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Understanding abiotic ferrihydrite re-mineralization by ferrous ions
Raghav, M.; Sáez, A. E. & Ela, W. P.
Abstract
In this work, we study the abiotic re-mineralization
of ferrihydrite under reducing conditions, obtained
by adding zero-valent iron (ZVI) to a suspension of ferrihydrite
particles. Under similar conditions, the system
(ferrihydrite and ZVI) proceeded along two different
transformation pathways differentiated by whether a magnetic
stirrer or an overhead stirrer was used for mixing.
X-ray diffraction characterization of the solid products
showed that magnetite was the sole product of ferrihydrite
transformation when a magnetic stirrer was used, whereas
both goethite and magnetite were formed when an overhead
stirrer was used. The system also behaved differently
in terms of transformation kinetics and amount of magnetite
formed. The quantification of magnetite generated was
performed using a procedure developed in this study. The
role of four mechanisms was investigated to explain these
observed differences, namely—(1) presence/absence of
high local Fe2+ concentrations, (2) mechanical abrasion,
(3) presence/absence of a magnetic field, and (4) presence/
absence of a crystalline ZVI surface. Ferrous ions are
expected to be concentrated near the magnetic bead on the
magnetic stirrer as opposed to a more dispersed distribution
with the overhead stirrer. This mechanistic study concluded
that the presence of high local Fe2+ concentrations in the
system leads to magnetite formation and the absence of the
same leads to mixed goethite/magnetite or magnetite-free
systems. These findings have significant implications for
the mobilization of arsenic from iron (III) hydroxides as the
conditions move from oxidizing to reducing, such as often
occurs in engineered landfills and natural carbon-rich
sediments.
Keywords
Arsenic; Iron compounds; Phase transformation; Water treatment; Landfills
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