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

Surface coatings (biofilms 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 oxide s and organic material to total observed Pb and Cd adsorption by the surfac e coating materials. Several alternative selective extraction techniques we re evaluated with respect to both selectivity and alteration of the residua l unextracted material. Pb and Cd adsorption was measured under controlled laboratory conditions (mineral salts solution with defined metal speciation , ionic strength 0.05 M, 25 degrees C and pH 6.0) before and after extracti ons to determine by difference the adsorptive properties of the extracted c omponent(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 remo ve metal oxides and organic materials. Several other extractants were evalu ated, but preliminary experiments indicated that they were not suitable for these experiments because of undesirable alterations of the residual, unex tracted material. The selected extraction methods removed target components with efficiencies between 71 and 83%, but significant amounts of metal oxi des and organic materials other than the target components were also remove d by the extractants (up to 39%). Nonlinear regression analysis of the obse rved Pb and Cd adsorption based on the assumption of additive Langmuir adso rption isotherms was used to estimate the relative contributions of each su rface coating constituent to total Pb and Cd binding of the biofilms. Adsor ption of Cd to the lake biofilms was dominated by Fe oxides, with lesser ro les attributed to adsorption by Mn and Al oxides and organic material. Adso rption of Pb was dominated by Mn oxides, with lesser roles indicated for ad sorption to Fe oxides and organic material, and the estimated contribution of Al oxides to Pb adsorption was insignificant. The fitted Pb adsorption i sotherm for Fe oxides was in excellent agreement with those obtained throug h direct experiments and reported in independent investigations. The estima ted Pb distribution between surface coating components also agreed well wit h that previously predicted by an additive adsorption model based on Pb ads orption isotherms for laboratory surrogates for Mn, Fe and Al oxides and de fined biological components, (C) 1999 Elsevier Science Ltd. All rights rese rved.