title: Cr(VI) removal from aqueous systems using pyrite as the reducing agent: batch, spectroscopic and column experiments.
abstract: 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 S2(2-) to SO4(2-) 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) reduction 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 [SO4(2-)]/[Fe(2+)] 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.
authors: Canakkale Onsekiz Mart University, Department of Environmental Engineering, 17100 Canakkale, Turkey. Electronic address: ckantar@comu.edu.tr.
authors: Cetin
authors: C
authors: Kantar
authors: Canakkale Onsekiz Mart University, Department of Environmental Engineering, 17100 Canakkale, Turkey.
authors: Cihan
authors: C
authors: Ari
authors: NanoMagnetics Instruments Ltd., Hacettepe - İvedik OSB Teknokent, 1368. Cad., No: 61/33, 06370 Ankara, Turkey.
authors: Selda
authors: S
authors: Keskin
authors: Mersin University, Department of Environmental Engineering, Ciftlikkoy Campus, Mersin, Turkey.
authors: Zeynep Gorkem
authors: ZG
authors: Dogaroglu
authors: Yildiz Technical University, Department of Environmental Engineering, Davutpasa, Turkey.
authors: Aykut
authors: A
authors: Karadeniz
authors: Canakkale Onsekiz Mart University, Department of Environmental Engineering, 17100 Canakkale, Turkey.
authors: Akin
authors: A
authors: Alten
mesh_terms: D002857:Chromium / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D019163:Reducing Agents / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods; D013440:Sulfides / Q000737:chemistry*; D062065:Wastewater / Q000737:chemistry; D018508:Water Purification / Q000379:methods*
chemical_list: D002857:Chromium; D007501:Iron; D019163:Reducing Agents; D013440:Sulfides; D062065:Wastewater; C011342:pyrite
keywords: Chromium; Oxidation; Permeable reactive barrier (PRB); Pyrite; Reduction; Sorption; Treatment