INVESTIGATION OF COBALT ENRICHMENT IN FERROMANGANESE CRUST: HIGH-RESOLUTION TEM OBSERVATION AND CO (II) SORPTION STUDY
Ferromanganese crusts (FMCs) are ubiquitous accumulates throughout the global ocean floor, especially on seamounts, ridges, and plateaus. They may potentially play an important role in controlling the accumulation and precipitation of trace elements such as Co, Ni and REE in seawater. X-ray PhotoElectron Emission Microscopy (X-PEEM) studies show that the main element in the FMCs is Mn which is in tetravalent state and Fe is in trivalent state. Sorption of cobalt ions from ferromanganese crust is currently of great interest for understanding the geochemical interface mechanism and also for economic considerations. It was suggested that Co is in trivalent state from X-ray absorption studies (Murray and Dillard, 1979). Uptake of Co(II) by FMCs from NW Pacific Ocean was studied as a function of pH and Co(II) concentration, using a combination of batch uptake experiments, High-resolution transmission electron microscopy (HRTEM) and X-ray energy-dispersive spectroscopy (EDS). The FMC samples are nanoporous with surface area around 500m2/g. Sorption isotherms of Co(II) at pH 5.0 and 7.0 show an initial steep slope at low Co(II) concentrations, followed by a plateau up to [Co(II)]<750 >µM, consistent with a Langmuir-type behavior. At [Co(II)]>750µM, a sharp rise in the pH 5.0 and 7.0 isotherms is interpreted as precipitation. The sorption isotherm at pH 9.0 shows a steep uptake step at [Co(II)] °Ü 250 µM, followed by an increasing linear trend up to [Co(II)] = 1mM, without any indication of a maximum, suggesting that precipitation is an important uptake mechanism at and above pH 9. Our results suggest that cobalt sorption on FMCs may play an important role in controlling the fate and transport of cobalt in aqueous system. It is proposed that Co(II) is directly associated with surfaces and interlayer positions of birnessite-like octahedral sheets in FMCs. This work is jointly supported by NSFC (No. 40473024, 40343019), Project of 10th Five Year Research and Development of International Seabed (No.PY105-2-01-04-13), and the Graduate School of University of Wisconsin_Madison.