A. R. Fraser

Characterization of the chemical structures of natural and synthetic aluminosilicate gels and sols by infrared spectroscopy

Infrared spectra in the 300–1400 cm−1 region indicate that the non-crystalline products of interaction between hydroxyaluminium species and orthosilicic acid in dilute aqueous solutions of pH 6. The compound formed in acid solution has an infrared spectrum similar to imogolite, and is termed proto-imogolite; like imogolite, it contains orthosilicate groups and 6-coordinated aluminium, and has an ideal Si:Al ratio of around 0.5, but it can incorporate some excess alumina or silica and it does not have the regular tubular structure of imogolite. Compounds formed in alkaline solutions that are not too dilute have infrared spectra resembling the feldspathoid group of minerals and are termed hydrous feldspathoids. They incorporate a condensed tetrahedral framework with an Si:Al ratio greater than one, but can also contain 6-coordinated aluminium. Natural allophanes of the proto-imogolite and hydrous feldspathoid types exist. The allophane of weathered pumice, however, contains a condensed silicate anion that incorporates little tetrahedral aluminium. Proto-imogolite forms stable sols at pH < 5 and must play an important role in the transport of aluminium in acidic natural waters containing dissolved silica.

A Moessbauer and I.R. spectroscopic study of the structure of nontronite

Mössbauer and i.r. spectra of a series of nontronites show that Fe3+ and Al3+ are distributed between tetrahedral and octahedral sites. The Mössbauer results have reaffirmed the occupation by Fe3+ of octahedral sites at which these ions are coordinated to pairs of OH groups in both cis and trans configurations. The distribution of Fe3+ between these two sites varies considerably but in all of the nontronites some Fe3+ occurs in the trans site in contrast to the all cis occupancy of the centro-symmetric structure proposed by Mering and Oberlin (1967). In one of the nontronites the distribution of Fe3+ between these two sites approaches that in the ideal non-centrosymmetric structure proposed for montmorillonite.

Podzolisation mechanisms and the synthesis of imogolite in northern Scandinavia

Abstract The nature of the short-range ordered Al and Fe minerals of the spodic B horizons of northern Scandinavia was studied by selective dissolution, transmission electron microscopy, infrared absorption and sulphate adsorption experiments. Imogolite-type materials (proto-imogolite allophane and well-developed imogolite) were the predominant oxalate-extractable Al minerals. Minor amounts of gibbsite and kaolinite were also found. The Fe oxides were less abundant and relatively well crystallized. Imogolite-type materials were the predominant source of positive charge, as evidenced by the amount of sulphate adsorbed. The Fe:AI ratio of the pyrophosphate extracts was positively correlated with the ratio of pyrophosphate-C to oxalate-extractable Al—this observation provides field evidence that imogolite-type materials were particularly unstable in the presence of organic substances. It is hypothesized that most accumulated Al and Fe originated from the congruent dissolution of primary minerals followed by the downward migration and degradation of metal-organic complexes, but incongruent weathering within the B horizon might have made additional contributions to the precipitation of the short-range ordered minerals. Historical climatic change might, to some extent, explain why metal-organic complexes were almost absent in many B horizons.

Role of quantitative mineralogical analysis in the investigation of sites contaminated by chromite ore processing residue

A range of techniques, normally associated with mineralogical studies of soils and sediments, has been used to characterise the solid materials found on sites contaminated with chromite ore processing residue (COPR). The results show that a wide range of minerals are present, many of which are found extensively in high-temperature synthetic systems such as cements and clinkers and their low temperature hydration products. Thus, the minerals in COPR can be divided into three main categories: unreacted feedstock ore (chromite); high temperature phases produced during chromium extraction (brownmillerite, periclase and larnite); and finally, minerals formed under ambient weathering conditions on the disposal sites (brucite, calcite, aragonite, ettringite, hydrocalumite, hydrogarnet). Apart from chromite, chromium occurs in brownmillerite, ettringite, hydrocalumite and hydrogarnet. Detailed study of the chemistry and stoichiometry of chromium-bearing phases in conjunction with phase abundance provides a quantitative description of the solid state speciation of Cr(III) and Cr(VI) in and amongst these minerals and in the COPR as a whole. Of the total chromium present in the samples most, approximately 60-70% is present as Cr(III) in chromite, whilst brownmillerite also represents a significant reservoir of Cr(III) which is approximately 15% of the total. The remaining chromium, between 20 and 25%, is present as Cr(VI) and resides mainly in hydrogarnet, and to a slightly lesser extent in hydrocalumite. In the latter, it is present principally in an exchangeable anionic form. Chromium (VI) is also present in ettringite, but quantitatively ettringite is a much less important reservoir of Cr(VI), accounting for approximately 3% of total chromium in one sample, but less than 1% in the other two. This description provides insight into the processes likely to control the retention and release of Cr(VI) from COPR-contaminated sites. Such information is of particular value in chemical modelling of the system, in risk assessment and in the development of methods of informed remediation.

