Recently, significant advances have been made in the theory and application of acoustic and electroacoustic spectroscopies for measuring the particle-size distribution (PSD) and zeta potential (ζ potential) of colloidal suspensions, respectively. These techniques extend or replace other techniques, such as light-scattering methods, particularly in concentrated suspensions. In this review, we summarize acoustic and electroacoustic theory and published results on clay mineral suspensions, detail theoretical constraints, and indicate potential applications for the study of environmentally significant clay mineral suspensions. Using commercially available instrumentation and suspension concentrations up to 45 vol.%, acoustic spectroscopy can characterize particle sizes from 10 nm to 10 µm, or greater. Electroacoustic spectroscopy can determine the ζ potential of a suspension with a precision and accuracy in the mV range. Despite the clear potential for their use in environmental settings, to date, acoustic methods have been used mainly on clay mineral colloids with industrial application, typically combined with similar measurements such as isoelectric point (IEP) determined from shear yield stress or ζ potential from electrophoretic mobility measurements. Potential applications in environmentally relevant suspension concentrations are significant, as PSD and ζ potential are important factors influencing the transport of mineral colloids and associated contaminants through porous media. Applications include determining the effects of suspension concentration, surfactants, electrolyte strength, pH and solution composition on soil clay properties and colloidal interactions, and determining changes in PSD, aggregation and ζ potential due to adsorption or variations in the clay mineralogy.

Acoustic and electroacoustic measurements of particle-size distribution (PSD) and zeta potential (ζ potential), respectively, were used to obtain in situ measures of the effects of suspension concentration and pH on interactions between mixed-charge clays and clay minerals from a highly weathered sediment. Measurements were obtained in concentrated suspensions as a function of weight fraction and as a function of pH during titrations. Standard dispersion and centrifugation methods were used to obtain a comparative measure of PSD. Thermogravimetric analysis and X-ray diffraction patterns were used to obtain semi-quantitative and descriptive analyses, respectively, of the sediment, which is composed of Fe oxide minerals, kaolinite, gibbsite, quartz, crandallite, chlorite and traces of other clay minerals. Acoustic measurements showed that the PSD of the clay fraction varied with suspension concentration, and electroacoustic measurements showed the ‘bulk’ ζ potential increased in absolute value as the suspension concentration decreased. Titration results were also sensitive to suspension concentration. Acoustic measurements indicated that the suspensions became unstable at ∼pH 7.5–8.0, as the attenuation spectra changed character near this pH and the calculated PSD shifted to a larger particle size. This pH value is near the points of zero charge of goethite and gibbsite, as verified by titrations on mineral standards. The results confirm the central role oxide minerals play in regulating clay mineral interactions in highly weathered sediments, and indicate that the average ζ potential of a suspension may be a poor indicator of controls on suspension stability.

Bromide-containing impurities were found to decrease the thermal stability of quaternary alkyl ammonium-modified layered silicates. Improved purification procedures completely removed bromide and led to a 20°C to >100°C increase in organic modified layered silicate thermal stability. Using mass spectrometry and thermal and electrochemical analysis, N,N-dimethyl-N,N-dioctadecyl quaternary ammonium-modified montmorillonite and fluorinated synthetic mica were found to degrade primarily through elimination and nucleophilic attack by these anions. The nature of residual bromides was identified and quantified, and the efficiency of removing these anions was found to be solvent dependent; sequential extraction, first ethanol then tetrahydrofuran, gave the best results. This exhaustive extraction method represents a viable alternative to the use of expensive, more thermally stable oniumion treatments for layered silicates.

X-ray diffraction with a position-sensitive detector (XRD-PSD) was used to make a time-resolved study of the dynamics of deposition and dehydration of Na-montmorillonite crystallites on flat substrates from deionized water suspensions. The static PSD geometry and simultaneous counting procedure allowed the acquisition of high-resolution data on the dynamics of interlayer and interparticle arrangements during dehydration. Three experimental datasets of Na-smectite dehydration are presented, each one representing different initial sample states (suspension, slurry and re-wetted thin film). The computer program NEWMOD was used to simulate one of the three datasets (dehydration of a smectite suspension) and thus obtain the apparent changes in relative proportions of different 00l values as smectite crystallites formed and dehydrated. Two types of diffracting domains formed: water-dispersed ‘packets’ of 1–2 smectite layers gaining long-range order in the c axis direction as water was lost to evaporation, and smectite layers deposited as hydrated crystallites with variable interlayer water contents. The experimental patterns show the rapid step-wise transition of Na-montmorillonite layers from d values of ∼55 to 18.5, 15.4 and 12.5 Å, with variations that depended upon how the hydrated smectite sample was prepared. The simulations show that there was a wide range of d values whose frequency distribution changed as dehydration proceeded and that transient d values occurred between the peaks observed experimentally. The data obtained in this study illustrate that XRD-PSD instruments have great potential in providing detailed data on the rapid kinetics of interlayer reorganization.

