Ion Adsorption at Clay Mineral Surfaces: The Hofmeister Series for Hydrated Smectite Minerals

Many important properties of clay minerals are defined by the species of charge-balancing cation. Phenomena such as clay swelling and cation exchange depend on the cation species present, and understanding how the cations bind with the mineral surface at a fundamental level is important. In the present study the binding affinities of several different charge-balancing cations with the basal surface of the smectite mineral, montmorillonite, have been calculated using molecular dynamics in conjunction with the well-tempered metadynamics algorithm. The results follow a Hofmeister series of preferred ion adsorption to the smectite basal surfaces of the form: Embedded Image The results also revealed the energetically favorable position of the ions above the clay basal surfaces. Key features of the free-energy profiles are illustrated by Boltzmann population inversions and analyses of the water structures surrounding the ion and clay surface. The results show that weakly hydrated cations (K+ and Cs+) preferentially form inner-sphere surface complexes (ISSC) above the ditrigonal siloxane cavities of the clay, while the more strongly hydrated cations (Na+) are able to form ISSCs above the basal O atoms of the clay surface. The strongly hydrated cations (Na+, Ca2+, and Ba2+), however, preferentially form outer-sphere surface complexes. The results provide insight into the adsorption mechanisms of several ionic species on montmorillonite and are relevant to many phenomena thought to be affected by cation exchange, such as nuclear waste disposal, herbicide/pesticide–soil interactions, and enhanced oil recovery

Interaction of Natural Organic Matter with Layered Minerals: Recent Developments in Computational Methods at the Nanoscale

The role of mineral surfaces in the adsorption, transport, formation, and degradation of natural organic matter (NOM) in the biosphere remains an active research area owing to the difficulties in identifying proper working models of both NOM and mineral phases present in the environment. The variety of aqueous chemistries encountered in the subsurface (e.g., oxic vs. anoxic, variable pH) further complicate this field of study. Recently, the advent of nanoscale probes such as X-ray adsorption spectroscopy and surface vibrational spectroscopy applied to study such complicated interfacial systems have enabled new insight into NOM-mineral interfaces. Additionally, due to increasing capabilities in computational chemistry, it is now possible to simulate molecular processes of NOM at multiple scales, from quantum methods for electron transfer to classical methods for folding and adsorption of macroparticles. In this review, we present recent developments in interfacial properties of NOM adsorbed on mineral surfaces from a computational point of view that is informed by recent experiments

Crystal chemistry of natural layered double hydroxides. 5. Single-crystal structure refinement of hydrotalcite, [Mg6Al2(OH)16](CO3)(H2O)4

Hydrotalcite, ideally [Mg6Al2(OH)16](CO3)(H2O)4, was studied in samples from Dypingdal, Snarum, Norway (3R and 2H), Zelentsovskaya pit (2H) and Praskovie–Evgenievskaya pit (2H) (both Southern Urals, Russia), Talnakh, Siberia, Russia (3R), Khibiny, Kola, Russia (3R), and St. Lawrence, New York, USA (3R and 2H). Two polytypes, 3R and 2H (both ‘classical’), were confirmed on the basis of single-crystal and powder X-ray diffraction data. Their chemical composition was studied by electron-microprobe analysis, infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The crystal structure of hydrotalcite-3R was solved by direct methods in the space group R3m on three crystals (two data collections at 290 K and one at 120 K). The unit-cell parameters are as follows (290/290/120 K): a = 3.0728(9)/3.0626(3)/3.0617(4), c = 23.326(9)/23.313(3)/23.203(3) Å and V = 190.7(1)/189.37(4)/188.36(4) Å3 . The crystal structures were refined on the basis of 304/150/101 reflections to R1 = 0.075/0.041/ 0.038. Hydrotalcite-2H crystallises in the P63/mmc space group; unit-cell parameters for two crystals are (data collection at 290 K and 93 K): a = 3.046(1)/3.0521(9), c = 15.447(6)/15.439(4) Å, V = 124.39(8)/124.55(8) Å3 . The crystal structures were refined on the basis of 160/142 reflections to R1 = 0.077/0.059. This paper reports the first single-crystal structure data on hydrotalcite. Hydrotalcite distribution in Nature, diagnostic features, polytypism, interlayer topology and localisation of M2+–M3+ cations within metal hydroxide layers are discussed

Molecular Dynamic Simulations of Montmorillonite-Organic Interactions under Varying Salinity: An Insight into Enhanced Oil Recovery

Enhanced oil recovery is becoming commonplace in order to maximize recovery from oil fields. One of these methods, low-salinity enhanced oil recovery (EOR), has shown promise; however, the fundamental underlying chemistry requires elucidating. Here, three mechanisms proposed to account for low-salinity enhanced oil recovery in sandstone reservoirs are investigated using molecular dynamic simulations. The mechanisms probed are electric double layer expansion, multicomponent ionic exchange, and pH effects arising at clay mineral surfaces. Simulations of smectite basal planes interacting with uncharged nonpolar decane, uncharged polar decanoic acid, and charged Na decanoate model compounds are used to this end. Various salt concentrations of NaCl are modeled: 0‰, 1‰, 5‰, and 35‰ to determine the role of salinity upon the three separate mechanisms. Furthermore, the initial oil/water-wetness of the clay surface is modeled. Results show that electric double layer expansion is not able to fully explain the effects of low-salinity enhanced oil recovery. The pH surrounding a clay’s basal plane, and hence the protonation and charge of acid molecules, is determined to be one of the dominant effects driving low-salinity EOR. Further, results indicate that the presence of calcium cations can drastically alter the oil wettability of a clay mineral surface. Replacing all divalent cations with monovalent cations through multicomponent cation exchange dramatically increases the water wettability of a clay surface and will increase EOR

