Mineralogy Laboratory Research Highlights
- Environmental Nanomaterials
- Interlayer clay-organic complexes in New Zealand soils
- Allophane and Imogolite
- Interactions of halloysites with amides
- Differentiating halloysite from kaolinite by intercalation of formamide
- Estimating surface areas of soils using para-nitrophenol
- Soil surface area and water adsorption
Environmental Nanomaterials
Allophane and smectite are nanomaterials of geological and pedological origins. Because of their small particle size, nanomaterials have an extensive surface area on which virtually every important chemical reaction takes place. Furthermore, a high proportion of the atoms in nanomaterials are located at or near surfaces, accounting for the charge characteristics and reactivity of such materials. Natural nanomaterials in soil and sediment perform a number of life-supporting services, such as regulating water and nutrient availability to plants, buffering element fluxes, and storing genetic information.
Nanomaterials in their natural state, or after surface modification, are good adsorbents for a variety of environmental pollutants and bioactive compounds, such as toxic heavy metals, pesticides, industrial organic effluents, and pharmaceutical compounds. Nanomaterials are abundant in New Zealand, inexpensive, and essentially non-toxic. They may be used for nutrient recycling, the treatment of industrial, pharmaceutical, and medical wastewater, and the remediation of contaminated air and soils. They may also serve as carriers or storage media for economically or environmentally important molecules. Interested individuals and investors should contact Dr Guodong Yuan for potential collaborations or technology development opportunities.
Bibliography
Yuan, G. 2004: Environmental nanomaterials (Occurrence, Syntheses, Characterization, Health Effect, and Potential Applications). A special issue of Journal of Environmental Science and Health A39 (10) (in press).
Yuan, G. 2003: Defining the distribution coefficient of heavy metals introduced to soils. Communications in Soil Science and Plant Analysis 34: 2315–2326.
Yuan, G.; Percival, H.J.; Theng, B.K.G.; Parfitt, R.L. 2002: Sorption of copper and cadmium by allophane-humic complexes. Developments in Soil Science 28A: 37–47.
TopInterlayer clay-organic complexes in New Zealand soils
Expanding layer silicates, notably smectites, can take up a wide range and variety of organic compounds between their layers to form interlayer clay-organic complexes or 'organo-clays'. Intercalation essentially occurs by one of two mechanisms: 1) exchanging the interlayer charge-balancing ions (Na+, Ca2+) by organic cations; and 2) replacing the interlayer water by uncharged organic molecules (Theng 1974, 1979). It seems surprising, therefore, that organo-clays are rarely found in nature. We were able to demonstrate their occurrence in certain New Zealand soils, using XRD in combination with carbon-13 NMR spectroscopy and chemical/heat pre-treatments (Theng et al. 1986). A combination of soil factors is apparently required for interlayer complex formation. This includes a smectitic clay mineralogy, an accumulation of organic matter, and a fairly acidic (low) pH. The interlayer organic compound is dominantly composed of polymethylene structures, and has an age of more than 6000 years. The persistence or 'inertness' of this material is very likely due to physical protection against microbial decomposition between the clay mineral layers.
Bibliography
Theng, B.K.G. 1974: The Chemistry of Clay-organic Reactions. London, Adam Hilger/New York/John Wiley. 343 pp.
Theng, B.K.G. 1979: Formation and Properties of Clay-polymer Complexes. Amsterdam, Elsevier. 362 pp.
Theng, B.K.G.; Churchman, G.J.; Newman, R.H. 1986: The occurrence of interlayer clay-organic complexes in two New Zealand soils. Soil Science 142: 262–266.
Allophane and Imogolite
Allophane and imogolite are clay-sized minerals commonly associated with soils derived from volcanic ash. They are also found in some sediments, non-volcanic soils, streambeds and drains.
Definitions
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| Allophane and imogolite – amorphous soil clay minerals commonly formed from volcanic parent materials. Photo: R Parfitt |
Ross and Kerr (1934) showed that allophane was an X-ray amorphous material commonly associated with the clay-mineral halloysite. They suggested, "the name allophane should be restricted to mutual solid solutions of silica, alumina, water and minor amounts of bases but should include all such materials, even though the proportions of these constituents may vary." Although this definition is still generally acceptable, some changes are required to a) exclude imogolite, b) allow for broad X-ray diffraction lines that are shown by allophane, and c) allow for synthetic allophanes, which may not contain bases at low pH. The definition given by Parfitt (1990a) is "Allophane is the name of a group of clay-size minerals with short-range-order which contain silica, alumina and water in chemical combination". There are three main types of allophane: aluminium-rich; silicon-rich; and stream deposit allophanes. Imogolite, a mineral made up of bundles of fine tubes, is excluded from this definition because it has long-range-order in one dimension.
