George W. Bailey

OPTICAL ABSORPTION SPECTRA OF CLAY MINERALS

A preliminary survey of electronic absorption spectra of clay minerals reveals the utility of u.v.-visible spectroscopy in the elucidation of structural, physical, and chemical properties of such systems. Spectra, which were obtained in the suspension, film, and single crystal states (where applicable), are interpreted in terms of iron-associated transitions. Microcrystalline clay minerals typically show Fe(lll) in octahedral oxo-ligand geometry whereas mica-type minerals may show a range of iron species, including octahedral Fe(III), tetrahedral Fe(III), and octahedral Fe(II). Iron affects the local site geometry and in “high iron” minerals may dictate layer geometry and subsequently the crystalline form.Résumé Un tour d’horizon préliminaire des spectres d’absorption électronique des minéraux argileux révèle l’utilité de la spectroscopic ultraviolet-visible si l’on veut élucider les propriétés structurales, physiques et chimiques de tels systèmes. Les spectres, obtenus avec des suspensions, des films ou des monocristaux (quand c’est possible) sont interprétés en terme de transitions associées au fer. Les minéraux argileux microcristallins montrent d’une façon typique le Fe (III) dans une géométrie octa-édrique d’oxo-ligands, tandis que les minéraux du type mica peuvent montrer toute une série d’espèces du fer, comprenant Fe (III) octaédrique, Fe III tétraédrique et Fe (II) octaédrique. Le fer affecte la géométrie locale du site et dans les minéraux “riches en fer” peut imposer une géométrie au feuillet et par là, imposer la forme cristalline.KurzreferatEine Voranalyse der elektronischen Absorptionsspektren von Tonmineralien erweist den Wert ultraviolett-sichtbarer Spektroskopie für die Erforschung der strukturellen, physischen und chemischen Eigenschaften derartiger Systeme. Spektren, die in den Suspensions-, Film- und Einkristallzuständen (je nach Verfügbarkeit) erzielt wurden, werden als Übergangsphasen in Verbindung mit Eisen interpretiert. Mikrokristalline Tonmineralien weisen typisch Fe(III) in oktahedralem Oxoverband auf, während glimmerartige Mineralien eine Reihe von Eisenarten wie oktahedralem Fe(III), tetrahedralem Fe(III) und oktahedralem Fe(II) enthalten mögen. Eisen übt auf die lokale Lagerstättengeometrie einen Einfluß aus und kann in eisenreichen Mineralien für die Schichtengeometrie und daher die Kristallform entscheidend sein.РезюмеПредварительное исследование спектра электронного поглощения глинистых минералов выявило ценность видимой ультрафиолетовой спектроскопии в разъяснении структурных физических и химических особенностей таких систем. Спектры полученные от суспензии, на пленке и на сростке отдельных кристаллов (где относится) расшифровывались как соединения с железом. Микрокристаллические глинистые минералы обычно включают Fe(III) в восьмигранной оксо-лигиндной конфигурации, в то время как слюдистые минералы могут состоять из ряда железистых групп, включая октаэдральный Fe(III) тетраэдральный Fe(III) и октаэдральный Fe(II). Железо влияет на конфигурацию места заложения и в минералах с высоким содержанием железа оно может влиять на конфигурацию слоев и на форму кристаллов.

Life after death: Lignin‐humic relationships reexamined

Abstract In the last decade, application of modern degradative and nondegradative analysis techniques to both lignin of living plants and humic substances of soil has demonstrated characteristic similarities in the structures of these two types of natural polymers. Recognition of the similarities resulted in a revival of an earlier hypothesis concerning the genesis of soil organic matter from the aromatic parts of wood and nonwoody plants. The hypothesis assumes functionalization and restructuring but not complete depolymerization of lignin during its biotransformation into humic and fulvic acids in the environment. The biotransformation process results in the preservation of certain structural features during the humification of dead plants. A genetic approach is useful in the analyses of structure, morphology, and chemical reactivity of humic substances.

