ABSTRACT

The series variations of logarithms of apparent distribution coefficients for rare earth elements (REE), log K_d(REE), between Fe-Mn deposit and deep water have been examined theoretically based on the experimental log K_d(REE) between Fe oxyhydroxide precipitate and NaCl solution doped with NaHCO₃ (Ohta and Kawabe, 2000). The experimental log K_d(REE) values are strongly affected by REE(III)-carbonate complexation. Those experimental ones in the system with the carbonate ion concentration similar to seawater reproduce the characteristics of log K_d(REE) evaluated from field data except for large positive Ce anomaly. REE speciation calculation in seawater by using our REE(III)-carbonate complexation constants indicates that the main REE species is REECO₃⁺_(aq) rather than REE(CO₃)₂^-_(aq), except for heavy REE. This is different from the result based on previous literature data for REE(III)-carbonate complexation constants. Series variations of log {m(REE(OH)₃·nH₂O)/[REE(CO₃)₂^-, _aq]} and log {m(REE(OH)₃·nH₂O)/[REECO₃⁺, _aq]} evaluated from field data have been compared with those from our experimental data. We have confirmed that our data of carbonate complexation constants are better to explain experimental and natural systems simultaneously than the previous literature data. The refined spin-pairing energy theory (RSPET) can explain the tetrad effects observed in experimental log K_d(REE) and REE(III)-carbonate complexation constants: Racah (E¹ and E³) parameters decrease in the order that REE³⁺_{(aq, octahydrate)} > REECO₃⁺_(aq) > REE(CO₃)₂^-_(aq) ≥ REE(OH)₃·nH₂O_(ss). This relationship is also compatible with field data. The tetrad effects observed in log K_d(REE) between marine Fe-Mn deposit and seawater and in REE(III)-carbonate complexation constants can be explained by the systematic differences in Racah parameters among the REE(III) species.