The retention behavior of cobalt(II) by nanofiltration from ammoniacal solutions used for the simultaneous absorption of SO2/NOx, was investigated. The cobalt(II) rejections in aqueous ammoniacal solutions were higher than 99% by the membrane with alkaline stability and a molecular weight cut-off of similar to 200 Da. The presence of small and mobile NH4+ in the feed solution improved the cobalt(II) retention. It was found that NH4NO3 had little effect on the permeation flux or cobalt(II) rejection while (NH4)(2)SO4 lowered both. The effects of initial cobalt(II) concentration (0.006-0.03 mol/L) in the feed on cobalt(II) rejection depended on the compositions of the solution. An increase in transmembrane pressure (0.4-0.8 MPa) led to higher cobalt(II) rejections for all solutions. Cobalt precipitates were generated during the filtration process due to the loss of chelating agent NH3 center dot H2O. The decline in permeation flux and cobalt (II) rejection could be recovered by effective cleaning. Adding NH3 center dot H2O during the filtration process could prevent the formation of cobalt precipitates so as to avoid deterioration in the membrane performance. (C) 2016 Elsevier Ltd. All rights reserved.