Supervisors info:
Στέφανος Κίλιας, Καθηγητής, Γεωλογίας και Γεωπεριβάλλοντος, ΕΚΠΑ
Αθανάσιος Γκοντελίτσας, Αναπληρωτής Καθηγητής, Γεωλογίας και Γεωπεριβάλλοντος, ΕΚΠΑ
Ιωάννης Μήτσης, Επίκουρος Καθηγητής, Γεωλογίας και Γεωπεριβάλλοντος, ΕΚΠΑ
Original Title:
Surface area and porosity determination of natural todorokite-rich manganese (Mn) oxides, Cape Vani, Milos island, Greece. Implications for potential industrial applications.
Translated title:
Surface area and porosity determination of natural todorokite-rich manganese (Mn) oxides, Cape Vani, Milos island, Greece. Implications for potential industrial applications.
Summary:
Natural todorokite-rich Mn oxide ore samples derived from the Cape Vani Mn oxide
deposit, Milos Island, Greece, were studied by means of N2 BET specific surface area
(SSA), and pore volume measurements. Cape Vani Mn oxide ores occur in a Pliocene-
Pleistocene intravolcanic marine basin, they consist primarily of nanocrystalline and
crystal defective todorokite, and δ-MnO2, hollandite-cryptomelane-coronaditemanjiroite
and pyrolusite that cement volcaniclastic sandstone, and are suggested to
be derived from biogenic Mn oxide precursors . Todorokite which possesses many
potential technological and industrial applications, has been identified in Cape Vani
employing primarily high-resolution transmission electron microscopy (HR-TEM) ;
moreover, the Milos todorokite has identical nanoparticle morphology to
experimentally produced todorokite from birnessite (Atkins et al., 2014) under
conditions analogous to marine diagenetic and hydrothermal settings. The
determined SSA and porosity were compared to available relevant data from the
published literature, i.e. natural and synthetic todorokite in order to evaluate its
industrial and/or environmental application prospects.
The measured N2 BET surface areas of the sandstone-hosted Cape Vani
material range from 10.7 to 53.1 m2/g, and correspond largely to Mn oxides and
primarily todorokite. The measured pore size that ranges 87.78 and 168.29 Å (8.7-16.8
nm) may suggest a tendency to mesoporosity (containing pores >20Å), and even for
microporosity with rather complicated (the so called "bottleneck") type of pores.
Generally, the SSA of the investigated todorokite-rich material is moderate, compared
to other Mn oxide and oxyhydroxide sorbents in natural systems (≤360m2/g), and
deep-sea manganese nodules (102-130 m2/g). However, the obtained SSA values fall
within the range of 19-59 m2/g reported in literature for natural and synthetic
todorokites, as well as synthetic abiotic δ-MnO2. Moreover, the determined SSA
maximum value (53.1 m2/g) is very close to measured N2 BET surface areas of
experimentally produced nanocrystalline todorokite from birnessite (65.82m2/g).
These results have important implications. Considering that birnessite has not
been detected in the analyses, the Milos nanocrystalline todorokite may represent a
time series product of birnessite transformation during diagenesis in a marine
hydrothermal environment, in concurrence with the geological setting of the Cape
Vani Mn ores. In cases of natural marine ore-grade Mn oxides precipitates containing
intermixed other tunnel-structured Mn-oxides (hollandite-cryptomelane-coronaditemanjiroite,
pyrolusite, ramsdellite) and predominant todorokite, like Cape Vani, the
overall surface area of the rock is controlled by the predominant Mn oxide phase and
may be used as a proxy for modal mineralogical composition. Mn oxide ore material
from Milos is not characterized by very large SSA and therefore it probably does not
hold particularly attractive sorptive properties compared to other Mn oxide and
oxyhydroxide sorbents in natural systems. However, the combined: (1) moderate
specific areas of up to 53.1 m2/g; (2) presence of abundant nanocrystalline todorokite;
(3) total porosity that may include meso and micropores; and, (4) presence of biogenic
3
Mn oxides in the Milos material that may suggest strong sorbent quality for a variety
of toxic trace metal cations, provide a scientific basis for further research, e.g. using Xray
absorption spectroscopy, in order to bringing new insights into the molecular-scale
structure and reactivity of the Milos Mn oxide minerals for more promising results.
This new knowledge may have important implications for the remediation of trace
metal contamination.
Further research is needed to fully characterize the trace metal adsorption and other
physical and chemical properties of biologically oxidized Mn in Milos and to establish
the importance of biologically oxidized Mn in natural aquatic systems, using Cape Vani
as a natural laboratory.
Keywords:
natural manganese oxides, todorokite, Cape Vani, Milos island, Greece
