Mineral Dissolution under Electric Stimulation

Abstract

Although mineral dissolution and precipitation have been extensively studied, the role of electric stimulation on these processes remains unclear. We reveal the effects of subcritical electric potential (i.e., lower than the breakdown potential of water) on the bulk dissolution rates of calcite (carbonate; CaCO 3 ) using a custom-built three-electrode cell. The effects of applied potential depend on the pH, ionic strength, and temperature. For calcite, the enhancement in dissolution ratesunder isothermal conditions is explained by enhanced ion transport. Thus, at acidic to nearneutral pH (pH 4−6) wherein calcite's dissolution is mass transfer or mixed mode controlled, dissolution rates increase with increasing potential. But, under alkaline conditions (pH 10), wherein surface reactions limit calcite's dissolution, its dissolution rate is unaffected by electric potential. This suggests that subcritical applied potentials do not appear to alter the distribution of charged surface sites within the inner Helmholtz plane (IHP) of the electric double layer (EDL) at the mineral−solution interface. Rather, applied potential acts to enhance transport-controlled dissolution by enhancing the ion flux in to and out of the diffusion boundary layer (DBL). This results in a reduction in the activation energy of mineral dissolution in proportion to the applied potential, indicating a Butler−Volmer mechanism of dissolution stimulation. This mechanism is confirmed by comparison to orthoclase (KAlSi 3 O 8 ), which dissolves exclusively under surface controlwhose dissolution is unaffected by applied potential. As a result, applied potentials are only effective when the mass transfer of ions is the rate-limiting step of mineral dissolution.

Simulation Study of the Conformational Properties of Diblock Polyelectrolytes in Salt Solutions

“…In our simulation study, one observes that for both monovalent and divalent counterions, α is positive at all concentrations of the divalent salt which further implies that there is no overcharging of the polyelectrolyte in our simulations. The phenomenon of overcharging that can lead to chain re‐expansion at high salt concentrations has been observed in experiments and predicted by theory and has been reproduced in simulations where the salt‐dependent behavior of dilute flexible polyelectrolytes in solution has been studied [15j,k] . At low salt concentrations, an increase in C S induces conformational collapse of individual polyelectrolyte chains, and decreases.…”

“…This chain re‐expansion has been linked to charge reversal (or overcharging ) [15f,28] when there is charge overcompensation by condensed salt cations that will lead to negative values of α. This depends on the ion size and also to some extent on the valency of the salt [15i,k] and was found to be weak if the valency of added salt was less than or equal to 2, in comparison with a tetravalent salt [15j] . In our simulation study, the valency of salt is equal to 2, so this overcharging effect is weak and not displayed in comparison with the cases with added tetravalent salt.…”

“…The addition of salt leads to the screening of the electrostatic interactions between charged groups on the polymer backbone and the chains favor to collapse with the increase in salt concentration. In the presence of an excess of multivalent salt present in the solution, it was also observed that chains can undergo re‐expansion when the salt concentration is increased to a higher value [15f,i–k,16] . In all these simulation studies, one could make a connection between ion distributions and chain conformations and how the latter is determined by the salt concentration.…”

Calcination-free production of calcium hydroxide at sub-boiling temperatures

“…[26][27][28][29] Therefore, with a specic goal of enhancing waste utilization, resource recovery and reutilization, and circular economy, we demonstrate an approach to extract Ca-from alkaline industrial wastes, 30,31 and produce Ca(OH) 2 via a calcination-free process. While herein we focus on alkaline industrial wastes, of course, the broad contours of this process are also applicable to alkaline rock species, which although harder to solubilize unless externally stimulated, 32,33 offer the potential to offer a near-limitless source for alkaline element extraction.…”