Spectroscopic and modeling investigation of U(VI) removal mechanism on nanoscale zero-valent iron/clay composites

Nanoscale zero-valent iron (nZVI)-based composites have been widely utilized in environmental cleanup due to their low cost, high adsorption performance and strong redox activity. Herein, removal mechanism of U(VI) on nZVI/clay composites was demonstrated by batch, XPS and modeling techniques. The batch experiments showed that nZVI/clay composites exhibited the high removal capacity (88.90 mg/g at pH 4.0) and good regeneration towards U(VI) from aqueous solution. The adsorbed U(VI) was mostly reduced to U(IV) by nZVI/clay composites according to XPS analysis. The removal process of U(VI) on nZVI/clay composites was satisfactorily fitted by surface complexation modeling using strong and weak sites, indicating the high chemisorption of U(VI) on nZVI/clay composites. However, the fitting results underestimated U(VI) adsorption at pH 7.0-9.0 due to the reduction of U(VI) into U(IV), whereas the overestimation of U(VI) at pH 4.0-6.0 could be attributed to fewer surface complexation reaction involved. These findings are crucial for the application of nZVI-based composites for the highly efficient removal of radionuclides in actual environmental remediation.