Nanoscale Zero-Valent Iron (nZVI) assembled on magnetic Fe_3O_4/graphene for Chromium (VI) removal from aqueous solution

摘要

Nanoscale Zero-Valent Iron (nZVI) assembled on magnetic Fe_3O_4/graphene (nZVI@MG) nanocomposites was synthesized for Cr(VI) removal from aqueous solution. nZVI particles were perfectly dispersed either among Fe_3O_4 nanoparticles (Fe_3O_4 NPs) or above the basal plane of graphene. This material shows Cr(VI) removal efficiency of 83.8%, much higher than those of individuals (18.0% for nZVI, 21.6% for Fe_3O_4 NPs and 23.7% for graphene) and even their sum of 63.3%. The removal process obeys pseudo-second-order adsorption model, suggesting that adsorption is rate-controlling step. Maximum Cr(VI) adsorption capacity varies from 66.2 to 101.0 mg g~(-1) with decreasing pH from 8.0 to 3.0 at 30℃. Negative DG and DH indicate spontaneous tendency and exothermic nature. Robust performance of nZVI@MG arises from the formation of micro-nZVI-grapheneZVI-Fe_3O_4 batteries and strong adsorption capability of broad graphene sheet/Fe_3O_4 surfaces. Electrons released by nZVI spread all over the surfaces of graphene and Fe_3O_4, and the adsorbed Cr(VI) ions on them capture these floating electrons and reduce to Cr(III). Fe_3O_4 NPs also served as protection shell to prevent nZVI from agglomeration and passivation.