Oxidation-reduction mechanisms in iron-bearing phyllosilicates

Abstract Reviewed in this article are the effects of structural Fe oxidation states on the physicochemical properties of smectite clay minerals. Reducing agents selected were dithionite (S 2 O 4 2− ), sulfide (S 2− ), thiosulfate (S 2 O 3 2− ), hydrazine (N 2 H 4 ), ascorbic acid (C 6 H 8 O 6 ), hydroquinone (C 6 H 6 O 2 ), and sodium oxalate (Na 2 C 2 H 2 O 4 ). Clay samples were prepared as aqueous suspensions of 2 O 4 2− > S 2− > C 6 H 8 O 6 > S 2 O 3 2− > C 6 H 6 O 2 ≈ C 2 H 2 O 4 . The heat of reaction of three of these reducing agents with the clay was measured, and decreased in the order S 2 O 4 2− > S 2 O 3 2− > S 2− . Compared to the order of reductive strength, the heats of reaction with S 2 O 3 2− and S 2− are reversed, suggesting that entropy changes are greater in the S 2− treatment. Electron spin resonance (ESR) revealed that free radicals may be responsible for the greatest levels of reductive potential, which provides an important attribute by which potential reducing agents can be screened. Measurements of rheological properties of oxidized and reduced clay suspensions indicated that structural Fe(II) increases the viscosity of clay suspensions as a result of greater attractive forces between clay particles. The type of bonding between particles has yet to be ascertained. Microbial reduction of the clay produces moderate to high levels of reduction and causes changes in physico-chemical properties similar to chemical reduction of the clay.

NON-BONDED ORGANO-MINERAL INTERACTIONS AND SORPTION OF ORGANIC COMPOUNDS ON SOIL SURFACES : A MODEL APPROACH

Abstract Molecular mechanics and molecular dynamics calculations have been performed on organo-mineral composites that model the sorption of high-molecular-weight humic polymers on mineral surfaces and the sorption of low-molecular-weight organic contaminants on both mineral and organic surfaces in soil. Muscovite mica was chosen as a mineral model; an oxidized topological lignin-carbohydrate complex was chosen as a humic model; benzene, sodium benzoate, atrazine, and DDT represent different classes of contaminants. Sorption energies were estimated based on molecular mechanics calculations. Flexible linear polymers undergo drastic conformational changes when approaching the mineral surface, to ensure a gain in the interaction energy that outweighs a loss in the conformational energy proper. Therefore, the gas-phase conformation composi tion of environmental organic polymers is not directly related to their spatial organization in soil composites. Molecular dynamics simulation suggests high stability of the organic polymer coatings of mineral surfaces in the environment. Low-molecular-weight organic molecules demonstrate much less affinity for the mineral surface, which implies unhindered exchanges between the surface and its near environment. Ionizable compounds, e.g. salts of organic acids, are different, because they can form strong associations with a mineral surface through cation bridges. Sorption energies are compound-specific and depend on the sorbate-sorbent orientation. The calculations suggest some preference for the edges of a model muscovite sheet in comparison with the basal oxygen surface as a sorption site. Coating of mineral surfaces with organic polymers does not hinder the sorption of organic molecules except in the special case of organic ions.

Reduction of structural iron in ferruginous smectite by free radicals

The oxidation state of structural iron greatly influences the physical-chemical properties of clay minerals, a phenomenon that may have significant implications for pollutant fate in the environment, for agricultural productivity, and for industrial uses of clays. Knowledge of redox mechanisms is fundamental to understanding the underlying basis for iron’s effects on clays. Past studies revealed that the extent of Fe reduction varied depending on the reducing agent used, but this variation may not have been a simple function of the reduction potential of the reducing agent. The objective of this study was to identify the relationship between the Fe reduction mechanism and free radical activity in the reducing agent. Several reducing agents and their mixtures with the Na-saturated, 0.5 to 2 μm size fraction of ferruginous smectite (SWa-1) were analyzed by electron spin resonance (ESR) spectroscopy to determine the presence of unpaired electrons or free radicals. Only Na2S2O4 exhibited paramagnetic free-radical behavior with a signal at about g = 2.011, which was attributed to the sulphoxylate (S02− ·) free radical. The free radical was labile in aqueous solution, and the ability of Na2S2O4 solution to reduce structural Fe in the smectite decreased with age of the solution and paralleled the disappearance of the free radical signal in the ESR spectrum. The paramagnetic species was preserved and enhanced if Na2S2O4 was added to the clay suspension, indicating that either the clay surface stabilized the SO2− · radical or the additional unpaired electrons were produced in the clay structure.

PROTONATION OF ORGANIC BASES IN CLAY-WATER SYSTEMS

The extent of protonation of organic bases in clay—water systems depends upon the adsorptive properties of the organo-clay species involved, and upon the structure and degree of hydration of the clay system. Organic molecules that can disperse cationic charge over two or more condensed aromatic rings give rise to greater surface-induced protonation than do single-ring organic molecules with similar solution pKa. Protonation in clay suspensions is frequently far in excess of that predicted on the basis of electrolytic suspension pH and solution pKa of the organic base. For a given organic base, protonation in a clay film exceeds that in the suspended clay system. Protonation in an organo-clay film increases as the film moisture content decreases. The extent of protonation in organo—clay systems varies with cationic species, cationic saturation, and clay type.

