Geochemical
reactive transport processes in natural mineral–fluid
systems may produce a wide array of emergent phenomena that are difficult
to predict from basic principles and to reproduce in model systems.
Here, we present experimental results obtained from a simple microfluidic
system with which we explored the consequences of reacting the calcite
(104) cleavage surface with an acidic Pb-bearing solution (pH = 3.5,
[Pb]total = 5 mM) as a function of flow rate. This system
is relevant to passive remediation systems for Pb-rich acid mine drainage.
We observed periodic banding in the amounts of Pb sorption at flow
velocities ≥926 μm s–1, where the band
spacing was spatially correlated with the amount of calcite dissolution
and the development of micropyramidal topography on the calcite (104)
surface. The equivalent coverage of Pb deposited in these Pb-rich
bands was at least several monolayers per unit cell, yet there was
no evidence for precipitation of any secondary Pb phase implying incorporation
of Pb within the near-surface calcite lattice. We also observed spatial
variations in nucleation and growth of euhedral secondary Pb-carbonate
minerals hydrocerusite and cerussite at flow rates ≤278 μm
s–1. These findings demonstrate potential for exploiting
the rich phenomenology afforded by the interplay among transport phenomena
and chemical kinetics in experimental systems designed to yield deeper
insights into geochemical self-organization.
Calcium carbonate (CaCO3) polymorphs, calcite, aragonite,
and vaterite, serve as a major sink to retain various metal ions in
natural and engineered systems. Here, we visualize the systematic
trends in reactivities of calcite, vaterite, and aragonite to Pb2+ dissolved in acidic aqueous solutions using in situ optical
microscopy combined with ex situ scanning electron and transmission
X-ray microscopies. All three polymorphs undergo pseudomorphic replacement
by cerussite (PbCO3) but with distinct differences in the
evolution of their morphologies. The replacement of calcite and aragonite
occurs through the formation of a pseudomorphic cerussite shell (typically
5–10 μm thick) followed by a slower inward propagation
of reaction fronts through a thin solution gap (∼0.1 μm
wide) between the shell and the CaCO3 substrate. The replacement
of vaterite is characterized by the formation of a thinner cerussite
shell (≤1 μm thick) and a larger cavity between the shell
and the host mineral. These systematic differences in cerussite morphology
for different CaCO3 polymorphs are explained by the relative
dissolution and precipitation rates of the reactant and product minerals,
coupled with the role of ion transport through the cerussite shells.
We also find that the replacement of calcite by cerussite is the slowest
when all three polymorphs coexisted. Our results provide mechanistic
insights into the growth mode of cerussite on dissolving calcium carbonate
and demonstrate these CaCO3 polymorphs as promising substrate
materials for removal and recycling of Pb from acidic polluted water
and industrial effluents.
Although previous studies have demonstrated redox transformations of selenium (Se) in the presence of Fe-bearing minerals, the specific mechanism of magnetite-mediated Se electron transfer reactions are poorly understood. In this study, the redox chemistry of Se on magnetite is investigated over an environmentally relevant range of Eh and pH conditions (+0.85 to-1.0 V vs. Ag/AgCl; pH 4.0 to 9.5). Se redox peaks are found via cyclic voltammetry (CV) experiments at pH conditions of 4.0 to 8.0. A broad reduction peak centered at-0.5 V represents a multi-electron transfer process involving the transformation of selenite to Se(0) and Se(-II) and the comproportionation reaction between Se(-II) and Se(IV). Upon anodic scans, the oxidation peak centered at-0.25 V is observed and is attributed to the oxidation of Se(-II) to higher oxidation states. Deposited Se(0) may be oxidized at +0.2 V when pH is below 7.0. Over a pH range of 4.0 to 8.0, the pH dependence of peak potentials is less pronounced than predicted from equilibrium redox potentials. This is attributed to pH gradients in the microporous media of the cavity where the rate of proton consumption by the selenite reduction is faster relative to mass transfer from the solution. In chronoamperometry measurements at potentials ≥-0.6 V, the current-time transients show good linearity between the current and time in a log-log scale. In contrast, deviation from the linear trend is observed at more negative potentials. Such a trend is indicative of Se(0) nucleation and growth on the magnetite surface, which can be theoretically explained by the progressive nucleation model. XPS analysis reveals the dominance of elemental selenium at potentials ≤-0.5 V, in good agreement with the peak assignment on the cyclic voltammograms and the nucleation kinetic results.
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