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

Abstract

Distribution coefficients of REE between Fe oxyhydroxide precipitates and NaCl 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 NaHCO3 concentration was changed in the range of (0∼12) × 10-3 M under a constant NaCl concentration (0.45 M). Kd(Sc) rapidly approaches Kd(Lu) as [NaHCO3] increases. Kd(Y) is lower than Kd(Ho) even in solutions with high [NaHCO3]. The convex tetrad effect in logKd(Ln) becomes less conspicuous with increasing [NaHCO3]. We proposed a method to determine REE-carbonate complexation constants from observed variations of Kd(REE) with increasing [CO32-, aq]. In solutions with [NaHCO3] ≥ 1 × 10-2 M, the dominant dissolved REE species are REE(CO3)2-(aq) except for La. We have tentatively determined the stability constants of β2 for REE(CO3)2-(aq) from our preliminary data set, although β1 for REECO3+(aq) could not be estimated. The series variation of logβ2 are compatible with literature values. On the basis of the refined spin-pairing energy theory (RSPET), we have analyzed the series variations of logβ2 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(CO3)2-(aq), whereas Racah E3 parameter of Ln(CO3)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(β1/β2) values for light Ln are slightly smaller than those ought to be.