Emergent Behavior at the Calcite–Water Interface during Reactive Transport in a Simple Microfluidic Channel

Abstract

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.

Non-equilibrium Composition of Mixed Metal Hydroxide Membranes Grown in Flow Systems

“…To explain the observed gradient, we explore the established fact that metal cations in a crystal lattice can be replaced by ions in solution . This cation-exchange mechanism has been observed for the replacement of Ca 2+ in CaCO 3 precipitates by Fe 2+ , Pb 2+ , or Mn 2+ − and can be shape-preserving for both nanoparticles and macroscopic precipitates. − Furthermore, Ni enrichment of minerals has been regularly observed in nature and has been attributed to several factors relating to solubility and the availability of low-energy orbitals for complex formation. , We evaluate these exchange reactions for our precipitates by exposing bulk precipitates of pure Mg­(OH) 2 and Ni­(OH) 2 to 1.0 M solutions of NiCl 2 and MgCl 2 , respectively. XRD and infrared analyses (Figure S6 of the Supporting Information) reveal that Mg­(OH) 2 precipitates exposed to Ni 2+ solutions indeed convert to Ni­(OH) 2 , whereas the Ni­(OH) 2 material appears unaltered after exposure to Mg 2+ .…”

Dynamic Inhibition of Calcite Dissolution in Flowing Acidic Pb2+ Solutions

“…Here, θ([Pb 2+ ]) is the fractional Pb coverage with respect to maximum saturation coverage (θ max , previously determined to be ∼2−4 Pb per calcite surface unit-cell area = ∼20 Å 2 ), 22 and R 0 is the calcite dissolution rate at [Pb 2+ ] = 0 (Figure S7). The surface coverage can be expressed using a Frumkin equation:…”

“…Recent observations reported complex reactivity patterns of single-crystal calcite (104) cleavage surfaces under far-from-equilibrium conditions in a microfluidic cell . The experiments were performed in acidic solutions (pH = 3.5) with ∼5 × 10 –3 M dissolved Pb 2+ in which the initial undersaturation of calcite led to calcite dissolution and the surface incorporation of 2–4 monolayers (MLs) of Pb 2+ where 1 ML corresponds to the theoretical coverage of adsorbate cations that substitute for all Ca 2+ atoms in one calcite unit-cell layer (∼3 Å thick).…”

“…Ex situ observations with state-of-the-art micro/nanoanalytical tools showed that, at the early stages of the reaction, this confined geometry led to the formation of microscale roughness and spatially variable patterns in terms of the amount of dissolved calcite as well as the coverage of Pb 2+ incorporated into the calcite surface. These patterns were interpreted as a manifestation of emergent behavior that occurs under the combined influence of interfacial reactivity (i.e., calcite dissolution and Pb 2+ sorption) and advective–diffusive reactive transport within the flow channels . However, these experimental results were obtained by ex situ observations after extended reaction times, and therefore, little understanding was achieved concerning how these reaction patterns evolved with time.…”

Virtual Special Issue of ACS Earth and Space Chemistry in Honor of Prof. Michael F. Hochella, Jr.

“…In addition, the three Hochella Symposium organizers were invited by Prof. Joel Blum, Editor-in-Chief of ACS Earth and Space Chemistry , to organize and serve as Guest Editors of a virtual special issue of this journal in honor of Mike Hochella. A total of 20 peer-reviewed manuscripts were accepted for publication in this virtual special issue (), with topics ranging from mineral/water interface chemistry and mineral/organic matter nanoparticles to organic matter/soil chemistry and microbial geochemistry studies. − These papers highlight the importance of nanogeoscience, mineral/aqueous solution interface chemistry, biogeochemistry, and organic matter/soil chemistry in understanding the Earth system.…”