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 K_d(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). K_d(Sc) rapidly approaches K_d(Lu) as [NaHCO₃] increases. K_d(Y) is lower than K_d(Ho) even in solutions with high [NaHCO₃]. The convex tetrad effect in logK_d(Ln) becomes less conspicuous with increasing [NaHCO₃]. We proposed a method to determine REE-carbonate complexation constants from observed variations of K_d(REE) with increasing [CO₃^2-, 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 logK_d(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 logK_d(Ln) diminishes as Ln(III)-carbonate complexation proceeds. Our experimental logK_d(Ln) values and apparent logK_d(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.