Chromium (Ⅵ) Reduction In Aqueous Solutions By Fe₃O₄-stabilized Fe⁰ Nanoparticles

Groundwater which contains hexvalent chromium (Cr(â…¥)) remediation by ironand iron nanoparticles has received increasing interest in recent years. Agglomerationof iron nanoparticles takes place, and then reduces the specific surface area, therebydiminishing particle reactivity. Laboratory batch experiments were conducted toinvestigate the feasibility of using magnetite (Fe₃O₄) stabilized zero-valent iron (ZVI)nanoparticles for the reduction and mitigation of hexavalent chromium Cr(â…¥) speciesin aqueous solutions.In the present paper is reported an efficient method of synthesizingFe₃O₄-stabilized Fe⁰ nanoparticles. They were observed by environmental scanningelectron microscope (ESEM). The results of using the Fe₃O₄-stabilized Fe⁰nanoparticles for the treatment of Cr(â…¥) are showed. The results are as following:1. The Cr(â…¥) reduction efficiency increased when Fe₃O₄ was ground with Fe⁰,different particle sizes of Fe⁰ resulted in different maximum Cr(â…¥) removalefficiencies. when Fe₃O₄ was ground with 5-8 mesh iron particles the reductionefficiency increased to 17 % for a Fe₃O₄ proportion of 80wt%, and when ground with20-40 mesh iron particles, the highest reduction efficiency reached 47.8 % for theFe₃O₄ proportion of 40 wt %.2. Because the reaction of chromium removal by Fe⁰ nanoparticles is a surfacereaction, so the smaller the Fe⁰ particles, the higher the surface area, the faster thereaction rate. Fe⁰ nanoparticles can attach to the surface of Fe₃O₄ if adding Fe₃O₄ intothe reaction solution during the preparation of Fe⁰ nanoparticles. The introduction ofFe₃O₄ prevents the aggregation of Fe⁰ nanoparticles and keeps the high efficiency ofthe nanocomposite for Cr(â…¥) reduction. Our results suggest that higher proportions ofFe₃O₄ in the nanocomposites could increase the rate of Cr(â…¥) reduction, and theoptimal ratio of Fe₃O₄: Fe⁰ for Cr(â…¥) reduction was determined to be 40 : 1.3. The effect factors of Cr(â…¥) removal contain Fe₃O₄ addition, pH value andreaction temperature. The removal efficiency increased with the increasing of Fe₃O₄addition, Low reaction temperature and low pH value could accelerate the rate of Cr(â…¥) removal.4. The mechanism of Cr(â…¥) reduction by Fe⁰ was as following: Cr(â…¥) reactedwith atomic H, the product of Fe⁰ corroding. And Cr(â…¥) was also removed directly byFe⁰ and Fe(â…¡). The product was Cr(â…¢)&Fe(â…¢) hydroxides, which cover the surfaceof Fe⁰ to slow down the reaction rate. But in Fe₃O₄-stabilized Fe⁰ nanoparticles, Fe⁰nanoparticles can attach to the surface of Fe₃O₄, which makes them hard to aggregate,so they can keep a larger surface area; further more, The Fe₃O₄ could act as anelectron mediator which can facilitate the electron transport from Fe⁰ to reducibleChromium. It overcomes the surface passivation problem which limits the electrontransfer to Cr(â…¥) and thus enhances the efficiency of the deoxidation process.