• Journal of Soil and Groundwater Environment
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• v.11 no.6
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• pp.35-42
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• 2006

Experimental study was conducted to identify the active agent for reductive dechlorination of TCE in cement/Fe(II) systems. Several potential materials-hematite (${\alpha}-Fe_2O_3$), lepidocrocite (${\gamma}$-FeOOH), akaganeite (${\beta}$-FeOOH), ettringite ($Ca_6Al_2(SO_4)_3(OH)_{12}$)-that are cement components or parts of cement hydrates were tested if they could act as reducing agents by conducting TCE degradation experiments. From the initial degradation experiments, hematite was selected as a potential active agent. The pseudo-first-order degradation rate constant ($k\;=\;0.637\;day^{-1}$) for the system containing 200 mM Fe(II), hematite and CaO was close to that ($k\;=\;0.645\;day^{-1}$) obtained from the system containing cement and 200 mM Fe(II). CaO, which was originally added to simulate pH of the cement/Fe(II) system, was found to play an important role in degradation reactions. The reactivity of the hematite/CaO/Fe(II) system initially increased with increase of CaO dosage. However, the tendency declined in the higher CaO dosage region, implying a saturation type of behavior. The SEM analysis revealed that the hexagonal plane-shaped crystals were formed during the reaction with increasing degradation efficiency, which was brought about by increasing the CaO dosage. It was suspected that the crystals could be portlandite or green rust ($SO_4$) or Friedel's salt. The XRD analysis of the same sample identified the peaks of hematite, magnetite/maghemite, green rust ($SO_4$). Either instrumental analysis predicted the presence of the green rust ($SO_4$). Therefore, the green rust ($SO_4$) would potentially be a reactive agent for reductive dechlorination in cement/Fe(II) systems.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2001.06a
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• pp.38-39
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• 2001

The seasonal changes in pH, Fe, Al and SO$_4$$\^$2-/ contents of acid drainage released from coal mine dumps play a major role in precipitation of metal hydroxides in the Taebaek coal field area, southeastern Korea. Precipitates in the creeks underwent a cycle of the color change showing white, reddish brown and brownish yellow, which depends on geochemical factors of the creek waters. White precipitates consist of Al-sulfate (basaluminite and hydrobasaluminite) and reddish brown ones are composed of ferrihydrite and brownish yellow ones are of schwertmannite. Goethite coprecipitates with ferrihydrite and schwertmannite. Ferrihydrite formed at higher values than pH 5.3 and schwertmannite precipitated below pH 4.3, and goethite formed at the intermediate pH range between the two minerals. With the pH being increased from acid to intermediate regions, Fe is present both as schwertmannite and goethite. From the present observation, the most favorable pH that basauluminte can precipitate is in the range of pH 4.45-5.95. SEM examination of precipitates at stream bottom shows that they basically consist of agglomerates of spheroid and rod-shape bacteria. Bacteria species are remarkably different among bottom precipitates and, to a less extent, there are slightly different chemical compositions even within the same bacteria. The speciation and calculation of the mineral saturation index were made using MINTEQA2. In waters associated with yellowish brown precipitates mainly composed of schwertmannite, So$_4$ species is mostly free So$_4$$\^$2-/ ion with less AlSo$_4$$\^$+/, CaSo$\sub$(aq)/, and MgSo$\sub$4(aq)/. Ferrous iron is present mostly as free Fe$\^$2+/, and FeSo$\sub$4(aq)/ and ferric iron exists predominantly as Fe(OH)$_2$$\^$+/, with less FeSo$\sub$4(aq)/, Fe(OH)$_2$$\^$-/, FeSo$_4$$\^$-/ and Fe$\^$3+/, respectively Al exists as free Al$\^$3+/, AlOH$_2$$\^$-/, (AlSo$_4$)$\^$+/, and Al(So$_4$)$\^$2-/. Fe is generally saturated with respect to hematite, magnetite, and goethite, with nearly saturation with lepidocrocite. Aluminum and sulfate are supersaturated with respect to predominant alunite and less jubanite, and they approach a saturation state with respect to diaspore, gibbsite, boehmite and gypsum. In the case of waters associated with whitish precipitates mainly composed of basaluminite, Al is present as predominant Al$\^$3+/ and Al(SO$_4$)$\^$+/, with less Al(OH)$\^$2+/, Al(OH)$_2$$\^$+/ and Al(SO$_4$)$\^$2-/. According to calculation for the mineral saturation, aluminum and sulfate are greatly supersaturated with respect to basaluminite and alunite. Diaspore is flirty well supersaturated while jubanite, gibbsite, and boehmite are already supersaturated, and gypsum approaches its saturation state. The observation that the only mineral phase we can easily detect in the whitish precipitate is basaluminite suggests that growth rate of alunite is much slower than that of basaluminite. Neutralization of acid mine drainage due to the dilution caused by the dilution effect due to mixing of unpolluted waters prevails over the buffering effect by the dissolution of carbonate or aluminosilicates. The main factors to affect color change are variations in aqueous geochemistry, which are controlled by dilution effect due to rainfall, water mixng from adjacent creeks, and the extent to which water-rock interaction takes place with seasons. pH, Fe, Al and SO$_4$ contents of the creek water are the most important factors leading to color changes in the precipitates. A geochemical cycle showing color variations in the precipitates provides the potential control on acid mine drainage and can be applied as a reclamation tool in a temperate region with four seasons.