2006 年 40 巻 1 号 p. 13-30
Chemical characteristics of deep-sea ferromanganese nodules from the Pacific and the Antarctic were studied, especially for rare earth elements (REE). The Ce anomalies of Pacific and Antarctic nodule samples vary coherently with their logarithmic Co/(Ni+Cu) ratios. The systematics strongly suggest that: (i) the Ce anomaly and log [Co/(Ni+Cu)] are similar geochemical indexes showing how effectively oxidative uptake of Ce and Co occurred in each nodule relative to non-oxidative uptake of nutrient-type metals in the respective metal groups, and (ii) there exists an initial source supplying metals common to all the types of ferromanganese nodules. It is inferred from various reasons that the common initial source is biogenic particulates delivered from overlying surface water. The efficiency of oxidative uptake of Ce and Co by ferromanganese coating on sinking biogenic particulates is dependent on their sinking velocities. The removal of REE by Fe hydroxide and Mn dioxides (δ-MnO2) from seawater is an important geochemical process. In order to investigate the incorporation of seawater REE into nodules and crusts, REE distribution coefficients, Kd(REE), between Fe hydroxide and 0.5 M NaCl solutions with NaHCO3 (0.0-12.0 mM) at 25℃ have been determined. The Kd(REE) values are strongly affected by REE(III)-carbonate complexation. REE partitioning data between Fe hydroxide and solution with similar carbonate ion concentrations to seawater can reproduce the characteristics of Kd(REE) evaluated from field data except for large positive Ce anomaly. REE speciation calculation in seawater using REE(III)-carbonate complexation constants determined in this study indicates that the main REE species is REECO3+(aq) rather than REE(CO3)2-(aq), except for heavy REE. The tetrad effects observed in log Kd(REE) values of experimental data and field data, and in REE(III)-carbonate complexation constants can be explained by the refined spin-pairing energy theory (RSPET). Their tetrad effects are expressed by the difference of Racah (E1 and E3) parameters, which decrease in the following order: REE3+(aq)>REECO3+(aq)>REE(CO3)2-(aq)>REE(OH)3・nH2O(ss). The REE adsorption experiments onto δ-MnO2 suspending in 0.5 M NaCl solutions have also been carried out. The Kd(REE) between δ-MnO2 and solutions show extremely large positive Ce anomalies compared to those between Fe hydroxide and solutions. Apparent oxidation states of Mn in δ-MnO2 precipitates with Ce(IV) (0.0-0.5 mg) by iodometric titration reveal that Ce(III) is oxidized to Ce(IV) by δ-MnO2 as an oxidizing agent. The convex tetrad effects of the series variations of log Kd(REE) between δ-MnO2 and solutions become conspicuous with increasing of pH in the range of 4.8<pH<6.8. This reflects the changing ratio of hydroxyl ion to water molecule coordinating REE(III) adsorbed on δ-MnO2 with increasing experimental pH values.