Structures
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Allophane. Photo: S-I Wada |
Imogolite is a tubular mineral; the walls of individual tubes are 0.7 nm thick; the outer surface is a gibbsite-like curved sheet, and the inner surface consists of O3SiOH, with the oxygen replacing the inner hydroxyls of the gibbsite sheet. Allophane is made up of hollow spherules with a diameter of 4–5 nm. Al-rich allophane has an Al/Si ratio of about 2 and an imogolite-like structure. Si-rich allophane contains polymerised silicate, and has an Al/Si ratio of about 1. Stream deposit allophanes have Al/Si ratios of 0.9–1.8, with Al substituting for some Si in the polymeric tetrahedra. Allophane and imogolite have large specific surface areas (700–1500 m2/g) and react strongly with phosphate, arsenate and soil organic matter.
Identification and Estimation
In the field, allophane deposits may be identified by their characteristic greasy feel. As little as 2% allophane can be detected in this way. Allophane and imogolite can best be estimated by dissolving in acid-oxalate, and measuring concentrations of Al and Si (Parfitt 1990a). Imogolite can be estimated by electron microscopy and differential thermal analysis (Parfitt 1990b). However, if the sediment contains more than about 0.5% carbon, the contribution of Al from Al-humus complexes that dissolve in oxalate must be accounted for. This can be achieved by using pyrophosphate reagent (Parfitt 1990a).
Processes of Formation
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| Imogolite. Photo: C Hedley |
The rate of formation of allophane is chiefly controlled by environmental factors, together with the composition of the parent deposits. The effect of time is less important. The activity of silicic acid, the availability of Al species, and the opportunity for co-precipitation are very important. The Si and Al are controlled by leaching, organic matter and pH. Generally, allophane forms at pH values between 5 and 7, and a pH value of at least 4.8 is required for allophane to precipitate. Allophane is commonly found in tephra layers under humid climates, where volcanic glass dissolves to produce allophane. On the face of open soil pits containing rhyolitic tephra, allophane has been observed to precipitate within a time frame of months. The mineral can also precipitate in drains. Allophane has been found in tuffs, lacustrine sediments, and Silurian sediments. It is involved in the formation of indurated layers or pans in soils and sediments. Imogolite is usually found accompanying allophane, but the classical pure imogolite in Japan occurs as gel films over the surface of lapilli (Yoshinaga & Aomine 1962).
Bibliography
Parfitt, R.L. 1990a: Allophane in New Zealand – A Review. Australian Journal of Soil Research 28: 343–360.
Parfitt, R.L. 1990b: Estimation of imogolite in soils and clays by DTA. Communications in Soil Science and Plant Analysis 21: 623–628.
Ross, C.S.; Kerr, P.F. 1934: Halloysite and allophane. U.S. Geological Survey Professional Paper. Pp. 185–189.
Yoshinaga, N.; Aomine, S. 1962: Imogolite in some Ando soils. Soil Science and Plant Nutrition 8(3): 22–29.
Interactions of halloysites with amides
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| Te Akatea halloysite – a member of the kaolinite subgroup with two associated water molecules. Photo: B Theng |
Halloysites of different origin, particle shape, and crystallinity are found in New Zealand. To assess their reactivity toward organic compounds, we investigated the intercalation of some amides using XRD. The process essentially occurs through replacement by amide molecules of the interlayer water in halloysite. All the amides tested give single-layer complexes with basal spacings between 1.04 and 1.24 nm. When the interlayer space of halloysite has already been expanded by water (as in fully hydrated halloysite forms), the rate and extent of intercalation depend more on the properties of the amide intercalant than on the mineralogy of the clay. With partially and completely dehydrated halloysites, however, intercalation is influenced by the particle size, layer ordering, and iron content of the minerals.
Bibliography
Churchman, G.J.; Theng, B.K.G. 1984: Interactions of halloysites with amides: Mineralogical factors affecting complex formation. Clay Minerals 19: 161–175.
TopDifferentiating halloysite from kaolinite by intercalation of formamide
In mixtures with kaolinite (with a basal spacing of about 0.7 nm), hydrated halloysite can easily be identified by its basal reflection near 1.0 nm in the XRD pattern. However, halloysite readily loses its interlayer water, reducing the basal spacing to 0.7 nm. This makes it difficult to differentiate halloysite from kaolinite by XRD in mixtures where the halloysite component is largely dehydrated. A rapid and simple test to distinguish between halloysite and kaolinite has been developed based on differences in the rate and extent of formamide intercalation.
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| Kaolinite – a common soil clay mineral.
Photo: B Theng |
Halloysite – tubular spicules. Photo: C Ross |
Bibliography
Churchman, G.J.; Whitton, J.S.; Claridge, G.G.C.; Theng, B.K.G. 1984: Intercalation methods using formamide for differentiating halloysite from kaolinite. Clays and Clay Minerals 32: 241–248.
