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.