Characteristics and Effects of Metal(II)-Doped Iron Oxides on the Reactivity of the Activated Iron Media Treatment System

The activated iron media (AIM) treatment system is an innovative ZVI-based water treatment technology for removing heavy metals, reactive oxyanions and nutrients from a contaminated water stream. The AIM treatment system consists of zero-valent iron (ZVI), iron oxides represented by magnetite and ferrous iron. When the AIM immobilizes heavy metals from wastewaters, the heavy metals are entrapped as impurities in the iron oxides. The metal impurities may alter the physical/chemical properties of the iron oxides, and thus affect the reactivity of the AIM system. In this study, Mn^2+ and Zn^2+, two heavy metals commonly found in industrial wastewaters, were chosen to evaluate their effect on the magnetite-coated ZVI for nitrate and selenate removal in batch experiments. The results demonstrated that a magnetite-coated ZVI system with Mn^2+ additions could effectively reduce both nitrate and selenate. Mn^2+ could substitute structural Fe(II) of the magnetite, releasing dissolved Fe^2+. The co-existence of Mn^2+ and Fe^2+ with magnetite-coated ZVI can retain the media reactivity for both nitrate and selenite reduction, and the inverse spinel crystal structure of magnetite is not altered by the introduction of Mn(II) into its structure. Unlike Mn^2+ additions, a magnetite-coated ZVI system with added Zn^2+ effectively reduces selenate, but nitrate reduction is significantly inhibited. Zn^2+ was incorporated into the iron oxide coating, but the original magnetite structure was altered. Both aqueous Zn^2+ and structural Zn(II) could effectively suppress nitrate reduction in a magnetite-coated ZVI system. The role of Zn^2+ addition on nitrate reduction inhibition was also confirmed by the continuous stirred-tank reactor (CSTR) experiments. The results showed that adding metal impurities into the AIM treatment system could significantly influence the removal performance for particular contaminants. The effect of metal ions other than Mn^2+ and Zn^2+ should be investigated to understand more potentials of the AIM treatment system for applications in the future research