Pb scavenging from a freshwater lake by Mn oxides in heterogeneous surface coating materials

Selective extraction techniques were used to assay the importance of specific solid phases in Pb binding by heterogeneous surface coating materials (biofilms) in Cayuga Lake, NY. Hydroxylamine hydrochloride (NH2OH . HCl) was used to extract easily reducible Mn oxides, and sodium dithionite (Na2S2O4) was used to extract Mn and Fe oxides in two sets of biofilm samples retrieved from the lake. Pb remaining after extraction was removed by extraction with 10% HNO3, determined by analysis of Pb-208 using a sector field mass spectrometer with an inductively coupled plasma ion source (ICP-MS), and compared to the total extractable Pb. The results indicate that the greatest contribution to total Pb binding to the heterogeneous surface coating materials was from Mn oxides. Pb adsorption capacity of Mn oxides exceeded that of Fe oxides on a molar basis by approximately an order of magnitude. The high reactivity observed for natural Mn oxides indicates that they are biogenic in origin, consistent with expectations based on the relative biotic and abiotic rates of Mn(H) oxidation under circumneutral conditions. Collectively, these results confirm expectations based on prior observations of adsorption of added Pb by Cayuga Lake biofilms before and after selective extraction, and also confirm predictions for Pb phase association in the lake based on the behavior of laboratory surrogates for adsorptive surfaces. (C) 2003 Elsevier Science Ltd. All rights reserved.