Comparison of different chelating agents to enhance reductive Cr(VI) removal by pyrite treatment procedure Cetin Kantar a,* , Cihan Ari a , Selda Keskin b a Canakkale Onsekiz Mart University, Department of Environmental Engineering, 17100, Canakkale, Turkey b NanoMagnetics Instruments Ltd., Hacettepe e Ivedik OSB Teknokent, 1368. Cad., No: 61/33, 06370, Ankara, Turkey article info Article history: Received 29 December 2014 Received in revised form 22 February 2015 Accepted 27 February 2015 Available online 7 March 2015 Keywords: Pyrite Chromium Organic ligands Complexation Treatment Oxidation abstract New technologies involving in-situ chemical hexavalent chromium [Cr(VI)] reduction to trivalent chromium [Cr(III)] with natural Fe(II)-containing minerals can offer viable solu- tions 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 investi- gated 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 z tartrate > EDTA > salicylate z non-ligand system. Organic ligands enhanced Cr(VI) removal by 1) removing surface oxide layer via the formation of soluble Fe eCr-ligand complexes, and 2) enhancing the reductive iron redox cycling for the regener- ation of new surface sites. While citrate, oxalate and tartrate eliminated the formation of surface Cr (III)eFe(III)-oxides, the surface phase Cr (III) species was observed in the pres- ence of EDTA and salicylate indicating that Cr(III) complexed with EDTA and salicylate sorbed or precipitated onto pyrite surface, thereby blocking the access of CrO 2 4 to pyrite surface. The binding of Fe(III) with the disulfide reactive sites (≡FeeSeSeFe(III)) was essential for the regeneration of new surface sites through pyrite oxidation. Although Fe(III)eS 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 di- sulfide reactive sites due to the formation of non-sorbing Fe(III)-ligand complexes. The absence of surface Fe(III)eS species indicated that no new reactive sites were generated through Fe redox cycling in the presence of salicylate and EDTA. © 2015 Elsevier Ltd. All rights reserved. * Corresponding author. Canakkale Onsekiz Mart University, Faculty of Engineering, Department of Environmental Engineering, Can- akkale, Turkey. Tel.: þ90 286 218 0018x2355; fax: þ90 286 218 0541. E-mail address: ckantar@comu.edu.tr (C. Kantar). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 76 (2015) 66 e75 http://dx.doi.org/10.1016/j.watres.2015.02.058 0043-1354/© 2015 Elsevier Ltd. All rights reserved.