Polyphenol-modified zero-valent iron prepared using ball milling technology for hexavalent chromium removal: Kinetics and mechanisms

•Polyphenol-modified ZVI presented much higher Cr(VI) removal compared with ZVI.•Cr(VI) removal increased with increasing ortho-phenol hydroxyl groups.•Polyphenols adsorbed on ZVI promoted the reduction of Fe(III) to Fe(II).•Polyphenols-modified ZVI had higher electron transfer rate than ZVI.•The main species in TA-ZVI and EGCG-ZVI for Cr(VI) removal were Fe(II) and Fe0. We used polyphenols to modify zero-valent iron (ZVI) by ball milling technology to obtain new catalysts TA-ZVI, EGCG-ZVI and pyGA-ZVI. The polyphenols included tannis acid (TA), epigallocatechin gallate (EGCG) and pyrogallic acid (pyGA). Scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, etc. were used to characterize the ZVIs. The results indicated that polyphenols were successfully coated on the surface of ZVI. TA-ZVI removed 98.2% of 20 mg·L−1 hexavalent chromium (Cr(VI)) within 2 min. In contrast, ZVI achieved only 10.5% removal of Cr(VI) within 2 min. High Cr(VI) removal was obtained by TA-ZVI at pH of 3–9 or with the presence of several ions including NO3−, Mg2+, SO42− and Cl−. Polyphenol-modified ZVI promoted the electron transfer by reducing the resistance, thus increasing the rate of electron transfer of the materials. The number of ortho-phenolic hydroxyl groups was crucial for modifying ZVI to remove Cr(VI). This work presents a new approach to synthesize modified ZVI using polyphenols for the removal of Cr(VI).