Abstract: Chilean soils derived from volcanic ashes are a natural source of iron oxides. Due to their properties, mineralogy, and surface characteristics, iron oxides from Chilean soils are potential candidates for technological applications such as heterogeneous catalysis. However, before a direct application in catalysis, pre-treatment methods are necessary to concentrate iron oxides from bulk volcanic soils. Here, we provide a comprehensive review of pre-treatment strategies for iron oxide concentration including physical separation and selective chemical dissolutive methods for application in catalytic processes, such as the water gas shift (WGS) and Fenton reactions. For preparation of WGS catalyst from volcanic soils, thermal treatment has been demonstrated to be effective, yielding enhanced results for Andisols compared to Ultisols. Based on mineralogical characterisation, it seems that WGS reaction efficiency depends on mineralogical phase shift and the changes of Fe2+/Fe3+ ratio, produced through heating. In addition, Ultisols have shown as efficient catalysts in Fenton and Fenton-like processes, after application of physical and chemical pre-treatments to different size-fractions of the soil sample, improving the yield performance of catalysts. Magnetic separates from the Ultisol sand fraction (compared to the silt+clay fractions) demonstrate the best catalytic performance as Fenton reagent due to their natural magnetite and titanomagnetite content. Application of NaOH selective dissolutive treatment to silt+clay fraction of volcanic soils also produces Fenton and Fenton-like catalyst with improved performance. Our review indicates that catalytic performance can be explained not only by the iron oxide mineral content but also by their characteristics and magnetic properties. The application of the appropriate physical and chemical pre-treatment methods can modulate and enhance the catalytic capabilities of iron oxides from volcanic soils.

On this work, the iron oxide mineralogy of Chilean volcanic ashes derived soils have been reviewed, emphasizing on new finding linked to the application of Mössbauer spectroscopy. It has been established that free iron oxide layer contributes with positive variable surface charge to the clay-size soil particle, at soil pH. However, the importance of such contribution seems to depend on the evolutionary stage of the different volcanic soil orders, which defines the crystalline degree of their iron oxide contents. Mössbauer spectroscopy complemented with different physical, chemical and instrumental techniques revealed key aspects of iron oxide mineralogy on these Chilean volcanic soils. For instance, results for Ultisol revealed that the evolution of the soil particle could be followed just analysing the main component of their iron oxide mineralogy; thus, the iron oxide mineralogy change when passing from the volcanic ashes (magnetite), to sand-size magnetic separates (partially-oxidized magnetite), to silt-size (strongly-oxidized magnetite), and finally to clay-size (maghemite) soil samples. Therefore, it would seem that physical weathering of the Ultisol produces smaller and more oxidized particles. On the other hand, the Andisol samples, a young volcanic soil compared to Ultisol, seems to have a constant iron oxide mineralogy among all its different particle size fractions: paramagnetic Fe2+ and Fe3+ species with low crystalline degree, the last one possibly assignable to a ferrihydrite-like mineral. Therefore, on the case of Andisols, it seems that the high contents of organic matter somehow prevent mineral evolution towards higher oxidation and more crystalline levels, in agreement with past studies.

Composites based on nanosized particles of magnetite and natural microsized zeolite were prepared and their efficiency to adsorb arsenate from aqueous systems was evaluated through kinetic data and adsorption isotherms, considering the proportion of magnetic iron oxide, composite:solution ratio and zeolite particle size. A well-crystallized nanomagnetite with an averaged particle size of 50 nm when supported on zeolite was obtained; the nanomagnetite enhances the arsenate adsorption of zeolite. The composite prepared with a nanomagnetite:zeolite mass ratio of 0.43:1 and 0.30:1 presented on zeolite milled 20 and 120 min, respectively, showed a similar adsorption capacity as that observed for pure magnetite. Supported nanomagnetite prevents risks of the nanoparticles contamination. The smaller the mean particle size of zeolite the lower the mass proportion of nanomagnetite in the composite required to reach the maximum adsorptive efficiency. Acidic conditions improve arsenate remotion, in agreement with the development of positive surface charge in the composite.

The renewed interest in imogolite nanotubes, with structural similarities with carbon nanotubes (CNTs), has led to a new search of the potential of imogolite in the field of nanoscience. The applications of imogolite started being studied at the beginning of the 1970s, but the lack of technological development prevented a detailed study of this aluminosilicate. Furthermore, the strong incursion CNTs, which show exceptional structural characteristics and physical properties, left imogolite as the subject of secondary studies and applications in nanotechnology. This review, revise the scientific interest on imogolite from its discovery until the present, showing the main areas of research and development for this nanotube from a nanotechnological point of view.

Pore volume, specific surface area (SSA), and total intragranular porosity (TIP) of Chilean soils derived from volcanic materials were studied. Soil samples involving the 0-15 and 15-30 cm depth of virgin and cultivated Collipulli (Ultisol) and Diguillín (Andisol) soils at two particle size fractions (<1 mm and <2 µm) were considered. From mercury porosimetry and N2 adsorption, mainly mesopores (pore diameter, dpore, about 10 nm) were determined for <1 mm Collipulli samples. Diguillín <1 mm soil shows macroporosity with dpore from 70 nm to 7000 nm. The clay fraction of Collipulli has macropores (dpore from 2000 nm to 40000 nm) and mesopores (dpore from 3 nm to 23 nm), while for Diguillín clay-size fraction most of the porosity comes from macropores (dpore from 50 nm to 800 nm). For all samples the SSA linearly correlates with the mesopore volume (r²=0.781; n=16) determined by N2 adsorption, and with the mesopore + macropore volume (r²= 0.771; n=12) when Collipulli <1mm samples are excluded; an inverse relationship between SSA and organic carbon content was found (r²=0.854; n=14). Thus, the SSA defined mainly by mesopores and macropores is probably related to the soil organic matter content. Mesopores and macropores mainly give the TIP, which increases as particle size decreases. No important changes in micropore and macropore volume, and in TIP were seen as result of cultivation. Mesopore volume is more important in samples dominated by kaolinite than in samples dominated by allophane (4 to 20 times). In general the soil pore distribution, its SSA and TIP are related to its mineralogy and organic matter content

The disposal of sewage sludge from domestic wastewater treatment plants is a growing problem worldwide. Because of the total P content and other characteristics, it is highly probable that in Chile important amounts of the sewage sludge will be disposed of on agricultural lands. The distribution of P forms in Chilean soils and sewage sludge was studied by chemical fractionation and 31P-NMR spectroscopy. The 0-15 cm depth of soils derived from volcanic materials (one Ultisol and two Andisols), alluvial materials (Mollisol), and sewage sludge from a domestic wastewater treatment plant located in Santiago were considered. The total P concentration is 6 to 18 times higher in the sewage sludge than in soils, but a similar 31P-NMR pattern was found in all samples. The most important signals correspond to inorganic orthophosphate and monoester P, with small signals assigned to pyrophosphate and diester P. The disposal of sewage sludge might not succeed as soil P fertilizer, contributing to an increase in the accumulation of P in volcanic soils, and it might be a potential pollutant in soils and water

The phosphorus distribution in soil (<2 mm) and in clay size separates (<2 mu m) of two uncultivated volcanic soils from southern Chile (one Ultisol and one Andisol) was studied by phosphorus-31 nuclear magnetic resonance spectroscopy (31P-NMR). The total P content ranges of the Ultisol samples were 733-1108 mg kg-1 (soil) and 746-896 mg kg-1 (clay), lower than 2163-2216 mg kg-1 (soil) and 2683-3362 mg kg-1 (clay) obtained for Andisol samples. In both fractions the 31P-NMR spectra of a single alkaline extraction, using 0.25 M NaOH and Chelex 100, showed the presence of inorganic orthophosphate, monoester-P, diester-P and pyrophosphate. Phospholipids-teichoic acids P were observed, but only in the clay size separates of the Andisol. Important differences in spectra of soils and clay size separates were observed only in samples with high organic matter content. The clay size separates enriched with organic matter have more and better defined signals of organic P forms. This allows the identification of P forms that might not be detected by 31P-NMR in the whole soil extracts

The thermal properties of soils, as thermal conductivity, have been usually determined by different designs of the quasi-permanent hot wire method. In the present paper the hot disk method was used to determine the effective thermal diffusivity (ETD) and thermal conductivity (TC) of soils derived from volcanic materials (one Ultisol and two Andisols) as a function of temperature from 25C to 280C. The volumetric heat capacity was also calculated from ETD and TC estimations. The TC values at 25C ranges from 0.84-0.94 for Andisols to 1.27 Wm-1K-1 for the Ultisol, and are similar to those reported by the hot-wire method. Values decrease down to 0.28-0.32 for Andisols and 0.67 Wm-1K-1 for Ultisol when heating temperature increases from 25C to 280C. The TC increases when soil density or water content increases, and decreases when soil organic matter content increases. The values of the thermal properties depend on water and organic carbon content, and mineralogy of samples

