Metal ion adsorption to complexes of humic acid and metal oxides: Deviations from the additivity rule

The adsorption of cadmium ions to a mixture of Aldrich humic acid and hemat ite is investigated. The actual adsorption to the humic acid-hematite compl ex is compared with the sum of the cadmium ion adsorptivities to each of th e isolated components. It is shown that the sum of the cadmium ion adsorpti vities is not equal to the adsorption to the complex. In general, the adsor ption of a specific metal ion to the complex can be understood and qualitat ively predicted using the adsorptivities to each of the pure components and taking into account the effect of the pH on the interaction between humic acid and iron oxide on the metal ion adsorption. Due to the interaction bet ween the negatively charged humic acid and the positively charged iron oxid e, the adsorption of metal ions on the mineral oxide in the complex will in crease as compared to that on the isolated oxide, whereas the adsorption to the humic acid will decrease as compared to that on the isolated humic aci d. As a result, the overall adsorption of a specific metal ion to the compl ex will be smaller than predicted by the additivity rule when this metal io n has a more pronounced affinity for the humic acid than for the mineral ox ide, whereas it will be larger than predicted by the additivity rule when t he metal ion has a higher affinity for the oxide than for the humic acid.