THE CONFORMATIONAL DYNAMICS OF HUMIC POLYANIONS IN MODEL ORGANIC AND ORGANO-MINERAL AGGREGATES

Abstract Molecular mechanics calculations and simulated annealing were applied to model humic polyanions originating from lignin. The dynamic behavior of such oxidized lignins in model soil organic complexes, such as an oxidized lignin-carbohydrate complex (LCC) and humic (oxidized LCC)-clay aggregates, was analyzed. Neither ionization nor hydrogen bonding bring significant changes in the conformational properties of oxidized lignin and LCC. Oxidized lignin and LCC oligomers (humic substances in soil) bind to the mineral surfaces, a process that was exemplified in computational experiments on complexes with muscovite. Upon ionization, a lignin-derived oligomer develops strong attractive organo-mineral interactions through cation bridges. Without metal cations, electrostatic repulsion between negatively charged anions and the oxygen-mineral surface prevails, and the two parts of the organo-mineral complex drift apart. This tendency is typical of an oxidized lignin oligomer but not of a topological oxidized LCC.

THE MYSTERY OF THE LIGNIN-CARBOHYDRATE COMPLEX : A COMPUTATIONAL APPROACH

Abstract Molecular mechanics calculations were performed on model linear chains of lignin and carbohydrate polymeric structures of living plants and soil humic materials. Among possible lignin chain conformers that result from minimum-energy conformers of dimeric units, a broad regular helix may contain a carbohydrate ribbon inside itself. This structure provides an energy gain in comparison with other ways of mutual adjustment of lignin and carbohydrate polymers. Oxidation of lignin into respective humic acids sharply changes the spatial organization of the initial polymer only if it is not associated with a carbohydrate.

Morphology of lead(II) and chromium(III) reaction products on phyllosilicate surfaces as determined by atomic force microscopy

Redox and acid-base reactions play important roles in the fate of metal contaminants in soils and sediments. The presence of significant amounts of Cr, Pb and other toxic heavy metals in contaminated soils and sediments is of great environmental concern. Oxidation states and dissolution characteristics of the heavy metals can exert negative effects on the natural environment. Atomic force microscopy (AFM) was used to follow the changes in morphology and structure of reaction products of Cr and Pb formed on mineral surfaces. Nitrate salts of Cr(III) and Pb(II) were used to replace the native exchangeable cations on muscovite and smectite surfaces and the metal-mineral systems were then reacted at different pH’s and redox conditions.For Pb, aggregate morphological forms were found at pH 6.1 and 12.4. At pH 6.1, the mean roughness value was 0.70 nm, and at pH 12.4 it was 5.30 nm. The fractal dimensions were 2.03 at pH 6.1 and 2.05 at pH 12.4. For Cr(III), both layered and aggregate morphological forms were found at pH 6.8 and 10.8. The mean roughness values were 0.90 nm at pH 6.8 and 4.3 nm at pH 10.8. Fractal dimensions for both were 2.00. The effect of redox conditions on morphological characteristics was studied on a smectite substrate. The reduced clays were more compacted than oxidized ones and the reduced clay could reduce Cr(VI) to Cr(III), forming new minerals on the surfaces.A geochemical equilibrium model, MINTEQA2, was used to simulate the experimental conditions and predict possible reaction products. Simulation results agreed well with data from experiments, providing evidence that modeling can provide a useful “reality check” for such studies. Together, MINTEQA2 and AFM can provide important information for evaluating the morphologies and chemical reactivities of metal reaction products formed on phyllosilicate surfaces under varying environmental conditions.

Reactivity of basal surfaces, steps and edges of muscovite; an AFM study

The reactivity of basal surfaces, steps and edges of muscovite was studied by imaging surface precipitates of PbCl2 using atomic force microscopy (AFM). We reacted PbCl2 solution with freshly cleaved muscovite surfaces and found that PbCl2 precipitates were formed on the basal surfaces, steps and edges. It was observed that PbCl2 precipitated preferentially along the steps compared to the basal surfaces and that PbCl2 precipitates at multiple-layer edges were needle-shaped and oriented in different directions. One of the muscovite samples we cleaved had muscovite fragments sitting on the freshly cleaved surfaces. These fragments resulted from previously formed cracks. Thus, we were able to compare the reactivity of the weathered surfaces with that of freshly cleaved surfaces. It was found that PbCl2 was not precipitated along the edges of previously cracked muscovite fragments. These results clearly demonstrated that the edges of freshly cleaved muscovite are the most reactive surface sites, whereas the edges of weathered muscovite are not as reactive. We believe that the surface reactivity of the edges of freshly cleaved muscovite is likely due to terminal Al-OH21/2+ or Al-OH1/2− groups on these crystalline surfaces, which favor adsorption of Pb2+ ions and the subsequent nucleation and precipitation reactions. We also investigated the effect of drying rate on the morphology of the surface precipitates. Fast drying resulted in a nearly complete covered surface with a leaflike morphology, whereas slow drying resulted in more isolated surface clusters.