facilitated heavy metal transport should be carefully considered when biochar is applied to the remediation of heavy metal contaminated sites. (c) 2021 Published by Elsevier B.V.

Automated summary

This study investigates the influence of colloidal biochar on U(VI) transport in the environment. The results show that the transport of U(VI) is pH-dependent and insensitive to ionic strength, while the transport of biochar colloids is more sensitive to changes in ionic strength. In the presence of biochar, the transport of U(VI) is significantly facilitated due to U(VI) adsorption. However, at high ionic strength, the transport of biochar is impeded. Colloid size exclusion effect and kinetic attachment/detachment model are observed in the transport of biochar. This work emphasizes the importance of considering the risk assessment of biochar-facilitated heavy metal transport in the remediation of contaminated sites.