Silicate Sites Research Articles

The interplay between climate and bedrock type determines litter decomposition in temperate forest ecosystems

At regional scale, it is dominantly considered in the European literature that litter decomposition is higher in soils from calcareous than siliceous bedrocks because of higher pH in the former forest ecosystems. We tested the alternative hypothesis that this should rather be due to differences in physical characteristics of the bedrock, with likely higher decomposition on calcareous than siliceous bedrocks in wet climates because of more favourable texture in the former and nutrient leaching in the latter, and the opposite in dry climates because of higher drought stress in calcareous soils. We assessed, four consecutive years with contrasting climates, the litter decomposition of a unified litter of Abies alba needles with the litterbag method in 70 forest sites located on both bedrocks and in wet oceanic and dry continental climates of the European Alps and the Pyrenees. Average and annual climate data were analysed with principal component analyses and the effects of bedrock type, average and annual climate drought stresses on litter decomposition were analysed, separately in the Alps and the Pyrenees, with analyses of variance. We found, in both mountain ranges, a highly significant bedrock type by average climate drought stress interaction on litter decomposition, due to a strong decrease in litter decomposition from wet oceanic to dry continental sites on calcareous bedrocks only. Although litter decomposition did not change over all years in siliceous sites with increasing climate drought stress, it increased during wet years in the dry continental sites only, in particular in the Pyrenees where interannual climate variability was higher. Together our results strongly support the physical hypothesis and not the chemical hypothesis. We argue that the chemical hypothesis was proposed based on studies only conducted in low elevation wet temperate sites comparing mull humus types from deciduous forests on calcareous soils to mor and moder humus types from evergreen forests on highly sandy siliceous bedrocks. Our study conducted on a wider range of climate and bedrock conditions bring strong evidence that litter decomposition is rather dependent on the physical characteristics of the bedrocks.

Efficient olefin epoxidation with high turnover rates over ordered mesoporous silica nanoparticles with uniform manganese sites

Olefin epoxidation has been studied over a series of mesoporous manganese silicate nanoparticles and tBuOOH. Partial substitution of Si by Mn in silica framework leads to highly dispersed, distorted Mn(III) sites, confirmed by various physicochemical analyses. High conversion and stable styrene oxide, stilbene oxide and epoxy cyclohexane yield have been achieved with MS75 with 1:1.5 ratio of olefin: oxidant. However, at faster reaction rates lower styrene oxide yield is observed at lower Mn-containing catalyst (MS100). High TON, TOF over manganese sites shows accessible monomeric Mn sites. Attributed to high reactant density at redox-active Mn silicate sites due to facile diffusion across shallow pore depth channels of nanoparticulate om-MSx and surface hydrophobic characteristics. The catalysts were found to be stable under recycling and filtration experiments. The excellent stability against leaching of active manganese species indicates stable sites and good textural properties in om-MSx, essential for extended activity.

Uniformly stable hydroxylated cobalt(II) silicate species embedded within silicalite-1 zeolite for boosting propane dehydrogenation

Co-based catalysts are promising alternatives to replace expensive Pt- and toxic Cr-based catalysts for propane dehydrogenation (PDH), while the catalytic stability and propylene selectivity have been subjected to the diversity of Co speciations. Given the situation, uniformly stable hydroxylated Co2+ silicate sites (depicted as {OH–Si–(OH)–Co–O–Si-(OH)3}) are successfully embedded into the silicalite-1 zeolite framework located in the straight and intersection channels (Co@S-1) with the assistance of Co precursors and geminal silanols via a facile hydrothermal synthesis, as revealed by detail characterizations. Under comparable conditions, a superior catalytic PDH performance with attractive propylene formation rate of 14.6 mmol·gcat−1·h−1, propylene selectivity higher than 93% and very low deactivation rate of 0.0206 h−1 after 7 h is achieved over the Co@S-1(EDA) catalyst. Furthermore, the catalytic performance can be restored to the same as that of the fresh catalyst after 3 successive regenerations. In situ FTIR spectra analysis further reveals the dynamic changes in the characteristic absorption bands associated with the hydroxylated Co2+ silicate sites and the intermediate product (2-propyl) during the PDH reaction and a reaction mechanism is proposed accordingly. This work provides a new insight for designing high-efficiency nonprecious metal PDH catalysts.

