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    Effect of pH-buffering on Cr(VI) reduction with pyrite in the presence of various organic acids: Continuous-flow experiments
    (Elsevier Science Sa, 2016) Kantar, Cetin; Bulbul, Muhammet Samet
    The effects of pH and pH buffering on catalytic hexavalent chromium (Cr(VI)) reduction by pyrite were investigated in a series of column experiments. The experiments show a strong dependence of Cr(VI) removal by pyrite on solution pH, with Cr(VI) removal decreasing with increasing solution pH. While organic buffers e.g., (acetic acid/acetate) showed higher Cr(VI) removal by pyrite under acidic conditions (e.g., pH 4) relative to unbuffered systems, the addition of an organic buffer (MOPS) decreased Cr(VI) removal by pyrite at pH 8 relative to systems buffered with an inorganic buffer (bicarbonate). Similarly, in systems buffered with acetate buffer at pH 4, organic acids (e.g., citrate and tartrate) significantly improved Cr(VI) removal by pyrite due to removal of surface oxidation products (e.g., Fe(OH)(3(s))). Unlike acidic conditions, organic acids such as citrate and EDTA adversely affected Cr(VI) removal under alkaline pH conditions relative to systems containing 0.01 M MOPS buffer only. This inhibitory effect observed under alkaline pH conditions may be explained through competitive adsorption of organic buffers (MOPS) and acids (citrate, EDTA) onto pyrite surface, which, not only, modified pyrite surface properties, but also, blocked access of Cr(VI) to pyrite surface. Overall, it is clear that while pH buffering under acidic conditions may significantly improve the process efficiency of pyrite-based Cr(VI) treatment, it may have adverse effects on process efficiency under alkaline conditions (e.g., pH 8) since organic buffers (e.g., MOPS) and acids (e.g., EDTA) may directly interact with pyrite surface, and change its surface properties. (C) 2015 Elsevier B.V. All rights reserved.
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    Role of Humic Substances on Cr(VI) Removal from Groundwater with Pyrite
    (Springer, 2017) Kantar, Cetin; Bulbul, Muhammet Samet; Keskin, Selda
    Groundwater composition may have a pronounced impact on long-termperformance of permeable reactive barriers (PRBs). Here, batch and column experiments were conducted to investigate the effects of humic acid (HA) on Cr(VI) removal by pyrite in systems containing cations such as Ca2+ and Mg2+. HAwas observed to have inhibitory effect on Cr(VI) uptake by pyrite under the experimental conditions studied (e. g., pH 3 to 8). HA sorbed onto pyrite surface and thus (1) competed against Cr(VI) for pyritic surface sites and/or (2) increased electrostatic repulsion between Cr(VI) and pyrite. In systems with HA and Ca2+/Mg2+, the Cr(VI) uptake by pyrite decreased drastically relative to HA alone due to the aggregation of HA with Ca2+/Mg2+. The formation of such HA aggregates/precipitates blocked Cr(VI) ions to reach its binding sites, thereby resulting in a substantial decrease in Cr(VI) uptake. Overall, the results have major implications for proper design and operation of PRBs with pyrite as the reactive material.
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    Role of Major Groundwater Ions on Reductive Cr(VI) Immobilization in Subsurface Systems with Pyrite
    (Springer International Publishing Ag, 2016) Bulbul, Muhammet Samet; Kantar, Cetin; Keskin, Selda
    Laboratory batch and column experiments were performed to better understand the effects of Ca2+, Mg2+, and HCO3- on Cr(VI) removal from aqueous systems with pyrite. Batch results show that increasing HCO3- concentration led to an increase in Cr(VI) removal by pyrite due to pH buffering capacity of HCO3-. However, while Ca2+ and Mg2+ individually had no effect on Cr(VI) removal at pH 4, the addition of Ca2+ or Mg2+ to systems containing HCO3- resulted in a significant decrease in Cr(VI) removal at pH 8 relative to the systems containing HCO3- alone. The XPS data proved that while Ca2+ precipitated as CaCO3(S) onto pyrite surface, Mg2+ sorbed and/or accumulated as Mg(OH)(2(S)) onto oxidized pyrite surface. The formation of surface precipitates (e. g., CaCO3) inhibited further Cr(VI) reduction by blocking electron transfer between Cr(VI) and pyritic surface sites. While the precipitation of Ca2+ as CaCO3 led to a significant decrease in effluent pH, the decrease in effluent pH was very low in systems containing Mg2+, most probably due to much higher solubility of Mg2+ at pH 8. Zeta potential measurements provided further evidence that while Ca2+ or Mg2+ had no effect on zeta potential of pyrite particles under acidic conditions (e. g., pH< 7), the addition of Ca2+ or Mg2+ to systems containing Cr(VI) reversed the pyrite surface potential from negative to positive under alkaline pH conditions (e. g., pH> 8) relative to system containing only Cr(VI), suggesting the sorption and/or accumulation of surface precipitates on pyrite surface.