Vitamin C mediates the activation of green tea extract to modify nanozero-valent iron composites: Enhanced transport in heterogeneous porous media and the removal of hexavalent chromium.

Both green tea (GT) extract and vitamin C (VC) were used for the reduction of Fe3+ to Fe0 using a green synthesis method. Modified nanozero-valent iron (GT-nZVI@VC nanocomposites) was successfully obtained and characterized as α-Fe0-iron oxide/VC by multiple analytical methods. The GT-nZVI@VC nanocomposites showed better transportability than nZVI, in that transport behavior was slightly dependent on various ratios of sand/soil in water-saturated heterogeneous porous media. Breakthrough curves of GT-nZVI@VC nanocomposites in paddy soil exhibited "blocking effects" and were well described using a first-order straining coefficient (k2) on site 2 obtained from a two-site kinetic attachment model. In particular, GT-nZVI@VC (VC/Fe = 0.6) showed higher Cr(VI) removal (especially reducibility) in both paddy soil and water compared to that of nZVI and VC. It is likely that the synergistic effects of VC (ascorbic acid) and tea polyphenols can increase the released free electrons into solution, favoring the high reduction of Cr(VI) into Cr(III) (i.e., FeOCr2O3, Cr(OH)3 and Cr2O3), where Cr(III) is prone to be immobilized by the nanocomposites in soil. This research highlights that VC can mediate the activation of GT extract to successfully modify nZVI, which could be beneficial for efficient transport in subsurface and remediation of Cr(VI)-contaminated soil and underground water.