Soil phosphorus status and P nutrition strategies of European beech forests on carbonate compared to silicate parent material

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry,https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENIP) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition.

The impact of landscape evolution on soil physics: evolution of soil physical and hydraulic properties along two chronosequences of proglacial moraines

Abstract. Soil physical properties highly influence soil hydraulic properties, which define the soil hydraulic behavior. Thus, changes within these properties affect water flow paths and the soil water and matter balance. Most often these soil physical properties are assumed to be constant in time, and little is known about their natural evolution. Therefore, we studied the evolution of physical and hydraulic soil properties along two soil chronosequences in proglacial forefields in the Central Alps, Switzerland: one soil chronosequence developed on silicate and the other on calcareous parent material. Each soil chronosequence consisted of four moraines with the ages of 30, 160, 3000, and 10 000 years at the silicate forefield and 110, 160, 4900, and 13 500 years at the calcareous forefield. We investigated bulk density, porosity, loss on ignition, and hydraulic properties in the form of retention curves and hydraulic conductivity curves as well as the content of clay, silt, sand, and gravel. Samples were taken at three depths (10, 30, 50 cm) at six sampling sites at each moraine. Soil physical and hydraulic properties changed considerably over the chronosequence. Particle size distribution showed a pronounced reduction in sand content and an increase in silt and clay content over time at both sites. Bulk density decreased, and porosity increased during the first 10 millennia of soil development. The trend was equally present at both parent materials, but the reduction in sand and increase in silt content were more pronounced at the calcareous site. The organic matter content increased, which was especially pronounced in the topsoil at the silicate site. With the change in physical soil properties and organic matter content, the hydraulic soil properties changed from fast-draining coarse-textured soils to slow-draining soils with high water-holding capacity, which was also more pronounced in the topsoil at the silicate site. The data set presented in this paper is available at the online repository of the German Research Center for Geosciences (GFZ; Hartmann et al., 2020b). The data set can be accessed via the DOI https://doi.org/10.5880/GFZ.4.4.2020.004.

Soil Phosphorus Heterogeneity Improves Growth and P Nutrition of Norway Spruce Seedlings

Phosphorus (P) nutrition of forest trees depends on soil P supply, which is poor on sites with sandy, quartz-rich soils and shallow calcareous soils. Soil P is distributed heterogeneously at different spatial scales. I tested the hypothesis that in P-limited forest ecosystems soil P enrichment in hotspots results in improved tree P nutrition and growth compared to soils where the same P amount is distributed evenly. In two field P fertilization experiments, Norway spruce (Picea abies) and European beech (Fagus sylvatica) seedlings were grown in soils with experimental homogeneous or heterogeneous P enrichment. The soils were amended with P to equal amounts of total P, but different patterns of P enrichment and/or as different P forms (orthophosphate [oPO4]; inositol hexaphosphate [IHP]). One experiment (Achenpass) was conducted with P-poor calcareous soil material of a Rendzic Leptosol (low-P soil), the other (Mitterfels) with Bw horizon material of a Cambisol that had formed on silicate bedrock and was characterized by moderate P concentrations (moderate-P soil). Half of the spruce seedlings additionally received experimentally elevated N deposition. At the low-P calcareous site, shoot and foliage biomass as well as foliar P, N, and S amounts of Norway spruce seedlings were considerably (+70–80%) and significantly larger on soils with spatially heterogeneous compared to homogeneous P distribution. Elevated N deposition reduced soil P heterogeneity effects on spruce by improving seedling growth on soils with homogeneous P distribution. No soil P form diversity effects were observed for spruce seedlings on calcareous soil. At the silicate site with moderate P supply, all seedlings showed excellent P nutrition. Here, only marginal, insignificant positive effects of heterogeneous soil P distribution were observed for the growth of spruce and beech seedlings without elevated N deposition, but foliar P concentrations of spruce seedlings increased significantly. Elevated N deposition resulted in a positive effect of heterogeneous vs. homogeneous soil P distribution on spruce growth (+39%; eta squared: 0.163; p=0.135) and P nutrition. Our results showed that soil P concentration heterogeneity is beneficial for spruce growth and P nutrition at sites with P-poor calcareous soils, and at silicate sites with ongoing N eutrophication.

