Comparison of Arsenate and Molybdate Speciation in Hydrogenetic Ferromanganese Nodules

Abstract

Marine ferromanganese oxides contain a large amount of trace elements, such as arsenic (As) and molybdenum (Mo). However, the host phases of tetrahedral AsO43– and MoO42– oxyanions therein have not been clearly identified thus far. In this work, we explored the mineralogical components of hydrogenetic (HG) ferromanganese nodules and compared the distribution behaviors of As and Mo. The X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analyses showed that the predominant manganese and iron phases were vernadite (δ-MnO2) and ferrihydrite, respectively. According to the sequential extraction examination, both As and Mo were associated with the iron (oxyhydr)oxide phases. However, the XAS analyses indicated that As was retained by the ferrihydrite phase via double corner-shared complexation, while Mo was preferentially adsorbed on δ-MnO2 via edge-shared complexation. The immobilization of As and Mo by HG ferromanganese samples was attributed to specific chemical binding (ΔGchem) rather than Cou...