Adsorption of free lead (Pb2+) by pedogenic oxides, ferrihydrite, and leafcompost

Iron oxides and organic matter have a high capacity to adsorb ph and concom itantly maintain a low free Pb2+ activity in solution. It is, therefore, im portant to assess the adsorption capacity of naturally occurring materials and evaluate their potential to reduce toxicity. The free Pb2+ activity was measured in the solution in equilibrium with ferrihydrite (a synthetic Fe oxide), two field-collected pedogenic amorphous oxides, and leaf compost. T he experiment used a factorial design varying total ph loading and solution pH. The results show that ferrihydrite was more efficient in lowering Pb a queous concentration than the two pedogenic oxides. Furthermore, of the two pedogenic oxides examined, a higher Pb2+ activity was maintained in soluti on at equilibrium with the most crystalline phase, which also has a lower s urface area relative to the other adsorbents. Leaf compost maintained a sig nificantly higher free Pb2+ activity, relative to the various oxides. The e xperimental data could be fitted to a semi-mechanistic model predicting fre e Pb2+ activity as a function of total Ph loadings and pH, with R-2 varying from 0.77 to 0.92.