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Öğe Comparison of different chelating agents to enhance reductive Cr(VI) removal by pyrite treatment procedure(Pergamon-Elsevier Science Ltd, 2015) Kantar, Cetin; Ari, Cihan; Keskin, SeldaNew technologies involving in-situ chemical hexavalent chromium [Cr(VI)] reduction to trivalent chromium [Cr(III)] with natural Fe(II)-containing minerals can offer viable solutions to the treatment of wastewater and subsurface systems contaminated with Cr(VI). Here, the effects of five different chelating agents including citrate, EDTA, oxalate, tartrate and salicylate on reductive Cr(VI) removal from aqueous systems by pyrite were investigated in batch reactors. The Cr(VI) removal was highly dependent on the type of ligand used and chemical conditions (e.g., ligand concentration). While salicylate and EDTA had no or little effect on Cr(VI) removal, the ligands including citrate, tartrate and oxalate significantly enhanced Cr(VI) removal at pH < 7 relative to non-ligand systems. In general, the efficiency of organic ligands on Cr(VI) removal decreased in the order: citrate >= oxalate tartrate approximate to EDTA > salicylate approximate to non-ligand system. Organic ligands enhanced Cr(VI) removal by 1) removing surface oxide layer via the formation of soluble Fe-Cr-ligand complexes, and 2) enhancing the reductive iron redox cycling for the regeneration of new surface sites. While citrate, oxalate and tartrate eliminated the formation of surface Cr (III) Fe(III)-oxides, the surface phase Cr (III) species was observed in the presence of EDTA and salicylate indicating that Cr(III) complexed with EDTA and salicylate sorbed or precipitated onto pyrite surface, thereby blocking the access of Cr-4(2-) to pyrite surface. The binding of Fe(III) with the disulfide reactive sites (equivalent to Fe-S-S-Fe(III)) was essential for the regeneration of new surface sites through pyrite oxidation. Although Fe(III)-S species was detected at the pyrite surface in the presence of citrate, oxalate and tartrate, Fe(III) complexed with EDTA and salicylate did not strongly interact with the disulfide reactive sites due to the formation of non-sorbing Fe(III)-ligand complexes. The absence of surface Fe(III)-S species indicated that no new reactive sites were generated through Fe redox cycling in the presence of salicylate and EDTA. (C) 2015 Elsevier Ltd. All rights reserved.Öğe 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, AkinLaboratory 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.Öğe Oxidative degradation of chlorophenolic compounds with pyrite-Fenton process(Elsevier Sci Ltd, 2019) Kantar, Cetin; Oral, Ozlem; Urken, Ozge; Oz, Nilgun Ayman; Keskin, SeldaBatch experiments, in conjunction with chromatographic and spectroscopic measurements, were performed to comparatively investigate the degradation of various chlorophenolic (CP) compounds (e.g., 2-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP, 2,3,4,6-TeCP) by a modified Fenton process using pyrite as the catalyst The batch results show that the CP removal by pyrite-Fenton process was highly dependent on chemical conditions (e.g., pH, CP and pyrite concentration), CP type, number and location of chlorine atoms on the aromatic ring. With the exception of 2,3,4,6-TeCP and 2,3-DCP, the CP removal decreased with increasing the number of chlorine constituents. While the main mechanism responsible for monochlorophenol removal (e.g., 2-CP and 4-CP) was the hydroxyl radical attack on aromatic rings, the CP removal for multichlorophenolic compounds (e.g., 2,3,4,6-TeCP) was driven by both: (1) hydroxyl radical attack on aromatic rings by both solution and surface-bound hydroxyl radicals and (2) adsorption onto pyrite surface sites. The adsorption affinity increased with increasing the number of Cl atoms on the aromatic ring due to enhanced hydrophobic effect. The TOC removal was not 100% complete for all CPs investigated due to formation of chemically less degradable chlorinated intermediate organic compounds as well as low molecular weight organic acids such as formic and acetic acid. Spectroscopic measurements with SEM-EDS, zeta potential and XPS provided evidence for the partial oxidation of pyrite surface Fe(II) and disulfide groups under acidic conditions. (C) 2019 Elsevier Ltd. All rights reserved.Öğe Role of citric acid on reduction and immobilization of chromium (VI) by pyrite under variable chemical conditions(Amer Chemical Soc, 2013) Kantar, Cetin; Karadeniz, Aykut; Ari, Cihan; Dogaroglu, Zeynep G.; Keskin, Selda[Anstract Not Available]Öğe Role of Humic Substances on Cr(VI) Removal from Groundwater with Pyrite(Springer, 2017) Kantar, Cetin; Bulbul, Muhammet Samet; Keskin, SeldaGroundwater 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.Öğe 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, SeldaLaboratory 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.