SUSTAINABLE ENVIRONMENTAL CHEMISTRY AND TECHNOLOGY WITH FOCUS ON THE MEDITERRANEAN AREA Reductive precipitation and removal of Cr(VI) from groundwaters by pipe flocculation-microfiltration Stylianos Stylianou 1 & Konstantinos Simeonidis 2 & Manassis Mitrakas 2 & Anastasios Zouboulis 1 & Mathias Ernst 3 & Ioannis A. Katsoyiannis 1 Received: 11 May 2017 /Accepted: 14 August 2017 # Springer-Verlag GmbH Germany 2017 Abstract Chromium (Cr(VI)) is a very toxic and carcinogen- ic element, which is widely present in groundwaters, mainly due to geogenic conditions. The limit of Cr(VI) in drinking water is expected to be reduced to 10 μg/L in both the USA and the European Union. Recent literature findings indicated that the most efficient process in reducing Cr(VI) levels to below 10 μg/L proved to be Cr(VI) reduction by Fe(II), by applying a molar ratio Fe(II)/Cr(VI) of around 9. In the present work, we investigated the reduction of Cr(VI) by Fe(II) in pipe flocculation reactors followed by filtration of insoluble prod- ucts by microfiltration. The proposed technology involves re- circulation of a part of the sludge in the pipe reactors, in order to improve kinetics and efficiency of the process. The obtain- ed results showed that with a Fe(II) dose of around 1 mg/L, Cr(VI) was reduced to below 10 μg/L, by even an initial concentration as high as 300 μg/L of Cr(VI), corresponding to a molar ratio Fe(II)/Cr(VI) of around 3, thus reducing the overall quantity of reductive reagents and of the produced sludge. This ratio was also confirmed by the XPS analysis, which also showed that Cr(VI) was reduced to Cr(III) and then precipitated either as Cr(OH) 3 or associated with the produced iron oxides. Keywords Hexavalent chromium . Pipe flocculation . Coagulation . Microfiltration Introduction Recent reports concerning the wide dispersion of hexavalent chromium (Cr(VI)) in drinking water across the world and the concomitant health risks related to the consumption of Cr(VI)- polluted water, triggered an open discussion about the con- tamination origin, the regulation update, and the optimization of removal methods (Costa 2003; Linos et al. 2011). Depending on the water source (surface, groundwater) and the relevant environmental conditions (pH, dissolved oxygen concentration, solid surfaces), water pollution by Cr(VI) can be ascribed to both anthropogenic and natural activities (Fantoni et al. 2002). Industrial and mining activities are gen- erally associated with the pollution of surface waters while groundwaters can be adversely affected in the long term. Responsible editor: Bingcai Pan * Ioannis A. Katsoyiannis katsogia@chem.auth.gr Stylianos Stylianou s.stulianou@gmail.com Konstantinos Simeonidis ksime@physics.auth.gr Manassis Mitrakas manasis@eng.auth.gr Anastasios Zouboulis zoubouli@chem.auth.gr Mathias Ernst mathias.ernst@tuhh.de 1 Laboratory of Chemical and Environmental Technology, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 2 Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 3 Institute for Water Resources and Water Supply, Hamburg University of Technology, Am Schwarzenberg-Campus 3, 21073 Hamburg, Germany Environ Sci Pollut Res DOI 10.1007/s11356-017-9967-4