Analytical solution for clay plug swelling experiments

Clay swelling experiments frequently involve monitoring the one-dimensional displacement with time of an initially dry clay plug as it imbibes water from a supply at its base. This article presents a new analytical solution for interpreting such experiments based on Richards' equation for flow in a partially saturated porous medium combined with a linear empirical function relating moisture ratio with void ratio. The analytical solution is described by just two parameter groups. The first parameter group describes the swelling potential of the clay. The second parameter group describes the rate at which the swelling plug reaches equilibrium, which is controlled by permeability and capillary pressure. Application of the analytical solution is demonstrated by calibration to one-dimensional displacement data from clay swelling experiments for an illite and bentonite clay

Osmium uptake, distribution, and 187Os/188Os and 187Re/188Os compositions in Phaeophyceae macroalgae, Fucus vesiculosus: Implications for determining the 187Os/188Os composition of seawater

The osmium isotopic composition (187Os/188Os) of seawater reflects the balance of input from mantle-, continental- and anthropogenic-derived sources. This study utilizes the Phaeophyceae, Fucus vesiculosus, to analyse its Os abundance and uptake, as well as to assess if macroalgae records the Os isotope composition of the seawater in which it lives. The data demonstrates that Os is not located in one specific biological structure within macroalgae, but is found throughout the organism. Osmium uptake was measured by culturing F. vesiculosus non-fertile tips with different concentrations of Os with a known 187Os/188Os composition (∼0.16), which is significantly different from the background isotopic composition of local seawater (∼0.94). The Os abundance of cultured non-fertile tips show a positive correlation to the concentration of the Os doped seawater. Moreover, the 187Os/188Os composition of the seaweed equaled that of the culture medium, stongly confirming the possible use of macroalgae as a biological proxy for the Os isotopic composition of the seawater

Layered double hydroxide minerals as possible prebiotic information storage and transfer compounds

One of the fundamental difficulties when considering the origin of life on Earth is the identification of an emergent system that not only replicated, but also had the capacity to undergo discrete mutation in such a way that following generations might inherit and pass on the mutation. We speculate that the layered double hydroxide (LDH) minerals are plausible candidates for a proto-RNA molecule. We describe a hypothetical LDH-like system which, when intercalated with certain anions, forms crystals with a high degree of internal order giving rise to novel information storage structures in which replication fidelity is maintained, a concept we use to propose an explanation for interstratification in terephthalate LDHs. The external surfaces of these hypothetical crystals provide active sites whose structure and chemistry is dictated by the internal information content of the LDH. Depending on the LDH polytype, the opposing external surfaces of a crystal may give rise to reactive sites that are either complementary or mirror images of each other, and so may be chiral. We also examine similarities between these proposed “proto-RNA” structures and the DNA that encodes the hereditary information in life today, concluding with a hypothetical scenario wherein these proto-RNA molecules predated the putative RNA-world

Ab initio transition state searching in complex systems: Fatty acid decarboxylation in minerals

Because of the importance of mineral catalyzed decarboxylation reactions in both crude oil formation and, increasingly, biofuel production, we present a model study into the decarboxylation of the shortest fatty acid, propionic acid C2H5COOH, into an alkane and CO2 catalyzed by a pyrophillite-like, phyllosilicate clay. To identify the decarboxylation pathway, we searched for a transition state between the reactant, comprised of the clay plus interlayer fatty acid, and the product, comprised of the clay plus interlayer alkane and carbon dioxide. Using linear and quadratic synchronous transit mechanisms we searched for a transition state followed by vibrational analysis to verify the intermediate found as a transition state. We employed a periodic cell, planewave, ab initio density functional theory computation to examine total energy differences, Mulliken charges, vibrational frequencies, and the frontier orbitals of the reactants, intermediates, and products. The results show that interpretation of vibrational data, Mulliken charges and Fermi-level orbital occupancies is necessary for the classification of a transition state in this type of mixed bulk surface plus interlayer species, clay-organic system

Synthesis of organo-layered double hydroxides by an environmentally friendly co-hydration route

Environmentally attractive synthetic routes are studied for the synthesis of MgAl-layered double hydroxides (LDHs) incorporating a series of organic dicarboxylate anions. The reaction conditions are systematically explored to ascertain the effect of acid chain length and layer charge (i.e. Mg/Al ratio). The materials are characterised by powder X-ray diffraction, scanning electron microscopy, infra-red spectroscopy and thermal methods. The effect of reaction conditions on product purity and the packing arrangement of the intercalated organic molecules is reported, with impurity free organo-LDHs observed at Mg/Al = 2 and increased interlayer packing as Mg/Al approaches 1