This study focuses on the removal mechanisms effective in the sequestration of Pb(II) and Hg(II) on nonviable activated sludge biosolids at batch biosorption tests. The raw biosolids were supplied from a nearby municipal wastewater treatment plant activated sludge aeration unit. The biosolids were conditioned by residing the dried biosolids in NaOH solution for 15 min and subsequently autoclaving at 18 psi at 121 degrees C for 30 min. IR spectral analysis of the conditioned biosolids disclosed that significant binding sites in biosorption were carboxyl, amine and phosphoryl groups. The functional groups on the conditioned biosolids were then stabilized by chemical methods and thus contribution of each functional group to metal ion sequestration in biosorption was disclosed. The effective binding mechanism in sequestration was further analyzed by metal ion fractionation and leaching tests. The results demonstrated that ion exchange was the most effective removal mechanism contributing to sequestration of metal ions on the biosolids. Desorption tests demonstrated that hydrochloric acid is a powerful agent to leach the sequestered metal ions from the biosolids. Regenerations carried out with NaOH solution did not result in any remarkable capacity decrease in the subsequent biosorption cycles. (C) 2008 Elsevier B.V. All rights reserved.