Marine Science Faculty Publications

Sorption of Yttrium and Rare Earth Elements by Amorphous Ferric Hydroxide:  Influence of Temperature

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The sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated between 10 and 40 °C over a range of pH (4.7−7.1) in the absence of solution complexation. Distribution coefficients, defined as iKFe = [MSi]T/([M]T[Fe3+]S), where [MSi]T is the concentration of sorbed YREEs, [M]T is the total dissolved YREE concentration, and [Fe3+]S is the concentration of precipitated iron, increased with increasing temperature over the entire investigated pH range. The observed increase in iKFe was largest for the heavy REEs, indicating that relative log iKFe values (i.e., YREE patterns) vary somewhat with temperature. The pH dependence of YREE sorption was described by a surface complexation model of the form iKFe = (Sβ1[H+]-1 + Sβ2[H+]-2)/(SK1[H+] + 1), where Sβn are stability constants for sorption of free YREE ions (M3+) and SK1 is a surface protonation constant for amorphous ferric hydroxide. The influence of temperature on the YREE surface stability constants (Sβ1 and Sβ2) was characterized by calculating molar enthalpies for M3+ sorption (ΔH10 and ΔH20) using the van 't Hoff equation. The ΔH10 values appropriate to Sβ1 range from 11.8 to 13.4 kcal/mol, whereas the ΔH20 values appropriate to Sβ2 range between 7.7 and 12.3 kcal/mol. These values are on the same order of magnitude as enthalpies of the first hydrolysis step for a variety of cations.

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Environmental Science & Technology, v. 41, issue 2, p. 541-546