The objective of this study was to investigate the influence of humic acid (HA) and salinity on adsorption of Hg on the amorphous and crystalline of iron and manganese hydroxides. The results show that the adsorption of Hg2+ on Fe and Mn hydroxides was inhibited in marine system due to the formation of stable, nonsorbing aqueous HgCl2 complexes in solution. Moreover, Cl- inhibited the Hg2+ adsorption more severely on amorphous than crystalline hydroxides. The addition of HA inhibited Hg2+ adsorption on Fe and Mn hydroxides in freshwater system might be attributed to the competition between Hg2+ and HA on adsorption to Fe and Mn hydroxides. In contrast, the addition of HA promoted Hg2+ adsorption on Fe and Mn hydroxides in the marine system, which might be due to the addition of humic acid resulted in the reaction between Cl- and HA, and therefore the reducing of Cl- promoted more Hg2+ on Fe and Mn hydroxides. In addition, the influence of HA on Hg2+ adsorption on Fe and Mn hydroxides are more visible for crystalline than amorphous hydroxides. (C) 2012 Elsevier B.V. All rights reserved.