Theoretical study of tetrad effects observed in REE distribution coefficients between marine Fe-Mn deposit and deep seawater, and in REE(III)-carbonate complexation constants

The series variations of logarithms of apparent distribution coefficients f or 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(RE E) 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 la rge positive Ce anomaly. REE speciation calculation in seawater by using ou r REE(III)-carbonate complexation constants indicates that the main REE spe cies 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 RE E(III)-carbonate complexation constants. Series variations of log {m(REE(OH )(3). nH(2)O)/[REE(CO3)(2)(-),(aq)] } and log {m(REE(OH)(3). nH(2)O)/ [REEC O3+,(aq)]} evaluated from field data have been compared with those from our experimental data. We have confirmed that our data of carbonate complexati on constants are better to explain experimental and natural systems simulta neously 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 (E-1 and E-3) parameters decrease in the order that REE(aq,octahydrate)3+ > REECO3(aq)+> REE(CO3)(2)(-)((aq)) greater than or e qual to REE(OH)(3). nH(2)O((ss)). This relationship is also compatible with field data. The tetrad effects ob served 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 d ifferences in Racah parameters among the REE(III) species.