HR: 11:05h
AN: B12A-04    [Abstracts]
TI: Sorption of Yttrium and the Rare Earth Elements on Non-Living Macroalgal Tissue
AU: Schijf, J
EM: schijf@cbl.umces.edu
AF: University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, P.O. Box 38, Solomons, MD 20688, United States
AU: * Straka, A M
EM: 82STRAKA@cua.edu
AF: University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, P.O. Box 38, Solomons, MD 20688, United States
AU: * Straka, A M
EM: 82STRAKA@cua.edu
AF: Catholic University of America, Chemistry Department, 620 Michigan Avenue NE, Washington, DC 20064, United States
AB: We have investigated sorption of yttrium and the rare earth elements (YREEs) on tissue of the green macroalga Ulva lactuca, commonly known as sea lettuce. Due to its nearly worldwide distribution in coastal waters, very simple morphology, and prodigious capacity for trace metal uptake from seawater, members of the Ulva genus serve as a basic but representative model of marine organic substrates in this type of study. In order to exclude active biological uptake effects, allowing us to focus on passive chemical mechanisms, we performed our initial experiments with sea lettuce Certified Reference Material consisting of a dehydrated, powdered tissue homogenate. A small quantity of this powder was suspended in NaCl solutions containing all YREEs, except Pm, at pH 3 and T = 25°C. The extent of YREE sorption was determined as a function of pH at constant temperature by titrating the solution with dilute NaOH and measuring the YREE concentrations of 0.2-μm filtered aliquots with an ICP-MS at regular time intervals after each pH adjustment. In NaCl solutions with an ionic strength approaching that of seawater, distribution coefficients, which quantify the proportion of sorbed and dissolved metal concentrations, are a highly linear function of pH in the range 3-8. The slope of the line suggests a sorption mechanism that involves ion exchange with both H⁺ and Na⁺ on surface functional groups. The shape of solution YREE patterns indicates that these functional groups are probably carboxylates at low and intermediate pH, but that other groups may contribute at high pH. The identification of carboxylate functional groups appears to be confirmed by preliminary results from EXAFS spectroscopic analyses of individual REE sorbed on the surface of Ulva lactuca tissue under similar conditions, conducted at the ANL Advanced Photon Source. In dilute NaCl solutions the distribution coefficient is largely independent of pH. We believe that prolonged exposure of the tissue to a low ionic strength solution may modify the chemical structure of the cell wall and make it permeable to organic ligands that otherwise sequester the YREEs in the cell interior. Chemical extraction of filtered solutions from the low ionic strength experiment with silica-bonded C₁₈, showing that a substantial fraction of dissolved YREEs is distinctly hydrophobic, seems to support this hypothesis. Additional experiments to clarify these observations, including acid-base titrations of the Ulva lactuca tissue to assess the number of different functional groups and their surface densities, are currently ongoing.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0461 Metals
SC: Biogeosciences [B]
MN: 2007 Fall Meeting