Clay minerals such as smectites are one of the most abundant silicate minerals, and is a major phase controlling the Earth's surface and near-surface materials' physical and chemical conditions. Their chemical composition highly varies due to extensive isomorphic substitution that depends much on environmental conditions. Numerous researches have focused on their natural formation and to duplicate these natural processes in the laboratory for highly tailored functionalities. Despite our ability to duplicate certain natural conditions in the laboratory, not all types of clays can be synthesized at low temperatures - a puzzle that is still undergoing heavy scrutiny. In a natural low-temperature setting, the physicochemical characteristics of waters circulating through Earth's materials (i.e. aqueous speciation) and the time involved to accumulate a detectable amount of precipitated clay materials are the main issues. Precipitation from low temperature solutions in conditions of low supersaturation minimizes kinetic factors and the composition of the precipitated smectites could approach maximum stability. On a microscopic scale, such may be the ideal scenario in natural systems and how it precipitates smectites at natural environmental conditions involves a larger volume of materials to accumulate detectable amounts of smectite. Laboratory experiments have shown that even at highly dilute solutions, precipitation takes place rapidly but requires a larger volume of solutions to precipitate a few milligrams of bulk nanomaterials. The inherent variability in composition and stability of smectites as shown by the natural system has also been exploited by duplicating specific types that can be used for novel applications. One of them is transition-metal-bearing smectites; they are rare and only forms in specific geochemical environments. At their nanometer-scale size, they have been shown to exhibit photocatalytic activity which has not been seen in bulk natural materials. Besides the well-known increase in chemical reactivity of smaller particles, this smectite nanomaterial could also exhibit properties akin to quantum-size effects.

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We examined the effect of clay mineralogy and interfacial chemistry of clay suspensions on the performance of a flocculant prepared from weathered volcanic ash rich in allophane and imogolite. Upon addition of the flocculant into the clay suspension and stirring, fluffy flocs formed and settled after a standing time. The absorbance of the supernatant gradually decreased to near zero at an optimum addition and then increased again when more flocculant was added. The optimum flocculant addition differed significantly among the clay suspensions. Inspection of the results of the flocculation tests and characterization of the sample clays, particularly stepwise multiple regression analyses, suggested that the major factors affecting the performance were the effective cation exchange capacity (ECEC), content of micaceous minerals and average particle size, of which the contribution of the former two factors seemed weightier. This was interpreted as indicating the importance of the amount of negative charge on the external surfaces of particles in the clay suspensions.

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Decomposition of the organic dye Rhodamine B (RhB) in aqueous suspensions of synthetic Zn-substituted phyllosilicates (smectites, vermiculite-like phase and mica) under UV radiation was monitored using UV-visible absorption spectroscopy and compared to that in aqueous suspensions of Mg-substituted phyllosilicates as reference. A reduction in the intensity of the absorption spectra was observed for RhB with phyllosilicates suspensions, although the shape and width of the absorption bands were similar to those in aqueous solution. The absorption behavior indicates that RhB molecules are adsorbed onto the surface of phyllosilicate particles without discernible aggregation and the adsorbed RhB molecules are well dispersed. Zn-smectites, which exhibit exfoliation behavior, have higher photocatalytic activity among the synthetic samples. The samples that do not exhibit exfoliation behavior, such as the Zn-vermiculite-like phase and Zn-mica, do not exhibit photocatalytic activity. The hybridization of RhB with the exfoliated Zn-smectite particles was confirmed to result in exponentially enhanced photodegradation. The obtained photocatalytic activity is attributed to the presence of a large amount of Zn in the crystal structure combined with the nanostructure of the synthesized Zn-smectites.

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Fluoride leaching from alkaline solid compounds especially from steel slag and coal fly ash which are currently utilized in construction has been considered a potential threat to public health and the environment. In hyperalkaline environments, it is difficult to remove fluoride from geomedia since most of the naturally-occurring minerals are negatively charged. To prevent fluoride leaching, it is necessary to identify stable secondary mineral phases where control of the pore water chemistry could be possible, and that can be generated and act as sorbing solids for fluoride under highly alkaline conditions in the further utilization or when dumping these alkaline solids as landfill. In this study, mineral phases synthesized in Si-Al-Mg systems under hyperalkaline conditions at room temperature were investigated. Fluoride co-precipitation and adsorption experiments were conducted to study the characteristics of the fluoride sorption by these minerals during and after mineral precipitation. The results showed that brucite, high Mg-containing hydrotalcite-like phases (HT), and smectite have high fluoride removal efficiency. Fluoride removal efficiency was much higher in co-precipitation experiments than in adsorption experiments. The mechanism of the fluoride removal was studied with FT-IR measurements, XRD analysis, and zeta potential determinations, and indicated that fluoride can be structurally fixed via ion exchange during and after the mineral formation. Hence, this study demonstrates that the leachability of fluoride at alkaline conditions can be minimized by incorporation into the structure of Mg containing minerals. Generation of Mg-containing minerals as the sorption sink for fluoride by controlling the pore water chemistry of alkaline solid compounds with high reactivity must be taken into account in the utilization and disposal of alkaline solid compounds.

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