ADSORPTION OF Pb AND Cd ONTO METAL OXIDES AND ORGANIC MATERIAL IN NATURAL SURFACE COATINGS AS DETERMINED BY SELECTIVE EXTRACTIONS: NEW EVIDENCE FOR THE IMPORTANCE OF Mn AND Fe OXIDES

ÐSurface coatings (bio®lms and associated minerals) were collected on glass slides in the oxic surface waters of Cayuga Lake (New York State, U.S.A.) and were used to evaluate the relative contributions of Fe, Mn and Al oxides and organic material to total observed Pb and Cd adsorption by the surface coating materials. Several alternative selective extraction techniques were evaluated with respect to both selectivity and alteration of the residual unextracted material. Pb and Cd adsorption was measured under controlled laboratory conditions (mineral salts solution with de®ned metal speciation, ionic strength 0.05 M, 258C and pH 6.0) before and after extractions to determine by di€erence the adsorptive properties of the extracted component(s). Hydroxylamine hydrochloride (0.01 M NH2OH HCl+0.01 M HNO3) was used to selectively remove Mn oxides, sodium dithionite (0.3 M Na2S2O4) was used to remove Mn and Fe oxides, and 10% oxalic acid was used to remove metal oxides and organic materials. Several other extractants were evaluated, but preliminary experiments indicated that they were not suitable for these experiments because of undesirable alterations of the residual, unextracted material. The selected extraction methods removed target components with eciencies between 71 and 83%, but signi®cant amounts of metal oxides and organic materials other than the target components were also removed by the extractants (up to 39%). Nonlinear regression analysis of the observed Pb and Cd adsorption based on the assumption of additive Langmuir adsorption isotherms was used to estimate the relative contributions of each surface coating constituent to total Pb and Cd binding of the bio®lms. Adsorption of Cd to the lake bio®lms was dominated by Fe oxides, with lesser roles attributed to adsorption by Mn and Al oxides and organic material. Adsorption of Pb was dominated by Mn oxides, with lesser roles indicated for adsorption to Fe oxides and organic material, and the estimated contribution of Al oxides to Pb adsorption was insigni®cant. The ®tted Pb adsorption isotherm for Fe oxides was in excellent agreement with those obtained through direct experiments and reported in independent investigations. The estimated Pb distribution between surface coating components also agreed well with that previously predicted by an additive adsorption model based on Pb adsorption isotherms for laboratory surrogates for Mn, Fe and Al oxides and de®ned biological components. Key wordsÐselective extraction, adsorption, lead, cadmium, iron oxide, manganese oxide