EXTENDED X-RAY ABSORPTION FINE STRUCTURE (EXAFS) STUDY OF REE SORPTION ON A MARINE MACROALGA

The coordination chemistry of yttrium and the nine heaviest rare earth elements (Eu–Lu) sorbed on the marine macroalga Ulva lactuca (sea lettuce) was investigated by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Advanced Photon Source electron synchrotron of Argonne National Laboratory. Single elements were sorbed from 0.5 M NaCl solution at pH ~ 6.5 on 0.3 g (dry weight) of rehydrated biomass homogenate, yielding concentrations of about 1500 ppm. Around 10–25 scans were collected for each element in fluorescence mode at the L_III edge (K edge for Y), or at the L_II edge to avoid overlap with the K edge of abundant natural Mn and Fe (~2500 ppm). Good χ(k)·k³ data were obtained out to k ~ 9.

Prior EXAFS studies of REE sorption on bacterial cells, on a fern, and on polysaccharide extracts from tea leaves, have been explained in terms of P–O–M (phosphate) and C–O–M (carboxylate) bonds (depending on element and pH), N–M (porphyrin) bonds, and S–O–M (sulfate) bonds, respectively, sometimes without providing much analytical detail. Whereas this divergence could reflect compositional differences in the cell material of these organisms, our data indicate that it may be at least partly due to the difficulty of distinguishing similar light elements in the first (N, O) and second (C, P, S) coordination shell of the metal M, as well as to a paucity of EXAFS data for suitable REE standard compounds.

Early analyses of U. lactuca have found an abundance of sulfated hexose and pentose sugars in the exopolysaccharide sheath. Our own research has shown that alkalimetric titrations of U. lactuca biomass can be modeled with three distinct functional groups, which have been tentatively identified as a carboxylic acid, a phosphate ester, and an amine, based on their pK_a values. While the amine group was neglected at the pH of our EXAFS experiments, preliminary modeling suggests that the data can be fit with various combinations of the other three groups. Our observations emphasize that models for fitting EXAFS data of REE coordination on organic matter should explore all reasonable combinations of functional groups derived from a detailed knowledge of sample composition and, even when based on high-quality spectra and validated with sorption experiments over a range of pH, must be interpreted with appropriate caution.