The applicability of Rothmund-Kornfeld semiempirical approach to describe the binary cationic exchange equilibria K-Ca, K-Mg, Ca-Mg, and Ca-Zn carried out on variable surface charge synthetic aluminosilicates with and without coating (10 samples at all), was studied. The equivalent fraction of adsorbed ions and the equilibrium constan results were compared with those obtained by using the Gaines-Thomas approach, widely used and validated in cation exchange studies carried out on variable surface charge systems. The synthetic aluminosilicates (with and without coatings of iron oxides and organic matter) present a higher cationic exchange capacity (CEC) and K selectivity than similar soils and fractions. The equivalent fraction of adsorbed ions obtained by both aproaches correlates, r² ranges between 0.962-0.998. The natural logarithm of the thermodynamic constant determined by both approaches correlates with r²=0.992. The slight observed differences are probably due to integration errors associated with the resolution of equation involved in the Gaines-Thomas calculations. The Rothmund-Kornfeld model can be applied to the describe the binary cation exchange equilibria carried out on synthetic variable surface charge systems, with independence of K selectivity and CEC magnitude of samples.

Se estudió la aplicabilidad del modelo empírico de Rothmund-Kornfeld en el análisis de equilibrios de intercambio catiónico binarios K-Ca, K-Mg, Ca-Mg y Ca-Zn en muestras de aluminosilicatos sintéticos con y sin recubrimientos que presentan carga variable (10 diferentes equilibrios binarios en total). Los resultados se compararon con aquellos obtenidos a través del modelo termodinámico de Gaines-Thomas, ampliamente usado y validado para el estudio del equilibrio de intercambio catiónico en sistemas de carga variable. Los aluminosilicatos sintéticos son sistemas con mayor capacidad de intercambio catiónico (CIC) que los suelos y sus correspondientes fracciones, presentando asimismo una mayor selectividad por K. Los valores de fracciones equivalentes de los iones adsorbidos determinados por Gaines-Thomas y calculados por Rothmund-Kornfeld correlacionan entre si, con valores de r² en el rango 0.962-0.998 según sea el equilibrio binario considerado. Los logaritmos naturales de las constantes termodinámicas para los equilibrios binarios determinadas por ambos métodos presentan un r²=0.992 considerando las constantes de los 10 equilibrios binarios estudiados. Las diferencias observadas en las constantes se deben, probablemente, a la indeterminación en la resolución de la integral que involucra el cálculo de la constante de equilibrio planteada en el modelo de Gaines-Thomas. El modelo de Rothmund-Kornfeld resultó ser un método válido para describir el equilibrio de intercambio catiónico binario en sistemas sintéticos de carga variable de alta CIC y alta selectividad por K.

The effect of K and Ca homoionization in chloride, phthalate-chloride, phosphate-chloride, and phthalate-phosphate-chloride adsorption on the <2&micro;m size fraction of the Osorno B unfertilized soil sample is studied. Results show that chloride adsorption on Ca2+ homoionized sample is higher than on K+ homoionized sample. Phthalate adsorption on Osorno-K+ is higher than on Osorno-Ca2+ when phthalate is equilibrated in presence of chloride, due to the high affinity of Ca2+ homoionized surface for chloride. These results showed the importance of electrostatic factors as one of the mechanisms involved in phthalate adsorption. Minimal differences due to homoionization were observed in phosphate adsorbed in competence with chloride, showing that phosphate adsorption mechanisms are highly selective. In phosphate-phthalate-chloride competence, no effect of phthalate on phosphate adsorption was observed, but phthalate adsorption is drastically reduced from 46 to 17 cmolkg-1 due to phosphate

En el presente trabajo se estudia el efecto de la homoionización catiónica con K+ y Ca2+ en la adsorción de cloruro y las competencias ftalato-cloruro, fosfato-cloruro y ftalato-fosfato-cloruro, en la fracción fina de suelo Osorno sin fertilizar, Horizonte B. Los resultados demuestran que la adsorción de cloruro es mayor en el suelo Osorno homoionizado en Ca2+, con respecto a K+. En los estudios de competencia entre ftalato y cloruro se observa una mayor adsorción de ftalato sobre el suelo Osorno homoionizado en K+, debido al alto grado de competitividad del ion cloruro causado por la presencia del catión Ca2+, lo que disminuye su adsorción. A partir de estos resultados se demuestra la importancia de los factores electrostáticos como uno de los mecanismos de adsorción del anión ftalato. En la competencia fosfato-cloruro sólo se observa una leve reducción en la adsorción de fosfato en la muestra homoionizada en Ca2+, con respecto a K+. La presencia de ftalato no afecta la adsorción de fosfato en competencia con cloruro, en la triple competencia, sin embargo la presencia de fosfato reduce de 46 cmolkg-1 a 17 cmolkg-1 la adsorción de ftalato en competencia con cloruro, demostrándose la intensidad de los mecanismos de adsorción de fosfato en suelos de carga variable derivados de materiales volcánicos