Zero-valent Iron Reduction, The Removal Of Hexavalent Chromium Contamination In Water

In the present paper is researched the removal efficiency and effects of hexvalent chromium (Cr(Ⅵ)) by iron powder or iron nanoparticles. The kinetics of Cr(Ⅵ) removal by iron powder was also analyzed. Mechanism was discussed using the pe-pH diagram of Fe-Cr-H₂O system combined with products analysis. The starch-stabilized iron nanoparticles was synthesized to improve the Cr(Ⅵ) removal efficiency. Furthermore vitamin C was applied to the Cr(Ⅵ) removal creatively. The results are as following:1. The effect factors of Cr(Ⅵ) removal contain Fe⁰ addition, initial Cr(Ⅵ) concentration, pH value, reaction temperature and the size of Fe⁰ particles. The removal efficiency increased with the increasing of Fe⁰ addition and reaction temperature, low pH value could accelerate the rate of Cr(Ⅵ) removal. The smaller the Fe⁰ particles, the higher the surface area, the faster the reaction rate.2. The reduction of Cr(Ⅵ) by iron powder was pseudo-first-order reaction.First-order rate coefficient k_Obs increased with increasing Fe⁰ addition but inversely with initial pH. According to the Arrehenius equation, the superficial active energy E=26.5 kJ/mol and k₀=3330 min^-1 was obtained in this research.3. Because of the large surface area(12.4 m²/g), the Cr(Ⅵ) reduction rate by Fe⁰ nanoparticles was about 5 times as well as that by iron powder. And starch-stabilized Fe⁰ nanoparticles' removal rate was one time higher than Fe⁰ nanoparticles' due to the prevention of nanoparticles agglomeration by starch.4. The mechanism of Cr(Ⅵ) reduction by Fe⁰ was as following: Cr(Ⅵ) reacted with atomic H, the product of Fe⁰ corroding. And Cr(Ⅵ) was also removed directly by Fe⁰ and Fe(Ⅱ). The product was Cr(Ⅲ)&Fe(Ⅲ) hydroxides, which cover the surface of Fe⁰ to slow down the reaction rate. Through the analysis of pe-pH diagram, the conclusion was made that the dominant product was Cr(Ⅲ)&Fe(Ⅲ) hydroxides.5. VC could reduce Cr(Ⅵ) in water effectively. The reducing rate was affected by factors such as VC addition, reaction temperature and dissolved oxygen. In the reaction iron powder and VC did not cooperate with each other. The reduction of Cr(Ⅵ) by VC was second-order reaction. Second-order rate coefficient k_obs increased linearly with the increasing of VC addition or reaction temperature.