FTIR spectroscopy of peat in and bordering Scots pine woodland: relationship with chemical and biological properties

Rapid methods of analysis, such as infrared spectroscopy, have advantages where chemical or biological analyses are time-consuming or require fresh material. FTIR (Fourier transform infrared) spectroscopy gives information on specific functional chemical groups present in organic matter. However, the whole spectrum may also contain information that relates to more general chemical or biological properties. Peat samples were taken along a transect from moorland, through a wetter bog area, into native pine at Abernethy Forest, Scotland. FTIR spectra were determined in addition to other chemical, physical and microbiological properties. Canonical variate analysis showed that the FTIR spectra of the peat from three major vegetation zones were significantly different. Partial least squares (PLS) analysis gave a good correlation between the zero-order FTIR spectra and physical/chemical properties such as pH (variance accounted for, r2=78%), ash (r2=69%) and total N (r2=60%). In particular, the regions in the spectrum corresponding to two secondary amide groupings explained much of the variation in total N. Correlations with biological parameters were poorer except for microbial biomass C (r2=75%) and total phospholipid fatty acids (r2=65%). However, the PLS models were not suitable for predicting the chemical and microbiological properties of the moorland and forest peat soils investigated in this study reliably.

Adsorption of carbon dioxide on goethite (α-FeOOH) surfaces, and its implications for anion adsorption

Carbon dioxide is strongly adsorbed as CO2–3 on moist goethite surfaces, but as both CO2–3 and HCO–3 on dry surfaces. On the basis of a model of the surface, sites for the adsorption of the carbonate species are suggested where these ions would play a role in redistributing surface charge. A similar role is proposed for strongly adsorbed anions such as phosphate.

INFRARED AND MOSSBAUER STUDIES OF REDUCED NONTRONITES

Infrared and Mössbauer spectroscopy show that the extent of the reduction of nontronite is dependent on the chemical composition of the nontronite and on the nature of the reducing agent. Hydrazine reversibly reduces about 10% of the iron in all of the nontronites studied irrespective of composition and it is suggested that the resulting ferrous iron occurs only in distorted octahedral sites. Similar conclusions are reached for the dithionite reduction of the nontronites containing little tetrahedral iron, but for those with more than one in eight silicons replaced by iron, changes brought about by dithionite treatment are irreversible due to dissolution of appreciable quantities of iron. Results from both spectroscopic techniques suggest that iron in tetrahedral sites is preferentially dissolved and that up to 80% of the structural iron can be reduced.Evidence is presented for the formation in these extensively reduced nontronites of a small amount of a mica-like phase resembling celadonite or glauconite, and, as dithionite is used for the pretreatment of soils, the implication of this observation is briefly discussed.The use of deuterated hydrazine as a reducing agent has enabled the nontronite absorption band near 850 cm-1 to be assigned to a Si-O (apical) stretching vibration, which is inactive in the infrared for perfect hexagonal symmetry, but which is activated by distortions in the tetrahedral layer.РезюмеСпектроскопия инфракрасных лучей и Моссбауэра показывает, что сте-пень восстановления нонтронита зависит от его химического состава и приро-ды восстановления. Гидразин обратимо восстанавливает около десяти процен-тов железа во всех изученных нонтронитах, независимо от состава, и предпола-гается, что двухвалентное железо встречается только в деформированных окта-эдрических формах. Подобные заключения были сделаны и для дитионитового восстановления нонтронитов, содержащих немного тетраэдрического железа, но для тех, в которых больше, чем 1 из 8 силиконов, были замещены железом, изме-нения вызванные дитионитовой обработкой являются необратимыми, ввиду раство-рения значительного количества железа. Результаты исследований с помощью обоих спектроскопических методов показывают, что железо в тетраэдрических формах растворяется легче и что до восьмидесяти процентов структурного же-леза может быть восстановлено.Приводятся доказательства, что в этих сильно восстановленных нонтронитах образуется небольшое количество слюдо-подобной фазы, напоминающей селадонит или глауконит, и, если дитионит используется для предварительной обработки почв, коротко обсуждается значение этих наблюдений.Использование дейтерированного гидразина в качестве восстановителя спо-собствует совмещению нонтронитовой абсорбционной связи вблизи 850см−1 с Si-O /апикальной/ растягивающей вибрацией, которая инертна в инфракрасных лучах благодаря совершенной гексагональной симметрии, но которая активизи-руется в результате деформаций в тетраэдрическом слое.KurzreferatInfrarot-und Mössbauerspektroskopie zeigen, daß der Ausmaß der Reduktion von Nontronit von der chemischen Zusammensetzung des Nontronit und von der Art des Reduktionsmittels abhängt. Hydrazin reduziert in umkehrbarer Reaktion ungefähr 10% des Eisens in allen Nontroniten, unabhängig von der Zusammensetzung und es wird vorgeschlagen, daß das resultierende Fe(II)-Eisen nur in verformten oktahedrischen Plätzen vorkommt. Ähnliche Beschlüsse wurden für die Reduktion mit Dithionit von Nontronit, welches ein wenig tetrahedrisches Eisen enthält, gemacht. diejenigen Nontroniten, die mehr als 1 unter 8 Silizium durch Eisen ausgewechselt haben, ist die Veränderung hervorgebracht durch Dithionitbehandlung, nicht reversibel wegen Auflösung von nennenswerten Mengen von Eisen. Resultate von beiden spektrokopischen Techniken schlagen vor, daß das Eisen in den tetrahedrischen Plätzen bevorzugt aufgelöst wird, und daß bis zu 80% des strukturellen Eisens reduziert werden kann. Beweise werden presentiert für die Formation in diesen ausgedehnt reduzierten Nontroniten, von einer glimmerartigen Phase, welche Keladonit oder Glaukonit gleichen und weil Dithionit für die Behandlung der Erden benutzt wird, wird die Implikation dieser Beobachtung kurz diskutiert. Das Benutzen von Deuterium-Hydrazin als Reduktionsmittel macht es möglich, das Nontronitadsorptionsband bei 850 cm−1 einer Si-O Streckschwingung zuzuschreiben. Für perfekte, hexagonale Symmetrie sind diese Schwingungen inaktiv, aber für Verzerrungen in der tetrahedrischen Schicht sind sie aktiv.RésuméLa spectroscopie infrarouge et de Mössbauer montre que l’étendue de la réduction de la nontronite dépend de la composition chimique de la nontronite et de la nature de l’agent réducteur. L’hydrazine réduit réversiblement à peu près 10% du fer dans toutes les nontronites étudiées irrespectivement de leur composition, et il est suggéré que le fer ferreux qui en résulte ne se trouve que sur des sites octaèdres déformés. Des conclusions semblables ont été tirées pour la réduction par la dithionite de nontronites contenant peu de fer tétraèdre, mais pour celles avec plus d’un silicium sur huit remplacé par le fer, les changements amenés par le traitement à la dithionite étaient irréversibles à cause de la dissolution de quantités appréciables de fer. Les résultats des deux techniques spectroscopiques suggèrent que le fer sur les sites tétraèdres est préferentiellement dissolu et que jusqu’à 80% de fer de composition peut être réduit. Des preuves de la formation dans ces nontronites d’une petite quantité d’une phase pareille au mica ressemblant à de la celadonite ou à de la glauconite sont présentées, et, comme la dithionite est utilisée pour le pré-traitement des sols, l’implication de cette observation est brièvement discutée. L’emploi de l’hydrazine deutérée comme agent réducteur a permi à la bande d’adsorption de la nontronite près de 850 cm−1 d’être assignée à une vibration allongeante Si-O (apique), qui est inactive dans l’infrarouge pour la symmétrie hexagonale parfaite, mais qui est activée par des distortions dans le feuillet tétraèdre.