Permeability is often the limiting factor in clay-based barrier systems designed to attenuate miscible or immiscible contaminant transport. One critical aspect of barrier design is prediction of the effects of permeant conditions on physicochemical properties (e.g swelling) of the clay component and thus, ultimately, the permeability of the barrier. To this end, the permeability of an organically modified bentonite to ethanol-water solutions was determined to approximate the effects of organo sorption-induced swelling. The bentonite was modified with a substituted alkylammonium cation: benzyloctadecyl-dimethylammonium (BODMA). Powder X-ray diffraction (XRD) measurements were used to measure interlayer expansion and to estimate aggregate particle densities at any given ethanol concentration. Permeability measurements were conducted on samples under confining overburden stresses of 43 kPa (∼2.5 m of saturated soil) using a hydraulic gradient of ∼300. Sample thickness was continuously monitored during measurements of volumetric flow so that sample strain, void ratio and porosity could be calculated from changes in sample pore volumes. For calculations of porosity and void ratio, the swelling solid volume, determined from XRD, was assumed to include the sorbed layer (crystalline water/ethanol), the thickness of which is not normally considered in engineering applications. Exposure of the BODMA bentonite to increasing ethanol concentrations increased the swell volume of the clay. Measured coefficients of permeability for the BODMA bentonite decreased from ∼1 × 10−7m s−1 for pure water to ∼8 × 10−10m s−1 for pure ethanol. Exposure of the organo clay to ethanol-water solutions containing 0.2 M NaCl increased the effect of ethanol on clay swelling and permeability. Probable consequences of the effects of sorption-induced swelling on permeability and the performance of organo clay-based permeable reactive barriers are discussed.

X-ray basal reflections of several HDTMA-vermiculites show that when these contain co-adsorbed molecules of HDTMA-Br, their interlayer spacings are ∼29.5 Å. After HDTMA-Br has been leached out with ethanol, spacings decrease to ∼26 Å, but when exposed to toluene vapor the spacings increase to ∼36 Å, and to ∼45 Å in toluene liquid. Transmission X-ray diffraction photographs of HDTMA-vermiculites, both with and without co-adsorbed HDTMA-Br, indicate that they have highly ordered interlayer structures. The structure of HDTMA-Br-free vermiculite flakes is more open with the interlayer cations arranged in a 3a × b superstructure. In the HDTMA-Br containing HDTMA vermiculite flakes, HDTMA-Br molecules occupy interstitial positions between the HDTMA cations. Swelling in toluene vapor does not disorder the 2-dimensional arrangement of interlayer cations, but disorder does occur in toluene liquid. In toluene vapor, the aliphatic chains of the cations remain partially interdigitated, but in toluene liquid the interdigitation disappears. In response to the layer-charge density, the organization of the interlayer HDTMA units is caused by the self-assembly process responsible for hydrocarbon close packing.

The Eocene Huber Formation, exposed in the CFI Hall mine south of Irwinton, in Wilkinson County, Georgia, displays colored zones that are a consequence of an oxidative weathering front. These zones vary from very light gray (gray) to pinkish white (pink) to very pale yellow (cream) (Munsell N8, 5YR 8/2 and 2.5Y 8/2, respectively) with increasing degree of oxidation. A representative sample from each zone was collected and analyzed for its chemical, crystallographic and Raman spectroscopic properties. The comparison of these genetically related samples allowed for a quantitatively accurate assessment of anatase’s contribution to the total TiO2 content. All samples contain ∼1.5 wt.% TiO2. Progressing from gray to pink to cream, anatase contents range from half to nearly all the TiO2. The a lattice parameter for anatase is constant in all three kaolins (3.7916 Å), suggesting a constant 4.6 mol.% isomorphous substitution of Fe for Ti.

Phase characterization and mass-balance considerations of the gray, pink and cream kaolins indicate that ilmenite and pyrite are precursor Ti- and Fe-bearing phases in the gray kaolin that undergo oxidation. Pseudorutile is a proposed intermediate phase resulting from the anodic corrosion of ilmenite. Hematite, goethite and anatase are the dominant end-products via dissolution-precipitation from the reactants pyrite and pseudorutile. The removal of Fe sulfides and organic matter and addition of hematite and goethite causes kaolin colors to change from gray to pink. Oxygen diffusing from groundwater in permeable overlying and underlying formations facilitates a process that probably involves aerobic and anaerobic bacteria that utilize Fe from pyrite, hematite and goethite. We postulate that the end result is the mobilization of Fe by siderophores and a kaolin color change from pink to cream.

