REE partitioning between Fe-Mn oxyhydroxide precipitates and weakly acid NaCl solutions: Convex tetrad effect and fractionation of Y and Sc from heavy lanthanides

抄録

Distribution coefficients of K_d (REE: ppt./sol.) with REE = lanthanides (Ln), Y, and Sc between Fe-Mn oxyhydroxide precipitates and NaCl solutions have been determined in pH = 5.4-6.4 at room temperature and atmospheric pressure. The logK_d(Ln) values show convex tetrad effect variations across the series and positive Ce anomalies. K_d(Y) values are significantly lower than those for Ho and Er, whereas K_d(Sc) values are higher than those for Lu. Behaviors of Y³⁺ and Sc³⁺ are distinct from heavy Ln³⁺ when REE coprecipitate with Fe-Mn oxyhydroxides in weakly acid solutions. The observed convex tetrad effect variations of logK_d(Ln) are commonly modified from theoretically expected ones in light Ln and heavier Ln. The variations of logK_d(Ln) across the series are related to {ΔG^o_f(Ln³⁺, aq)-ΔG^o_f(Ln(OH)₃.nH₂O) } for the following reactions between Ln³⁺(aq) and Ln(OH)₃.nH₂O as precipitate: Ln(OH)₃.nH₂O(ss) + 3H⁺(aq) = Ln³⁺(aq) + (3 + n)H₂O(l). Hence, the variations of logK_d(Ln) reflect (i) the tetrad effect originating from differences in Racah E¹ and E³ parameters for 4f electron repulsion in Ln³⁺ ions between the two Ln(III) series, and (ii) the changes of coordination states of Ln³⁺ ions in the respective Ln(III) complex series. The structural changes of the first-coordination spheres of Ln³⁺ ions in light Ln³⁺(aq) and heavy Ln(OH)₃.nH₂O series make the series variation of logK_d(Ln) irregular. When we correct the logK_d(Ln) values for the structural changes, they become regular tetrad effect variations compatible with the refined spin-pairing energy theory. Racah parameters for 4f electron repulsion in Ln³⁺ ions of octahydrate Ln³⁺(aq) series are inferred to be larger than those of Ln(OH)₃.nH₂O.