Understanding hydrogen bonding in calcium silicate hydrate combining solid-state NMR and first principle calculations

Mechanisms of hydrogen bonding and molecular structure in the C-S-H are investigated by combining results of 29Si, 1H solid SSNMR analysis and first principle calculations. First principle calculations for 1H chemical shifts are used to correlate the observed 1H SSNMR spectra with the underlying structure of various hydrogen bonds between different silicon moieties and hydrogen groups in the C-S-H gels. The results show that strong hydrogen bonds are formed between water and terminal silicate sites or silanols, between adjacent terminal silanols and terminal silicate sites, between water or terminal silanols and hydroxyl groups bonded with interlayer calcium ions. Strong hydrogen bonds are favored to form in C-S-H with high Ca/Si molar ratios. The hydrogen bonding in terminal silicate sites is stronger than those in the paring or bridging silicate sites. New insights into the changes of basal spacing, Ca/Si ratio and H2O/Si ratio of in the molecular structure of C-S-H are presented based on the mechanism of hydrogen bonding.

Characterizing the Mineral Assemblages of Hot Spring Environments and Applications to Mars Orbital Data.

Certain martian hydrated silica deposits have been hypothesized to represent ancient hot spring environments, but many environments can produce hydrated silica on Earth. This study compares the mineral assemblages produced in terrestrial hot springs to those observed in silica-producing volcanic fumarolic environments to determine which diagnostic features of hot springs could be remotely sensed on Mars. We find that hot spring environments are more likely to produce geochemically mature silica (i.e., opal-CT and microcrystalline quartz) in addition to opal-A, whereas volcanic fumarolic environments tend to produce only opal-A, potentially reflecting differences in water-to-rock ratios. Neutral/alkaline hot springs contain few accessory minerals (typically calcite and Fe/Mg clays), while acidic hot springs commonly contain accessory kaolinite. By comparison, mineral assemblages at volcanic fumaroles contain protolith igneous minerals and a diversity of alteration minerals indicative of acidic conditions. Based on these terrestrial observations, the presence of opal-CT and/or microcrystalline quartz could be more diagnostic of a hot spring origin rather than a fumarolic origin, and accessory mineralogy could provide information on formation pH. On Mars, we observe that most orbital opal detections in outcrop are opal-A, sometimes accompanied by Fe/Mg clays, suggestive of neutral/alkaline conditions. However, these observations do not uniquely distinguish between hot springs and fumarolic environments, as opal-A can occur in both environments. Many martian silica detections occur in regionally extensive units, and sometimes in association with fluvial landforms suggesting a detrital or lower temperature authigenic origin. Thus, only a few martian opal detections may be mineralogically, spatially, and morphologically consistent with a hot spring origin. However, although it is difficult to unambiguously identify martian hot spring environments from orbital data sets, the orbital data are still valuable for identifying siliceous sites that are consistent with higher biosignature preservation potential, that is, sites with opal-A (not opal-CT), for future in situ investigations.

