Mineral surface charge development in mixed electrolyte solutions

Structural Properties of Aqueous Solutions at the (100) and (101) Goethite Surfaces by Molecular Dynamics Simulation

Trace metal concentrations in soils and sediments are often controlled by adsorption to iron oxides such as goethite in both natural and contaminated systems. Because of goethite's importance as an adsorbent, its interaction with aqueous solutions has been studied extensively. Nonetheless, despite the use of numerous analytical and computational tools, the properties of goethite-aqueous solution interfaces are not fully understood. In this research, we investigate the interaction of water and aqueous NaCl, MgCl₂, and BaCl₂ solutions ranging in concentration from 0.1 to 4 M, with two goethite surfaces, (100) and (101), using classical molecular dynamics simulation. In the past, the (100) surface has been studied the most because of its simplicity; however, goethite crystals in the environment exhibit other prominent surfaces like the (101) surface which may exhibit very different adsorption properties than the (100) surface. The (100) surface has three surface sites; one is an under-coordinated Fe³⁺ which interacts strongly with water affecting the interfacial water structure, another site remains deprotonated and forms a hydrogen bond to the only hydroxylated surface site. The (101) surface is terminated with five hydroxyl groups that do not interact as strongly with water as the under-coordinated Fe³⁺ ion and that form a more corrugated surface structure. As a result, the (101) goethite-solution interface exhibits less water structure, weaker electric double layer oscillations, and more inner-sphere ion adsorption, especially for Cl^- and Ba²⁺ ions. The fundamental differences in interfacial properties for these surfaces suggest that the adsorption properties of one goethite surface cannot be averaged to represent goethite interfaces present in soils and sediments.

The pH dependent surface charging and points of zero charge. VII. Update

The pristine points of zero charge (PZC) and isoelectric points (IEP) of metal oxides and IEP of other materials from the recent literature, and a few older results (overlooked in previous searches) are summarized. This study is an update of the previous compilations by the same author [Surface Charging and Points of Zero Charge, CRC, Boca Raton, 2009; J. Colloid Interface Sci. 337 (2009) 439; 353 (2011) 1; 426 (2014) 209]. The field has been very active, but most PZC and IEP are reported for materials, which are very well-documented already (silica, alumina, titania, iron oxides). IEP of (nominally) Gd₂O₃, NaTaO₃, and SrTiO₃ have been reported in the recent literature. Their IEP were not reported in older studies.

Isoelectric points and points of zero charge of metal (hydr)oxides: 50years after Parks' review

The pH-dependent surface charging of metal (hydr)oxides is reviewed on the occasion of the 50th anniversary of the publication by G.A. Parks: "Isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems" in Chemical Reviews. The point of zero charge (PZC) and isoelectric point (IEP) became standard parameters to characterize metal oxides in aqueous dispersions, and they define adsorption (surface excess) of ions, stability against coagulation, rheological properties of dispersions, etc. They are commonly used in many branches of science including mineral processing, soil science, materials science, geochemistry, environmental engineering, and corrosion science. Parks established standard procedures and experimental conditions which are required to obtain reliable and reproducible values of PZC and IEP. The field is very active, and the number of related papers exceeds 300 a year, and the standards established by Parks remain still valid. Relevant experimental techniques improved over the years, especially the measurements of electrophoretic mobility became easier and more reliable, are the numerical values of PZC and IEP compiled by Parks were confirmed by contemporary publications with a few exceptions. The present paper is an up-to-date compilation of the values of PZC and IEP of metal oxides. Unlike in former reviews by the same author, which were more comprehensive, only limited number of selected results are presented and discussed here. On top of the results obtained by means of classical methods (titration and electrokinetic methods), new methods and correlations found over the recent 50years are presented.

Kinetics and Mechanisms of Ciprofloxacin Oxidation on Hematite Surfaces

Adsorption of antibiotics at mineral surfaces has been extensively studied over the past 20 years, yet much remains to be learned on their interfacial properties and transformation mechanisms. In this study, interactions of Ciprofloxacin (CIP), a fluoroquinolone antibiotic with two sets of synthetic nanosized hematite particles, with relatively smooth (H10, 10-20 nm in diameter) and roughened (H80, 80-90 nm in diameter) surfaces, were studied by means of liquid chromatography (LC), mass spectrometry (MS), and spectroscopy (vibration and X-ray photoelectron). Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy provides evidence for inner-sphere bidentate complex formation of CIP at hematite surfaces in 0.01 M NaCl, irrespective of pH and particle size. ATR-FTIR spectroscopy also revealed that the sorbed mother CIP molecule decayed to other surface species over a period of at least 65 h. This was supported by the detection of three daughter products in the aqueous phase by LC/MS. The appearance of NH3(+) groups during the course of these experiments, revealed by cryogenic XPS, provides further evidence that CIP oxidation proceeds through an opening of piperazine ring via N-dealkylation. Additional in vacuo FTIR experiments under temperature-programmed desorption also showed that oxidation of sorbed byproducts were effectively degraded beyond 450 °C, a result denoting considerably strong (inter)molecular bonds of both mother and daughter products. This work also showed that rougher, possibly multidomainic particles (H80) generated slower rates of CIP decomposition but occurring through more complex schemes than at smoother particle surfaces (H10). This work thus uncovered key aspects of the binding of an important antibiotic at iron oxide surfaces, and therefore provided additional constraints to our growing understanding of the fate of emerging contaminants in the environment.

The variable capacitance model: a strategy for treating contrasting charge-neutralizing capabilities of counterions at the mineral/water interface

Thermodynamic models predicting ion adsorption at mineral/water interfaces can have limitations from the simplifying assumptions that compact plane thicknesses and capacitance values are constant, and that charge densities of electrolyte counterions of different charge-to-size ratios lie at the same planes of adsorption, or split between different planes. To address these limitations a thermodynamic adsorption modeling framework was developed to account for coexisting compact planes for each type of counterion complexes formed on a single mineral surface. This framework was developed to predict charge development at lepidocrocite (γ-FeOOH) particle surfaces suspended in aqueous solutions of NaCl and NaClO4. The model incorporates properties of Cl(-), ClO4(-), and Na(+) complexes formed at the (001) and (010) faces of this mineral obtained by molecular dynamics (MD) simulations. This concept was incorporated in a thermodynamic adsorption model that predicts an overall variable compact plane capacitance in terms of a linear combination of the capacitances of ion-specific EDL structures scaled for their relative surface loadings. These capacitance values are in turn constrained by compact plane thicknesses of every Cl(-), ClO4(-), and Na(+) complex, based on their MD-derived structures and atomic densities. The model predicts experimental potential-determining (H+, OH-) data for submicrometer-sized synthetic lepidocrocite particles exhibiting both (001) and (010) faces. It also isolates electrostatic contributions from these faces. A computer code solving for this Variable Capacitance Model--VCM--is provided in the Supporting Information section of this article, and can be readily modified to predict molecular-level details of any other mineral/water interface systems using this methodology.