연세대학교 환경지구화학연구실

Arsenic (As), which is deleterious to human health, exists in +III and +V oxidation states as prevalent inorganic species in natural water. Because As(III) is known to be more toxic and more mobile than As(V), the processes of removing As from drinking water have conventionally utilized the pre-oxidation of As(III) to As(V) followed by a process of coprecipitation or adsorption. Permanganate (MnO4-) has widely been used as an effective oxidant of As(III) for drinking water treatment systems, as well as for in-situ treatment of groundwater, with a benefit of fast oxidation kinetics. There has been a debate regarding the reaction stoichiometry of As(III) oxidation by permanganate due to the complexity of the reaction intermediates, such as Mn(III),-(IV), -(V), and -(VI) species involved with permanganate reduction. In addition, to our knowledge, the solid product of the permanganate oxidation of As(III) has not yet been extensively characterized. The purposes of this study are to (1) determine the stoichiometric ratio of permanganate oxidation of As(III) with varying doses of reactants under circumneutral pH conditions, (2) characterize the Mn solid product from the reaction, and (3) examined the feasibility of secondary heterogeneous oxidation of As(III) by the solid product. We also discussed the potential reaction pathway of As(III) oxidation by permanganate.

Chromium (Cr) usually exists as Cr(III) or Cr(VI) in the natural environment, and is susceptible to redox reactions. Its oxidation state determines the chemical characteristics and toxicity of this element. While Cr(III) is less toxic and exists commonly as Cr(OH)3(s) which has a low solubility under circumneutral pH conditions, Cr(VI) is highly toxic and usually exists as an oxyanion. Cr(VI) has been known to be the 2nd most common inorganic contaminant of groundwater. Since Cr in the mineral is usually in the oxidation state of +III, the contamination of groundwater by Cr(VI) has been generally considered to be anthropogenic. However, soil and groundwater contamination by Cr(VI) without any anthropogenic sources have been reported in several regions over the world. As Mn oxides are the only naturally occurring effective oxidant of Cr(III) in the environment, this study examines the Mn geochemical processes causing the natural contamination of Cr(VI).

Recently, zero-valent magnesium (ZVMg) has been regarded as an effective reductant of environmental contaminants for potential applications in the remediation processes. Utilization of ZVMg for the treatment of contaminants would have several advantages; (1) ZVMg (E0SHE = -2.37 V) has the highest reduction potential among zero-valent metals; (2) the nuisance effect of surface passivation may be insignificant due to the substantially high solubility of ZVMg corrosion products; and (3) ZVMg can be applied under both aerobic and anaerobic conditions. Despite the potential advantages of ZVMg, research of removing contaminants with ZVMg is uncommon. Therefore, we are examining the reactivity of ZVMg in aqueous solution as a strong and efficient reducing agent of various contaminants and trying to elucidate the corresponding reaction pathways and mechanisms of the contaminant removal processes. In addition, we have been investigating the feasibility of practical application of ZVMg wasted from industrial manufacturing processes using metallic Mg and Mg alloy.

Manganese (oxyhydr)oxides (hereafter referred to as ‘Mn oxides’) are widely distributed in most geological environments and found in more than 30 different types. In particular, each type of Mn oxides is characterized by distinct reactivity depending on its physicochemical properties (i.e., crystal structure and the valence state of structural Mn). In the natural environment, Mn oxides may play a key role in controlling the fate and transport of various inorganic and organic substances by participating in complex (bio)geochemical processes. It is well known that during (bio)geochemical processes including homogeneous, surface catalyzed, or microbial Mn(II) oxidation processes, formation and phase transformation of numerous types of Mn oxides may occur. Here, we are attempting to experimentally determine abiotic geochemical processes and factors controlling the pathways of the formation and the phase transformation of various Mn oxides which still have not been clearly understood yet.

: a precursor in the abiotic formation of diverse Mn oxides when exposed to oxygen

Diverse aspects of Mn and its compounds have been extensively studied because of their far-reaching roles in a wide range of biogeochemical processes in natural systems. The (ad)sorption behavior of Mn(II), however, is poorly understood despite its important role as the primary reaction step for surface-catalyzed Mn(II) oxidation that is the principal abiotic process forming various Mn (oxyhydr)oxides in nature. The purposes of the present study were to (1) elucidate the Mn(II) (ad)sorption behavior on the goethite surface under strictly oxygen-free conditions; (2) evaluate the effects of dissolved carbonates at a range of concentrations prevailing in natural waters on Mn(II) (ad)sorption processes; and (3) elucidating the molecular configuration of surface complexes of Mn(II) adsorbed to goethite using XAFS spectroscopy.

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