Synthesis of imogolite: a tubular aluminium silicate polymer

Heating weakly acidic dilute solutions containing hydroxyaluminium orthosilicate complexes gives a fibrous polymer, each fibre consisting of tubes about 2·2 nm external diameter and 1·0 nm internal diameter, essentially identical with those of the natural mineral, imogolite.

Surface reactivity of poorly-ordered minerals in podzol B horizons

The surface reactivity of mineral soil horizons from three podzolised forest soils in Scandinavia was examined. The amount of accumulated C was low, between 1.8 and 2.3% in the top of the B horizon ...

An IR spectroscopic study of soil humin and its relationship to other soil humic substances and fungal pigments

Abstract Humin, with a relatively low ash content, was isolated from a sandy soil by exhaustively dissolving humic and fulvic fractions in cold 0.5 M NaOH, and the mineral components in 20% HF. The infrared spectrum of the insoluble humin was similar to that of humic acid derived from the same soil, and resembled those of the humic-like pigments obtained from the soil fungi Coniothyrium minitans and Rhizoctonia solani . It was less like the spectrum of the pigment from Aspergillus niger which closely resembled that of the melanin pigment from Sclerotinia sclerotiorum . The brown debris remaining after extracting some of the fungi with NaOH, produced, on acid hydrolysis, fractions having IR spectra similar to acid hydrolysed humin. It is concluded from the similarity of humin, humic acid and fungal pigments, that well humified soil organic matter is to a large extent microbial in origin. When 14 C-labelled humin from a sandy loam was incubated with fresh soil, about 20% of the radioactivity was lost after 3 months and 25% after 6 months, but subsequently there was virtually no further loss. A similar pattern of stability was obtained using 14 C-labelled humic acid and this result, in conjunction with those from infrared spectroscopy, indicates a close relationship between humin and humic acid in terms of their structure, and their turnover in the soil. It is concluded that the composition of a soil humin is likely to be determined by the soils microbial population.