Biological reduction of Fe(III) to Fe(II) is a key step in nitrogen oxide (NOx) removal by the integrated chemical absorption-biological reduction process. NOx removal efficiency strongly depends on the concentration of Fe(II) in the scrubbing liquid. In this study, a newly isolated strain, Enterococcus sp. FR-3, was used to reduce Fe(III) chelated with citrate to Fe(II). Strain FR-3 reduced citrate-chelated Fe(III) with an efficiency of up to 86.9% and an average reduction rate of 0.21 mM h(-1). SO42- was not inhibitory whereas NO2- and SO32- inhibited cell growth and thus affected Fe(III) reduction. Models based on the Logistic equation were used to describe the relationship between growth and Fe(III) reduction. These findings provide some useful data for Fe(III) reduction, scrubber solution regeneration and NOx removal process design. (C) 2010 Elsevier Ltd. All rights reserved.