Inhibition of the nitrification process in activated sludge by trivalent and hexavalent chromium, and partitioning of hexavalent chromium between sludge compartments Breda Novotnik a,b , Tea Zuliani a , Janez Šc ˇanc ˇar a,b , Radmila Milac ˇic ˇ a,b,⇑ a Department of Environmental Sciences, Joz ˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia b Joz ˇef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia highlights  Inhibition of nitrification by Cr(III) and Cr(VI) was studied in activated sludge.  Quantification of Cr(VI) was performed by speciated ID-ICP-MS.  Cr(VI) concentration higher than 2.5 mg L 1 severely inhibited nitrification.  Cr was distributed between adsorbed, intracellular and intercellular sludge compartments.  Reduction of Cr(VI) occurred almost solely within the intercellular sludge compartment. graphical abstract Inhibition of nitrification by Cr(VI) in activated sludge 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 Time (h) Concentration of NH 3 -N (mg/L) article info Article history: Received 3 September 2013 Received in revised form 1 December 2013 Accepted 25 December 2013 Available online 24 January 2014 2;titioning Keywords: Activated sludge Iinhibition of nitrification Chromium species Speciated isotope dilution inductively coupled plasma mass spectrometry Partitioning abstract The input of wastewater treatment plants (WWTPs) may contain high concentrations of Cr(III) and Cr(VI), which can affect nitrogen removal. In the present study the influence of different Cr(III) and Cr(VI) con- centrations towards activated sludge nitrification was studied. To better understand the mechanisms of Cr(VI) toxicity, its reduction, adsorption and uptake in activated sludge was investigated in a batch growth system. Quantification of Cr(VI) was performed by speciated isotope dilution inductively coupled plasma mass spectrometry. It was found that Cr(VI) concentrations above 1.0 mg L 1 and Cr(III) concen- trations higher than 50 mg L 1 negatively affected nitrification. Speciation studies indicated almost com- plete reduction of Cr(VI) after 24 h of incubation when Cr(VI) concentrations were lower than 2.5 mg L 1 , whereas for Cr(VI) added to 5 mg L 1 around 40% remained unreduced. The study of the partitioning of Cr in the activated sludge was performed by the addition of Cr(VI) in concentrations of 2.5 and 5.0 mg L 1 . Results revealed that Cr was allocated mainly within the intercellular compartments, whereas intracel- lular and adsorbed Cr represented less than 0.1% of the Cr sludge concentrations. Cr(VI) was reduced in all compartments, the most efficiently (about 94%) within the intracellular and intercellular fractions. The extent of reduction of adsorbed Cr was 92% and 80% for 2.5 and 5.0 mg of Cr(VI) L 1 , respectively. The results of present investigation provide a new insight into the toxicity of Cr species towards activated sludge nitrification, which is of significant importance for the management of WWTPs in order to prevent them from inflows containing harmful Cr(VI) concentrations. Ó 2014 Elsevier Ltd. All rights reserved. 0045-6535/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.12.096 ⇑ Corresponding author at: Department of Environmental Sciences, Joz ˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. Tel.: +386 1 477 3560; fax: +386 1 2519 385. E-mail address: radmila.milacic@ijs.si (R. Milac ˇic ˇ). Chemosphere 105 (2014) 87–94 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere