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Some amorphous constituents of New Zealand soils

Part 1 – Allophanes derived from volcanic ash Soils formed on weathered volcanic ash in the central and northern North Island of New Zealand contain the amorphous aluminosilicate allophane as the predominant clay mineral which is produced by the weathering of volcanic glass. Allophane has a characteristic DTA curve and I.R. spectrum which serve as a useful means of identifying the clay mineral. Fine clays were separated from soils in the Waikato area and these showed typical allophane patterns of variable cation exchange capacity as well as DTA and I.R. spectra. Under the electron microscope these clays presented a picture of very small, ill-defined particles with indistinct edges and it was apparent that most of the properties of allophane could be explained by the presence of a colloidal system. The colloidal properties of allophane were investigated and it was shown to be remarkable among clay minerals in having a high isoelectric point (∼6). This means that allophane is positively charged under normal soil pH’s (4.5 – 5.5). Synthetic aluminosilicates were prepared with varying SiO₂/Al₂O₃ ratios and it was found that with a SiO₂/Al₂O₃ ratio between 2 and 1.5 the isoelectric point was in the range 4.5 - 5.0. Increasing the ratio resulted in a lowering of the isoelectric point. If the allophane samples were deferrated using sodium dithionite and sodium citrate then the isoelectric point of the resulting material was below 2. The very high chemical reactivity normally associated with allophane was thus reduced, although the general properties such as the NaF/phenolphthalein teat and I.R. spectra were unaffected. The results show that the ferric oxide, despite its small proportion, contributes significantly to the high isoelectric point of the allophane found in natural soils. A definition for allophane is proposed which includes its remarkable properties of high isoelectric point, high surface area, high ion exchange capacity along with its very high phosphate retention. The presence of imogolite was noted in New Zealand soils for the first time in one sample. Since New Zealand has a cheap, abundant supply of this unusual material, work was initiated to find possible commercial uses for allophane. One possible use described is the removal of protein from slaughterhouse effluent. The main proteins found in slaughterhouse effluent, hemoglobin, serum albumin and gelatin, were adsorbed onto allophane and adsorption isotherms determined for each. Allophane adsorbed up to 10% by weight of protein from a solution with the protein concentration of the order 1 mg/ml, after which the clay became saturated and would adsorb no further protein. At this stage the protein-clay complex became “jelly-like” and sank to the bottom of the flask. The actual amount of uptake was dependent on the particular allophane used and the solution pH. Allophane shows indications of having antibiotic properties. The breakdown of protein by bacterial activity was greatly inhibited by the presence of allophane in the protein solutions. These solutions could be left open to the air for several days without any change in protein concentration measured by U.V. absorption. Also, when samples of allophane were examined under the electron microscope there were no signs of bacteria, unlike samples from non-volcanic soils in which bacteria could be readily seen. This phenomenon appears coupled with the fact that allophane from volcanic areas bears a positive charge at a pH less than about 6. The bacteria, being negatively charged, become surrounded by a coating of minute allophane particles which inhibits growth. Part II – Synthetic aluminosilicates Synthetic aluminosilicates were prepared to investigate the colloidal properties of alumina and silica. Positively charged hydrated alumina will adhere to the negatively charged surface of freshly cleaved mica and will readily absorb phosphate from a solution containing ³²P phosphate. Negatively charged hydrated silica was adsorbed onto the surface of alumina-treated mica. This would be expected to block the alumina to phosphate uptake, but this was not the case, the uptake being only slightly less than the alumina treated mica. If further alumina was added, little difference in phosphate uptake was again noted suggesting a uniform surface is presented to the phosphate solution. With hydsrated ferric oxide and hydrated silica alternately adsorbed on the mica surface, an oscillating pattern was noted showing that layers can be built up. The experiment was repeated using ⁵⁹Fe to ensure that layers were built up and not being removed and then replaced. These experiments showed that amorphous hydrous silica and alumina will react in some way and it is suggested that a reaction takes place between the colloidal hydrous aluminia and silica with the formation of Al-O-Si bonds. The ferric oxide system does not however for Fe-O-Si bonds, but it is suggested the colloidal particles are held together by electrostatic bonds in a form of “colloid complex”. Fresh colloidal suspensions of hydrated alumina and silica prepared from hydrolysis of corresponding organo-derivatives, when mixed and freeze dried, provided an amorphous product with the properties or allophane. But if the solutions were allowed to age by standing tor several days, mixed and then freeze dried, the DTA pattern and X-ray spectrum showed the presence of bayerite (a crystalline form of Al(OH)₃). Addition of acid or base to the hydrolysing solutions increased the rate of crystallisation of hydrated aluminia to either bayerite or gibbsite. Part III – Investigation of other “allophane-like” clay minerals An allophane-like clay mineral has been reported by the pedologists in the High Country soils of the South Island of New Zealand. These soils have been derived from greywacke. In this thesis an attempt was made to bridge the gap between the pedological and chemical description of these soils. Pedologists place great stock by the feel of a soil when it is rubbed between the fingers. When investigated it was found that it was the presence of amorphous oxides or materials less than 10⁻⁵cm that produces a slippery, non-sticky feel, usually associated with the presence of allophane. These High Country soils give this characteristic test and on this basis have been thought to contain allophane. The fine clays from both these soils, and the soils derived from chlorite-schist under similar climatic conditions, were separated and investigated. The soil properties of cation exchange, phosphate retention as well as the I.R. spectra, DTA curves and chemical composition were determined. These soils all gave a positive NaF/phenolphthalein test and up to 30% of the fine clay dissolved in boiling 0.5 M NaOH. However the DTA pattern showed a large additional exotherm and an additional endotherm which are not characteristic of allophane derived from volcanic parent material. The electron micrographs showed the fine clays to be sharp, well-defined particles. Rocks taken from the soil and finely ground in a Tema Mill, showed similar properties. The samples were high in silica and had isoelectric points around three which means the particles bear a negative charge in soil under most field conditions. These results showed the clays to be less reactive chemically than volcanic allophanes. A mechanism of soil formation is proposed whereby the rock is first split by ice-wedging and the pieces further ground by solifluction processes to produce very finely divided rock. The ground rock contains two X-ray amorphous components. One is micro-crystalline whose size is too small to diffract X-rays or to produce an I.R. pattern, and which can only be studied by DTA and electron microscopy. The only weathering that has taken place in this component is the oxidation of much of the iron from ferrous to ferric and a certain amount of leaching. The second component is rock whose original crystal lattice has been destroyed and so it is truly amorphous having a random structure. It is proposed that these amorphous fractions may be a common soil constituent in alpine and polar regions. This material, which it is proposed is a variation of allophane, is given the name “Alpinite”. “Alpinite” will usually weather to the more usual clay minerals. Under periglacial conditions, however, the rate of formation is high and the rate of transformation to other clay minerals is low so that a high steady state concentration is reached. So high that sample of the fine clays (< 0.2μ) from soils derived at high altitudes give no X-ray pattern whatsoever.
Type of thesis
The University of Waikato
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