Geochemical fates and unusual distribution of arsenic in natural ferromanganese duricrust

- Unusual arsenic enrichment in manganese oxides in ferromanganese duricrust was revealed by EMPA and μ-XRF mappings.
- All the arsenic occurs as oxidized state As(V) which adsorbs on Fe/Mn oxides in bidentate binuclear bridging complexes.
- Extremely high levels of As enrichment has been found in Mn-rich hematite micro-bands.
- A conceptual model for arsenic attenuation in natural ferromanganese duricrusts has been proposed.

Preferential enrichment of arsenic in iron oxides relative to manganese oxides has been well documented. In this study, however, a distinct arsenic enrichment is revealed in natural ferromanganese duricrusts, which are commonly found in natural weathering profiles of manganese-bearing carbonate rocks. In the studied ferromanganese duricrust covering Carboniferous carbonates at Qixia Mountain in eastern China, stromotalite-like structures composed by hematite, goethite, pyrolusite and hetaerolite have been observed. Electron microprobe analysis (EMPA) mapping and synchrotron-based micro-scanning X-ray fluorescence (μ-XRF) analyses reveal that the arsenic content in manganese oxides is elevated with respect to iron oxide phases. For example, the arsenic content of pyrolusite is approximately 5 times as much as that of hematite or hetaerolite. However, the highest arsenic content (0.58 wt% As2O5) occurs in 2.75 (±0.96, ±σ) μm micro-bands of hematite ((FexMnIII1-x)2O3, 0.75 < x < 0.83). Although arsenic contents in the Mn-rich hematite micro-bands are extraordinarily high, the amount of hematite with a high Mn content is very low in the duricrust. Hence manganese oxides are suggested to be the major arsenic sink in the ferromanganese duricrust. Extended X-ray absorption fine structure spectra (EXAFS) further shows that all arsenic is present as oxidized As(V) and are bound to Fe/Mn oxides in bidentate binuclear bridging complexes with AsFe and AsMn bond distances of 3.24 Å and 3.23 Å, respectively. In addition, it is found that zinc is also more enriched in Mn oxides (besides hetaerolite) than in Fe oxides. The fine hematite crust with low contents of heavy metals could act as a protective seal to separate Mn oxides core with high Zn and As from environmental fluids. This separation could reduce the interaction between them and decrease the release of Zn and As from this ferromanganese duricrusts, which ensures long-term sequestration of heavy metals. The unique structural and mineralogical constraints on the distribution of heavy metals can provide insights into novel strategies for environmental remediation of heavy metals contaminations.

335