Enhanced Cr(VI) removal in the synergy between the hydroxyl-functionalized ball-milled ZVI/Fe3O4 composite and Na2EDTA complexation

The application of zero-valent iron (ZVI) for Cr(VI) removal often presents limitations due to its easy passivation. In this study, a column packed with hydroxyl-functionalized ball-milled ZVI/Fe3O4 (HFB-ZVI/Fe3O4) composite, assisted with Na2EDTA complexation, was developed to achieve sustainable treatment of Cr(VI)-containing wastewater. The results showed that at influent Cr(VI) concentration of as high as 50 mg/L, effluent Cr(VI) concentration below 0.5 mg/L was sustained for bed volume of as high as 1271 BV along with 2 g/L Na2EDTA concentration. Through the hydroxyl-functionalized treatment, FeOOH coating containing surface-adhering Fe(II) on the composite surface can be formed. Hydrolysis and auto-reduction of Fe(II)/Fe(III) species caused proton accumulation, resulting in remarkable attraction of anionic Cr(VI) via electrostatic interaction and exchange. Therefore, the reduction of Cr(VI) to Cr(III) was considerably enhanced. On the other hand, Na2EDTA complexation suppressed the growth of passivation layer and generated fresh corrosion products, thereby providing additional reactive sites for Cr(VI) removal. Additionally, Na2EDTA recovery and spent column regeneration were successfully proven. The remarkable effectiveness of Cr(VI) removal from actual electroplating wastewater confirmed the potential of the HFB-ZVI/Fe3O4 composite-packed column and Na2EDTA assistance-coordinated system in full-scale application.