Ferrihydrite is a poorly crystalline Fe oxide of which 2-(XRD)line and 6-line varieties are commonly used in experiments, although species with intermediate numbers of peaks have been found in nature. To simulate nature, we synthesized two continuous series of ferrihydrites with between 2 and 6–7 peaks at room temperature in two different ways: (1) by varying the rate of hydrolysis of an Fe(NO3)3 solution (HR series); and (2) by oxidizing an FeCl2 solution containing up to 73 mmol Si/L (Si series), both at pH 7. Mössbauer spectra at 4.2 K showed that the ferrihydrites of the HR series had a constant magnetic hyperfine field (Bhf) at 4.2 K of 48.8 T whereas Bhf in the Si series dropped from 49.4 to 46.7 T as the Si content of the ferrihydrites increased from 0 to 74.7 g/kg of Si. Temperature scans between 4 and 170 K illustrate that the magnetic order breaks down at a temperature which is lower the higher the hydrolysis rate and the Si concentration in the ferrihydrite.

A 2 m thick brecciated zone containing magnesian minerals is present at the contact of tectonites and cumulates. Tectonites below this zone comprise serpentinized orthopyroxenite and serpentinite. An alteration zone with vein-type bedding comprises four different levels; from bottom to top they are: (1) green-brown serpentinite with dolomite (0.9 m), (2) light greenish-white dolomite with serpentine (0.5 m), (3) white dolomite with sepiolite (0.4 m), and (4) greenish-white dolomite with smectite-chlorite (0.2 m). The first level has a mineral association of serpentine + dolomite ± calcite ± aragonite, the second level consists of dolomite + serpentine ± calcite or dolomite + magnesite + serpentine, the third level comprises dolomite + sepiolite, and the fourth level is made up of dolomite + chlorite + smectite + serpentine. Dolomite, the main mineral of the alteration zone, occurs as coarse crystals (microsparitic-sparitic) in fractures and as small crystals (microsparitic-micritic) in the matrix, which includes serpentine and gabbro relics. Sepiolite developed at the edges and surfaces of dolomite and as fibrous forms in voids. Cumulate rocks above this zone comprise uralitic gabbros. The occurrences of magnesian minerals developed in three stages: the first stage was the serpentinization of olivine; the second stage was the dissolution of serpentine by groundwater and/or meteoric water containing carbon dioxide; and the last stage was the synthesis of neoformed minerals.

An O2 oxidation and freeze-dry procedure has been used to synthesize birnessite through the oxidation of Mn2+ in alkali media. The effects of O2 flow rate, the fluxion velocity of reaction suspension, the reaction temperature, pretreatment the reaction solutions with N2, and the hydration conditions on the purity of birnessite, the crystallinity, the ion-exchange properties, and the phase transformation of intermediate phases were examined. Buserite with a 1 nm basal spacing, produced after the oxidation, is transformed to 0.7 nm Na birnessite by complete freeze drying. Increasing the fluxion velocity of the reaction suspension and the O2 flow rate facilitated oxidation of Mn(OH)2. Prephase I (a phase related to hausmannite, γ-Mn3O4 (Luo and Suib, 1997; Luo et al., 1998), and feitknechtite (β-MnOOH) were formed as intermediates during the synthesis. Mechanical stirring was used to change the fluxion velocity of the reactive suspension. When the speed of stirring and the O2 flow rate were raised to 250 rpm and 3.0 L/min, respectively, or 450 rpm and 2.0 L/min, respectively, birnessite was the only phase in the final product. Irrespective of temperature in other reactions, pure birnessite was synthesized as long as the temperature during the initial mixing of the reaction solutions was maintained below 10°C. Increasing the reaction temperature led to a larger crystal size, better crystallinity and lower surface area. The pretreatment of solutions with N2 or O2 had little effect on the synthesis; synthesized birnessites had the same purity (100%) as, and similar crystallinity to, that of the no-pretreatment control. Dehydration of the buserite by freeze drying and heating at 60°C did not affect the production of birnessite; however, the latter caused partial loss of ion-exchange capacity with Mg2+. The pathways of the birnessite formation in this study might be:

1. (1) Mn(OH)2 (amorphous) → feitknechtite → buserite → birnessite, and

2. (2) Mn(OH)2 (amorphous) → prephase I → feitknechtite → buserite → birnessite

Mn(OH)2 existed in an X-ray amorphous state, not in the form of ‘pyrochroite’, during the synthesis.

The adopted conditions for synthesis of pure birnessite were NaOH to Mn molar ratio of 13.7, O2 flow rate of 2 L/min and oxidation for 5 h during vigorous stirring at 450 rpm at room temperature. The birnessite synthesized had a hexagonal platy morphology with good crystallinity, an average composition of Na0.25MnO2.07.0.66H2O, and a surface area of 38 m2/g.