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<br />., <br /> <br />2018 <br /> <br />. <br /> <br />A rnnge of Fe concentrations was tested to detennine the impact of the adsorption when large <br /> <br />concentrations (700 ugJL) of dissolved Fe were present and when small concentrations (25 ugJL) of dissolved Fe <br /> <br />were present. The order for adsorption of dissolved metals to colloidal ferrihydrite was: <br /> <br />Cd < Zn < Cu < Pb. <br /> <br />For samples with large concentrations of dissolved Fe. the results indicated that less than two percent of <br /> <br />the available Cd became bound to the ferrihydrite surfaces. About one percent was contributed from the Cd+2 <br /> <br />species and about 1 percent was contributed from the CdC03 species. For samples with small concentrations of <br /> <br />dissolved Fe. the impact was negligible. For samples with large concentrations of dissolved Fe, about twelve <br /> <br />percent of the available Zn became bound to the ferrihydrite surfaces. About five percent was contributed from the <br /> <br />Zn+2 species and about seven percent was contributed from the Znco3 and other complexed species. For samples <br /> <br />with small concentrations of dissolved Fe. the impact was negligible. Cu and Pb speciation was more sensitive to <br /> <br />. the addition of the adsorption model. For samples with large concentrations of dissolved Fe, the model predicted as <br /> <br />much as 20 percent of the dissolved Cu and 75 percent of the dissolved Pb was sorbed. For samples with small <br /> <br />concentrations of dissolved Fe, the model predicted about 10 percent of the Cu and about 40 percent of the <br /> <br />dissolved Pb was sorbed. The sorbed Cu was contributed primarily by Cu(OH)2 and the sorbed Pb was contributed <br /> <br />by PbC03.The results of this testing suggest that the predicted activities of aqueous species represent a worst-case <br /> <br />scenario for the dissol ved species of Cu and Pb, and Zn to a lesser extent The impact of adsorption on Cd <br /> <br />speciation is considered negligible. Since most of the Cu and Pb species that were predicted to be sorbed were <br /> <br />contributed from metal complexes. which are generally considered less toxic. the net effect of sorption on <br /> <br />bioavailability and toxicity probably is small. <br /> <br />. <br /> <br />17 <br />