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    Cr(VI) removal from aqueous systems using pyrite as the reducing agent: Batch, spectroscopic and column experiments
    (Elsevier Science Bv, 2015) Kantar, Cetin; Ari, Cihan; Keskin, Selda; Dogaroglu, Zeynep Gorkem; Karadeniz, Aykut; Alten, Akin
    Laboratory batch and column experiments, in conjunction with geochemical calculations and spectroscopic analysis, were performed to better understand reaction mechanisms and kinetics associated with Cr(VI) removal from aqueous systems using pyrite as the reactive material under both static and dynamic flow conditions similar to those observed in in situ permeable reactive barriers (PRBs). The X-ray photoelectron spectroscopy (XPS) and geochemical calculations suggest that the Cr(VI) removal by pyrite occurred due to the reduction of Cr(VI) to Cr(III), coupled with the oxidation of Fe(II) to Fe(III) and S-2(2-) to SO42- at the pyrite surface. Zeta potential measurements indicate that although the pyrite surface was negatively charged under a wide pH range in the absence of Cr(VI), it behaved more like a metal oxide surface with the surface potential shifting from positive to negative values at pH values >pH 6 in the presence of Cr(VI). Batch experiments show that increasing solution pH led to a significant decrease in Cr(VI) removal. The decrease in Cr(VI) removal at high Cr(VI) concentrations and pH values can be explained through the precipitation of sparingly soluble Cr(OH)(3(s)), Fe(OH)(3(s)) and Fe(III)-Cr(III) (oxy) hydroxides onto pyrite surface which may, then, lead to surface passivation for further Cr(VI) reduction. Batch results also suggest that the reaction kinetics follow a first order model with rate constants decreasing with increasing solution pH, indicating proton consumption during Cr(VI) reducdon by pyrite. Column experiments indicate that nearly 100% of total Fe in the column effluent was in the form of Fe(II) species with a [[SO42-]/[Fe2+]] stoichiometric ratio of 2.04, indicating that the reduction of Cr(VI) by pyrite produced about 2 mol of sulfate per mole of Fe (II) release under excess surface sites relative to Cr(VI) concentration. Column experiments provide further evidence on the accumulation of oxidation products which consequently led to a significant pressure build up in pyrite packed columns over time. (C) 2015 Elsevier B.V. All rights reserved.
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    Reductive Immobilization of Chromium in Soils Containing Heterogeneous Fe-Bearing Minerals
    (Taylor & Francis Inc, 2016) Dogaroglu, Zeynep Gorkem; Kantar, Cetin
    Cr(VI) immobilization in systems containing Fe-bearing soil minerals was studied in batch and column systems. Batch experiments showed that water chemistry such as solution pH and Cr(VI) concentration had a pronounced impact on Cr(VI) removal by Fe-bearing soil minerals. Acidic conditions were observed to be more favorable for enhanced Cr(VI) removal. The dependence of Cr(VI) removal on Cr(VI) concentration indicated that there were limited numbers of surface sites on Fe-bearing minerals responsible for Cr(VI) removal. A complexing agent, citrate, significantly enhanced both Cr(VI) removal and total Fe-dissolution from the mineral surfaces relative to non-citrate containing systems, and the iron drm, implying that Cr(VI) removal occurred mainly on mineral surfaces, andissolved from the mineral surfaces was in Fe(III) oxidation fo the surface Fe(II) sites played an active role in Cr(VI) reduction. The results from column experiments showed that the accumulation of surface precipitates resulted in clogging of pore spaces, thereby creating preferential flow paths within the column. However, the addition of citrate significantly prevented the accumulation of surface precipitates due to the formation of highly soluble Fe-citrate complexes. SEM images revealed that the precipitates accumulated in the column had sponge-like shapes. The energy-dispersive spectroscopy analysis provided further evidence that the surface precipitates formed also contained Cr species as well as Fe. Overall it is clear that Fe-bearing minerals may serve as an effective reducing agent for in-situ reductive immobilization of hexavalent chromium in subsurface systems.