The influence of geology and season on macroinvertebrates in Belizean streams: implications for tropical bioassessment

Considerable attention has been paid to the potentially confounding effects of geological and seasonal variation on outputs from bioassessments in temperate streams, but our understanding about these influences is limited for many tropical systems. We explored variation in macroinvertebrate assemblage composition and the environmental characteristics of 3rd- to 5th-order streams in a geologically heterogeneous tropical landscape in the wet and dry seasons. Study streams drained catchments with land cover ranging from predominantly forested to agricultural land, but data indicated that distinct water-chemistry and substratum conditions associated with predominantly calcareous and silicate geologies were key determinants of macroinvertebrate assemblage composition. Most notably, calcareous streams were characterized by a relatively abundant noninsect fauna, particularly a pachychilid gastropod snail. The association between geological variation and assemblage composition was apparent during both seasons, but significant temporal variation in compositional characteristics was detected only in calcareous streams, possibly because of limited statistical power to detect change at silicate sites, or the limited extent of our temporal data. We discuss the implications of our findings for tropical bioassessment programs. Our key findings suggest that geology can be an important determinant of macroinvertebrate assemblages in tropical streams and that geological heterogeneity may influence the scale of temporal response in characteristic macroinvertebrate assemblages.

First principles characterization of silicate sites in clay surfaces.

Aluminosilicate clays like Montmorillonite (MMT) and Muscovite Mica (MT) have siloxane cavities on the basal plane. The hydroxyl groups localized in these cavities and van der Waals (vdW) forces contribute significantly to adsorption processes. However, the basal sites are found to be difficult to characterize experimentally. Here, (001) surfaces of MMT and MT clays were investigated using first-principles calculations to understand how these silicate surface sites are influenced by hydroxyl groups and the effective role of inner layer vdW interactions. Based on density-functional theory (DFT) within the generalized gradient approximation (GGA), different types of exchange-correlation functionals were tested to check the effect of vdW dispersion correction. Noncontact atomic force microscopy (nc-AFM), X-ray absorption spectroscopy (XAS) in the near-edge region and solid-state nuclear magnetic resonance (SS-NMR) spectroscopy were simulated. In both clays, the oxygen surface sites are directly affected by the intralayer interaction through hydroxyl groups. Our results indicated that the chemical environment of the hydroxyl groups is distinct in the MMT and MT structures. The vdW correction was essential for a better description of the surface oxygen sites and correctly describes the similarity between both clays. Particularly, the bulk apical oxygen sites in the MT structure are less influenced by vdW interaction. Compared to MMT, the silicon surface sites of MT are more sensitive to the intralayer changes in Si-Oapical-Al and with less effect of the hydroxyl groups. These results provide a clear understanding of influence of the siloxane cavity on the oxygen and silicon surface sites in aluminosilicates.

The Role of Al in Cross‐Linking of Alkali‐Activated Slag Cements

The structural development of a calcium (sodium) aluminosilicate hydrate (C–(N‐)A–S–H) gel system, obtained through the reaction of sodium metasilicate and ground granulated blast furnace slag, is assessed by high‐resolution 29Si and 27Al MAS NMR spectroscopy during the first 2 yr after mixing. The cements formed primarily consist of C–(N‐)A–S–H gels, with hydrotalcite and disordered alkali aluminosilicate gels also identified in the solid product assemblages. Deconvolution of the 27Al MAS NMR spectra enables the identification of three distinct tetrahedral Al sites, consistent with the 29Si MAS NMR data, where Q3(1Al), Q4(3Al), and Q4(4Al) silicate sites are identified. These results suggest significant levels of cross‐linking in the C–(N‐)A–S–H gel and the presence of an additional highly polymerized aluminosilicate product. The mean chain length, extent of cross‐linking, and Al/Si ratio of the C–(N‐)A–S–H gel decrease slightly over time. The de‐cross‐linking effect is explained by the key role of Al in mixed cross‐linked/non‐cross‐linked C–(N‐)A–S–H gels, because the cross‐linked components have much lower Al‐binding capacities than the noncross‐linked components. These results show that the aluminosilicate chain lengths and chemical compositions of the fundamental structural components in C–(N‐)A–S–H gels vary in a way that is not immediately evident from the overall bulk chemistry.

