Rational construction of micron-sized zero-valent iron/graphene composite for enhanced Cr(VI) removal from aqueous solution

Abstract

Micron-sized zero-valent iron (mZVI) has been widely used in wastewater treatment. However, the readily formation of FeCr hydroxide passivation layer limited its application in Cr(VI)-containing wastewater treatment. This study reports the rational construction of mZVI/graphene composite (mZVI/G) through a simple and environmental-friendly mechanochemical method. Owing to the extraordinary mechanical strength, graphene nanosheets intertwine with the mZVI particles to form a strong coupled system, leading to the decoupling of water corrosion of Fe0 to two half reactions:1) Fe0 as anode lose electron to produce Fe(II); 2) Graphene nanosheets serve as cathode for hydrogen gas formation involving atomic hydrogen (H*) as intermediate. Compared to carbon nanotubes and activated carbon, graphene nanosheets could stabilize H* due to the electron sponge effect related to large π conjugation system. When mZVI/G was used for Cr(VI) removal, the Cr(VI) species were preferentially adsorbed onto the graphene nanosheets, on which to be reduced to Cr(III) by Fe(II) as well as H*. The mZVI/G achieved a stable Cr(VI) removal capacity for eight consecutive cycles, reaching 102.35 mg·g-1 with the optimal composition, which was 11, 3.12 and 2.17 times higher than that of physical mixed mZVI and GO, mZVI/AC and mZVI/CNT, respectively. Moreover, mZVI/G can achieve efficient removal of Cr(VI) over a wide pH range (3.0 – 9.0). Given the low cost and green chemistry nature, the mechanochemical synthesized mZVI/G was considered to be a promising material for the treatment of Cr(VI) contaminated water.