Distribution coefficients of REE between Fe oxyhydroxide precipitates and NaCl solutions affected by REE-carbonate complexation.

Distribution coefficients of REE between Fe oxyhydroxide precipitates and NaCI solutions doped with NaHCO3 have been determined in pH = 8.1-8.6 and at room temperature and pressure.The coefficient is defined as Kd(REE: ppt./sol.),where REE designates each lanthanide (Ln), Y or Sc.The NaHC0.3 con centration was changed in the range of (0-12) x 10-3 M under a constant NaCI concentration (0.45 M).Kd(Sc) rapidly approaches Kd(Lu) as [NaHC031 increases.Kd(Y) is lower than Kd(Ho) even in solutions with high [NaHC03].The convex tetrad effect in logKd(Ln) becomes less conspicuous with increasing [NaHC031.We proposed a method to determine REE-carbonate complexation constants from observed variations of Kd(REE) with increasing [C032-,aq].In solutions with [NaHC031 > 1 x 10-2 M, the domi nant dissolved REE species are REE(C03)2-(aq) except for La.We have tentatively determined the stability constants of B2 for REE(C03)2 (aq) from our preliminary data set, although 61 for REECO3+(aq) could not be estimated.The series variation of 10962 are compatible with literature values.On the basis of the refined spin-pairing energy theory (RSPET), we have analyzed the series variations of 10962 and logKd(Ln) values with and without Ln(III)-carbonate complexation effect.Racah E1 parameter is approximately the same between Ln(OH)3•nH2O as the precipitate and Ln(C03)2(aq), whereas Racah E3 parameter of Ln(C03)2 (aq) is only slightly larger than that of Ln(OH)3•nH2O.This is the reason that the convex tetrad effect of logKd(Ln) diminishes as Ln(III)-carbonate complexation proceeds.Our experimental logKd(Ln) values and apparent logKd(Ln) ones from marine Mn-Fe deposit/seawater pairs suggest that reported log(B1/B2) val ues for light Ln are slightly smaller than those ought to be.