Potential drivers of spatial structure of leaf-litter food webs in south-western European beech forests

Spatial heterogeneity – in terms of topography, and macro- and microclimatic conditions, among others – results in habitat diversity, which in turn may promote species diversity. Thus, the spatial structure (i.e. spatial variance partitioning at different scales and spatial arrangement of drivers of abundance) of the different feeding guilds within food webs may reflect relevant differences in the way populations interact and how these interactions affect ecosystem processes. Our study focuses on the spatial distribution of animals living in the leaf litter layer of beech forests in the National Parks of northern Spain, across sites which differ in precipitation. Using Generalised Linear Mixed Models we estimated the spatial variance components at different scales and for three feeding guilds in leaf litter food webs: saprophagous and microphytophagous mesofauna, saprophagous macrofauna and zoophagous macrofauna. We found that the only consistently significant source of variation for the three feeding guilds was that at the level of “among valleys within Parks”, which, among other potential explanations, could reflect meso-climatic differences among valleys. We also studied which factors may control spatial variation in these food webs through a model selection approach. Controlling for all other relevant factors, we still found strong differences in abundances among National Parks. Also, invertebrates were more abundant in micro-sites located at the base of slopes – where nutrients and water accumulate – than in micro-sites located on the slopes – which, due to surface runoff, are drier and accumulate fewer nutrients. Also, as found in beech forests of Central Europe, limestone sites have higher abundances of fauna than siliceous sites with lower pH. The macro- and micro-scale dependence of these food webs on water availability could have important consequences for the persistence of these forests under global warming.

Comment on E. Prud'Homme et al., “Structural characterization of geomaterial foams — Thermal behavior”, J. Non-Cryst. Solids, 2011

We provide a discussion of a recent paper in Journal of Non-Crystalline Solids (E. Prud'Homme et al., “Structural characterization of geomaterial foams — Thermal behavior”, J. Non-Cryst. Solids, 2011, in press), and in particular propose a revised interpretation of the NMR spectra in terms of silicate site connectivity. This new interpretation is consistent with the infrared spectroscopic and thermogravimetric data presented in the same study, and also with known trends in geopolymer gel development as a function of the nature of the alkali cations present.

Origins of saccharide-dependent hydration at aluminate, silicate, and aluminosilicate surfaces

Sugar molecules adsorbed at hydrated inorganic oxide surfaces occur ubiquitously in nature and in technologically important materials and processes, including marine biomineralization, cement hydration, corrosion inhibition, bioadhesion, and bone resorption. Among these examples, surprisingly diverse hydration behaviors are observed for oxides in the presence of saccharides with closely related compositions and structures. Glucose, sucrose, and maltodextrin, for example, exhibit significant differences in their adsorption selectivities and alkaline reaction properties on hydrating aluminate, silicate, and aluminosilicate surfaces that are shown to be due to the molecular architectures of the saccharides. Solid-state (1)H, (13)C, (29)Si, and (27)Al nuclear magnetic resonance (NMR) spectroscopy measurements, including at very high magnetic fields (19 T), distinguish and quantify the different molecular species, their chemical transformations, and their site-specific adsorption on different aluminate and silicate moieties. Two-dimensional NMR results establish nonselective adsorption of glucose degradation products containing carboxylic acids on both hydrated silicates and aluminates. In contrast, sucrose adsorbs intact at hydrated silicate sites and selectively at anhydrous, but not hydrated, aluminate moieties. Quantitative surface force measurements establish that sucrose adsorbs strongly as multilayers on hydrated aluminosilicate surfaces. The molecular structures and physicochemical properties of the saccharides and their degradation species correlate well with their adsorption behaviors. The results explain the dramatically different effects that small amounts of different types of sugars have on the rates at which aluminate, silicate, and aluminosilicate species hydrate, with important implications for diverse materials and applications.

