Zeta potential of artificial and natural calcite in aqueous solution

Abstract

Despite the broad range of interest and applications, controls on calcite surface charge in aqueous solution, especially at conditions relevant to natural systems, remain poorly understood. The primary data source to understand calcite surface charge comprises measurements of zeta potential. Here we collate and review previous measurements of zeta potential on natural and artificial calcite and carbonate as a resource for future studies, compare and contrast the results of these studies to determine key controls on zeta potential and where uncertainties remain, and report new measurements of zeta potential relevant to natural subsurface systems. The results show that the potential determining ions (PDIs) for the carbonate mineral surface are the lattice ions Ca, Mg and CO. The zeta potential is controlled by the concentration-dependent adsorption of these ions within the Stern layer, primarily at the Outer Helmholtz Plane (OHP). Given this, the Iso-Electric Point (IEP) at which the zeta potential is zero should be expressed as pCa (or pMg). It should not be reported as pH, similar to most metal oxides. The pH does not directly control the zeta potential. Varying the pH whilst holding pCa constant yields constant zeta potential. The pH affects the zeta potential only by moderating the equilibrium pCa for a given CO partial pressure (pCO). Experimental studies that appear to yield a systematic relationship between pH and zeta potential are most likely observing the relationship between pCa and zeta potential, with pCa responding to the change in pH. New data presented here show a consistent linear relationship between equilibrium pH and equilibrium pCa or pMg irrespective of sample used or solution ionic strength. The surface charge of calcite is weakly dependent on pH, through protonation and deprotonation reactions that occur within a hydrolysis layer immediately adjacent to the mineral surface. The Point of Zero Charge (PZC) at which the surface charge is zero could be expressed as pH, but surface complexation models suggest the surface is negatively charged over the pH range 5.5-11. Several studies have suggested that SO is also a PDI for the calcite surface, but new data presented here indicate that the value of pSO may affect zeta potential only by moderating the equilibrium pCa. Natural carbonate typically yields a more negative zeta potential than synthetic calcite, most likely due to the presence of impurities including clays, organic matter, apatite, anhydrite or quartz, that yield a more negative zeta potential than pure calcite. New data presented here show that apparently identical natural carbonates display differing zeta potential behaviour, most likely due to the presence of small volumes of these impurities. It is important to ensure equilibrium, defined in terms of the concentration of PDIs, has been reached prior to taking measurements. Inconsistent values of zeta potential obtained in some studies may reflect a lack of equilibration. The data collated and reported here have broad application in ...

Zeta potential in oil-water-carbonate systems and its impact on oil recovery during controlled salinity water-flooding

“…The zeta potential of calcite depends on the concentration-dependent adsorption of the lattice ions Ca 2+ , Mg 2+ and CO 3 2− in the Stern layer (the inner part of the EDL)303132333435363738394041. At the high Ca 2+ and/or Mg 2+ concentration typical of formation brines, the zeta potential of the calcite surface is positive.…”

“…At the high Ca 2+ and/or Mg 2+ concentration typical of formation brines, the zeta potential of the calcite surface is positive. Reduction of Ca 2+ and/or Mg 2+ concentration, either selectively or by bulk dilution, can invert the polarity, yielding negative zeta potential404142434445. Addition of SO 4 2− can also yield more negative zeta potential8304046.…”

Wetting Behavior of Tight Rocks: From Core Scale to Pore Scale

“…F DL between charged surfaces can be either attraction (negative) or repulsion (positive) depending on the pH and salinity of the electrolyte medium (Al Mahrouqi et al, 2017;Alshakhs & Kovscek, 2016;Xie et al, 2014). The double-layer forces for a constant potential can be calculated using the reduced surface potentials of fluid 2/fluid 1 and rock/fluid 1 pairs as (Gregory, 1975)…”

“… F DL between charged surfaces can be either attraction (negative) or repulsion (positive) depending on the pH and salinity of the electrolyte medium (Al Mahrouqi et al, ; Alshakhs & Kovscek, ; Xie et al, ). The double‐layer forces for a constant potential can be calculated using the reduced surface potentials of fluid 2/fluid 1 and rock/fluid 1 pairs as (Gregory, ) where n b , k B , T , ψ r 1 , ψ r 2 , κ , and h are the ionic density of fluid 1, Boltzmann constant (1.38 × 10 −23 J/K), absolute temperature (293.15 K in this study), reduced surface potential for fluid 2/fluid 1 pair, reduced surface potential for rock/fluid 1 pair, the reciprocal Debye‐Huckel length, and the separation distance between rock surface and fluid 2 layer (m), respectively.…”

A Surface Complexation Model of Alkaline-SmartWater Electrokinetic Interactions in Carbonates

“…Some of these plausible mechanisms include electric double layer [4], in-situ soap generation (saponification effect) [8], and multi-ion exchange [9]. The surface charges of carbonate/brine and crude oil/brine are altered in such pore-scale processes, which affects the zeta-potential measurements used in understanding the rock wettability [10,11].…”