Cr(VI) removal by micron-scale iron-carbon composite induced by ball milling: The role of activated carbon

Abstract The oxide film coated on the surface of zero-valent iron (ZVI) greatly limits the reducibility of ZVI. Application of iron-based materials especially microscale or nanoscale for water treatment can be more effective when modified with activated carbon (AC). In the study, the removal efficiency of Cr(VI) was significantly enhanced by mZVI/AC composite synthesized by ball milling (BM) under acidic and anaerobic conditions, and reached 94.01% within 120 min. And BM promoted effective contact or collision between mZVI and AC, which was a necessary condition for strengthening Cr(VI) removal by mZVI/AC. By detecting pH changes and the dissolution of total iron during batch experiments, it was shown that AC enhanced the corrosion of mZVI, accompanied by consumption of H+. Tafel scans further confirmed that the corrosion of mZVI was promoted when coupled with AC. Compared with mZVI, AC enhanced the adsorption-reduction ability, and also enhanced the homogeneous reduction reaction (the role of the secondary reducing agent Fe2+) in the process of Cr(VI) removal by mZVI/AC. Based on X-ray photoelectron spectroscopy (XPS), valence analysis of chromium and 1,10-phenanthroline shielding experiments, the Cr(VI) removal mechanism by mZVI/AC was also unraveled. Furthermore, the reusability of mZVI/AC was limited by the accumulation of (hydr)oxide on its surface and the loss of surface AC, but can be easily rejuvenated by BM again with a small amount of AC. In conclusion, mZVI/AC composite synthesized by BM are a promising green material to remove Cr(VI) from wastewater.