Relevance of genetics for conservation policies: the case of Minorcan cork oaks.

Marginal populations of widely distributed species can be of high conservation interest when they hold a significant or unique portion of the genetic diversity of the species. However, such genetic information is frequently lacking. Here the relevance of genetic surveys to develop efficient conservation strategies for such populations is illustrated using cork oak (Quercus suber) from Minorca (Balearic Islands, Spain) as a case study. Cork oak is highly endangered on the island, where no more than 67 individuals live in small, isolated stands in siliceous sites. As a consequence, it was recently granted protected status. Two Bayesian clustering approaches were used to analyse the genetic structure of the Minorcan population, on the basis of nuclear microsatellite data. The different groups within the island were also compared with additional island and continental populations surrounding Minorca. Very high genetic diversity was found, with values comparable with those observed in continental parts of the species' range. Furthermore, the Minorcan oak stands were highly differentiated from one another and were genetically related to different continental populations of France and Spain. The high levels of genetic diversity and inter-stands differentiation make Minorcan cork oak eligible for specific conservation efforts. The relationship of Minorcan stands to different continental populations in France and Spain probably reflects multiple colonization events. However, discrepancy between chloroplast DNA- and nuclear DNA-based groups does not support a simple scenario of recent introduction. Gene exchanges between neighbouring cork oak stands and with holm oak have created specific and exceptional genetic combinations. They also constitute a wide range of potential genetic resources for research on adaptation to new environmental conditions. Conservation guidelines that take into account these findings are provided.

Regeneration of spent bleaching earth and its adsorption of copper (II) ions from aqueous solutions

This study was designed to provide a comprehensive investigation into heat and acid reactivation of spent bleaching earth (SBE) and adsorption of Cu(II) ions from aqueous solutions. Heat treatment was the master variable in SBE regeneration. Dilute acid treatment did not constitute an effective SBE reactivation protocol for this purpose. Solvent extraction of residual oil using excess methylethyl ketone followed by heating at 370 °C was, therefore, the most effective reactivation procedure. Highly adsorptive materials with > 98% removal of Cu(II) ions from solution were obtained. Thus, > 80% Cu adsorption was reversible at SBE silicate sites because of their higher proportion in the adsorbent.

Sorption of Am3+ cations on suspended silicate: Effects of pH, ionic strength, complexing anions, humic acid and metal ions

Sorption of tracer Am3+ to silicate particles was studied as a function of pcH (4 to 9) and of ionic strength [0.20M to 1.50M (NaClO4)] at 298 K. The sorption increased with increased pcH from 4 to 6 above which saturation was observed. The insensitivity of Am3+ sorption to increased ionic strength indicates inner-sphere complexation with the surface silicate sites. The effects of different complexing anions such as carbonate, acetate, oxalate, phosphate, citrate, EDTA and humic acid, on Am3+ sorption were investigated. Synergistic enhancement in Am3+ sorption was observed in the presence of phosphate (4≤pcH≤7) and acetate (4≤pcH≤5) ligands at 0.20M NaClO4. The presence of the other ligands inhibited Am3+ sorption in the order: EDTA > citrate > oxalate > carbonate. Am3+ sorption in the presence of HA (25.00 mg/l) increased in the pcH range of 4.0 to 5.5, then decreased. Increased ionic strength enhanced Am3+ sorption in the presence of 25.00 mg/l HA for 4≤pcH≤9. The sorption increased in the presence of a mixture of HA (25.00 mg/l) and phosphate (1.00·10−3M) as compared to that of HA (25.00 mg/l) alone. The presence of Fe3+ (1.00·10−4M) enhanced Am3+ sorption at pcH∼4 but suppressed it from pcH of 5 to 9; 1.00·10−4M of Ca2+ and of UO22+ ions had no effect on the sorption profile.

