Sophorolipid modification enables high reactivity and electron selectivity of nanoscale zerovalent iron toward hexavalent chromium

Nanoscale zero-valent iron is considered to be a promising nanostructure for environmental remediation, while increasing the electron selectivity of nanoscale zerovalent iron (nZVI) during target contaminant removal is still a challenge (electron selectivity, defined as the percentage of electrons transferred to the target contaminants over the number of electrons donated by nZVI). In this study, the strategy for increasing the reactivity and electron selectivity of nZVI via sophorolipid (SL-nZVI) modification was proposed. The results showed that the removal efficiency and electron selectivity of SL-nZVI toward Cr(VI) was 99.99% and 56.30%, which was higher than that of nZVI (61.16%, 25.91%). Meanwhile, the particles were well characterized and the mechanism for enhanced reactivity and electron selectivity was investigated. Specially, both the morphology and BET specific surface area characterization suggested that stability against aggregation was higher in SL-nZVI nanoparticles than in nZVI. Besides, X-ray photoelectron spectroscopy (XPS), Tafel polarization curves, and Electrochemical impedance spectroscopy also indicated that the introduction of sophorolipid successfully prevent the nanoparticles from oxidation and benefit the electron transferring. In addition, a water contact angle test revealed that SL-nZVI nanoparticles were less hydrophilic (contact angle = 34.8°) than nZVI (contact angle = 23.9°). Therefore, in terms of reactivity, sophorolipid modification inhibited the aggregation of the nanoparticles and enhanced the electrical conductivity. For electron selectivity, the introduction of sophorolipid not only benefited Cr(VI) adsorption and the electron transfer from Fe0 to the surface-adsorbed Cr(VI) that followed but also reduced the possibility of side reactions between Fe0 and H2O. This study demonstrates that the introduction of sophorolipid is an effective strategy for developing a highly efficient nZVI-based nanocomposite system and highlights the potential role of sophorolipid in improving the electron selectivity of nZVI.