G. Jock Churchman

Halloysite nanotubes: prospects and challenges of their use as additives and carriers – A focused review

Abstract There is increasing research interest in potential applications of halloysite as fillers for polymer composites, controlled drug delivery, carriers for the supply and sustained release of active agents for anticorrosion coatings, in nanoreactors or nanotemplates, and for the uptake of contaminants or pollutants and support for catalysts. In this review, recent findings in terms of the prospects and challenges of using halloysite nanotubes (HNTs) in different polymeric matrices targeting a range of old and new applications are discussed and evaluated. The compositions include chitosan/halloysite membranes as bone-tissue scaffolds, polylactic acid (PLA)/halloysite membranes for food-packaging applications and their antimicrobial activities, instrumented impact properties of epoxy/halloysite nanocomposites and the role of halloysite in the self-healing of epoxy composites, polyacryronitrile (PAN)/halloysite membranes for use in water filtration as well as a review of some recent applications of halloysite/alginate beads in the adsorption of contaminants such as lead.

Unique but diverse: some observations on the formation, structure, and morphology of halloysite

Abstract New insights from the recent literature are summarized and new data presented concerning the formation, structure and morphology of halloysite. Halloysite formation by weathering always requires the presence of water. Where substantial drying occurs, kaolinite is formed instead. Halloysite formation is favoured by a low pH. The octahedral sheet is positively charged at pH < ∼8, whereas the tetrahedral sheet is negatively charged at pH > ∼2. The opposing sheet charge would facilitate interlayer uptake of H2O molecules. When halloysite intercalates certain polar organic molecules, additional (hkl) reflections appear in the X-ray diffraction pattern, suggesting layer re-arrangement which, however, is dissimilar to that in kaolinite. Associated oxides and oxyhydroxides of Fe and Mn may limit the growth of halloysite particles as does incorporation of Fe into the structure. Particles of different shape and Fe content may occur within a given sample of halloysite.

The origin of spheroidal halloysites: a review of the literature

Abstract Tubular halloysite has many applications as a nanomaterial. Spheroidal halloysite (SPH) is the other most common form of halloysite. Its mode of formation has had different explanations, including association with allophane, or more generally, following weathering of volcanic glass. Some SPHs have formed from minerals in crystalline rocks, sometimes as an early stage of evolution into plates and/or tubes of halloysite and ultimately to kaolinite. Spheroidal halloysites can show a range of Fe contents and can occur with other forms of halloysite; they have often formed in confined environments whereas tubular halloysites apparently form in more open spaces. They have also formed on microbes or where there is a significant amount of organic matter. Generally, SPHs have often formed by rapid dissolution of volcanic glass and primary minerals. The SPHs can persist over time. They have few active edges, so interparticle interaction is poor, causing low viscosities in clay-water suspensions, poor soil stability and low adsorption capacities.

Carbon Storage and DNA Adsorption in Allophanic Soils and Paleosols

Andisols and andic paleosols dominated by the nanocrystalline mineral allophane sequester large amounts of carbon (C), attributable mainly to its chemical bonding with charged hydroxyl groups on the surface of allophane together with its physical protection in nanopores within and between allophane nanoaggregates. C near-edge X-ray absorption fine structure (NEXAFS) spectra for a New Zealand Andisol (Tirau series) showed that the organic matter (OM) mainly comprises quinonic, aromatic, aliphatic, and carboxylic C. In different buried horizons from several other Andisols, C contents varied but the C species were similar, attributable to pedogenic processes operating during developmental upbuilding, downward leaching, or both. The presence of OM in natural allophanic soils weakened the adsorption of DNA on clay; an adsorption isotherm experiment involving humic acid (HA) showed that HA-free synthetic allophane adsorbed seven times more DNA than HA-rich synthetic allophane. Phosphorus X-ray absorption near-edge structure (XANES) spectra for salmon-sperm DNA and DNA-clay complexes indicated that DNA was bound to the allophane clay through the phosphate group, but it is not clear if DNA was chemically bound to the surface of the allophane or to OM, or both. We plan more experiments to investigate interactions among DNA, allophane (natural and synthetic), and OM. Because DNA shows a high affinity to allophane, we are studying the potential to reconstruct late Quaternary palaeoenvironments by attempting to extract and characterise ancient DNA from allophanic paleosols.

Clay addition and redistribution to enhance carbon sequestration in soils

The association of organic carbon (SOC) with clay in soils means that additions of clay to soils can increase the capacity of the soils for storage, and, eventually, sequestration of C. Addition of a fine-textured waste from bauxite processing to sandy soils for up to 29 years has led to increases of about 12 Mg C ha−1, with a strong (r 2 = 0.93, P < 0.001) correlation between clay content and SOC. An increase of 2.2 Mg C ha−1 has also occurred after 8 years in a sandy topsoil amended with subsoil clay-rich material. Bentonite addition increased plant yield in degraded and light-textured soils in tropical Australia. In Thailand, addition of clay-rich materials, particularly bentonite, but also clayey termite mound material, greatly increased the productivity of a degraded light-textured soil.

