The predominant clay mineral in many soils formed on weathered volcanic ash in the central and northern part of the North Island of New Zealand is the amorphous aluminosilicate allophane. Although it can be obtained cheaply, allophane is the only common clay mineral not yet exploited commercially. This may be because large deposits of it only occur in a few areas of small extent on recent andesitic and rhyolitic parent material. These areas are usually remote from the industrial areas of the world. Allophane has been studied by soil chemists who have been concerned with its influence on the properties of soils. Their work suggested that allophane has unusual ion exchange and colloidal properties but much of the surface and colloid chemistry allophane was not well understood. The aim of this project was to investigate the properties of allophane with the view of developing commercial uses and also to understand better how it might behave in the natural environment. Amongst the physical properties determined were density, surface area and particle size distribution, for which a new centrifugal method was developed. From these properties it was shown that allophane is extensively hydrated and has a much larger surface area than could be associated with its external surfaces, confirming that it has a large internal surface. The surface charge of an allophane sample was determined by potentiometric titration; its point of zero charge was found to be below pH 5. However, determination of the surface charge below pH 6 was complicated by the dissolution of aluminium. The isoelectric point of the same material, as determined by electrophoresis, was near pH 6. This is about 3 pH units higher than for clay minerals such as kaolinite, halloysite and montmorillonite and so allophane is one of the few natural colloids which is neutral or positively charged at soil pH’s. The difference between the isoelectric point and the point of zero charge was due to the influence of the two different surface components; aluminosilicate, which tends to be negatively charged at near neutral pH, and aluminium hydroxide which tends to bear a positive charge under the same conditions. Dissolution studies showed that allophane derived from Tirau soil had aluminium in a state similar to that in aluminium hydroxide gel; that from Ohaupo soil had aluminium in a less soluble form but it was still more readily available than that generally associated with halloysite. Similar studies suggested that the iron found with allophane might exist as a separate and discrete phase. Some support for this was obtained from the uv spectra of allophane suspensions where weak absorbance bands were observed which coincided with those of colloidal iron (III) hydroxide. The “site-binding” model for ion adsorption at the hydrous oxide/aqueous interface was studied and it was developed and extended to cover multivalent cations, mixtures of cations, and cation adsorption in the presence of a slightly soluble metal hydroxide. The predictions made of the expected selectivity, selectivity changes with pH, and the effect of the presence of aluminium hydroxide on the adsorption of cations by a hydrous oxide were substantiated by the results of a study of cation adsorption by allophane. Cation adsorption was enhanced by increasing the pH. It was generally greater for cations of greater ionic charge, radius, and ease of hydrolysis. The selectivity shown by allophane between any two cations was found to vary with pH; at low pH the more favoured ion was that with the larger radius and lower charge but with pH increase the selectivity changed in a regular manner to favour small highly charged ions. The selectivity sequence at pH 4.0 was Pb²⁺ >> T1⁺ > Sr²⁺ ~ Co²⁺ ~ K⁺ > Ca²⁺ > Na⁺ but at pH 6.0 it had changed to Pb²⁺ >> Sr²⁺ ~ Co²⁺ > Ca²⁺ > T1⁺ > K⁺ > Na⁺. Monovalent thallium was found to bind more strongly, at lower pH, than any divalent species except hydrolysed lead. In much of its surface and colloid chemistry allophane behaves as a hydrous oxide. This is particularly evident from its interactions with ionic materials and from its colloid properties, i.e., its flocculation and electrophoretic mobility. Because of possible use in water treatment and pollution abatement the interactions of allophane with biopolymers such as protein and ribonucleic acid were studied. It was found to bind both strongly. The strength of adsorption of these was controlled by the pH and maximum adsorption occurred at the polymers isoelectric point. Viruses were shown to behave like proteins in their interactions with allophane; very efficient removal from suspensions of low concentration was shown to be possible. To investigate whether allophane could have any effect on the nutrient state of local lakes, studies were undertaken of the interactions of allophane with suspensions of the small green algae, Chlorella vulgaris. It was shown that these algae could be successfully flocculated by small amounts of allophane; in a suspension of 10⁶ cells per ml the first signs of extensive floc formation were visible after the addition of only 2 μg of clay per millilitre. The efficiency increased with increasing allophane concentration. The flocculation also proceeded at pH 7.2 where each colloid was separately stable. It was shown microscopically that the allophane was acting as a bridge between the algal cells to which it became bound, probably by the polysaccharides of the cell slime layer. The interactions of allophane with phosphates, biopolymers and microorganisms indicate that when it was able to enter lakes and rivers as a consequence of erosion, either naturally or assisted by man, it was capable of retarding, or even reversing, eutrophication. As a result of its high isoelectric point the colloid and surface chemistry of allophane differs from that of the other common clay minerals. The major differences are its greater ability to adsorb anionic materials whilst retaining a high capacity for cations, the ease with which it can be flocculated from neutral suspensions when the other clay minerals form stable sols, and its ability to flocculate stable negatively charged biocolloids. On the basis of these properties, and its high surface activity, allophane shows promise for use as an adsorbent and flocculant for effluents from such industries as slaughterhouses, canning factories, geothermal power stations and sewage treatment works (particularly those using oxidation ponds growing algae). A sound chemical case exists for investigating the use of allophane for effluent treatment at the pilot-plant stage. From this the possibility of utilising some of the billion tons of readily accessible allophane in New Zealand could be decided.