Evolution of the local environment of lanthanum during simplified SON68 glass leaching

The evolution of the short- and medium-range local environment of lanthanum was determined by L III-edge X-ray absorption spectroscopy (XAS) during leaching of simplified SON68-type glasses. In glass without phosphorus, lanthanum is found in a silicate environment, and its first coordination sphere comprises eight oxygen atoms at a mean distance of 2.51 Å. When this glass was leached at a high renewal rate, the lanthanum local environment was significantly modified: it was present at hydroxycarbonate and silicate sites with a mean La–O distance of 2.56 Å, and the second neighbors consisted of La atoms instead of Si for the glass. Conversely, in the gel formed at low renewal rates, lanthanum was found in a silicate environment similar to that of the glass. In phosphorus-doped glass, lanthanum is found in a phosphate environment, although the Si/P atomic ratio is 20:1. Lanthanum is surrounded by seven oxygen atoms at a mean distance of 2.37 Å. When phosphorus-doped glass is leached, regardless of the leaching solution flow rate, the short- and medium-range lanthanum local environment remains almost constant; the most significant change is a 0.05 Å increase in the La–O distance.

Modeling Speciation in Highly Concentrated Alkaline Silicate Solutions

A model describing the distribution of silicate sites in concentrated alkali metal silicate solutions used in the synthesis of geopolymeric gels is presented. The model is based on a free energy minimization algorithm, using free energy of formation data obtained from the literature. A simplified form of the Pitzer method (Pitzer, K. S. J. Phys. Chem. 1973, 77, 26) is used to calculate activity coefficients, with interaction parameters calculated from known values for ions of similar size and charge type. Ion pairing effects are incorporated into the model formulation by a simple approximation, whereby larger cyclic or cagelike ions form strong ion pairs with dissolved alkali metal cations, and smaller monomeric (Q0) or end-group (Q1) sites pair less strongly with the cations. The model is able to describe accurately the speciation data obtained from a systematic 29Si NMR investigation of sodium, potassium, and mixed (1:1) sodium/potassium silicate solutions. The model is also tested against a large data set from the literature on sodium silicate solutions with a wide range of compositions. The model provides understanding of the speciation of silicate solutions and a basis for further understanding and modeling of the geopolymerization process.

Possible mechanism of interaction among the components in MAPP modified layered silicate PP nanocomposites

Model reactions were carried out with components frequently used for the preparation of intercalated or exfoliated polypropylene (PP) nanocomposites. The results prove that maleinated polypropylene (MAPP) can react chemically with the surfactant applied for the organophilization of the filler, if this latter contains active hydrogen groups. The reaction of hexadecylamine (HDA) and MAPP was detected by MALDI-TOF spectroscopy, DSC measurements and FTIR spectroscopy. Anhydride groups are consumed and mainly amide groups form in the reaction. The formation of cyclic imides could not be proved by the techniques used. MAPP reacts also with the surfactant adsorbed on the surface of the silicate in ionic form. On the other hand, N-cetylpyridinium chloride (CPCl) not containing active hydrogen atoms does not react with maleinated PP. Intercalated or exfoliated composites could be prepared from the silicate organophilized with HDA, while microcomposites formed from the filler treated with CPCl. Chemical reactions remove the surfactant from the surface of MMT and hydrogenated silicate sites are left behind. The high energy surface interacts either with the anhydride or the amide groups by dipole–dipole interactions. Even the unmodified polypropylene chains may be attached much stronger to the surface by London dispersion forces than to the silicate covered with aliphatic chains. Although the effect of competitive adsorption (MAPP, HDA) and mutual solubility of the components (PP, MAPP, surfactant, reaction products) cannot be neglected, chemical reactions play a crucial role in structure formation in PP nanocomposites containing a functionalized polymer. Direct interaction of the silicate surface and the functionalized polymer as well as the formation of hydrogen bridges seem to play a lesser role, but the relative influence of processes may change with the type of surfactant, functionalized polymer, surface coverage and processing conditions.