Discovery of halloysite books in altered silicic Quaternary tephras, northern New Zealand

Abstract Hydrated halloysite was discovered in books, a morphology previously associated exclusively with kaolinite. From ∼1.5 to ∼1500 μm in length, the books showed significantly greater mean Fe contents (Fe2O3 = 5.2 wt.%) than tubes (Fe2O3 = 3.2 wt.%), and expanded rapidly with formamide. They occurred, along with halloysite tubes, spheroids and plates, in highly porous yet poorly permeable, silt-dominated, Si-rich, pumiceous rhyolitic tephra deposits aged ∼0.93 Ma (Te Puna tephra) and ∼0.27 Ma (Te Ranga tephra) at three sites ∼10-20 m stratigraphically below the modern landsurface in the Tauranga area, eastern North Island, New Zealand. The book-bearing tephras were at or near saturation, but have experienced intermittent partial drying, favouring the proposed changes: solubilized volcanic glass + plagioclase→halloysite spheroids→halloysite tubes→halloysite plates→ halloysite books. Unlike parallel studies elsewhere involving both halloysite and kaolinite, kaolinite has not formed in Tauranga presumably because the low permeability ensures that the sites largely remain locally wet so that the halloysite books are metastable. An implication of the discovery is that some halloysite books in similar settings may have been misidentified previously as kaolinite.

Halloysite from the Eucla Basin, South Australia – Comparison of Physical Properties for Potential New Uses

The tubular form of halloysite, a kaolin group mineral, is a natural nanotube that has attracted research interest in development of new products as fibre reinforcement in polymers and as micro-containers for controlled delivery of active agents. The principal source of commercial halloysite is from Northland, New Zealand, with large resources also available at the Dragon Mine, Utah, USA. These formed by hydrothermal activity. Physical characteristics of these halloysites are compared with those of halloysites from a potential new source in the Eucla Basin, South Australia, formed by the action of acid groundwater on fine-grained sediments. Nitrogen adsorption and transmission electron microscopy show that the hydrothermal halloysites have a low specific surface area and are thicker and more varied in pore dimensions than the more regular, thinner tubes associated with the acid groundwater deposit. Tailoring the choice of clay to meet specific requirements should assist in optimising the performance of new products.

Size of subsoil clods affects soil-water availability in sand–clay mixtures

Clay delving in strongly texture-contrast soils brings up subsoil clay in clumps ranging from large clods to tiny aggregates depending on the equipment used and the extent of secondary cultivation. Clay delving usually increases crop yields but not universally; this has generated questions about best management practices. It was postulated that the size distribution of the subsoil clumps created by delving might influence soil-water availability (and hence crop yield) because, although the clay increases water retention in the root-zone, it can also cause poor soil aeration, high soil strength and greatly reduced hydraulic conductivity. We prepared laboratory mixtures of sand and clay-rich subsoil in amounts considered practical (10% and 20% by weight) and excessive (40% and 60% by weight) with different subsoil clod sizes (<2, 6, 20 and 45 mm), for which we measured water retention, soil resistance, and saturated hydraulic conductivity. We calculated soil water availability by traditional means (plant-available water, PAW) and by the integral water capacity (IWC). We found that PAW increased with subsoil clay, particularly when smaller aggregates were used (≤6 mm). However, when the potential restrictions on PAW were taken into account, the benefits of adding clay reached a peak at ~40%, beyond which IWC declined towards that of pure subsoil clay. Furthermore, the smaller the aggregates the less effective they were at increasing IWC, particularly in the practical range of application rates (<20% by weight). We conclude that excessive post-delving cultivation may not be warranted and may explain some of the variability found in crop yields after delving.

Rapid in situ conversion of late-stage volcanic materials to halloysite implicated in catastrophic dam failure, Hawaii

Abstract Ka Loko Dam, in Kauai, Hawaii, failed suddenly and catastrophically on March 14, 2006. The resulting breachwas marked by three topographic benches, the lowest of which exposed native volcanic deposits once resident in the dam foundation. These deposits were found to contain outcrops of a waxy, gel-like material that appeared to result from in situ weathering processes. This unusual material was found to be highly enriched in halloysite. Gravel-size pieces in the hydraulic fill of the embankment derived from these materials also exhibited significant in situ weathering and significant halloysite content. Engineers and geologists generally recognize that bedrock materials weather progressively into soil constituents over ‘geological time’, and that this process is accelerated in tropical environments. Still, the strength, stiffness and durability of bedrock, earth and embankment materials are not expected to vary significantly over the geologically short life of a dam. In the case of Ka Loko Dam, however, the volcaniclastic sediments that comprise the local bedrock experienced substantial in situ weathering over its geologically brief 115-year operational lifetime. Prolonged exposure to seepage of anoxic water weathered the sediments completely to saprolite, including weak, sensitive, fine, spherical halloysite.