Influence of humic acid on sorption of Co(II), Sr(II), and Se(IV) on goethite

This work examined the influence of a leonardite humic acid on the sorption of Co(II), Sr(II), and Se(IV) onto goethite as a function of pH. The sorpt ion of humic acid and ions alone was first studied. The humic acid sorbs ap preciably to the goethite surface according to a reversible process, with a maximum sorption of 19 mg of total organic carbon per g of goethite reache d at lower pH values (approximate to 5). Cobalt and selenium are significan tly sorbed on goethite while strontium is only weakly sorbed. In the presen ce of humic acid, the cobalt sorption is enhanced in acidic media due to th e humic acid sorption on goethite, which modified the surface properties by making it more negative. However, it decreases in alkaline media due to th e complexation of Co(II) by soluble humic acid. The sorption of Sr(II) is s ignificantly increased in the presence of humic acid even in alkaline media . The sorption of selenite ions (SeO32-) decreases in the presence of humic acid because a competition with humate ions occurs at the surface sites of the goethite. The results presented here constitute a good set of data for modelling the effect of humic acid on the sorption of ions on goethite.