Estimating surface areas of soils using para-nitrophenol
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| Electron micrograph of Aokautere Ash showing a matrix of amorphous clay minerals. Photo: C Hedley |
The reactivity of a soil is closely related to its surface area. Surface area is therefore a key parameter for understanding many soil processes. However the surface area of a known weight of soil is difficult to measure. A method based on the adsorption of para-nitrophenol (pNP) from an organic solvent (Ristori et al. 1989) has been developed to estimate the surface area of soils varying in clay content and mineralogy. Monolayer coverage by pNP is identified by a plateau in the adsorption isotherm (Theng 1995). pNP intercalates into smectites, although it is unable to access interspherule pores of allophane. Since the adsorption of pNP is unaffected by organic matter (Theng et al. 1999), the method can be used to estimate the surface area of topsoils (Hedley et al. 2000).
Bibliography
Ristori, G.G.; Sparvoli, E.; Landi, L.; Martelloni, C. 1989: Measurement of specific surface areas of soils by p-nitrophenol adsorption. Applied Clay Science 4: 521–32.
Theng, B.K.G. 1995: On measuring the specific surface area of clays and soils by adsorption of para-nitrophenol: use and limitations. In: Churchman, G.J.; Fitzpatrick, R.W.; Eggleton, R.A. eds Clays controlling the environment: Proceedings of the 10th International Clay Conference, Adelaide, Australia 1993. Melbourne, CSIRO Publishing. Pp. 304–310.
Theng, B.K.G.; Ristori, G.G.; Santi, C.A.; Percival, H.J. 1999: An improved method for determining the specific surface area of topsoils with varied organic matter content, texture and clay mineral composition. European Journal of Soil Science 50: 309–316.
Hedley, C.B.; Saggar, S.; Theng, B.K.G.; Whitton, J.S. 2000: Surface area of soils of contrasting mineralogies using para-nitrophenol adsorption and its relation to air-dry moisture content of soils. Australian Journal of Soil Research 38: 155–67.
Soil Surface Area and Water Adsorption
The measurement of the specific surface area of soils may be useful for ranking soils for their ability to sorb organic compounds such as pesticides and pollutants. For A horizons with varied mineralogy, particularly those containing large amounts of soil organic matter, this is problematic. We estimate the surface area of a range of topsoils from water adsorption using the BET equation. The values we obtain are greater than those measured from the adsorption of para-nitrophenol (pNP). There is a good relationship between the BET water area and the cation exchanges capacity (CEC) of the samples (r2 = 0.83). There is a better relationship between the BET surface area and the water content of air-dry topsoils (r2 = 0.98). We suggest that the specific surface area of topsoils estimated from their air-dry water contents could be used to rank soils in the order of their surface reactivity.
Parfitt, R.L.; Whitton, J.S.; Theng, B.K.G. 2001: Surface reactivity of A horizons towards polar compounds estimated from water adsorption and water content. Australian Journal of Soil Research 39: 1105–1100.
Bibliography
Childs, C.W.; Inoue, K.; Seyama, H.; Soma, M.; Theng, B.K.G.; Yuan, G. 1997: X-ray photoelectron spectroscopic characterization of Silica Springs allophane. Clay Minerals 32: 565–572.
Coyne, L.M.; Costanzo, P.M; Theng, B.K.G. 1989: Luminescence and electron spin resonance studies of relationships between O-centres and structural iron in natural and synthetically hydrated kaolinites. Clay Minerals 24: 671–693.
Parfitt, R.L. 1990: Allophane in New Zealand - A Review. Australian Journal of Soil Research 28: 343–360.
Soma, M.; Churchman, G.J.; Theng, B.K.G. 1992: X-ray photoelectron spectroscopic analysis of halloysites with different composition and particle morphology. Clay Minerals 27: 413–421.
Soma, H.; Seyama, H.; Yoshinaga, N.; Theng, B.K.G.; Childs, C.W. 1996: Bonding state of silicon in natural ferrihydrites by X-ray photoelectron spectroscopy. Clay Science 9: 385–391.
Theng, B.K.G. 1974: The chemistry of clay-organic reactions. London, Adam Hilger/New York, John Wiley. 343 p.
Theng, B.K.G., 1979: Formation and properties of clay-polymer complexes. Amsterdam/Oxford/New York, Elsevier Scientific Publishing Company. 362 p.
Theng, B.; Churchman, J. 1993: Taita research centre – four decades of science, 1951-93. Mineralogy. NZ Soil News 41: 123–127.
Theng, B.K.G.; Wells, N. 1995: The flow characteristics of halloysite suspensions. Clay Minerals, 30: 99–106.
Theng, B.K.G.; Russell, M.; Churchman, G.J.; Parfitt, R.L. 1982: Surface properties of allophane, halloysite, and imogolite. Clays and Clay Minerals 30: 143–149.
Whitton, J.S.; Churchman, G.J. 1987: Standard methods for mineral analysis of soil survey samples for characterisation and classification in NZ Soil Bureau. NZ Soil Bureau Scientific Report 79. Lower Hutt, New Zealand, NZ Soil Bureau. 27 p.